Novel passive phase noise cancelling technique for interferometric fibre optic sensors

A novel technique for the cancellation of phase noise in a fibre interferometer caused by laser frequency emission jitter is described. The scheme is based on the use of a reference interferometer and phase generated carrier passive homodyne phase detection. Signal processing involving cross-multiplication of the sine and cosine components of the sensor and reference interferometer outputs is shown to provide cancellation of phase noise.<>     

Input-polarisation scanning technique for overcomingpolarisation-induced signal fading in interferometric fibre sensors

Results of theoretical and experimental studies are presented, which show that polarisation-induced signal fading, caused by random polarisation fluctuations in an interferometric sensor, can be overcome by interrogating the sensor at three pre-defined input SOPs     

干涉型光纤传感器偏振衰落动态补偿技术

为了消除单模光纤制作的光纤传感器常会因外部环境扰动而引起相位衰落和偏振衰落,通过改进传统的马赫-泽德干涉仪,利用高频振荡的PZT结合相位生成载波零差解调电路用来补偿相位衰落,引入一个可随干涉信号的幅值变化旋转的半波片来调整输出的干涉信号,据此可实现偏振衰落的动态补偿.通过理论分析可以得出由偏振衰落造成的信号幅度波动不大于3dB.     

Combined optical fiber interferometric sensors for the detection of acoustic emission

A type of combined optical fiber interferometric acoustic emission sensor is proposed. The sensor can be independent on the laser source and make light interference by matching the lengths of two arms, so it can be used to monitor the health of large structure. Theoretical analyses indicate that the system can be equivalent to the Michelson interferometer with two optical fiber loop reflectors, and its sensitivity has been remarkably increased because of the decrease of the losses of light energy. PZT is powered by DC regulator to control the operating point of the system, so the system can accurately detect feeble vibration which is generated by ultrasonic waves propagating on the surface of solid. The amplitude and the fre- quency of feeble vibration signal are obtained by detecting the output light intensity of interferometer and using Fourier transform technique. The results indicate that the system can be used to detect the acoustic emission signals by the fre-quency characteristics.     

Combined optical fiber interferometric sensors for the detection of acoustic emission

A type of combined optical fiber interferometric acoustic emission sensor is proposed. The sensor can be independent on the laser source and make light interference by matching the lengths of two arms,so it can be used to monitor the health of large structure. Theoretical analyses indicate that the system can be equivalent to the Michelson interferometer with two optical fiber loop reflectors,and its sensitivity has been remarkably increased because of the decrease of the losses of light energy. PZT is powered by DC regulator to control the operating point of the system,so the system can accurately detect feeble vibration which is generated by ultrasonic waves propagating on the surface of solid. The amplitude and the frequency of feeble vibration signal are obtained by detecting the output light intensity of interferometer and using Fourier transform technique. The results indicate that the system can be used to detect the acoustic emission signals by the frequency characteristics.     

Demodulation scheme for polarimetric optical fibre sensors usingderivative technique

A demodulation method for polarimetric optical fibre sensors is described and demonstrated. Two signals with a π/2 phase difference are generated from the sensor output signal and its first derivative with respect to the phase. This method does not require strict modulation conditions for the laser source     

Passive demodulation of optical interferometric sensors

Two techniques for passive (no-feedback) demodulation of signals from a remote two-beam interferometric sensor are discussed. Termed "synthetic heterodyne" and "quadrature recombination," both methods are based on forced sinusoidal modulation of phase within the interferometer. If the phase modulation is generated by linearly proportional frequency modulation of the laser light source, then the interferometer, deployed by means of optical fibers, can be a completely passive remote sensor, and an array of identical sensors can be operated from one light source. The former technique, proposed by Cole, Danver, and Bucaro permits standard FM demodulation. The latter technique, proposed by Cable and Green, employs arithmetic operations that are appropriate to digital implementation. Constraints on application of the technique, especially frequency distribution and amplitude of phase noise (out-of-band) versus signal amplitude (in-band), are analyzed and an implementation is proposed that is consistent with Contemporary analog-to-digital conversion technology.     

光纤传感器检测系统的自适应相位跟踪

本文结合光纤传感器测弱磁场,讨论了含源零差检测系统的模型、电路、自适应相位跟踪及有关分析,并给出了相应的仿真结果。     

Demodulation scheme fibre interferometric sensors employing laser frequency switching

A demodulation technique suitable for use with fibre interferometric sensors is described. The scheme employs a frequency switched diode laser and an unbalanced Mach-Zehnder interferometer. Minimum phase sensitivity ? 2× 10-5 rad is obtainable, and can be shown to be limited by the laser phase noise.     

Statistical modelling of polarisation induced fading in interferometric fibre sensors

The probability functions associated with the polarisation parameters of an optical fibre interferometer, and the concomitant polarisation fading problems which arises have been experimentally and theoretically investigated. Good agreement between experimental results and theoretical models are obtained.<>     

光纤传感系统评述

光纤传感器对于传感各种物理量有着很大的潜力，其中包括声音、磁场、电流、温度、荧光、加速度、速度和旋转率等等。国外已形成一个新的有前途和研究领域，估计到八十年代后期将会进入实用阶段。本文主要介绍光纤传感器的优点和基本原理。     

对光纤位移传感器耦合效率的修正

采用积分的方法，算出了两光纤位移传感器的耦合效率，导出了光纤位移传感器耦合效率的解析表达工，并与常规理论计算的结果进行了比较。     

New polarisation-insensitive detection technique for coherent optical fibre heterodyne communications

A new technique for overcoming polarisation-induced fading in coherent optical fibre communications is described. The technique generates two IFs, which fade in antiphase, by mixing the signal light with orthogonally polarised local oscillator modes which have been separated in the frequency domain.     

New nonlinear phase transduction method for DC measurand interferometric fibre sensors

A new approach to the measurement of displacement using optical fibre interferometry is described. The technique is based on a geometrical arrangement in which linear displacement is transposed nonlinearly into axial fibre strain. The technique can be used in the development of true DC sensors for the displacement, acceleration, pressure and other variables. A displacement sensitivity of ?4 ? is demonstrated.     

Interferometric optical fibre sensors using internal mirrors

Reflectively monitored Fabry-Perot interferometers which make use of dielectric mirrors in continuous lengths of single mode fibre are characterised for use as temperature and wavelength sensors     

干涉型光纤传感器直流相位跟踪检测系统的改进

对干涉型光纤传感器直流相位跟踪检测系统进行了分析和改进，通过在检测系统中加入简单的电路模块，使系统的幅频响应特性得到改善。建立了Michelson光纤干涉型传感系统，实验结果与理论分析相吻合。     

Elliptical-core, two-mode optical fibre sensors as vibration-mode filters

Reggedised two-mode elliptical-core fibre sensors are shown to be selective in sensing the different modal frequencies of vibration from a clamped-clamped beam. Sensor placement on a flexible structure is shown to be critical in the design of fibre sensors that measure an integral of strain over their entire sensing lengths. Experimental results confirm theoretical predictions and efficient vibration-mode selectivity is demonstrated.<>     

Transmissive quasidistributed optical fibre sensing scheme for two-state sensors

A coherence multiplexed sensing scheme for two-state sensors is demonstrated. The system uses conventional white light interferometry to resolve polarisation mode cross coupling sites along a highly birefringent bus fibre. The limitations of such a system, based on a multi-longitudinal mode laser diode source, are identified.<>     

Noise in interferometric optical systems: an optical Nyquisttheorem

The index of refraction of an optical medium at a temperature above absolute zero undergoes statistical fluctuations, which in turn introduce fluctuations in the phase of an optical signal propagating in the medium. The magnitude and spectral density of these phase fluctuations are calculated, and it is shown that they can be larger than the phase uncertainty due to quantum noise. Barring the onset of nonlinearities, these fluctuations and not quantum noise set the limit to the measurement of optical phase     

Electronic technique for temperature compensation of fibre Bragg gratings sensors

Optical sensors composed of fibre Bragg gratings (FBG) are becoming a choice for a wide range of sensing applications. However, in order to manufacture temperature independent sensors, it is necessary to compensate for the intrinsic temperature dependence of the FBG. In this letter we present an extremely effective, simple and low cost novel technique for stabilising the bias point of an interrogation current amplitude detection system which uses a laser diode source and a pair of matched FBGs. An electronic feedback control circuit measures the wavelength shift due to temperature variations in a reference FBG and actuates on the laser diode temperature, to induce changes in its wavelength output and compensate for the temperature dependence of the FBG sensor.