Kiyoto Takahata

Characteristics of InAlAs/InGaAs high-electron-mobility transistors under illumination with modulated light

The optical response of InAlAs/InGaAs HEMTs under illumination with modulated light from a 1.3-/spl mu/m semiconductor laser diode onto the backside of the substrate is measured by using an optical-signal analyzer. It is clear that the response is composed of two signals. One signal is dominant at a low frequency and is due to the photovoltaic effect that causes excess holes photogenerated in the InGaAs channel to accumulate in the source region. This accumulation thus causes a decrease in the threshold voltage of the HEMTs. To explain this mechanism, a theory is given which connects the change in threshold voltage with that in the Fermi energy of the two-dimensional electron gas (2-DEG). The other signal is dominant at a high-frequency and is due to the photoconductive effect in the InGaAs channel beneath the gate. In this case, a large optical gain is produced since electrons at the source region are replenished in the gate channel. This leads to the first clear observation of a photoconductive signal. The bandwidth due to the photovoltaic effect is as low as 45 MHz and is dominated by the lifetime of the excess holes. The bandwidth due to the photoconductive effect is as high as 37 GHz and is dominated by the gain-bandwidth product of transistors rather than the intrinsic transit-time of electrons.

1.3-μm, 4 × 25-Gbit/s, EADFB laser array module with large-output-power and low-driving-voltage for energy-efficient 100GbE transmitter

A 1.3-μm, 4 × 25-Gbit/s, EADFB laser array module with large output power and low driving voltage is developed for 100GbE. A novel rear grating DFB laser is introduced to increase the output power of the laser while keeping the single mode lasing, which is desirable for a monolithic integration. Also, InGaAlAs-based electroabsorption modulators make very-low-driving-voltage operation possible due to their steep extinction curves. With the module, very clear 25-Gbit/s eye openings are obtained for four wavelengths with the driving voltage of only 0.5 V while securing the dynamic extinction ratio required by the system. These results indicate that the presented module is a promising candidate for energy-efficient future 100GbE transmitter.

A Compact EADFB Laser Array Module for a Future 100-Gb/s Ethernet Transceiver

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.

Ultra-Compact 100 GbE Transmitter Optical Sub-Assembly for 40-km SMF 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.

46.5-GHz-bandwidth monolithic receiver OEIC consisting of a waveguide p-i-n photodiode and a HEMT distributed amplifier

A large bandwidth monolithically integrated photoreceiver for 1.5-/spl mu/m wavelength operation was fabricated using a stacked structure of waveguide p-i-n photodiode layers and InAlAs-InGaAs high-electron mobility transistor (HEMT) layers grown using a single-step metal-organic vapor-phase epitaxy. The monolithic receiver optoelectronic integrated circuit (OEIC) consists of a waveguide p-i-n photodiode with a high responsivity of 0.5 A/W and a HEMT distributed amplifier. It has a bandwidth of 46.5 GHz, which is the largest yet reported for a long-wavelength receiver OEIC, and exhibits a clear eye opening at 40 Gb/s. This excellent performance is very attractive for use in high-speed optical transmission systems and millimeter-wave fiber-radio systems.

Characteristics of InAlAs/InGaAs high electron mobility transistors under 1.3-μm laser illumination

The current-voltage (I-V) characteristics of InAlAs/InGaAs high electron mobility transistors (HEMTs) under illumination are investigated. The change of the drain current caused by the illumination can be explained by using the photovoltaic effect so that the excess holes photo-generated in the InGaAs channel layer accumulate at the source-electrode region and cause an effective decrease in the potential barrier for electrons between the source and the channel. The basic equations describing this phenomenon are derived on the basis of the experimental results. In addition, our experimental results are shown to support the barrier-induced hole pile-up model in which holes generated by the impact ionization accumulate in the InAlAs barrier on the source side and cause the kink effect In InAlAs/InGaAs HEMTs.

10-Gb/s two-channel monolithic photoreceiver array using waveguide p-i-n PDs and HEMTs

Two-channel side-illuminated receiver OEIC arrays comprising waveguide p-i-n photodiodes and InAlAs-InGaAs-HEMT transimpedance amplifiers were fabricated with good uniformity on a 2-in wafer. The performances of the two channels were measured by on-wafer probe measurements and showed well-matched 3-dB bandwidth of 8.0 GHz, transimpedance of 155 /spl Omega/, and crosstalk less than -20 dB. The OEIC array operates at 10 Gbit/s with sensitivities of -66.1 dBm and -15.3 dBm for each channel for a bit error rate of 10/sup -9/ at a wavelength of 1.55 /spl mu/m. This is the highest bit rate yet reported for a long-wavelength monolithic photoreceiver array.

Ultrafast monolithic receiver OEIC composed of multimode waveguide p-i-n photodiode and HEMT distributed amplifier

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.

Flip-Chip Interconnection Lumped-Electrode EADFB Laser for 100-Gb/s/

We have achieved the 100-Gb/s/λ operation of a flip-chip interconnection 1.3-μm 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-pulseamplitude modulation operation after a 10-km single-mode fiber transmission.

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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.