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。     

相位敏感光时域反射系统低频响应性能优化

相位敏感光时域反射系统以其分布式光纤传感技术的优势在分布式水听、压裂微地震检测、自然灾害预警等低频监测领域具有极高的应用前景。文中对系统中脉冲斩波信号与频率调制信号时钟不同源的问题予以验证,并对其产生的影响进行理论分析;设计双路同步时钟源驱动产生脉冲斩波信号和频率调制信号,降低每个脉冲重复周期中频率调制信号的随机低频相位噪声,提高探测脉冲光的相位稳定性;采用时钟同源和时钟非同源两种方式对典型的基于外差相干检测的相位敏感光时域反射系统的声光调制器进行驱动,由信号发生器驱动缠有光纤的压电陶瓷,产生不同频段的扰动信号。实验结果表明:在同一测试条件下,前者在低频段的信噪比、相位解调质量、频率响应方面均优于后者,最小响应频率为0.1 Hz,相对提高两个数量级,降低了系统中低频噪声干扰。该方法易于实现,可与现有的低频性能优化方法或结构兼容,进一步提高系统低频响应性能。     

Frequency estimation in slowly fading multipath channels

This paper concerns the estimation of a frequency offset of a known (pilot) signal propagated through a slowly fading multipath channel, such that channel parameters are considered to he constant over the observation interval. We derive a maximum-likelihood (ML) frequency estimation algorithm for additive Gaussian noise and path amplitudes having complex Gaussian distribution when covariance matrices of the fading and noise are known; we consider in detail the algorithm for the white noise and Rayleigh fading, in particular, for independent fading of path amplitudes and pilot signals with diagonal autocorrelation matrices. For the latter scenario, we also derive an ML frequency estimator when the power delay profile is unknown, but the noise variance and bounds for the path amplitude variances are specified; in particular, this algorithm can be used when path delays and amplitude variances are unknown. Finally, we consider frequency estimators which do not use a priori information about the noise variance; these algorithms are also operable without timing synchronization. All the frequency estimators exploit the multipath diversity by combining periodograms of multipath signal components and searching for the maximum of the combined statistic. For implementation of the algorithms, we use a fast Fourier transform-based coarse search and fine dichotomous search. We perform simulations to compare the algorithms. The simulation results demonstrate high accuracy performance of the proposed frequency estimators in wide signal-to-noise ratio and frequency acquisition range.     

Advances in optical time domain reflectometry

Advances in optical time-domain reflectometry (OTDR), such as enlargement of dynamic range, enhancement in resolution, reduction of noise intrinsic to single-mode fibers, and increase in user friendliness of the equipment, are reviewed. The gated detection technique and recent progress in OTDR are presented. Future optical network diagnostics are discussed     

A performance enhanced Rayleigh Brillouin optical time domain analysis sensing system

Aiming at the problem of large fading noise in Rayleigh Brillouin optical time domain analysis system, a wavelength scanning technique is proposed to enhance the performance of the temperature sensing system. The principle of the proposed technique to reduce the fading noise is introduced based on the analysis of Rayleigh Brillouin optical time domain analysis system. The experimental results show that the signal-to-noise ratio (SNR) at the end of optical fiber with length of 50 m after 17 times wavelength scanning is 5.21 dB higher than that with single wavelength, the Brillouin frequency shift (BFS) on the heated fiber with length of 70 m inserted at the center of sensing fiber can be accurately measured as 0.19 MHz, which is equivalent to a measurement accuracy of 0.19 °C. It indicates that the proposed technique can realize high-accuracy temperature measurement and has huge potential in the field of long-distance and high-accuracy sensing.     

An Exact Error Probability Analysis of OFDM Systems with Frequency Offset

In this paper, we derive exact closed form bit error rate (BER) or symbol error rate (SER) expressions for orthogonal frequency division multiplexing (OFDM) systems with carrier frequency offset (CFO). We consider the performance of an OFDM system subject to CFO error in additive white Gaussian noise (AWGN), frequency flat and frequency selective Rayleigh fading channels. The BER/ SER performances of BPSK and QPSK modulation schemes are analyzed for AWGN and frequency-flat Rayleigh fading channels while BPSK is considered for frequency-selective Rayleigh fading channels. Our results can easily be reduced to the respective analytical error rate expressions for the OFDM systems without CFO error. Furthermore, the simulation results are provided to verify the accuracy of the new error rate expressions.     

Multipath effects and adaptive transmission in presence of indoor power line background noise

The paper studies the background noise characteristics of a typical indoor power line (PL) in India. The noise decreases rapidly with frequency and can be considered as white beyond 5 MHz. The time variation of the background noise at every frequency in the non‐white region is modeled and compared in different environments which include laboratory and factory setups. In all cases, the background noise can be modeled by density functions related to multipath fading i.e. Nakagami‐m, Rayleigh and Rician. Fading is worse than the Rayleigh fading when the PL network has more connected loads and impedance discontinuities. Single frequency noise shows Rician fading when the background noise is narrowband Gaussian. The results are compared with those obtained in other countries in an attempt to check the validity of the model worldwide. A PL channel noise simulator is developed using the density functions to simulate the background noise. A technique is proposed that adapts the carrier frequency according to the channel property at a particular instant. The performance of such a system is tested in a simulated environment and shows that the method enables transmission of signals at very low transmission power compared with that in the AWGN channel. Copyright © 2009 John Wiley & Sons, Ltd.     

On the characterization of optical fiber network components withoptical frequency domain reflectometry

The basic features of coherent optical frequency domain reflectometry are presented. The ultimate limits of range and sensitivity were discussed, as well as polarization effects. It is shown that this technique is very suitable for optical network components characterization, and accurate measurements of a number of such components are presented     

基于相位-强度调制转换的光纤色散精确测量方法

在分析光纤色散导致的相位-强度调制转换的基础上,提出了一种利用电光相位调制的光纤色散扫频测量方法。方法的原理为光纤色散使相位调制信号获得附加相移并产生周期性衰落,从衰落曲线的特征性凹陷频率确定出光纤色散。实际运用中,由于凹陷频率附近的信号弱,因此噪声大且不稳定。为了解决这一问题,通过衰落曲线的多项式拟合,进一步提高凹陷频率和光纤色散的测量精度。实验中,对长光纤或者短光纤分别测试以验证本文方法对于不同色散的适应性。实验结果表明,本文方法的相对误差小于0.22%。使用矢量网络分析仪(VNA)和相位调制器进行测试,可工作于不同光波长,适用于测量不同种类的光纤的色散;并且可以利用简单的实验系统,实现光纤色散的大小和符号的测量。     

Linearizing optical frequency-sweep of a laser diode for FMCWreflectometry

We propose and demonstrate a novel linearizing method of optical frequency-sweep of a laser diode for frequency-modulated continuous-wave (FMCW) reflectometry. In order to linearly sweep the optical frequency, we adopt a reference interferometer and an electric phase comparator. The interference beat signal of the reference interferometer is phase-compared with an external reference rectangular signal having a fixed frequency near the interference beat signal frequency by a lock-in amplifier. The error signal from the lock-in amplifier is fed back to the modulating signal of the injection current of the laser. Thus, a phase-locked loop composed of optical and electric circuits can be established, and the beat signal frequency is locked to the frequency of the reference signal. The optical frequency of the laser diode is, therefore, excellently linearly swept in time. In order to experimentally confirm the linearity of the proposed method, we apply this frequency-swept laser diode to the FMCW reflectometry. Resultingly, the improvement of the linearity is estimated to be about 10 dB. And the theoretically limited spatial resolution of the FMCW reflectometry is achieved. The backscattered light in optical waveguide devices is measured by the FMCW reflectometry using the proposed light source, and the propagation loss of a single-mode glass waveguide is successfully evaluated     

Coherent optical frequency domain reflectometry (OFDR) using afiber grating external cavity laser

An optical frequency domain reflectometry (OFDR) system containing a narrow linewidth fiber grating external cavity laser is demonstrated to have 62-dB of sensitivity when detecting Fresnel backreflection and 2 m of resolution at a 115 m range in optical fiber. With this system we were able to detect Rayleigh backscattering in optical fiber with 20-dB signal-to-noise ratio. The phase noise limitation on the distance range for the OFDR was investigated, and the measured signal-to-noise ratio (SNR) data followed the theoretical simulation over the ranges measured. This technique has potential to be applied to the OFDR at 1550 mm with very high dynamic range by using an erbium doped fiber laser     

BER Analysis of OFDM Systems Impaired by Phase Noise in Frequency-Selective Fading Channels

In this paper, we study the effect of finite-power, phase-locked loop based phase noise on the bit-error-rate (BER) performance of orthogonal frequency division multiplexing (OFDM) systems in frequency-selective fading channels. It is well known the impact of phase noise on the performance of an OFDM system can be divided into a multiplicative term called common phase error (CPE) and an additive term called intercarrier interference (ICI). Based on the conditional Gaussian approximation technique, we first derive the BER formulas for BPSK, QPSK, 16-QAM, and 64-QAM modulated OFDM signals in frequency-selective Rayleigh fading channels. To further quantify the individual influence of the CPE and the ICI on system performance for different phase noise spectra, we derive the BER expressions for perfect CPE compensation cases. The analytical results obtained for frequency-selective Rayleigh fading channels are then generalized to frequency-selective Rician fading channels. Simulation results not only validate the accuracy of our analysis but also show the dependency of BERs on the shapes of phase noise spectra.     

An Enhanced Inter-carrier Interference Reduction Scheme for OFDM System with Phase Noise

Inter-carrier interference (ICI) reduction techniques achieve a better carrier-to-interference ratio (CIR) in OFDM system in the presence of synchronisation errors. However, the frequency diversity available on the frequency-selective channel has not been utilised by conventional ICI reduction techniques. In this paper, the frequency diversity of ICI reduction methods in the presence of phase noise over frequency-selective fading channels is analysed. Based on the analysis, an ICI reduction technique is proposed, enhanced symmetric data-conjugate (ESDC) technique, to enhance the frequency diversity in multipath fading channel. The carrier-to-interference ratio (CIR) and common phase error (CPE) of the proposed ICI reduction scheme are derived and the BER performance of the proposed system is compared with the conventional ICI reduction methods such as adjacent data-conjugate (ADC) and symmetric data-conjugate (SDC) methods. Simulation results reveal that the proposed ICI reduction scheme provides an improvement in BER performance over a fading channel and it is also better than conventional ICI reduction techniques in the presence of ICI due to phase noise.     

Performance of FH SS radio networks with interference modeled as amixture of Gaussian and alpha-stable noise

We consider the performance of frequency-hopping spread spectrum (FH SS) radio networks in a Poisson field of interfering terminals using the same modulation and power. The problem is relevant to wireless random-access communication systems where little information about transmitters requires stochastic modeling of their positions. Assuming that the signal strength is attenuated over distance r on average as 1/r ^m, we show that the interference in the network could be modeled as a mixture of Gaussian and α-stable noise. Based on this modeling, we derive expressions for the probability of error (P_e ) for systems with M-ary frequency shift keying (FSK) which use conventional envelope detectors. Because conventional envelope detectors are optimum only in Gaussian noise and are suboptimum in the noise considered, we also investigate noncoherent detectors which offer improved performance. We examine receivers with limiting nonlinearities and detectors which are optimal in Cauchy noise. Numerical calculations and Monte Carlo simulations are provided to confirm the accuracy of the analysis presented. The results obtained are useful in the performance evaluation of multiple-access radio networks in environments varying from urban settings to office buildings with deterministic and stochastic propagation laws such as lognormal shadowing and Rayleigh fading     

基于OFDR技术的深层土体水平位移场监测研究

深层土体水平位移场监测是基坑安全开挖的重要指标,现有测斜法耗费大量人力,且精度较低,但仍是当前获得水平位移的主要方法。该文提出了一种基于光频域反射(OFDR)技术的分布式光纤测量法,推导了应变-挠曲变形转换关系,通过室内实验进行了验证,证实了分布式光纤测斜的可行性和准确性,且基于OFDR的测量方法在苏州某基坑工程中得到了应用,准确详细地获取了基坑在开挖过程深层土体位移信息。     

Interferometer-less coherent optical range finder

A new coherent optical detection technique employing coherent frequency-domain reflectometry and a novel optical frequency sensor is demonstrated for high-precision optical path-length measurements. Using pulsed laser sources, an improvement of more than two orders of magnitude in spatial resolution over conventional optical coherent frequency domain reflectometry techniques is demonstrated. Varying degrees of spatial resolution ranging from several centimeters to a few hundred nanometers are achieved. High-precision distance measurement with long baseline is also presented     

Improved Frequency Domain Channel Estimation and Equalization for MIMO Wireless Communications

This paper introduces an improved frequency domain channel estimation method based on interpolation vectors for single carrier frequency domain equalization (SC-FDE) with the multiple-input multiple-output (MIMO) scheme. The proposed algorithm is derived by employing the least squares (LS) criterion, and a specified application for the wide sense stationary uncorrelated scattering (WSSUS) Rayleigh fading channel is presented. The channel frequency domain responses estimated at two adjacent pilot blocks are used to track the time-variant channel information, which can effectively improve the accuracy of channel estimation without significantly increasing complexity. Maximum mean square error (MMSE) frequency domain equalization based on the estimated channel is employed in the receiver to recover transmitted signals. This paper also investigates a training sequence design method for multiple transmit antennas and a noise variance estimation method. Numerical simulation results show that the proposed methods can perform very well for fading channels with long multipath delay and high Doppler spread.     

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...     

Modulation and demodulation techniques in optical heterodyne PSKtransmission systems

Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described