Shigeru Kanazawa

Tuning power consumption of a tunable laser (TL) increasingly becomes an issue as the application range of wavelength division multiplexing grows in optical communications. On-chip Joule heat of a micro-heater commonly employed for a recent coherent TL needs to be exhausted out of the laser chip. It results in an increase in power consumption of laser control electronics with a thermal electric cooler (TEC) driver. An electro-optically TL whose laser wavelength is tuned by the electro-optic (EO) effect is advantageous for power-efficient tuning caused by little Joule heat during tuning unlike a micro-heater-based TL. We developed an electro-optically TL using a reflection-type transversal filter (RTF) whose reflection spectrum is tuned by the quantum-confined Stark effect (QCSE) known as a kind of the EO effect. In this paper, we describe an RTF laser with power-efficient control electronics thanks to little Joule heat from an RTF laser, which suppresses TEC driver power consumption. While an issue of control electronics of an RTF laser is a larger negative voltage (-15 V) for the QCSE than the industrial standard +3.3 V voltage supply, we found low-noise laser operation of an RTF laser to which reverse voltages are supplied from control electronics with an inverting switching converter based on the Ćuk topology. As a result, we successfully achieve wavelength-channel-dependent power consumption of less than 150 mW of an RTF laser with control electronics. Practical light source performance for a 400G digital coherent modulation format is also demonstrated.

In this work, we propose a wire interconnection AXEL CoC designed to increase the modulation bandwidth for 200-Gbit/s/l operation and simplify the CoC fabrication process. We first developed an electro-absorption modulator (EAM) that maintains a balance between the extinction ratio and the modulation bandwidth, and then, to increase the modulation bandwidth, we designed a termination circuit that can be integrated with the EAM and the CoC to reduce the bonding wire length. We obtained simulated 3-dB bandwidths of over 80 GHz and over 98 GHz with the longest and shortest bonding wire lengths, respectively. The 3-dB bandwidth of the AXEL CoC was over 95 GHz, which is enough for 226-Gbit/s/l operation. Moreover, under 226.875 Gbit/s 4-PAM operation, the average output power exceeded +9.0 dBm thanks to the integrated SOA in the chip.

We present a vertical-illumination InP/InGaAs photodiode (PD) with a monolithically integrated micro-lens having a 165-GHz bandwidth. Interference-based absorption enhancement resulted in a 0.53-A/W responsivity and a record 70-GHz external bandwidth-efficiency product, while maintaining a broader absorption spectral range compared with conventional resonant cavity enhanced PDs. The PD features an electric field confinement by an inverted p-down structure, which results in a low dark current of approximately 1 pA and low manufacturing variability. In addition, an InP microlens is monolithically integrated to increase the effective aperture by up to 20 μm, which improves assembly tolerance.

This paper presents a four-channel (4-ch) transmitter consisting of a 4-ch 65-nm CMOS cascode shunt laser diode (LD) driver and flip-chip-bonded 1.3-μm membrane LD-array-on-Si that has the highest power efficiency yet reported for 4-ch over-25-Gbps/ch transmitters that use distributed-feedback (DFB)-LDs and vertical-cavity surface emitting lasers. In order to transmit over-25-Gbps/ch NRZ PRBS-31 signals on four channels simultaneously, the driver incorporates electro-optic (EO)-response-improvement functions. It was designed with our photonic-electronic conversion system in SPICE, and the transmitter was implemented with a flip-chip-bonding interconnection between the driver and LD array. The effects of the EO-response-improvement functions were clarified mathematically and the optical output waveforms of a transmitter with and without the functions were simulated and measured. Furthermore, the bit error rate (BER) was measured. It was found that the functions improved the eye openings and reduced the jitter of the simulated and measured optical output waveforms. Moreover, the crosstalk penalties between discrete and simultaneous operation decreased, and 30 (31) Gbps/ch NRZ PRBS-31 signals could be transmitted error-free on four channels over 10-km-long standard single-mode fiber (or in the back-to-back (BtoB) configuration) with a power efficiency of 1.19 (1.15) mW/Gbps. Without the improvement functions, error-free simultaneous 4-ch transmissions were possible in the BtoB configuration for 25-Gbps/ch NRZ PRBS-31 signals but not for higher rates per channel.

We propose the method to estimate the zero-dispersion wavelength (ZDW) and to optimize the emission wavelength for high-speed intensity modulation and direct detection (IM-DD) transmission. The feature of this method can estimate the ZDW by using the unique notch frequency caused by chromatic dispersion from the frequency spectrum of the received signal, and reduce chromatic dispersion by tuning the emission wavelength to the estimation value of the ZDW in a configuration that using only general transmitters/receivers without additional dedicated equipment. With the proposed method, the limitation of transmission distance can be relaxed in high-speed IM-DD transmission without using a dispersion compensation device or an equalizer. We experimentally demonstrated that the notch frequency was observed at a condition of the pseudorandom binary sequence (PRBS) 2 ¹⁵ -1 and 2 ³¹ -1 non-return-to-zero (NRZ) signals at 53.125 Gbps for a total 80 and 100 km, and the ZDW was estimated with fast Fourier transformation sizes of 256, 512, and 1024. The results indicated that the proposed method can be estimated with an accuracy of about ±1 nm from the true value of the ZDW. In addition, to confirm the feasibility of the proposed method, we experimentally demonstrated that PRBS 2 ¹⁵ -1 and 2 ³¹ -1 NRZ signals at 53.125 Gbps were transmitted over 100 km by tuning the emission wavelength to the estimated ZDW value. For the ZDW with a true value of 1312.20 nm, the chromatic dispersion penalty was up to 1.5 dB under the conditions of a 53.125 Gbps NRZ signal with an emission wavelength of 1303.73 nm and a transmission distance of 100 km. Penalty-free and equalizer-free transmission was demonstrated by applying this method.

We propose advanced maximum likelihood sequence estimation (MLSE) methods for increasing the capacity of short-reach intensity-modulated direct detection (IM-DD) transmissions for economically realizing next generation Ethernet such as 800GbE or 1.6TbE. It is particularly important to conquer the traffic growth in data center networks, which has become a major issue. In this article, we introduce a method for reducing the complexity of the Viterbi algorithm, which is a problem in the actual implementation of MLSE, by using a penalty-less truncated trellis diagram with temporal decision results. Next, we introduce a method for improving the demodulation performance by precisely emulating the nonlinear response of the transmission system with low-cost devices using a nonlinear filter. Finally, we introduce a method for reducing the computational complexity with a decision feedback function. Transmission experiments in the O-band with transmission rates of 200 Gbps/lane or higher using each of the proposed methods are also presented. Finally, we present a 200Gbps/lane 4-LAN-WDM 800G 2km transmission experiment using a 4-channel LAN-WDM TOSA designed for 400G transmission.

The authors developed a mirror‐based out‐of‐plane optical coupler (MOOC) for InP photonic integrated circuits (PICs). The coupler has the coupling loss and alignment tolerance comparable to those of a conventional edge coupler. An asymmetric Mach–Zehnder interferometer (a‐MZI) with the MOOC exhibits little wavelength‐dependent loss (WDL) and a high extinction ratio (ER) of >20 dB over 100‐nm wavelength bandwidth. In addition, the temperature‐independent transmittance spectra were obtained. Finally, the authors fabricated a 4‐ch MOOC array with the pitch of 40 μm, which corresponds to the core pitch of practical multi‐core fibre (MCF).

We previously developed a semiconductor optical amplifier (SOA) assisted extended reach electroabsorption modulator integrated distributed feedback (EADFB) laser (AXEL) to increase the optical modulation output power and a high-frequency integrated design based on the flip-chip interconnection technique (Hi-FIT) to obtain a high modulation bandwidth. We achieved a high-output power 224-Gbit/s 4-level pulse-amplitude-modulation (4-PAM) operation of a Hi-FIT AXEL module. In this paper, we developed a compact DC block circuit integrated into Hi-FIT AXEL module to decrease chip-power consumption without increasing the sub-assembly size and degrading the modulation bandwidth. We designed a Hi-FIT AXEL sub-assembly, that include a flip-chip interconnection board with a DC block circuit that affects the modulation bandwidth. We achieved a 3-dB bandwidth of up to 64 GHz for this designed sub-assembly. The fabricated Hi-FIT AXEL module has a 3-dB bandwidth of more than 62 GHz and a flat frequency response up to 50 GHz. For the 224-Gbit/s 4-PAM operation, the fabricated Hi-FIT AXEL module has a chip-out average output power of +10.0 dBm with a transmitter eye-closure quaternary (TECQ) of 1.9 dB. The integrated compact DC block circuit can reduce chip power consumption by up to 17%.

We investigate the temperature dependence of the dynamic characteristics of a 1.3-μm InGaAlAs-based directly modulated membrane laser on a SiC substrate. The laser is fabricated by combining direct wafer bonding of SiC and InP substrates using a very thin (∼10 nm) oxide layer and epitaxial regrowth of a membrane InP layer on the InP/SiC template. The membrane laser structure on SiC with a high thermal conductivity (490 Wm ^-1 K ^-1 ) and a moderate refractive index (∼2.6) ensures both high thermal dissipation and a large optical confinement factor in the active region, which are ideal for increasing the relaxation oscillation frequency even at high operating temperatures. We further leverage the optical feedback effect from the output facet of a waveguide to enhance the bandwidth and enable signal modulation at 100 GBaud and beyond. Owing to the high relaxation oscillation frequency and the photon-photon resonance effect, it is possible to obtain a flat frequency response at temperatures ranging from 25 to 85°C. The fabricated membrane laser on SiC with an active-region length of 50 μm exhibits 3dB bandwidths of >110, 97, and 74 GHz at 25, 55, and 85°C, respectively. We successfully demonstrate 2-km transmissions of 100-Gbit/s NRZ signals under uncooled conditions (25 to 85°C). Furthermore, the laser is capable of 112-Gbit/s NRZ modulation at 85°C.