Hisanao Sato

Broadening of the phase-matching bandwidth in quasi-phase-matched second-harmonic generation

We report on theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG) in LiTaO/sub 3/. QPM waveguides consisting of segments, where each segment has a phase-matching condition different from the others, are proposed to obtain a broad bandwidth and simultaneously, a high SHG efficiency. The waveguides were fabricated by adjusting the phase-matching condition either by changing the grating period of the SHG coefficient or by controlling the width of the waveguide. Consequently, the bandwidth of QPM-SHG was broadened to 0.35 nm with an SHG efficiency of 57 percent/W by modulation of the waveguide width and to 1.12 nm with an SHG efficiency of 29 percent/W by modulation of the grating period. The SHG efficiency and phase-matching characteristics in both types of waveguides showed good agreement with theoretical results. >

Intensity noise suppression and modulation characteristics of a laser diode coupled to an external cavity

Static and dynamic properties of a GaAlAs laser diode with an external cavity (1.5-50 cm) have been studied experimentally, it is found that the optical feedback induced intensity noise is strikingly suppressed by 30 dB when a single frequency oscillation is realized with good phase matching. The threshold of modulation depth, over which intensity noise increases abruptly, has been measured as a function of the modulation frequency in a range of 10-400 MHz. It has the maximum at about 40-70 MHz and increases with the decrease of the external cavity length. On the basis of the compound cavity model the phase and gain conditions for the single frequency oscillation are discussed and the threshold modulation depth is calculated, which is shown to be in good agreement with the experimental results.

Electrical and Optical Properties of CdS-MnS Single Crystals

Single crystals of Cd 1- x Mn x S ( x ≤0.4) were prepared by Bridgman-Stockbarger method under high gas pressure and electrical and optical properties of these crystals were measured. The mobility of electrons was found to be significantly influenced by the alloy scattering. In the optical absorption spectra, three absorption peaks due to Mn 2+ ions were observed at 455, 480, and 510 mµ, similar to the case of Zn 1- x Mn x S crystals. The band gap of Cd 1- x Mn x S varies anomalously with x ; it shows a minimum at x ∼0.03 and increases linearly with x for x ≥0.1. The reason for the appearance of this minimum at small x has not been understood.

Chaos in a directly modulated semiconductor laser

The fundamental possibility of chaos generation in directly modulated semiconductor lasers is studied on the basis of the rate equation with the form of a driven nonlinear oscillator. The origin of chaos generation and the effect of the spontaneous emission factor, which affects the nonlinearity of the dumping force and the restoring force in the rate equation, have been clarified.

Theory of spectral linewidth of external cavity semiconductor lasers

A new formula of the spectral linewidth of external cavity semiconductor lasers is proposed, with which linewidth narrowing with the optical feedback is discussed. It is shown that in the limit of large external cavity length, the linewidth caused by the phase diffusion due to spontaneously emitted photons becomes dominant and the linewidth decreases proportionally to the inverse of the external cavity length.

Intensity fluctuation in semiconductor lasers coupled to external cavity

Broad peaks are observed in the intensity noise spectrum for external cavity AlGaAs lasers. Oscillating longitudinal mode spectrum measured with a high resolution Fabry-Perot interferometer indicates that the intensity noise peaks result from the lasing of several closely spaced external cavity modes. A new mechanism is proposed to explain the observed noise spectrum; the intensity fluctuation is induced by modulation of the dielectric constant in the semiconductor cavity due to the beating of the electric fields for lasing modes. Qualitative agreements between the theory and experiments are obtained.

Domain inversion in LiTaO3 using proton exchange followed by heat treatment

We report on fabrication of a first‐order periodically domain‐inverted structure in −Z‐cut LiTaO3 using proton exchange followed by heat treatment. The expansion of the domain inversion width can be suppressed within the first‐order intervals by shortening the heat‐treatment time. To perform this suppression, we propose quick heat treatment, which features rapid rising rate and short soak time. This technique was used to study the formation process and the expansion characteristics of domain inversion during heat treatment. This produced a periodically domain‐inverted structure with interval of 4 μm and depth of 1.8 μm over a 10 mm length in the LiTaO3. The potential for this structure was evaluated by examining the characteristics of a first‐order quasi‐phase‐matched second‐harmonic generation device with a proton‐exchanged waveguide. We also discuss the nucleation and expansion of domain inversion, based on experimental studies.

Fabrication of periodic domain inversion in an x‐cut LiTaO3

We report on the fabrication of periodic domain inversion on the x face of LiTaO3 by selective proton exchange and quick heat treatment. Observation of domain formation in –c‐cut LiTaO3 indicates an inside field model where electric field caused by proton diffusion reverses the orientation of domains. Based on this model, it has been found that an effective inside field to form domain inversion is also induced in x‐cut LiTaO3 by using selective proton exchange. Accordingly, periodic domain inversion at 10 μm intervals has been successfully formed on the x face of LiTaO3.

Growth of ZnSe/ZnS strained‐layer superlattices on Si substrates

The high‐quality ZnSe‐ZnS strained‐layer superlattices (SLSs), as well as the ZnS layers, were successfully grown for the first time on Si substrates. Photoluminescence (PL) spectra of ZnS layers grown on Si showed an intense excitonic‐emission line. In transmission electron microscopy analyses, no misfit dislocations and no moire fringes were observed on the ZnS layer with a thickness of less than about 500 A. We have also characterized ZnSe‐ZnS SLSs grown on Si substrates. By the PL measurements, an intense excitonic‐emission line, and no emissions due to deep levels, were observed. As the ZnSe well‐layer thickness decreased, the peak of the line largely shifted towards the higher‐energy side. This behavior may be related to the quantum size effect. In the temperature dependence of PL intensity, there appeared the thermal quenching process, which may be related to the thermal release of excitons in quantum wells. As the thickness of the ZnSe well‐layer decreased, the activation energy abruptly increased...

Dependence of elastic strain on thickness for ZnSe films grown on lattice‐mismatched materials

We present a new approach for calculation of the dependence of elastic strain on layer thickness concerning ZnSe films grown on GaAs and on ZnS. The basic concept involved in our model is that the interfacial misfit dislocations are generated only when the areal strain energy density exceeds an energy barrier. It is found that the elastic strain calculated by this model is quite in agreement with experimental results for the ZnSe/GaAs system with the energy barrier of 79 erg/cm2. To compare the theory with experiments, the difference of thermal expansion coefficient between the epitaxial layer and the substrate must be taken into consideration. Applications of the model to other systems are also discussed.