Clock & Data Recovery

An investigation into the effects of varying levels of chromatic dispersion on a Mode-Locked Laser Diode optical clock recovery process is presented. Results demonstrate that this technique is invariant to input dispersion varying between ±75ps/nm.

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In this work, we numerically investigate the performances of optical regenerators based on self-phase modulation and spectral offset filtering at 40 Gbit/s. We outline the different effects affecting the device performances and explain the choice of the optimal working power. The impact of the regenerator on the output signal is also analysed through a statistical approach. Both single and double stage configurations are investigated.

We report on the impact of the sampling-source extinction ratio (ER) in a linear optical sampling system. To this effect, an analytical model is developed and contrasted with experimental data and numerical simulation, to determine the operational limits of the monitoring system. The ER of the reconstructed signal consists of a combined contribution from the magnitude of the symbol under test and the magnitude of the product between its neighboring symbols and the corresponding subpulses of the sampling source. The sampled signal can be completely reconstructed from its samples for ERs of the sampling source above 35 dB.

Basic principles of all-optical signal regeneration are presented, and main state-of-art techniques are reviewed. Optical fiber and semiconductor based devices are addressed, and some recently reported 2R and 3R signal regeneration experiments are discussed.

Optical switching offers the potential to significantly scale the capacity of data center networks (DCN) with a simultaneous reduction in switching time and power consumption. Previous research has shown that end-to-end switching time, which is the sum of the switch configuration time and the clock and data recovery (CDR) locking time, should be kept within a few nanoseconds for high network throughput. This challenge of low switching time has motivated research into fast optical switches, ultra-fast clock and amplitude recovery techniques. Concurrently, the data rate between server-to-server and server-to-switch interconnect is increasing drastically from the current 100 Gb/s (4×25 Gb/s) to 400 Gb/s and beyond, motivating the use of high order formats such as 50-GBaud four-level pulse-amplitude modulation (PAM-4) for signalling. Since PAM-4 is more sensitive to noise and distortion, digital equalizers are generally needed to compensate for impairments such as transceiver frequency rolloff, dispersion and optical filtering, adding additional time for equalizer adaptation and power consumption that are undesired for fast optical switching systems. Here we propose and investigate an equalizer state caching technique that reduces equalizer adaptation time and computation power consumption for fast optical switching systems, underpinning optically-switched DCNs using high baud rate and impairment-sensitive formats. Through a proof-of-concept experiment, we study the performance of the proposed equalizer state caching scheme in a three-node optical switching system using 56 GBaud PAM-4. Our experimental results show that the proposed scheme can tolerate up to 0.8-nm (100-GHz) instantaneous wavelength change with an adaptation delay of only 0.36 ns. Practical considerations such as clock phase misalignment, temperature-induced wavelength drift, and equalizer precision are also studied.

This paper presents an experimental performance characterization of all-optical subsystems at 40 Gb/s using interconnected hybrid integrated all-optical semiconductor optical amplifier (SOA) Mach-Zehnder interferometer (MZI) gates and flip-flop prototypes. It was shown that optical gates can be treated as generic switching elements and, when efficiently interconnected, can form larger and more functional network subsystems. Specifically, this paper reports on all-optical subsystems capable of performing on-the-fly packet clock recovery, 3R regeneration, label/ payload separation, and packet routing using the wavelength domain. The all-optical subsystems are capable of operating with packet-mode traffic and are suitable for all-optical label-switched and self-routed network nodes. The intelligent functionality offered, combined with the compactness and stability of the optical gates, verifies the potential that all-optical technology can find application in future data-centric networks with efficient and dynamic bandwidth utilization. This paper also reports on the latest photonic integration breakthroughs as a potential migration path for reducing fabrication cost by developing photonic systemson-chip utilizing multiple SOA-MZI optical gates on a single chip.

In this letter, we demonstrate clock extraction from 10-Gb/s asynchronous short data packets. Successful clock acquisition is achieved from data packets arriving at time intervals of only 1.5 ns, irrespective of their precise phase relation. The clock recovery circuit used consists of a Fabry-Pérot filter and an ultrafast nonlinear interferometer gate and requires very short time for synchronization.

This paper presents a 1.5 to 10 Gb/s SATA/SAS/FC receiver in 65 nm CMOS. The multiple constraints set by industry standards ask for a receiver architecture capable of simultaneously addressing channel loss impairments, high frequency-difference tracking and low serial to parallel latency. An adaptive 3-tap DFE data recovery is based on a direct-feedback topology to provide a continuous equalized signal assuring a robust clock-data self alignment. A latch-based DFE topology has been developed to overcome the classical DFE feedback loop-delay issue. A digital early-late clock recovery has been proven for 5000 ppm SSC tracking. Extensive digital features allow self-calibration and internal eye analysis. The device, realized in a 65 nm technology, supports more than 36 FR4 at 6 Gb/s with SSC and 28 at 8.5 Gb/s while keeping 0.4 UI of additional sinusoidal jitter tolerance, consuming 140 mW from 1 V.

We present an all-optical multi-wavelength clock recovery circuit for operation with short optical data packets at 40 Gb/s and 160 Gb/s. The proposed scheme comprises a low-finesse Fabry-Perot Filter (FPF) followed by a Quantum-Dot Semiconductor Optical Amplifier (QD-SOA) and exploits the memory properties of the passive filtering element and the nonlinear gain characteristics of the homogeneously broadened spectral regions within the QD-SOA in order to acquire the clock signal of multiple incoming wavelengths on a per packet basis. Simulation-based performance analysis shows successful operation both at 40 Gb/s and 160 Gb/s with data packets at four wavelengths entering simultaneously the circuit, yielding a packet clock signal for every incoming data packet with only a few bits rise time and with significantly reduced timing jitter compared to the jitter of the original data.

We present a novel scheme for high-speed multiwavelength clock recovery using a low finesse Fabry-Pérot filter and a quantum-dot semiconductor optical amplifier. Performance evaluation based on extensive numerical analysis reveals that the proposed configuration can acquire clock from 40-and 160-Gb/s input data packet streams at four wavelengths with very short rise-and fall-times.

transmultiplexing and all-optical dual-wavelength regeneration at the OTDM rate are presented. By using the asynchronous retiming scheme, we achieve error-free buffer-less data grooming with time-slot interchange (TSI) capability for OTDM meshed networking. We demonstrate excellent performance from the system, discuss scalability, applicability, and the potential reach of the asynchronous retiming scheme for transparent OTDMdomain interconnection. Index Terms—Optical communication, synchronization, optical signal processing, time division multiplexing, ultrafast optics. A I.

In this paper we present some characterizations of passively mode-locked semiconductor lasers. These lasers are multimode and they exhibit a modulation of their output power even though they are DC-biased. The modulation frequency corresponds to their free-spectral range. We demonstrate their potential: for generation of ultra-fast modulation for THz wave generation at room temperature without any direct modulations applied, in the radio frequency range they synchronise their modulation to the bit-rate of an incoming signal used for clock extraction. Therefore these devices can be used for all-optical clock recovery.

An experimental analysis of the timing jitter associated with sub-picosecond pulses produced from a quantum dash passively mode-locked Fabry-Pérot laser is produced. Moreover, application as an all-optical clock recovery function for a 40Gb/s data rates with non-return-to-zero (NRZ) modulation format is demonstrated using this laser.

We report, to the best of our knowledge, the first demonstration of 320 Gb/s all-optical clock recovery and all-optical time de-multiplexing after 51 km transmission by exploiting single-quantum dash mode-locked laser diode (QD-MLLD). Based on injection locking of the QD-MLLD, the 40 GHz synchronized optical clock pulses were recovered from the 320 Gb/s with a pulse width of 1.9 ps and timing jitter of 135 fs, which allowed directly time de-multiplexing of 320-40 Gb/s without additional complex optoelectronic circuitry. The 320-40 Gb/s all-optical de-multiplexing was achieved with averaging a power penalty of 4.5 dB at BER of 1E-6.

Wavelength tunability of an all-optical clock recovery operation based on a quantum dash mode-locked Fabry-Pérot laser diode is experimentally investigated. Synchronization of the device to the injection of 40 Gb/s NRZ incoming data is assessed by analyzing both the carrier-to-noise ratio and the linewidth of the 40 GHz beat-tones measured at the mode-locked laser output. Under optical injection, beat-tone linewidths below 10 Hz are measured. Recovered clock pulses featuring a width of 1.6 ps are obtained irrespective of the wavelength detuning between the laser spectra and the optical carrier of the incoming data stream.

It is experimentally shown an original optical pulse generator able to produce pulses at MHz rate with few ns timewidth at 1/3 of input period. It is based on the self-switched Semiconductor Laser Amplifier Loop Mirror (SLALOM) configuration. The input is a sinusoidal signal in the C-band and the output is amplified pulses. Numerical simulations are also carried out and shown that GHz & ps pulse generation is possible. Besides their usefulness as an optical pulse generator, application in self-sampling is also discussed.

We investigate the behavior of a Ge-on-Si light source and demonstrate its feasibility for polarization-encoded discrete-variable quantum key distribution following the BB84 protocol, enabling a potential "all-silicon" QKD scheme which can operate well below the necessary QBER limit and successfully generate secret keys.

A novel scheme of optical label encoding by wavelength conversion based on electroabsorption modulators (EAMs) is reported. Based on the experimental observations, the chirp properties of the wavelength-converted signal are discussed and a wide dynamic range of the chirp -parameter is found allowed. Compared with cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA), the EAM has several advantages, which make it attractive for optical label encoding or other applications as a wavelength converter.

We describe recent results of the Advanced Research Projects Agency (AFWA) sponsored Consortium on Wideband All-Optical Networks which is developing architectures, technology components, and applications for ultrafast 100 Gb/s timedivision multiplexing (TDM) optical networks. The shared-media ultrafast networks we envision are appropriate for providing low-access-delay bandwidth on demand to both future highburst rate (100 Gb/s) users as well aggregates of lower-rate users (i.e., a heterogeneous user population). To realize these goals we are developing ultrafast network architectures such as HLAN, described here, that operate well in high-latency environments and require only limited processing capability at the ultrafast bit rates. We also describe results on SO-Gb/s, 90-km soliton transmission, lOO-Gb/s soliton compression laser source technology, picosecond short-pulse fiber ring lasers, picosecondaccuracy optical bit-phase sensing and clock recovery, all-optical injection-locked fiber figure-eight laser clock recovery, shortpulse fiber loop storage, and all-optical pulse width and wavelength conversion. I sponsored Consortium made up of AT&T Bell Laboratories, Digital Equipment Corporation, and the Massachusetts Institute of Technology was formed to develop architectures and technologies for future high-speed high-capacity wavelengthdivision multiplexing (WDM) optical networks and to develop architecture and technology for ultrafast, 100 Gb/s, timedivision multiplexing (TDM) networks. This paper describes our optical TDM architecture and details key technology milestones toward realization of this system. A second separate paper in this special issue addresses Consortium work on WDM networks.

This paper demonstrates optical 3R regeneration distance is demonstrated utilizing a reconfigurable for a 10 Gb/s nonreturn-to-zero (NRZ) data format. recirculating loop transmission setup that facilitates Semiconductor optical amplifier based Mach-Zehnder transmission with variable 3R regeneration spacings. interferometer (SOA-MZI) wavelength converters, synchronous modulator, and Fabry-Perot filter (FPF) are used for 3R regenearation including all-optical clock 2. Experimental Setup recovery. Recirculating loop transmission experiments compare transmission performances with different 3R 2.1 Reconfigurable Recirculating Transmission Setup regenation spacings. Transmission with the 3R regenerator shows significant performance improvement over that Figure 1 shows the setup for recirculating loop transmission without 3R regeneration in terms of eye diagram, Q-factor, to evaluate transmission performance with cascaded 3R and transmission distance. Q-factors around 20 dB are regenerators. The transmitter generates 10 Gb/s NRZ obtained up to a transmission distance of 5000 km when the signal. The 100-km dispersion-compensated transmission 3R regeneration spacing is 100 or 500 km. line consists of two sets of 50-km LEAF fibers followed by a two-stage erbium-doped fiber amplifier (EDFA) and a dispersion compensating fiber (DCF). To reduce optical Keywords: All-optical clock recovery, Fabry-Perot filter, signal-to-noise ratio (OSNR) degradation while maintaining nonreturn-to-zero format, optical regeneration, the linear optical transmission regime, the input power semiconductor optical amplifier based Mach-Zehnder levels to the LEAF and DCF fibers are set moderately high interferometer at 5 dBm and -3 dBm, respectively. The measured OSNR and the estimated residual dispersion at the 3R regenerator input are 37.7dB (0.1 nm bandwidth) and 22 ps/nm, 1. Introduction respectively. Three acoustic optical modulator (AOM)

We demonstrate an all-optical clock-and-data recovery technology for 10-Gb/s NRZ-DPSK signals. With a relatively simple configuration, the clock-recovery scheme achieves less than 1.5-ps RMS jitter for signals after fiber transmission. The pattern-dependence of data-recovery is within 0.5-dB at BER 10 -9 .

This paper proposes and demonstrates a simulation model to systematically investigate jitter accumulations in cascaded all-optical 2R regenerators. The simulation results indicate that when the pattern dependence from the memory effect is minimized, the jitter accumulation depends critically on the degree of the regenerative nonlinearity. Studies of tradeoffs between the jitter from bandwidth limitation and the signal-to-noise-ratio degradation help identify the optimized regenerator bandwidth for various degrees of regenerative nonlinearity. The simulation then considers the pattern dependence from the memory effect and finds that it can severely degrade the cascadability of an optical 2R regenerator and can make it worse than that of a linear optical amplifier (optical 1R). The simulation results show good matches to the experimental results of an optical 2R regenerator based on a semiconductor optical amplifier based Mach-Zehnder interferometer. To overcome the jitter accumulation associated with the optical 2R regeneration, we experimentally demonstrate an optical 3R regenerator for optical nonreturn-to-zero signals with all-optical clock recovery. The experiments achieve more than 1000-hop cascadability for pseudorandom binary sequence 2 31 1 inputs with a 100-km recirculation loop using lab fiber. Field trial experiments then demonstrate a more than 1000-hop cascadability for a 3R spacing of 66 km and a 100-hop cascadability for a 3R spacing of 264 km.

This paper proposes and demonstrates optical 3R regeneration techniques for high-performance and scalable 10-Gb/s transmission systems. The 3R structures rely on monolithically integrated all-active semiconductor optical amplifier-based Mach-Zehnder interferometers (SOA-MZIs) for signal reshaping and optical narrowband filtering using a Fabry-Pérot filter (FPF) for all-optical clock recovery. The experimental results indicate very stable operation and superior cascadability of the proposed optical 3R structure, allowing error-free and low-penalty 10-Gb/s [pseudorandom bit sequence (PRBS) 2 23 -1] return-to-zero (RZ) transmission through a record distance of 1 250 000 km using 10 000 optical 3R stages. Clock-enhancement techniques using a SOA-MZI are then proposed to accommodate the clock performance degradations that arise from dispersion uncompensated transmission. Leveraging such clock-enhancement techniques, we experimentally demonstrate error-free 125 000-km RZ dispersion uncompensated transmission at 10 Gb/s (PRBS 2 23 -1) using 1000 stages of optical 3R regenerators spaced by 125-km largeeffective-area fiber spans. To evaluate the proposed optical 3R structures in a relatively realistic environment and to investigate the tradeoff between the cascadability and the spacing of the optical 3R, a fiber recirculation loop is set up with 264-and 462-km deployed fiber. The field-trial experiment achieves error-free 10-Gb/s RZ transmission using PRBS 2 23 -1 through 264 000-km deployed fiber across 1000 stages of optical 3R regenerators spaced by 264-km spans.

This letter demonstrates optical 3R regeneration in 10-Gb/s nonreturn-to-zero field transmission. The 3R regenerator utilizes semiconductor-optical-amplifier-based Mach-Zehnder interferometer wavelength converters, a synchronous modulator, and a Fabry-Pérot filter to realize optical 3R regeneration including all-optical clock recovery. The cascadablity of the 3R regenerator is investigated in recirculating loop transmission experiments with various regeneration spacings up to 462 km (corresponding to an input optical signal-to-noise ratio (OSNR) of 22 dB). Transmission with the 3R regenerator shows significant performance improvement over that without 3R regeneration. A 66-km-spaced 3R regeneration with a 33-dB input OSNR achieves 1000-hop cascaded error-free transmission (66 000 km in distance) with no hop-to-hop power penalties. A 264-km-spaced 3R regeneration with a 25-dB input OSNR also achieves 100-hop cascaded transmission (26 400 km in distance) with a bit-error-rate error floor at 1 10 8 .

In this paper, a class of high-data-rate battery-free yet active miniature radio-frequency identification tag without any external components (except antenna) operating at millimeter (mm)-wave frequencies is proposed and demonstrated. This fully embedded tag consists of a recently proposed CMOS-based zero-intermediate-frequency self-oscillating mixer, a high power conversion efficiency mm-wave-to-dc rectifier, and an ultralowpower voltage regulator on a single chip, integrated with ceramicbased antennas. Interconnection between the CMOS die and the antenna is realized using a wire-bonding technique, which is compensated and optimized to match the antenna input impedance and also to minimize the wire-bond associated losses at mm frequencies. The 10 × 10 mm 2 tag wirelessly harvests its energy from an incoming signal at 24 GHz, receives, and recovers the data sent by reader on an amplitude modulation (AM)-modulated 40-GHz carrier, and transmits its data back to the reader on a 40-GHz carrier, using AM modulation as well. The tag exhibits a bit rate of about 500 kb/s during the reader-to-tag communication and 10 Mb/s during the tag-to-reader communication, solely relying on the rectified energy for powering its operation. To the best of our knowledge, such an mm-wave identification tag at mm-wave frequencies has never been reported in the literature.

In this paper, we consider underwater quantum key distribution (QKD) systems with time-gated single photon avalanche photodiodes (SPADs) and present a comprehensive performance analysis and optimization. We utilize a deep learning model for the system optimization. We first perform Monte Carlo simulations for a subset of possible scenarios and train the deep learning model on them. Then, we use this model to instantly extract the optimum values for possible underwater scenarios. Our study provides critical insights on determining the optimal bit time, field of view (FoV), and gate time in underwater QKD. Through a meticulous analysis of the propagation delay and angle of arrival results, we determine the bit time and FoV, respectively, by striking a balance between the average number of received photons and background noise. Afterwards, using the proper bit time and FoV from the previous steps, we determine the optimal gate times in the sense of minimizing the quantum bit error rate (QBER). Our findings reveal that the proposed method significantly improves QBER, with enhancements reaching up to two orders of magnitude across different link lengths.

INDEX TERMS Quantum key distribution, single photon avalanche diode, deep learning, underwater quantum communication, quantum bit error rate.

The application of adaptive power-supply regulation is extended to serial links. The adaptive supply maximizes the energy-efficiency of the I/O circuits and serves as a global bias to scale the link properties optimally with the bitrate. Parallelism in transceivers and the use of multiphase clocks increase the bitrate to a multiple of the clock frequency and, hence, enable the lowfrequency low-voltage operation to reduce power while meeting the specified bitrate. Two key designs to enable this power saving are presented: parallelized transceivers for low-voltage operation and dual-loop architecture phase/delay-locked loop for multiphase clock distribution. A prototype chip fabricated in 0.25-m CMOS process operates at 0.65-5.0 Gb/s while dissipating 9.7-380 mW.

We evaluate the asynchronous Q factor from a dense-wavelength-division-multiplexed (DWDM) optical signal distorted by chromatic dispersion, amplified-spontaneous-emission noise, nonlinear effects, and intersymbol interference, using amplitude histograms. It was ...

It has historically been ditficult to distribute a well-aligned hardware clock throughout the physical extent of a synchronous processor. 'haditionally, this task has been accomplished by distributing the output of a central oscillator over a tree-like network, with repeaters at necessary intervals. While straightforward in concept, this method suffers from poor reliability, poor scalability and high skew. In this paper, we present an alternative approach-Distributed Synchronous Clocking-that maintains the simplicity of synchronous operation without suffering the drawbacks of centralized clocking. A network of independent oscillators takes the place of the centralized clock source, providing separate clock signals to the physically distant parts of a computing system. A distributed error correction algorithm effects global phase alignment by utilizing local comparisons of neighboring oscillator phase. In contrast to centralized clock distribution, distributed clocking has the inherent potential for complete scalability and graceful degradation. However, because oscillator phase is a modular quantity, a naive implementation of distributed synchronous clocking can suffer from mode-lock-the trapping of local oscillator phase in undesirable stable equilibria where global phase is not aligned [%I. We present a simple method for eliminating this problem in k-ary Cartesian meshes and give a proof of its correctness for two-dimensional networks. An electronic implementation is also presented and several engineering issues relating to error tolerance are discussed.