Tohru Takiguchi

Selective-area MOCVD growth for 1.3 μm laser diodes with a monolithically integrated waveguide lens

A tapered thickness profile and a high thickness enhancement ratio of 3.5 in the waveguide which are necessary for a narrow beam have been realized by selective-area metalorganic chemical vapor deposition (MOCVD) growth using a tapered shape twin mask. A multiple quantum well (MQW) active layer with high strain of 0.9% which is effective for a low threshold current has also been successfully grown by the control of the compositional modulation of InGaAsP in the selective-area growth. Using these techniques, a narrow beam and a low threshold current have been realized for a 1.3 μm laser diode monolithically integrated with a tapered thickness waveguide lens.

Improvement of crystal quality and laser characteristics by zero net strain structure

Zero net strain structure is very useful for strained multiple quantum well (MQW) lasers for which many numbers of quantum wells are required. We demonstrate the effectiveness of the zero net strain structure to prevent the degradation of the crystal quality up to 15 wells, which cannot be realized by the conventionally strained MQW (well: 1.5% compressive strain, 30 A thick). We also show that the zero net strain structure has finite critical thickness and dislocations other than 60° misfit dislocations are introduced into the zero net strain structure beyond the critical thickness. Narrower distribution of threshold current than that of the conventionally strained lasers, high relaxation oscillation frequency of 5.5 GHz/√mW and high differential gain of 6.3 × 10 −16 cm 2 have been realized for the 1.55 μm laser with zero net strain MQW with 15 wells by the suppression of degradation of crystal quality

40G RZ-DQPSK transmitter monolithically integrated with tunable DFB laser array and Mach-Zehnder modulators

A 40G RZ-DQPSK transmitter monolithically integrated with a tunable DFB laser array and Mach-Zehnder modulators was developed for the first time. The optical waveform of 40Gb/s DQPSK was successfully demonstrated.

High quality p-Cd0.22Hg0.78Te grown by liquid-phase epitaxy

Abstract We have systematically studied the growth conditions of Cd 0.22 Hg 0.78 Te liquid-phase epitaxy using an open-tube horizontal slider apparatus. Optimizing each parameter, we have reproducibly obtained high-quality p-Cd 0.22 Hg 0.78 Te epitaxial layers having an area of 15 × 20 mm 2 with the following characteristics: surface roughness of ± 0.5 μm, compositional uniformity of epitaxial layer of 0.220 ± 0.001, carrier concentration of 1 × 10 16 cm -3 at 77 K, and Hall mobility of 650 cm 2 /V·s at 77 K. These characteristics are suitable for production of high-performance integrated detector arrays.

Uncooled directly modulated 1.3-μm AlGaInAs-MQW DFB laser diodes

We have proved that short cavity length and large coupling coefficient structure can increase the relaxation oscillation frequency of directly modulated 1.3mm DFB-LDs for 10Gb/s operation. The grating with large coupling coefficient can be made of high refractive index InGaAsP material (=narrow band gap energy material), which has to be fabricated in the n-type InP cladding layer to prevent large hole and electron accumulation in the grating layer. By using this grating, 1.3mm AlGaInAs quarter lambda phase shifted DFB-LDs successfully revealed excellent eye diagrams with over 10% margin for OC-192 mask at the temperature range of 25-95 deg.C. Furthermore, we newly propose multi-phase shifted gating to 10Gb/s quarter lambda phase-shifted DFB-LDs to suppress the deterioration of eye diagrams due to non-uniform longitudinal distribution of light intensity and carrier density.

MOCVD growth of heavily p-type doped InGaAs using bismethylcyclopentadienyl-beryllium

Beryllium (Be) has been investigated as p-type dopant for InGaAs layers grown using bismethylcyclopentadienyl-beryllium ((MeCp)/sub 2/Be) by low-pressure metalorganic chemical vapor deposition. It is found that hole concentration is in proportion to the (MeCp)/sub 2/Be flow rate, which results in excellent doping controllability. Moreover, the doping efficiency of (MeCp)/sub 2/Be is one order of magnitude higher than that of diethylzinc (DEZn). InGaAs increases with increasing growth temperature in the temperature range from 570 to 625/spl deg/C. Heavily Be-doped InGaAs layers with specular surface morphology have been attained with a hole concentration in excess of 10/sup 19/ cm/sup -3/ by optimization of growth condition. (MeCp)/sub 2/Be is promising precursor as p-type dopant source for optoelectronic devices.

Uncooled DFB lasers for CATV

Strained-layer MQW-DFB lasers at a wavelength of 1.3 micrometers operating from -40 degree(s)C to 85 degree(s)C without any coolers are demonstrated. On the basis of the leakage current analysis, a laser structure including the active layer and the current blocking layer is chosen to achieve low distortion over wide-temperature-range. Extremely low threshold current of 17 mA at 85 degree(s)C and operation current of 37 mA at 5 mW and 85 degree(s)C are obtained. The lasers realize low distortion of less than -50 dBc at 65 degree(s)C in 78-channel CSO measurements. Furthermore, a fabricated laser module with a coaxial package also achieved low CSO values of -55 dBc under 16-channel loading in the temperature range from -40 degree(s)C to 85 degree(s)C. This uncooled DFB laser module is very useful for return path application in CATV systems.