Impact of Reflection Induced Noise on AM-VSB CATV Lightwave Systems

1 Introduction The lightwave CATV systems recently can provide the extremely high-fidelity transmission required by the AM-VSB video format. These systems can also provide the satisfactory performance of CNR, CSO and CTB. Such performance has been realize…     

Impact of Reflection Induced Noise on AM-VSB CATV Lightwave Systems

The effect of reflection on the performance of AM-VSB CATV lightwave systems is analyzed. Usually, there are three kinds of reflection existing in the AM-VSB CATV lightwave systems: multipath interference (MPI), double Rayleigh backscattering (DRB) and external optical feedback. The MPI and external optical feedback can be neglected owing to using the FC/APC connectors and isolated DFB lasers in the system link. It is shown theoretically and experimentally that the DRB is the limitation of the noise induced by reflection.     

DFB FL Sensor Cross-Coupling Reduction

The effect of weak external reflections on distributed feedback (DFB) fiber laser (FL) sensor frequency noise has been studied. For multiplexed DFB FL arrays, it was demonstrated that out-of-band reflections from DFB FL raise the noise level of adjacent lasers by up to 50 dB. It was further demonstrated that by incorporating a complex phase- and amplitude-apodized FL grating design with high sidelobe suppression, cross coupling due to out-of-band reflections can be reduced to negligible levels, and Rayleigh backscatter limited noise performance can be achieved. Practical limitations to acoustic sensitivity of DFB FL sensors due to increased laser frequency noise are discussed, and ways to reduce DFB FL sensitivity to external reflections are proposed     

采用级连反向泵浦喇曼光纤放大器的长距离光纤传输系统中双重瑞利散射噪声的抑制

给出了反向泵浦喇曼光纤放大器中信号和双重瑞利散射噪声的计算方法,分析了双重瑞利散射噪声的特性,计算了采用光隔离器进行双重瑞利散射噪声抑制时,光隔离器的最佳位置。研究了使用多级反向泵浦喇曼光纤放大器的1080km光纤通信系统的传输特性,比较了信号和双重瑞利散射噪声在加入光隔离器前后沿传输光纤的功率变化。加入光隔离器以后,信号所受的影响不大,但双重瑞利散射噪声能得到有效抑制,可减少约三个数量级。1     

Intensity noise characteristics of erbium-doped distributed-feedback fiber lasers

We present an experimental and theoretical investigation into the low-frequency intensity noise characteristics of erbium-doped distributed feedback (DFB) fiber lasers. The intensity noise characteristics of six nonidentical erbium-doped DFB fiber lasers are presented along with the characteristics of the grating and doped fibers. An analytical model has been used to predict the intensity noise generated in a linear fiber laser and explain the observed noise characteristics. Overall we find good agreement between our analytical model and observations. In particular, we find the intensity noise at frequencies close to the relaxation oscillation frequency significantly elevated due to excess noise from either spontaneous emission or cavity loss modulation. These results can be used to optimize the fiber laser design for sensor applications.     

Experimental and theoretical investigations of coherent OFDR withsemiconductor laser sources

Experimental and theoretical investigations of coherent optical-frequency-domain reflectometry using semiconductor laser sources are presented. Good agreement was found between the analysis of the signal-to-noise ratio due to the phase noise and the experimental results. The sensitivity limit due to the quantum noise is also described. Limitations due to the nonlinearity in the optical frequency sweep produced by the thermal-response time of the laser and mode hopping are investigated and compared with experimental results. Two interferometric methods to characterize the thermal-response time of the laser and their implementations are described. The effects of mode hopping in the optical-frequency sweep are compared to numerical simulations. A simple formula to predict the position of spurious peaks due to mode hopping are presented. A spatial resolution of 400 μm over 10 cm was obtained by correcting the nonlinearity in the optical-frequency sweep by using an auxiliary interferometer. The Rayleigh backscattering was observed for the first time over more than 400 m of fiber using a DFB laser coupled to an external cavity     

光纤激光传感技术及其应用

超高灵敏度的信号探测在石油勘探、地震预报和安全监测等领域都具有重要的应用价值.近年来出现了一种以分布反馈(DFB)光纤激光器为传感元件的新一代光纤传感器,它具有尺寸小、输出激光信号极窄的光谱线宽和极低的噪声等优势,与高分辨率波长解调技术结合可以达到极高的探测灵敏度.介绍了在光纤激光传感技术及其应用技术方面的研究进展,包括线宽仅为3 kHz、尺寸仅为3.6 cm的窄线宽低噪声DFB光纤激光器的研制及其测试,波长分辨率达3.5×10~(-7) pm/Hz~(1/2)的超高分辨率波长解调系统,基于密集波分复用的光纤激光传感网络,以及相关技术在水声和地震波探测中的应用研究.     

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。     

A DFB Fiber Laser Sensor System with Ultra-High Resolution and Its Noise Analysis

A distributed feedback fiber laser （DFB FL） sensor system with ultra-high resolution is described. The sensor is made by writing distributed feedback structures into a high gain active fiber, and the system employs an unbalanced Michelson interferometer to translate laser wavelength shifts induced by weak measurands into phase shifts. A digital phase generated carrier demodulation scheme is introduced to achieve ultra-high resolution interrogation. A detailed noise analysis of the system is presented, and it is shown that the system resolution is limited by the frequency noise of the DFB FL.     

一种具有迈克耳逊结构的新型光时域反射仪

利用传输光纤中后向瑞利散射光携带有光纤位置、外界振动等信息的特点,采用光纤迈克耳逊结构构成干涉型光时域反射仪,有望以分布方式传感沿光纤的微小振动信号.文中设计了一种新型结构的分布式光纤振动传感器,该传感器用3dB耦合器将波长为1310nm的红外激光分为两路相干光:参考光和传感光,它们中的后向瑞利散射光在耦合器处发生干涉,干涉信号的变化点对应于振动点所在的位置.将没有振动信号、加微小振动信号和加损耗干扰三种情况分别作用于传感光纤并进行了试验对比.结果表明,在微小振动的情况下,光强变化微弱;加损耗时,光强变化显著,定位准确.     

单模光纤中高相干光源的瑞利散射光的统计特性

基于光纤中高相干光源(相干长度大于100km)的瑞利散射光的统计特性,采用随机相向量模型,推导了光纤中瑞利散射光的功率分布和相位分布,指出瑞利散射光功率的概率密度函数(PDF)符合修正的莱斯分布,而瑞利散射光的相位分布近似为高斯分布。采用高灵敏度的光电探测器对不同长度的光纤的瑞利散射光功率进行了测试,实验结果与理论吻合,同时发现不同长度光纤的瑞利散射光功率的最大值随平均值线性增加。     

Spectral properties of Rayleigh backscattered light fromsingle-mode fibers caused by a modulated probe signal

The statistical properties of Rayleigh backscattering from single-mode fibers are investigated, taking into account the modulation and the coherence of the source light. The autocorrelation function and the power spectral density of the backscattered field and intensity are derived. To describe the noise due to Rayleigh backscattering in bidirectional systems, the spectrum of the interference of backscattered light with reflected source light is also calculated. The measurements confirm the theoretical results. It is shown that, if no optical isolator is used and the laser diode is exposed to Rayleigh backscattering, the interaction between laser and fiber depends strongly on the modulation of the light     

Narrow linewidth fiber laser for 100-km optical frequency domain reflectometry

A diode-pumped single-frequency piezoelectrically tuned fiber laser with narrow spectral linewidth has been used as a light source in applications for long-range coherent frequency-domain reflectometry. Frequency-modulated continuous-wave (FMCW) measurements of Rayleigh back-scattering and Fresnel reflection from a 95-km-long fiber have been demonstrated without the use of an optical amplifier. This is, to our knowledge, the longest distance measurement with FMCW. The high sensitivity and dynamic range of the long-range backscattering measurements benefit from the extremely long coherence length of the narrow linewidth fiber laser, which has been estimated to be 210 km in air.     

Analysis and observation of coupling ratio dependence of Rayleighbackscattering noise in a fiber optic gyroscope

Noise due to Rayleigh backscattering in a fiber-optic gyroscope is observed in quadrature with the Sagnac signal. This noise can be nulled by adjustment of the coupling ratio of the loop coupler and the state of polarization in the gyro loop. A rigorous analysis of the phases of, and the modulations applied to, the primary and scattered beams shows that no coherent Rayleigh backscatter noise should be detected on the Sagnac signal even with relatively coherent semiconductor laser sources     

Numerical simulation of intensity and phase noise from extracted parameters for CW DFB lasers

A self-consistent numerical approach is demonstrated to analyze intensity and phase noise from experimentally extracted parameters for a continuous-wave distributed feedback (DFB) laser. The approach takes into account the intrinsic fluctuations of the photon number, carrier number, and phase. Values for the parameters appearing in the rate equations are extracted from the measured relative intensity noise spectra and linewidth of the laser. The simulation of the frequency spectra of intensity and phase noise of the DFB laser are performed by fast Fourier transform and exhibit good agreement with experimental results. The model presented here can readily be extended for the purpose of system simulations.     

低掺杂铒纤上分布反馈布拉格光纤激光器的制作

采用准分子激光器成功地在低掺杂普通铒纤上制作出5 cm的光纤光栅分布反馈布拉格(DFB)激光器,铒纤的峰值吸收率为5 dB/m,在100 mW,980 nm抽运光条件下,光纤激光器的输出功率为50μW,边模抑制比为50 dB。使用耦合模理论分析了一段5 cm带相移的分布反馈布拉格光纤激光器输出光强同腔内损耗及相移量的关系,计算结果表明,光纤腔内的损耗对激光器的输出具有非常重要的影响,大的损耗对应获得最大功率的光栅耦合强度相应减小,因此,在低掺杂铒纤上制作分布反馈布拉格激光器必须正确估计光纤激光器的腔内损耗,选择合适的光栅耦合强度,可以获得较大的输出功率。     

Statistical properties of Rayleigh backscattering in single-modefibers

An analytical model that takes into account the influence of the laser linewidth on Rayleigh backscattering is given. For an unmodulated source the power spectral density is found to be identical to the delayed self-homodyne spectrum of the laser. The backscattered signal was measured for different laser linewidths, and the results are compared with the theory. The effects of Rayleigh backscattering noise on fiber gyroscopes and bidirectional optical systems are discussed     

An Ultralow Noise and Narrow Linewidth λ/4-Shifted DFB Er-Doped Fiber Laser With a Ring Cavity Configuration

We report a polarization-maintaining lambda/4-shifted distributed feedback (DFB) Er-doped fiber laser with a ring cavity configuration. The ring cavity suppressed the self-pulsation of the stand-alone Er-doped DFB fiber laser. The laser with a 57-m-long ring cavity achieved single-longitudinal-mode operation, a linewidth as narrow as 6 kHz, and relaxation-oscillation-free noise characteristics.     

Fiber-reflection-induced impairments in lightwave AM-VSB CATVsystems

Impairments due to multiple fiber reflections are evaluated for amplitude-modulated (AM) vestigial-side-band (VSB) lightwave cable TV (CATV) systems using directly modulated distributed feedback (DFB) lasers or externally modulated diode-pumped YAG laser transmitters. For DFB lasers, the spectral broadening caused by laser chirp results in a Gaussian optical spectrum with a width near 5 GHz. Square-law detection of multipath signals from two -30-dB reflections results in an effective intensity noise near -150 dB/Hz. For externally modulated YAG lasers, the same multiple reflections result in interference that cannot be described by an effective RIN. Although the total interference power is comparable to that of the DFB lasers, the signal and interference are highly correlated. As a result of this correlation, multipath interference for YAG-based systems cannot be measured using the standard test procedure with unmodulated carriers     

Fundamental Thermal Noise in Distributed Feedback Fiber Lasers

A detailed analysis of fundamental thermal frequency noise in distributed feedback fiber lasers (DFB FL) is presented. An expression relating the laser frequency noise spectrum to the spectrum of thermal (temperature) fluctuations in the fiber, accounting for the nontrivial DFB FL mode shape, is derived. A new expression for the thermal fluctuation spectrum of single-mode optical fiber is derived which predicts high-frequency thermal noise 3 dB higher than previously published results. Theoretical predictions are compared against experimental measurements for a well characterized DFB FL. Anomalous 1/f noise at low frequency is discussed