From the analysis of layer number dependence of response time and current gain, there is a tendency that with decreasing layer number, response time becomes higher and current gain becomes larger. The current gain band-width product , is estimated theoretically to be 640 GHz (current gain is to be 34), when metal layer number is one and layer thickness is 10 Å.

It is shown that the resonance-like phenomena or the relaxation oscillations in the light output of the directly modulated injection lasers is suppressed by a non-resonant π-type electric circuit connected to the laser diode. Application of this suppressor circuit is not sensitive to the bias current of laser diode and the cut-off frequency of modulation is sufficiently large. It is pointed out that the use of this method at the narrow stripe laser is the most effective.

The wavelength shift of a dynamic-single-mode (DSM) laser directly modulated by sinusoidal current has been investigated using the large-signal analysis. A rate equation which includes carrier diffusion terms is solved numerically. The variation of the carrier density as an origin of the dynamic wavelength shift is calculated. The calculated dynamic wavelength sifts are in good agreement with experiments. At high modulation frequency range, the dynamic wavelength shift is mainly caused by relaxation oscillations and the shift is almost constant to the stripe width of active guide. It shows that the dynamic wavelength shift in pulse modulation is also independent of the stripe width of active guide. An analytical expression of dynamic wavelength shift is given at low modulation frequency range. When the modulation depth is below 30 percent, the small-single analysis is in agreement with large-signal analysis. By considering the effect of spontaneous emission, a modified small-signal analysis presented here is applicable to a modulation depth of 100 percent in the case of low frequency limit.

Phase-matched film thickness are studied precisely for SHG in a nonlinear quartz-glass waveguide. The glass film waveguide deposited upon the surface of quartz suffers non-uniform stress due to the thermal expansion. This effect generates an anisotropic property in the film. Taking this anisotropy of the glass film into account, the observed film thickness for phase-matched condition results in good agreement with the theory.

Wavelength-tunable 1.5 µm GaInAsP/InP bundle-integrated-guide distributed Bragg reflector (BIG-DBR) dynamic-single-mode lasers are presented. Tuning is due to free carrier plasma effect generated by the injected tuning current at the monolithically integrated tuning regions. Low threshold current CW operation of 28 mA was obtained in junction-up mounting. Wide-range continuous wavelength tuning more than 9 was demonstrated for the first time.

Field induced refractive index variation and the ratio of index to loss variations p' which is an important parameter deciding the loss characteristics of an intersectional optical switch, was analyzed for GaInAs/InP quantum box structure. We found that large index variation and large | p| value(10) with low fundamental absorption, which are required for low insertion loss switch(1 dB), can be attained. Further the usable wavelength range for low loss switching operation is shown to be around 8 nm.

The harmonic and phase distortions in the direct modulation of injection lasers are given theoretically with help of a large signal analysis by a Fourier-series-expansion method. It is shown that the distortions are relatively small when there is no-kink in the I-L curve and the magnitudes of the distortions are inversely proportional to the bias current above threshold and proportional to the photon lifetime in laser cavity. It is also shown that the maximum modulation frequency in the analog-modulation is limited by the phase distortion rather than the amplitude distortion.

Single axial mode oscillation in Integrated Twin-Guide AlGaAs laser fabricated by sputter-etching was observed by current injection pumping at room temperature, as predicted by theory.

The basic properties of 1.55µm GaInAsP/InP distributed reflector (DR) laser were analyzed from the view points of active region structure and π/2 phase shift position. By introducing SCH structure with thin active layer, differential quantum efficiency could be improved up to 60-70% without sacrificing threshold current density. Optimum π/2 phase shift position for high power operation was found to be located inside the active region at a distance of 0.3 times of active region length from the joint portion between active and passive regions.

Current leakage along longitudinal direction, from active region to connected external waveguide or neighboring opto-electronic (O-E) functional region, commonly occurs in integrated type semiconductor lasers. This current leakage degrades not only lasing characteristics but also interrupts operation of neighboring functional devices. In this paper, the longitudinal current leakage is analytically given for an integrated laser by introducing an effective length of leakage current along the longitudinal direction. The minimum lengths of active region and isolation region to minimize the influence of longitudinal current leakage were clarified. As the results, shortening of current injection region with respect to the active region length as well as increasing sheet resistance were found to be effective for reduction of leakage current.

We propose and analyze a new type of intersectional optical switch using positive refractive index variation in quantum well structure. The switch structure has a built-in refractive index difference in the waveguide, due to which the incident light is reflected to cross port at the OFF state. When the electric field applied to the electrode (ON state), the built-in refractive index difference vanishes by the positive refractive index variation in the quantum well, and the light transmits to straight port. Low insertion loss of lesser than 1 dB and high extinction ratio of more than 20 dB can be obtained at both cross and straight port.

Bundle-integrated-guide (BIG) structure distributed-Bragg-reflector (DBR) lasers based on 1.5-1.6 µm GaInAsP/InP system are presented. The significance of this structure is the suitability for the planar fabrication process of integrated optical devices, such as DBR lasers, which comprise active and passive waveguide regions. The BIG structure enables easier fabrication of buried hetero-structure (BH) of such an integrated waveguide device. High coupling efficiency between those waveguides of 95-99 percent is theoretically available with sufficiently large tolerance in thickness and composition of waveguide layers. Devices with different lengths of the active region, such as 200 µm, 100 µm and 50 µm, were fabricated and tested both for DC operation and rapid direct modulation. Threshold current as low as 28 mA and output power of 6.5 mW/facet were obtained for BH-BIG-DBR lasers with 100 µm long and 3 µm wide active region. Side-mode suppression ratio (SMSR) of more than 32 dB was obtained at the bias current of 1.2 times the threshold and it was not much degraded by rapid direct modulation.

A novel electronic device named Space Charge Limited Insulator Tetroide (SCLIT), which uses space charge limited current in insulator of a metal-insulator structure is proposed. A possibility for high speed response (fγ1.1 THz) is estimated theoretically. The static characteristics are also shown.

Ultimate limit of the bandwidth of the multimode optical fiber with the index profile consisting of α-power and the fourth-order terms of radius is derived, rejecting the effects of the cladding and compensating the material dispersion. The 3 dB-bandwidth can be increased up to 1.9 GHz at 10 km length.

A dynamic-single-mode bundle-integrated-guide distributed-bragg-reflector (BIG-DBR-DSM) laser with short active region was experimentally demonstrated for low threshold current and high sub-mode suppression. CW threshold current of 22 mA was obtained for the case of active region length of 50 µm. Sub-modes were suppressed more than 35 dB.

The spontaneous emission factor in the rate equation of the injection laser is derived with help of the classical electromagnetic theory. The spontaneous emission is treated as the radiations from dipoles located in the active region. The value of the spontaneous emission factor given theoretically is in good agreement with the measured results. The magnitude of the spontaneous emission factor is inversely proportional to the volume of the active region and the spectral width of the spontaneous emission, and is proportional to the energy confinement factor and the fourth power of the wavelength. The possibility of the suppression of the relaxation oscillation by reducing the volume of the active region is predicted theoretically.

Transfer function of mode-coupled graded-index multimode optical fibers is given theoretically with an algebraic method, which can be applied to arbitrary index profile and distance. Using the transfer function thus obtained, the relation between the transmission characteristics and the mode coupling due to the random bending of the fiber axis is investigated. As a result it is shown that the length-dependent transmission characteristics such as frequency characteristics, coupling-induced loss and bandwidth can be represented in a unified manner using the coupling length defined in this paper. The frequency characteristics is determined dominantly by the ratio of the fiber length to the coupling length. Further, the length dependencies of both the bandwidth and the coupling-induced loss are represented by a normalized form which does not depend upon the index profile. For the distance which is far from the coupling length, the bandwidth is inversely proportional to the square root of the product of the fiber length and the coupling length, and the coupling-induced loss is proportional to the ratio of the fiber length to the coupling length.

This paper gives an experimental result on the measurement of the maximum coupling coefficient and the coupling length between the active waveguide and the passive output waveguide in GaAs/AlGaAs integrated twin-guide (ITG) injection lasers prepared by LPE. Very high coupling efficiency between the active and the passive waveguides was experimentally confirmed. The maximum coupling coefficient and the coupling length were measured to be about 90% and 250 µm, respectively. These values were in agreement with theory. At properly chosen parameters, the threshold current density on an ITG-type" laser was as low as 3 kA/cm2 at room temperature.

The harmonic distortion and the intermodulation distortion of a laser diode are calculated to be severely influenced by the coherent interaction with reflected waves and are given to be about 2030 dB larger than intrinsic values of the laser diode without reflected waves.

The effect of the intraband electronic relaxation on the relaxation oscillation of the light output from the injection laser modulated directly is examined theoretically. Assuming the k-selection rule, carriers are devided into two parts; that is, carriers which contribute to the laser gain directly, and the rest. The intraband electronic relaxation is taken into account as the coupling of the two parts of the carriers in rate equations. As the result of the small signal analysis of rate equations, the height of the resonance-like" peak in the frequency response of the direct modulation depends on the electronic relaxation time remarkably in the range from 10-13 to 10-12 s. The intraband electronic relaxation possibly concerns with the suppression of the relaxation oscillation of the light output from the GaAs injection lasers.