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We fabricated a flip-chip interconnection lumped-electrode EADFB laser module and an optical receiver that includes a broadband electrical amplifier. Both have a 3-dB bandwidth of more than 50 GHz. Under 53.2-Gbaud 4-PAM (107 Gb/s) operation, clear eye openings were obtained without an equalizer even after a 2-km SMF transmission, and we obtained a BER of less than 2 × 10−4 under error-free conditions using KP4 FEC without an equalizer also after a 2-km SMF transmission.

We fabricated a Hi-FIT LE-EADFB laser module. Hi-FIT, which is a wire-free interconnection technique, provides a higher modulation bandwidth and a flatter frequency response than a conventional wire interconnection technique. The fabricated module has a 3-dB bandwidth of about 59 GHz and a sufficiently flat frequency response of less than 45 GHz. Using this module, we demonstrated a single-wavelength single-polarization direct-detection 4-PAM transmission with a record net data rate of 200 Gb/s. And with 4-PAM operation at 214 Gb/s, we obtained a BER of less than 3.8 × 10–3, which is an error-free condition using a 7%-OH HD-FEC code, even after a 10-km SMF transmission.

Digital coherent transmission systems with multilevel and multicarrier modulation formats have been moving beyond 100-G systems. In this paper, we report a one-chip 400-G coherent receiver founded on a silica-based planar lightwave circuit (PLC) integration platform. To integrate a polarization beam splitter (PBS), two polarization rotators, four-optical hybrids, and 16 high-speed photodiodes (PDs) in one chip, we employed a 5%-Δ waveguide with a minimum bending radius of 300 μm, and the heterogeneous integration of InP-PDs. To achieve a compact receiver without any degradation in optical passive performance, we also investigated a three-branched spot size converter with a low loss and a high-fabrication tolerance and a waveplate type PBS with a high-polarization extinction ratio. Using these techniques, we fabricated a dual-carrier dual-polarization (DP) 16-level quadrature amplitude modulation (QAM) receiver PLC, and successfully demonstrated 32-Gbaud DP-16QAM signal demodulation for each channel.

This paper reviews our work on high-speed avalanche photodiodes (APDs) with a unique vertical-illumination structure, targeting 100-Gb/s systems and beyond. The APDs feature easy lateral and vertical scaling for achieving the required operation speed and responsivity. For 100-Gb/s systems, a 3-dB bandwidth of 18.5 GHz with multiplied responsivity of 9.1 A/W is obtained. For larger capacity applications such as 400-Gb/s systems, a maximum 3-dB bandwidth of over 35 GHz and responsivity at a gain of unity of almost 0.7 A/W are obtained by reducing diameter of an n-contact mesa and optimizing thickness and configuration of an absorption layer.

Device design for unitraveling-carrier photodiodes (UTC-PDs) and their derivative structures is reconsidered from the point of view of terahertz (THz) applications. A key design procedure for maximizing their bandwidth is optimization by incorporating hybrid absorbers. The effect of quasi-field in p-type absorber is carefully examined. It has been shown that the initial velocity transient must be taken into account to evaluate the effective average velocity. Photomixers integrating a hybrid-absorber UTC-PD and a bow-tie antenna were fabricated and characterized. THz-wave generation by the photomixers in a frequency range of up to around 2.5 THz was confirmed. The observed THz-wave output exhibits significant changes with bias voltage, where the decrease in the output with increasing negative bias voltage is more pronounced at higher frequencies. This output behavior is due to the change in electron velocity in the diode depletion layer associated with the overshoot effect. From the dependence of the output power on frequency, effective electron velocity is found to be as high as $6 \times 10^{7}$ cm/s at optimum bias voltage of -0.4 V.

The extension of an optical transmission system with a combination of quadrature phase-shift keying and a digital coherent technique has led to the need for economical and compact receivers. In this paper, we demonstrate the heterogeneous integration of InP PDs on a silica-based planar lightwave circuit (PLC). By utilizing adhesive InP chip bonding and tilt dry-etching, we have successfully fabricated high-speed photodiodes (PDs) and a low-loss micro-mirror on a small integration area on the PLC. We have also evaluated the receiver performance. We realized 32 Gbaud DP-QPSK signal demodulation using a compact coherent receiver PLC integrated with eight high-speed PDs, a monitor PD, a polarization beam splitter, two optical 90° hybrids, a variable optical attenuator and micro-mirrors.

A 25-Gbit/s avalanche photodiode (APD) using an inverted p-down structure and triple-mesa structure for eliminating an edge breakdown and reducing an electric field at a surface of the device is proposed and examined. Active area dependence of dark current and activation energy as small as 0.17 eV strongly suggest that the surface leakage current is negligible, and the edge breakdown does not occur. As for RF characteristics, sufficient 3-dB bandwidth of 18 GHz for 25-Gbit/s operation is obtained. We experimentally find that our triple-mesa inverted structure successfully eliminates the possible components which affect the reliability and stability of the APDs.

We have developed a flip-chip interconnection technique to solve the problem of the degradation of the modulation bandwidth of an optical transmitter due to the parasitic inductance of bonding wire. And we have fabricated an LE-EA-DFB laser module that has a 3-dB bandwidth of 59 GHz thanks to our flip-chip interconnection technique. The fabricated optical receiver, which includes a wideband electrical amplifier, has a 3-dB bandwidth of 51 GHz. Using these optical modules with a 3-dB bandwidth of more than 50 GHz, we have obtained a bit-error rate of less than 2 × 10−4, which is an error-free condition using KP4 forward error correction, under 53.2-Gbaud 4-level pulse amplitude modulation (4-PAM) operation without an equalizer even after a 2-km transmission through single-mode fiber.

Using a lumped-electrode EADFB laser with a modulation bandwidth of ∼59 GHz, we demonstrated single-wavelength single-polarization direct-detection 4-PAM transmission with the record net data rate of 200 Gbit/s.

We fabricated a lumped-electrode type EADFB laser module using a flip-chip interconnection technique which provides a large modulation bandwidth. We obtained clear 56-Gbaud 4-PAM signal after 10-km SMF transmission with no equalizer using this module.

Photodiodes have been commonly used for generation of continuous terahertz (THz) waves. In this paper, we proposed the use of photodiodes also for detection of THz waves in order to realize CW THz spectroscopy system based on 1.55-"m flber-optics. We experimentally demonstrated two detection schemes based on the square-law detection and downconversion, and compared them with respect to sensitivity and dynamic range at 260{420GHz.

Uni-traveling-carrier (UTC) photomixers and Schottky barrier diodes (SBDs) are described for continuous THz-wave generation and detection, respectively. A newly designed narrowband UTC photomixer based on a bowtie antenna was found to have a resonance feature with enhanced THz output at 0.5 THz. Quasi-optical packaging was used to fabricate an SBD detector with 1080 V/W sensitivity at 250 GHz, the highest yet reported for InP-based quasi-optical broadband SBD modules.

We demonstrate the first 4 × 28 Gbaud PAM4 data reception with an integrated APD-ROSA. A minimum receiver sensitivity of ™21.2dBm OMA of the inner-eye was obtained, which enable us to achieve 30-km transmission, assuming a SSMF.

A polarization-independent high-efficiency edge-illuminated refracting-facet photodiode (RFPD) with a uni-traveling-carrier (UTC) structure has been developed. The efficiency is maximized by using a ring-shaped electrode, and the polarization dependence is minimized by designing the layers so that the interferences between the lights reflected from the different interfaces compensate each other. The fabricated RF-UTC-PD shows a responsivity as high as 1.0 A/W with a polarization dependence of less than 0.2 dB. A large 3-dB bandwidth of over 50 GHz at a high operation current of 20 mA is also observed

The degradation behavior of avalanche photodiodes (APDs) with a mesa structure during bias-temperature aging is investigated. The dark-current variation using the optical-beam-induced current (OBIC) imaging technique is analyzed. A rise in the induced photocurrent was observed when the diodes were biased below the reach-through voltage indicating degradation, and the current path localized at the mesa edge of the absorption layer. The APD dark-current deterioration is well correlated with the OBIC observation, and the surface potential change near the mesa-edge surface is the most plausible cause of the deterioration. An APD with a lifetime exceeding 200 000 h at 85 degC, which is a sufficient reliability for receiver applications, has been achieved by preventing the degradation

The unitraveling-carrier photodiode (UTC-PD) is a novel photodiode that utilizes only electrons as the active carriers. This unique feature is the key for its ability to achieve excellent high-speed and high-output characteristics simultaneously. To date, a record 3-dB bandwidth of 310 GHz and a millimeter-wave output power of over 20 mW at 100 GHz have been achieved. The superior capability of the UTC-PD for generating very large high-bit-rate electrical signals as well as a very high RF output power in millimeter/submillimeter ranges can lead to innovations in various systems, such as broadband optical communications systems, wireless communications systems, and high-frequency measurement systems. Accomplishments include photoreceivers of up to 160 Gb/s, error-free DEMUX operations using an integrated UTC-PD driven optical gate of up to 320 Gb/s, a 10-Gb/s millimeter-wave wireless link at 120 GHz, submillimeter-wave generation at frequencies of up to 1.5 THz, and photonic frequency conversion with an efficiency of -8 dB at 60 GHz. For the practical use, various types of modules, such as a 1-mm coaxial connector module, a rectangular-waveguide output module, and a quasi-optic module, have been developed. The superior reliability and stability are also confirmed demonstrating usefulness of the UTC-PD for the system applications.

A compact unitraveling-carrier photodiode (UTC-PD) module with a WR-8 rectangular waveguide output port for operation in the F-band (90-140 GHz) has been developed. A resonating matching circuit integrated with a UTC-PD and a microstrip-line-to-rectangular-waveguide transformer are designed to have high output powers with a wide bandwidth covering the F-band. The module size and configuration are equivalent to those of conventional optoelectronic devices, which enables the use of standard assembly technology. The fabricated module exhibits a record millimeter-wave output power of 17 mW at 120 GHz for a bias voltage of -3 V. The 3-dB bandwidth is as wide as 55 GHz, which fully covers the F-band. An optical input stress test at a photocurrent of 10 mA performed to confirm the long-term stability of the module showed that the dark current stays below 1 /spl mu/A for more than 4000 hours.

In this letter, we demonstrate over 1 mW power generation at 300 GHz with a uni-travelling-carrier photodiode (UTC-PD) packaged in a WR-3 waveguide module. To increase the maximum power, two identical UTC-PDs were monolithically integrated along with a T-junction to combine the power from the two PDs. The UTC-PD module exhibited peak saturated output power of approximately 1.2 mW at 300 GHz with photocurrent of around 20 mA per PD and bias voltage of -3.9 V. In addition, 3 and 10 dB bandwidths were measured to be around 70 GHz or 23% and over 150 GHz or 50%, respectively.

We propose a new hybrid integrated optical front-end module for a 100-Gb/s-class coherent receiver using silica-based planar lightwave circuit (PLC) technology. We have developed an integrated structure composed of multichannel microcollimator optics between PLC waveguides and photodiodes (PDs), and compact hermetically sealed optical-to-electrical converters with a PD array and a transimpedance amplifier array to maintain reliability. We used the technology to fabricate an all-in-one optical front-end module for a dual polarization quadrature phase-shift-keying (DP-QPSK) coherent receiver, and confirmed that the module operated successfully for 112-Gb/s DP-QPSK signal detection.

A K- to Ka-band photonic microwave generator (PMG) consisting of a uni-traveling-carrier photodiode (UTC-PD) and a monolithically integrated bias circuit utilizing a 1/4-wavelength coplanar waveguide is presented. The device exhibits a high-saturation-output power of +14 dBm at 26 GHz for a bias voltage of -4 V. The output power is almost constant within a frequency range from 23 to 29 GHz. The 3-dB down bandwidth of the generator is as wide as 20 GHz, which is in good agreement with the circuit model calculation. Two types of devices, one with and one without a dc-cut capacitor, exhibit almost the same input-output characteristics. The device for the operation in the 38-GHz band is also fabricated, and it shows a maximum millimeter-wave (mm-wave) output power of +9 dBm. The PMG is suitable for the broad-band high-output fiber-radio applications.

This talk reviews our work with avalanche photodiodes (APDs) with unique vertical-illumination structure. The APD features 3-dB bandwidth of 18.5 GHz with multiplied responsivity of 9.1 A/W, satisfying requirements for 100-Gb/s systems. Further high-speed characteristics targeting 50-Gb/s are also discussed.

We achieved a high-performance InAlAs avalanche photodiode (APD) for beyond 25-Gbit/s applications, such as 50-Gbit/s. The large optical tolerance of the APD's vertical-illumination structure makes optical coupling easy compared with waveguide-type structures. The fabricated APD exhibits responsivity of 0.69 A/W with a large 3-dB bandwidth of 30 GHz at a multiplication factor (M) of 4.6, which enables 50-Gbit/s highly sensitive operation.

A multimode waveguide p-i-n photodiode (WGPD) and a distributed baseband amplifier consisting of high-electron mobility transistors (HEMTs) were monolithically integrated on InP substrate using a stacked layer structure for both components. The multimode WGPD has a 3-dB bandwidth of 49 GHz. The distributed baseband amplifier has a 3-dB bandwidth of 47 GHz, though its 0.5-/spl mu/m gate-length HEMTs have modest cutoff frequencies f/sub T//f/sub max/ of 47/100 GHz. The receiver optoelectronic integrated circuit has a bandwidth of 46.5 GHz. It was packaged into a fiber-pig-tailed module, and the WGPD in the module has a high responsivity of 0.62 A/W for 1.55-/spl mu/m wavelength. The module achieves a sensitivity of -22.7 dBm at 40 Gb/s and exhibits a clear eye-opening at 50 Gb/s.

Recent developments in high-speed photodetectors show a trend toward high efficiency and high output voltage. The waveguide photodiode is a powerful tool for achieving both the objectives – it has a bandwidth of more than 100GHz and an efficiency of 50%. The recently developed uni-traveling-carrier structure has boosted the output voltage of the waveguide photodiode up to 1.3V, which is high enough for directly operating a digital circuit. Another trend is toward reducing the receiver cost. As opto-electronics-integrated-circuit technology based on the waveguide photodiode becomes mature and its speed reaches the 40-GHz region, it will play an important role in constructing future high-speed systems at low cost.

A planar lightwave circuit (PLC) platform for optoelectronic hybrid integration shows potential for achieving 10 Gb/s operation. It uses AuSn bump-type bonding pads on a silica layer to decrease parasitic capacitance, which limited the CR time constant in the optical chip assembly region, and two-layer electrical wiring to reduce parasitic inductance, which caused resonance in the electrical circuit region. An arrayed receiver module fabricated by integrating a two-channel monolithic opto-electronic integrated circuit (OEIC) chip on the PLC platform demonstrated a 3 dB-bandwidth of 8 GHz in both channels, which is equal to the bandwidth of the OEIC chip. This shows the feasibility of using this PLC platform for multichannel 10 Gb/s operation. Furthermore, this PLC platform can combine the versatile optical circuit functions of a PLC, such as an arrayed-waveguide grating wavelength multiplexer, with the high-speed signal processing function of mature electronic IC circuits. Consequently, this platform is a key device that will lead to high-capacity optical signal processing systems using optical wavelength/frequency routing.

To realize a DP-QPSK receiver PLC, we heterogeneously integrated eight high-speed PDs on a silica-based PLC platform with a PBS, 90-degree optical hybrids and a VOA. The use of a 2.5%-Δ waveguide reduced the receiver PLC size to 11 mm x 11 mm. We successfully demonstrated 32 Gbaud DP-QPSK signal demodulation with the receiver PLC.

A sub-millimeter(mm)-wave detector module implementing an InP-based zero-biased Schottky-barrier diode (SBD) has been fabricated. The SBD is monolithically integrated with a short-stub resonant matching circuit for increasing the detection sensitivity as well as providing a biasing circuit. The SBD chip is mounted in a compact J-band (WR-3) rectangular-waveguide-input module for practical use. The module exhibits a peak sensitivity at around 350 GHz due to the characteristics of the matching circuit, and a good linearity of the output voltage against the input sub-mm-wave power. A record sensitivity of 1460 V/W is obtained for the InP-based zero-biased SBD at 350 GHz.

This work investigated the field confinement behavior and junction size scaling for an inverted p-down avalanche photodiode, which requires no selective diffusion or ion-implantation and regrowth techniques. With this structure, the small spread of electric field of only 1 µm is obtained. The maximum 3-dB bandwidth of 23 GHz at the multiplication factor of 4.5 is obtained with the diameter of 20 µm, with responsivity as high as 0.91 A/W. These results indicate that the structure is favorable for high-speed and high-sensitivity operation.

We propose an in-band OSNR monitor based on high-speed polarization analysis. We fabricated a high-speed integrated Stokes polarimeter with a PLC-based polarization filter, high-speed photodiodes and trans-impedance amplifiers, and carried out proof-of-concept experiments with 100-Gb/s DP-QPSK signals.

In this letter, we present an experimental characterization of third-order intermodulation distortion (IM3) at 5.8 GHz in a unitraveling-carrier refracting-facet photodiode (UTC-RFPD) and a p-i-n refracting-facet photodiode (pin-RFPD). The IM3 in the pin-RFPD is considerably reduced compared to the waveguide-type p-i-n photodiode. The third-order intercept point (IP3) of the UTC-RFPD does not decrease as the photocurrent increases, indicating that space-charge-induced nonlinearity is well suppressed in this photodiode. The two-tone IP3 of the UTC-RFPD estimated from the measurement reaches a very high level of 40.5 dBm.

This paper describes device and integration technologies for 100-Gbit/s polarization division multiplexed quadrature phase shift keying integrated coherent receiver front-end. The silica-based planar lightwave circuits for the optical passive circuits, InP-based photodiode for the opto-electrical conversion, and InP HBT for the transimpedance amplification are used to achieve high-performance. A module-level hybrid integration technology based on a chip-scale packaged O/E converter is applied as the integration technology. The fabricated receiver front-end has a wide dynamic range of around 20dB with a constant local power of 13.5 dBm and an excellent common-mode rejection ratio of better than -25dB up to 25GHz.

A photonic transceiver module integrating a uni-traveling-carrier photodiode, a Schottky barrier diode, and a planar circulator circuit has been developed for sub-terahertz(THz)-wave reflection-geometry imaging. All the components are assembled in a compact rectangular-waveguide-output package for operation in the J-band (220 - 325 GHz), and continuous sub-THz waves are generated by the photomixing. The frequency dependence of the detection sensitivity of the transceiver module exhibits clear resonant behavior at around 270 GHz. The peak internal signal-to-background (S/B) ratio is measured to be as large as about 10. The characteristic of the fabricated transceiver is evaluated by measuring two-dimensional images of a test sample at frequencies from 240 to 310 GHz. Although the image resolution degrades with signal frequency deviation from the resonant condition due to the S/B ratio decrease, it is confirmed that a practical contrast can be obtained for a bandwidth of about 40 GHz even with an S/B ratio below one. Based on these results, the reflection-geometry in-vivo imaging of a human finger at 270 GHz is successfully demonstrated.

We report the design and fabrication of a hybrid-integration-type coherent receiver. We optimize the receiver building blocks, and achieve a -25-dB common-mode rejection ratio and a 20-dB signal input power dynamic range.

We demonstrate the first heterogeneous integration of high-speed InP PDs on a silica-based PLC platform. 28 Gbaud QPSK signal detection is realized using a coherent detector integrated with four PDs and a 90° optical hybrid.

A compact sub-terahertz transceiver module integrating a uni-traveling-carrier photodiode, a Schottky barrier diode, and a planar circulator circuit has been developed. A 180-degree rat-race hybrid circuit fabricated on a quartz substrate is employed as the circulator. All the components are assembled in a compact rectangular-waveguide-output package for operation in the J-band. The frequency dependence of the detection sensitivity of the transceiver module showed clear resonant behavior at around 270 GHz while the output power showed no distinct resonance. The insertion loss of the circulator for the input and output signals at 270 GHz were estimated to be 4.6 dB and 11.4 dB, respectively. The isolation between the output port and the SBD port of the circulator was evaluated to be 14.3 dB at 270 GHz, indicating that the fabricated module functioned properly as a transceiver. The linearity of the output power and the input sensitivity of the transceiver were also confirmed to be very good. This is the first demonstration of a photonic mm-wave transceiver module for continuous waves operating at around 300 GHz.

We present a terahertz wave wireless link operating at 300 GHz which has a potential for use in ultra fast future wireless services in short range. Terahertz wave was generated and modulated with photonic technologies in the transmitter, allowing us to use radio on fiber system concept as well. For the receiver, we used a Schottky barrier diode detector integrated with a planar antenna. With the link, error free data transmission at 12.5 Gbps was experimentally demonstrated. Taking the performance margin of the transmitter and receiver into consideration, we believe that even up to 20-Gbps data can be transmitted.

We fabricated a 40-Gbit/s CAN-type ROSA using a maximized-induced-current photodiode (MIC-PD). 40-Gbit/s error-free operation was achieved for the first time with a CAN-type ROSA, owing to the high-speed capability of the MIC-PD and impedance matching of glass feedthrough.

Spectroscopic images were obtained using a system combining a tunable continuous-wave terahertz laser with an InP-based Schottky barrier diode detector integrating a broadband log-periodic antenna. The distribution of polymorphic forms in pharmaceutical tablets was observed in obtained at images over 1 THz.

We fabricated 25-Gbit/s receiver optical subassemblies (ROSAs) using a maximized-induced-current photodiode (MIC-PD). By employing a high-speed and high-responsivity MIC-PD, we achieved successful operation at 25 Gbit/s using a flexible printed circuit board and a coaxial package designed for a 10-Gbit/s ROSA.

A novel maximized-induced-current-photodiode (MIC-PD) structure with a composite field depletion layer achieves high responsivity of 0.8 A/W and a wide 3-dB bandwidth of 30 GHz at a low reverse bias voltage of 2 V for optical input power of +7 dBm.