The semiconductor lasers operating with self-sustained pulsation are under developing to be lasers which are less disturbed by the optical feedback from a surface of optical disk. Structures setting saturable absorbing regions utilizing the multi-layer configuration become popularly used for giving stronger pulsation. However, the quantum (intensity) noise in these lasers tends to be enhanced. The ridge stripe structure, of which almost self-sustained pulsation lasers consist, seems to give a leak current flowing along plane of the cladding region. Such leak current also increases the quantum noise. In this paper, theoretical calculations of operating characteristics, such as the self-sustained pulsation, the optical output, the quantum noise as well as the transverse filed profile, are theoretically analyzed by including the above mentioned several phenomena.

Digital neural networks are suitable for WSI implementation because their noise immunity is high, they have a fault tolerant structure, and the use of bus architecture can reduce the number of interconnections between neurons. To investigate the feasibility of WSIs, we integrated either 576 conventional neurons or 288 self-learning neurons on a 5-inch wafer, by using 0.8-µm CMOS technology and three metal layers. We also developed a new electron-beam direct-writing technology which enables easier fabrication of VLSI chips and wafer-level interconnections. We fabricated 288 self-learning neuron WSIs having as many as 230 good neurons.

As an estimating method of the threshold current level on experimental data of the semiconductor injection laser, finding peak position on second order derivative of I-L (injection current vs. light output) characteristics is popularly in use. A theoretical proof of this method is given in this letter.

Intensity noise of an injection laser enhanced by the optical feedback was reduced by application of electric negative feedback. The reduced noise level was lower than that under operation free from the optical feedback.

The oscillating linewidth of the semiconductor injection lasers is broadened by the mode competition effect which tends to be induced by the optical feedback. The broadening phenomena are theoretically analyzed by taking into account both interactions among lasing modes and fluctuations caused by the spontaneous emission. Operating conditions to avoid these phenomena are also given. For example in case of BH type AlGaAs laser, the re-injection ratio of going into the laser cavity to the emitted light should be less than 10-7 to keep the stable single mode operation without the hopping phenomena.

Microwave modulation in frequency region of 5.9 to 10 GHz was determined in TJS and BH lasers. Incident microwave power was less than 32 mW at 10 GHz. Modulation mechanism was regarded to be caused by driving of the injected-carrier-dencity even at such high frequency.

Stabilization of longitudinal mode was obtained by a technique of compensation for temperature at oscillating condition. Oscillation with fixed single mode was continued over twice level of the threshold in Channeled-Substrate-Planar lasers.

Characteristics of the optical feedback noise in semiconductor lasers under superposition of the HF (High Frequency) current were experimentally examined and theoretically analyzed. The feedback noise was mostly suppressed by superposition of HF current, but still remained when frequency of the HF current coincided with a rational number of the round trip time period for the optical feedback in experimental measurement. Theoretical analysis was also given to explain these characteristic based on the mode competition theory of the semiconductor laser.

A theoretical prediction that the lasing gain must increase by breaking the electrically neutral condition in the active layer is given. A possible structure to give a very low threshold current less than 1 mA was discussed.

Operation of a newly proposed semiconductor optical modulator based on absorption control by electron depletion around a p-n junction is demonstrated, forming preliminary structures of waveguide-type as well as panel-type (or surface-illuminated type) devices. The optical absorption is occurred at the intrinsic energy levels in the band structure not at the extended state into the band-gap. Performance of 35 dB on-off extinction ratio for 4 V variation of the applied voltage was obtained in a waveguide type device. Validity of the proposed mechanism were confirmed by getting large change of the absorption coefficient of around 5000 cm-1 over wide wavelength range of 30 nm.

Behavior of the longitudinal mode is analyzed for injection laser with undoped active region. Injection laser shows multi-longitudinal-mode-operation at near the threshold current level, and becomes to single-longitudinal-mode-operation with increasing of the injection current beyond the threshold current level, due to strong suppression effect among the longitudinal modes. Such behavior is analyzed quantitatively for AlGaAs DH laser with consideration of some factors such as mixing of the spontaneous emission, carrier diffusion, threshold current level and field profile. The single-longitudinal-mode-operation is achieved in lower injection level with smaller spontaneous emission factor, lager effect of the carrier diffusion, lower threshold current level and transverse-fundamental-mode.

An integrated laser having increased rate of the spontaneous emission is proposed a apply this device as a light source giving weaken optical coherency. This device consists of two sections, one is a Light-Emitting-Diode (LED)-section and another is a Laser-section. The spontaneous emission is generated and amplified in the LED-section, then injected into the Laser-section. Power of the incoherent light in the Laser-section becomes more than ten times of that in the conventional single section laser with the help of the light injection. Wider half width of the output spectrum more than 1.5 nm is obtained at injection current level of I/Ith=1.5 in GaAs DH structure. Mechanism and fundamental properties of this device are analyzed with the help of both classical Maxwell's equation and quantum mechanics.

Theoretical calculations of the pulsing operation and the intensity noise under the optical feedback are demonstrated for operation of the self-sustained pulsation lasers. Two alternative models for the optical feedback effect, namely the time delayed injection model and the external cavity model, are applied in a combined manner to analyze the phenomena. The calculation starts by supposing the geometrical structure of the laser and the material parameters, and are ended by evaluating the noise. Characteristics of the feedback induced noise for variations of the operating parameters, such as the injection current, the feedback distance and the feedback ratio, are examined. A comparison to experimental data is also given to ensure accuracy of the calculation.

Generating conditions of the optical feedback noise in self-pulsing lasers were experimentally examined. The noise charcteristics were determined by changing the operating power, the feedback distance and the feedback ratio for several types of self-pulsing lasers. The idea of the effective modulation index was introduced to evaluate the generating conditions in an uniform manner based on the mode competition theory. Validity of the idea was experimentally confirmed for generation of noise.

An optical detection system using a DFB laser with a very small aperture is theoretically proposed. The threshold gain level in DFB laser is sensitively varied with combined reflections by the facet and the corrugation as well as with the optical injection reflected at the surface of the optical disk. Variation of the threshold gain level is counted as the voltage change on electrodes of the laser. It is found that sensitivity of the optical detection with a well-designed DFB laser becomes six times larger than that with conventional Fabry-Perot ones. Field distribution around the small aperture is analyzed taking into account both the near-field and the radiation field. Numerical data on the voltage change are given as examples of the detection system.

The generation mechanism for excess intensity noise due to optical feedback is analyzed theoretically and experimentally. Modal rate equations under the weakly coupled condition with external feedback are derived to include the mode competition phenomena in DFB and Fabry-Perot lasers. We found that the sensitivity of the external feedback strongly depends on design parameters of structure, such as the coupling constant of the corrugation, the facet reflection and the phase relation between the corrugation and the facet. A DFB laser whose oscillating wavelength is well adjusted to Bragg wavelength through insertion of a phase adjustment region becomes less sensitive to external optical feedback than a Fabry-Perot laser, but other types of DFB lasers revealing a stop band are more sensitive than the Fabry-Perot laser.

This paper shows an improved rate equation by which the phase relation of the carriers and inhomogeneous property are taken into account for injection lasers. The direct modulation characteristics were analyzed as an application of this improved rate equation. A more general discussion was obtained for the resonance-like phenomenon and its suppression effect. For the undoped AlGaAs DH laser with index guiding structure, it is found that the resonance-like phenomenon is mainly suppressed by effect of spatial diffusion of injected carriers, reduction of the thresh-old level and lateral singlemode-operation. We also found that other mechanisms, such as mixing effect of the spontaneous emission, variation of longitudinal mode number and the intra-band relaxation, hardly affect to suppress the resonance-like phenomenon.

Potential Controlled (POCO) Laser is a new type of laser which utilizes a character that the lasing gain effectively increases and the threshold current level can be reduced by putting a large number of holes into the active layer. The basic idea of this laser is very close with the so called modulation doping in the multi-quantum-well structure. In this paper, the optimum configuration of the POCO laser is theoretically examined in detail. Possibility to get a very low threshold current lower than 100µA is predicted.

Reduction of the intensity noise in semiconductor lasers is an important subject for the higher performance of an application. Simultaneous usage of the superposition of high frequency current and the electric negative feedback loop was proposed to suppress the noise for the higher power operation of semiconductor lasers. Effective noise reduction of more than 25 dB with 80 mW operation was experimentally demonstrated.

The effects of impurity atoms and energy gap profiles in the optical guiding layers of GaAs-AlGaAs Separate Confinement Hetero (SCH) structure on the threshold current density are theoretically analyzed. Impurity doping in the optical guiding layers is not effective in reducing the threshold current, because impurity atoms induce non-effective current in those layers. The threshold current densities in three types of band gap profiles of the optical guiding layers are examined to find the most profitable structre of the SCH laser. Examined shapes of the profile are step, parabolic and linear types. The lowest threshold current densities calculated in each profile are almost identical if the design parameters such as the thickness of each layer and the alloy ratio are suitably chosen.