基于脉冲调制的Φ-OTDR研究综述

基于相位敏感光时域反射计（Φ-OTDR）的分布式光纤振动传感系统向传感光纤中注入探测光脉冲,通过测量光脉冲传输过程中产生的背向瑞利散射光的波动实现对入侵扰动的识别和定位,故探测光脉冲对Φ-OTDR系统性能有重要影响。介绍了Φ-OTDR基于光脉冲调制方面的研究进展,其中包括光脉冲的宽度、频率、形状以及消光比等参数对Φ-OTDR系统性能影响机理,以及通过脉冲调制优化Φ-OTDR系统信噪比、报警率、空间分辨率、响应带宽等参数以及恢复振动信号波形方面的研究进展。旨在通过光脉冲调制实现Φ-OTDR系统多项性能指标同步提升提供新思路,进一步推动Φ-OTDR的工程应用。     

基于100 MHz晶体振荡器的再生二分频低相位噪声点频源研究

低相噪时钟在电子系统中往往作为参考或者相位参考信号,在系统中起着“心脏”的作用。电子系统比如高速采样测试系统、低噪声合成频率源、雷达系统以及高频时钟同步系统都要求其内部参考源向高频化的方向发展,但是系统中又会用到低频同源时钟信号。为了系统降低复杂性,需要将高频时钟信号进行变换得到所需低频时钟信号。针对传统下变频会引入交调组合杂散,数字分频噪底不够低等传统的低频信号实现方式的缺陷。本文提出了一种基于100MHz晶体振荡器的再生二分频技术的低相噪50MHz信号产生方法。实际测试结果表明,在偏离载波频率几十KHz以内,相位噪声基本按理论值优化;同时,在偏离载波频率100KHz及其以外,相位噪声的噪底达到了-175dBc。实验证明,该种技术方式是确实可行的,具有非常重要的实用价值。     

相位敏感光时域反射系统相位模糊及解卷绕

对相位敏感光时域反射系统相位模糊问题和解卷绕进行了研究,推导了系统中扰动引入的相位变化的整个过程,分析了相位敏感光时域反射系统相位解调过程中产生相位模糊的原因。实验采用压电陶瓷作为扰动源,通过数字相干解调方法进行了相位解调。实验结果表明,相位模糊同时存在于每条相位解调曲线上和曲线之间,因此需要进行两次相位解卷绕消除相位模糊现象;同时,对实验中产生的相位错乱进行了分析,指出相位解卷绕阈值和扰动位置相位的剧烈变化导致了扰动位置相位解卷绕结果不准确,提出了采用相位差变化峰之后的临近位置处的相位变化来还原扰动。实验表明,这种方法能够正确还原扰动信号,能够对10 Hz~1.5 kHz范围内的扰动信号进行准确解调,可同时响应并解调光纤沿线多点扰动,且相位变化幅值与扰动强度具有良好的线性关系。     

一种基于相位敏感光时域反射计的多参量振动传感器

提出了一种基于数字相干检测和维纳滤波技术的相位敏感光时域反射计(φ-OTDR)。在该技术中,实时解调出了沿光纤链路散射回的瑞利信号电场的振幅和相位。通过分析解调出的振幅和相位,检测到了位于3.5km传感光纤上的一个由压电陶瓷(PZT)所产生的正弦振动信号的位置、频率和强度。应用维纳滤波降低了由激光器的相位噪声和探测器加性噪声引起的相位波动。系统的空间分辨率为5m。     

基于电子相位补偿的光纤频率传递技术

针对远距离光纤频率直接传递由机械应力引入的相位噪声,分析了机械应力对频率传递性能的影响,设计了基于电子相位补偿的光纤频率传递方案,进行了光缆晃动模拟试验,并在实际架空光缆上进行试验,补偿了大部分由光缆环境变化引起的相位抖动,时钟源输出相位噪声为-120 dBc/Hz@1 Hz的10 MHz频率信号,经过基于电子相位补偿光纤频率传递系统后相位噪声达-111 dBc/Hz@1 Hz,与未进行电子相位补偿光纤频率传递相比相位噪声提高12 dB,实现10 MHz频率信号高质量传输。     

单频光纤激光器相位噪声的影响因素

单频光纤激光器是光纤通信中最具前途的一种光源,在光纤传感、激光雷达和激光测距等方面具有重要的应用,而相位噪声是衡量其性能的关键指标之一。采用示波器和动态信号分析仪方法对单频光纤激光器的相位噪声进行测量。利用功率谱、积分谱对不同泵浦源功率、种子源功率和波长的光纤激光器的相位噪声进行定性分析,结果表明:泵浦功率越大,低频相位噪声所占比重越大;而对于不同种子源功率,相位噪声在测试频段的比例基本处于同一水平;对于测试的不同种子的波长,相位噪声高频段所占比例不同,其中种子波长为1 560.48 nm比例最大。     

基于内调制啁啾脉冲的高信噪比低成本直接探测型φ-OTDR系统

提出一种新型的基于内调制啁啾脉冲的高信噪比(SNR)直接探测型相位敏感光时域反射计(φ-OTDR)。通过理论分析可得,利用啁啾脉冲的方法并通过提高受激布里渊阈值可提高注入光能量,提高探测信号中的直流分量可使信号与噪声分离,进而提升SNR。提出一种φ-OTDR系统信噪比的综合评判方法,对SNR的统计特性进行分析。同时,开展了基于分布式反馈激光器的内调制直接探测型φ-OTDR系统实验研究,利用可调谐滤波器对内调制产生的频率啁啾范围进行控制,经对比实验验证,可得所提系统与传统φ-OTDR系统的信噪比期望相当,分布方差更小,测量可靠度更高的结论。此外,该系统还具有成本低、结构简单等优点,实际应用时可以利用光纤布拉格光栅代替可调谐滤波器,提升其应用的灵活性,为工程应用提供新的解决方案。     

基于相干合成的可调全光纤脉冲激光源

基于光束相干合成的原理,提出了脉冲激光产生的新方法,设计并构造了相应的全光纤实验系统.该方法通过对光纤耦合器中相干合成的两束相干光的相位差进行有效地控制,实现重复频率、脉宽、占空比均可调的脉冲激光输出.实验中利用闭环工作点控制法进行噪声相位补偿,以得到稳定的脉冲光输出.实验分别利用矩形波、三角波、正弦波进行相位调制,得到了相应波形的激光输出;利用不同参量矩形波进行相位调制,可以得到重复频率从1～500 kHz,占空比从20%～80%可调的脉冲光输出,在重复频率为500 kHz时,脉宽可达300 ns.这种脉冲激光产生技术为大功率、可调脉冲激光的产生提供了一种新的途径.     

原子干涉仪中拉曼激光的相位-频率双调制稳频

为降低拉曼激光的频率噪声,提出了一种相位-频率双调制稳频技术。用光纤电光相位调制器对激光进行调制并产生大失谐边带;用射频信号对光纤相位调制器的微波驱动信号进行频率调制,通过锁相放大法将一个大失谐边带锁到铷原子的饱和吸收谱线上。利用该技术实现了拉曼种子激光的稳频和2 GHz的移频,拉曼激光的线宽大幅压窄到56 kHz,预期拉曼激光频率噪声引起的原子干涉重力仪的单次测量噪声可降低到7×10-9/s2。     

CATV光传输中相位调制抑制相位干涉噪声的理论分析

对附加相位调制抑制相位干涉噪声的方法进行了理论分析与计算,结果表明：在光纤AM－CATV外调制传输系统中,对传输光载波信号引入附加相位调制,当相位调制信号的频率为CATV频带的2倍以上时,可以有效地抑制相位干涉噪声的发生,相位调制度越大,抑制效果越好。     

Scheme for all-optical clock division based on period doubling insemiconductor lasers

All-optical clock repetition-frequency division is realised by using period doubling in a semiconductor laser under modulation. A frequency-halved optical clock signal with low phase noise of -87 dBc/Hz is generated from a 12.4 GHz input optical clock. The generated clock signal can exhibit low phase noise over a large input frequency range (300 MHz)     

A performance analysis of an all-optical clock extraction circuitbased on Fabry-Perot filter

A performance analysis of an optical clock extraction circuit based on a Fabry-Perot filter (FPF) is presented. Two analytical methods, time-domain and frequency-domain analysis, are developed in this paper. Time-domain analysis shows that there is no phase jitter in the extracted optical clock if the free spectral range (FSR) of the FPF is exactly equal to the signal clock frequency. Based on this, we obtain an analytical expression for root mean square (rms) amplitude jitter of the extracted optical clock in time domain, in which we have taken the impacts of carrier frequency drift and carrier phase noise into account. When the FSR of the FPF deviates from the signal clock frequency, both phase jitter and amplitude jitter will occur in the extracted optical clock. In this situation, a more general frequency-domain method is developed to deal with the timing performance under the assumption that carrier phase noise is negligible. This method allows us to calculate both rms phase jitter and rms amplitude jitter of the extracted optical clock. Using the developed two methods, we present a detailed numerical investigation on the impacts of finesse of the FPF, carrier frequency drift, resonator detuning, carrier phase noise, and optical pulse chirp on the timing performance. Finally, the application of this circuit in multiwavelength clock recovery is discussed     

FMCW反射计检测距离影响因素分析

频率调制连续波（FMCW）反射计以其高动态范围和窄的空间分辨率优势,在光网络检测、集成光路诊断和光纤传感等领域有着广泛的应用前景。在介绍FMCW反射计基本原理的基础上,分析了光源扫描重复频率、光源功率与FMCW反射计检测距离之间的关系,并探讨了光源相干长度和相位噪声对检测距离的影响。理论分析表明,当待测光纤的长度接近光源的相干长度时,中频信号和相位噪声之间的信噪比会急剧下降,所以FMCW反射计要使用高相干性的光源和一定的相位噪声补偿方法才能应用于长距离的光纤检测。     

Ultrahigh-speed clock recovery with phase lock loop based onfour-wave mixing in a traveling-wave laser diode amplifier

A new phase lock loop (PLL) is proposed and demonstrated for clock recovery from an ultrahigh-speed time-division multiplexed (TDM) optical signal. A traveling-wave laser-diode amplifier (TW-LDA) is used as a phase detector, and the cross-correlation component between the optical signal and an optical clock pulse train is detected as a four-wave-mixing (FWM) signal generated in the TW-LDA. A timing clock from a TDM signal is extracted as a prescaled electrical clock, and this prescaled clock is directly recovered from a randomly modulated TDM optical signal. A prescaled 6.3 GHz clock is successfully extracted from a 100 Gb/s signal using the timing comparison output obtained as the cross-correlation between the optical signal and a short (<10 ps) 6.3 GHz optical clock pulse train in the generated FWM light. A comparison of the PLL phase noise with a previously reported gain modulation method is also shown, and the possibility of the Tbit/s operation of this PLL is also considered in the experiments     

脉冲参量对时间门控光频域反射仪性能的影响

为了对时间门控光频域反射仪系统参量设置进行优化,采用光脉冲压缩原理搭建了时间门控光频域反射仪试验系统,进行了理论分析与实验验证,取得了仿真与实验数据。结果表明,当探测光脉冲信号的峰值功率和本振光信号功率不变时,系统信噪比只与啁啾率相关,且与其成反比;当扫频范围保持80MHz不变时,系统相位噪声随脉宽增加呈现先减小后增大的趋势,且最小值处于2.5μs附近;在实际应用中,满足系统最大扫频范围的条件下,通过调节扫频信号的脉宽,可以获得最优系统相位噪声(最小值)。这一结论对时间门控光频域反射仪系统参量设置与优化是有帮助的。     

Novel Technique to Improve Spatial Resolution in Brillouin Optical Time-Domain Reflectometry

A novel Brillouin optical time-domain reflectometry (BOTDR) system, called a double-pulse BOTDR (DP-BOTDR) system, is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution. Our experiment confirmed that the DP-BOTDR system enables us to measure the distributed Brillouin frequency shift, i.e., the distributed strain and temperature, with a spatial resolution of 20 cm. This spatial resolution is five times better than that provided by the conventional single-pulse BOTDR system.     

Origin of clock-frequency components in NRZ-formatted optical signals

We show that a conventional optical nonreturn-to-zero modulator based on a Mach-Zehnder interferometer always produces optical amplitude modulation at the clock frequency of the modulating signal, which in turn can generate a large clock-frequency component (CFC) in the received electrical signal when the optical signal has experienced excessive group velocity dispersion (GVD). We calculate the amplitude of these CFCs as a function of GVD for various optical bit sequences and pulse shapes.     

The performance limit of coherent OTDR enhanced with optical fiberamplifiers due to optical nonlinear phenomena

This paper theoretically and experimentally clarifies the limit of incident optical pulse power in coherent optical time-domain reflectometry (C-OTDR) enhanced with optical fiber amplifiers. The critical pulse power, at which the performance of C-OTDR is degraded by the effect of optical nonlinear phenomena in a single-mode optical fiber, depends on the amplified optical pulse waveform and the pulse width. For a pulse width of 1 μs or longer, the incident pulse power is limited by the effect of self-phase modulation (SPM). When an optical pulse having a power gradient within the pulse width is incident to a single-mode optical fiber, the optical frequency of the backscattered signal is shifted by SPM, and the center frequency of the signal moves outside the receiver band, so the sensitivity of C-OTDR is degraded. For a pulse width of 100 ns, the incident optical pulse power is limited by four-wave mixing (FWM) which transfers the energy from the incident optical pulse to Stokes and anti-Stokes light as a result of the interaction between the incident optical pulse and amplified spontaneous emission. This paper also demonstrates the high performance of C-OTDR enhanced with EDF A's with 48, 44, 39, and 29 dB single-way dynamic ranges for pulse widths of 10 μs, 4 μs, 1 μs, and 100 ns, respectively, limited by the effect of SPM or FWM. These results are believed to be the best performance of C-OTDR with EDFA's