Takuya Fujii

Compact high-power wavelength selectable lasers for WDM applications

Wavelength selectable lasers with record +1O dBm fiber coupled output power and uniform characteristics have been realized by integration of eight-channel distributed feedback laser arrays with low-loss multi-mode interference combiners and optical amplifiers with a length of only 2 mm.

A theory for metalorganic vapor phase epitaxial selective growth on planar patterned substrates

Abstract We developed a theory for selective area metalorganic vapor phase epitaxy on planar patterned substrates. The theory includes effects of vapor phase diffusion and surface migration. We found that vapor phase effects, such as vapor phase diffusion and readsorption of desorbed source materials, can be included in a two-dimensional surface equation. The surface equation revealed that three surface parameters, which are renormalized by the vapor phase effects, determine the growth rate distribution: diffusion length on the epilayer, diffusion length on the mask, and the lifetime ratio for desorbing and solidifying of the mask and the epilayer. When surface migration has a significant effect, these parameters are independent. If vapor phase diffusion dominates, however, the number of independent parameters is reduced from three to two. Our theory accurately predicted experimental results. Experiments indicated that surface migration has a significant effect.

Origin of compositional modulation of InGaAs in selective area metalorganic vapor phase epitaxy

We investigated the in‐plane compositional modulation of InGaAs epilayers in selective area metalorganic vapor phase epitaxy using a stagnant layer model. The growth pressure dependence of selective area growth of InAs and GaAs constituents using trimethylindium and triethylgallium revealed that the origin of the compositional modulation is in the mask region. A larger escaping probability of desorbed Ga source materials from the mask surface to the fluid layer and a slightly larger sticking coefficient defined at the mask surface of Ga source materials enrich the In composition of InGaAs epilayers near the mask edge. The escape probability is determined by the length ratio of the vapor phase mean‐free path of source materials to the stagnant layer thickness. The finite sticking coefficient at the mask surface corresponds to a chemical reaction which produces nonreactive species. With the larger escape probability of Ga source materials and the introduction of finite sticking coefficients at the mask surf...

Macroscopic mechanism of group V interdiffusion in undoped InGaAs/InP quantum wells grown by MOVPE

Abstract We studied group V interdiffusion in undoped InGaAs/InP quantum wells using photoluminescence measurements. After thermal annealing, the photoluminescence energy shifted to a higher energy than the as-grown sample in a manner inversely proportional to the quantum well width and proportional to the annealing time. Analyzing these results using microscopic diffusion theory, we concluded that group V species interdiffuse easily inside both the InP and the InGaAs layers, but interdiffuse with difficulty through InGaAs/InP hetero-interfaces. The interdiffusion rate was determined at the InGaAs/InP hetero-interfaces.

A novel method for designing chirp characteristics in electroabsorption MQW optical modulators

We propose a novel method to control the chirp parameter characteristics in electroabsorption (EA) multiquantum-well (MQW) optical modulators. We demonstrate that a large decrease of absorption on the short wavelength side enables us to achieve a negative chirp parameter that is independent of the extinction characteristics. This is achieved by enhancing the decrease in the absorption by higher states, which can be brought about by a shallow heavy-hole band discontinuity. It is also experimentally shown that a negative chirp operation from a near-zero-bias voltage in GaInAsP MQW optical modulators is realized by applying this new concept.

Growth pressure dependence of selective area metalorganic vapor phase epitaxy on planar patterned substrates

We investigated the growth mechanism of selective area metalorganic vapor phase epitaxy (MOVPE) on planar mask-patterned substrates by analyzing the growth pressure dependence of the in-plane growth rate distribution using an equation we reported previously [T. Fujii, M. Ekawa and S. Yamazaki, J. Crystal Growth 146 (1995) 475]. Experiments showed that the lateral diffusion constants defined at the epilayer and mask surfaces are inversely proportional to growth pressure. This dependence is evidence of negligible surface migration both on the epilayer and mask surfaces. The growth rate enhancement produced by mask patterning increased and saturated as growth pressure increased. Our theory predicts, however, that enhancement is proportional to growth pressure when the growing probability on the mask surface is exactly zero. This contradiction suggests that chemical reaction at the mask surface reduces the vapor phase concentration of group III source materials. This assumption successfully quantitatively predicted the experimental results.

Single-stripe tunable laser with Chirped Sampled Gratings fabricated by nanoimprint lithography

Fabrication of diffraction gratings of Chirped Sampled Grating Distributed Reflector (CSG-DR) laser by nanoimprint lithography (NIL) has been demonstrated. The diffraction gratings with highly uniform linewidth and period have been successfully fabricated by the combination of the reverse-tone NIL and precise etching techniques. The fabricated CSG-DR laser using NIL shows sufficiently wide tuning range of 40 nm as we designed.

Dependence of differential quantum efficiency on the confinement structure in InGaAs/InGaAsP strained-layer multiple quantum-well lasers

An internal efficiency of 91% was obtained with In/sub 0.7/Ga/sub 0.3/As/InGaAsP strained-layer multiple quantum well (MQW) lasers emitting at a wavelength of 1.5 mu m. The dependence of the reciprocal differential quantum efficiency on the length of the laser cavity shows that the absorption loss in the InGaAsP ( lambda =1.3 mu m) confinement layer caused by carrier overflowing into the confinement layer reduces the internal efficiency.<<ETX>>

Narrow-beam divergence 1.3-/spl mu/m multiple-quantum-well laser diodes with monolithically integrated tapered thickness waveguide

This paper describes the optimum design, fabrication, and performance of a 1.3-/spl mu/m multiple-quantum-well (MQW) laser diode monolithically integrated with a tapered thickness spot-size transformer. The dependence of the lasing characteristics on the thickness distribution of the core layer and on the current injection profile of the device were analyzed. This integrated laser with its optimized structure performed at a low threshold current of 22.2 mA, even at 85/spl deg/C. The integrated spot-size transformer effectively reduced the lateral and vertical far-field FWHMs to 8/spl deg/ and 9/spl deg/, respectively. A very long lifetime of over 1/spl times/10/sup 5/ h was estimated at 85/spl deg/C and 8 mW under CW operation.

High-temperature operation of InGaAs/InGaAsP compressive-strained QW lasers with low threshold currents

We investigated the influence of the band gap wavelength of barrier layers and separate confinement heterostructure (SCH) layers lambda /sub SCH/ on the high-temperature operation of InGaAs/InGaAsP compressive-strained quantum-well (QW) lasers. The optimum lambda /sub SCH/ was 1.2 mu m, at which carriers were sufficiently confined into quantum wells. The QW laser with lambda /sub SCH/ = 1.2 mu m exhibited low threshold currents of 2.3 mA at 20 degrees C and 9.7 mA at 100 degrees C and CW lasing up to 150 degrees C.<<ETX>>