Osamu Imafuji

600 mW CW single-mode GaAlAs triple-quantum-well laser with a new index guided structure

A very-high-power, single-mode GaAlAs triple-quantum-well (TQW) laser with a novel real refractive index guided structure has been developed. The maximum output power of 720 mW CW and single transverse- and longitudinal-mode operation up to 600 mW CW have been achieved in a single-element device. This excellent mode operation is attained by using the real refractive index guided structure with a GaAlAs current-blocking layer which gives a low effective refractive index step, suppressing the spatial hole-burning effect in the high-power range. >

Structural investigation of sapphire surface after nitridation

Abstract Nitridation process of a sapphire surface was investigated with atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), and X-ray photoelectron spectroscopy (XPS) in order to reveal a general nitridation mechanism. It was found that the nitridation process consists of two steps. First, inter-mixing between nitrogen-related species and sapphire surface occurs forming hydrogenated Al oxynitride. This step does not change the surface morphology significantly. Second, crystalline AlN islands are gradually formed by further nitridation of hydrogenated Al oxynitride, resulting in a very rough surface.

100-mW high-power angled-stripe superluminescent diodes with a new real refractive-index-guided self-aligned structure

We have developed 100-mW high-power angled-stripe superluminescent diodes with a new angled-stripe real refractive-index guided self-aligned structure. The structure has a GaAlAs optical confinement layer on a planar active layer and an inclined current injection stripe by 5/spl deg/ with respect to the facet normal. The structure gives small internal loss (/spl sim/10 cm/sup -1/) and facet power reflectivity less than the order of 10/sup -6/. As a result, the output power as high as 105 mW at a low operating current of 270 mA is obtained with less than 3% spectral modulation and 10.5 nm full width at half maximum (FWHM) spectral width.

Optical-loss suppressed InGaN laser diodes using undoped thick waveguide structure

We propose optical-loss suppressed thick-optical-waveguide (TOW) InGaN laser diodes (LDs) without operatingvoltage increase. A record high continuous-wave (CW) output of 7.2W for a single-emitting InGaN LD is achieved without thermal peak-out in the light-current curve. The TOW enables to confine major part of the propagating light into a transparent undoped region, and thus significantly reduces the optical-loss. An electron-overflow-suppression (EOS) layer placed between the waveguide layer and a p-cladding layer plays an important role to reduce the operating voltage after introduction of the undoped TOW layer. We executed a self-consisted calculation of voltage-current characteristics taking into account Schrödinger and Poisson equations in conjunction with a carrier continuity equation. The calculation result indicates possible presence of conductivity-modulation in the waveguide filled with electrons reflected backward by the EOS layer and holes injected from the p-type cladding layer. We successfully demonstrated the optical-loss suppressed operation resulting in the slope efficiency (SE) increase from 2.0W/A to 2.5W/A. It is noted that the operating voltage of the TOW LD is nearly identical to the conventional LD thanks to the above conductivitymodulation phenomenon. The presented result suggests that our TOW structure can overcome the optical-loss drawback of the InGaN LDs, and hence will lead them to the applications requiring high wattage light sources.

Low-Noise and High-Power GaAlAs Laser Diodes with a New Real Refractive Index Guided Structure

We demonstrate for the first time a real refractive index guided GaAlAs laser structure that has both very low-noise characteristics at a low output power (reading mode) and a stable fundamental transverse mode up to a high power level (writing mode) sufficient for optical read-write systems. The low-noise characteristics are realized by self-sustained pulsation which is induced by large saturable absorbers outside a stripe, and the high power is achieved by using a thin active layer which confines the optical flux very loosely. In order to optimize the device parameters, numerical simulation of the structure is proposed with consideration of interaction between optical field and carrier diffusion under gain nonlinearity and carrier diffusion taken into account. The calculated results are verified experimentally. The lasers showed relative intensity noise level less than -135 dB/Hz under 0-20% optical feedback and output power higher than 60 mW.

Low operating current self‐sustained pulsation GaAlAs laser diodes with a real refractive index guided structure

We have developed a new real refractive index guided GaAlAs laser which satisfies both low operating current and low noise characteristics by self‐sustained pulsation. The structure has a planar active layer and a GaAlAs optical confinement layer which give large saturable absorber, and small internal loss. As a result, as operating current as low as 19.7 mA at an output power of 3 mW is obtained under self‐sustained pulsation, leading to a relative intensity noise level less than −135 dB/Hz.

Quantitative study of nitridated sapphire surfaces by x-ray photoelectron spectroscopy

The characteristics of an amorphous oxynitride layer formed by nitridation are studied with x-ray photoelectron spectroscopy in order to reveal the role of nitridation before GaN growth in metalorganic chemical vapor deposition. The intensity variation of the N 1s peak with nitridation time is analyzed quantitatively using a diffusion-based model. The effective diffusion coefficient of nitrogen-related species in the amorphous layer is estimated to be 4×10−18 cm2/s at 1000 °C. The thickness of the amorphous layer is on the order of monolayers because of this small diffusion coefficient. The nitrogen concentration in the layer, however, is as high as the order of the site density of O atoms on the sapphire surface. These characteristics imply that the role of the amorphous layer formed by nitridation is to modify surface energy with high-density N atoms rather than to form a buffer-like layer.

High power subterahertz electromagnetic wave radiation from GaN photoconductive switch

The authors present apparently the observation of subterahertz electromagnetic wave emission from GaN based large aperture photoconductive switches (LA-PCSWs) excited by ultraviolet femtosecond laser pulses. The photoconductive layer of the GaN LA-PCSW is doped with carbon giving rise to a resistivity as high as 60MΩcm. The absolute energy of the emitted radiation pulses is measured to be 93.3pJ/pulse under 500V bias voltage. The Fourier spectrum of the measured pulse by the time-domain-spectroscopy shows the main components lie in 0.1–0.2THz regime.

Low operating current and high-temperature operation of 650-nm AlGaInP high-power laser diodes with real refractive index guided self-aligned structure

Low operating current and high-temperature operation has been demonstrated in 650-nm AlGaInP high-power laser diodes with real refractive index guided self-aligned (RISA) structure. The RISA structure features an Al/sub 0.5/In/sub 0.5/P current blocking layer, which leads to small internal loss in the waveguide and substantially reduced operating carrier density. The resultant operating current for 50-mW continuous-wave at 70/spl deg/C is as low as 98 mA, which is almost a half of the lowest value ever reported.

Inheritance of zinc-blende structure from 3CSiCSi(001) substrate in growth of GaN by MOCVD

GaN has been grown on 3CSiCSi(001) at temperatures from 600 to 800°C by low pressure MOCVD using TMGa and NH3. The grown films were characterized by X-ray diffraction signals from the zinc-blende GaN(002) and the wurtzite GaN(0002) planes. An X-ray diffraction signal detected at 36.8° indicated the wurtzite GaN(1011) plane. The growth of the wurtzite GaN was suppressed by increasing the growth temperature. The annealing of the thick polycrystalline GaN in NH3 ambient at 800°C was found to cause the atomic displacement and sublimation. Zinc-blende GaN with suppressed wurtzite domain was obtained on 3CSiCSi(001) at 800°C by inserting a thin polycrystalline GaN buffer layer deposited at 600°C.