光纤法-珀传感器并联复用的互相关解调

为了实现光纤法-珀传感器并联复用信号的高精度、绝对腔长解调,提出了一种改进的互相关解调法。该方法采用了特殊的相关因子,使得它与复用输出信号的互相关函数在传感器各腔长处为单峰从而有利于腔长比较接近的传感器的分辨;采用分段黄金搜索法减少了计算量,解调速度快。在宽带光源、光谱仪等组成的解调系统中对两只传感器复用进行了实验验证,结果表明:该系统可以实现光纤法-珀传感器并联复用信号的解调,能分辨的传感器之间的最小腔长差小于6ìm,腔长波动小于0.07ìm。     

Stabilization of an embedded fiber optic Fabry-Perot sensor forultrasound detection

A method is proposed to stabilize an intrinsic fiber optic Fabry-Perot interferometric sensor embedded in a solid material for the purpose of detecting ultrasound. Without stabilization the interferometer drifts out of quadrature due to the presence of low-frequency dynamic strains. Stabilization is achieved by using active homodyne stabilization which tunes the laser frequency to maintain quadrature. A control loop shifts the laser frequency by 10 GHz at rates less than 25 Hz in order to compensate for induced drifts. The stabilization procedure was tested for a sensor embedded in an epoxy plate. Ultrasonic pulses, generated by a 5 MHz piezoelectric transducer, were detected with the embedded fiber sensor stabilized in the presence of applied low frequency strains. Improvements in sensitivity which result from stabilization are demonstrated. Additionally, a simulated acoustic emission signal, generated by a lead pencil break (Hsu-Neilson source), was detected with the sensor stabilized in the presence of dynamic strains     

High-resolution interrogation technique for fiber optic extrinsic Fabry-Perot interferometric sensors by the peak-to-peak method

An improved peak-to-peak method is developed for interrogating the absolute cavity length of fiber optic extrinsic Fabry-Perot interferometric (EFPI) sensors with high resolution. A fiber Fabry-Perot tunable filter (FFP-TF) is used to scan the optical spectrum of an EFPI, and the problems caused by the nonlinear performance and poor repeatability of the FFP-TF are removed by using a wavelength calibration technique. A linear fitting is used to calculate the wavelength spacing between two adjacent apexes in the optical spectrum, and the cavity length can be retrieved using this wavelength spacing. The experimental results show that the measuring resolution is improved from 25 to 1 microm, and a linear output is also obtained.     

Fabry-Perot resonator composed of a photoinduced birefringent fiber grating

The periodic mode coupling between the x- and y-polarization modes that is induced by a photoinduced birefringent fiber grating is analyzed through the use of the coupled-mode theory. A novel Fabry-Perot resonator composed of a photoinduced birefringent fiber grating is proposed and theoretically investigated. In the ideal case, the x- and y-polarization resonances of the resonator coincide with each other because of the polarization power exchange by the photoinduced birefringent grating, which results in the total resonator output's being polarization independent. However, if there are some external perturbations (e.g., temperature change or strain), the phase matching is detuned, which could destroy the coincidence between the two polarization resonances, which can further result in the total output resonance's having two peaks. The separation between these two resonant peaks depends on perturbations of the environment (temperature changes or strain).     

Optical acoustic detector based on a fiber Fabry-Perot interferometer

We describe a novel optical acoustic detector based on a bias-controlled fiber Fabry-Perot interferometer. The detector has a broad bandwidth from 10 Mhz to a few gigahertz and higher sensitivity than conventional systems, which are useful for noncontact characterization of microsamples based on laser ultrasound.     

非本征光纤法珀传感器的振动特性研究

针对光纤法珀(Fabry-Perot, FP)超声波传感器振动特性,将传感器薄板振动简化为具有集中参数的二阶振动,推导二阶振动方程的集中参数(力阻抗、等效质量、弹性系数、集中力),获得传感器共振频率、振幅灵敏度与结构参数设计关系。测试直径2.52 mm、厚度150 μm玻璃振动薄板在空气、水中的共振频率分别为205 kHz及115 kHz,水介质中共振频率振幅灵敏度约18 pm/Pa。制作的传感器可测试局放产生超声波的最小声压约1 Pa。     

Using a validated transmission model for the optimization of bundled fiber optic displacement sensors

A variety of intensity-modulated optical displacement sensor architectures have been proposed for use in noncontacting sensing applications, with one of the most widely implemented architectures being the bundled displacement sensor. To the best of the authors' knowledge, the arrangement of measurement fibers in previously reported bundled displacement sensors has not been configured with the use of a validated optical transmission model. Such a model has utility in accurately describing the sensor's performance a priori and thereby guides the arrangement of the fibers within the bundle to meet application-specific performance needs. In this paper, a recently validated transmission model is used for these purposes, and an optimization approach that employs a genetic algorithm efficiently explores the design space of the proposed bundle sensor architecture. From the converged output of the optimization routine, a bundled displacement sensor configuration is designed and experimentally tested, offering linear performance with a sensitivity of -0.066 μm(-1) and displacement measurement error of 223 μm over the axial displacement range of 6-8 mm. It is shown that this optimization approach may be generalized to determine optimized bundle configurations that offer high-sensitivity performance, with an acceptable error level, over a variety of axial displacement ranges. This document has been approved by Los Alamos National Laboratory for unlimited public release (LA-UR 11-03413).     

Extrinsic Fabry-Perot cavity optical fiber liquid-level sensor

An optical fiber liquid-level sensor based on an extrinsic Fabry-Perot (FP) cavity is proposed and demonstrated. The FP cavity consists of the end of the single-mode optical fiber and the elastic silicon layer. Liquid pressures act on the mechanical construction to change the cavity length, resulting in differential phase shifts that may be observed as variations of the output signal intensity. Self-compensated steps have been taken to obtain high accuracy and long-term stability in realistic circumstances. Experimental results indicate that accuracy of 2 mm over a full scale of 3.5 m (water) is obtained under ambient temperature 10-38 degrees C. The sensor can be used to measure liquid levels continuously and accurately in explosive and flammable environments.     

Analysis of a low-finesse Fabry-Perot sensing interferometer illuminated by a multimode optical fiber

A model of the reflected fringe system for an ideal plane-parallel, low-finesse Fabry-Perot (FP) cavity illuminated by a multimode optical fiber has been developed and experimentally validated. This showed that the phase dispersion within the cavity arising from the divergent nature of the incident illumination significantly degrades the visibility of the reflected fringes. Departures from the ideal FP cavity are also examined. The effect on fringe visibility when the plane of the FP cavity is tilted with respect to the fiber axis and when the cavity surfaces are no longer perfectly parallel to each other has been explored. The analysis described is relevant to the design and the optimization of multimode optical-fiber sensors that use FP sensing cavities.     

Optical vernier filter with fiber grating Fabry-Perot resonators

A simple optical vernier filter with fiber grating Fabry-Perot resonators is proposed. This vernier filter comprises two isolated Fabry-Perot fiber resonators in tandem; each is formed with two parallel fiber reflection gratings. A comparison between this vernier filter and one that uses fiber (or waveguide) ring resonators is made. It is found that this vernier grating filter is simpler and usually has better output characteristics than the others.     

Resonator fiber optic gyro employing a semiconductor laser

Resonator fiber optic gyro (RFOG) based on the Sagnac effect has the potential to achieve the inertial navigation system requirement with a short sensing coil. Semiconductor laser is one of the key elements for integration and miniaturization of the RFOG. In this paper, an RFOG employing a semiconductor laser is demonstrated. The model of the laser frequency noise induced error in the RFOG is described. To attenuate the laser frequency noise induced error, active frequency stabilization is applied. An online laser frequency noise observation is built, as a powerful optimum criterion for the loop parameters. Moreover, the laser frequency noise observation method is developed as a new measurement tool. With a fast digital proportional integrator based on a single field programmable gate array applied in the active stabilization loop, the laser frequency noise is reduced to 0.021 Hz (1σ). It is equivalent to a rotation rate of 0.07°/h, and close to the shot noise limit for the RFOG. As a result, a bias stability of open-loop gyro output is 9.5°/h (1σ) for the integration time 10 s in an hour observed in the RFOG. To the best of our knowledge, this result is the best long-term stability using the miniature semiconductor laser.     

光纤耦合问题的研究及球形端面光纤的应用

介绍了光源与光纤耦合的几种方法以及球形光纤的制作方法，采用射线理论分析了圆球形端面多模光纤的端面效应，给出了半导体激光器（LD）与圆球形端面光纤耦合的应用实例。实验结果表明该方法结构简单、调整方便，有实际应用的潜力。     

Error estimate of a fiber optic displacement transducer installation

Translated from Izmeritel'naya Tekhnika, No. 5, pp. 19–20, May, 1990.     

Exact analysis of low-finesse multimode fiber extrinsic Fabry-Perot interferometers

A straightforward theory is presented to accurately model the light inferences in a low-finesse multimode fiber extrinsic Fabry-Perot (FP) interferometer. The effect on the fringe visibility of the gap length, sensor structure imperfections, and modal power distributions is explored. The analysis is particularly useful in the design and optimization of sensors that use an extrinsic FP cavity as the sensing element.     

光纤流量计初探

基于光纤的性能优势和光纤传感器的发展前景，文章利用光的强度受流量调制的特性，在传统涡街流量传感器基础上，设计了一种光纤流量计。文章阐述了光纤微弯损耗机理和光纤涡街流量传感器的工作原理，在此基础上提出了光纤涡街流量计系统设计模型，并进行了光纤涡街流量计的稳定性分析。     

Intermodal beat length measurement with Fabry-Perot optical fiber cavities

We present a new technique for measuring the intermodal beat length of a two-mode optical fiber. We formed a Fabry-Perot fiber cavity by depositing reflective mirrors on the fiber tips. As the fiber is stretched, two series of resonance peaks are observed. One series is due to the resonance of LP(01 q) modes and the other is due to the LP(11 q) modes. From the separation of resonance peaks as a function of cavity length, we deduce the intermodal beat length of the fiber. The measurement principle and the experimental confirmation are discussed.     

Frequency-estimation-based signal-processing algorithm for white-light optical fiber Fabry-Perot interferometers

A novel signal-processing algorithm based on frequency estimation of the spectrogram of single-mode optical fiber Fabry-Perot interferometric sensors under white-light illumination is described. The frequency-estimation approach is based on linear regression of the instantaneous phase of an analytical signal, which can be obtained by preprocessing the original spectrogram with a bandpass filter. This method can be used for a relatively large cavity length without the need for spectrogram normalization to the spectrum of the light source and can be extended directly to a multiplexed sensor system. Experimental results show that the method can yield both absolute measurement with high resolution and a large dynamic range. Performance analysis shows that the method is tolerant of background noise and variations of the source spectrum.     

System identification application using Hammerstein model

Generally, memoryless polynomial nonlinear model for nonlinear part and finite impulse response (FIR) model or infinite impulse response model for linear part are preferred in Hammerstein models in literature. In this paper, system identification applications of Hammerstein model that is cascade of nonlinear second order volterra and linear FIR model are studied. Recursive least square algorithm is used to identify the proposed Hammerstein model parameters. Furthermore, the results are compared to identify the success of proposed Hammerstein model and different types of models.     

A multi-purpose reflective fiber optic sensor

Fiber optic sensors, including reflective ones, have widespread applications in sensing technology. In this study, we first formulated the gain of a general reflective fiber optic sensor in terms of five major sensor parameters, and for two patterns of input intensity: namely uniform and Gaussian. By using these formulae, we have analyzed and plotted the variations of gain versus one or two of these parameters. The results obtained have exact compatibility for the two patterns of input intensity, although they have been obtained by different methods. The results can be suitably used in the optimum design of a wide variety of reflective fiber optic sensors.