ALBERT

Notes: This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency.For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three-level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.

Notes: Fe/DyFe multilayers were prepared by radio-frequency magnetron sputtering, where DyFe layers were amorphous and nonmagnetic at room temperature. When the Fe layer thickness dFe decreases, the crystallographic structure of the Fe layer changes from body-centered-cubic to amorphous at around 20 A(ring). At this critical thickness, the spontaneous magnetization and the Kerr rotation angle are nearly zero and the magnetization curve does not show hysteresis or saturation indicating its spin-glass-like or superparamagnetic nature. For dFe 〈 20 A(ring), the saturation magnetization and the Kerr rotation angle increase again due to the Fe-rich-DyFe amorphous alloy formed at the vicinity of the layer boundary with decreasing dFe. The perpendicular anisotropy increases with decreasing dFe because of the interfacial anisotropy Ks = 0.62 erg/cm2 in the crystalline region of dFe (approximately-greater-than) 20 A(ring).

Notes: An accurate correspondence between the local optical responses and the structures of semiconductor light-emitting devices is demonstrated by using an illumination-mode photon scanning tunneling microscope with noncontact atomic force microscope technique. We study the novel-structured lateral p-n junctions grown on patterned GaAs(111)A substrate. Measuring the spatially resolved photoluminescence spectra with a 200 nm apertured probe, we precisely determine the position and the width of the transition region of p-n junctions. The illumination-collection hybrid mode is also employed to map the two-dimensional emission efficiency with higher resolution, which is not affected by carrier diffusion. © 1995 American Institute of Physics.

Notes: We find that using the technique of optical feedback locking, to narrow semiconductor linewidths, does not sacrifice the ability to modulate the laser's frequency via the injection current. The frequency of a laser is stabilized to a separate Fabry–Perot reference cavity using resonant optical feedback and can be modulated efficiently at frequencies related by rational fractions to the free-spectral range of the reference cavity. This system can provide an array of narrow-linewidth, frequency-stable laser lines and shows promise for applications in frequency-division-multiplexed coherent communications, as well as laser frequency control and precision measurement systems.

Notes: Recently, correlated spontaneous emission in a laser of three-level atoms has been analyzed theoretically and has been demonstrated experimentally using a three-level He-Ne Zeeman laser. We demonstrate that correlated spontaneous emission is also feasible in a semiconductor laser with a long extended cavity. As a result, the heterodyned spectral width between two lasing modes in a grating extended-cavity laser was reduced to below the spontaneous emission noise level. The laser can be used for high-precision optical-heterodyne sensing.

Notes: The bistability problem, common to scanning microscopes employing lateral dithering of the probe for image formation (i.e., shear force microscope) or probe-sample distance control (i.e., near-field optical microscope) is shown to stem from the two nearly degenerate vibration degrees of freedom possessed by a laterally dithered fiber. Controlling the fiber vibration direction by means of a four-sectioned piezo was found to be a simple and effective solution of the problem. An image of a microtubule is presented to demonstrate the improved imaging ability. © 1997 American Institute of Physics.

Notes: We propose and demonstrate a novel method to enhance the near-field optical intensity on the apertured probe with its foot removed asymmetrically by using a focused ion beam. The spatial distribution of the near-field optical intensity on the asymmetric probe was observed by scanning another sharpened symmetric probe over the aperture. The observed spatial distribution profile was in good agreement with a numerical result corresponding to the HE11 mode. Furthermore, compared with the symmetric apertured probe, a 10 times enhancement of the near-field optical intensity on the asymmetric apertured probe was observed. © 1997 American Institute of Physics.

Notes: Transmission efficiency of a fiber probe used in photon scanning tunneling microscope is evaluated as a function of aperture diameter. The apertured probe has been fabricated by chemical etching technique and metal coating. By comparing two types of probes with different cone angles, we determine the most influential factor in the transmission property of the metal-cladding tapered waveguide. A long tip with high efficiency is developed by a multistep etching method so as to be suitable during actual scanning operation. Photoluminescence imaging of lateral p-n junctions on the GaAs substrate is demonstrated in the illumination-collection hybrid mode operation of photon scanning tunneling microscope. © 1996 American Institute of Physics.

Notes: A technique that excites a localized plasmon on the apex of probe for near-field optical microscope and spectroscope is presented. Vapor deposition and heating of several tens nm thick gold films on 1 nm thick germanium-covered tapered optical fiber probe make highly adherent and smooth overlayers. A "tunneling-photon-mode" experimental system combined with a dye laser is used to excite a localized plasmon. The enhancement of near-field intensity was measured varying incident photon energies with its maximum occurring at 2.11 eV. This local field enhancement could be estimated to generate only in the close proximity of the substrate. © 1998 American Institute of Physics.