Optical Signal Processing by Phase Modulation and Subsequent Spectral Filtering Aiming at Applications to Ultrafast Optical Communication Systems

This paper describes optical signal processing based on optical phase modulation and subsequent optical filtering, which is applicable to 160-Gb/s optical time-division multiplexed (OTDM) subsystems. Ultrafast phase modulation of an optical signal is done by self-phase modulation (SPM) and cross-phase modulation (XPM) when an optical pulse passes through a nonlinear optical fiber. Such phase modulation induces the spectral shift of the optical signal. Various types of optical signal processing are realized simply by filtering out the spectral-shifted component. Using SPM-based pulse reshaping in a 500-m-long silica-based highly nonlinear fiber (HNLF), we demonstrate highly stable generation of a 10-GHz 2-ps optical pulse train tunable over the entire C band. A phase-locked loop (PLL) can suppress the slow phase drift of the output pulse train induced by fluctuations of the nonlinear fiber length, enabling the application of the pulse generator to a 160-Gb/s OTDM transmitter. Based on XPM in a 2-m-long photonic crystal fiber, optical time-division demultiplexing of 160-Gb/s optical signals is demonstrated. The long-term stability is drastically improved as compared with the device composed of a conventional silica-based HNLF, because the short fiber length reduces the phase fluctuation between the signal and control pulses. Instead of nonlinear fibers, an electrooptic modulator such as a (LN) modulator also performs the phase modulation in a more practical manner. We propose and demonstrate an optoelectronic time-division demultiplexing scheme for a 160-Gb/s OTDM signal, which consists of an LN phase modulator driven by a 40-GHz electrical clock and an optical bandpass filter (BPF). We also demonstrate base-clock recovery from a 160-Gb/s optical signal with an optoelectronic PLL. The phase comparator is simply composed of an LN phase modulator and an optical BPF, ensuring the stable and reliable operation in the 160-Gb/s receiver.     

Highly Stable 160-Gb/s OTDM Technologies Based on Integrated MUX/DEMUX and Drift-Free PLL-Type Clock Recovery

This paper reports 160-Gb/s optical time-division-multiplexed (OTDM) technologies including an all-optical integrated multiplexer (MUX) providing all-channel independent modulation, an all-optical integrated demultiplexer (DEMUX) that offers all-channel simultaneous demultiplexing, and a drift-free phase-locked-loop (PLL)-type clock recovery circuit for ultrahigh-speed OTDM signals. We present the configuration of each technology and the results of experiments on those technologies. Highly stable operation is successfully demonstrated by using a MUX based on periodically-poled lithium niobate (PPLN) hybrid integrated planer lightwave circuit (PLC), a DEMUX based on semiconductor optical amplifier hybrid integrated PLC, and clock recovery circuit based on a PLL with an optical phase modulator and a PPLN waveguide     

Terabus: Terabit/Second-Class Card-Level Optical Interconnect Technologies

In the “Terabus” optical interconnect program, optical data bus technologies are developed that will support terabit/second chip-to-chip data transfers over organic cards within high-performance servers, switch routers, and other intensive computing systems. A complete technology set is developed for this purpose, based on a chip-like optoelectronic packaging structure (Optochip), assembled directly onto an organic card (Optocard). Vertical-cavity surface emitting laser (VCSEL) and photodiode arrays (4$,times,$12) are flip-chip bonded to the driver and receiver IC arrays implemented in 0.13-$mu$m CMOS. The IC arrays are in turn flip-chip assembled onto a 1.2-cm$^2$silicon carrier interposer to complete the transmitter and receiver Optochips. The organic Optocard incorporates 48 parallel multimode optical waveguides on a 62.5-$mu$m pitch. A simple scheme for optical coupling between the Optochip and the Optocard is developed, based on a single-lens array etched onto the backside of the optoelectronic arrays and on 45$^circ$mirrors in the waveguides. Transmitter and receiver operation is demonstrated up to 20 and 14 Gb/s per channel, respectively. The power dissipation of 10-Gb/s single-channel links over multimode fiber is as low as 50 mW.     

Pulse generation for soliton systems using lithium niobatemodulators

We describe the principle of operation and performance of several soliton pulse sources and also a complete soliton transmitter based on lithium niobate modulators. Subsystems based on lithium niobate modulators are attractive because the modulators are now commercially available, qualified for system use, can operate up to very high speeds, and can operate over a wide wavelength range. The pulse sources we describe are based on two techniques. The first is the chirped pulse compression technique in which one or two sinusoidally driven modulators generate frequency chirped pulses that are subsequently compressed to the desired width using dispersion in a fiber. In the second technique, sinusoidally driven modulators are cascaded serially to form pulses. Using these techniques we produced nearly transform-limited pulses at repetition rates up to 15 GHz with a FWHM pulsewidths from 10-33% of the pulse period. A complete soliton transmitter using a single modulator to simultaneously generate optical pulses and encode data is also discussed. The performance of this compact transmitter in a 2.5-Gb/s soliton system experiment is comparable to other more common soliton transmitters     

Fully stabilized electroabsorption-modulated tunable DBR lasertransmitter for long-haul optical communications

We report on a fully functional 2.5-Gb/s electroabsorption (EA)-modulated wavelength-selectable laser module meeting all long-haul transmission requirements for stability, chirp, power and linewidth over 20 channels on a 50-GHz grid. Based on a highly integrated InP chip comprising a distributed Bragg reflector (DBR) laser, semiconductor optical amplifier, power monitor, and EA-modulator, the compact transmitter module also contains optics and control circuits necessary to ensure simultaneous long-term wavelength and mode stability. We have achieved 2.5-Gb/s transmission on all 20 channels over 680 km of standard fiber     

Large-scale photonic integrated circuits

100-Gb/s dense wavelength division multiplexed (DWDM) transmitter and receiver photonic integrated circuits (PICs) are demonstrated. The transmitter is realized through the integration of over 50 discrete functions onto a single monolithic InP chip. The resultant DWDM PICs are capable of simultaneously transmitting and receiving ten wavelengths at 10 Gb/s on a DWDM wavelength grid. Optical system performance results across a representative DWDM long-haul link are presented for a next-generation optical transport system using these large-scale PICs. The large-scale PIC enables significant reductions in cost, packaging complexity, size, fiber coupling, and power consumption.     

Optical receivers for optoelectronic VLSI

We describe our work on the design and testing of optical receivers for use in optoelectronic VLSI. The local nature of the optoelectronic VLSI system permits novel receiver designs, incorporating multiple optical beams and/or synchronous operation, while the requirement of realizing large numbers of receivers on a single chip severely constrains area and power consumption. We describe four different receiver designs, and their different operating modes. Results include 1-Gb/s high-impedance, two-beam diode-clamped FET-SEED receivers, single and dual-beam transimpedance receivers realized with a hybrid attachment of multiple-quantum well devices to 0.8-μm linewidth CMOS operating to 1 Gb/s, and synchronous sense-amplifier-based optical receivers with low (~1 mW) power consumption. Finally, we introduce a measure of receiver performance that includes area and power consumption     

Performance of 20 Gb/s quaternary intensity modulation based on binary or duobinary modulation in two quadratures with unequal amplitudes

2/spl times/10 Gb/s quaternary intensity modulation signals (4-IM) can be generated by combining two modulation signals with unequal amplitudes in quadrature phases or orthogonal polarizations. Two 10-Gb/s nonreturn-to-zero (NRZ) amplitude-shift keying (ASK) signals and a quadrature phase-shift keying (QPSK) modulator allow to generate 4-IM with the same bandwidth as an NRZ-ASK signal (QASK). Measured sensitivity at a bit error rate (BER) of 10/sup -9/ and chromatic dispersion (CD) tolerance are -21.6 dBm and /spl sim/+130 ps/nm, respectively. Two duobinary 10-Gb/s data streams and a QPSK modulator allow to generate a 9-constellation point quaternary intensity signal (QDB), with the same bandwidth as a duobinary signal. A stub filter with frequency response dip at 5 GHz was used to generate the duobinary signals. Detected as a 4-IM, this scheme features a sensitivity and a CD tolerance of -21.2 dBm and /spl sim/+140 ps/nm, respectively. By combining the two duobinary 10-Gb/s data streams with unequal amplitudes in orthogonal polarizations, a 9-constellation point quaternary intensity signal was also obtained (QPolDB). Sensitivity and CD tolerance were -20.5 dBm and /spl sim/+340 ps/nm, respectively. They became -18.4 dBm and /spl sim/+530 ps/nm, respectively, when the frequency response dip of the stub filter was changed to 6 GHz. A polarization and phase-insensitive direct detection receiver with a single photodiode has been used to detect all generated quaternary signals as 4-IM signals.     

High-Speed OTDM and WDM Networks Using Traveling-Wave Electroabsorption Modulators

We describe the design and performance of a number of elements based on traveling-wave electroabsorption modulators (TW-EAMs) in optical time-division-multiplexing (OTDM) and wavelength-division-multiplexing (WDM) networks. The incorporation of traveling-wave (TW) electrode design into electroabsorption modulators (EAMs) relieves the resistance-capacitance (RC ) bandwidth limitation common to lumped components, enabling higher operation speed without shortening the device. As a result, high-speed operation can be combined with essential modulator characteristics such as modulation efficiency and extinction ratio. While significant modulation bandwidth has been achieved, a lesser known aspect is that the TW electrode also provides an extra dimension for improving and enabling functionalities beyond broadband modulation. This new dimension originates from the distributed effect of the TW design and its interactions with distinctive EAM properties. This paper reviews such developments in recent years with specific applications for optical signal processing in OTDM and WDM networks. The covered functionalities include various optical gating operations for OTDM, regenerative wavelength conversions for WDM, and clock recovery     

Development of Packaging Technologies for High-Speed ($≫40$Gb/s) Optical Modules

We developed high-speed optoelectronics packaging technologies for a waveguide photodiode and a traveling wave electro-absorption modulator device for 40-Gb/s digital communication systems. The effects of the device and the packaging designs on the broadband performance were investigated to optimize broadband characteristics. For the receiver, inductive peaking was used for bandwidth control and an internal bias tee was implemented; in addition, two types of preamplifier devices were used to develop high-gain receiver and wide-bandwidth receiver. In the optical-to-electrical response, a 3-dB bandwidth of the high-gain module was about 32 GHz as compared to 42 GHz for the wide-bandwidth module. The clear 40-Gb/s nonreturn-to-zero (NRZ) eye diagrams showed a good system applicability of these modules. In addition, an optimized modulator module showed a 3-dB bandwidth of 38 GHz in the electrical-to-optical response, an electrical return loss of less than 10 dB at up to 26 GHz, an rms jitter of 1.832 ps, and an extinction ratio of 5.38 dB in a 40-Gb/s NRZ eye diagram.     

40-Gb/s Widely Tunable Transceivers

We present the first monolithic widely tunable 40-Gb/s transceivers. The devices integrate sampled grating distributed Bragg reflector (SG-DBR) lasers, quantum-well electroabsorption modulators (EAM), low-confinement semiconductor optical amplifiers (SOA), and uni-traveling carrier (UTC) photodiodes for state-of-the-art light generation, modulation, amplification, and detection. A relatively simple high-flexibility fabrication scheme combining quantum-well intermixing (QWI) and blanket metal-organic chemical vapor deposition (MOCVD) regrowth was used to integrate components with performance rivaling optimized discrete devices. The SG-DBR/EAM transmitters demonstrate 30 nm of tuning, 39-GHz bandwidth, low-drive voltage, and low power penalty 40-Gb/s transmission through 2.3 km of fiber. The SOA/UTC photodetector receivers provide 23-28 dB of gain, saturation powers up to 18.6 dBm, and -20.2 dBm of chip-coupled sensitivity at 40 Gb/s. By connecting the transmitters and receivers off-chip, we demonstrate 40-Gb/s wavelength conversion     

Phase-diversity homodyne detection of multilevel optical modulation with digital carrier phase estimation

This paper describes a phase-diversity homodyne receiver that which can cope with multilevel modulation formats. The carrier phase drift is estimated with digital signal processing (DSP) on the homodyne-detected signal, entirely restoring the complex amplitude of the incoming signal. Our DSP-based phase-estimation scheme consists of a simple and demultiplexable architecture that allows the system to reach significantly higher performance than conventional optical delay detection. Since the whole optical signal information is preserved with our receiver, various kinds of postprocessing of the received signal become possible. For example, we can demultiplex wavelength-division/optical time-division multiplexed channels and compensate for group velocity dispersion of fibers as well as the nonlinear phase noise in the electrical domain. We also experimentally evaluate the performance of our receiver. Our offline bit-error rate experiments show the feasibility of transmitting polarization-multiplexed 40-Gb/s quadrature phase-shift keying signals over 200 km with channel spacing of 16 GHz, leading to spectral efficiency of 2.5 b/s/Hz.     

Monolithic Wavelength Converters for High-Speed Packet-Switched Optical Networks

This paper describes the design and demonstration of advanced 40-Gb/s return-to-zero (RZ) tunable all-optical wavelength converter technologies for use in packet-switched optical networks. The device designs are based on monolithic integration of a delayed interference Mach-Zehnder interferometer (MZI) semiconductor optical amplifier (SOA) wavelength converter with a sampled-grating distributed Bragg reflector tunable laser and an on-chip waveguide delay. Experimental results are presented demonstrating error-free wavelength conversion with 1-dB power penalty at 40-Gb/s data rates. By incorporating label modulation functionality on-chip along with a fast tunable 40-Gb/s wavelength converter, fully monolithic packet-forwarding chips are realized that are capable of simultaneous error-free wavelength conversion of 40-Gb/s payloads, remodulation of 10-Gb/s packet headers, and data routing through fast wavelength switching     

Reliability of InGaAs waveguide photodiodes for 40-Gb/s optical receivers

The reliability of 1.55-/spl mu/m wavelength InGaAs waveguide photodiodes (WGPDs) fabricated by metal-organic chemical vapor deposition is investigated for 40-Gb/s optical receiver applications. Reliability for both high-temperature storage and accelerated life tests obtained by monitoring both the dark current and the breakdown voltage is examined. The median device lifetime and the activation energy of the degradation mechanism are extracted for WGPD test structures. The device lifetimes are examined via statistical analysis which is highly reliable in predicting the device lifetime under practical conditions. The degradation mechanism for the WGPD test structures can be explained by the formation of leakage current path by ionic impurities in the passivation layer on the exposed p-n junction. Nevertheless, it can be concluded that the WGPD test structures exhibit sufficient reliability for practical 40-Gb/s optical receiver applications.     

Benefit of SPM-based all-optical reshaper in receiver for long-haul DWDM transmission systems

The effectiveness of a self-phase modulation (SPM)-based all-optical reshaper with optically time-division-demultiplexing receiver was experimentally investigated using 42.7-Gb/s carrier-suppressed return-to-zero (CS-RZ) signals. We have confirmed that this scheme is quite effective to suppress the waveform degradation due to optical signal bandlimitation. We have demonstrated 80% spectral efficiency without using polarization demultiplexing by using the all-optical reshaper. We have also demonstrated 50-GHz-spaced 55/spl times/42.7 Gb/s signals transmission over 2500 km, using an optically bandlimited CS-RZ signal and the SPM-based all-optical reshaper in receiver without using polarization demultiplexing. A Q-factor improvement of about 1.5 dB was obtained by using the all-optical reshaper.     

A 43-Gb/s clock and data recovery OEIC integrating an InP-InGaAs HPT oscillator with an HBT decision circuit

This paper presents a 43-Gb/s clock and data recovery (CDR) optoelectronic integrated circuit (OEIC) that consists of a 43-GHz heterojunction phototransistor (HPT) oscillator as an optoelectronic clock recovery circuit and a 40-Gb/s-class heterojunction bipolar transistor (HBT) decision circuit. The layer and fabrication process of the HPT and HBT are fully compatible, and the HPT has a photocoupling window in the emitter electrode for optical access from the top. When the HPT is directly illuminated, the HPT oscillator successfully extracts a 43-GHz electrical clock signal from a 43-Gb/s optical data stream by itself. The OEIC regenerates the data signal input into the HBT decision circuit by using the electrical clock signal optoelectronically extracted by the HPT oscillator. The CDR OEIC achieves error-free operation for a 2/sup 31/-1 PRBS data signal. The power dissipation of the OEIC is only 0.79 W, which is less than half that of a fully electrical 40-Gb/s-class CDR IC. This is the first successful demonstration of HPT-based OEICs integrated with HBT digital circuits operating at such a high bit rate.     

Simple Elegant Optical Coupling Method for 20-GHz and 40-b/s Photoreceiver

In this paper, we describe a new design of a lens-tip fiber which facilitates the optoelectronic packaging of wideband photoreceivers for 10.7-Gb/s return-to-zero (RZ) and 40-Gb/s nonreturn-to-zero (NRZ) applications. After a brief presentation of what a photoreceiver is and what are the packaging specifications required, we describe the optical coupling technology used here. Finally, we demonstrate its efficiency on a 20-GHz-bandwidth photoreceiver.     

Monolithically integrated eight-channel WDM modulator with narrow channel spacing and high throughput

We demonstrate an eight-channel wavelength division multiplexing (WDM) modulator module that monolithically integrates arrayed waveguide gratings and semiconductor optical amplifiers and electroabsorption optical modulators arrays. The compact module can generate individual optical signals for each WDM channel with low optical and electrical crosstalk. We show two configurations for the narrow channel spacing of 25 GHz and high throughput of beyond 80 Gb/s. Combining this WDM modulator with a multi-wavelength light source is a promising approach to creating a compact WDM optical transmitter.     

High-speed optoelectronic VLSI switching chip with >4000 opticalI/O based on flip-chip bonding of MQW modulators and detectors tosilicon CMOS

We present the first high-speed optoelectronic very large scale integrated circuit (VLSI) switching chip using III-V optical modulators and detectors flip-chip bonded to silicon CMOS. The circuit, which consists of an array of 16×1 switching nodes, has 4096 optical detectors and 256 optical modulators and over 140K transistors. All but two of the 4352 multiple-quantum-well diodes generate photocurrent in response to light. Switching nodes have been tested at data rates above 400 Mb/s per channel, the delay variation across the chip is less than ±400 ps, and crosstalk from neighboring nodes is more than 45 dB below the desired signal. This circuit demonstrates the ability of this hybrid device technology to provide large numbers of high-speed optical I/O with complex electrical circuitry     

Demonstration of negative dispersion fibers for DWDM metropolitanarea networks

We present a detailed experimental and theoretical study, showing that a novel nonzero dispersion-shifted fiber with negative dispersion enhances the capabilities of metropolitan area optical systems, while at the same time, reducing the system cost by eliminating the need of dispersion compensation. The performance of this dispersion-optimized fiber was studied using different types of optical transmitters for both 1310- and 1550-nm wavelength windows and for both 2.5-and 10-Gb/s bit rates. It is shown that this new fiber extends the nonregenerated distance up to 300 km when directly modulated distributed feedback (DFB) laser transmitters at 2.5 Gb/s are used. The negative dispersion characteristics of the fiber also enhance the transmission performance in metropolitan area networks with transmitters that use electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers, which are biased for positive chirp. In the case of 10 Gb/s, externally modulated signals (using either EA-DFBs or external modulated lasers using Mach-Zehnder modulators), we predict that the maximum reach that can be accomplished without dispersion compensation is more than 200 km for both 100- and 200-GHz channel spacing. To our knowledge, this is the first demonstration of the capabilities of a nonzero dispersion-shifted fiber with negative dispersion for metropolitan applications