Takeshi Kamiya

Characteristics of optical guided modes in multilayer metal-clad planar optical guide with low-index dielectric buffer layer

The attenuation characteristics of a multilayer metal-clad optical guide, which is suitable for a mode filter or electrooptic devices, is investigated by the exact theory and the analytical approximation based on a perturbation technique. By using this approximation the dependences of the ohmic loss on the various waveguide parameters and the condition for the absorption peak of the TM mode are derived in closed form. Some remarks concerning the waveguide material and dimension for the design of the mode filter are also presented. The insertion loss at the abrupt junction between a normal (dielectric-clad) optical guide region and a metal-clad optical guide region is treated. Also the transformation of optical guided modes in the two kinds of tapered structures between the above two regions is examined.

Effect of Photon Recycling on Diffusion Length and Internal Quantum Efficiency in AlxGa1-xAs–GaAs Heterostructures

The photon recycling process, i.e., the excitation of electron-hole pairs by the reabsorption of luminescent light, has large effects on the determination of the minority carrier diffusion length L and the internal quantum efficiency ηin GaAs with a high quantum efficiency. In this report, we show that the separate treatments of L and ηin cause the erroneous results for materials with high quantum efficiency. We developed a consistent method of evaluating the photon recycling effects on both ηin and L, based on the simultaneous measurements of the external quantum efficiency ηex and the photo-excited current under the short circuit condition Isc. This method was applied to the specimens with single heterestructure (p-AlxGa1-xAs-p-GaAs-n-GaAs) prepared by the liquid phase epitaxial (LPE) process.

Improved coupled mode analysis of corrugated waveguides and lasers

In the previous paper [1] we proposed an accurate formalism for analyzing corrugated waveguide devices. This formalism, developed for the TE modes, is extended here to TM modes. Characteristics of TM modes in distributed feedback (DFB) lasers, DBR reflectors, and grating beam couplers are investigated with emphasis on the effect of the radiation loss and they are compared with the TE case.

Lasing Emission from an In0.1Ga0.9N Vertical Cavity Surface Emitting Laser

Lasing action in an In0.1Ga0.9N vertical cavity surface emitting laser was successfully achieved, for the first time, at a wavelength of 381 nm. The 3λ vertical cavity comprising an In0.1Ga0.9N active region was grown on a GaN/Al0.34Ga0.66N quarter-wave reflector by metal organic chemical vapor deposition (MOCVD), and covered with a TiO2/SiO2 reflector by electron-beam evaporation. The laser was operated at 77 K under optical excitation. We have observed a significant narrowing of the emission spectrum from 2.5 nm below the threshold to 0.1 nm (resolution limit) above the threshold, which is a clear signature of lasing action.

Band filling in GaAs/AlGaAs multiquantum well lasers and its effect on the threshold current

Existing theoretical models for multi-quantum-well laser operation are not always accurate in evaluating band-filling characteristics. This, as a result of the partial omission of some significant physical processes or simplified modeling, sometimes leads to excessively optimistic prediction of threshold-current reduction in these types of lasers. An improved model which includes the effects of band nonparabolicity, revised band offsets at the heterojunction interface, nonradiative processes in the barrier and waveguide regions, and the energy-dependent spectral linewidth has been developed. It is tested satisfactorily by comparing its numerical prediction to the experimental results on cavity-length dependence of threshold current for lasers with a different number of quantum wells. >

Propagation Characteristics of a Partially Metal-Clad Optical Guide: Metal-Clad Optical Strip Line

Experimental investigations of the partially metal-clad optical guide were performed. The optical guide consisted of a silicon substrate, an SiO(2) low index dielectric layer, an Al(2)O(3) high index dielectric layer, and a partial cladding layer of Al. The far-field patterns of the output beam from the second prism coupler were consistent with the calculated transverse field distributions using the effective index-of-refraction method. The experimental results on the coupling angles, confinement at the curved section, and attenuation constants are also reported.

Recombination lifetime of carriers in GaAs‐GaAlAs quantum wells near room temperature

The lifetime τ of carriers in undoped GaAs‐GaAlAs quantum well structures was studied at room temperature by using the photoluminescence phase shift method. We have found that τ is inversely proportional to the carrier concentration under the excitation levels of 1016–1018 cm−3. The comparison of these results with both the theory of band‐to‐band recombination and the measured dependence of τ on the carrier concentration at 77 K suggests that the carrier recombination near room temperature is dominated by this band‐to‐band recombination process. We also discussed the dependence of τ on the quantum well thickness.

Chirp and stability of mode-locked semiconductor lasers

A numerical study of mode-locked semiconductor lasers is presented with special attention to the chirp characteristics and to dispersion-related criteria for stable pulse-train emission. The dependence of the pulse chirp upon the refractive-index change, both with carrier density and carrier temperature changes, is discussed. The experimental observation of blue-chirped pulses for passive mode-locking in contrast to red-chirped pulses for active mode-locking is found to be due to the different contributions of gain and absorber media to the refractive-index change. In addition, it is revealed that the boundary of the stable operation regime is critically influenced by the spectral characteristics of laser and external cavity. Design considerations toward the achievement of high pulse energy, narrow spectral bandwidth, and linear chirp are given.

Subharmonic hybrid mode-locking of a monolithic semiconductor laser

Performance of a subharmonically hybrid mode-locked (SH-ML) monolithic semiconductor laser is investigated. A 33-GHz passively mode-locked distributed Bragg reflector semiconductor laser is stabilized by the injection of an electrical signal with a subharmonic frequency of the laser cavity resonance. Systematic measurements on the phase noise, timing jitter, amplitude modulation, and locking bandwidth are performed for the second- and third-order SH-ML conditions, and the results are compared with the fundamental hybrid mode-locking (FH-ML) case. Low timing jitter of less than 0.6 ps, comparable to that under the FH-ML case, is achieved for the both SH-ML cases. The amplitude modulation imposed by the subharmonic driving frequencies is found to be very small (<-24 dBc) for the second-order SH-ML because of the sufficiently low modulation response of the laser at 16.5 GHz. The third-order SH-ML is found to exhibit a very unique locking characteristics, leading to a maximum locking bandwidth of 56 MHz that is even larger than that for the FH-ML case.

Ultralow chirping short optical pulse (16 ps) generation in gain‐coupled distributed feedback semiconductor lasers

We describe optical short pulse generation in novel gain‐coupled distributed feedback (GC‐DFB) semiconductor lasers by using a simple electrically pumped gain switching method. An optical pulse as short as 16 ps has been obtained from a 130 ps current pulse. The most exciting result is that the lasers keep single longitudinal mode oscillation with very low wavelength chirping (∼0.12 nm) during the gain switching operation. From the optical pulse width and the amount of the wavelength chirping, we have estimated the α parameter of the GC‐DFB laser to be less than 1.6.