Takeshi Akagawa

High-speed 1.1-μm-range InGaAs VCSELs

We have developed 1.1-μm-range high-speed VCSELs based on InGaAs-QWs for high-speed and highly reliable optical interconnections. 3-dB bandwidth up to 20 GHz and error-free 30-Gbps 100 m operations were demonstrated with oxide confined VCSELs. We also developed buried tunnel junction VCSELs to further improve the relaxation oscillation frequency limit. 3-dB bandwidth up to 24 GHz and error-free 40-Gbps operations were demonstrated with these VCSELs.

Compact PIN-Diode-Based Silicon Modulator Using Side-Wall-Grating Waveguide

We developed PIN-diode-based silicon Mach-Zehnder modulators, which have side-wall-gratings in the phase-shifter sections. Such passive waveguides with gratings were fabricated using ArF immersion lithography, which showed a small scattering loss of 0.4 dB/mm. We extensively investigated the forward-biased operation of the modulators by using equivalent circuit analysis and the measurement of the fabricated devices. We argue carrier recombination time only plays a minor role for the overall performance of the modulator. Dependences of the modulation efficiency on other various critical parameters are discussed. In particular, if we use relatively short phase shifter, the forward-biased operation provides smaller VπL than reversed one even at high frequency of 20 GHz, at the expense of the narrow bandwidth. Our approach enables high-speed operation up to 50 Gb/s, by using phase shifter as short as 250 μm and preemphasis signals. For 12.5-Gb/s operation, the modulator cell size was only 300 μm × 50 μm, which was suitable for the applications of high-density optical interconnects.

Highly Reliable High-Speed 1.1-

In this paper, we describe high-speed 1.1-μm-range oxide-confined vertical-cavity surface-emitting lasers (VCSELs) for large-scale optical interconnection applications. For achieving high data rates up to 25 Gbit/s under high temperature, we applied InGaAs/GaAsP strain-compensated multiple quantum wells (SC-MQWs) as the active layer. The developed device showed 25 Gbit/s error-free operation at 100°C. We also examined reliability of the VCSELs through accelerated life tests. The result showed an extremely long lifetime of about 10 thousand hours in MTTF under an ambient temperature of 150°C and a current density of about 19 kA/cm2. The level of reliability either equaled or surpassed that of conventional 0.85-μm VCSELs. Moreover, we revealed a typical failure mode of the device, which was caused by ≪110≫ dark line defects (DLDs) generated in the n-DBR layers under the current aperture area.

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We developed VCSELs based on InGaAs-QWs for high-speed optical interconnections. Error-free operations were achieved up to 25 Gbps. We also proposed VCSELs with type-II tunnel junctions to reduce electrical resistance for higher speed operation

m-Range VCSELs With InGaAs/GaAsP-MQWs

We developed a high-speed Mach-Zehnder modulator with the shortest phase shifter (100-μm length) reported so far. Our modulator exhibited 25-Gbps eye-openings with an extinction ratio of 4.3 dB and on-chip insertion loss of 4.7 dB.

High-speed InGaAs VCSELs

We review our recent progress in novel planar blue-violet laser diodes (BV-LDs). The planar BV-LDs are characterized by an inner-stripe waveguide formed with a buried AlN current-blocking layer and a wide regrown cladding layer that also acts as a current and heat spreader. These features enable high-power operation for BV-LDs thanks to their low electrical and low thermal resistance even with a narrow-stripe waveguide. In this paper, we report successful demonstration of the planar inner-stripe BV-LDs by utilizing low-temperature-grown AlN and the regrown cladding layer. Low electrical resistance of the regrown cladding layer was confirmed by scanning spread resistance microscopy. Heat spreading characteristics were also investigated by 2-dimensional thermal simulation. The fabricated BV-LDs with a 1.4-&mgr;m-wide stripe achieved a low threshold current of 32 mA, a low threshold voltage of 4.1 V and greater than 200- mW kink-free output power under CW operation. Moreover, the kink-free output level surpassed 1,000 mW for the 1.0- &mgr;m stripe BV-LDs under 0.03%-duty-pulsed operation. The BV-LDs operated stably for more than 1,000 hours at a high output power of 200 mW at 80oC under a 50%-duty-pulsed condition. After the reliability test, transmission electron microscopy revealed no defect near the regrown interface of the tested LDs.

25-Gbps operation of silicon p-i-n Mach-Zehnder optical modulator with 100-μm-long phase shifter

We demonstrated a 25-Gbps error-free data link on a silicon optical interposer. The experimental results verified that our interposer is compatible with high-performance FPGAs, and transmitter pre-emphasis and receiver equalization reduced bit error rate of the optical data link.

High-power operation of inner-stripe GaN-based blue-violet laser diodes

We developed 1.1-mum-range oxide-implant VCSELs with InGaAs/GaAsP strain-compensated MQWs. 25 Gbit/s-100degC error-free operation and high reliability over 3000 hours under 150degC operation were successfully demonstrated.

Demonstration of 25-Gbps optical data links on silicon optical interposer using FPGA transceiver

In this paper, we describe high temperature operation of high speed 1.1μm-range oxide-confined vertical-cavity surfaceemitting lasers (VCSELs) for optical interconnection applications. For achieving high speed of over 25 Gbit/s under a high temperature, we applied InGaAs/GaAsP strain-compensated multiple quantum wells (SC-MQWs) as the active layer. The developed device showed 25 Gbit/s error-free operation at 100°C. We also examined reliability of the VCSELs via accelerated life tests. The result showed extremely long MTTF lifetime of about 10 thousand hours under an ambient temperature of 150°C and a bias current of about 19 kA/cm2, a reliability that either equals or surpasses that of conventional 850-nm VCSELs with 10 Gbit/s. Moreover, we revealed a typical failure mode of the device; the result of analysis indicated that the failure was caused by <110> dark line defects (DLDs) generated in the n-DBR layers under the current aperture area.

25 Gbit/s-100 °C operation and high reliability of 1.1-μm-range VCSELs with InGaAs/GaAsP strain-compensated MQWs

We propose a highly wearable, upper-arm type, oscillometric-based blood pressure monitoring technology with low-stress. The low-stress is realized by new developments in the hardware and software design. In the hardware design, conventional armband; cuff, is almost halved in volume thanks to a flexible plastic core and a liquid bag which enhances the fitness and pressure uniformity over the arm. Reduced air bag volume enables smaller motor pump size and battery leading to a thinner, more compact and more wearable unified device. In the software design, a new prediction algorithm enabled to apply less stress (and less pain) on arm of the patient. Proof-of-concept experiments on volunteers show a high accuracy on both technologies. This paper mainly introduces hardware developments. The system is promising for less-painful and less-stressful 24-hour blood pressure monitoring in hypertension managements and related healthcare solutions.