Hideo Itoh

Optical tristability using a twin-stripe laser diode

Optical tristability is demonstrated with a positive feedback circuit composed of a twin‐stripe laser diode, a p‐i‐n photodiode, and a transistor. A novel nonlinearity in the current/light characteristics of the twin‐stripe laser diode is the key point in realizing the tristability. By changing the circuit parameters, optical bistability and differential gain characteristics can also be performed.

Laser beam scanning using a local deflector integrated with an effective mode filter

Deflection of a diode laser beam using an injection‐type deflector monolithically integrated with a very long waveguide is demonstrated. The deflector and the waveguide constitute a laser cavity. The waveguide filters out higher order modes, and only the fundamental‐mode light is incident on the deflector. Because the incident light is of a fixed single mode, the deflection of the output beam is very smooth. The deflection mechanism is primarily due to interference between constituent modes for short deflectors, and phase‐front tilting of each mode for long deflectors.

Ultraviolet absorption spectra of metalorganic molecules diluted in hydrogen gas

Abstract Ultraviolet absorption spectra of trimethyl gallium, triethyl gallium, and trimethyl aluminum diluted in hydrogen gas were measured as a function of the wavelength (185–350 nm) and the concentration of the molecules (4.8×10−6 -1.6×10−4 mol/liter). Their absorbances changed linearly with the concentration of the molecules, which allowed us to calculate the molar absorption coefficients of the molecules on the basis of the Beer-Lambert law.

Influence of carrier spreading on the current-controlled lateral-mode behavior of twin-stripe lasers

Near‐ and far‐field patterns, lasing efficiency, and spectra of twin‐stripe lasers as a function of the injected current ratio were systematically examined with particular interest in their dependence on the carrier spreading which was controlled by the interstripe etched depth. As for lasers lightly etched not deeper than 0.5 μm to the active layer, the near‐field peak shifts with current ratio at a higher rate for deeper etching, which leads to the more drastic change of the lasing efficiency. At the same time, the peak direction of the far‐field pattern and the peak wavelength also changes at a higher rate for deeper etching. The dependence of these characteristics on the etching depth can be attributed to the higher current isolation for deeper etching. These results mean that laser beam deflection and nonlinear light‐current characteristics, which are particularly important for information processing, can be controlled by the interstripe etched depth. As for lasers deeply etched down to 0.3 μm to the active layer, the regions below the stripes lases independently from each other which are ascribed to both the strong isolation of carriers and the considerable dip in the built‐in index in the interstripe region.

Optoelectronic implementation of real-time control of an inverted pendulum by fuzzy-logic-control units based on a light-emitting-diode array and a position-sensing device.

We have realized a novel optoelectronic implementation of an analog fuzzy-inference architecture, using the movement of the center of gravity of a far-field pattern. In the inference process Gaussianlike membership functions and a product-sum-gravity method are used. Antecedent membership functions and the grade evaluations are realized by combining light-emitting-diode (LED) arrays for sequential light emission and position-sensing devices for center-of-gravity detection. Consequent and defuzzification operations are realized by a LED array and a position-sensing device. High-speed fuzzy inference of more than several tens of mega fuzzy logical inference per second is possible with these units. The usefulness of the system is demonstrated by the real-time standing control of an inverted pendulum.

Mechanism of the negative slope in the current-light characteristics of twin-stripe lasers

The mechanism of the negative slope in the current‐light characteristics of twin‐stripe lasers is explained by the dependence of a near‐field pattern on the current distribution. A guiding principle for designing the laser structure for a more effective negative‐slope characteristic is discussed.

A semiconductor laser with a converging output beam

Semiconductor lasers with converging output beams are reported for the first time. The lasers have a low‐gain region on the center of the mesa stripe near one cleaved facet. The low‐gain region distorts the phase front of the guided light and beam convergence results. The width of the laser beam in the direction parallel to the junction plane decreases with distance from 30 μm at the facet down to 7 μm at 111 μm off the facet. Moreover, the one‐dimensional optical power density increases with distance in accordance with the decrease in beam width.

AlGaAs/GaAs buried quantum well laser diodes by one time selective metalorganic chemical vapor deposition growth on dielectric window stripes

AlGaAs graded‐index separate‐confinement heterostructure lasers with lateral mode confinement are realized by one time metalorganic chemical vapor deposition with silicon dioxide stripes as selective masks and current blocking layers. The effect of the active region width on lasing characteristics is investigated. The threshold current and external quantum efficiency are around 12 mA and 70%, respectively, for narrow lasers. Elevated waveguide structure formed by the growth‐rate difference between (100) and sidewall planes was effective for stable lateral mode characteristics.

LPE Growth of AlGaAs–GaAs Quantum Well Heterostructures

AlGaAs-GaAs single quantum wells with thicknesses d down to 6 nm are prepared by LPE at 742°C. The transmission electron microphotographs and the luminescence/photocurrent spectra of these wells indicate that the interface roughness and the transition layer thickness L of the wells are about 1.5 nm and 3 nm, respectively. Energy levels of E1h, E1l, and E2h are calculated as a function of d and L, which allows us to estimate these values from luminescence or photocurrent spectra measured at room temperatures.

Integration of a diode laser and an electronic lens for controlling the beam focus position

An electronic lens is developed for controlling the focus position of an optical beam without mechanical movement. The lens utilizes the dependence of the lateral index profile on carrier concentration profile in the waveguide layer. The lens is integrated with a diode laser by the cleave‐couple technique, and the control of the focus position of the output beam of this device is demonstrated.