Phase noise cancellation in a carrier recovered optical heterodynereceiver

The performance and application of an optical heterodyne receiver which uses a carrier recovery demodulator are described. Phase sensitive demodulators used in coherent optical transmission are compared, and the suppression of both phase noise and frequency instability of light sources by a carrier recovery (CR-) demodulator is described. A carrier recovered PSK (CR-PSK) demodulator and a phase noise canceling circuit (PNC) for a coherent SCM receiver are introduced as examples of CR-demodulators. The relationship between laser diode spectral linewidths and the delay time difference between the two paths in the CR-PSK demodulator necessary to keep the system performance within a certain power penalty is then derived. In a preliminary experiment using 560-Mb/s CR-PSK transmission, a receiver sensitivity of -51.6 dBm was obtained, and a laser phase noise suppression of about ¹/₂ that of DPSK was confirmed. The results suggest the possibility of constructing a heterodyne receiver which has no AFC-loop. Applications of a CR-demodulator to an optical frequency division multiplexing (OFDM) system and to a multivalue modulation scheme are discussed     

Subcarrier multiplexed coherent lightwave systems for videodistribution

The application of subcarrier multiplexing techniques to coherent lightwave systems designed for video transmission is described. The theory of the intrinsic receiver sensitivity of multichannel coherent SCM (subcarrier multiplexing) systems is presented. Specific implementations of SCM coherent systems transmitting 60 FM video channels and 20 100 Mb/s FSK (frequency shift keying) channels are described. It is shown that a simple phase noise canceling circuit can be implemented, which allows these systems to be built with wide-linewidth DFB (distributed feedback) lasers. Several applications of multichannel coherent SCM systems to video distribution networks are proposed     

Coherent FM-SCM system using DFB lasers and a phase noisecancellation circuit

A multichannel coherent subcarrier multiplexing (SCM) system using standard distributed-feedback lasers is discussed. A simple phase noise canceling circuit yields excellent quality FM video transmission (SNR=56 dB), even though the 50-MHz IF linewidth exceeds the 30-MHz signal bandwidth     

Strain measurement using frequency modulation fiber optic interferometer

The method for measuring the strain of an object using an optical fiber and a frequency modulation(FM)coupled carity semiconductor laser is pro-posed.This method uses the coherent FM heterodyne principle of the Michelson in-terferometer and can avoid the π/2 nonreciprocal phase bias and phase shifting problem existing in general fiber optic interferential sensors,the maximum detec-tion range is limited by the coherent length of the semiconductor laser and its rela-tive factor.     

FM-FDM optical CATV transmission experiment and system design forMUSE HDTV signals

FM-FDM (frequency division multiplexing) optical transmission equipment has been developed for 34-channel MUSE HDTV (high-definition television) signals to realize optical CATV (cable television) systems. The equipment uses an LD (laser diode) with a 1.3 μm wavelength, a single-mode optical fiber, and an avalanche photodiode (APD). A good picture is received after a 42 km transmission. A part of the multiplexed signals is distorted near or below the threshold of an LD. When the number of transmission channels is small and the total optical modulation depth is large, this nonlinearity governs the power ratio of an FM signal to one distortion component-the DU ratio. However, when the number of transmission channels is large, the DU ratio is determined by the effective optical modulation depth rather than the total optical modulation depth. Furthermore, the method of system design is clarified for an optical trunk line CATV system. If no restriction on the transmission bandwidth of optical devices exists, approximately 30 km transmission of 100-channel MUSE HDTV signals is available with a received CN ratio of 17.5 dB     

Coherent optical tapping using semiconductor optical amplifier

Semiconductor optical amplifiers are attractive not only as optical repeaters but also as functional devices, since carrier density modulation in amplifiers causes a nonlinear phenomenon. Utilizing the effect of the carrier density modulation on the semiconductor optical amplifier junction voltage, a coherent optical tapping is proposed for signal monitoring or control signal extraction. A 155 Mb/s FSK (frequency shift keying) signal tapping was realized with a simple configuration using heterodyne single-filter detection with -24.4 dBm sensitivity. Many applications for this coherent optical tapping are discussed, and basic characteristics for frequency-selective tapping from FDM (frequency division multiplexing) signals and optical amplifier gain control are examined     

Spectral efficiency of optical FDM systems impaired by phase noise

The required frequency spacings between channels in an optical frequency division multiplexing (FDM) network are considered. The minimum permissible spacings consistent with meeting bit error rate (BER) objectives are derived. The assumed transmission uses on-off keying (OOK), at a data rate 1/T (in bits per second), via external modulation of a laser source having linewidth β (in hertz). The assumed receiver consists of an optical channel selection filter followed by a p-i-n photodiode and a postdetection integrate-and-dump circuit. The analysis estimates the adjacent channel interference (ACI)-induced floor on BER for the middle of three FDM channels, as a function of frequency spacing and linewidth-to-bit rate ratio (βT). For BER=10^-9 and βT ranging from 0.32 to 5.12, the required channel spacing ranges from 5.2 to 27.5 bit rates. The multiplying factors associated with using (wide-deviation) frequency shift keying (FSK), coherent (heterodyne) detection, and infinitely many FDM channels, respectively, are estimated to be 2.0, at most 3.0, and at most 1.37     

Coherent transmission of 60 FM-SCM video channels

A fiber-optic 60-channel FM video system using coherent subcarrier multiplexing (SCM) is described. Two receiver implementations were studied. For the 60-channel coherent experiment, a carrier-to-noise ratio of 17 dB and a corresponding signal-to-noise ratio of 56 db was achieved with a detected optical power of -32 dBm and a phase modulation index of β-0.07. This represents a 15.2-dB improvement over a similar 60-channel direct-detection FM-SCM system and a 5.6 dB improvement compared to the performance of the system when a semiconductor optical preamplifier was used     

Two-channel coherent optical FDM transmission system with a new frequency stabilisation scheme

A two-channel 140 Mbit/s coherent optical FDM transmission system with a new frequency stabilisation scheme has been developed. In this scheme, low frequency modulation is applied to transmitter laser diodes to lock them at the resonances of a Fabry-Perot interferometer, and a high speed IF AFC circuit is used to eliminate the system performance degradation due to this low frequency modulation. The measured BER is 10/sup -9/ for a received signal power of -41 dBm and no crosstalk between channels is observed with a channel spacing of 4.0 GHz.<>     

Dynamic range of coherent analog fiber-optic links

We investigate the performance of coherent analog optical links employing amplitude modulation (AM), phase modulation (PM), and frequency modulation (FM). The performance of these coherent links is compared to that of AM direct-detection (DD) links. The signal-to-noise ratios, nonlinearities, and-spurious-free dynamic ranges (SFDR's) of the foregoing links are evaluated. We calculate the SFDR for links using DFB and Nd:YAG lasers with typical linewidths of 10 MHz and 5 kHz, respectively. The performance of PM and FM links is dominated by phase noise above a critical value of received optical power. For a linewidth of 10 MHz, and SFDR's of PM and FM links are 30 and 31 dB, respectively, for a received optical power above -27 dBm in a 1 GHz bandwidth. For a linewidth of 5 kHz, the corresponding SFDR's above a received power level of 0 dBm are 51 and 53 dB. The performance of DD and AM links is dominated by RIN above a critical value of received optical power. For a RIN level of -155 dB/Hz, the SFDR's of DD and AM links are 49 and 47 dB, respectively, for a received optical power of 10 dBm in a 1 GHz bandwidth. The SFDR's of the DD and coherent links used for transmission of subcarrier-multiplexed (SCM) signals are also derived. We evaluate target laser parameters needed by a number of different applications. For AM video and antenna remoting applications, linewidths of <1 and <3 kHz are required to use PM and FM links, respectively. For FM video, linewidths of <150 and <350 MHz are required to use PM and FM links. For SCM digital applications, linewidths of <80 and <200 MHz are required to use PM and FM links. The paper concludes with a discussion of system implementation issues, including linearization, optical frequency modulation, balanced receivers, and IF issues     

外差锁定式相干光通信

提出并研制了一种新的外差锁定激光频率调制系统,可用于相干光通信,并进行了闭环调频、调幅实验。对实用的外差锁定式半导体激光器光纤通信系统的技术条件作了讨论。     

相位调制光外差稳频信号检测技术

在高功率条件下,由于受到多种因素的影响,单频光纤激光器输出激光的谱线宽度大幅展宽,输出激光的稳定性也不高。相位调制光外差稳频（PDH）技术在高功率条件下可以实现高频率稳定性。为实现对中心波长为1 064 nm的单频光纤激光器的稳频,理论分析了PDH稳频系统的原理并搭建PDH稳频系统。实验发现100 MHz相位调制光外差信号的检测是稳频系统的关键。实验中首先利用自行设计的探测器前置放大电路,基于Si探测器,实现了信号的探测和放大;其次,设计解调电路,通过将光电转换后的信号与参考信号进行混频实现解调,得到鉴频曲线,实现对光外差信号的检测。     

1-Gsymbol/s 64-QAM Coherent Optical Transmission Over 150 km

Quadrature amplitude modulation (QAM) is an excellent modulation format for realizing optical communication systems with a high spectral efficiency of much greater than 1bit/s/Hz. We describe QAM coherent optical communication that we achieved by using heterodyne detection with a frequency-stabilized fiber laser and an optical phase-locked loop (OPLL) technique. The phase error variance of the intermediate frequency signal of the OPLL was 6.1times10^-3 rad. A 1-Gsymbol/s 64-QAM coherent signal was successfully transmitted over 150km     

A full-duplex optical access system with hybrid 64/16/ 4QAM-OFDM downlink

A full-duplex optical passive access scheme is proposed and verified by simulation, in which hybrid 64/16/4-quadrature amplitude modulation (64/16/4QAM) orthogonal frequency division multiplexing (OFDM) optical signal is for downstream transmission and non-return-to-zero (NRZ) optical signal is for upstream transmission. In view of the transmitting and receiving process for downlink optical signal, in-phase/quadrature-phase (I/Q) modulation based on Mach-Zehnder modulator (MZM) and homodyne coherent detection technology are employed, respectively. The simulation results show that the bit error ratio (BER) less than hardware decision forward error correction (HD-FEC) threshold is successfully obtained over transmission path with 20-km-long standard single mode fiber (SSMF) for hybrid downlink modulation OFDM optical signal. In addition, by dividing the system bandwidth into several sub-channels consisting of some continuous subcarriers, it is convenient for users to select different channels depending on requirements of communication.     

基于FPGA数字相位调制光外差激光稳频系统设计

为了实现中心波长为1064nm的单频光纤激光器的稳频,采用相位调制光外差（PDH）激光稳频技术,搭建稳频系统光路。分析了相位调制光外差稳频信号以及误差信号特征;设计基于现场可编程门阵列（FPGA）的数字式解调和反馈控制电路,在FPGA中实现对相位调制光外差稳频信号的数字解调,再经数模转换器输出获得误差信号。结果表明,在FPGA中能成功实现对相位调制光外差信号的解调,经Allan方差计算,频率漂移的方差值可达10-11,即所设计的数字系统实现了较高的稳频精度。     

Optically coherent direct modulated FM analog link with phase noisecanceling circuit

This paper demonstrates a new phase noise canceling (PNC) circuit for use with an optically coherent analog frequency modulation (FM) link employing directly modulated distributed feedback (DFB) lasers. Direct frequency modulation of a semiconductor laser is a highly efficient optical-to-electrical conversion process, which can lead to very low noise figure (NF) and high dynamic range (DR). However, the large laser phase noise found in semiconductor lasers significantly degrades the FM link performance. The PNC circuit is a simple means for canceling the laser phase noise while extracting the modulated signal and taking advantage of the high FM conversion efficiency of semiconductor lasers. The theoretical performance of the PNC PM link is discussed in terms of the signal-to-noise ratio (SNR), NF, and DR, and is compared to a Mach-Zehnder modulated link which uses high-power, solid-state lasers. Phase noise cancellation is demonstrated in an experimental PNC FM link. Comparison of a PNC FM link to an externally modulated AM (coherent) link shows a 31 dB improvement in the NF of the FM link, and a 10 dB improvement in DR. However at higher received optical powers phase-to-intensity noise limits the performance of both links to well below the theoretical calculations     

Requirements for high-frequency LD FM response in an optical CPFSKheterodyne delay demodulation system

The required high-frequency laser diode (LD) FM responses for a continuous phase frequency shift keying (CPFSK) heterodyne delay modulation system were investigated. The degradations due to insufficient high-frequency FM response are evaluated, considering the FSK waveform distortion, which gives undesired phase error at the demodulation circuit. From the phase error, bit error rate (BER) degradation is calculated. The calculated results indicate that the delay-time adjustment for the demodulation circuit is effective in minimizing the power penalty. The theoretical evaluation, including the demodulation circuit optimization, explains the experimental results fairly well. Using the same evaluation procedure, required high-frequency LD FM responses are derived. The results, together with the required low-frequency FM responses, give guidelines for transmitter LD selection for coherent CPFSK systems     

Overmodulation in subcarrier multiplexed video FM broad-bandoptical networks

The probability of an SCM (subcarrier multiplexed) signal falling outside the linear region of a laser diode is calculated in order to determine the overmodulation that may be practically employed for multichannel video distribution in single-octave SCM networks. The overmodulation is predicted as a function of the number of subcarriers in single-octave SCM systems supporting up to about 70 FM video channels, for SNRs (signal-to-noise ratios) of 47 dB (I/PAL), and 56 dB (M/NTSC). The RMS optical modulation index (OMI) is demonstrated to be a more useful performance parameter than the linear OMI in SCM systems carrying a large number of channels. A 47 dB weighted SNR (with 4 dB margin) is deemed sufficient for acceptable picture quality in cable distribution systems, whereas a 56 dB value would be required for long-distance point-to-point optical links. RMS OMI values of around 70 and 50, respectively, can be employed     

Performance implications of the thermal-induced frequency drift infast wavelength switched systems with heterodyne detection

The frequency modulation response and tuning dynamics of a two-section DFB laser are determined by carrier and thermal modulation of the active layer refractive index. In response to a step change in the injection current for switching between channels in a wavelength division multiplexed system, the optical frequency changes rapidly due to the carrier effect, and then slowly drifts toward a steady state value due to the thermal effect. For wavelength switched applications with heterodyne detection, the drifting of the optical frequency broadens the spectrum of the IF signal and may impose a limit on the time that the signal remains within the IF passband (residency time). The IF spectral broadening and residency time are investigated theoretically and experimentally. Based upon a minimum mean square error fit between experimental and theoretical FM responses, the dependence of the spectral broadening and residency time on the bias condition of the laser and the optical frequency switching interval is characterized