This paper proposes a design of high-speed interconnection between optical modules and electrical modules via bonding-wires and coplanar waveguide transmission lines on printed circuit boards for 400 Gbps 4-channel optical communication systems. In order to broaden the interconnection bandwidth, interdigitated capacitors were integrated with GSG pads on chip for the first time. Simulation results indicate the reflection coefficient is below -10 dB from DC to 53 GHz and the insertion loss is below 1 dB from DC to 45 GHz. Both indicators show that the proposed interconnection structure can effectively satisfy the communication bandwidth requirements of 100-Gbps or even higher data-rate PAM4 signals.

A Mach-Zehnder optical modulator with the tunable multimode interference coupler was fabricated using Ti-diffused LiNbO3. The modulation extinction ratio could be voltage controlled to maximize up to 50 dB by tuning the coupler. Optical single-sideband modulation was also achieved with a sideband suppression ratio of more than 30 dB.

Silicon photonics technology is a promising candidate for small form factor transceivers that can be used in data-center applications. This technology has a small footprint, a low fabrication cost, and good temperature immunity. However, its main challenge is due to the high baud rate operation for optical modulators with a low power consumption. This paper investigates an all-Silicon Mach-Zehnder modulator based on the lumped-electrode optical phase shifters. These phase shifters are driven by a complementary metal oxide semiconductor (CMOS) inverter driver to achieve a low power optical transmitter. This architecture improves the power efficiency because an electrical digital-to-analog converter (DAC) and a linear driver are not required. In addition, the current only flows at the time of data transition. For this purpose, we use a PIN-diode phase shifter. These phase shifters have a large capacitance so the driving voltage can be reduced while maintaining an optical phase shift. On the other hand, this study integrates a passive resistance-capacitance (RC) equalizer with a PIN-phase shifter to expand the electro-optic (EO) bandwidth of a modulator. Therefore, the modulation efficiency and the EO bandwidth can be optimized by designing the capacitor of the RC equalizer. This paper reviews the recent progress for the high-speed operation of an all-Si PIN-RC modulator. This study introduces a metal-insulator-metal (MIM) structure for a capacitor with a passive RC equalizer to obtain a wider EO bandwidth. As a result, this investigation achieves an EO bandwidth of 35.7-37 GHz and a 70 Gbaud NRZ operation is confirmed.

A compact silicon photonic crystal waveguide (PCW) slow-light modulator is presented. The proposed modulator is capable of achieving a 64 Gbps bit-rate in a wide operating spectrum. The slow-light enhances the modulation efficiency in proportion to its group index ng. Two types of 200-µm-long PCW modulators are presented. These are low- and high-dispersion devices, which are implemented using a complementary metal-oxide-insulator process. The lattice-shifted PCW achieved low-dispersion slow-light and exhibited ng ≈ 20 with an operating spectrum Δλ ≈ 20 nm, in which the fluctuation of the extinction ratio is ±0.5 dB. The PCW device without the lattice shift exhibited high-dispersion, for which a large or small value of ng can be set on demand by changing the wavelength. It was found that for a large ng, the frequency response was degraded due to the electro-optic phase mismatch between the RF signals and slow-light even for such small-size modulators. Meander-line electrodes, which bypass and delay the RF signals to compensate for the phase mismatch, are proposed. A high cutoff frequency of 55 GHz was theoretically predicted, whereas the experimentally measured value was 38 GHz. A high-quality open eye pattern for a drive voltage of 1 V at 32 Gbps was observed. The clear eye pattern was maintained for 50-64 Gbps, although the drive voltage increased to 3.5-5.3 V. A preliminary operation of a 2-bits pulse amplitude modulation up to 100 Gbps was also attempted.

We fabricated an InP-based dual-polarization In-phase and Quadrature (DP-IQ) modulator consisting of a Mach-Zehnder (MZ) modulator array integrated with RF termination resistors and backside via holes for high-bandwidth coherent driver modulators and revealed its high reliability. These integrations allowed the chip size (Chip size: 4.4mm×3mm) to be reduced by 59% compared with the previous chip without these integrations, that is, the previous chip needed 8 chip-resistors for terminating RF signals and 12 RF electrode pads for the electrical connection with these resistors in a Signal-Ground-Signal configuration. This MZ modulator exhibited a 3-dB bandwidth of around 40 GHz as its electrical/optical response, which is sufficient for over 400 Gbit/s coherent transmission systems using 16-ary quadrature amplitude modulation (QAM) and 64QAM signals. Also, we investigated a rapid degradation which affects the reliability of InP-based DP-IQ modulators. This rapid degradation we called optical damage is caused by strong incident light power and a high reverse bias voltage condition at the entrance of an electrode in each arm of the MZ modulators. This rapid degradation makes it difficult to estimate the lifetime of the chip using an accelerated aging test, because the value of the breakdown voltage which induces optical damage varies considerably depending on conditions, such as light power, operation wavelength, and chip temperature. Therefore, we opted for the step stress test method to investigate the lifetime of the chip. As a result, we confirmed that optical damage occurred when photo-current density at the entrance of an electrode exceeded threshold current density and demonstrated that InP-based modulators did not degrade unless operation conditions reached threshold current density. This threshold current density was independent of incident light power, operation wavelength and chip temperature.

To meet the demand for continuous increase in data traffic, full usage of polarization freedom of light is becoming inevitable in the next-generation optical communication and datacenter networks. In particular, Stokes-vector modulation direct-detection (SVM-DD) formats are expected as potentially cost-effective method to transmit multi-level signals without using costly coherent transceivers in the short-reach links. For the SVM-DD formats to be practical, both the transmitter and receiver need to be substantially simpler, smaller, and lower-cost as compared to coherent counterparts. To this end, we have recently proposed and demonstrated novel SV modulator and receiver circuits realized on monolithic InP platforms. With compact non-interferometric configurations, relatively simple fabrication procedures, and compatibility with other active photonic components, the proposed devices should be attractive candidate in realizing low-cost monolithic transceivers for SVM formats. In this paper, we review our approaches as well as recent progresses and provide future prospects.

We have proposed and developed new electro-optic modulators for the pre-equalization of signal distortion caused by the optical fiber chromatic dispersion effect. We found that the synthesis of an almost arbitrary impulse response function is obtainable by utilizing an electro-optic modulator composed of a Mach-Zehnder waveguide and travelling-wave electrodes on a ferro-electric material substrate with polarization-reversed structures. In this paper, the operational principle, design and simulation results of the pre-equalization modulator are presented. Some preliminary experimental results are also shown with future prospects.

A quantum dot (QD) electro-absorption device was successfully developed with a highly stacked InAs/InGaAlAs QD structure. A 1.55-µm waveband electro-absorption effect and a quantum confined Stark effect of approximately 22 meV under the application of a 214-kV/cm reverse bias electric field are clearly observed in the developed QD device.

A millimeter-wave radar receiver using a z-cut LiNbO3 optical modulator with orthogonal-gap-embedded patch-antennas on a low-k dielectric material is proposed. A millimeter-wave from a reflected radar signal can be received by the patch-antennas and converted directly to a lightwave through electro-optic modulation. A low-k dielectric material is used as a substrate for improving antenna gain. Additionally, an interaction length between millimeter-wave and lightwave electric fields becomes long. As a result, large modulation efficiency can be obtained, which is proportional to sensitivity of the millimeter-wave radar receiver. Optical millimeter-wave radar beam-forming can be obtained using the proposed device with meandering-gaps for controlling interaction between millimeter-wave and lightwave electric fields in electro-optic modulation. Analysis and experimentally demonstration of the proposed device are discussed and reported for 40GHz millimeter-wave bands. Optical millimeter-wave radar beam-forming in 2-D is also discussed.

A new wavelength-selective optical modulator was proposed and discussed. The modulator consists of three kinds of distributed Bragg reflectors (DBRs) integrated in a single straight waveguide. The waveguide can guide TE$_0$ and TE$_1$ modes, and an in-line Michelson interferometer is constructed by the three DBRs. An operation-wavelength wave among incident wavelength-division-multiplexed TE$_1$ guided waves is split into TE$_0$ and TE$_1$ guided waves by one of DBRs, and combined by the same DBR to be TE$_0$ output wave with interference after one of waves is phase-modulated. A modulator using an electro-optic (EO) polymer is designed, and the static performance was predicted theoretically. An operation principle was confirmed experimentally by a prototype device utilizing a thermo-optic effect instead of the EO effect.

A reduction in the intensity deviation of a nine-channel optical frequency comb block (OFCB) is demonstrated, by adopting an asymmetric differential drive method for an InP-based dual drive Mach-Zehnder modulator. The generation of a tailored OFCB with an intensity deviation of less than 0.8dB is confirmed by using the modulator.

Quasi-phase-matching (QPM) electro-optic modulators using gap-embedded patch-antennas were proposed for improving wireless microwave-optical signal conversion. The proposed QPM devices can receive wireless microwave signals and convert them to optical signals directly. The QPM structures enable us to have twice antenna elements in the fixed device length. The device operations with improved conversion efficiency of 10 dB were experimentally demonstrated at a wireless signal frequency of 26 GHz. The proposed QPM devices were also tested to a wireless-over-fiber link.

We demonstrate high baud-rate DQPSK modulation with full-ETDM technique using a novel high-speed optical IQ modulator consisting of a ridge-type optical waveguide structure on a thin LiNbO3 substrate. Our fabrication technique achieves a drastic extension of the modulator's bandwidth and a reduction of half-wave voltage. Demonstration of 90-Gbaud NRZ-DP-DQPSK signal generation with the modulator successfully achieved a bit rate of 360-Gb/s under full-ETDM configuration.

We present recent progress of high-speed Mach-Zehnder modulator technologies for advanced modulation formats. Multi-level quadrature amplitude modulation signal can be synthesized by using parallel Mach-Zehnder modulators. We can generate complicated multi-level optical signals from binary data streams, where binary modulated signals are vectorially summed in optical circuits. Frequency response of each Mach-Zehnder interferometer is also very important to achieve high-speed signals. We can enhance the bandwidth of the response, with thin substrate. 87 Gbaud modulation was demonstrated with a dual-parallel Mach-Zehnder modulator.

For the uplink of a radio-on-fiber system or an electromagnetic field sensor, a resonant type optical modulator array connected with antennas can effectively convert a micro/millimeter-wave to a light wave. We designed and fabricated 10 GHz band resonant modulators and micro-strip antennas. And we demonstrated the simultaneous operation of four modulators using power received by micro strip antennas connected to each modulator. We confirmed that the optical phase change induced by the received power could be proportionally increased with the number of arrays.

Complex thin oxide films with electro-optic (EO) properties are promising for use in advanced optical devices because of their large EO effect. We developed a method of aerosol deposition (AD) for fabricating EO films. The mechanism for AD is based on the solidification by impact of submicron particles onto a substrate. Since particles in AD films preserve their crystalline structure during the formation of film, epitaxial growth is not necessary for exhibiting the EO effect. Highly transparent Pb(Zr, Ti)O3 films, which have acceptable transmittance loss for use as optical devices, were directly deposited on glass substrates by AD. We found the Pb(Zr, Ti)O3 film by AD produced a fairly high EO coefficient (>150 pm/V), approximately 10 times larger than that of LiNbO3. A Fabry-Perot (FP) optical modulator was developed with EO films fabricated by AD. We demonstrated the modulation of optical intensity with an electrical field applied to an EO film made of ferroelectric Pb (Zr, Ti)O3.

We propose a simple method for the chromatic dispersion measurement of optical fibers by using bi-directional modulation of a Mach-Zehnder electro-optical modulator embodied in a fiber loop mirror. The detected output of the bi-directionally modulated light, with time difference, creates fading in the RF domain. Dispersion is found by measuring the period of fading at different wavelengths.

We have designed and fabricated air-bridged modulators with bandwidths exceeding 10 GHz, the highest yet realised to date for InGaAsP/InGaAsP multiple-quantum-well (MQW) asymmetric Fabry-Perot modulators (AFPMs). Microwave modulation, measurements of intermodulation between the photodetected downlink and modulated uplink signals, and bi-directional broadband data over fibre transmission experiments have been performed to verify the potential of the AFPM as a single electrical/optical transceiver. We also report the first direct integration of this AFPM with a microstrip patch antenna and present results of a preliminary microwave signal transmission experiment over a distance of 1.4 m in free-space at 5.2 GHz with the integrated AFPM as a photodetector.

An electric filed sensor using Mach-Zehnder interferometers has been designed to operate more than 10 GHz. The velocity of optical wave on the waveguide is investigated to determine the electrode length, and the characteristics of frequency response are analyzed using the moment method to determine the sensor element length. The electrode length of 1 mm and the element length of 8 mm are settled by these investigations. An isotropic electric field sensor is constructed using three sensors. The minimum detectable electric field strength is 22 mV/m at frequency bandwidth of 100 Hz. This is about 100 times for the conventional electric field sensor using the similar element. The sensitivity deviation is within 3 dB when temperature changes from 0 degree to 40 degree. The deviation of directivity can be tuned within 1 dB to calibrate the sensitivity of the each element. The sensitivity degradation is within 6 dB up to 5 GHz and within 10 dB up to 10 GHz. This is almost agree with the calculated results. The sensor can measure almost the same waveform as the applied electric field pulse whose width is 6 ns and rise time is less than 2.5 ns.

LiNbO3 optical modulators for band-operation with a resonant modulating electrode are investigated in this paper. We propose an asymmetric resonant structure consisting of two arms of modulating electrodes, where one arm is open-ended and the other arm is short-ended. The voltage standingwave was enhanced by the resonance of the electrodes, so that effective optical modulation was achieved, while the length of the modulating electrode was much shorter than the conventional travelingwave-type electrodes. The optical response at 6.2 GHz of a resonant modulator designed by maximizing the normalized induced phase was 4.94 of the response at dc with a non-resonant modulator.