Ultrashort Optical Pulse Detection for High-Speed Asynchronous Optical CDMA Networks

Like its wireless counterpart, optical code-division multiple access (optical CDMA) offers greater scalability than other optical multiplexing schemes and provides flexible quality of service, physical layer privacy and asynchronous access . However, unlike wireless CDMA, high bit-rate optical CDMA networks use much higher bandwidth, which cannot be effectively processed with modern electronics rendering many earlier developed detection schemes inapplicable. In this paper we show both theoretically and experimentally that conventional electronics-based detection is inefficient in optical CDMA networks and limits the total network throughput by the bandwidth of the photodetector used. As a solution, we show that network performance can be greatly improved using ultrafast all-optical signal processing for signal detection. Recently developed all-optical thresholding devices performing cubic transformation allow for more than seven times increase in throughput for typical network parameters. A comprehensive comparison of different detection methods for optical CDMA including optimized electronics-based and all-optical signal processing-based is given for the first time.      

Self-Clocked All-Optical Add/Drop Multiplexer for Asynchronous CDMA Ring Networks

In this paper, we demonstrate a novel self-clocked code-drop unit for incoherent optical code-division multiple-access (CDMA) networks. The unit is based on an all-optical thresholder with internal wavelength conversion that creates a control signal for the drop gate from the incoming data stream. This architecture does not require an external clock signal for drop operation and can be used in asynchronous ring networks. The proposed unit is experimentally demonstrated for two-dimensional time-wavelength optical CDMA codes at a bit rate of 2.5 Gbit/s with two different types of drop gates: terahertz optical asymmetric multiplexer and nonlinear fiber-based loop mirror. Error-free operation is achieved in both configurations. The development of a self-clocked add/drop multiplexer demonstrates a novel concept of asynchronous node operation in multiple access networks.      

10 Gbit/s all-optical regenerator

A 10 Gbit/s optical data stream has been regenerated all-optically by clock recovery and remodulation. The regenerator consists of a fibre ring laser, modelocked by the incoming data stream, and a Kerr shutter acting as an all-optical AND gate to recode the ring laser pulses with the incoming data. Error ratio measurements for an all-optical regenerator are presented for the first time and the future potential of all-optical regeneration is discussed     

利用TOAD实现10 Gbit/s全光非归零码到归零码的转换

利用从非归零（NRZ）信号中全光提取的时钟，采用太赫兹光非对称解复用器（TOAD）实现了10 Gbit／s非归零码到归零（RZ）码的码型转换。非归零信号采用半导体光放大器（SOA）进行时钟分量增强并用平面波导阵列（AWG）滤出相应的伪归零（PRZ）信号，然后采用半导体光放大器注入锁模光纤环形激光器进行时钟提取，提取的时钟信号和待转换的非归零信号分别作为抽运光和探测光输入太赫兹光非对称解复用器，在其中进行码型转换。转换后输出的归零信号的质量仅由恢复的时钟信号和非归零信号的质量决定，受太赫兹光非对称解复用器中半导体光放大器增益恢复时间的影响极小。实验测得转换后的归零信号消光比为8．7dB，码型效应非常低，其光谱明显展宽．并且出现谱间隔为0．08nm的多峰结构，与10 Gbit／s的比特速率相对应。该方法对时钟信号的码型效应有一定的容忍度。     

Word timing recovery in direct detection optical PPM communicationsystems with avalanche photodiodes using a phase lock loop

A technique for word timing recovery in a direct detection optical pulse position modulation (PPM) communication system is described. It tracks on back-to-back pulse pairs in the received random PPM data sequences with the use of a phase locked loop. The experimental system consisted of an AlGaAs laser diode transmitter (λ=833 nm) and a silicon avalanche photodiode photodetector, and its used Q=4 PPM signaling at a source data rate of 25 Mb/s. The mathematical model developed to characterize system performance is shown to be in good agreement with the experimental measurements. Use of this recovered PPM word clock, along with a slot clock recovery system described previously, caused no measurable penalty in receiver sensitivity when compared to a receiver which used common transmitter/receiver clocks. The completely self-synchronized receiver was capable of acquiring and maintaining both slot and word synchronizations for input optical signal levels as low as 20 average detected photons per information bit. The receiver achieved a bit error probability of 10^-6 at less than 60 average detected photons per information bit     

Bit rate and wavelength transparent all-optical clock recovery scheme for NRZ-coded PRBS signals

All-optical clock recovery from 40-Gb/s nonreturn-to-zero (NRZ) pseudorandom binary sequence data streams based on self-pulsating lasers is presented. A compact preprocessing circuit is utilized to convert an NRZ signal to a pseudoreturn-to-zero sequence before injecting into the optical clock. It comprises a semiconductor optical amplifier followed by a periodical wavelength-division-multiplexing demultiplexer filter. A stable sinusoidal clock signal with a root-mean-square jitter below 700 fs is detected at the output of the self-pulsating laser within data dynamic range of more than 8 dB. The performance of the all-optical clock recovery scheme is investigated by varying the bit rates between 39.81 and 43.02 Gb/s as well as for various wavelengths in the C-band.     

Performance analysis of direct-detection optical asynchronous CDMAsystems with double optical hard-limiters

Performance of optical asynchronous code-division multiple-access (CDMA) systems with double optical hard-limiters is analyzed under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Optical orthogonal codes (OOC's) are employed as signature sequence codes. In the analysis, chips are assumed to be synchronous among users, that is, the chip synchronous case, because the effect of the interference is largest in the chip synchronous case and thus the performance in the chip synchronous case results in the upper bounds on the performance of the asynchronous system. The performance is evaluated under average power and bit rate constraints. The results show that, differing from the optical synchronous CDMA systems with double optical hard-limiters, the optical asynchronous CDMA systems with double optical hard-limiters have good performance even when the number of simultaneous users is large     

All optical signal regularizing/regeneration using a nonlinearfiber Sagnac interferometer switch with signal-clock walk-off

All-optical signal regularizing/regeneration using a nonlinear fiber Sagnac interferometer switch (NSIS) that employs signal-clock walk-off is investigated. The NSIS realizes all-optical signal regeneration, including timing and amplitude regularizing, by switching clock pulses with amplified input signals using a walk-off-induced, wide, square switching window and intensity-dependent transmittance of the device. First, characteristics (in both the temporal and spectral domains) of the all-optical signal regeneration achieved with the NSIS are investigated theoretically and experimentally. They certify that if clock pulses are within the square switching window obtained with signal-clock walk-off, the clock pulses can be modulated according to the data that the input signals carry and retain their temporal and spectral profiles. This means that if clock pulses can be prepared that meet the system requirements, the NSIS can convert input signals that may not satisfy system requirements into high-quality output signals. Limitations on the switching contrast due to the cross-phase modulation of counterpropagating reference pulses is also discussed. Second, two possible applications of NSIS-based all-optical signal regularizing/regeneration, 1) an all-optical multiplexer with an optical clock and 2) an all-optical regenerative repeater, are discussed. Preliminary experiments with ~10-ps pulses at bit rates of ~5 Gb/s that use locally prepared optical clock pulses, show that the NSIS provides an error-free regeneration function with a certain tolerance for pulse-period irregularity if a proper optical clock is obtained     

Slot clock recovery in optical PPM communication systems withavalanche photodiode photodetectors

Slot timing recovery in a direct-detection optical PPM communication system can be achieved by processing the photodetector output waveform with a nonlinear device whose output forms the input to a phase-locked loop. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The RMS phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode (λ=834 nm) and a silicon avalanche photodiode photodetector. The system used Q =4 PPM signaling and operated at a source data rate of 25 Mb/s. The mathematical model developed to compute the RMS phase error of the recovered clock is shown to be in good agreement with results of actual measurements of phase errors. The use of the recovered slot clock in the receiver resulted in no significant degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10^-6 at a received optical signal energy of 55 detected photons per information bit     

All-optical clock recovery using a modelocked figure eight laserwith a semiconductor nonlinearity

A novel geometry for all-optical clock recovery is demonstrated using a semiconductor laser amplifier in a nonlinear optical loop mirror to modelock a figure eight laser. A clock signal can be recovered over a broad locking bandwidth of 0.5 MHz when random data at 1 Gbit/s are input to the system     

Optical Noise Tolerance in a Polarization-, Wavelength-, and Filter-Free All-Optical Clock Recovery System Based on a Monolithic Mode-Locked Laser

We experimentally investigated optical noise tolerance of a polarization-, wavelength-, and filter-free all-optical clock recovery system based on a monolithic mode-locked laser diode. The results showed that for input signal degradation, the jitter of the recovered clock did not degrade to approximately 8 dB/nm of the optical SNR. We also compared the clock recovery performance as functions of the wavelength and the modulation format (on-off keying and phase-shift keying) of the input data signals. We revealed that no significant changes were measured in the clock recovery performance despite such changes in the parameters of the input data signals, indicating that noise-tolerable clock recovery is expected, retaining the polarization- and the filter-free operations, regardless of the wavelength and the modulation format of the input data signals, by using our new scheme.     

All-optical regenerator

An all-optical regenerator has been demonstrated using two self-electro-optic effect devices (SEED) and a local pump laser. The regenerator's optical signal processing included clock recovery, data retiming and signal amplification. A 2 dB optical gain was attained at a 5 kbit/s data rate     

40-Gb/s Polarization-Insensitive All-Optical Clock Recovery Using a Quantum-Dot Fabry–PÉrot Laser Assisted by an SOA and Bandpass Filtering

We experimentally demonstrate polarization- insensitive all-optical clock recovery using a passively mode-locked quantum-dot Fabry–PÉrot (QD-FP) semiconductor laser at 40 Gb/s. Polarization insensitivity is achieved by using a wavelength conversion stage based on cross-phase modulation in a semiconductor optical amplifier and optical bandpass filtering. A clock signal with a root-mean-square timing jitter of 300 fs is recovered for an input data signal with a scrambled state-of- polarization (SOP). This is comparable to that from the QD-FP laser alone for an input signal with a fixed SOP. Moreover, an improvement in the tolerance to a degradation in the optical signal-to-noise ratio of the input signal is achieved by the use of the wavelength conversion stage.      

Very-high-speed fibre-optic networks for broadband communications

Single-mode optical fibres have an enormous transmission bandwidth which can support ultra-high-speed digital transmission and networking. The use of electrical signal-processing, however, ultimately limits the network capacity. To eliminate the throughput bottleneck, all-optical processing techniques should be employed in a fibre-optic network. This paper discusses several schemes for implementing optical time division multiple access (OTDMA) networks with the emphasis on optical clock distribution, synchronisation and all-optical detection. The use of a fast-switching bistable laser diode as an all-optical threshold detector and data regenerator is suggested. A dual-wavelength OTDMA technique is investigated, which can be used for digital TV or future HDTV distributions. Moreover, an efficient multiple access scheme, called wavelength division multiple access with optical time division multiplexing (WDMA-OTDM), is proposed for broadband communication services. It is shown that WDMA-OTDM has all the advantages possessed by the individual OTDMA and WDMA schemes but offers improved flexibility and capacity     

All-Optical Clock Recovery From NRZ-DPSK Signal

All-optical clock recovery (CR) from 20-Gb/s nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signal is demonstrated, with an all-fiber delay interferometer (DI) and a mode-locked semiconductor optical amplifier (SOA) fiber laser. The tunable DI serves as an all-optical DPSK demodulator and the phase-modulated NRZ-DPSK signal is converted into the intensity-modulated pseudoreturn-to-zero (PRZ) signal, with the enhancement of the clock component. Followed SOA fiber-laser is used to achieve CR from the PRZ signal. Fixed bit pattern and 2³¹-1 pseudorandom binary sequence NRZ-DPSK signals are used to test the performance of the proposed system. It is shown that the recovered clock signal with the extinction ratio over 10 dB and the root-mean-square timing jitter of 800 fs can be achieved     

全光纤锁模腔结构的全光时钟提取实验研究

分析并实验验证了一种全光纤锁模激光器结构的全光时钟提取方案。方案中,采用高非线性光纤(HNLF)替代传统结构中的半导体光放大器(SOA),利用光纤中的交叉相位调制(XPM)效应实现腔内的非线性调制,避免了以SOA作为非线性光调制器件的锁模激光器全光时钟提取方案中,由于载流子恢复时间较长从而限制工作速率的缺点,以达到突破"电子瓶颈"的目的。理论分析了光纤中交叉相位调制的特性以及环形锁模腔的时钟提取原理,并通过实验,从40Gbit/s的光归零码(RZ)信号中提取出了高质量的光信号时钟。该方案可以直接在更高速率条件下工作。     

Meeting the need for fiber-optic transmission speed

Review current OTDMA technology, with the emphasis on optical clock recovery and time demultiplexing. Various optical switching techniques are surveyed, and they are compared by taking account of the potential feasibility of engineering implementation. The architectures for OTDMA networks are described, and we look at all-optical clock recovery schemes that are based on traveling-wave (TW) semiconductor laser amplifiers, self-pulsating laser diodes, EO switches, mode locking of a fiber ring laser, and four-wave mixing (FWM). We also review various optical switching techniques for ultrafast time demultiplexing     

Electrical and optical clock and data recovery in optical access networks: a comparative study

Clock and data recovery (CDR) is an essential part in high‐speed telecommunication systems. The CDR is used to extract the clock and re‐time the received data, which allows a synchronous operation to recover the transmitted signal. In optical access networks, electrical CDR or optical CDR implementations can be used. However, there are no clear guidelines or recommendations on which CDR implementation should be adopted for better performance. These missing clear recommendations are because the electrical CDR requires electronics design expertise whereas the optical CDR requires optical design expertise. Consequently, in this paper, an all‐digital CDR, designed and implemented on the field‐programmable gate array platform, and an optical CDR, developed by using fiber Bragg grating technology on the OptiSystem platform, are presented. Furthermore, the integration of these 2 CDR implementations with the optical access network is implemented, and their performance is evaluated for various transmission rates and communication distances. Finally, a comparative study in terms of the bit error rate between the all‐digital CDR and the optical CDR is presented.     

Delay-and-multiply clock regeneration in APD-based direct-detectionoptical OOK communication systems

A clock recovery scheme for direct-detection optical on-off keying (OOK) communication systems with nonreturn-to-zero pulse shaping is proposed and investigated. In the suggested model, the optical field is detected with the aid of an avalanche photodiode (APD) photodetector, which is followed by a clock regeneration subsystem. The proposed clock recovery subsystem consists of a delay-and-multiply nonlinearity followed by a conventional phase-locked loop (PLL), tuned to the slot frequency of the desired optical OOK signal. Performance of the proposed system is obtained in terms of the signal-to-noise ratio (SNR_L) of the linearized PLL device (or, equivalently, the inverse of phase, or timing, error variance) when background noise and receiver thermal noise are present. Numerical results are presented in order to explain the influence of noise processes on the performance of the proposed clock recovery subsystem. The performance of this system is also compared to that of an early-late gate and square-law symbol synchronizers     

A 40 Gb/s Clock and Data Recovery Module with Improved Phase‐Locked Loop Circuits

A 40 Gb/s clock and data recovery (CDR) module for a fiber‐optic receiver with improved phase‐locked loop (PLL) circuits has been successfully implemented. The PLL of the CDR module employs an improved D‐type flip‐flop frequency acquisition circuit, which helps to stabilize the CDR performance, to obtain faster frequency acquisition, and to reduce the time of recovering the lock state in the event of losing the lock state. The measured RMS jitter of the clock signal recovered from 40 Gb/s pseudo‐random binary sequence (2³¹‐1) data by the improved PLL clock recovery module is 210 fs. The CDR module also integrates a 40 Gb/s D‐FF decision circuit, demonstrating that it can produce clean retimed data using the recovered clock.