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We fabricate a membrane InP-based electro-absorption modulator (EAM), in which an InGaAsP-based multiple-quantum-well (MQW) absorption region is buried with an InP layer, on Si-waveguide circuits. By optical coupling between the MQW absorption region and Si core, a low-loss and large-absorption-length (300-μm-long) supermode waveguide is designed to suppress electric-field screening at high optical input power. The EAM is fabricated by combining direct bonding of the MQW layer and regrowth of the InP layer on a thin InP template bonded on a silicon-on-insulator wafer. The fabricated membrane EAM shows an on-chip loss of less than 4 dB at wavelengths over 1590 nm and temperatures from 25 to 50 °C. Since the membrane lateral p-i-n diode structure is beneficial for reducing the RC time constant of a lumped-electrode InP-based EAM, the EO bandwidth of the EAM is around 50 GHz without a 50-ohm termination up to fiber-input power of 10 dBm. Using the device, we demonstrate clear eye openings for 56-Gbit/s NRZ and 112-Gbit/s PAM4 signals at temperatures from 25 to 50 °C.

A product-sum photonic integrated circuit consisting of SOI-based cascaded microring modulators and a 4×1 multimode interference coupler is proposed for CNN computing. A basic product-sum operation is numerically demonstrated for 2×2 matrix.

A 300-µm-long membrane InGaAsP electro-absorption modulator (EAM) integrated on Si-waveguide circuits exhibits a 3-dB bandwidth of 50 GHz. The EAM shows high linearity, low loss (< 4 dB), and eye openings for 112-Gbit/s PAM4 signals at wavelengths from 1570 to 1600 nm.

We fabricated the first compact 100-gigabit Ethernet (100GbE) transmitter optical sub-assembly (TOSA) using a directly modulated DFB laser (DML) array monolithically integrated with an optical multiplexer. Specially designed InGaAlAs/InGaAlAs multiple quantum wells make it possible to obtain a large gain in the 1295–1310-nm wavelength range and to operate at a high temperature of 55 °C. The four-channel DML array chip is only $2.0 \,\text{mm} \times 2.4 \,\text{mm}$ in size, and the fabricated TOSA is only $7.2 \,\text{mm} (W) \times 14.3 \,\text{mm} (L) \times 6.5 \,\text{mm} (H)$ , which fits a CFP2 or smaller 100-GbE transceiver. For all the lanes, the 3-dB bandwidths of the TOSA exceeded 17 GHz, and it could operate at 25.8 Gb/s. With 25.8-Gb/s × four-channel simultaneous operation, error-free transmissions over a 30-km single-mode fiber were demonstrated for all the lanes.

We have developed a compact hybrid-integrated four-lane $\times 25.8$ Gb/s transmitter optical sub-assembly (TOSA) for a 100 Gb/s transceiver for 40-km transmission. The TOSA has a simple configuration in which a four-channel electroabsorption modulator integrated distributed feedback (EADFB) laser array is directly attached to the input waveguide end-faces of a silica-based arrayed waveguide grating (AWG) multiplexer. We integrated a lateral tapered spot-size converter for the EADFB laser to achieve a high optical butt coupling efficiency between the EADFB lasers array and the AWG multiplexer. We confirmed that the TOSA, which is $19.9~\text {mm} \,\times \, 6~\text {mm} \,\times \, 5.8$ mm, achieved error-free operation for a 40-km transmission with a high average optical output power of over 0.3 dBm at an operating temperature of 55 °C.

A compact hybrid-integrated 100 Gb/s (4 lane × 25.78125 Gb/s) transmitter optical sub-assembly (TOSA) has been developed for a 100 Gb/s transceiver for 40-km transmission over a single-mode fiber. The TOSA has a simple configuration in which four electro-absorption modulator-integrated distributed feedback (EADFB) lasers are directly attached to the input waveguide end-face of a silica-based arrayed waveguide grating (AWG) multiplexer without bulk lenses. To achieve a high optical butt coupling efficiency between the EADFB lasers and the AWG multiplexer, we integrated a laterally tapered spot-size converter (SSC) for the EADFB laser and employed a waveguide with a high refractive index difference of 2.0% for the AWG multiplexer. By optimizing the laterally tapered SSC structure, we achieved a butt-coupling loss of less than 3 dB, which is an improvement of around 2 dB compared with a laser without an SSC structure. We also developed an ultracompact AWG multiplexer, which was 6.7 mm × 3.5 mm in size with an insertion loss of less than 1.9 dB. We achieved this by using a Mach–Zehnder interferometer-synchronized configuration to obtain a low loss and wide flat-top transmission filter spectra. The TOSA body size was 19.9 mm (L) × 6.0 mm (W) × 5.8 mm (H). Error-free operation was demonstrated for a 40-km transmission when all the lanes were driven simultaneously with a low EA modulator driving voltage of 1.5 V at an operating temperature of 55 °C.

We developed a flip-chip interconnection technique that is suitable for an EADFB laser array transmitter operating at more than 100 Gb/s (4 × 25 Gb/s). The flip-chip interconnection technique provides a good thermal flow comparable to that of the conventional wire interconnection technique but with a higher modulation bandwidth and lower electrical crosstalk. For a flip-chip interconnection 112-Gb/s (4 × 27.9 Gb/s) module, the OTU4 mask margin is only 4% worse for simultaneous four-channel operation than for single-channel operation. For a flip-chip interconnection 400-Gb/s (8 × 50 Gb/s) module, we obtained clear eye openings for all eight lanes with simultaneous eight-channel operation.

We have developed a novel electroabsorption modulator (EAM)-integrated distributed feedback laser (EA-DFB laser), in which an EAM functioning as an external modulator is integrated in a Mach–Zehnder interferometer (MZI). The MZI-type EAM (MZEA) modulates light intensity by both optical absorption and interferometric extinction. As a result, it improves the extinction ratio (ER) of a conventional EAM, which allows us to reduce the modulation voltage ( $V_{{\rm m}}$ ), and thus reduce the power consumption of the driver circuit. A fabricated MZEA-integrated DFB laser (MZEA-DFB laser) exhibited high-ER operation and an output power of about 8 dBm, which is comparable to that of a conventional EA-DFB laser, with a 100-mA laser current. Thanks to the high ER and sufficient output power, the MZEA-DFB laser operating at half the $V_{{\rm m}}$ of a conventional EA-DFB laser is able to generate optical signals with a given optical modulation amplitude, and we achieved a 25.8-Gb/s 40-km transmission with the novel laser at a $V_{{\rm m}}$ of $1 {\rm V_{pp}}$ .

We fabricated a 400-G flip-chip interconnection eight-channel EADFB laser array module. The flip-chip interconnection technique provides a higher modulation bandwidth and lower electrical crosstalk than the wire interconnection technique. The 3-dB bandwidth of the flip-chip interconnection subassembly was about 50 GHz. And the electrical crosstalk of the flip-chip interconnection module was suppressed to less than −20 dB below 35 GHz. We obtained clear eye openings for all eight lanes under eight-channel 50-Gb/s simultaneous operation after a 10-km SMF transmission.

A transmitter optical subassembly (TOSA) has been developed for a 100-Gbit/s Ethernet system for a long optical transmission distance over a 40-km single-mode fiber (SMF). To obtain a high optical output power and a high dynamic extinction ratio, the TOSA consists of two two-channel distributed feedback laser diode array chips integrated with electro-absorption modulators (EADFB laser array chips) with a $2 \times 1$ multimode interference (MMI) optical coupler and a polarization beam combiner for wavelength multiplexing. The TOSA is $8.7\; {\rm mm} \times 28.7\; {\rm mm} \times 6.5 \;{\rm mm}$ including an LC-type receptacle. The TOSA has a twin collimator path for two EADFB laser array chips. The wavelength multiplexer consists of a mirror, a half-wavelength plate, and a polarization beam combiner. The TOSA could improve the optical output power by around 2 dB compared with a TOSA with a four-channel EADFB laser array chip and a $4 \times 1$ MMI optical coupler. An optical modulation amplitude of over 0.5 dBm and a mask margin exceeding 21% were obtained for each lane when all lanes were operated simultaneously with an EA modulator driving voltage of 1.5 V at a bit rate of 25.78125 Gbit/s, which fully satisfies the 100GBASE-ER4 specifications. We demonstrated error-free operation for an SMF transmission of over 40 km. Moreover, we confirmed that the TOSA was capable of error-free SMF transmission over 60 km with a power penalty of less than 1.4 dB.

The first compact and high-performance optical transmitter using flip-chip interconnects has been developed for a 112-Gbit/s transceiver. The flip-chip interconnects provide lower crosstalk and a higher modulation bandwidth than wire interconnects. The flip-chip interconnection module has 3-dB frequency bandwidths of over 30 GHz. This value is up to 5 GHz better than that of a wire interconnection module. Under four-channel simultaneous operation, the OTU4 mask margins for all four lanes of the flip-chip interconnection module exceed 37%. These values are degraded up to only 4% compared with those for single-channel operation. In addition, an error-free transmission through 10 km of single-mode fiber at 112 Gbit/s is demonstrated.

A 4 × 25-Gb/s electroabsorption modulator integrated with a distributed feedback (DFB) laser (EADFB laser) array monolithically integrated with a low-loss optical multiplexer (MUX) is developed for 100 GbE transmitters for the first time. A cascaded Mach-Zehnder (MZ) filter, which has no intrinsic loss, is used as the MUX. The design and fabrication of the MZ-MUX for 100 GbE are thoroughly described. The measured loss of fabricated MZ-MUX is 2-3 dB, which is improved by 4 dB compared with that of the conventional 1 × 4 multimode interference MUX. The MZ-MUX is monolithically integrated with a four-EADFB-laser array and the output power of the chip is increased by 3 dB compared with that of a conventional 4 × 25-Gb/s monolithically integrated chip. With the folded layout, the size of the chip is similar to that of the conventional chip. The fabricated chip is packaged into a very compact transmitter optical subassembly (TOSA). The system requirements are satisfied with semi-cooled operation, and very clear eye openings are obtained after 10-km single-mode-fiber transmission. The adjustment of filter spectra by current injection is also shown, and fine tuning is possible with very low injection current.

We have achieved the 100-Gb/s/ $\lambda $ operation of a flip-chip interconnection 1.3- $\mu \text{m}$ lumped-electrode electroabsorption modulator integrated with a distributed feedback laser module for the first time. The flip-chip interconnection provides a flatter frequency response characteristic and a higher modulation bandwidth. Clear eye opening was achieved for 103-Gb/s nonreturn to zero and equalizer-free 56-GBd 4-pulse-amplitude modulation operation after a 10-km single-mode fiber transmission.

The first ultra-compact transmitter optical sub-assembly (TOSA) has been developed for the metro-area 100 Gbit/s Ethernet (100 GbE) system. Four DFB lasers integrated with electro-absorption modulators (EADFB lasers) operated at 25.8 Gbit/s and an optical multiplexer are monolithically integrated on one chip. The chip was assembled in the compact TOSA using a three-dimensional (3D) RF circuit board supported by a spacer. The new structure provides compact and high-density interconnection with low crosstalk between RF signal lines. The TOSA is very small at just 8.0 mm × 35 mm × 6.5 mm including the receptacle and the flexible printed circuit (FPC) and could be installed in next-generation small form 100 GbE transceivers. In addition, an error-free transmission is demonstrated through 40 km of single-mode fiber at 100 Gbit/s with a modulation voltage of 2 V.

Using a lumped-electrode EADFB laser with a modulation bandwidth of ∼59 GHz, we demonstrated single-wavelength single-polarization direct-detection 4-PAM transmission with the record net data rate of 200 Gbit/s.

We fabricated a lumped-electrode type EADFB laser module using a flip-chip interconnection technique which provides a large modulation bandwidth. We obtained clear 56-Gbaud 4-PAM signal after 10-km SMF transmission with no equalizer using this module.

A compact electroabsorption modulators integrated distributed feedback (EADFB) laser array module has been developed for 100-Gb/s Ethernet. Four 25-Gb/s EADFB lasers and an optical multiplexer are monolithically integrated on one chip in an area of only 2 mm × 2.6 mm. As a result, a compact 12 mm × 20 mm EADFB laser array module is achieved. A bridge-type RF circuit board is employed to transmit high-frequency, 4 × 25-Gb/s, electrical signals effectively in a narrow package and improve the electrical/optical response of the module. The module has a 3-dB frequency bandwidth of 20 GHz for the four lanes assigned in 100-Gb/s Ethernet. Furthermore, the possibility of realizing low-driving voltage operation is investigated to reduce the power consumption of the module. An error-free transmission through 10 km of single-mode fiber at 100 Gb/s with a modulation voltage of only 1 V is demonstrated.

High-performance 1.3-µm electroabsorption modulators integrated with DFB lasers are developed for long-reach 100Gbit Ethernet. The dynamic extinction ratio of over 8-dB with the voltage swing of 2V are achieved for the four LAN-WDM lanes (14nm wavelength range) used in 100Gbit Ethernet with the same modulator structure. The fabricated devices are packaged in butterfly modules and four-lane 40-km single mode fiber transmission at 25-Gbit/s operation is demonstrated. Further, a can-type transmitter optical subassembly is fabricated to reduce the cost and size of transmitter modules. The use of a low-dielectric-constant liquid crystal polymer transmission line makes the 3-dB bandwidth larger and enables 25-Gbit/s operation with CAN-TOSA module.

A compact transmitter optical sub-assembly (TOSA) has been developed for a 100 Gb/s Ethernet system with a long transmission distance of 40 km over single-mode fiber. The TOSA consists of electro-absorption modulator integrated distributed feedback laser diode arrays and a polarization beam combiner for wavelength multiplexing to obtain a high optical output power and a high dynamic extinction ratio. We confirmed that the TOSA successfully achieved an error-free four-lane ×25.8 Gb/s transmission, i.e., a total bit-rate of 100 Gb/s, over 40 km.

We developed a compact hybrid-integrated 100 Gbit/s TOSA using an EADFB laser array with a spot-size converter and a silica-based AWG multiplexer. Error-free operation for a 40-km transmission was demonstrated at an operating temperature of 55 °C.

We present a high-linearity avalanche photodiode (APD) for 28-Gbaud PAM4, namely. 56 Gbit/s/λ,. With an optical receiver incorporating the APD, a 40-km transmission with −17 dBm receiver sensitivity was successfully demonstrated for the first time.

We developed a new type of EAM-integrated DFB laser that employs both optical absorption and interferometric extinction. And, we achieve a 25.8-Gbit/s 40Gbit/s 40-km transmission with the km novel laser at a modulation voltage of 1 Vpp.

A multimode waveguide p-i-n photodiode (WGPD) and a distributed baseband amplifier consisting of high-electron mobility transistors (HEMTs) were monolithically integrated on InP substrate using a stacked layer structure for both components. The multimode WGPD has a 3-dB bandwidth of 49 GHz. The distributed baseband amplifier has a 3-dB bandwidth of 47 GHz, though its 0.5-/spl mu/m gate-length HEMTs have modest cutoff frequencies f/sub T//f/sub max/ of 47/100 GHz. The receiver optoelectronic integrated circuit has a bandwidth of 46.5 GHz. It was packaged into a fiber-pig-tailed module, and the WGPD in the module has a high responsivity of 0.62 A/W for 1.55-/spl mu/m wavelength. The module achieves a sensitivity of -22.7 dBm at 40 Gb/s and exhibits a clear eye-opening at 50 Gb/s.

We developed the first compact 100GbE optical transmitter to use flip-chip interconnects for the first time. The flip-chip interconnects provide low crosstalk and a high modulation bandwidth. Under four-channel simultaneous operation, the 100GBASE-LR4 mask margin of the flip-chip interconnection module was improved by 10% to 27% compared with that of a wire interconnection module.

The first ultracompact transmitter optical subassembly for long-distance, beyond-100-Gbit/s data communication systems based on a monolithically integrated light source is developed. The light source consists of four InGaAlAs-based electroabsorption modulator integrated DFB lasers (EADFB lasers) and their optical multiplexer and these components are monolithically integrated on one chip, resulting in very small chip size of 2.4 × 3.3 mm2. This small chip makes it possible to reduce the size of the transmitter module. Very small transmitter optical subassemblies (TOSAs), using the chip and employing three-dimensional interconnection board, is fabricated, whose volume is only 1.82 cc. By using the TOSA, 40-Gbit/s × 4 operation with the clear eye openings up to 10-km single mode fibre transmission is demonstrated. Electrical crosstalk under the multi-lane operation is also evaluated, and the error-free transmission is achieved with the power penalty to the discrete operation of 1 dB.

A 4×25-Gbit/s EADFB laser array chip monolithically integrated with a cascaded Mach-Zhender multiplexer is developed for CFP4-class 100GbE transmitters for the first time. Our compact TOSA containing this chip satisfies system requirements with semi-cooled operation.

We developed a 100GbE transmitter optical sub-assembly using electro-absorption modulator integrated distributed feedback laser diode arrays and a polarization beam combiner for wavelength multiplexing. It provides sufficient optical modulation amplitude for 100GbE of over 10 km.

This paper proposes a novel model for estimating power dissipation of optical/electrical interconnections as a function of transmission bit error rate. This model is applied to a simplified optoelectronic transmitter and receiver configuration in which a photodetector is directly connected to the decision circuit. It is analytically verified that this configuration can achieve error-free operation with low power under practical operating conditions. A comparison between optical and electrical interconnections based on this simplified configuration is performed. This result shows the interconnection length and bit rate at which optical interconnection is superior in terms of power dissipation to electrical interconnection, Only optical interconnections achieve error-free operation with 40 mW power dissipation at a transmission bit rate of 10 Gb/s and an interconnection length over 7 m.

An extremely compact 1.3-μm, 4×25-Gbit/s TOSA was made by using monolithically integrated EADFB laser array with rear-grating DFB lasers. The dynamic extinction ratio and optical modulation amplitude are high enough for 10-km 100GbE.

The first ultracompact transmitter optical subassembly for long-distance, beyond-100-Gbit/s data communication systems based on a monolithically integrated light source is developed, and 40-Gbit/s× 4 operation with the clear eye openings up to 10-km single mode fibre transmission is demonstrated.

A 1.3-µm, 4×25G, EADFB laser array with a novel rear grating DFB laser was developed and packaged in a compact module. It provides enough dynamic extinction ratio and averaged optical power for 10-km 100GbE.

We have developed 1.3-µm, 50-Gbit/s EADFB lasers for future 400Gbit Ethernet. 10-and 40-km SMF transmission with clear eye openings over 14-nm wavelength range under 50-Gbit/s operation are demonstrated for the first time.

A large-output-power 1.3-μm, 4×25G, EADFB laser array with a novel rear grating DFB laser is developed for 100GbE. Ultralow-driving-voltage operation (0.5 V pp ) is achieved for the first time with clear eye openings after 10-km transmission.

Ion Bernstein wave (IBW) heating at ion cyclotron half-harmonic frequencies was performed in hydrogen (H) discharges in the WT-3 tokamak. The bulk ion temperature increase was measured by a charge exchange analyser and a spatially scanning polychrometer. The latter diagnostics can provide radial ion-temperature profiles. The incremental ion temperature has shown a peak at the exact location where the IBW frequency omega equals 3/2 of the ion cyclotron frequency, indicating the nonlinear direct acceleration of H ions. The wave electric field was measured by a Langmuir probe in the scrape-off layer. The electric field at the second harmonic of the excited IBW frequency was observed to vary depending on the power level and the toroidal magnetic field. These experimental results are compared with the theoretical prediction of nonlinear IBW damping. The measured dependences of the ion heating and the wave amplitude at 2 omega on the toroidal magnetic field are in reasonable agreement with the prediction.

We developed a 4×25.8-Gbit/s EADFB laser array module for use as a 100-Gbit/s Ethernet transceiver. We demonstrated error-free transmission over a 10-km single-mode fiber at 100-Gbit/s with a modulation voltage of 1 V.