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In order to support continuous growth of global IP traffic, highly energy-efficient networks are demanded. One of the key components is an optical multicast switch which has large bandwidth with small power consumption. When using multicast switches, however, intrinsic losses will be an issue especially for large port count switches. Erbium doped fiber amplifiers (EDFAs) are typically used for the loss compensation, although it would much increase the number of components and footprint of the system. One of the options to solve this problem would be an integration technology of semiconductor optical amplifiers (SOAs) on the silicon photonics switches which we have recently developed. In the previous work, we have demonstrated a gain-integrated 4 × 4 silicon matrix switch. For the practical use of our technology, it is very important to show further scalability of the platform. Moreover, it is also important to show applicability to other switch topologies, such as multicast switches. In this article, we demonstrate an 8 × 8 silicon photonics multicast switch with on-chip integrated 2 × 4-ch. SOAs. The on-chip SOAs exhibit a net gain of ∼9 dB, which almost compensates for the intrinsic loss. We observe the crosstalk of less than −35 dB. Loss/crosstalk reduction and chip power consumption are also discussed.

A precise flip-chip bonding (FCB) technology for indium phosphide semiconductor optical amplifiers (InP-SOAs) on a silicon photonics platform within less than ±1- μ m alignment accuracy was developed. For efficient optical coupling and a relaxed alignment tolerance, the mode field on both the InP-SOAs and the Si waveguides was expanded by spot-size converters (SSCs). On the InP-SOAs, width-tapered SSCs were used to obtain an isotropic mode-field having an approximately a 3- μ m diameter. On the silicon photonics platform, dual-core SSCs were used to expand the same mode-field size of 3 μ m as for the SSCs on SOAs. Using the FCB technology and the SSCs, an in-line optical amplification of 15 dB was achieved by in-line integrated SOAs with angled waveguides. The optical coupling losses were 7.7 dB, which included 5.1-dB excess losses by misalignment and a gap between InP-SOA and Si waveguides. A 4 × 4 Si switch with a hybrid-integrated 4-ch SOA array was fabricated, and achieved the first demonstration of a lossless Si switch.

We experimentally demonstrate the lossless transmission of wavelength division multiplexing (WDM) signals through a silicon-photonics 4 × 4 switch with a flip-chip bonded 4-channel semiconductor optical amplifier (SOA). We first optimized the input power and gain of the SOA-integrated switch to obtain the optimum operation point in terms of the transmitted signal quality. We then performed simultaneous transmission of 8-ch, 32-Gbaud, SP 16-QAM WDM (800 Gb/s) signals through all the four paths of the switch. The effect of crosstalk on the switch was very small, and thus could not be observed. We also examined multistage (up to four stages) transmission of the signals with circulating configurations. We show that even for a 4-stage transmission, the bit error rate of the transmitted signal is below the 20% forward-error-correction limit. Finally, we discuss approaches to improve the optical signal-to-noise ratio of the transmitted signals to enlarge the signal quality margin and increase the possible number of the cascading stages and/or WDM channels for wide applications.

We fabricated waveguide germanium photodetectors (Ge-PDs) using silicon photonics technology at a commercial CMOS foundry. The fabricated Ge-PDs exhibited low dark current, high responsivity (1.1 A/W) and 40-GHz O/E response with TIA up to high input power of +5.8 dBm.

We review the hybrid-integration of an InP semiconductor optical amplifier on a silicon photonics chip, and our in-line SOA integration using precise flip-chip bonding technology. We demonstrate the loss-less operation of a 4×4 switch with a 4ch-SOA array.

1.3-μm-wavelength AlGaInAs distributed-reflector laser arrays on semi-insulating InP substrate for high-speed direct modulation are investigated to realize compact and low-power-consumptive optical modules. Combination of AlGaInAs quantum wells with large differential gain and semi-insulating buried-heterostructure for reduction of the volume in active region achieved high-speed direct modulation. The fabricated lasers in the array lase with different wavelengths with stable single-longitudinal mode. Clear eye-opening and large mask margin were obtained in push-pull driving at 25.8 Gb/s direct modulation with coplanar electrode and semi-insulating InP substrate. Comparison of modulation waveform under simultaneous operation of neighbor lasers shows that crosstalk between lasers in the array is small. Clear eye-openings, large mask margins, and 10-km fiber transmission of four different wavelength lasers on LAN-WDM grid are demonstrated under 28-Gb/s operation at 50 °C. For further high-speed operation, 43-Gb/s direct modulation were performed. These results indicate that the 1.3-μm AlGaInAs DR laser arrays are promising as light sources for 100-Gb/s Ethernet and higher speed transmission.

A novel gain control scheme of semiconductor optical amplifiers (SOAs) using a heater-mounted SOA was investigated. In our proposal, a simple SOA gain control using an on-chip heater provides a high saturation output power over an entire gain control range of 10 dB. This scheme widely expands the usable power range in output level control of a 10 Gbps NRZ-OOK signal, compared to that of a conventional gain control operated by drive current. As a result, the heater-mounted SOA exhibited a large input power dynamic range (IPDR) of > 17 dB up to the output power of +8.0 dBm in level control operation.

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.

1.3-μm quantum-dot lasers integrated with a thickness tapered waveguide as a spot-size converter have been developed to improve coupling efficiency to silicon photonics devices. Improvements of far-field patterns and coupling efficiency were experimentally demonstrated.

Temperature-dependence of a 1.3 µm external-cavity quantum-dot laser was evaluated. Four-wavelength simultaneous oscillation and wide-eye openings under 25-Gbps NRZ signal using the filtered individual longitudinal mode were demonstrated between 40 °C and 80 °C.

High-speed directly modulated single-mode lasers are attractive light sources for compact, low-power-consumptive optical modules. For next-generation optical modules, attempts for higher modulation speed at high temperatures are energetically studied.

rdquoWe developed a broadband polarization-insensitive multiquantum-well (MQW) semiconductor optical amplifier (SOA) for application as an optical switch in broadcast-and-select-type optical packet switching systems. We adopted a GalnNAs-GalnAs MQW structure active layer to decrease the noise figure (NF) around the gain peak wavelength together with a small gain tilt in the C-band. The device exhibited a smaller NF on the shorter wavelength side of the gain peak compared with a GalnNAs strained-bulk SOA. The reduced NF resulted in a large effective gain bandwidth of up to 90 nm (1510-1600 nm). The device also exhibited the small gain tilt (

A high-power eight-channel semiconductor optical amplifier gate array was developed for application as an optical switch in broadcast-and-select-type optical packet switching systems. The device exhibited a low polarization-dependent gain ( 60 dB), a low noise figure ( +18.2 dBm) for all eight channels. It also exhibited very high interchannel uniformity of less than 0.3 dB for all characteristics.

40-Gbps direct modulation of 1.3-μm wavelength AlGaInAs distributed-reflector laser arrays on semi-insulating InP substrate are investigated. Clear eye-opening is demonstrated under simultaneous operation of two lasers.

AlGaInAs distributed-reflector-laser arrays with four different wavelengths on LAN-WDM grid are demonstrated. Each laser provided output power of over 10 mW at 50°C under simultaneous operation and operated at 25.8 Gbps under push-pull driving configuration.

AlGaInAs-MQW distributed reflector (DR) laser with semi-insulating buried-heterostructure (SI-BH) has been developed as a high speed light source for short reach data transmissions. The DR laser has DBR mirrors on both sides of a DFB active region. Owing to the increased optical feedback with these DBR mirrors, we can obtain short-cavity lasers without increasing threshold gain. The small active region of the DR laser, which comes from the combination of the short cavity structure and the SI-BH waveguide, makes driving current lower. The fabricated DR lasers have shown excellent characteristics such as stable single mode operation, and high values of relaxation oscillation frequencies under low driving current in a wide temperature range. We have achieved 25- and 40-Gbps direct modulation with low driving current. Using four different wavelength DR lasers, whose lasing wavelengths coincide with LAN-WDM grid, 25.8- Gbps direct modulations with sufficient mask margins was obtained. Using 1.55-μm-wavelength DR lasers, we achieved 40-Gbps direct modulation with 5 dB dynamic extinction ratio with the driving current less than 50 mA even at 85°C. 40-Gbps fiber transmission over 10-km single mode fiber under the operation conditions up to 70°C was also confirmed in 1.3-μm wavelength.

50-Gbps direct modulation is demonstrated for the first time using a short cavity 1.3-μm wavelength distributed-reflector laser. Clear eye openings with dynamic extinction ratios of 5 dB were obtained up to 50°C and 10-km fiber transmissions were confirmed.

A world's first uncooled, polarization-insensitive 1.3-μm SOA module was developed using an AlGaInAs MQW-SOA. A very small gain variance of 3.8 dB including PDG was demonstrated over a wide temperature range from 25 to 75°C at a constant current.

40-Gbps transmission over 5-km single-mode fiber using a directly modulated 1.3-µm AlGaInAs MQW distributed-reflector laser was demonstrated. Error-free operations up to 70°C have been achieved.

We developed a compact 1.3-µm optical amplifier module using AlGaInAs MQW-SOA operated at 45°C. The module exhibited very low electric power consumption of 0.84W over wide temperature range (0–75°C) and excellent performance as pre-amplifiers in 40 or 100 Gb/s systems.

1.3-µm-wavelength AlGaInAs distributed reflector lasers were investigated for uncooled high-speed direct modulation. 40-Gbps modulation under low driving current of 50 mA up to 50°C and 40-Gbps eye opening at 85°C are demonstrated.

We developed a highly uniform monolithically integrated 8-input and 1-output (8:1) SOA gate switch for a large-scale high-speed switching system. The gain fluctuation due to an internal interference was suppressed with a low loss 8:1 tapered MMI coupler. The device exhibited a very small gain deviation of 50 dB and an ON-state gain of >10 dB. We also demonstrated a penalty-free amplification of 8×10 Gb/s WDM signals with a large input power dynamic range of >9.8 dB

We introduce our recent works on directly-modulated AlGaInAs quantum-well lasers with semi-insulating buried-heterostructure for ultra-high-speed transmission. The short-cavity 1.3-µm-wavelength DFB lasers showed low-penalty transmission up to 13 km under direct modulation at 25 Gbps, as well as clearly-opened eye patterns by 40-Gbps direct modulation. For further reduction of driving current in the high-speed directly-modulated lasers, we developed the distributed reflector lasers with the active-region having the length of 100 µm or less, sandwiched by the passive reflectors. The fabricated distributed reflector lasers exhibited very high slope value of relaxation oscillation frequency of 4.0 GHz/mA½ and more. The distributed reflector lasers emitting in 1.3-µm wavelength region achieved 40-Gbps direct modulation with the driving current of 2/3 of that by the DFB lasers. By the distributed reflector lasers of 1.55-µm-wavelength, high temperature 40-Gbps operation was realized as well as reduction of driving current.

1.3-μm semi-insulating buried-heterostructure AlGaInAs-MQW distributed reflector lasers have shown very high slope value of relaxation oscillation frequency of 4.0 GHz/mA½. 25- and 40-Gb/s direct modulation has been achieved with low driving current.

We developed a monolithic 8:1 SOA gate switch for use in a broadband optical packet-switching system. The device exhibited a large gain of > 12.3 dB and a high extinction ratio of > 65 dB.

We designed a 1×8 mode conversion-type optical coupler and experimentally demonstrated that the monolithically integrated SOA gate switch that uses it has low interchannel gain imbalance ( 14.5 dB).

We developed MOVPE growth of height-controlled InAs columnar QDs with an improved TM-mode sensitivity on InP(011) substrate by directly stacking SK-mode QDs whose tops were planarized under hydrogen ambient annealing to make each QD a uniform height.

A wavelength-tunable EA-modulated laser (wavelength-tunable EML) using tunable distributed amplification (TDA) structure was demonstrated. An 80 km transmission of 10 Gb/s signals was confirmed on 8 channels on ITU grid using the device's simple, continuous tuning mechanism.

rdquoWe developed a broadband polarization-insensitive multiquantum-well (MQW) semiconductor optical amplifier (SOA) for application as an optical switch in broadcast-and-select-type optical packet switching systems. We adopted a GalnNAs-GalnAs MQW structure active layer to decrease the noise figure (NF) around the gain peak wavelength together with a small gain tilt in the C-band. The device exhibited a smaller NF on the shorter wavelength side of the gain peak compared with a GalnNAs strained-bulk SOA. The reduced NF resulted in a large effective gain bandwidth of up to 90 nm (1510-1600 nm). The device also exhibited the small gain tilt (

A GaInNAs / InP tensile-strained bulk polarization-insensitive SOA was developed for the first time. It exhibited a red shift of gain peak, 50 nm, resulting in a low gain tilt of 1.5 dB in the wavelength of 1530-1570 nm.

An electrically wavelength-tunable laser array with tunable distributed amplification (TDA) DFB structure has been developed for the first time. A tuning range of 39.5 nm has been achieved by monolithic array of 8-channel mode-hop-free lasers

This paper reviews recent developments in optical signal processing based on quantum-dot semiconductor optical amplifiers. With an ultrafast gain response unique to quantum dots, an optical regenerator having receiver-sensitivity improving capability of 4 dB at a BER of 10-9 and operating speed of > 40 Gb/s has been successfully realized with an SOA chip. This performance achieved together with simplicity of structure suggests a potential for low-cost realization of regenerative transmission systems.

Uncooled, low-driving-current 25.8 Gbit/s direct modulation with clear eye-opening up to 85°C is demonstrated using 1.3 µm AlGaInAs multiple quantum well (MQW) distributed-reflector lasers. The peak driving currents were as low as 30.0 and 38.9 mA at 25 and 85°C, respectively.

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.

A polarisation-insensitive semiconductor optical amplifier module with a thin tensile-strained multiple quantum well active layer is developed. The module exhibited a record high saturation output power of +20.0 dBm and a low noise figure of 6.0 dB at 1550 nm together with a small polarisation-dependent gain of 0.6 dB.

We calculated the interband optical transition energies in self-assembled In x Ga1-x As quantum dots on InP as functions of the strain, composition, and height of dots on the basis of the k·p perturbation theory. We found that a transverse-magnetic (TM)-mode-sensitive optical transition at wavelengths in the range from 1.5 to 1.6 µm is achieved due to the light-hole valence band when the crystal lattice is expanded in the same direction of growth as the in-plane lattice matched to the substrate at x≤0.4, or when the lattice is compressed in the growth direction with the in-plane lattice relaxed at x≥0.6. We propose barrier structures covering the dots in order to realize these conditions, leading to TM-mode-sensitive quantum dots for polarization-independent optical amplifiers.

We investigated hybrid-integration of InP semiconductor optical amplifier on silicon photonics chip using flip-chip bonding, and achieved in-line optical amplification of 10 dB. Silicon 1×8 splitter and selector are fabricated and operated with on-chip loss-compensation.

We demonstrate lossless transmission of 8-channel 32-Gbaud 16-QAM WDM signals through a 4×4 silicon PILOSS switch with a flip-chip bonded 4-ch InP-based SOA. Transmission up to 4 stages was confirmed by optimizing nonlinearity and OSNR.