A dual-core spot size converter (DC-SSC) is integrated with a lateral grating assisted lateral co-directional coupler (LGLC) tunable laser by using no additional complicated fabrication processes. The excess loss due to the DC-SSC is only 0.5 dB, and narrow full width half maximums (FWHMs) of vertical and horizontal far-field patterns (FFPs) produced by the laser are about 25° and 20°. This integration causes no degradations of the performance of the LGLC laser; in other words, it maintains good lasing characteristics, namely, wide tuning range of over 68 nm and SMSR of over 35 dB in the C-band under a 50 semi-cooled condition.

We have developed a high-speed electroabsorption modulator integrated distributed feedback (EA/DFB) lasers. Transmission performance over 10 km was investigated under 25 Gbps and 43 Gbps modulation. In addition, the feasibility of wide temperature range operation was also investigated. An uncooled EA/DFB laser can contribute to the realization of low-power-consumption, small-footprint and cost-effective transceiver module. In this study, we used the temperature-tolerant InGaAlAs materials in an EA modulator. A wide temperature ranged 12 km transmission with over 9.6 dB dynamic extinction ratio was demonstrated under 25 Gbps modulation. A 43 Gbps 10 km transmission was also demonstrated. The laser achieved a clear, opened eye diagram with a dynamic extinction ratio over 7 dB from 25 to 85. The modulated output power was more than +2.9 dBm even at 85. These devices are suitable for next-generation, high-speed network systems, such as 40 Gbps and 100 Gbps Ethernet.

Our recent progress in improving the performance of the GaInNAs laser is fully reviewed here. We improved the crystal quality of GaInNAs by optimizing the conditions for its grown by gas-source molecular beam epitaxy (MBE) using N radicals as a N source. We found that the temperature window for obtaining GaInNAs with high crystal quality, good surface morphology, and good photoluminescence (PL) characteristics is smaller than that for obtaining this kind of GaInAs. Like dopant atoms such as Si or Be in GaAs, the N radicals produced by an RF discharge have a high sticking coefficient. Their use is therefore effective when we want to increase and control the N content of GaInNAs. We found that the AsH3-flow-rate mainly affected crystal quality of GaInNAs rather than incorporation of nitrogen atoms. We also investigated the effects of thermal annealing on the optical properties of as-grown GaInNAs layers and found that it greatly increased the PL intensity and produced the large shift in the PL wavelength. The absorption spectra of the GaInNAs bulk layer revealed that the large shift in the PL wavelength is probably caused by a bandgap shift in the GaInNAs well layer, and cathodeluminescence measurements revealed that the increased PL intensity is due to the improved emission being more uniform spatially: uniformity from the entire region; in comparison, nonuniform dot-like regions exist in an as-grown GaInNAs layer. Optimizing the growth conditions and using thermal annealing effect, we made a 1.3-µm GaInNAs/GaAs single-quantum-well laser that has a high characteristic temperature (215 K) under pulsed operation. To our knowledge, this is the highest characteristic temperature reported for a 1.3-µm band-edge emitter suitable for used in optical-fiber communication systems. The use of GaInNAs as an active layer is, therefore, very promising for the fabrication of long-wavelength laser diodes with excellent high-temperature performance.

This paper describes 40-Gbit/s operation of 1.55-µm electro-absorption (EA) modulators applicable to compact and low-cost transmitters for very-short-reach (VSR) applications. We start by identifying factors that make a multi-quantum-well (MQW) design suitable for high levels of output power and for uncooled operation. From the basic experimental results, we determine that a valence-band discontinuity ΔEv at around 80 meV is optimal in terms of combining high-output-power operation and a good extinction ratio. We then apply the above findings in an InGaAsP-MQW EA modulator that is monolithically integrated with a distributed feedback (DFB) laser, and thus obtain operation with high output power (+1.2 dBm), a high ER (10.5 dB), and a low power penalty (0.4 dB after transmission over 2.6 km of single-mode-fiber). These results confirm the applicability of our EA modulator/DFB laser to VSR applications. After that, we theoretically demonstrate the superiority in terms of ER characteristics of the InGaAlAs-MQW over the conventional InGaAsP-MQW. InGaAlAs-MQW EA modulators are fabricated and demonstrate, for the first time, 40-Gbit/s operation over a wide temperature range (0 to 85).