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).

A new type of a dynamic-single-mode laser, which has distributed reflector (DR) both in active and passive regions, is proposed and analyzed to attain high output efficiency with the superior single mode property without increase of threshold current density.

We fabricated a p-i-n photodiode (PD) with an integrated microlens, and demonstrated its high performance capabilities including high speed (35 GHz), high responsivity (0.8 A/W), and large misalignment tolerance (26 µm), and an error-free 25-Gbit/s 10-km single-mode fiber transmission by using a 100-Gbit/s Ethernet quadplexer receiver module with the PDs.

This paper describes the use of direct wafer bonding technique to implement the novel concept of "free-material and free-orientation integration" which we propose. The technique is applied for various wafer combinations of an In-Ga-As-P material system with lattice- and orientation-mismatches. The properties of the bonded structures are studied in terms of the crystalline and electrical characterization. The high crystalline quality of the bonded structures with those mismatches is proved by transmission electron microscopy, and good electrical conduction was attained in some bonded structures of InP and GaAs. (001) InP-based 1.55-µm wavelength lasers are fabricated on (110) GaAs substrate by direct wafer bonding. The light-current characteristics of the lasers are almost identical to those of lasers fabricated on (001) InP and (001) GaAs substrates, while the turn-on voltage is a little bit higher due to the higher barrier height at the bonded interface. The practicability in those lasers are also examined. Furthermore, we show direct wafer bonding of a (001) InP-based structure and a (110) Si substrate with a GaAs buffer layer, aligning the cleavage planes of the InP and the Si. The results demonstrate the remarkable feasibility of using the direct wafer bonding technique to obtain integrated structures of material- and orientation-mismatched wafers with satisfactory quality.

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.

Waveguide characteristics of beam-expanders integrated with laser diodes were numerically analyzed by the beam propagation method (BPM) or the finite-difference time-domain (FD-TD) method. It was demonstrated that the vertically and horizontally hybrid tapered structure or an optimized refractive index in the cladding layer improve the trade-off relationship between fiber coupling efficiency and lasing characteristics. It was also demonstrated that exponentially tapering stripe width can reduce device length without sacrificing device properties.