ALBERT

Description: Heterodyne generation of parallel random bit streams from chaotic emission of an optically injected semiconductor laser is investigated. The continuous-wave optical injection invokes chaotic dynamics in the laser. The broadband chaotic emission is detected through optical heterodyning and electrical heterodyning into different channels. The channels digitize the signals into parallel independent random bit streams. Because of efficient utilization of different portions of the chaos bandwidth, heterodyne detections enable parallel generation of random bit streams, offer high total output bit rates, and require no high-bandwidth analogue-to-digital converters. In the experiment, two optical heterodyne channels and four electrical heterodyne channels are implemented. Each channel is required to digitize only 2.5 GHz of a much broader chaos bandwidth. The sampling rate is 10 GHz with five least significant bits selected from every 8-bit sample. The total output bit rate reaches 100 Gb/s and 200 Gb/s for optical and electrical heterodyning, respectively. The standard test suite of the National Institute of Standards and Technology verifies the randomness of both individual and interleaved output bit streams.

Description: We investigate the operation of directionally bistable semiconductor-ring lasers as all-optical flip-flops. We demonstrate fast switching between the two lasing directions by injection of optical pulses acting as set and reset control signals with switching times as fast as 20 ps, delay times as short as 60 ps, and switching energy of 150 fJ.

Description: Mode beating via third order nonlinearity in semiconductor ring lasers has been analyzed using a frequency-domain multimode rate equation model. Compared with Fabry–Perot lasers, semiconductor ring lasers are 1.33, 2, and 4 times more efficient in self-gain compression, cross-gain compression, and four-wave mixing processes, respectively, due to its travelling-wave nature. It is shown that, using dual (pump and signal) external optical injections into the ring laser cavity, multiple modes can be locked in phase via the strong four wave mixing phenomenon. This results in modulation of the light wave at the mode beating frequencies which could be used for RF optical carrier generation.

Description: The increasing use of optoelectronic devices in high data rate communication systems drives the need of precise electrical circuit modeling that allows the study of important parameters on link performances such as nonlinearity and noise level. In 60-GHz band radio over fiber system, the millimeter-wave signal generation offers simple configuration for the base station. Various techniques have been proposed such as optical heterodyning where the frequency difference between two optical carriers mixed in a photodetector generates the desired electrical carrier. Phase noise and linewidth of the optical sources determine the purity of the generated signal. In this paper, the optical phase noise is integrated into an electrical equivalent model of the laser diode to simulate radio over fiber systems in an electrical simulator. The laser output is represented here in the optical field with both intensity and phase noises. The influence of optoelectronic devices on the modulated analog or complex digital signals can be also analyzed. Two uncorrelated laser diodes are used to generate a millimeter-wave signal. Physical parameters of these lasers are determined from static response and relative intensity noise measurements. Phase noise contribution of individual lasers to the millimeter-wave signal is performed and compared with theoretical expectations.

Description: Lasers emitting in the 600 nm wavelength range have gained attention for a number of important applications, including optical information processing, plastic fiber communication systems, optical storage, and full color (RGB) laser displays and laser projectors. Visible lasers are currently realized with GaN-based heterostructures having InGaN/GaN quantum wells as the gain media. The performance of these devices, particularly at longer wavelengths, is limited by materials inhomogeneity and effects related to a large strain-induced polarization in the quantum wells. A laser emitting in the red $(lambdasim 630~{rm nm})$ has not been realized. Here, we demonstrate lasers which emit at 630 nm, the longest wavelength achieved with the nitride system, by incorporating InGaN/GaN self-organized quantum dots as the gain media. Strain relaxation during dot formation results in reduced polarization fields and consequently low threshold current density, ${rm J}_{rm th}=2.5~{rm kA}/{rm cm}^{2}$ , small blue shift of the emission peak, very weak temperature dependence of ${rm J}_{rm th}~({rm T}_{0}=236~{rm K})$ , and linearly TE polarized output.

Description: We show the evolution of two fundamental bright solitons after being simultaneously launched into nonlinear triangular parity time (PT) symmetric waveguides, at mirrored transverse input positions. Numerical simulations show, whether below or above the phase transition, in general the retarded soliton propagating along the gain side and the advanced soliton evolving along the loss side of either waveguide interfere incoherently, within the interacting regime. In non-interacting regime, however, each soliton evolves as it has been launched individually. When beyond the phase transition, the unstable solitons become self-trapped after traversing relatively short distances. The incoherent interference alters the projectile of each soliton from that of the one launched individually at the same initial input position. The temporal dynamics of interacting solitons along PT symmetric waveguide, due to the interplay of nonlinearity and the active elements, are not periodic. This is in contrast to the interaction dynamics along the purely index guided cell. The results suggest that appropriately designed nonlinear PT symmetric waveguides, below or above the phase transition, can play the role of key elements in design and fabrication of optical gates, isolators, and switches.

Description: We design and implement a chaotic-based system, enabling ultra-fast random bit sequence generation. The potential of this system to realize bit rates of 160 Gb/s for 8-bit digitization and 480 Gb/s for 16-bit digitization is demonstrated. In addition, we provide detailed insight into the interplay of dynamical properties, acquisition conditions, and post-processing, using simple and robust procedures. We employ the chaotic output of a semiconductor laser subjected to polarization-rotated feedback. We show that not only dynamics affect the randomness of the bits, but also the digitization conditions and postprocessing must be considered for successful random bit generation. Applying these general guidelines, extensible to other chaos-based systems, we can define the optimal conditions for random bit generation. We experimentally demonstrate the relevance of these criteria by extending the bit rate of our random bit generator by about two orders of magnitude. Finally, we discuss the information theoretic limits, showing that following our approach we reach the maximum possible generation rate.

Description: A radio-over-fiber system with frequency 12-tupling optical millimeter-wave (mm-wave) generation using an integrated nested Mach–Zehnder modulator (MZM) is proposed and demonstrated by simulation. Through properly adjusting the direct current bias voltages of two sub-MZMs, the RF local oscillator (LO) voltages and phases, and the gain of base-band signal, the frequency 12-tupling optical mm-wave with data only carried by one sixth-order sideband is generated. As the signal is transmitted along the fiber, there is no periodical fading and bit walk-off effect caused by chromatic dispersion. The eye diagrams stay open even when the signal is transmitted over 60 km and the power penalty is 0.67 dB at a BER of $10^{-10}$ . Furthermore, it is also proved to be valid that the BER is insensitive to the deviation of modulator extinction ratio and RF LO voltage.

Description: The nonlinear optical properties of ZnO nanorods (NRs) synthesized by coprecipitation method were investigated using a focused femtosecond laser light. The excitation wavelength was tuned from 750 to 795 nm so that excitons could be selectively generated via two-photon absorption above or below the exciton ground state of ZnO NRs. Both second harmonic generation (SHG) and two-photon-induced luminescence (TPL) were observed in the nonlinear response spectrum of ZnO NRs. The relative intensities of SHG and TPL were found to depend not only on excitation wavelength, but also on excitation intensity. At high excitation intensities, the nonlinear response spectrum became dominated by TPL for excitation wavelengths shorter than 770 nm, whereas it was still governed by SHG for excitation wavelengths longer than 770 nm. In addition, the intensities of SHG and TPL did not scale quadratically with excitation intensity but exhibited different slopes in different excitation intensity regimes, implying the existence of competition between them. More interestingly, a negative slope, which indicates a reduction of SHG with increasing excitation intensity, was observed at high excitation intensities for excitation wavelengths longer than 770 nm, implying the energy redistribution or energy transfer between SHG and TPL. Meanwhile, a slope much ${>}{2.0}$ was identified for TPL at high excitation intensities. It is suggested that the reduction in the bandgap energy resulting from the effects of bandgap renormalization and temperature rise were responsible for the rapid increase of TPL. A weak exciton emission was resolved for excitation wavelengths longer than 770 nm and it was explained by the existence of Rabi oscillation.

Description: Injection locking of polarization modes of 1.3 $mu{rm m}$ wavelength vertical-cavity surface-emitting lasers (VCSELs) fabricated via wafer fusion is investigated. Maps of the injected modes in the frequency-power plane reveal the detailed response of the slave VCSEL to the optical injection, in agreement with numerical simulations of the system.

Description: We investigate the dynamical properties of nanolasers comprising a few two-level emitters coupled to an optical cavity. A set of rate equations is derived, and is shown to agree very well with a solution of the full master equation model. Using a linearized version of these rate equations, we can analytically express the response of the nanolaser to a modulation of the pumping rate. These results are compared with the modulation response obtained directly from the master equation using a novel method. We show that contrary to conventional semiconductor lasers, the nanolaser is typically over-damped and displays a dip in the modulation bandwidth as the two-level systems become inverted. Both these features can be traced back to the use of discrete emitters that are incoherently pumped.

Description: In this paper, we numerically demonstrate that the performances of private transmission using chaotic lasers can be substantially improved by double masking the message by a suitable scheme. In addition to standard optical chaos, our method makes use of the chaotic voltage waveform available across the transmitter laser junction. With respect to the standard setup, the new one requires only one more electrical amplifier and a passive adder at the transmitter, while the receiver remains unchanged. It has been found that with this new scheme, synchronization quality is significantly improved for the twin lasers, while this is not the case for an unmatched laser pair. This results in an increased bit error rate differential between the authorized user and an eavesdropper, thus enhancing privacy. Simulations have been performed with the Lang–Kobayashi model.

Description: The optimization of the Yb:YAG gain medium and absorbing clad parameters was investigated for efficient heat removal in cryogenically-cooled multislab amplifiers operating in the kilowatt average power range (100 J/10 Hz). The 3-D distributions of temperature, stress, strain, and birefringence were calculated by a finite element analysis. Based on these data, the space-resolved optical path difference and depolarization losses were determined considering eight slabs, two laser heads, and four passes. We have found that a combination of properly designed (doping/width) index matching material and helium cryogenic cooling leads to a quasi-constant transverse temperature distribution in the pump area $({〈}{rm 0.5}~{rm K})$ and a very small axial thermal gradient $({〈}{rm 1}~{rm K})$ in the slab. It is shown that the resulting thermally induced phase aberrations, stresses, and average depolarization are rendered insignificant.

Description: A theoretical analysis for resonant cavity enhanced p-type quantum dot (QD) infrared photo-detector that uses intervalence subband transitions in ${rm In}_{rm x}{rm Ga}_{rm 1-x}{rm As}/{rm GaAs}$ QDs is presented. Multiband effective mass k.p model with the strain effect is used to calculate valance subband energy levels. Photocurrent spectra, response wavelength, and dark current density of QD infrared detector have been calculated. The calculations have been performed for a wide range of dot sizes, compositions, dot height, bias voltages, and temperatures. The effect of QD height, radius, and composition on the response of the photodetectors has been analyzed and some criteria for performance improvement have been suggested.

Description: We report the use of Al-doped ZnO (AZO) as the current spreading layer for AlGaInP-based light-emitting diodes (LEDs). It was found that AZO could form good ohmic contact with AuBe-diffused p-GaP. It was also found that the specific contact resistance could be further reduced from $2.68times 10^{-4}$ to $1.52times 10^{-4}Omegahbox{-}{rm cm}^{2}$ by performing rapid thermal annealing at 400 $^{circ}{rm C}$ for 5 min in ${rm N}_{2}$ ambient. Furthermore, it was found that output power of the LEDs with AZO current spreading layer was 6.2% larger than that of the LEDs with indium-tin-oxide current spreading layer. It was also found that LEDs with AZO current spreading layer also exhibit good electrical properties and good reliability.

Description: An adaptive controller is demonstrated that is capable of both obtaining and maintaining high-energy, single-pulse states in a mode-locked fiber laser. In particular, a multi-parameter extremum-seeking control (ESC) algorithm is used on a nonlinear polarization rotation (NPR) based laser using waveplate and polarizer angles to achieve optimal passive mode-locking despite large disturbances to the system. The physically realizable objective function introduced divides the energy output by the kurtosis of the pulse spectrum, thus balancing the total energy with the coherence of the mode-locked solution. In addition, its peaks are high-energy mode-locked states that have a safety margin near parameter regimes where mode-locking breaks down or the multipulsing instability occurs. The ESC is demonstrated by numerical simulations of a single-NPR mode-locked laser and is able to track locally maximal mode-locked states despite significant disturbances to parameters such as the fiber birefringence.

Description: The optical properties of a plasmonic slot waveguide that comprises a metallic nanowire supported by a thin-dielectric-coated metal surface are investigated at the telecom wavelength. The fundamental plasmonic mode sustained by the waveguiding configuration is shown to be strongly confined inside the gap between the nanowire and the metallic substrate, along with moderate propagation loss and deep-sub-wavelength mode size achievable simultaneously. Studies on the effects of key geometric parameters on the modal properties reveal that the tradeoff between confinement and loss could be well controlled through tuning the dimensions of the nanowire and the gap. Through conducting 3-D numerical simulations, we further show that its guided plasmonic mode can be directly excited using similar strategies to those of the ${rm TM}_{0}$ plasmonic nanowire mode. The nice optical performance, in conjunction with significant field enhancement inside the nanoscale low-index gap region, could facilitate potential applications in sensing, nonlinear light processing, optical manipulation as well as the implementations of numerous ultra-compact passive, and active nanophotonic devices.

Description: We study the dynamics of a vertical-cavity surface-emitting laser with continuous-wave orthogonal optical injection from a tunable laser. We use the dynamical properties of polarization bistability and the interplay with internal or external noise to demonstrate numerically a reliable logic output to two logic inputs encoded in the optical frequency of the injected light. This all-optical configuration is more than ten times faster than the electro-optical implementation and has the advantage of operating at constant injection power.

Description: The modal analysis is needed for the design of photonic waveguide devices to determine propagation constants of guided modes and their field pattern. Rigorous methods based on finite-difference or finite-element methods for the approximation of wave equations rely on the solution of eigenvalue problems. Imaginary-distance beam propagation methods require the solution of matrix equations of high order. The computation time needed for both approaches can be tedious. A modal analysis method based on a beam propagation scheme is proposed, which allows an efficient computation of field patterns and propagation constants, as the resulting algorithm is based on simple matrix vector multiplications. This allows the choice of a large number of discretization points and a dense discretization. The beam propagation operator is designed to include the eigenvalue spectrum of corresponding guided modes only. For this reason, the method can be interpreted as a subdomain propagation method. The concept will be validated by utilizing a single-mode and multimode rib waveguide.

Description: We have investigated the dependence of modal behavior on the structures of 2-D implant-defined coherently coupled vertical cavity surface-emitting laser (VCSEL) arrays experimentally and theoretically. 2-D VCSEL arrays with different interelement spacing have been designed and fabricated. Through the experimental analysis of near-field and far-field distribution, in-phase and out-of-phase mode are obtained from the arrays, depending on the interelement spacing. Considering the effect of thermal and carrier injection, the refractive index distribution of the arrays under operation is calculated. Then, FDTD Solutions software is employed to calculate the near-field distributions of the arrays through establishing a simplified refractive index model. The simulation results agree well with the experiments. Meanwhile, according to the phase characteristics of in-phase and out-of-phase mode, as observed from the near-field profiles, the far-field distributions are calculated, in an excellent agreement with the experiments.

Description: We present experimental investigations of the intensity noise properties of injection-locked midinfrared emitting quantum cascade lasers. Following the theoretical approach by Simos et al. , the injection locking is realized below and near the threshold of the free running slave laser, resulting in an efficient technique to achieve low-noise operation. We find that below the threshold, the locking characteristics as locking range, shape and bandwidth, are different in comparison with those above threshold. In addition, we also investigate injection locking into longitudinal side modes of the slave laser apart by several longitudinal mode hops, and observe similar characteristics, however, with the potential to achieve higher relative intensity noise suppression. The measurements are confirmed by additional numerical simulations with a new model, which considers the multimode spectrum of the slave laser and the spectral profile of the material gain. Under the actual experimental conditions, a reduction of the relative intensity noise of the slave laser of up to 10 dB (above threshold) and up to 20 dB (below threshold) in comparison to the free running slave laser noise level is achieved.

Description: A theoretical and experimental study of all-solid-state barium borate (BBO)-based optical parametric oscillator with wide tunability from visible to near-infrared by means of intracavity second-order cascaded nonlinear interaction has been demonstrated for simultaneous generation of multicolor radiation. Radiations in the wavelength range of 600–661 nm in the visible and 892–1322 nm in the near-infrared range are generated by a combination of optical parametric oscillation and simultaneous second harmonic generation in BBO crystal. The effective third-order susceptibility is measured to be 885.18 $mathrm{pm}^{2}$ / $mathrm{V}^{2}$ , which is greater than intrinsic third-order susceptibility by one order of magnitude.

Description: In finite-size high-contrast gratings (HCGs) guided modes with low $Q$ values can occur. On the one hand, this is a resonant mode with a $Q$ value of some hundreds only, on the other hand it can transform part of the finite-size incident wave into the in-plane direction. Such finite-size HCGs with guided modes are proposed here for optical sensors with an on-chip integrated p-i-n photodiode in the same HCG layer. The mode characteristics of the guided mode and the performance of the proposed integrated HCG-based optical sensors are investigated. The proposed integrated HCG-based optical sensors are compact, cost-effective, label-free, and highly sensitive, and can be extended to an array format increasing the throughput.

Description: Mode characteristics of AlGaInAs/InP square resonator microlasers laterally confined by a bisbenzocyclobutene layer, with an output waveguide connected to one vertex or the midpoint of one side, are investigated numerically and experimentally. Numerical simulation results indicate that the square resonator with the midpoint output waveguide has a better single-mode property, but a lower mode ({Q}) -factor than that with the vertex output waveguide. For a 20- (mu ) m-side-length square microlaser with a 1.5- (mu ) m-wide vertex output waveguide, lasing spectra with multiple transverse modes are observed with the mode wavelength intervals agreeing with the 2-D-finite difference time domain simulation results. In contrast, single-mode operation with a side-mode suppression ratio of 42 dB is obtained for a 20- (mu ) m-side-length square microlaser with a 1.5- (mu ) m-wide midpoint output waveguide. The results indicate that output waveguide can result in a mode selection in addition to realizing unidirectional emission.

Description: Properly designed oxide-confined vertical cavity surface emitting laser (VCSEL) allows leakage of the optical modes from the all-semiconductor core region to the selectively oxidized periphery if the orthogonality between the core mode and the modes on the periphery is broken by the oxidation-induced optical field redistribution. The leakage losses are stronger for high-order transverse modes, which have a higher field intensity close to the oxidized region. Single mode lasing in the fundamental mode can thus proceed up to large aperture diameters. The 850-nm GaAlAs thick oxide aperture VCSEL based on this concept is designed, modeled, and fabricated, showing single-mode lasing with the aperture diameters up to 5 (mu mathrm {m}) . Side mode suppression ratio >20 dB is realized at the current density of (sim 10) kA/cm (^{2}) in devices with the series resistance of 90 (Omega ) .

Description: A GaN-based light-emitting diode (LED) grown on a nanocomb-shaped patterned sapphire substrate (PSS) is fabricated and studied. Nanocomb-shaped patterns are transferred on a sapphire substrate using a well-ordered anodized aluminum oxide (AAO) thin film as a mask for the inductively coupled plasma etching process. This well-ordered AAO thin film with a high aspect ratio is grown on a sapphire substrate by an oxalic acid-based electrochemical system and a three-step anodization. The strain state generated during epitaxial growth could be effectively alleviated by the use of nanocomb-shaped PSS. The treading dislocation density could be reduced. Thus, the enhanced crystalline quality is obtained. In addition, due to the presence of photonic crystal-like air buffer layer, part of reflected photons upward the top side could be scattered by this layer. Therefore, more photons could be extracted outside. Experimentally, at 20 mA, as compared with a conventional LED grown on a planar sapphire substrate, the studied LED grown on a nanocomb-shaped PSS shows 53.8% and 43.7% enhancements in light output power and external quantum efficiency as well as a reduced leakage current.

Description: We report detailed experimental bifurcation diagrams of an external-cavity semiconductor laser. We have focused on the case of a DFB laser biased up to 1.6 times the threshold current and subjected to feedback from a distant reflector. We observe bifurcation cascades resulting from the destabilization of external-cavity modes that appear successively when the feedback is increased, and explain, in light of the Lang and Kobayashi (LK) model, how the cascading is influenced by various laser operating parameters (current, delay, and feedback phase) and experimental conditions. The semiquantitative agreement between experiments and simulations validates over a large range of operating parameters, the LK model as a tool for reproducing the salient aspects of the dynamics of a DFB laser subjected to external optical feedback.

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Description: We present an experimental and numerical study of the dynamics of a semiconductor laser subject to filtered optical feedbacks (FOFs) from two separate external cavities. Our results show that the inclusion of a second FOF introduces a rich control over the frequency of the dynamics. The period of frequency oscillations of the laser light is determined by the two time scales related to the feedback loops, and the frequency corresponding to this period is given by the average of the fundamental frequencies of the two cavities. The average frequency is dependent on the relative feedback from the cavities. Proper adjustment of the cavity lengths leads to the oscillations at a frequency which represents the average of the higher harmonics of one cavity and the fundamental frequency of the other cavity. The amplitudes of the frequency components in the single-FOF can be controlled by adding a second FOF, and in particular, the amplitude of the fundamental frequency is suppressed while the amplitude of the second harmonic becomes larger than that of the fundamental frequency. A cascade of period doubling bifurcations leads the dynamics to a chaotic state.

Description: The photovoltaic characteristics of Ga-face GaN/InGaN p-i-n solar cells are investigated numerically. The severe polarization and barrier effects induced by the GaN/InGaN hetero-interfaces are demonstrated to be detrimental for the carrier collection. The conversion efficiency could be degraded to be out of application when the degree of polarization and/or indium composition are high. To efficiently eliminate both critical issues, the solar cell structure with appropriate band engineering is introduced. In the proposed structure, the photovoltaic characteristics not only show high-grade performance but also become insensitive to the degree of polarization, even in the situation of high indium composition.

Description: The n-side-up vertical light emitting diodes (LEDs) have the advantage of carving the surface of the thick n-GaN layer to improve light extraction and to adjust radiation profiles. In this paper, a two-step surface patterning is employed with the focus on understanding angular light diffractions from both nanopatterns and truncated microdomes. Light output enhancement of the LEDs with hybrid surface texturing is investigated experimentally and theoretically. The results suggest that light is diffracted through the grating effect and curved sidewalls when interacting with truncated microdomes, resulting in a maximum enhancement 64 $^{circ}$ away from the normal surface. On the other hand, nearly omni-directional enhancement was found from the randomly scattered nanopatterns. As for the hybrid structure, since guided modes in the semiconductor layers are diffracted by either nanopatterns or microdomes, the percentage increase of light extraction from the hybrid structure is approximately the linear superposition of both types of surface textures. The results suggest an interesting guideline to improve LED light output and to adjust angular radiation with the mutlistep surface patterning.

Description: In this paper, we present a theoretical study on the optimization of passively mode-locked quantum dot lasers based on an alternative cavity design. In particular, we investigate a geometry in which the saturable absorber is located near the low reflection facet of the chip (output facet). The investigation is carried out by means of a time-domain traveling wave numerical model for quantum-dot active medium for both the gain and absorbing sections. The analysis shows superior performance in terms of pulsewidth and peak power of devices based on the new geometry compared to devices based on the conventional geometry, where the saturable absorber is placed near the high reflectivity facet. The optimization relies on the enhanced bleaching of the saturable absorber when the latter is located near the output facet, which prevents the generation of colliding or self-colliding pulse effects.

Description: We report full digital-alloy ${rm In}({rm Ga}_{1-z}{rm Al}_{z}){rm As}/{rm InP}$ multiple-quantum well 1.35- $mu{rm m}$ laser diodes using molecular beam epitaxy. The wells and barriers consist of five pairs of InGaAs/InAlAs (1.5 nm/0.375 nm) and four pairs of InGaAs/InAlAs (0.66 nm/0.98 nm). The separate confinement layer consists of two 60 pairs of InGaAs/InAlAs (0.66 nm/0.98 nm) and makes for an optical confinement factor of 7.07%. We obtained a continuous wave of 200 mW from a single cleaved facet of 1.6-mm long broad area LDs, with 100- $mu{rm m}$ aperture width at 10 $^{circ}{rm C}$ , and high characteristic temperature $T_{0}$ of 70 K. In this paper, we find that, with the MBE, the full digital-alloy technique makes bandgap engineering possible through the entire LD structure with only InGaAs/InAlAs short-period superlattices pairs.

Description: The performance of metal-insulator-metal (MIM) diodes is investigated. In this paper, we derive an analytical model that uses the Transfer Matrix Method based on the Airy Functions (AF-TMM) for the tunneling transmission probability through any number of insulating layers. The fast-computing AF-TMM simulator results show a complete matching with the numerical Non-Equilibrium Green's Function and a reasonable matching with previously published experimental results. This study shows the effect of the work function difference and insulator thickness on MIM diode performance. The advantage of using two insulator layers on enhancing the diode responsivity, resistance, and nonlinearity is also investigated.

Description: We discuss the design of mid-infrared quantum cascade lasers in which the emission frequency can be rapidly modulated by applied bias voltage. The devices are based on integrating a layer with voltage-variable refractive index, based on Stark-tunable inter-sub-band transitions, below the laser active region. A three-terminal configuration is used to independently bias the laser active region and the refractive index variation layer. The ultimate performance of the proposed scheme is analyzed theoretically and proof-of-principle devices are demonstrated experimentally. Experimentally, $lambdaapprox 10~mu{rm m}$ lasers demonstrate up to 4.5-GHz frequency tuning at a temperature of 80 K.

Description: A study of frequency stability and power scaling of single-mode GaSb-based 2.05- $mu{rm m}$ semiconductor disk laser (SDL) modules is presented. Single-mode emission and wavelength tuning over a range of 120 nm is accomplished by inserting a birefringent filter into the cavity. In order to evaluate the frequency stability, heterodyne beat-note linewidth measurements are performed for different time scales. Furthermore, a deeper insight in relevant noise mechanisms could be obtained by determining the Allan deviation and the frequency noise spectral density. For reducing the frequency noise, side-of-fringe and Pound–Drever–Hall locking are applied with the cavity length, and for the latter locking scheme also the pump diode current, controlled by the active feedback loop. That way, single-mode operation at a few kHz linewidth up to sampling times ${>}{rm 0.1}~{rm s}$ are achieved. Operating the SDL in a cavity configuration optimized for higher output powers, up to 1-W output power at a 100- $mu{rm s}$ sampled linewidth of 20 kHz were obtained when using active stabilization.

Description: Using an approach unlike that in the literature, the effects induced by waveguides on the gain in infrared free-electron lasers are analyzed for both the low and the high gain regimes. By analyzing the relation of the detuning parameter to the radiation frequency and the waveguide parameter, the radiation frequency and the slippage are analyzed not merely for the resonance case, but for more general cases. Their dependence on the waveguide parameter is analytically presented for an arbitrary value of the detuning, the features of the gain and the requirements for the waveguide parameter can be easily revealed. It is found that the derivative of the detuning parameter with respect to the radiation wavenumber is proportional to the slippage. The analysis shows that the zero slippage always exists, and to have gain at the zero-slippage frequency corresponds to obtaining a gain curve with a single broadband peak. This can be realized for both the high gain and the low gain regions by choosing the appropriate dimension of the waveguide. It is also shown that the higher order modes of the waveguide have greater effect on the high gain than on the low gain regime.

Description: We derive analytical approximations to the threshold gain, detuning, and power distribution ratios of distributed-feedback lasers with a variable phase shift at a variable longitudinal position. These closed-form approximations exhibit the direct influence of the grating parameters, and are in excellent agreement with the exact solution of the coupled mode equations for a wide range of phase shifts, positions, grating strengths and lengths. It is shown that the product of grating strength and phase shift offset distance from the grating center has a dominant influence on most threshold parameters. The threshold gain and output power splitting ratio grow exponentially with this product, whereas the intracavity peak power and the $Q$ -factor of the cavity decay exponentially with it. It is also shown that a nonvanishing sine of the phase shift leads to a linear change of the detuning, a quadratic increase of the threshold gain, and a quadratic decrease of the output power splitting ratio, intracavity peak power, and $Q$ -factor. Our results are independent of the source of the gain, such as semiconductor, rare-earth, and Raman fiber lasers, provided that the gain is approximately constant along the grating length and that reflections from facets can be neglected. As an example, we simulate a Raman DFB laser and demonstrate that our closed-form approximations are in perfect agreement with steady state results from involved time-domain simulations.

Description: $({rm Nd}_{x}{rm Y}_{0.9-{rm x}}{rm La}_{0.1})_{2}{rm O}_{3}~({rm x}=0.005hbox{--}0.04)$ transparent ceramics were fabricated by conventional solid-state processing. The radiative spectral properties of the ceramic samples were evaluated by fitting the Judd–Ofelt model with the absorption and emission data. $({rm Nd}_{x}{rm Y}_{0.9-{rm x}}{rm La}_{0.1})_{2}{rm O}_{3}$ ceramics have broad absorption and emission bands with a radiative decay time of 328 $mu{rm s}$ . The absorption cross section at 806 nm and stimulated emission cross section at 1078 nm are calculated to be 1.53 $,times 10^{-20}$ and 5.22 $,times 10^{-20}~{rm cm}^{2}$ , respectively. The product of quantum efficiency and the ionic concentration $(eta{rm N})$ exhibited a peak value at 1.5 at% ${rm Nd}^{3+}$ ion concentration, while the lifetime decreases dramatically from 300 $mu{rm s}$ (0.5 at% Nd) to 49 $mu{rm s}$ (4.0 at% Nd). With 1.0 at% ${rm Nd}{:}{rm Y}_{1.8}{rm La}_{0.2}{rm O}_{3}$ ceramics acting as a laser medium, continuous-wave output power of 1.03 W was obtained at 1079.5 nm under an absorbed pump power of 7.2 W, corresponding to a slope efficiency of 18.4%.

Description: Numerical modeling of cylindrical semiconductor nano-lasers has been undertaken accommodating local gain variations in the active region of the device. Analysis is performed using both the cylindrical transfer matrix method and the finite element method. Calculations have thereby been performed of the modal gain and the lasing condition for the device. The model has been applied to annular active core structures and a comparison has been made of the requirements for achieving lasing via the excitation of ${rm TE}_{01}$ and ${rm TM}_{01}$ modes. For representative structures, it is shown that TE and TM mode lasing can be supported in devices having cavity lengths in the order of 1 and 60 $mu{rm m}$ , respectively.

Description: For the first time, we study the fluorescence signals induced by spontaneous emission under three different kinds of atom densities (including proper, high, and low atom densities). Apart from the fluorescence signal, we also present the characteristics of images and spectral signals of the probe transmission and four-wave mixing under proper atom density, such as the Autler-Townes splitting of the spectral signals and the images. In addiiton, under other two improper atom densities, we give the observations of spectral signals of two super-fluorescence signals to understand the different contributions of high and low atom densities. The phenomena in spectral signals and images can be well controlled by adjusting the incident beam powers and frequency detunings. Such studies can be used in all-optical controlled spatial signal processing.

Description: The $^{13}{rm CHD}_{2}{rm OH}$ isotopic form of methanol has been investigated as a far-infrared laser medium for wavelengths above 100 $mu{rm m}$ using a recently constructed optically pumped molecular laser system. This system, which utilizes a transverse pumping geometry of the far-infrared laser medium, has generated 43 new far-infrared laser emissions ranging in wavelength from 106.4 to 700.3 $mu{rm m}$ . Along with the wavelength, each laser emission is reported with its optimal operating pressure, polarization with respect to the ${rm CO}_{2}$ pump laser, and relative intensity.

Description: GaN is grown on an Si substrate using metal–organic vapor–phase epitaxy. Compared with the full width at half maximum values from X-ray diffraction patterns and photoluminescence spectra of conventional GaN on the Si substrate, those of GaN on the Si substrate with the insertion of various-temperature AlN nucleation layers and ${rm Al}_{0.3}{rm Ga}_{0.7}{rm N}/{rm GaN}$ superlattice intermediate layers are reduced by 34.9% and 25.6%, respectively. In addition, Raman spectra show that residual stress on the GaN epilayers decreased by 0.35 GPa. The c-lattice parameter of the GaN epilayer is 5.1844 Å, which is close to that of an unstrained GaN layer. Ultraviolet metal–semiconductor-metal photodetectors are fabricated on an almost–crack–free GaN surface. The dark current of a photodetector on the Si substrate is 2.4 $,times 10^{-11}$ A at a 9 V applied bias, which is one order of magnitude smaller than that of a photodetector on a conventional sapphire substrate. The maximum quantum efficiency value of a photodetector on the Si substrate is ${sim}{97}%$ with an incident light wavelength of 360 nm and a 9 V applied bias.

Description: This paper presents, for the first time, a study on the pulse-to-pulse variation in the spatial, spectral, and energy characteristics (pointing instability, divergence, intensity, as well as the emission line-width and their fluctuations) of both green and yellow radiations of a copper-HBr laser (Cu-HBrL). The experimental study was performed in a high power (specific power: 70 W/L, total power: 72 W), 18 kHz repetition rate Cu-HBrL for different wall plug electrical input powers (6–9.5 kW) and HBr concentrations (5%–9%). These observations are supplemented with studies on near field spatial intensity and temporal profiles under all working conditions. It is observed that the characteristics are widely different for the two radiation components. The experimental observations are supported by comprehensive analysis in terms of input power coupling, laser gain and its radial distribution, thermally induced wave-front aberrations, optical noise contribution, and laser kinetics.

Description: A physical model is established to describe the kinetic process and the laser amplification mechanism of a diode side pumped alkali vapor laser MOPA system. The pump energy distribution inside the vapor cell is displayed. Taking into consideration the amplification saturation effect, the influences of the parameters, such as the temperature, cell length, and pump power on the output performance, are simulated and analyzed. The gain saturation effect in the direction of cell axis is also discussed. The calculations have a good agreement with observations, which shows the validity of this model. A group of optimal parameters is obtained for further improving the output characteristics of a cesium laser MOPA system.

Description: An InP L3 photonic crystal cavity on a suspended membrane has a characteristic time of ${sim}{rm 1}~mu{rm s}$ , which is $5.4times$ slower than the thermal cavity lifetime. To explain this mismatch, we use a lumped circuit model containing not only the cavity temperature, but also an average temperature of the membrane. Comparison of this model to a 1D COMSOL simulation, results in a thermal time constant of the membrane that is $7.9times$ slower than the thermal cavity lifetime, and slows down the transient response. The membrane behaves similar to other membrane cavity types.

Description: A theoretical analysis is proposed to investigate the Compton scattering as well as the inverse Compton scattering. In this analysis, the incident and scattered electromagnetic (EM) waves are classically described using Maxwell's equations, while the interacting electron is represented quantum mechanically by a wavepacket with a finite spreading length. The density matrix formalism is introduced to analyze the interaction between the electron wave and the EM field in a quantum mechanical manner. The interaction mechanism is caused by the effective coupling between EM and electron waves satisfying the energy and momentum conservation rules. The effects of the electron relaxation time due to Coulomb collisions among electrons, as well as the finite spreading size of an electron, are newly considered. Expressions for the radiated power of the scattered wave are derived for Compton and inverse Compton scattering.

Description: Quantum dot semiconductor optical amplifiers (QD-SOA) characteristics are analyzed by rate equations governing carrier dynamics in ground, excited, and upper continuum states and wetting layer. In this paper, using transfer matrix method, we analyze the picosecond pulse propagation in QD-SOA, considering cross gain modulation and gain saturation nonlinear effects. The effects of various parameters, including bias current and inhomogeneous broadening on pulse propagation, are researched. It is shown that for bias current densities beyond 8 ${rm kA}/{rm cm}^{2}$ , distortion-free pulse amplification can be achieved for input powers up to 80 mW. Lower inhomogeneous broadening leads to higher gain at central frequencies. In addition, our simulation procedure is capable of including co and counter multiwavelength signals. The simulation algorithm can handle various schemes of signal propagation in QD-SOA in reasonable simulation time, a fact that is extremely precious, especially in multiwavelength counter propagation, considering the numerous and lengthy equations that must be considered. In counter propagation, output shapes of signals are shown to be completely different from those of copropagation. The underlying reason for the differences is thoroughly researched.

Description: Oxide vertical cavity surface emitting lasers (VCSELs) have been known to have a premature failure mode when operated in the presence of atmospheric water vapor, due to corrosion failure. However, information on the precise chain of failure has not previously been available. In this paper, we start by showing the field failure results in a large particle-physics application with 6,000 deployed channels. Next, evidence is shown for the chain of failure between the previously observed oxide aperture tip cracking and delamination, and how that appears to cause dark-line defect networks in the GaAs active region beneath the aperture. In addition, we show how corrosion can be monitored through spectral width narrowing and discuss other symptoms of corrosion-based degradation that show up hundreds or thousands of hours before the rapid failure takes place. Finally, possible ways of preventing corrosion-based failure are discussed.

Description: A short pulse train with pulsewidth ${〈}{rm 1}~{rm ps}$ was generated in a quantum dot mode-locked laser (QD MLL). Due to the short dispersion length, it is required to include group-velocity dispersion (GVD) in modeling pulse train generation and evolution from QD MLLs. On the other hand, Kerr effect is also required to consider due to high peak power density in the laser cavity, and its induced self-phase modulation (SPM) also contributes to the pulse evolution. In this paper, a time domain traveling wave model, including the effect of GVD and SPM, combined with rate equations, is established to model the pulse evolution in a single-section QD MLL. It is shown that the pulse evolution calculated by this model is in reasonable agreement with the experiments. The contribution to the pulse evolution by the GVD and SPM impact is discussed.

Description: The switching dynamics of short optical pulses in a multimode interference (MMI) coupler made of Kerr materials are numerically investigated using generalized nonlinear Schrödinger equations. These propagation equations contain four-wave mixing (FWM) as well as self-phase and cross-phase modulation and dispersion of different orders to properly describe the nonlinear propagation in the multimode waveguide section. The FWM effects are more dominant than those of self-phase and cross-phase modulation, which cause power exchange among the modes in the waveguide. In numerical modeling, a 2 $,times,$ 2 MMI coupler is treated to operate as a nearly two-mode interference (TMI) coupler. For long TMI couplers, multiple switching takes place in a relatively narrow range of input powers. It is also found that the transmitted pulse becomes unstable because of the interplay between the modal dispersion (i.e., the group-delay difference) and the FWM effect when the duration of the input pulse is extremely short (typically sub-picoseconds). In general, the switching characteristics of the nonlinear TMI coupler are similar to those of a nonlinear directional coupler.

Description: The light-transmission resonant behavior of complex-shaped patterns can be difficult to estimate intuitively due to many possible resonant contours. In this paper, we propose a simple method to predict the effective resonant paths of onefold or twofold mirror-symmetry patterns, which exploits the antiphase property of certain field component along the resonant path and the symmetry requirement associated with the incident-wave polarization state. In addition, the resonant wavelengths for aperture-type patterns can further be estimated by a simple modified cutoff wavelength equation for a rectangular waveguide. Such prediction is validated by the simulated results of the finite-difference time domain method. In addition, we discuss how the separation distance between slit elements in the aperture affects the resonant wavelength, showing how the coupling between adjacent slits would play a role in the variation of the spectra. By studying the properties of such factors and how they interact in detail, we could manipulate the spectra with an additional degree of freedom, which could be important to structures with multielements in one unit cell.

Description: We propose and demonstrate a novel method to characterize the coupling coefficient and loss coefficient of a distributed Bragg reflector (DBR). The method is based on a coupled-mode analysis of periodic structures, and involves the linewidth comparison of a pair of DBR Fabry-Pérot resonators. The method is shown to be independent of coupling efficiency and wavelength dependence of the DBR parameters.

Description: Faraday isolators for high average power lasers operating at room temperature are surveyed. Three devices on $[001]$ oriented TGG crystals with the most known optical schemes are considered: traditional scheme, and schemes with compensation of thermally induced depolarization inside magnetic field and outside magnetic field. We report a unique 30-mm-aperture Faraday isolator with thermally induced depolarization compensation inside magnetic field. It provides 33.5-dB isolation ratio at 1.5-kW average laser power.

Description: We demonstrated the generation of stable topological insulator nanosecond $Q$ -switched erbium-doped fiber laser with wide pulse repetition rate range. By virtue of the excellent saturable absorption property of the topological insulator saturable absorber, the $Q$ -switched fiber laser with a section of high concentration erbium-doped fiber and two fiber Bragg gratings in a simple linear laser cavity can produce $Q$ -switched pulse with a wide range of pulse repetition rate from 12.6 to 177.7 kHz, and minimum pulse duration of 217 ns. This laser configuration enables laser operation free of self-mode-locking effects.

Description: The reinvestigation of $^{13}{rm CD}_{3}{rm OH}$ , $^{13}{rm CD}_{3}{rm OD}$ , ${rm CD}_{3}{rm CN}$ , and $^{13}{rm CD}_{3}{rm I}$ as far-infrared lasing media has been performed with an optically pumped molecular laser system utilizing a transverse pumping geometry. Thirty-four far-infrared laser emissions, with wavelengths ranging from 55.0 to 873.2 $mu{rm m}$ , have been discovered with this system. Each new laser emission is reported with its wavelength, optimal operating pressure, polarization with respect to the ${rm CO}_{2}$ pump laser, and power. When possible, the offset frequency of the ${rm CO}_{2}$ pump laser with respect to its center frequency was determined using the reported offset frequencies for known far-infrared laser emissions. Four of the laser emissions discovered for $^{13}{rm CD}_{3}{rm OH}$ and $^{13}{rm CD}_{3}{rm OD}$ are consistent with the wavelengths predicted from the tentative spectroscopic assignments of far-infrared laser schemes.

Description: In this paper, we report on the theoretical simulation and experimental investigation on a novel ultraviolet (UV) laser at 281 nm by second-harmonic generation (SHG) of a frequency-doubled nanosecond Nd:YAG laser in ${rm CsB}_{3}{rm O}_{5}$ (CBO) crystal. We firstly presented a numerical model that describes the nanosecond type-I SHG of CBO, accounting for pump depletion, beam divergence, beam spatial walk-off, linear absorption, and phase-mismatch in the Gaussian approximation of both spatial and temporal profiles. In experiment, we demonstrated the high average power UV at 281 nm up to 1.3 W by the cascaded SHG of an 1123-nm Nd:YAG laser to 561.5 nm in LBO at first SHG stage and then frequency-doubled again to 281 nm in CBO at second SHG stage. Moreover, we investigated the SHG efficiency and the UV 281-nm laser beam quality theoretically under the experimental conditions, and the calculations agreed well with the experimental results.

Description: In this paper, we theoretically study the impact of an intracavity filter based on a Mach–Zehnder interferometer (MZI) on the pulses emitted by InGaAsP/InP passively mode-locked quantum well ring lasers. The filter allows to control the net gain curvature in the device, hereby providing for control over the modes that participate in the dynamics. Simulations of a traveling-wave model indicate that the pulsewidth can be controlled and reduced down to 500 fs. We present and verify a simple algorithm, which can be used for calculating the optimum values of the MZI parameters. The optimum parameters are then used in the study of an MZI passive-mode-locked laser under various operating conditions.

Description: We discuss relevant design considerations for the fabrication of electromechanically tunable photonic crystal cavities based on double semiconductor slabs. A simple optical and electromechanical model of the device based on coupled-mode theory and electrostatics is discussed and used jointly with 3-D finite-element calculations of optical cavity modes to extract the tuning-range dependence on geometrical parameters. A design rule, which avoids the sticking of membranes due to capillary forces and keeps a large tunability, is defined. The details of the fabrication process and a summary of the experimental results on GaAs and InGaAsP/InP material systems are given. We also address the problem of nonsymmetric devices, where the thicknesses of the membranes are not exactly the same, resulting in an imbalanced power emission of coupled modes.

Description: The gain saturation factor describes the proportionality between the real and imaginary parts of the optical susceptibility associated with the circulating optical field in the laser cavity. This factor depends on where the laser operates relative to the gain peak. The effects of the gain saturation factor on the characteristics of the nonlinear phenomena induced by optical injection are studied to find the optimal operating conditions for various application requirements. Such requirements include high microwave frequency, low phase noise, broadband frequency tunability, high modulation bandwidth, optical single-sideband modulation, and stability. Optically injected semiconductor lasers of positive, negative, and zero gain saturation factors are compared through mappings of their nonlinear dynamical characteristics as functions of the detuning frequency and the injection strength. It is found that the instability of an optically injected laser is enhanced by a smaller or more negative gain saturation factor. In both stable locking and periodic oscillations, a laser of a positive gain saturation factor that is smaller than the linewdith enhancement factor is most suitable for optical transmission requirements.

Description: An optically pumped molecular laser system utilizing a transverse or zig-zag pumping geometry of the far-infrared (FIR) laser medium has enabled the reinvestigation of the ${rm CH}_{2}{rm DOH}$ , ${rm CHD}_{2}{rm OH}$ , and ${rm CH}_{3}{^{18}}{rm OH}$ isotopic forms of methanol for wavelengths ${>}{rm 100}~mu{rm m}$ . With this system, 28 FIR laser emissions have been discovered with wavelengths ranging from 117.2 to 744.7 $mu{rm m}$ . Along with the wavelength, each laser emission is reported with its optimal operating pressure, polarization with respect to the ${rm CO}_{2}$ pump laser, and relative intensity. Three of the laser emissions generated by ${rm CH}_{3}{^{18}}{rm OH}$ support the spectroscopic assignments of FIR laser transitions originally proposed through combination-difference loops. A fourth ${rm CH}_{3}{^{18}}{rm OH}$ laser emission should contribute to another, incomplete FIR laser scheme.

Description: The ideal diode approximation is used to evaluate the properties of many semiconductor diode-based devices. However, when it is applied to lasers and photovoltaics, which incorporate quantum dots (QDs), the ideality factors measured are difficult to interpret. Here, we show that a proper consideration of carrier escape from the QD confined states overcomes this problem. In addition, this approach suggests a novel method of empirically estimating the absolute value of the escape current at the threshold current of a laser diode. We argue that this technique will be of great importance in the experimental analysis of nonradiative losses in lasers and should also prove useful to the development of next-generation QD photovoltaics where thermal carrier escape must be minimized.

Description: Nitride-based light-emitting diodes (LEDs) grown with a structure of InN/GaN matrix quantum-wells (QWs) using metal–organic chemical vapor deposition are fabricated and characterized. The InN/GaN matrix QWs can significantly enhance the formation of phase-separated In-rich regions. Under an injection current of 500 ${rm A}/{rm cm}^{2}$ , the LED output power can be enhanced by 26%, and the efficiency droop can be improved as compared with a conventional LED. These improvements could be attributed to the strong localization effect to remove carriers from the confining potential and capture these carriers at nonradiative centers in the active layer.

Description: A detailed analysis of the noise spectrum of a semiconductor optical amplifier excited by an amplitude-modulated input signal is presented. This extends the well-established theory for the case of continuous-wave input signals and is relevant for various applications within optical signal processing. One important example is the analysis of noise in microwave photonic elements based on slow-light propagation in semiconductor optical amplifiers. Expressions for the noise spectra are derived and the dependence on important operation parameters, such as input power, modulation depth, and modulation frequency, is investigated and explained. We find several interesting modifications to the spectra compared with the continuous-wave case. In particular, the side-bands present in the input-signal lead via four-wave mixing effects to additional structure in the spectra as well as additional noise components.

Description: We investigated theoretically dynamics of a semiconductor optical amplifier (SOA) based on the quantum dot-in-a-well (QDWELL) structure. We modified the QDWELL laser rate equations for the case of SOA and solved the modified equation system numerically. We have shown that a strong pumping optical wave in the QDWELL SOA synchronizes the QD carrier dynamics, which results in the enhancement of the QDWELL SOA bandwidth and repetition frequency, fast gain recovery, cross-gain modulation, and chirp decrease.

Description: A Y-splitter based on hybrid multilayer plasmonic waveguide structure composed of silver–silica–silicon–silica–silver is introduced. Properties, such as the transmission rate and split ratio, are numerically investigated by changing the thickness of dielectric layers, bending radii of the bending parts, as well as input working wavelength. Results indicate that the proposed structure can have high transmission rates of >47% at the two output branches with split ratio close to 1:1. When the symmetry of the structure is broken by adjusting the bending radii of the bending parts, one may control the transmission rate of the two different output branches and then control the split ratio for a given working wavelength to a certain degree. Comparison reveals that the proposed Y-splitter is superior to the T-splitter within the wavelength range of 1000–1600 nm. These properties are helpful in the design and application of photonic devices and integration.

Description: Wide-stripe design for high-power laser diodes (LDs) permits multilateral-mode optical field distribution and the formation of an unstable filament pattern within the active waveguide. This is a serious drawback leading to a decrease in the emitted beam quality. The filaments cause spatial and time fluctuations in the LDs near- and far-field (FF) characteristics due to their random nature. The multimode optical field induces a widening of the FF pattern (known as FF blooming effects) as the drive current increases due to an increasing number of lateral modes. In this paper, the gain shaping in the LDs active region by ion implantation is proposed as a way to induce beam stability improvement. The optical resonator of the presented LDs consists of a series of interlaced narrow high- and low-gain stripes. The low-gain stripes are pumped only by spreading the current from neighboring stripes. The calculations show that under such conditions, one of the higher lateral modes can be excited at laser threshold due to the best spatial fit to the stripe’s periodicity. This should restrict the FF blooming effect. The predictions are confirmed by measurements of the LDs based on the multistripe design. A reduction of the optical field intensity fluctuations has also been observed.

Description: A novel InP-based multispectral photodetector chip, using multiple Fabry–Perot resonant cavities integrated with photodectors, was designed, fabricated, and characterized. The integrated devices are capable of simultaneous wavelength detection at four wavelengths in the near infrared/short-wave infrared regime such as 1.32, 1.41, 1.49, and 1.66 (mu ) m. Experimental results show highly distinguishable detection peaks from 22.8% to 37.4%. A little change in dark current density has been observed after filter integration ( (sim 2 times 10^{-2}) A/cm (^{2}) at −10 V). The experimental full-width half-maximum values are 2.18–2.9 times of the design’s, indicating light scattering caused by cavity etch and cavity thickness nonuniformities. The discrepancy between design and experimental results are analyzed for a better understanding of the filter properties and experimental variables.

Description: The unique properties of nonpolar GaN light-emitting diodes (LEDs) have the advantages of generating polarized light emission. The employment of asymmetric 2-D photonic crystals (PhCs) can further enhance the light polarization ratio. In addition, it was generally recognized that the Purcell effect can increase the internal quantum efficiency of the LEDs with PhCs. In this paper, we study the properties of optical modes from different crystal planes. The Purcell effect is analyzed based on the PhCs and material crystal orientations. With different transition probability of the polarized photons in valence bands, the corresponding Purcell effect enhancement on the quantum efficiency varies.

Description: We present the detailed characteristics of solid source molecular beam epitaxy (MBE) grown large format ( (640times 512) ) extended short wavelength infrared In 0.83 Ga 0.17 As sensor with desirable performance at both pixel and focal plane array (FPA) levels. The FPA pixels in the mesa structure grown on a graded AlInAs buffer layer with 2.65- (mu ) m, 300-K cutoff wavelength exhibited 300- and 200-K peak detectivities as high as (sim !! 2.5times 10^{10 }) and (sim !!1times 10^{12}) cmHz (^{1/2}) /W, which are both equivalent to the theoretical limits set by the Johnson noise of the detector. Dark current analysis of the pixels displayed no considerable tunneling component with the dark current being dominated by generation-recombination and shunt leakage mechanisms >200 K up to a reverse bias voltage of 3 V. Moreover, the noise measurements displayed no 1/ (f) noise in the FPA pixels. In spite of the large lattice mismatch, the FPA yielded very good response linearity, as well as impressively good responsivity nonuniformity and pixel operability of 5.5% and 99.8%, respectively. The results showing the feasibility of both solid source MBE and graded AlInAs buffer for the growth of extended InGaAs photodetectors are very encouraging for the large format FPA implementation of these detectors with desirable imaging performance and an extended cutoff wavelength as high as (sim 2.7)

Description: A finite-element code has been developed to study the vector acoustic modes in high index contrast silica optical waveguides. Detailed spatial variations of the transverse and longitudinal displacement vectors are shown for the bending, radial, and torsional modes. The variation of the frequency shift due to the stimulated Brillouin scattering and the overlaps seen between the acoustic and optical waveguides also are presented, together with the modal hybridness and modal dispersion.

Description: Electronic band structure and material gain are calculated using the 8-band kp Hamiltonian for three quantum well (QW) systems grown on GaAs substrates: 1) Ga 0.66 In 0.34 N y As 1–y /GaAs; 2) GaN y As 0.69–y Sb 0.31 /GaAs; and 3) GaN y P 0.46 Sb 0.54–y /GaAs QWs with various nitrogen concentrations. Bandgap and electron effective mass for Ga 0.66 In 0.34 N y As 1–y , GaN y As 0.69–y Sb 0.31 , and GaN y P 0.46 Sb 0.54–y alloys are determined within band anticrossing model using the proper alloying approximations. The intensity and spectral position of gain peak are directly compared for QWs with 2% of nitrogen and 2% compressive strain in the QW layer. The largest shift of gain peak has been observed for GaN 0.02 P 0.46 Sb 0.52 /GaAs QW. In this case, the gain peak (transverse electric (TE) mode) is located at 1.77 (mu ) m that is longer by 390 nm than the gain peak (TE mode) for Ga 0.66 In 0.34 N 0.02 As 0.98 /GaAs QW. In addition, for GaN 0.02 As 0.67 Sb 0.31 /GaAs QW the largest shift of gain peak (TE mode) is observed for Ga 0.66 In 0.34 N 0.02 As 0.98 /GaAs QW (1.67 versus (1.38~mu ) m). The intensities of TE modes of material gain are comparable for all three systems. The above indicates that GaN y As 0.69–y Sb 0.31 /GaAs and GaN y P 0.46 Sb 0.54–y /GaAs QW systems are promising candidates for GaAs-based lasers dedicated for emission in very long-wavelength telecommunication windows- The required nitrogen concentration for achieving gain peak position (TE mode) at 1.3 and (1.55~mu ) m are 0.5% and 1.5% N for GaN y As 0.69–y Sb 0.31 /GaAs QWs, respectively, while for GaN y P 0.46 Sb 0.54–y /GaAs QWs are 0.3% and 1.2% N. Those values are significantly less than in Ga 0.66 In 0.34 N y As 1–y /GaAs QWs. Considering a possible blue shift of QW emission upon annealing (i.e., the post grown procedure, which is usually used for improving optical quality of these QWs) the required nitrogen concentration can be larger by (sim 0.5) % N than the one calculated for as-grown QWs.

Description: A simple tool for evaluating the stability of the finite-element beam propagation method (FEBPM) for lossy waveguides is described. Current stability issues are explained by this method. The sufficient stability condition of the FEBPM is described. The stability of the analytical propagation function, the Crank–Nicolson propagation scheme, and various Padé approximants are analyzed using the tool. The dependence of the stability of the FEBPM on the propagation step size is also noted.

Description: In this paper, we designed the offset frequency locking and active frequency stabilization system for the continuous-wave (CW) and electro-optical (Q) -switched RF-excited waveguide CO 2 lasers to provide an ideal light source for coherent radar. The CW laser is used as a local laser, and the pulsed laser is used as a main laser, which can produce (Q) -switched and cavity-dumped laser synchronously. A closed-loop laser frequency control system was used to stabilize the (Q) -switched laser at the gain curve center frequency and to lock the two lasers at a selected heterodyne frequency. The system includes two parts: 1) pulsed laser frequency stabilization and 2) offset frequency locking. The frequency stabilization part can keep the (Q) -switched laser oscillating at the peak of the gain curve by measuring the pulse build-up time continuously and controlling the cavity length. The offset frequency locking circuits keep the heterodyne frequency around the selected frequency by measuring the frequency of the beat signals and controlling the CW laser cavity length. In the experiment, the long-term frequency shift of the pulse laser was measured to be less than ±16 MHz and the offset frequency fluctuation was less than ±2 MHz when the frequency control system was in a closed-loop operation.

Description: This paper investigates methods of suppressing higher order transverse modes in a GaInNAs/GaAs quantum-well GaAs-based vertical-cavity surface-emitting diode laser, with the aid of a 3-D self-consistent model. The inverted surface relief design, which creates the antiphase condition, proved efficient at suppressing higher order transverse modes, in spite of unfavorable thermal effects.

Description: The acceleration of gain recovery in quantum dot vertical cavity semiconductor optical amplifiers (QD-VCSOA) is investigated. The 1-Tb/s pattern effect free cross gain modulation in QD-VCSOA can be achieved by using an additional light beam. We can overcome the limitation of the slow transition of carriers from the wetting layer to the excited state by applying a light beam which increases the carrier density in the excited state and thus strongly improves the gain recovery time.

Description: We describe an unsaturated gain–loss model in regards to colloidal PbSe quantum dot (QD)-doped fiber laser realized previously. Based on experimental observation on the laser, the numerical simulation on the model indicates there exists a pumping threshold that depends on the QD doping concentration. There is a defined range of doping concentration, within which the laser can be generated. Available doping density depends on the fiber length, following an exponential decrease with the increased fiber length. Our model suggests a reasonable lifetime of 100 ns for the upper level of QDs in the background of UV curing adhesive, which is shorter than the lifetime reported in literatures.

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Description: We analyze the limit cycle, also known as period-one (P1), dynamics induced by strong optical injection to a semiconductor laser. Based on the well-established laser rate equations, an approximate solution of the P1 dynamics can be obtained by starting with a general limit-cycle solution and taking into account high-order harmonics of the P1 frequency. The results from this analysis are in a good agreement with numerical and experimental results. Our analysis shows that the strong coupling between the oscillating carrier density and the oscillating optical field is responsible for the high nonlinearity of these P1 dynamics.

Description: In this paper, we investigate the novel behavior of induced focusing in nonlinear metamaterials (MMs) through designing probe-pump configuration, where the coupled waves may simultaneously experience negative refractive index or positive refractive index, or one experiences positive refractive index and another experiences negative refractive index both analytically and numerically. It is shown that cross-phase modulation (XPM) in MM not only brings some new features to the behavior of induced focusing, but also makes it possible in the ordinary positive-index materials (OMs), where it is otherwise impossible. First, we reveal that weak probe beam can be focused by strong pump beam even when their centers are completely overlapping each other. Moreover, it is found that the separated distance between coupled waves has an optimal value, which results in the strongest focusing of probe beam. Finally, another most notable property is that a spatial induced focusing is found to occur for different combinations of the signs of refractive index decided by the probe and pump beams, respectively, implying that a spatial XPM in MMs can be used as a more powerful tool for controlling the inducing focusing, when compared with the corresponding case in OMs. Our findings demonstrate that MMs can provide us unique opportunities unattainable in OMs to manipulate the all-optical control of optical focusing behavior through the new physical mechanism of XPM.

Description: A numerical model of a ring cavity wavelength-swept laser based on semiconductor optical amplifier (SOA) is described. In this model, the SOA is restricted by a carrier density rate equation and set of traveling wave equations, which describe the amplified signal and spontaneous emission photon rates. In addition, fiber Fabry–Perot tunable filter, which is used to select the transmission wavelength, is modeled by a multibeam interference transmission function. Thereafter, a coupler, which outputs and feedbacks the laser is simply described by its splitting ratio. Finally, an isolator is idealized to ensure unidirectional lasing. By numerically solving the model, the spectra evolution, carrier density, signal, and amplified spontaneous emission forward and backward distributions along the transmission direction, the output power versus filtering center wavelength, injection current, and coupling ratio were studied, respectively. The results are used for deeply understanding the kinetic process of such swept laser and predicting the sweep range, the bandwidth, the output power, and the threshold current of the laser.

Description: In this paper, we examine for the first time how to design arbitrary shape modal fields within complex refractive index circular waveguides. In order to achieve this flexibility in field shape, we use complex refractive index profile waveguides, where we determine the required refractive index profile on both real and imaginary refractive index. We develop a technique for calculating directly and accurately the refractive index profiles of cylindrical waveguides from knowledge of the desired modal electric field. The method we use to solve this inverse problem is via modeling the waveguide transversely as a transmission line. We demonstrate this algorithm with a number of example reconstructions of different arbitrary modal electric fields. The refractive index profiles generated supporting the required modal electric fields are complex. We reconstruct complex refractive index profiles which support unusual electric field distributions. We expect this technique to be useful in designing special fibers for mode matching between dissimilar waveguides, in designing sensing optical fibers, in mode division multiplexing, in gain flattening optical fiber amplifiers, and also for high power optical fibers.

Description: We study polarization switching and hysteresis in a vertical-cavity surface-emitting laser (VCSEL) subject to an orthogonal optical injection, such that the polarization of the injected light is perpendicular to that of the free-running VCSEL. We use the framework of the spin-flip model to characterize the polarization state of the VCSEL as a function of the frequency detuning. With appropriated injection conditions, the orthogonal polarization turns on and locks to the injected field. Increasing and decreasing the detuning across the two locking boundaries results either in narrow or wide hysteresis cycles, or even in irreversible switching. These results are in a good agreement with recent experimental observations.

Description: Quantum-dot lasers can exhibit simultaneous ground- and excited-state lasing. With increasing pump current, a quenching of the ground-state lasing intensity is sometimes observed. The causes for this are investigated, and its dependence on temperature, gain, and electron–hole asymmetry is studied via an analytical approach. A numerical model based on the semiconductor Bloch equations with a set of rate equations for electrons and holes is used for validation. We also investigate the influence of doping and different cavity lengths on the two-state lasing dynamics. We find that ground-state quenching is more common in p-doped, short cavity devices with low gain.

Description: We evaluate the chirp reduction achievable using the detuned loading effect in the polymer-based tunable external-cavity laser (ECL) capable of operating at 10 Gb/s. In particular, we estimate the achievable linewidth enhancement factor (LEF) of this ECL considering the stable operation region increased by the nonlinear gain. The results show that, due to the nonlinear gain, we can detune the lasing mode further away from the nominal detuning limit (i.e., half of the mode spacing) by $sim 4$ GHz for an ECL with a mode spacing of $sim 30$ GHz under the continuous-wave (CW) operating condition. As a result of this additional detuning, the effective LEF is reduced from 1.3 to 0.8, although its material LEF is $sim 5.$ However, when we change the operating wavelength of this ECL, the minimum achievable LEF is deteriorated due to the parasitic reflection occurring at the antireflection (AR)-coated facet. For example, when the optical power reflected at the AR-coated facet is $sim 5$ % of the feedback power from the polymer Bragg grating reflector (PBR), the effective LEF is increased from 0.8 to 3.4. To verify these results experimentally, we fabricate a polymer-based tunable ECL and measure its chirp characteristics. For example, we directly modulate this ECL at 10 Gb/s and measure its effective LEF as a function of the detuned frequency. The result shows that we can reduce the effective LEF to $sim 1$ by detuning the operating mode by 18 GHz. This value is slightly larger than that achievable under the nearly CW operating condition (i.e., 0.8) due to the chirp-induced mode hopping. We also confirm - hat, by optimizing the detuning condition, this ECL can be used for the transmission of a 10-Gb/s signal over 20 km of a standard single-mode fiber (SSMF) with a power penalty of <2 dB. However, an error-free transmission cannot be achieved at some wavelengths due to the parasitic reflection. To avoid this problem, it is needed to suppress the parasitic reflection to be <0.1% of the feedback power from the PBR.

Description: An optimization study of quantum cascade lasers (QCLs) considering the laser instability condition is performed. To model current transport, the Pauli master equation is solved using a Monte Carlo approach. The effects of saturable absorber and pumping strength on the instability threshold are investigated. A particle swarm optimization algorithm is applied to increase the laser optical gain. A large optical gain below the instability threshold is achieved for optimized QCL designs. To analyze the optimized structure, a numerical calculation based on the Maxwell-Bloch equations is performed. The results indicate side-mode instabilities due to Risken-Nummedal-Graham-Haken-like instability.

Description: This paper reports the results of numerical and experimental investigations of the dynamics of an external-cavity diode laser device composed of a semiconductor laser and an external Bragg grating, which provides optical feedback. Due to the influence of the feedback, this system can operate in different dynamic regimes. The traveling-wave model is used for simulations and analysis of the nonlinear dynamics in the considered laser device. Based on this model, a detailed analysis of the optical modes is performed, and the stability of the stationary states is discussed. It is shown that the results obtained from the simulation and analysis of the device are in a good qualitative agreement with the experimental findings.

Description: We report on an experimental study of the injected optical power required for polarization switching (PS) in a vertical-cavity surface-emitting laser (VCSEL) subject to orthogonal optical injection. This power is found as a function of the frequency detuning between the injected light and suppressed linear polarization of the solitary VCSEL. Increasing the injected power, we found two types of PS when injecting light with wavelength close to the wavelength of the suppressed linear polarization and to that of the linear polarization of the solitary VCSEL. These two types of PS are, respectively: 1) single and double (with two minima of injected power necessary for PS) and 2) double PS (with two islands of PS and a separate region with a single minimum of injected power). Our experimental results confirm the theoretical predictions of Torre et al. Double PS is observed around the elliptically polarized injection locked states found by Lin et al. Different dynamics are found in the route to PS.

Description: Internal quantum efficiency (IQE) of a blue high-brightness InGaN/GaN light-emitting diode (LED) was evaluated from the external quantum efficiency measured as a function of current at various temperatures ranged between 13 and 440 K. Processing the data with a novel evaluation procedure based on the ABC-model, we have determined the temperature-dependent IQE of the LED structure and light extraction efficiency of the LED chip. Separate evaluation of these parameters is helpful for further optimization of the heterostructure and chip designs. The data obtained enable making a guess on the temperature dependence of the radiative and Auger recombination coefficients, which may be important for identification of dominant mechanisms responsible for the efficiency droop in III-nitride LEDs. Thermal degradation of the LED performance in terms of the emission efficiency is also considered.

Description: A rate-equation theory model with the amplified spontaneous emission (ASE) effect considered for the surface plasmon polariton (SPP) waveguide amplifier is presented for analyzing the SPP amplifications with both side and SPP pumping techniques. By partitioning the cross section of the waveguide to suitable small units and introducing the overlapping integrated factor in each unit, the 3-D coupled equations of the model can be simplified to 1-D equations, which could be solved more conveniently. With our model, the gain and noise properties of a dielectric-loaded SPP waveguide amplifier are simulated. The results show that the ASE effect reduces the signal gain and deteriorates the signal-to-noise ratio of the SPP amplifier. For both the pumping schemes, the gains and noises for SPP multimode amplifications depend on the waveguide structure. By optimizing the waveguide structure, the gain-equalized amplification for different SPP modes may be achieved.

Description: Relativistic third-harmonics of a laser in a magnetized plasma channel is studied. Due to relativistic self-focusing in plasma, a high-intensity laser pulse pushes the plasma electrons radially outward to create a plasma channel. The relativistic mass effect and the magnetic field contribute to the nonlinear dielectric response of plasma and create a plasma channel due to the laser self-focusing. Self-sustained plasma channel is very important and may affect the efficiency of harmonic generation of the interacting laser beam. The velocity and density perturbation associated with the self-focused laser beam can generate a nonlinear current at triple fold frequency of the fundamental laser. Our results show the significant effect of the self-sustained magnetized plasma channel on the efficiency of third-harmonic generation of the laser beam.

Description: We analyze amorphous Al 2 O 3 ( (alpha ) -Al 2 O 3 ) for use as a thick thermally conductive shield in metallo-dielectric semiconductor nanolasers, and show that the use of (alpha ) -Al 2 O 3 allows a laser to efficiently dissipate heat through its shield. This new mechanism for thermal management leads to a significantly lower operating temperature within the laser, compared with lasers with less thermally conductive shields, such as SiO 2 . We implement the shield in a continuous wave electrically pumped cavity, and analyze its experimental performance by jointly investigating its optical, electrical, thermal, and material gain properties. Our analysis shows that the primary obstacle to room temperature lasing was the device's high threshold gain. At the high pump levels required to achieve the gain threshold, particularly at room temperature, the gain spectrum broadened and shifted, leading to detrimental mode competition. Further simulations predict that an increase in the pedestal undercut depth should enable room temperature lasing in a device with the same footprint and gain volume. Through the integrated treatment of various physical effects, this analysis shows the promise of (alpha ) -Al 2 O 3 for nanolaser thermal management, and enables better understanding of nanolaser behavior, as well as more informed design of reliable nanolasers.

Description: The presented laser driver for a photoinjector of an electron linear accelerator delivers 10-ps long laser pulses with the energy of (1.85~mu ) J at the wavelength of 262 nm. Laser pulses irradiating with the repetition rate of 10 MHz form the rectangular 800- (mu ) s long pulse trains with 18.5-W mean power inside them. The pulse train repetition rate is 10 Hz. Short-lived absorption centers accumulating in BBO fourth-harmonic generator crystal cause significant distortion of the pulse train envelope. The envelope remains rectangular due to precompensating of these distortions in the fiber acousto-optical modulator with fast varying transmission.

Description: We have investigated the properties of In 0.51 Ga 0.49 N/GaN disk-in-nanowire light emitting diodes (LEDs) epitaxially grown on silicon substrates by plasma-assisted molecular beam epitaxy. The radiative efficiency of the nanowire ensemble, obtained from the temperature-dependent photoluminescence measurements, under optimized growth conditions is 43%, which increases to 55% after parylene passivation. From high resolution transmission electron microscopy, it is evident that there is significant coalescence between nanowires when the areal density approaches (10^{mathrm {mathbf {11}}}) cm (^{mathrm {mathbf {-2}}}) . We have identified and characterized deep level electron and hole traps in the GaN nanowires and it is found that the trap densities increase with nanowire density, or with the degree of coalescence. It is therefore believed that the deep levels originate from dislocations and stacking faults arising from nanowire coalescence. The best output characteristics are measured in a LED having a nanowire density of (2~times ~10^{mathrm {mathbf {10}}}) cm (^{mathrm {mathbf {-2}}}) , which exhibits a maximum internal quantum efficiency of (sim 55) % at an injection level of 10 A/cm (^{mathrm {mathbf {2}}}) . It is seen that the maximum efficiency would increase to 60% in the absence of deep level traps.

Description: Epitaxial ruby thin film is deposited via pulsed laser deposition technique. The charateristic R-lines in PL spectra confirmed ruby phase in the film. Raman spectra showed (C) -axis growth of the film. The nonlinear absorption coefficient and nonlinear refractive index of film were found to be (beta sim 5.2 times 10^{mathrm {mathbf {-5}}}) m/W and (n_{mathrm {mathbf {2}}}sim 3.1 times 10^{mathrm {mathbf {-5}}}) m (^{mathrm {mathbf {2}}}) /W, respectively. Further, the sub as well as superluminal propagation of 130-ns Gaussian pulse in 3.5- (mu ) m-thick PLD ruby thin film is reported in this paper. A delay of 17 ns is observed at film orientation of +45°, whereas pulse advancement of 12 ns is observed at −45°. Pulse broadening and compression are also observed with film rotation.