ALBERT

Beschreibung: Gold nanorods (GNRs) with surface plasmon resonance peak at 1063.8 nm were fabricated and experimentally exploited as the single and combined saturable absorber (SA) in a $Q$ -switched Nd:YAG laser for the first time. In the situation using GNRs as a single SA, the maximum pulse energy of 19 $muhbox{J} $ was achieved at a pulse repetition rate of 20 kHz. However, due to the small effective modulation depth of the GNRs, microsecond pulses were generated in such a GNRs-based passively $Q$ -switched laser. A novel configuration using GNRs as a combined SA in an acoustooptic $Q$ -switched Nd:YAG laser was proposed. The small nonlinear loss modulation induced by the GNRs played a distinctive role in flexibly manipulating the nanosecond pulse waveforms. Our study demonstrated the feasibility of generating high-energy pulses by singly using GNRs-based SA in all solid-state lasers. Meanwhile, a new application mode incorporating GNRs as a combined SA was studied, providing an alternative method to take advantage of the nanomaterial-based SA with a relatively small modulation depth.

Beschreibung: This paper proposes the improved design of an ultraviolet (UV)- and-blue-light-inhibited white light-emitting diode for use as a safe and practical light source. Covered with a glass substrate coated with a photocatalyst resin on one side and a reflectance film on the other side, wavelengths below 400 nm are reflected back to re-excite the red–green–blue phosphors and, consequently, enhance luminous efficiency. The absorption spectrum of bismuth oxide photocatalysts is below 521 nm, and the leaked UV and blue light can be absorbed, thereby exciting electron–hole pairs and producing the photocatalytic effect. Thus, blue light and UV leakage can be suppressed appreciably, and the luminous efficiency can be increased markedly. Experimental results showed a UV suppression ratio of 88.43% and a visible light increasing ratio of 21.66%. The Commission International de L'Eclairage chromaticity coordinates $(x,y)$ were (0.343, 0.404), the correlated color temperature and the deviation from de Blackbody locus were (5201 K, 0.0250), and the color rendering index was 93.16. In addition, the photocatalyst coating layer can act as a diffuser to provide a comfortable visual experience and facilitate environmental purification.

Beschreibung: Metamaterial-based perfect absorbers have attracted considerable attention due to their potential for practical applications. The existing absorbers, however, are mostly polarization insensitive or only sensitive to one direction, which is inapplicable in some areas. Polarization tunable or high absorption in two orthogonal directions is very useful and necessary. Herein, we present a polarization tunable absorber formed by an asymmetric patch and a dielectric layer on top of a metallic board. With this structure, the frequency of the absorber can be tuned by merely changing the polarization of the incident. The tunable mechanism originates from the different length of the patch along the two orthogonal directions. The concept is rather general and applicable to various absorbers, as long as the asymmetric design is valid. The absorber can find practical applications in manipulation of the polarization of the light and detecting waves with specific polarization.

Beschreibung: We demonstrate low-voltage waveguide-coupled germanium avalanche photodetectors (APDs) with a (wafer-scale mean) gain $times$ bandwidth product of 140 GHz at $-$ 5 V by utilizing a 185-nm-thick Ge layer. An optical receiver based on such an APD operating up to 25 Gb/s is demonstrated.

Beschreibung: The two-, three-, and four-channel mapping multiplexing technique (MMT) is demonstrated to increase the data capacity of multilevel intensity-modulated transmission formats to 2, 3, and 4 bits/symbol with a substantial reduction in power penalty. This paper outlines the $N$ -channel MMT design metric consideration influence on the performance enhancement of higher order amplitude modulated systems in terms of power penalty, receiver sensitivity, and spectral efficiency. The signal space model has been developed for $N$ -channel MMT, where the average electrical and optical power expressions are derived. Transmission of 2, 3, and 4 bits/symbol using MMT system shows better performance in terms of the average optical power penalty, in comparison with pulse amplitude modulation (M-PAM) formats, with respect to the information capacity. The proposed scheme can be considered as an alternative to M-PAM systems with enhanced power efficiency.

Beschreibung: Optical networks-on-chips (ONoCs) are burgeoning structures of communication that can give a solution to the bandwidth and latency for multiprocessor systems-on-chips. Furthermore, photonic devices are widely used in ONoCs but suffer from crosstalk noise. Therefore, crosstalk noise is the critical factor that affects signal transmission and limits the network scale of ONoCs. To reduce the crosstalk noise sharply, an angle optimization that uses the cross angle of 60 $^{circ}$ or 120 $^{circ}$ instead of the conventional angle of 90 $^{circ} $ is proposed to optimize the Crux router. Furthermore, we present a signal transmission system simulation model for the mesh-based ONoC. For different routers applied to the mesh-based ONoC, the simulation model can present the time-domain waveform, eye diagram, and bit-error-rate (BER) curve clearly. On the basis of the analytical model, the results show that the minimum signal-to-noise ratio (SNR) of a 7 $times$ 7 mesh-based ONoC is 20.9 dB when using optimized Crux routers, and the injection power is equal to 0 dBm, which is 2.1 dB better than using Crux routers. Therefore, the application of angle optimization to the Crux router and mesh network can minimize crosstalk noise and improve the scalability of mesh-based ONoCs.

Beschreibung: A working small-footprint asymmetrically and vertically coupled hybrid Si/GaN microring resonator (HMR) was demonstrated. The HMR of a minimum radius of 20 $muhbox{m}$ was fabricated in a high-yield ( $sim$ 90%) hybrid nanophotonic platform that allowed interactions between the GaN microring and the underlying Si waveguide circuit. The HMR's spectral response across the telecommunication C- and L-bands was measured. The high extinction ratio of up to 17 dB, the resonance linewidth as narrow as 40 pm, and the intrinsic quality factor of up to 70 000, which was the highest value for GaN microring reported so far, were achieved. The explicit analytical model for the high-index-contrast HMR was developed. Our basic study opened new possibilities for exploring GaN–Si nonlinear phenomena and for developing complex on-chip optical interconnects.

Beschreibung: In the light-emitting-diode (LED) industry, it is a great challenge to simultaneously achieve high efficiency values and excellent light quality in practical applications. In this paper, we have designed microscale-roughness-controlled-surface modules to optimize rectified mellowness index, quantum efficiency, correlated color temperature, and color rendering index; we have also discovered a mechanism that reduces the total internal reflection trapped within the diffuser, maximizes phosphor fluorescence probabilities, and randomizes photon emissions. The first effect subsequently leads to the photon-propagation-direction rectification, such that incident photons are allowed to escape from the top but are discouraged to reversely exit from the bottom. In addition to the discovery of this mechanism, we have proposed a previously undefined criterion that evaluates the uniformity of various optical parameters exemplified by wavelengths, solid angles, and spatial distributions. Current findings offer a guide on future research for human-eye-comfort-related uniformities.

Beschreibung: We experimentally investigate the wavelength-division multiplexing (WDM) transmission of single-carrier 400G based on an orthogonal optical time-division multiplexed (orth-OTDM) 80-GBd polarization-division multiplexed (PDM) 8 quadrature-amplitude modulation (QAM) signal. The 80-GBd 8QAM signal is generated using the OTDM method based on two tributaries of time-domain orthogonal pulses with 40-GBd 8QAM signals. Full-band coherent detection is utilized with digital signal processing based on a single broadband receiver for the 80-GBd signals. The WDM transmission performance of single-carrier 400G signals within different channel grids and carrier spacing is also investigated. Finally, four 400G channels, each carrying an 80-GBd PDM-8QAM signal within 100-, 80-, and 75-GHz grids, are successfully transmitted over 1600-, 800-, and 400-km standard single-mode fibers with Erbium-doped fiber amplifier (EDFA)-only amplification below a pre-Forward error correction (FEC) bit-error rate (BER) of $2.4times10^{-2} $ (20% soft-decision FEC) , respectively. The achieved spectral efficiency for the 400G channel can be 4, 5, and 5.33 bit/s/Hz, respectively.

Beschreibung: We design a series of plasmonic bowtie antennas with broadly varying resonant wavelengths from 1.34 to 3.36 $muhbox{m}$ under the same antenna footprint size. Based on the prototype of contour bowtie antennas (CBAs), we modify the geometry of CBAs with extensive resonant paths and, therefore, redshift resonances and maintain the gap enhancement factors, at least at the same level with that of the solid bowtie antenna.

Beschreibung: As data centers evolve to sustain the economic scale in network traffic, optical interconnections are adapted to support the vast amount of capacity requirements. Among emerging solutions is the incorporation of digital-signal-processing (DSP) functionalities at the receiver to compensate for the optical fiber and optoelectronic components impairments. The bottleneck in such architecture is often implementing a high-resolution and high-speed analog-to-digital-converter (ADC) device, which usually leads to massive power consumption and high costs. In this paper, we propose novel transmission schemes based on low-resolution ADCs, termed DSP-enhanced analog-to-digital conversion. The proposed methods are based on the mathematical principle of predictive quantization, combined with linear equalization theory. Mathematical analysis and performance bounds associated with common channel impairments are derived. Simulation results demonstrate that the DSP-enhanced ADC provides a significant signal-to-noise-ratio (SNR) gain and, subsequently, may enable heavy digital compensation with reduced-resolution ADCs.

Beschreibung: A monolithically integrated silicon coherent receiver based on a novel scheme with a crossing-free 90 $^{circ}$ hybrid optical coupler and two balanced germanium photodetectors is reported. The integrated receiver is compact (footprint is $0.8times 1.0 hbox{mm}^{2}$ ), and it is demonstrated by working with single-polarization 56-Gb/s quadrature phase-shift keying (QPSK) and 80-Gb/s 16-quadrature amplitude modulation (16-QAM) signals. In particular, QPSK transmission that is back-to-back at 28 Gbaud shows a bit-error rate (BER) below $10^{-4}$ for an optical signal-to-noise ratio (OSNR) lower than 17 dB, whereas 16-QAM at 20 Gbaud shows a BER not better than $3^{ast}10^{-2}$ because of the nonideal behavior of the device. Reasons for the performance limitations are discussed.

Beschreibung: We investigate lifetime (induced dipole–dipole interaction) of spontaneous parametric four-wave mixing (SP-FWM) and multiorder fluorescence (FL) signals controlled by polarization dressing in a heteronuclear-like molecule system of Pr $^{3+} $ :YSO. The comparisons between a lifetime of SP-FWM and multiorder FL controlled by single dressing, double dressing, and triple dressing polarization have been investigated. It is observed that FLs have a stronger dressing effect than SP-FWM. The polarization dressing is more obvious in the intensity signal in the spectral domain than that in the time domain. Such results have potential applications in quantum communication such as optical information storage, routing, and processing on a photonic chip.

Beschreibung: The limited band of optical and electrical devices in an intensity-modulation direct detection (IM/DD) system is considered to be a major bottleneck that hinders the increase in transmission capacity. Spectrally efficient modulation formats and advanced digital signal processing are key technologies to improving the performance of IM/DD for high-speed short-reach optical systems. In this paper, we propose and experimentally demonstrate a novel Nyquist eight-level, pulse-amplitude, single-carrier modulation with frequency-domain equalization (PAM8-SCFDE). In addition, 40-GBd Nyquist PAM8-SCFDE signals can be successfully transmitted over 2-km standard single-mode fiber or 20-km large effective area fiber based on 10-G-class DAC and photodiode. Therefore, the achieved aggregate data rate is 120 Gb/s, and the measured bit error rate (BER) is under 7% pre-forward-error-correction (FEC) threshold of $3.8times 10^{-3} $ . Compared with conventional PAM8, this scheme can provide double spectral efficiency and 6-dB receiver sensitivity improvement.

Beschreibung: An eight-channel integrated optical add-drop multiplexer with cascaded parent-sub microring resonators is experimentally demonstrated on a silicon-on-insulator (SOI) substrate. Through thermal tuning, each channel can be independently switched between the adding and dropping states. The measured thermal tuning efficiency is 0.15 nm/mW. Typically, all eight channels can be multiplexed and demultiplexed with a 2.6-dB drop loss, a 0.36-nm bandwidth ( $〉$ 40 GHz), and a $-$ 20-dB channel crosstalk. Moreover, the scheme to increase the number of operating channels is also discussed.

Beschreibung: We apply the principle of master-slave (MS) interferometry to a full-field swept source optical coherence tomography (OCT) setup equipped with a fast 2-D camera. MS interferometry does not involve Fourier transformations and, therefore, eliminates the need for spectrum data resampling required by the conventional spectral domain OCT. Using this method in a full-field OCT setup, en face images are acquired in vivo from finger skin, Drosophila melanogaster larvae, and pupae, with no spectrum resampling and no mechanical scanning. The signal processing is based on a comparison operation of the shapes of channeled spectra for each camera pixel, with a set of reference signals (masks). In this way, en face OCT images can be obtained with no need for the volumetric assembly and software cutting the en face images from an image volume, which are required by the conventional spectral domain OCT method. We demonstrate that the sensitivity and axial resolution of the MS method in a full-field swept source OCT setup are similar to the values obtained using the conventional Fourier-transformation-based swept source OCT method in a full-field setup. Multiple en face images can be produced in parallel by using multiple stored shapes of channeled spectra for the depths of interest. The full-field MS-OCT method presented here opens the possibility of parallel processing for all image points in a 3-D volume of the object.

Beschreibung: We reported a gas pressure sensor based on CO 2 -laser-induced long-period fiber grating (LPFG) in an air-core photonic bandgap fiber (PBF). The LPFG was inscribed in an air-core PBF by the use of an improved CO 2 laser system with an ultraprecision 2-D scanning technique, which induced periodic collapses of air holes along the axis of the PBF. Such an LPFG could be used to develop a promising gas pressure sensor with a sensitivity of −137 pm/MPa. Moreover, a simplified fiber model with a relatively similar elastic response was developed to qualitatively study the gas pressure response of the LPFG inscribed in the air-core PBF. A simulated stress distribution along the LPFG revealed that the gas pressure leads to a stress concentration at the collapsed area of the air holes in the fiber cladding, which finally results in a resonant wavelength shift of the LPFG through an elastooptical effect.

Beschreibung: Visible light communication (VLC) is attractive as a supplementary method for future fifth-generation (5G) wireless communication owing to the shortage of radio-frequency bandwidth in conventional wireless communications. The VLC systems reported in the literature are mainly based on PIN receivers (Rxs). It is highly desirable if these VLC signals can be detected by using built-in complementary metal–oxide–semiconductor (CMOS) cameras as Rxs to provide flexible and low-cost wireless communications. However, using the CMOS camera is challenging. In this paper, we propose and demonstrate a VLC link using a CMOS mobile phone camera as Rx. By using the rolling shutter effect of the CMOS sensor, the VLC data rate can be significantly enhanced. We first use a second-order polynomial fitting to mitigate the “blooming effect” (saturation of pixels) of the CMOS sensor. In order to extend the VLC transmission distance and mitigate the influence of the background noise, we also propose and demonstrate using histogram equalization and Sobel filter to enhance VLC signal performance. Finally, a third-order polynomial fitting is used to define the threshold. The experimental results show that significant improvement of bit error rate (BER) for about an order of magnitude under different illuminances can be achieved.

Beschreibung: Nd(TTA) 3 (TPPO) 2 complex (HTTA: 2-thenoyltrifluoroacetone, TPPO: triphenylphosphine oxide) doped SU-8 polymer waveguide amplifiers were demonstrated. The absorption and photoluminescence spectra were observed on a 100- $mutextrm{m}$ -thick film of the Nd(TTA) 3 (TPPO) 2 complex doped SU-8 polymer. Under excitation at 808 nm, the fluorescence was obtained at 904 nm, 1068 nm, and 1346 nm wavelengths. The full-width at half-maximum bandwidth was about 25 nm wide around 1068 nm. Using reactive ion etching technology, optical waveguides were fabricated by taking the Nd(TTA) 3 (TPPO) 2 complex doped SU-8 polymer and polymethylmethacrylate (PMMA) material as the core and cladding layers, respectively. According to fundamental rate equations and optical power propagation equations, the gain characteristic of the polymer waveguide amplifier was simulated to be 5.1 dB/cm. An experimental setup for the optical gain measurement was established. A relative optical gain of about 2.4 dB/cm at 1064 nm was obtained in a 20-mm-long multimode waveguide for an input signal power value of 0.1 mW and pump power of 140 mW.

Beschreibung: The nonlinearity of a light-emitting diode (LED) limits the system performance of a visible light communications (VLC) system. Many nonlinearity mitigation methods have been studied. Recent technology advances in the micro-LED array have provided a simple alternative to deal with the system nonlinearity. In this paper, we study the approach of providing multiple access for the VLC system with the micro-LED array. A code-division multiple-access (CDMA) scheme that enables multiple access with variable data rate is proposed. In addition, the proposed system architecture is immune to the nonlinearity of the micro-LED array, thus providing optimal system performance. Simulations and laboratory experiments show the robustness and consistency of the proposed scheme.

Beschreibung: We describe a compact Yb:YAG laser Q-switched by a graphene-based saturable absorber and pumped by a laser diode at 932 or 969 nm. The compact laser generates a maximum average output power value of 185 mW at 1032 nm with a slope efficiency value of 12%. The shortest duration of the Q-switched pulse achieved is 228 ns at a repetition frequency of 285 kHz. The maximum pulse energy amounts to 0.65 $mumathrm{J}$ .

Beschreibung: Orthogonal frequency-division multiplexing (OFDM) has been widely studied and adopted in visible-light communication (VLC) systems because of its high spectral efficiency and low-complexity implementation. However, OFDM-based VLC systems suffer from a high peak-to-average power ratio (PAPR), along with the nonlinear transfer characteristics of light-emitting diodes (LEDs). In this paper, a subcarrier grouping scheme for an OFDM-based VLC system is proposed. This scheme exploits the property that one LED lamp usually contains multiple independent LED chips. In the proposed scheme, all OFDM signal subcarriers are divided into groups, and each group is transmitted by an individual LED chip while the same receiver structure is maintained. The PAPR is reduced significantly because only some of the subcarriers are located on each LED chip. Hence, the achievable input power can be increased, leading to a higher signal-to-noise ratio (SNR) and a lower bit error rate (BER). Simulated and experimental results demonstrate that the BER performance can be improved by over two orders of magnitude. Moreover, an adaptive switching mechanism is also proposed in this paper, in which a subcarrier grouping scheme is employed adaptively when the required input power is high. Simulation results validate that the proposed mechanism can enhance system robustness significantly.

Beschreibung: Distributed fiber sensing technology based on a Brillouin optical time domain reflectometer is widely applied for health monitoring in engineering. However, it cannot perceive quick external interference. In this paper, we propose to use polynomial fitting on the inverse of an incomplete Brillouin spectrum and find the peak frequency by statistical analysis. Simulation results show that the peak frequency obtained by our method is only 1.096 MHz higher than that by using the Lorenz fitting method on a whole discrete Brillouin spectrum. The method has high accuracy, good repetitiveness, and high stability and is helpful in health monitoring for situations with interference that lasts for a short time.

Beschreibung: We establish an optical path difference (OPD) distribution model in a spatial scanned polarized low-coherence interferometry (LCI) considering the nonperpendicular incidence of light to explore the spatial frequency drift. Simulation shows that the OPD is no longer linearly distributed on a charged coupled device (CCD) and that the spatial frequency drifts approximately linearly as low coherent interference fringe (LCIF) shifts. A compensation process for spatial frequency domain analysis (SFDA)-based algorithm is proposed to avoid interference-order misidentification and consequent jump error. We verified our analysis and effectiveness of the proposed algorithm with an optical fiber Fabry–Pérot pressure sensing experiment. The measured spatial frequency drift agreed well with simulation results, and the interference-order misidentification was eliminated.

Beschreibung: Vehicle-to-vehicle (V2V) communication using visible light communication (VLC) technology under fog conditions is presented. Fog is known as one of the most detrimental atmospheric conditions that causes outdoor optical wireless communications to be unreliable. The effect of the fog conditions is experimentally analyzed in the VLC-based V2V system. Recognizing the least attenuation coefficient and a taillight color of a vehicle, a red light-emitting diode (LED) was employed in the experiment. In addition, a Fresnel lens and multiple photodiodes are utilized to efficiently counteract the impairment caused by fog. The experimental results demonstrate that the proposed VLC-based V2V system offers a reliable V2V data transmission over the fog-impaired optical channel with a relatively high signal-to-noise ratio (SNR), even under a heavy-fog condition.

Beschreibung: We examine and demonstrate a biosensor using single-mode tapered fiber that has been immobilized with biorecognition molecules to sense targeted proteins. Interaction of evanescent waves with the external medium surrounding the tapered region produces an interferometric-patterned spectrum, which shifts correspondingly to any changes of refractive index (RI) in the external medium. The proposed setup managed to obtain an RI sensitivity and concentration sensitivity of 2526.8 nm/RIU and 20.368 $text{nm}/mutext{M}$ , respectively, which, to our knowledge, is highly sensitive when compared with previous studies. The dynamic performance, good specificity, and high sensitivity of the proposed method highlight an immensely beneficial choice for immunological diagnostics.

Beschreibung: In this paper, a method to enhance the security of the optical stealth channel is proposed. The proposed method uses the techniques of polarization modulator based code-shift-keying (CSK) data modulation and optical code-division multiple-access (OCDMA) en/decoding, where an incoherent light source is needed. Our analysis and experimental results show that the CSK-OCDMA signal, where a wavelength selective switch is used for optical en/decoding, has been stealthily transmitted over a 40-km wavelength-division multiplexing optical fiber link. In addition, the stealth channel only causes a power penalty less than 0.48 dB to the public channel, whereas both of these two channels can achieve error-free transmission (with bit error rate less than $10^{-9}$ ).

Beschreibung: In this paper, a new technique to attain a tunable parametric amplifier for application in the communication wavelength, based on near-zero ultraflat dispersion with PCF, has been explored. The chromatic dispersion profile and the corresponding zero dispersion wavelength (ZDW) of the optimized PCF can be well tuned to provide a wide gain spectrum in the communication wavelength just by changing the temperature of the fiber. A detailed investigation reveals that the fiber retains its near-zero ultraflat nature when the temperature is increased by 2.50 $^{circ}hbox{C} $ from room temperature. While changing the temperature, the ZDWs can be well tuned, giving rise to a gain region of hundreds of nanometers around the pumping wavelength by changing the temperature of the fiber externally.

Beschreibung: Two kinds of novel plasmonic high sensitivity of refractive index (RI) sensors based on analyte-filled photonic crystal fiber (AF-PCF) are proposed in this paper. The metallic gold and silver is used as the surface plasmon resonance activity metal. A full-vector finite-element method is applied to analyze and investigate the sensing and coupling characteristics of this designed AF-PCF with the gold or silver layer. Phase matching between the second surface plasmon polariton and fundamental modes can be met at different wavelengths as the analyte of the RI is increased from 1.40 to 1.42. The phase-matching wavelength of the designed AF-PCF with the gold layer is shifted to the longer wavelength direction compared with that with the silver layer, and the resonance strength is much stronger. The average sensitivities of 7040 and 7017 nm/RIU in the sensing are arranged from 1.40 to 1.42 with high linearity are achieved for the designed sensors with the gold and silver layers, respectively, which are almost the same. However, the figure of merit with the silver layer is much better than that with the gold layer.

Beschreibung: A reflective semiconductor optical amplifier intensity modulator (RSOA-IM) frequency chirp considerably increases the system frequency response of an intensity-modulation and direct-detection (IMDD) PON system over the passband signal spectral region. Adaptive bit and power loading of optical OFDM (OOFDM) enables full use of the RSOA-IM frequency chirp-enhanced passband system frequency response characteristics. As a direct result, without requiring extra electrical and optical equalization schemes, an RSOA-IM with a 3-dB small-signal modulation bandwidth as low as 1 GHz can support 40 Gb/s OOFDM signal transmissions over 25 km SSMF IMDD PON systems free from optical amplification and chromatic dispersion compensation. This paper suggests that significantly relaxed requirements on both RSOA-IM's 3-dB small-signal modulation bandwidth and frequency chirp are feasible for achieving desired OOFDM transmission performances for next-generation PONs.

Beschreibung: The mucosal microanatomy of the large intestine is characterized by the presence of crypts of Lieberkühn, which is associated predominantly with goblet cells. Such cellular-level intestinal microstructures undergo morphological changes during the progression of bowel diseases, such as colon cancer or ulcerative colitis. As an indicator of gastric cancers, intestinal metaplasia in the large intestine is characterized by the appearance of goblet cells in gastric epithelium, and therefore, visualization of intestinal microstructure changes in cross-sectional view, particularly in vivo , in a high-speed fashion would assist early disease diagnosis and its treatment. In this paper, we investigated the capability of micro-optical coherence tomography $(mutext{OCT})$ for high-speed cellular-level crypt and goblet cell structures imaging ex vivo and in vivo . The adopted $mutext{OCT}$ system achieved a resolution of 2.0 $mutext{m}$ in both the lateral and axial directions in air. Ex vivo and video-rate in vivo images acquired in 3-D at respective imaging rates of 20 and 60 frames/s are presented and compared with the histology images. Imaging results show that the detailed microstructures, such as the crypt lumen and the goblet cells, could be clearly identified and are also comparable with those in histology images. Such comparisons also indicate that high-resolution $mutext{OCT}$ could be a powerful tool to perform “optical biopsy” in colorectal tissue. This is the first work, to the best of our knowledge, on cellular-level structure imaging in intestinal mucosa using spectral-- omain OCT.

Beschreibung: We demonstrate an omnidirectional laser from a 3-D Bragg microcavity, which is fabricated from a cholesteric liquid crystal that is doped with reactive mesogen. The wavelength of the laser is found to be determined by the position of the photonic bandgap that was formed within the 3-D Bragg microcavity. Compared with the laser from pure cholesteric liquid crystal microcavity, a rigid polymer network is formed within the microcavity due to the doped reactive mesogen. The polymer network successfully enhances the resistance ability of the 3-D Bragg microcavity to higher pump power by 1.5 times, thus leading to a more stable laser based on it. This laser may have potential use in optics, displays, and other photonic devices.

Beschreibung: A copper-nanowire (CuNWs)-based saturable absorber (SA) is utilized as a new Q-switcher for pulsed operations in visible regions. The plasmon resonance effect of CuNWs contributes to inducing the saturable absorption property. By incorporating the CuNWs-based SA into a red praseodymium $Pr^{3+}$ - doped ZBLAN fiber laser cavity, the red light shifts from a continuous wave (CW) state to a stable pulsed operation state. When it changes the injected pump power, the pulse repetition rate can be varied from 239.8 to 312.4 kHz. The narrowest pulse duration of this laser is 394 ns. Moreover, the maximum output power under a $Q$ - switching operation is 9.6 mW. The results indicate that a CuNWs-based SA is an available Q-switcher for the red laser, revealing the potential of metal nanomaterials for short-pulse generation in visible regions.

Beschreibung: In this paper, we propose a zeroth- and first-order-convertible fiber interleaving filter that has a frequency-interleaving capability in the zeroth- or first-order multiwavelength transmission spectrum. The proposed interleaving filter is a kind of polarization-diversity-loop-based comb filter composed of a polarization beam splitter, three half-wave plates (HWPs), and two polarization-maintaining fiber (PMF) segments, each of which is located between two adjacent HWPs. The HWP inserted between two PMF segments enables the interleaving filter to operate in the zeroth- or first-order transmission mode by adjusting the relative angular difference between the principal axes of two PMF segments. With the orientation angles of the three HWPs set as specific angles, first-order flat-top and narrow-band transmission spectra with a channel spacing of ∼0.792 nm could be obtained and be interleaved by controlling only the HWP between the two PMF segments. In particular, at other specific angle sets of the three HWPs, the zeroth-order transmission spectrum with the same channel spacing could be obtained, and their multiwavelength channels could be interleaved by adjusting only the orientation angle of the intermediary HWP. That is, a first-order comb spectrum could be converted into a zeroth-order one with the same fringe period, and each one could be interleaved through the proper control of the three HWP angles.

Beschreibung: We investigate the temporal dynamics of a multinonlinear electrooptical system with delayed feedback. We show that when a self-sustained electronic oscillator is introduced in the electric path of the feedback loop, a wide variety of complex periodic and chaotic states can be excited. In particular, the chaotic breather and pulse-package oscillations with slow–fast temporal dynamics are experimentally observed in the system, and they are characterized by excitablelike trajectories and scroll patterns in the state space. We show that a notable specificity of these chaotic states is the presence of three distinct and well-separated timescales in the temporal time traces. We analyze the interaction between the main nonlinearities of the system and the delayed feedback loop, and finally, we discuss the main dynamical features displayed by this family of photonic systems.

Beschreibung: Fog is considered to be a primary challenge for free space optics (FSO) systems. It may cause attenuation that is up to hundreds of decibels per kilometer. Hence, accurate modeling of fog attenuation will help telecommunication operators to engineer and appropriately manage their networks. In this paper, we examine fog measurement data coming from several locations in Europe and the United States and derive a unified channel attenuation model. Compared with existing attenuation models, our proposed model achieves a minimum of 9 dB, which is lower than the average root-mean-square error (RMSE). Moreover, we have investigated the statistical behavior of the channel and developed a probabilistic model under stochastic fog conditions. Furthermore, we studied the performance of the FSO system addressing various performance metrics, including signal-to-noise ratio (SNR), bit-error rate (BER), and channel capacity. Our results show that in communication environments with frequent fog, FSO is typically a short-range data transmission technology. Therefore, FSO will have its preferred market segment in future wireless fifth-generation/sixth-generation (5G/6G) networks having cell sizes that are lower than a 1-km diameter. Moreover, the results of our modeling and analysis can be applied in determining the switching/thresholding conditions in highly reliable hybrid FSO/radio-frequency (RF) networks.

Beschreibung: Based on the reconstruction-equivalent-chirp technique and tuning only by the injection current, a multisection DFB tunable laser is demonstrated experimentally. At the temperature of 20 °C, each laser can be tuned at about 3.4–3.6 nm with a side-mode suppressing ratio (SMSR) of over 35 dB from 30 to 170 mA, and the tuning speed is about 10–30 ms, which is much faster than that by the thermoelectric cooler. The results may help to realize very low cost tunable lasers in the future.

Beschreibung: A compact and ultrabroadband polarization beam splitter (PBS) utilizing a hybrid plasmonic Y-branch (HPYB) on a silicon-on-insulator (SOI) platform is proposed and numerically demonstrated. The HPYB consists of a vertical hybrid plasmonic waveguide (HPW) and a horizontal HPW formed by silicon (Si) and silver (Ag) strip waveguides sandwiched with a silicon dioxide (SiO 2 ) layer, in which the vertical and horizontal hybrid plasmonic modes (HPMs) are excited by the input transverse electric (TE) and transverse magnetic (TM) modes, respectively. The HPMs are split into different ports and coupled back to TE and TM modes to implement the polarization splitting function. A simplified and compact HPYB is robust for the HPMs' generation. The structure is wavelength insensitive since the HPMs' excitation is weakly correlated to the optical wavelengths. The simulation results show that the HPYB-based PBS has a compact footprint of $5 times 1.8 mutext{m}^2$ and an ultralarge working bandwidth of 285 nm, with the polarization crosstalk < −10 dB and the worst-case TE (TM) mode insertion loss of −1.53 (−2.35) dB. The device also exhibits a large fabrication tolerance of 210-nm variation (from −100 to 110 nm) to the waveguide width for both polarizations.

Beschreibung: Visible light communications (VLC) has emerged as one of the prominent technologies to cater to the ever-increasing high-speed-data demand for proposed fifth-generation (5G) systems. However, two main issues affect the performance of VLC in an indoor environment: a) nonlinearity of light-emitting diode, which renders the overall system nonlinear; and b) intersymbol interference due to the propagation channel, which closes the eye diagram of the transmit constellation and, hence, causes it to be unsuitable for detection. To counter these artifacts, complex post-distortion receivers such as the Volterra-decision feedback equalizer (DFE) have been proposed to recover the transmit symbols. In this paper, the use of a reproducing kernel Hilbert space-based minimum symbol error rate equalizer is proposed that provides performance comparable to a long Volterra-DFE, with much less computational cost. Simulations have been carried over IEEE 802.15 personal area network (PAN) channels, which suggest that the proposed approach gives equivalent performance in an indoor VLC channel, as compared with Volterra-DFE with far fewer computations. An analytical expression for mean square error dynamics over these channels is also derived, and it is observed that the theoretically derived expression matches the simulation results for the considered IEEE 802.15 PAN indoor VLC channels.

Beschreibung: Within the validity of the first-order Born approximation, the near-field evanescent wave is analytically obtained from the scattering of a monochromatic plane wave from a spatially deterministic hollow particle with a semisoft boundary. The scattered spectral density shows the fluctuating profile in any cross-sectional plane. The increase of either the outer or the inner radius of the scattering potential accelerates the decay of the spectral density value. Given a smaller integer number in the summation index of the scattering potential, the scattered wave experiences less spreading in the near field. The obtained results have potential applications to generate the unique scattered patterns in the near field and the guidance of tiny particles along the desired trajectories.

Beschreibung: Toward the next-generation exa-scale short-reach optical interconnects (OIs) supporting large-capacity data transmission, a compact computer-compatible 8-core heterogeneous trench-assisted multicore fiber (TA-MCF) is proposed, in which cores are arranged in a rectangular array. To analyze the crosstalk (XT) between adjacent cores of TA-MCF OI, a rigorous full-vectorial $mathbf{H}$ - field finite element method (FEM) and coupled power theory are applied. The impact of various trench design parameters on the mode-coupling coefficient $C_{mn}$ and the coupling length $L_{c}$ is discussed in detail. An accurate explicit condition for the achievement of low XT in an 8-core heterogeneous TA-MCF OI is obtained through numerical simulations. A rigorous modal solution approach based on the computationally efficient FEM and the least squares boundary residual method is employed to analyze the coupling loss caused by the misalignment to a butt-coupled TA-MCF OI.

Beschreibung: A new photonic microwave image rejection mixer that can achieve high conversion efficiency and has a wide bandwidth is presented. It is based on an integrated dual-parallel Mach–Zehnder modulator (MZM) with a 90° polarization rotator on one branch of the modulator. The two modulators connected in parallel are biased at the minimum transmission point to generate two sets of orthogonally polarized radio frequency (RF) and local oscillator (LO) modulation sidebands with the optical carrier being suppressed. Suppressing the optical carrier enables high RF and LO modulation sidebands to be detected by a photodiode without saturation. The LO modulation index is also designed to obtain a high conversion efficiency mixing operation. Experimental results demonstrate frequency downconversion with an image rejection function and a high conversion efficiency of > $-$ 5 dB over a wide 3 to 20 GHz frequency range. A technique for compensating the effect of coupler amplitude and phase imbalance to obtain a high image rejection ratio is also experimentally demonstrated.

Beschreibung: We propose and experimentally demonstrate an indoor positioning system which combines identity positioning and the radio frequency carrier allocation technique in order to reduce the signal interference from nearby light-emitting diodes (LEDs) and improve the accuracy of positioning. In addition, the solar cell is used as an optical receiver for the visible light positioning system. Due to the benefits of solar cells such as low-cost, high light sensitivity, and ease in integration with wearable devices, the proposed system could be an energy-efficient and environmentally friendly choice for indoor positioning in the future.

Beschreibung: A multiple-image encryption method that is based on a modified logistic map algorithm, compressive ghost imaging, and coordinate sampling is proposed. In the encryption process, random phase-only masks are first generated with the modified logistic map algorithm; multiple secret images are transformed to be sparsed by the 2-D discrete cosine transformation (DCT) operation and scrambled by different random sequences; the scrambled images are then grouped to one combined image with the help of the coordinate sampling matrices; finally, putting the combined image in the object plane of the compressive ghost imaging system, the ciphertext will be obtained from the buck detector and transferred to the receivers. During the decryption, which is the possession of the correct principle keys and the assistant geometrical parameter key, the original secret images can be successfully decrypted by the logistic map algorithm, measurement matrix generations, compressive sensing reconstruction, DCT distribution extraction, inverse DCT transform, and filtering operation.

Beschreibung: A new microwave photonic phase controlling structure that can realize a continuously tunable 0 $^{circ}$ to 360 $^{circ}$ phase shift using only one DC voltage control is presented. Unlike other phase controlling structures, the radio-frequency (RF) signal phase shift produced by the new phase controller has a predefined linear relationship with the control voltage. This enables users to easily determine the control voltage needed to realize the desired RF signal phase shift. The microwave photonic phase controller is based on an integrated dual-polarization dual-parallel Mach–Zehnder modulator (DPMZM) and a polarization-dependent optical phase shifter. It is suitable for realizing multiple RF signal phase shifts, which are required in beamforming applications. Experimental results are presented which demonstrate the realization of a continuously tunable $-$ 180 $^{circ}$ to 180 $^{circ}$ phase shift with little phase and amplitude variations over a wide frequency range, as well as a linear RF signal phase shift to control voltage relationship.

Beschreibung: A simple, high-sensitivity, and low-cost optical sensor for the measurement of temperature and salinity in seawater is demonstrated based on a microfiber directional coupler. The shifts of sensing dips are highly sensitive to the change of salinity and temperature in seawater with highest sensitivities at 930 pm/‰ and $-$ 160 pm/ $^{circ}$ C for coupler assembled by 3.79- and 2.20- $mutext{m}$ - diameter fibers. The temperature sensitivity can be further improved to $-$ 1130 pm/ $^{circ}$ C for smaller fibers with diameters of 3.50 and 1.35 $mutext{m}$ . To keep the stable structure in liquid, a methodology of coating using ethyl cellulose ethoce film is proposed. By ethyl cellulose ethoce solvent coating on the coupling area, the coupler can be operated stably in seawater. The sensor demonstrated here is easy to construct, is low cost, is high sensitivity, and is compatible with the optical fiber system, which may find applications in developing miniature and multifunction optical sensors used in seawater and other liquid surroundings.

Beschreibung: A photonic-assisted microwave mixer using a Sagnac-loop-based modulator and polarization-dependent modulation is proposed and experimentally demonstrated. In this scheme, a dual-parallel Mach–Zehnder modulator (DPMZM) is incorporated in a Sagnac loop, where light is single-sideband (SSB) modulated along one direction with no modulation along the opposite direction due to the traveling-wave velocity mismatch in DPMZM. The SSB-modulated light and unmodulated optical carrier (OC) with opposite propagation directions and orthogonal polarizations are combined via a polarization beam combiner, and thus, a partial orthogonal SSB-modulated signal is obtained, which is then injected into a Mach–Zehnder modulator (MZM) via a polarization controller to ensure the polarization of the unmodulated OC paralleling with the transverse electric (TE) principal axis of MZM. Thus, only the OC is modulated by the data, and the first-order sidebands pass through the MZM directly without modulation based on the polarization dependence of MZM. After a polarizer for polarization interference and a photodetector for optical-to-electrical conversion, a phase-stable microwave signal is obtained. By adjusting the bias point of MZM, clear eye diagrams, and waveforms of 1 Gb/s on-off keying (OOK), amplitude-shift keying (ASK), and binary-phase-shift keying (BPSK) signals at 8 GHz are experimentally achieved.

Beschreibung: This paper evaluates novel design strategies to enhance the performance of a recently proposed waveguide-based pulse-shaping method, i.e., discrete space-to-time mapping (D-STM), demonstrating the capability of the method to shape pulse waveforms with duration periods in the tens of picoseconds regime. In particular, we experimentally synthesize 70-ps high-quality flat-top pulses and a 40-ps-long 200-GBd 16-quadrature amplitude modulation (16-QAM) data sequence using D-STM in concatenated co-directional couplers. Our proposed devices have been fabricated on a silicon-on-insulator (SOI) technology platform using ultraviolet and single-etch electron-beam lithography processes. The fabricated devices are all-passive, functioning without needing post-fabrication tuning, which further proves the robust performance of the proposed scheme.

Beschreibung: We have demonstrated a scheme to realize the generation of frequency-sextupled signals with low phase noise (PN). The structure is based on a multimode optoelectronic oscillator (OEO) and two cascaded Mach–Zehnder modulators (MZMs). A drive source signal is modulated by the MZM1, which is biased at the minimum transmission point (MITP); the generated first-order sidebands are injected into the multimode OEO, which is contributed by the MZM2, the single-mode fiber, the photodetector, and the amplifier. The third-order sidebands are generated by the MZM2, which is also biased at the MITP. The modulated signal is then divided into reflection components and transmission components by a fiber Bragg grating (FBG). The reflection components, including the first-order sidebands, are sent to the photodetector and fed back to the MZM2 to form an optoelectronic hybrid loop. The transmission components, including the third-order sidebands, are heterodyned by another photodetector to generate a frequency-sextupled signal. Compared with the drive signal, the generated signal has a lower PN due to the selection of the oscillation mode. In the experiment, a frequency-sextupled signal from 15 to 24 GHz is generated by a drive signal, which is tuned from 2.5 to 4 GHz. The PN of the generated signal at 24 GHz is −103.6 dBc/Hz@10 kHz; 14.6 dB PN reduction is realized.

Beschreibung: Due to the ultralow saturation properties of the topological insulator (TI), stable and high-power TI-based solid state has become an important issue. In this paper, using a folding V-shaped laser cavity to minimize the residual pump to the broadband saturable absorber, passive Q-switched operations with high power not only at 1.06 $mutext{m}$ but at 1.34 $mutext{m}$ as well were demonstrated. The 1.06- $mutext{m}$ laser of 150 mW and the corresponding repetition rate and a pulse duration of 28.57 kHz and 576 ns, respectively, were performed. The pulse energy of 5.24 $mutext{J}$ and the corresponding peak power of over 9 W were also estimated. Additionally, the laser at 1.3- $mutext{m}$ exhibited a passive Q-switched operation with an average power of as high as 326 mW and a pulse duration of 673 ns. A tunable repetition rate of 75.5 to 116.6 kHz, a pulse energy of 2.8 $mutext{J}$ , and the corresponding peak power of 4.2 W were obtained. In addition, the stability of Q-switched pulses was analyzed. The results experimentally extend the promising application of the 2-D material, i.e., exfoliated TI Bi 2 Te 3 , in solid-state lasers.

Beschreibung: In this paper, we present the first tunable extended cavity integrated passively mode-locked laser realized in an anticolliding design. The laser is realized as an InP-based photonic integrated circuit. A detailed study of the laser performance under various operating conditions is presented. The evolution of the radio-frequency (RF) spectrum and optical spectrum with the injection current to the optical amplifier is investigated. Tuning over 9 nm is achieved by injecting current in a distributed Bragg reflector section. We demonstrate that tuning of the optical spectrum toward shorter wavelengths, which increases absorption in the saturable absorber section, leads to an improvement of the mode-locked laser performance in terms of reduction of fundamental RF linewidth and reduction of the pulsewidths.

Beschreibung: Color shift keying (CSK) is a visible light communication (VLC) intensity modulation scheme, which is outlined in IEEE 802.15.7, that transmits data imperceptibly through the variation of the color emitted by red, green, and blue light-emitting diodes (RGB LEDs). In this paper, digital CSK (DCSK) is presented, which represents CSK symbols by digitally controlled multiple multicolor LEDs to avoid the effect of LED nonlinearity. By using this scheme, the DCSK system using nine RGB LEDs can transmit IEEE 802.15.7 CSK constellations with an error-vector-magnitude (EVM) performance on an $x{-}y$ chromaticity of 3%. The bit-error-rate (BER) evaluation of both the conventional and the proposed DCSK system shows that the DCSK system achieves similar BER to that of CSK with a linear variable current driver including predistortion, with taking into account optical wireless channels including the LED response, undesired color shift, and channel crosstalk by the practice characteristics of RGB LEDs, RGB bandpass filters, and photodiodes.

Beschreibung: In this paper, we introduce speckle-shifting ghost imaging (SSGI) which uses several corresponding shifted groups of speckle patterns instead of random speckle patterns in “computational ghost imaging” (CGI) to improve the performance of edge detection. The shifting of speckle patterns makes SSGI directly achieve the edges of an unknown object without the clear “ghost” images. Numerical simulations and experiments are performed. It is seen that SSGI is applicable for both binary and gray-scale objects in noisy environments. This provides a great opportunity to pave the way for the real applications of CGI in remote sensing and biological imaging.

Beschreibung: An all-optical photonic microwave phase shifter that requires only a single dc voltage control to realize continuous 0° to 360° radio-frequency (RF) signal phase shift over a wide frequency range is presented. It is based on using an integrated dual-parallel Mach Zehnder modulator (DPMZM) with a 90° polarization rotator in one arm to generate an orthogonally polarized optical carrier and RF modulation sidebands and using a polarization-dependent optical phase shifter to introduce different optical phase shifts to these orthogonal polarization components. The phase shifter has a fast response time, a high resolution, and a compact structure. Experimental results are presented, which demonstrate a continuous 360° phase shift with a flat phase and amplitude response performance over a microwave frequency range of 6.5–26.5 GHz.

Beschreibung: Low-coherence interferometry (LCI) has been a useful tool to detect distributed polarization coupling (DPC) points in polarization-maintaining fiber (PMF). However, due to birefringence dispersion, two close coupling points in long PMF are usually difficult to be distinguished. A novel method in our paper has been presented to tackle the problem. When the product of birefringence dispersion and fiber length called accumulated dispersion (AD) is numerically enough, the coherence envelopes caused by two close coupling points will split into periodic multiple peaks in the overlapping region, and the distance of the two coupling points can be further demodulated from the frequency of splitting peaks. An experimental setup based on LCI has been established with two Panda PMFs of 600 and 950 m under test. Experimental results show that the measuring errors of two close coupling points distance were less than 5%. When AD reached as high as 29 and 46 fs/nm, the spatial resolution was increased by 13-fold and 20-fold, respectively. The constraint conditions to ensure the appearance of peak splitting are also discussed. It indicates that our method may show a potential application of spatial resolution enhancement in high dispersive LCI without dispersion compensation.

Beschreibung: In this paper, we present a new 3-D structure for the InP-based avalanche photodiode, aiming at decreasing the excess noise factor. To the best of our knowledge, it is the first time new device designs based on the recently developed 3-D spatial dead space model in 2014 have been proposed. In addition, we also propose a methodology, i.e., the 2-D planar absorption distribution projection technique, for further optimizing the 3-D model. According to our theoretical simulation results, by combining photonic crystal and selective area doping, the effective k values of InP and In 0.52 Al 0.48 As can be reduced to as low as ∼0.19 and as ∼0.13, respectively. Meanwhile, the optimal thickness of the multiplication region is larger than 0.45 $mutext{m}$ , which reduces the tunneling effect. The detailed parameter optimization process, including optics, electronics, and material, is comprehensively presented. The examples in this paper also provide a fresh idea for researchers to foretell and design new photodetectors with the 3-D structure.

Beschreibung: In this paper, a wavelength-locked 878.6-nm pumped acoustic-optically Q-switched Nd:YVO 4 laser is presented. A master oscillator power amplifier configuration with two-stage amplifiers is employed to scale the output power. Both the oscillator and the power amplifiers are directly pumped by a wavelength-locked 878.6-nm laser diode, reducing the thermal load of the laser crystal. The maximal output power is 120.8 W at the repetition rate of 100 kHz, with the $text{M}^{2}$ factor of 1.31 and 1.30 in two directions. The maximal pulse energy of 4.85 mJ and the shortest pulsewidth of 15 ns are achieved at the repetition rate of 20 kHz, with near diffraction limited output. The global optical–optical efficiencies are 37.7% and 47% at the repetition rate of 20 and 100 kHz, respectively. The laser has excellent output stability in a specific range of time and temperature. The output power is very stable during the five operating hours, and it is insensitive to the temperature variation of the water chiller from 15 °C to 35 °C.

Beschreibung: An effective tunable optical beam deflection device based on a periodic gradient refractive index (GRIN) profile is reported, which is capable of achieving a 5 $^{circ}$ beam deflection with an angle resolution of 0.5 $^{circ}$ . The operation voltage is around 6 $text{V}_{mathrm{pp}}$ , and the maximum operation frequency is 2 kHz. The GRIN beam deflection device is induced by periodically varying liquid crystal orientations. A highly resistive and patterned polymer film with a resistivity of 11 $text{G}Omega/text{sq}$ is deposited to achieve a linear potential distribution between the two addressing electrodes in every unit cell. The resulting electric field pattern across the device is sawtooth-like. Both the device fabrication and the driving interconnection circuit are very simple. The advanced materials and the techniques used in this device are also applicable to other tunable optical and photonic components (such as adaptive lenses and light intensity modulators) by adapting the electrode design and the voltage signals.

Beschreibung: This paper proposes and presents the experimental demonstration of 320-Gb/s free-space optical (FSO) communication based on dense-wavelength-division-multiplexing (DWDM) technology and afocal scheme. To the best of our knowledge, this is the first one that adopts DWDM technology and afocal scheme to successfully demonstrate 50-m/320-Gb/s FSO communication. Results show that the free-space transmission distance is greatly increased by the afocal scheme and that the free-space transmission rate is significantly enhanced by the DWDM technology. DWDM FSO communication over a 50-m free-space link with a total transmission rate of 320 Gb/s $(40;text{Gb/s}/lambda times 8lambda=320;text{Gb/s})$ is achieved. With the aid of a low-noise amplifier and clock/data recovery at the receiving site, good bit-error-rate (BER) performance and clear eye diagram are obtained at a 50-m/320-Gb/s operation. Such 50-m/320-Gb/s DWDM FSO communication provides the advantages of optical wireless communications for long transmission distance and high transmission rate, which is thoroughly useful for long-haul and high-speed light-based WiFi (LiFi) applications.

Beschreibung: Single-channel 112-Gb/s PAM-4 transmission based on low-cost intensity modulation and direct detection (IM/DD) optics is experimentally demonstrated over 1-km standard single-mode fiber. By employing a digital precompensation, duobinary encode/decoding with PAM-4 signal and 7-level training-sequence-aided least mean square (TS-LMS) algorithm, we successfully achieve a receiver sensitivity of about −2 dBm with 7% overhead HD-FEC. Chirp management is applied at the transmitter side to extend the reach to 1 km for transmission at 1500 nm. This is the first demonstration, to the best of our knowledge, that 112-Gb/s PAM-4 modulation is achieved using only a 18-GHz (3-dB-bandwidth) commercial directly modulated laser. The method proposed in this paper is both bandwidth and computationally efficient, which is thought to be feasible in the low-cost short-reach optical applications.

Beschreibung: A fiber laser-pumped high-power burst-mode-operated picosecond mid-infrared (IR) laser at 3.8 $mutext{m}$ is reported. A gain-switched distributed Bragg reflector laser diode with a pulse repetition rate (PRR) of 138 MHz and pulse duration around 200 ps was applied as the seed laser of a master oscillator power amplifier (MOPA)-structured Yb fiber laser. The PRR of the MOPA was increased to about 1.1 GHz through a pulse multiplier consisting of four cascaded 2 $times$ 2 fiber couplers. A fiber-pigtailed acousto-optic modulator was used to carve the pulse train into pulse bursts so that the peak power of the final linearly polarized fiber laser output could be optimized by adjusting the duty cycle of the pulse bursts correspondingly. The output of the fiber laser was directed to pump a periodically poled magnesium-oxide-doped lithium niobate-based optical parametric oscillator through the quasi-synchronized pump scheme. Efficient parametric conversion was realized with a maximum average power output of 7.3 W at 3.8 $mutext{m}$ under pump power of 45 W at 1.064 $mutext{m}$ with pump-to-idler conversion efficiency exceeding 16%.

Beschreibung: We present the signal-to-noise performance of a short-wave infrared detector, which offers an internal avalanche-free gain. The detector is based on a similar mechanism as the heterojunction phototransistor and takes advantage of a type-II band alignment. Current devices demonstrate a noise-equivalent sensitivity of ∼670 photons at 260 K and over a linear dynamic range of 20 dB. While this level of sensitivity is about an order of magnitude better than an ideal p-i-n detector attached to the same low-noise amplifier, it was still limited by the amplifier noise (∼2600 electrons root mean square) due to the insufficient device gain. Performance comparison with other SWIR detector technologies demonstrates that the so-called electron-injection detectors offer more than three orders of magnitude better noise-equivalent sensitivity compared with state-of-the-art phototransistors operating at similar temperature.

Beschreibung: An asymmetrical thin-core long-period fiber grating (ATC-LPFG) cascaded with an inline Mach–Zehnder interferometer (MZI) for dual-parameter sensing has been demonstrated. In addition, bending direction is also determined at the same time. A section of thin-core fiber (TCF) is fusion spliced between two single-mode fibers (SMFs). ATC-LPFG is 4 cm away from the first splicing point to form an inline MZI. The transmission spectrum consists of four dominant resonant wavelengths generated by multimode interference and loss peaks of ATC-LPFG. Two resonant wavelengths are the main loss peaks of ATC-LPFG, and additional two resonant wavelengths are caused by multimode interference. Bending direction is determined by former two resonant wavelengths; curvature and temperature sensitivities are measured by the resonant wavelength of multimode interference and ATC-LPFG. Cross sensitivity can be overcome because the resonant wavelength is generated by different mechanisms. The experimental results indicate that the sensitivities of curvature and temperature are 28.82 dB/m −1 and 67.3 pm/°C, respectively.

Beschreibung: We report on the development of efficient continuous wave (CW) and mode-locked ring fiber lasers utilizing an $text{Er}^{3+}$ - doped nanoengineered yttria-stabilized zirconia alumino (YSZA)-silicate fiber as the gain element. With the proper design of the material composition, the YSZA fiber host presents superior features to eliminate the $text{Er}^{3+}$ cluster effects and enhance the radiative emission over commercial silica fibers, thus significantly increasing the fiber laser efficiencies. Through cavity analysis and optimization, we successfully demonstrate a 975-nm pumped single-mode $text{Er}^{3+}$ - doped CW YSZA-silicate fiber laser with a slope efficiency of 43%, which is corresponding to the quantum efficiency of 69%. Because of the special dispersion property of the $text{Er}^{3+}$ - doped YSZA-silicate fiber, we are able to build a stable soliton mode-locked 1565-nm fiber laser with 3.2-nJ pulse energy, 644-fs pulsewidth, and 4.96-kW peak power.

Beschreibung: We propose and experimentally demonstrate an optical modulation format converter from a 25-Gbps quadrature phase-shift keying (QPSK) signal to two 12.5-Gbps binary phase-shift keying (BPSK) signals in a double-pass configuration, which is achieved by four-wave mixing in a single highly nonlinear fiber. The phase logic of the original QPSK signal is preserved on both of the converted BPSK signals. The clear constellation diagrams and the satisfactory bit-error-rate (BER) performance are successfully obtained both in numerical simulation and experimental demonstration. Moreover, the tolerance of phase mismatch among pumps and signals in the proposed format converter is also investigated.

Beschreibung: A wavelength tunable V-cavity laser (VCL) employing integrated thin-film heaters is reported. The laser is fabricated by a simple process with no grating or band-gap engineering. A single-electrode-controlled wavelength tuning of 32 channels is achieved by experiment, with a tuning efficiency of 0.13 nm/mW, which is about 1.5 times higher than that of the current injection based thermal tuning. By combining four single-electrode controlled tunings with different bias currents, we obtain 51 wavelength channels covering 43 nm. Well-open eye diagrams with extinction ratios above 6.6 dB are obtained in all channels under direct modulation at 2.5 Gbps. The power penalty over a 25-km fiber transmission is less than 0.3 dB. Clearly open eye diagrams with bit error rates (BERs) below $10^{-12}$ are also obtained at 5 and 8.5 Gbps after a 10-km single-mode fiber transmission.

Beschreibung: A structure made of tapered three-core fiber and CO 2 laser notch long-period fiber grating is proposed in this paper. By adjusting the taper waist diameter of a three-core fiber, this structure is made suitable for simultaneous strain and temperature measurements. The effectiveness of the proposed structure in realizing simultaneous measurement sensitivity is verified by measuring the sensitivities of the two peaks at the variation of the wavelength and the depth difference. Experimental results indicate that the wavelength sensitivities of the two peaks are $-2.96 text{nm m}varepsilon^{-1}$ , 42.9 pm °C −1 and $-1.52 text{nm m}varepsilon^{-1}$ , 47.4 pm °C −1 for strain and temperature, respectively, and the depth sensitivities of two peaks can reach 2.5 dB $text{m}varepsilon^{-1}$ , 0.007 dB °C −1 and $-2.8 text{dB m}varepsilon^{-1}$ , −0.013 dB°C −1 . It can therefore be concluded that the proposed structure can be realized by adjusting the taper waist diameter of a three-core fiber for simultaneous strain and temperature measurements.

Beschreibung: In this paper, a wafer-scale uniform silver nanocave array is fabricated by soft ultraviolet nanoimprint lithography. We investigate its plasmonic effects using far-field and near-field experimental approaches and illuminate the physics inside by theoretical analysis and computational simulation. The array shows robust multispectral features for various surrounding media and possesses the sensitivity up to 514.7 nm/RIU in the visible range, which is promising for the mass production of high-performance plasmonic refractive index sensors.

Beschreibung: In this paper, we proposed a cost-effective broadcast signal transmission scheme using a distributed feedback laser diode (DFB-LD) for wavelength-division-multiplexing passive optical network (WDM-PON) systems. The DFB-LD is directly modulated with a single-tone radio frequency signal to generate multiple optical carriers. An optical signal-to-noise ratio (OSNR) is enhanced using a delay interferometer as a double-pass interferometer. Orthogonal frequency division multiplexing (OFDM) and on-off keying (OOK) signals are experimentally demonstrated to test the broadcasting scheme. We measured the bit-error-rate (BER) and the power penalty after the broadcasting OFDM and OOK signals are transmitted through a 25-km single-mode fiber. An excellent performance of BER is achieved for both OOK and OFDM signals. The proposed scheme is a practical and cost-effective solution for broadcast signal transmissions in WDM-PON systems.

Beschreibung: We demonstrated a fast mechanical wheel lapping technique to fabricate a convenient and low-cost side-polished fiber (SPF) with a rough, rather than smooth, polished surface. Such a bare SPF can be directly used to develop a promising sensing device, because a Mach–Zehnder interference pattern with high fringe contrast above 10 dB in transmission spectrum was achieved due to the macro scratches on the rough polished surface. Fabrication parameters, e.g., thickness of remaining fiber, polished length, and roughness of abrasive paper, were optimized. The sensor exhibited strain and temperature sensitivities up to $-2.00 hbox{pm}/muvarepsilon$ and $29.37 hbox{pm}/^{circ}hbox{C} $ and can be used to realize simultaneous strain and temperature measurement.

Beschreibung: Exploiting the characteristics of spatially chirped ultrafast pulses has been an increasingly active area of research. In this paper, we review the developments in the past year of the microscopy, materials processing, and micromachining fields, where the spatiotemporal structure of these beams is important. We also summarize progress in theoretical and experimental work to better control and characterize spatially chirped beams.

Beschreibung: One-dimensional semiconductor colloidal nanostructures such as nanorods and core–shell dot-in-rods can now be synthesized with excellent chemical stability and photostability, large absorption cross section, reduced blinking, and high quantum efficiency. These structures have generated great interest because, compared to spherical nanocrystals, their morphology is attributed to their specific electronic and optical properties, such as increased charge transfer and large polarization anisotropy. They are very good candidates for application in photovoltaics, lighting, and visualization and even for biomedical applications.

Beschreibung: Integral imaging concept provides a fascinating tool for the capture and display of 3-D images or movies with full parallax, with no need for special glasses and without visual fatigue due to the convergence and accommodation conflict. The limitations and recent advances in integral imaging are discussed.

Beschreibung: This paper highlights the 2014 breakthroughs in the area of optical interconnection networks. As photonic components are being further integrated, particular attention is given to integrated optical interconnection networks for modern computer systems, beyond optical point-to-point interconnectivity. This field of research has become increasingly multidisciplinary with impressive system integration demonstrations.

Beschreibung: In this paper, we present a judicious combination of two renowned surface integral equation (SIE)-based techniques, namely, the multilevel fast multipole algorithm (MLFMA) and the method of moments (MoM), which synergize into a hybrid method that allows to address the analysis of large densely packed particle assemblies in an efficient and accurate way. This hybridization takes advantage of the repetition pattern inherent to these kinds of structures. Basically, the repeated self-coupling problems are squarely solved throughout the factorization of their MoM impedance matrix, whereas the cross-couplings through the surrounding medium are expedited via the MLFMA in the framework of a global iterative scheme. Some results are presented here to demonstrate the suitability of the proposed hybrid method to address large-scale nanoparticle arrays in the framework of nanoplasmonic biosensing applications.

Beschreibung: Network coding is a revolutionary technique that can enhance network throughput and protection. This paper introduces optical physical-layer network coding (OPNC) and the recent development of OPNC, with focused discussion on “common-channel” OPNC that can efficiently utilize network resources. It also describes the challenges ahead of OPNC, including incorporating more than two signals for OPNC; circumventing the higher signal processing complexity when higher order modulations are adopted; and exploiting OPNC in more sophisticated multichannel systems.

Beschreibung: A stable single-polarization narrow linewidth single-frequency distributed feedback (DFB) fiber laser is proposed and demonstrated. For the first time, to our knowledge, the tapered FBG written in the photosensitive large-mode-area (LMA) high-concentration erbium-doped fiber (EDF) is applied in the DFB fiber laser. Self-injection locking is used to make the proposed DFB fiber laser operate in the stable single-polarization state and reduce the linewidth of the DFB fiber laser. The LMA high-concentration EDF is fabricated by the modified chemical vapor deposition technique. The tapered FBG can be seen as the equivalent phase shift FBG, and the equivalent $pi$ phase shift is produced by tapering the uniform FBG directly. The threshold of the DFB fiber laser is about 145 mW, and the maximum output power is 43.55 mW at 450-mW pump power. The measured and modified slope efficiencies of the DFB fiber laser are approximately 14.27% and 15.82%, and the optical signal-to-noise ratio (OSNR) of the laser is over 55 dB. The stable single-polarization and single-frequency operation of the fiber laser is realized, and the measured 20-dB linewidth of DFB fiber laser with self-injection locking and without self-injection locking are approximately 10.3 and 52.3 kHz, respectively.

Beschreibung: A bilayered-twisted Fermat's spiral chiral metamaterial (FSCMM) is proposed for the realization of circular polarization of far-infrared waves and polarization filters. It is demonstrated through full-wave simulations that the proposed FSCMM can convert waves from linear to circular polarization (CP) around 2.5 THz. The reported FSCMM is sensitive to the handedness of circularly polarized light, where it can filter out either right-handed or left-handed CP light with an optical isolation of 35 dB around 2.5 THz. The proposed FSCMM is compact with a dimension of $lambda/4timeslambda/4timeslambda/9$ at the far-infrared operating waveband. The CP conversion and filtering functionalities of the proposed FSCMM are ensured for oblique incidences up to 20 $^{circ}$ with an optical isolation of more than 25 dB for both $x$ - and $y$ -oriented linear polarized excitations. The operation bandwidth of the proposed system is enhanced in the level of 0.058 THz to 0.11 THz for two- and three-unit-cell composite systems, respectively. Due to the strong circular dichroism, compactness, wide-angle functionalities, and appreciable bandwidth, the proposed FSCMM is useful for the development of integrated far-infrared spectroscopic and imaging devices.

Beschreibung: Multiple scattering of light in a disordered medium with gain may provide for the necessary feedback to achieve lasing action without an optical cavity. In addition to the fundamental interest raised by this regime of light-matter interaction in open cavity, the relatively simple design of these so-called “random lasers” and the possibility to control their emission open perspective of new applications in domains not yet covered by conventional lasers.

Beschreibung: We experimentally demonstrate apodized focusing subwavelength grating couplers for both the fundamental transverse electric ( ${rm TE}_{00}$ ) mode and the fundamental transverse magnetic ( ${rm TM}_{00} $ ) mode. A measured insertion loss of 3.2 dB with a 1-dB bandwidth of 36 nm has been obtained for the ${rm TE}_{00} $ mode, and a measured insertion loss of 3.3 dB with a 1-dB bandwidth of 37 nm has been obtained for the ${rm TM}_{00}$ mode. Back reflections of $-$ 24 dB and $-$ 21 dB have been obtained for the ${rm TE}_{00}$ and ${rm TM}_{00}$ modes, respectively.

Beschreibung: Coherently coupled vertical-cavity surface-emitting laser (VCSEL) arrays exhibit novel optical properties, which can be exploited for applications. After a brief review of prior semiconductor optically coupled microcavity laser arrays, we discuss recent advances in photonic crystal implanted vertical-cavity laser arrays. We report the ability to tune these VCSEL arrays with appropriate geometry into coherence by ensuring spectral overlap between the resonance of each element of the array. Using the independent current injection into the elements of the coherently coupled array, the relative phase of the array elements and the output beam coherence can be tuned. Coherently coupled microcavity arrays are shown to offer the potential for ultrahigh-speed digital modulation.

Beschreibung: A precise fiber-based time and frequency dissemination scheme for multiple users with a tree-like branching topology is proposed. Through this scheme, ultrastable signals can be easily accessed online without affecting other sites. The scheme is tested through an experiment, in which a modulated frequency signal and a synchronized time signal are transferred to multiple remote sites over delay-stabilized fiber optic links that are over 50 km long. Results show that the relative stabilities are $5times 10^{-14}$ at 1 s and $2times 10^{-17}$ at 10 4 s. Meanwhile, compared with each site, time synchronization precision is less than 80 ps. These results can pave the way to practical applications in joint time and frequency dissemination network systems.

Beschreibung: We demonstrate an all-fiber figure-eight mode-locked thulium-doped fiber laser with a wide tunable range in both pulsewidth and wavelength. A 45-m-long ultrahigh numerical aperture fiber is used to manage the cavity dispersion, and the net cavity dispersion is calculated to be 0.8585 ps 2 at 1900 nm. With net-normal dispersion, the experimental laser, operating in a dissipative soliton resonance region, generates stable rectangular pulses. The pulsewidth varies from 480 ps to 6.19 ns with the increasing pump power, and its center wavelength has a 28.95-nm tunable range (from 1940.22 to 1969.17 nm) by properly adjusting the polarization controllers. The maximum of average output power and pulse energy is 60.73 mW and 19.51 nJ, respectively. The rectangular pulses have a clamped peak power of about 3.16 W. The wavelength-tunable fiber laser with high-energy output operating at 2 $muhbox{m}$ has great potential in various application fields.

Beschreibung: In this paper, we proposed an ultrafast 3-D imaging system based on phase-shifting method and a hybrid dispersion scanning technique. The imaging system is capable of acquiring the 3-D shape of the object at an ultrafast scan rate. A theoretical model is built to describe the operation principle of the imaging system and to analyze the performance of the system. In the demonstration, the imaging system is designed with practical devices and components. The system has a scan rate of 25 MHz and a frame rate of 100 kHz. The field of view is $50 mu hbox{m}times 50 muhbox{m} $ , and the number of pixels of one frame is 260 pixel $times$ 1000 pixel. The reconstructed 3-D image has the peak signal-to-noise ratio (PSNR) of 30.11 dB. The performance of the imaging system is investigated by analyzing the influence of the input beam size and the resolution of the digitizer. The input collimated light with large beam size is preferred for high image quality. Moreover, the depth resolution can be improved by using high-resolution digital signal processing.

Beschreibung: The experimental detail of a random lasing action from dye-doped nematic liquid crystals (LCs) inside single-core capillaries with a core diameter below 50 $muhbox{m} $ was investigated. The resonant characteristics, including the number of emission spikes, the full width at half maximum (FWHM), and the estimated $Q$ -factor, were shown to depend on the core diameter of a capillary. In contrast with a capillary with a larger core diameter and having various emission spikes, only three emission spikes were excited from a capillary with a 10- $muhbox{m} $ core diameter, owing to the smaller effective area of the pump beam. However, the decrease in the amplitude of emission spikes and the broadening of the linewidth accompanying the higher lasing threshold from a capillary with a 50- $muhbox{m} $ core diameter are attributed to the decrease in the pump fluence and the increase in the scattering loss, respectively. In this paper, a random laser (RL) with the shortest FWHM emission peak of about 0.47 nm and the highest $Q$ -factor of about 1268 was generated from the capillary with a 20- $muhbox{m} $ core diameter. By means of temperature adjustment, the emission spectra of the RL that is related to the birefringence and alignment ordering of the LC molecules inside a capillary with a 20- $muhbox{m} $ core diameter can be effectively altered. Our experiments show that the RL, revealing adjustable output emission spectra, can be a promising device in using remote sensing application- .

Beschreibung: Indirect illumination with light-emitting diodes (LEDs) and a diffuse reflector is widely applied in various fields that require uniform illumination, whereas the high uniformity obtained is always at the cost of sacrificing the light efficiency. Hence, in this paper, the energy conservation is implemented to greatly compensate for an inefficient design. A mathematical model of a free-form reflective surface based on the Lambertian characteristic of LED, ideal diffuse surface, and energy conservation is established. The diffuse free-form surface is constructed by solving a series of equations numerically. Furthermore, several array configurations are used to illustrate the advantages of the free-form surface. Finally, not only in the far-field simulation but in the near-field simulation as well, the results show better uniformity and higher efficiency compared with traditional design methods (direct illumination and diffuse illumination with a hemispherical inner surface).

Beschreibung: An equivalent-distributed coupling coefficient (EDCC) distributed feedback (DFB) semiconductor laser is proposed, and a four-channel EDCC DFB laser array is experimentally demonstrated. In this paper, the EDCC grating profile is realized by varying the duty cycle of a certain subsection of the whole sampled Bragg grating (SBG) along the laser cavity, instead of the true DCC structure obtained by changing the height of the uniform grating pitch. Moreover, the EDCC grating array can be designed by stepping the sampling period of the SBG and fabricated by combining the conventional holographic exposure with a micrometer-level photolithography on the same wafer. The experimental results show that the uniform channel spacing in the array has been obtained and that the residual of the lasing wavelengths after linear fitting changes in the scope of $-$ 0.078 and 0.048 nm. Each laser has good single longitudinal mode operation with the minimum side-mode suppression ratio (SMSR) of about 47.65 dB and the average SMSR of 49.83 dB under the same injection current of 80 mA.

Beschreibung: Total internal reflection (TIR) is a ubiquitous phenomenon used in photonic devices ranging from waveguides and resonators to lasers and optical sensors. Controlling this phenomenon and light confinement are keys to the future integration of nanoelectronics and nanophotonics on the same silicon platform. We introduced the concept of relaxed TIR, in 2014, to control evanescent waves generated during TIR. These unchecked evanescent waves are the fundamental reason photonic devices are inevitably diffraction limited and cannot be miniaturized. Our key design concept is the engineered anisotropy of the medium into which the evanescent wave extends, thus allowing for skin depth engineering without any metallic components. In this paper, we give an overview of our approach and compare it to key classes of photonic devices such as plasmonic waveguides, photonic crystal waveguides, and slot waveguides. We show how our work can overcome a long-standing issue in photonics, namely, nanoscale light confinement with fully transparent dielectric media.

Beschreibung: The dynamical properties of InP-based quantum dot (QD) gain media are surveyed and analyzed for three time scales ranging from tens of picoseconds to less than 200 fs, where quantum coherent phenomena are observable. Each of these time scales determines the important properties of QD devices, i.e., modulation capabilities, nonlinear gain, and coherent light–matter interactions. Experimental investigations and modeling results are described, highlighting the state of the art in QD devices for the important 1550-nm telecom wavelength range.

Beschreibung: Plasmonic nanolasers are promising as nanoscale coherent sources of optical fields because they support ultrasmall sizes and show ultrafast dynamics. Major advances over the last several years have focused on nanocavity design, materials improvements in quality of both the plasmonic materials and gain media, and room-temperature operation. In 2014, the most significant results focused on tunable nanolasing emission and strategies to time-resolve the dynamics.

Beschreibung: Materials that undergo an electronic phase change in a reversible manner open up new directions for research in light–matter interactions and photonic devices. The highly tunable dielectric properties, along with the spatial control of insulating and metallic domains, create photonic-crystal-like environments to control light propagation. In this brief overview, recent advances in photonics that utilize solid-state phase-change systems such as vanadium dioxide ( $hbox{VO}_{2} $ ) and chalcogenides are discussed. The controllable optical properties such as the refractive index and the optical conductance between two distinct phases in these materials pave the way for switchable photonic devices with memory. Furthermore, the intermediate states with the coexistence of two phases in the vicinity of phase transition in these materials behave as a tunable metamaterial with extraordinary optical properties that is promising for applications such as perfect absorbers.

Beschreibung: We present a silicon photonic traveling-wave Mach–Zehnder modulator operating near 1550 nm with a 3-dB bandwidth of 35 GHz. A detailed analysis of traveling-wave electrode impedance, microwave loss, and phase velocity is presented. Small- and large-signal characterization of the device validates the design methodology. We further investigate the performance of the device in a short-reach transmission system. We report a successful 112-Gb/s transmission of four-level pulse amplitude modulation over 5 km of SMF using 2.2 ${rm V}_{{rm p} - {rm p}} $ drive voltage. Digital signal processing is applied at the transmitter and receiver. 56-GBaud PAM-4 and 64-Gb/s PAM-2 transmission is demonstrated below a pre-FEC hard decision threshold of $4.4 times 10^{-3}$ .

Beschreibung: We present a remote low-coherence fiber vibrometer, which is composed of a hybrid configuration of Mach–Zehnder and Sagnac interferometer. In the interferometer, the signal and reference beams travel along the same path but in opposite directions. Owing to the fiber differential interferometric theory and a passive demodulation algorithm, absolute amplitude at a distance more than 1 m could be measured. The experimental results demonstrate that the vibrometer has a wide dynamic range for amplitude measurement from 4.34 pm to 2.43 mm. The deviation of measured amplitude is less than 0.8 dB, and the frequency response range is 30 Hz–11 kHz. The proposed method is potentially suitable for applications that require noncontact and high-dynamic-range measurements.

Beschreibung: We experimentally realize seamless operations with below 100-fs timing jitter in a 10-GSample/s 3-bit photonic analog-to-digital converter (ADC) with an input 2.5-GHz sinusoidal electrical signal. To address the energy efficiency, it is necessary to explore some serial approaches to get most operations in a photonic ADC done before serial-to-parallel conversion to save the number of devices. To press forward with the work on subsequent operations after optical sampling in a photonic ADC, we have investigated optical quantization and coding and demonstrated their performances. The experimental results successfully demonstrated seamless operations in a photonic ADC, i.e., sampling, quantization, and coding, while keeping its parallel-configuration-free characteristics and low timing jitter below 100 fs. This demonstration could address the energy efficiency by reduction of the number of devices, including electrical ADCs for subsequent operations after optical sampling in existing high-performance photonic ADCs.

Beschreibung: We propose and experimentally demonstrate a robust mode converter based on liquid crystal-on-silicon (LCOS), which is not placed at the focal plane of the collimating lens. We present theoretical investigations to evaluate the performance of proposed configuration, and our experimental results verify the favorable function of mode-selective conversion with orthogonal linear polarizations, as well as two-mode multiplexing. The robust device configuration with an insertion loss of less than 3 dB, together with cascade capability, can be used for multidimensional fiber-optical transmission system. In particular, in terms of LP 11 mode conversion, the off-focus operation allows the range of position offset for input fiber with graded-index (GRIN) lens, LCOS, and few-mode fiber to be 7.5 mm, 151 $mu{rm m} $ , and 40.4 mm, respectively, indicating easy alignment and maintenance.

Beschreibung: In a visible light communications (VLC) system, the frequency response of the light-emitting diode (LED) limits the signal bandwidth and, thus, becomes the bottleneck of the transmission data rate. The recent technology breakthrough of the micro-LED array has the potential to improve the modulation bandwidth of the LED by ten times. On the other hand, the nonlinearity of the micro-LED array is significant. Nonlinear distortion degrades system performance in both the achievable data rate and the transmission range. In this paper, we propose a digitally controlled micro-LED array architecture for the VLC system, which enjoys the benefit of wide modulation bandwidth of the micro-LED array and avoids the disadvantage of the nonlinear distortion. Compared with existing multiple LED structures, the proposed micro-LED array architecture shows robustness against nonlinear distortion.

Beschreibung: We embody a miniature reflection-type optical displacement sensor tapping into a projected beam that features high structural tolerance. A code scale with periodic patterns spatially encrypts the projected beam, which is established by imaging a collimated beam from a vertical-cavity surface-emitting laser via an aspheric lens, and linearly expands it, so as to be decoded by a receiver that comprises four serially cascaded identical photodetector cells. For the modulated beam, we observe a pitch increase with a rate of 18 $muhbox{m/mm} $ along the propagation direction. The two quadrature signals Sig-A and Sig-B derived from the PD signals exhibit a 90 $^{circ} $ phase relationship. The proposed sensor has been designed via ray-optic simulations and embodied through passive alignment, combined with plastic injection mold. We achieve a positional resolution of $sim!! 10 muhbox{m} $ from either of Sig-A and Sig-B. By concurrently considering the two quadrature outputs, we efficiently enhance the resolution to 2.5 $muhbox{m} $ , with the discovered direction of displacement. Finally, we thoroughly investigate the crucial dependence of the sensor performance on the positional tolerance pertaining to the constituent elements, which is discovered to be as large as $+/!!-100 muhbox{m}$ , under a certain resolution.

Beschreibung: Assessment of tumor invasion depth prior to making therapeutic decisions in colorectal carcinoma is crucial for both the patient and the physician. In this paper, multiphoton microscopy (MPM) was used to simultaneously label freely image loose areolar connective tissue in the submucosa and intramuscular septa in the muscularis propria to perform assessment of colorectal carcinoma invasion depth. The results indicated that MPM can accurately exhibit whether colorectal carcinoma invades into the submucosa or the muscularis propria. Collagen content alteration and the presence of dirty necrosis can be extracted to serve as quantitatively intrinsic biomarkers for reflecting collagen degradation, the occurrence of desmoplastic reaction, and breakdown of cancer cells, which are tightly related to the prognosis of colorectal carcinoma. With the development and clinical applications of the multiphoton endoscope, in vivo histological-like diagnosis of tumor invasion depth may become its main application in the field of colorectal carcinoma and lead to faster and improved therapeutic decision making in the clinics.

Beschreibung: We propose a novel and compact plasmonic sensing structure based on a metal–insulator–metal (MIM) waveguide with a side-coupled hexagonal cavity. The sensing structure has been numerically and theoretically investigated using the finite- difference time-domain (FDTD) method and temporal coupled-mode theory. The numerical simulation results show that the resonance dips of the structure have a high resonant transmission contrast ratio and that the resonance wavelengths have a near-linear relationship with the refractive index of the dielectric material in the cavity. The numerical simulation results obtained from the transmission spectra are used to analyze the sensing characteristic of the structure. The effects of the geometrical parameters on the transmission and sensing characteristics of the structure are analyzed in detail. The sensitivity can be tuned to a value as high as 1562.5 nm per refractive-index unit (RIU) with a high figure of merit of $sim! 38.6 hbox{RIU}^{-1} $ around the resonance wavelength of 1550 nm using the novel structure and by optimizing the structural parameters. In addition, the temperature- sensing characteristic of the structure based on the refractive-index sensor is also discussed in this paper. The proposed structure may potentially be applied in optical networks-on-chip and on-chip nanosensors.