Ken Morito

High-output-power, single-wavelength silicon hybrid laser using precise flip-chip bonding technology

An Si/III-V hybrid laser oscillating at a single wavelength was developed for use in a large-scale Si optical I/O chip. The laser had an InP-based reflective semiconductor optical amplifier (SOA) chip integrated with an Si wavelength-selection-mirror chip in a flip-chip configuration. A low coupling loss of 1.55 dB at the Si-SOA interface was accomplished by both mode-field-matching between Si-SOA waveguides and accurately controlling the bonding position. The fabricated Si hybrid laser exhibited a very low threshold current of 9.4 mA, a high output power of 15.0 mW, and a high wall-plug efficiency of 7.6% at 20 °C. Moreover, the device maintained a high output power of >10 mW up to 60°C due to the high thermal conductance between the SOA chip and Si substrate. The short cavity length of the flip-chip bonded laser expanded the longitudinal mode spacing. This resulted in temperature-stable single longitudinal mode lasing and a low RIN level of <-130 dB/Hz.

A broad-band MQW semiconductor optical amplifier with high saturation output power and low noise figure

A broad-band semiconductor optical amplifier (SOA) that achieves both a high chip saturation output power and a low chip noise figure (NF) was developed by using a thin multiquantum well with low internal loss. The SOA exhibited a high chip saturation output power of >+19.6 dBm and a low chip NF of <4.5 dB over a 3-dB gain bandwidth of 120 nm (1450-1570 nm). For the amplification of optical signals modulated at 40-Gb/s nonreturn-to-zero format, a penalty-free amplification was confirmed up to an average chip output power of +18.1 dBm.

High-output-power polarization-insensitive semiconductor optical amplifier

A high-output-power 1550 nm polarization-insensitive semiconductor optical amplifier (SOA) was developed for use as a compact in-line optical amplifier. A very thin tensile-strained bulk structure was used for the active layer and active width-tapered spot-size converters (SSCs) were integrated on both input and output sides. The SOA module exhibited a high saturation output power of +17 dBm together with a low noise figure of 7 dB, large gain of 19 dB, and low polarization sensitivity of 0.2 dB for optical signals of 1550 nm wavelength. For the amplification of optical signals modulated at 10 Gb/s in the nonreturn-to-zero (NRZ) format, a good eye pattern without waveform distortion due to the pattern effect was obtained at an average output power of up to +12 dBm. Additionally, good amplification characteristics were demonstrated for the signal wavelength range corresponding to the C-band.

Compact high-power wavelength selectable lasers for WDM applications

Wavelength selectable lasers with record +1O dBm fiber coupled output power and uniform characteristics have been realized by integration of eight-channel distributed feedback laser arrays with low-loss multi-mode interference combiners and optical amplifiers with a length of only 2 mm.

Penalty-free 10 Gb/s NRZ transmission over 100 km of standard fiber at 1.55 μm with a blue-chirp modulator integrated DFB laser

We present transmission beyond the dispersion limit with a blue-chirp modulator integrated DFB laser for the first time. By suppressing the hole pile-up in the modulator, stable modulation characteristics were realized in negative /spl alpha/ regime. Blue-chirp operation was clearly observed by time-resolved dynamic chirp measurement. Penalty-free 10 Gb/s transmission over 100 km of standard fiber at 1.55 /spl mu/m was demonstrated with the device. This modulator integrated DFB laser is promising for use as a high-power blue-chirp transmitter in multigigabit transmission systems.

Cross-Gain Modulation in Quantum-Dot SOA at 1550 nm

We experimentally study the cross-gain modulation (XGM) in an InAs/InGaAsP/InP columnar quantum-dot semiconductor optical amplifier working in the 1550-nm spectral region. We report 12-nm wavelength conversion of 10, 20, 40, 80, and 160 Gb/s return-to-zero (RZ) signals. We find that pure XGM is effective for wavelength conversion without significant pattern effects up to around 40 Gb/s. At higher bit rates, distortion-free conversion can be obtained by exploiting jointly XGM and some cross-phase modulation that arises at the same time in the amplifier. This can be easily achieved by using a slightly blue-shifted output band-pass filter. By testing the signal integrity by means of the factor analysis, we also find that the XGM transfer function attenuates the input intensity noise. Finally, we show how the output inverse-RZ signals obtained in the XGM conversion can be format-converted back to RZ by side-band filtering.

Monolithically Integrated 8:1 SOA Gate Switch With Large Extinction Ratio and Wide Input Power Dynamic Range

A monolithic 8:1 SOA gate switch that integrates an 8-ch SOA gate array, an 8:1 optical coupler, and a 1-ch SOA gate was developed for use in a large scale optical packet switching system. A 250-mum-long compact field-flattened coupler (FFC) produces a very small channel-imbalance together with a compact total chip size of 3.0 times 1.0 mm. The device exhibited a large ON-state gain of >14.3 dB and a small total gain deviation of 3.0 dB. The optimized passive waveguide structure successfully suppressed the stray light, which resulted in a record-high ON-OFF extinction ratio of >70 dB. We used a thin tensile-strained multi-quantum well (MQW) active layer which can attain high-saturation output power, low noise, and polarization insensitivity for SOA gates. Due to the coexistence of a high saturation output power and a low noise figure, the device exhibited a very wide input power dynamic range of 20.5 dB for a 10-Gb/s NRZ signal.

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.

Novel Optical 90

We propose a novel optical 90° hybrid consisting of a paired-interference based 2 × 4 multimode interference (MMI) coupler, a phase shifter and a 2 × 2 MMI coupler. We theoretically clarified that the proposed device shows superior performances to a conventional 4 × 4 multimode interference coupler from the viewpoint of a wavelength sensitive loss and an operating bandwidth. We fabricated the proposed 90° hybrid using a deep-ridge waveguide with a GalnAsP/InP material system, and experimentally demonstrated quadrature phase response with a low interchannel imbalance, a high common-mode rejection ratio (> 20 dB) and a low phase deviation (< 5°) over 94-nm-wide spectral range.

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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.