Nami Yasuoka

Demonstration of transverse-magnetic dominant gain in quantum dot semiconductor optical amplifiers

We demonstrated transverse-magnetic (TM)-mode dominated gain at the 1.5μm wavelength in semiconductor optical amplifiers (SOAs) with columnar quantum dots (QDs). We show that we can control the polarization dependence of optical gain in QD-SOAs by changing the height and tensile-strained barrier of columnar QDs. The TM mode gain is 17.3dB and a gain of over 10dB was attained over a wide wavelength range of 200nm. The saturation output power is 19.5dBm at 1.55μm.

Quantum-Dot Semiconductor Optical Amplifiers With Polarization-Independent Gains in 1.5-

We have demonstrated a polarization-independent gain in semiconductor optical amplifiers that have columnar quantum dots surrounded by strained side barriers in 1.5-mum wavelength bands. We obtained a polarization-dependent gain of 0.5 dB with a gain of 10 dB and a saturation output power of 18 dBm at a wavelength of 1.55 mum.

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We demonstrate the use of diluted multiple quantum wells to achieve low‐loss fiber‐matched optical waveguides. A fiber mismatch loss of 0.2 dB/interface and propagation loss of 0.4 dB/cm are achieved at 1.54 μm wavelength using InGaAsP/InP materials.

m Wavelength Bands

Low-loss monolithic integration of a 3-dB waveguide coupler with twin photodetectors using large-mode size guides, mirror coupling, and low-capacitance lateral detectors is demonstrated. Waveguide-mirror integration introduces only approximately=1 dB optical insertion loss at 1.54 mu m wavelength, compared to top-illuminated detectors. Full balanced operation of the monolithic circuit with 33-dB local oscillator intensity noise suppression is demonstrated.<<ETX>>

Efficient fiber coupling to low‐loss diluted multiple quantum well optical waveguides

A novel ‘‘vertical impedance matching’’ approach is demonstrated to improve absorption in evanescently coupled, integrated waveguide/photodiodes by 500% over conventional structures. Our devices exhibit both high absorption at short length (90% at 190 μm) and efficient fiber butt coupling (42%) at 1.3–1.55 μm wavelengths. Unusual transient phenomena are observed in such impedance‐matched devices and discussed theoretically.

Low-loss monolithic integration of balanced twin-photodetectors with a 3 dB waveguide coupler for coherent lightwave receivers

An Al0.053Ga0.947Sb avalanche photodiode (APD) has been fabricated and tested. First, measurement of an excess noise factor as well as an ionization rate ratio has been demonstrated. A low excess noise factor of 3.8, which is 1.2 dB lower than the conventional GaInAs APD, has been obtained. From this excess noise factor, the effective value of the hole‐to‐electron ionization rate ratio (keff) has been determined as high as 5, being in good agreement with the hole‐to‐electron ionization rate ratio (β/α) given by the multiplication data. This keff value is the highest ever reported for long‐wavelength III‐V APDs.

Integrated waveguide/photodiodes using vertical impedance matching

Optical properties of polarization-controlled columnar quantum dots (QDs) grown by metalorganic vapor-phase epitaxy for semiconductor optical amplifier (SOA) applications are reported. The photoluminescence peak wavelength and polarization sensitivity depended on the time of AsH3 preflow before InAs growth as well as the InAs supply amount, and these changes in the optical properties are considered to be attributed to the change in the strain rather than the change in the height of the columnar QDs. Nearly polarization-insensitive QDs in the 1.5 μm region were obtained by 13-fold columnar QDs and finely controlling polarization of columnar-QD SOAs was demonstrated by changing barrier thickness by 0.05 ML steps.

AlGaSb avalanche photodiode exhibiting a very low excess noise factor

A polarization-insensitive quantum dot semiconductor optical amplifiers (QD-SOAs) have been studied for use in future optical communication systems. A part of our work shows that the optical polarization property in QDs depends on both their aspect ratio and strain. To control these two parameters, we propose the use of strain-controlled columnar QDs (SC-CQDs), which exhibit a high aspect ratio and have strain-controlled side barriers for polarization-insensitive operation in the 1.5-μ m wavelength band. QD-SOAs with these optimized SC-CQDs demonstrated polarization-insensitive characteristics. They showed a gain of 8.0 dB with polarization dependence of the gain as low as 0.4 dB, -3-dB saturation output power of 18.5 dBm at a wavelength of 1550 nm, and error-free amplification at a bit rate of 40 Gbit/s.

Optical properties of columnar InAs quantum dots on InP for semiconductor optical amplifiers

The optical polarization properties of columnar InAs quantum dots (QDs) on InP substrate grown by metalorganic vapor-phase epitaxy were investigated. The polarization of photoluminescence was found to strongly depend on the strain in QDs as well as the shape of QDs. We successfully changed the polarization properties from a transverse-electric-dominant to a transverse-magnetic-dominant regime by controlling the height of coupled QDs based on the stacking number and by controlling strain within QDs based on the thickness of 3.7%-tensile-strained barriers. Highly strained side barriers were required to change the polarization of QDs, which is considered to be due to wetting layers acting in maintaining biaxial-compressive strain in QDs. Polarization-insensitive QDs with the 1.55-µm telecom region were obtained, which promises to provide polarization-insensitive semiconductor optical amplifiers.

Polarization-Insensitive Quantum Dot Semiconductor Optical Amplifiers Using Strain-Controlled Columnar Quantum Dots

In this paper APD is fabricated with an 80 nm InP multiplication layer. The maximum bandwidth of 21 GHz, the multiplication-bandwidth product of 170 Hz and the intrinsic avalanche build up time of 0.4 ps (390 GHz) were obtained. The results shows that an APD for next generation 40 Gbit/s system can be achieved using a conventional InP multiplication layer.