Toshitaka Aoyagi

Extremely small-chirp electroabsorption-modulator integrated distributed feedback laser diode with a shallow quantum-well absorption layer

We have developed an extremely small-chirp electroabsorption modulator integrated with a distributed feedback laser diode (EAM-DFB-LD) with a novel shallow quantum-well (QW) absorption layer. By using a shallow QW absorption layer, the estimated lifetime of the photogenerated holes and thus the estimated concentration of the holes generated by optical absorption has been reduced to 9% of that for a conventional QW. It was experimentally confirmed that the excess chirp due to the pileup of carriers has been reduced to 10% of that for a conventional QW, and this result is consistent with the estimates. The shallow QW EAM-DFB-LD has shown a chirp parameter |/spl alpha/| less than 0.7 over the entire range of the EAM reverse bias voltage of 0-3 V. Finally, successful 10-Gb/s return to zero transmission has been confirmed through both positive dispersion (+425 ps/nm) and negative dispersion (-425 ps/nm).

120/spl deg/C 10-gb/s uncooled direct Modulated 1.3-/spl mu/m AlGaInAs MQW DFB laser diodes

A 1.3-/spl mu/m AlGaInAs multiquantum well ridge waveguide distributed feedback laser diode was developed. By forming n-InGaAsP grating in the n-InP cladding layer close to the active region, accumulation of the holes in the grating layer was reduced and over 5 mW of output power was obtained at 120/spl deg/C. Clear eye opening was confirmed with no mask hits for OC-192 under 10-Gb/s direct modulation at the temperature up to 120/spl deg/C.

Novel fiber alignment method using a partially-metal-coated fiber in a silicon V-groove

We propose a novel fiber alignment method using a partially metal-coated (PMC) fiber. A PMC fiber is a fiber whose side is partially metal coated, and a core of PMC fiber can be regarded as eccentric with respect to the outside of PMC fiber. Rotating the PMC fiber in a silicon V-groove, the core position can be aligned. Using this method, laser diode modules with high coupling efficiency were obtained.

High-Current Back-Illuminated Partially Depleted-Absorber p-i-n Photodiode With Depleted Nonabsorbing Region

We demonstrate a high-current back-illuminated InGaAs/InP p-i-n photodiode (PD), whose depleted region comprises partially depleted-absorbing, depleted-absorbing, and depleted-nonabsorbing layers to increase RF power output. The back-illuminated PD has an advantage of small thermal resistance between a photoabsorber and a heat sink for avoiding catastrophic thermal failure. The thermal resistance decreases with an increase in the detecting area; however, it simultaneously increases the capacitance, imposing a limitation on the RF response. To reduce the capacitance, we have incorporated a depleted-nonabsorbing layer into a partially depleted absorber PD structure that photogenerated electrons can drift trough. We have fabricated two samples, one with a detecting area of 50 μm, and the other with a detecting area of 70 μm in diameter. Both PDs show high RF power outputs of 28.7 and 29.0 dBm at a frequency of 5 GHz, and 25.7 and 26.7 dBm at each -3-dB frequency of 10.5 and 7 GHz, respectively.

1.3-/spl mu/m strained MQW-DFB lasers with extremely low intermodulation distortion for high-speed analog transmission

The intermodulation distortion and the noise characteristics of 1.3-/spl mu/m strained multiquantum-well distributed feedback (MQW-DFB) lasers have been investigated under the modulation frequency of 1.9 GHz in connection with the device structure. In this study, a strained MQW with strain-compensated layers has been introduced in order to increase in the quantum-well number and well width. This causes increase in the differential gain, resulting in increase of the resonance frequency (FR). The FR as high as 5.1 GHz/mW/sup 1/2/ has been obtained which is in good agreement with the theoretical calculation. In addition to the strained MQW structure, a new buried heterostructure entirely grown by MOCVD, named as FSBH (facet selective growth buried heterostructure), has been developed to minimize the leakage current which degrades L-I characteristics at high bias current causing the high distortion. The third-order-intermodulation distortion (IMD3) of -88 dBc and relative intensity noise (RIN) of -152 dB/Hz have been obtained under a two-tone test at 1.9 GHz. This suggests that this newly developed laser is quite suitable for high-speed-subcarrier multiplexing transmission.

Design and Characteristics of Guardring-Free Planar AlInAs Avalanche Photodiodes

This paper reports a guardring-free planar AlInAs avalanche photodiode (APD) for optical fiber communications. AlInAs APDs can achieve a larger gain-bandwidth product (GBP) with lower excess noise than commercial InP APDs, and the guardring-free planar structure enables these superior AlInAs APDs to have both a low dark current and high reliability for practical use. We present the structure, fabrication, designs, APD characteristics, and receiver sensitivity, systematically. The guardring-free planar structure and its peculiarities are described, and we show APD designs for 2.5-Gb/s and 10-Gb/s applications and their characteristics. A 0.2-mum -thick AlInAs multiplication layer achieves a GBP of 120 GHz and an excess noise factor of 2.9 at a multiplication factor of 10. Their dark currents are less than 20 nA and their lifetime is evaluated to be 25 million hours at 85degC. Lastly, we demonstrate that the guardring-free planar AlInAs APDs with a transimpedance amplifier achieve the remarkable sensitivity of -37.0 dBm at a bit error rate of 10-10 for 2.5-Gb/s signals and of -29.9 dBm at a bit error rate of 10-12 for 10-Gb/s signals. This performance indicates that the guardring-free planar AlInAs APDs have made great advances against commercial InP APDs and other AlInAs APDs.

High-power ultralow-chirp 10-Gb/s electroabsorption modulator integrated laser with ultrashort photocarrier lifetime

A high-power, ultralow-chirp electroabsorption modulator (EAM) integrated with a distributed-feedback laser diode (EML) having ultrashort lifetime of photogenerated holes in the EAM quantum-well (QW) structure is reported for the first time. A shallow QW structure having a small valence band offset to enhance the sweepout of photogenerated holes was employed as EAM absorption layer. The measured hole lifetimes were 7-11 ps, and the measured frequency chirp (/spl alpha/-parameter) was low or negative at low EAM reverse bias voltages even under high optical output power conditions. Successful 10-Gb/s 80-km normal-dispersion single-mode fiber transmission (chromatic dispersion D=1600 ps/nm) and the record average fiber optical output power (P/sub f/) of +5.3 dBm were achieved at 25/spl deg/C. In addition, semicooled operation of EML at enhanced bit rates has been demonstrated for application in small-form-factor protocol-agnostic optical transceivers. A 10.7-Gb/s 1600-ps/nm transmission was achieved at 45/spl deg/C and P/sub f/=+3.0 dBm.

40 GHz modulation bandwidth of electroabsorption modulator with narrow-mesa ridge waveguide

We have developed the EA modulator with narrow-ridge waveguide to obtain the sufficient extinction ratio with the shorter modulators. Narrowing the waveguide reduces device capacitance from 0.12 pF to 0.08 pF without decrease of the extinction ratio. The modulation bandwidth of 40 GHz at -1 V bias voltage, and the extinction ratio of 15 dB at 0 V to -2.5 V operation voltage are obtained. From the viewpoint of frequency bandwidth and extinction ratio for practical use, it may be concluded that the narrow mesa waveguide structure is suitable for 40 Gb/s optical transmitter devices.

Eye-opening improved electroabsorption modulator/DFB laser diode with optimized thickness of the separate-confinement heterostructure layers

The separate-confinement heterostructure (SCH) of an electroabsorption modulator integrated with a distributed feedback laser diode (EAM-LD) was optimized to obtain a clear optical waveform (eye opening) without penalty of chirp characteristics. The electric field applied to the multiple-quantum-well (MQW) structure was controlled by employing the proper thickness of undoped SCH layers to attain gentle absorption characteristics of the EAM at the optical mark level. As a result, the eye opening of the modulated emission at 10 Gb/s was improved and the intersymbol interference was reduced. No severe drawback on chirp characteristics was found in an /spl alpha/-parameter measurement. EAM-LDs with the optimized SCH structure exhibited good transmission characteristics of power penalties under 1.5 dB at 10 Gb/s, and the reliability of 10 Gb/s transmission characteristics was also confirmed. We have also investigated how the optical waveform after transmission was affected by the dependence of the /spl alpha/-parameter on bias voltage. Measurement and numerical calculation of the optical waveform after transmission showed that the optical waveform was severely affected by the /spl alpha/-parameters at deep reverse voltages.

High efficiency 10 Gbps InP/InGaAs avalanche photodiodes with distributed Bragg reflector

InP/InGaAs-based avalanche photodiodes for 10 Gbps application have been developed. By adopting the distributed Bragg reflector, high efficiency of 0.88 A/W and large gain-bandwidth product of 100 GHz are successfully demonstrated.