Yoshikazu Hori

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.

Field-controllable liquid-crystal phase grating

A new liquid-crystal phase-grating mode has been found. The grating is formed by the application of a spatially periodic electric field to a uniformly aligned liquid-crystal layer with dielectric and optical anisotropy. The fundamental diffraction characteristics of this liquid-crystal phase grating were measured by using a Schlieren optical system, and analyzed based on Fraunhofer diffraction theory with the assumption that the refractive indices vary periodically in the liquid-crystal layer. The incident light is diffracted under the electric field in particular directions determined by the spatial frequency of the periodic electrode, and the intensity of the diffracted light increases abruptly or gradually above the threshold depending on the cell thickness. The applicability of this liquid-crystal phase grating to a light valve in a projection display system is also considered.

Electroluminescent features of oligothiophenes dispersed as a dopant in host matrices

The electroluminescent (EL) features of oligothiophenes dispersed as a dopant in the host matrices comprising tris(8-hydroxyquinoline)aluminum have been investigated. We chose the oligothiophenes that are substituted with phenyl or methyl groups at both the molecular terminals and possess various degrees of polymerization. Regarding both the phenyl- and methyl-substituted materials, the EL spectra are progressively red-shifted with the increasing number of thiophenes. Comparing these spectra with the photoluminescent spectra, we have found out that the EL arises mostly from the dopant molecules of the oligothiophenes. The emission is dominated by energy transfer from host matrices to the dopant molecules, leading to the enhanced device efficiencies. The specific effects of the phenyl- or methyl-substitution and the extension of the π-delocalization in the molecules are also discussed.

Effect of device parameters on bistable semiconductor laser

The effect of device parameters on static and dynamic properties of inhomogeneously current‐injected bistable semiconductor lasers has been analyzed. It is shown that in addition to the fraction of current injection region in the cavity, the device parameters, such as dopant concentration in active layer, facet reflectivity, and cavity length, affect the hysteresis width and threshold current. The mechanism of transition between lasing and nonlasing states is discussed, in which a new interpretation is given about the bias dependence of switch‐on delay time and the driving condition for switching off is clarified. The carrier increasing rates due to the bias current and dopant concentration in the active layer are dominant factors for the switching on and switching off, respectively.

External-cavity semiconductor laser with focusing grating mirror

A novel external-cavity semiconductor laser with a focusing grating mirror (FGM), which enables a single-mode oscillation at a specified wavelength, is proposed. The optical properties of the FGM, which is a computer-generated holographic grating with chirp and bend structure, are numerically analyzed. An optimally designed FGM for realizing laser oscillation at a specific wavelength of 1.30 mu m is fabricated by using a computer-controlled electron-beam writing system. The fabricated FGM with grating area of 1*1 mm/sup 2/ is combined as an external feedback mirror with an InGaAsP-InP semiconductor laser of 1.3 mu m wavelength range, and the lasing characteristics are experimentally measured. Stable and single-mode oscillations with spectral line width less than 10 MHz are observed. >

Fabrication of aberration-free focusing grating couplers

An almost aberration-free focusing grating coupler with a grating area of 1 x 1 mm(2) and a 2-mm focal length was made by using an electron beam writing system with accurately calibrated electron beam deflection and an accurate parameter setting of the effective refractive index of the waveguide. The smallest focused spot size achieved was 1.2 microm (FWHM), which is 1.06 times the value of the diffraction-limited spot size.

Holographic optical element for an optical disk head with spot-size detection servo optics

A CGH construction of a holographic optical element (HOE) for an optical disk head with spot-size detection by means of superimposing two Fresnel zone plates is studied. Optimum parameter values for the ratio of segment to grating pitch (duty) and grating height are clarified from the viewpoint of total light power efficiency in the round trip optical system. The influences of higher-order diffracted beams near the first-order diffracted beams used to get signals on signal detection are shown to be small. The HOEs with almost optimum parameter values are fabricated and the desired diffracted spots, the expected total light power efficiency, and the small influence of higher-order diffracted beams are observed.

Fabrication of a focusing grating mirror by electron beam lithography

A focusing grating mirror (FGM), which is a new computer generated holographic grating with functions of focusing lens and diffraction grating mirror, is proposed and fabricated, we believe, for the first time. A high speed and accurate electron beam writing system, based on a multitask minicomputer, 16-bit D/A converters with 64K-word static memories, and a scanning electron beam microscope, is developed and used to fabricate the FGMs comprising grating corrugations with chirp and bend structures. Excellent properties of the FGM are obtained, giving almost diffraction-limited spot size and the expected wavelength dispersion. The fabricated FGM is combined with a laser diode as an external cavity mirror to confirm the functions of direct optical feedback and wavelength selection, and the fundamental operation of an external-cavity laser is demonstrated.

Integrated holographic optical pickup head with wide allowance range

We propose an integrated hybrid device based on spot-size detection method for holographic optical pickup head and discuss allowances of the misalignment with laser and photodetector. The integrated hybrid device with parallel-shaped photodetector is found to permit positional misalignment of over 200 µm.

Recent advances in optical pickup head with holographic optical elements

Recent advances in optical pickup heads using holographic optical elements (HOEs) are reviewed, with emphasis on a practical servo-signal detection system that can realize compact integration of HOE, laser diode, photodetectors, and objective lens. This paper presents a new type of holographic servo method that employs a blazed HOE placed in close proximity to the objective lens so that wide allowance range of the optical system is maintained, thus allowing the lens and HOE to move simultaneously. The blazed HOE and the integrated optical system are analyzed and discussed in detail.