Multiplexing of nonlinear fiber-optic interferometric sensors

The scheme is demonstrated with a three-sensor array consisting of magnetic-field, pressure, and displacement sensors exhibiting resolutions at 1 Hz of 20 μOe/√Hz (limited by ambient noise), 0.1 kPa/√Hz, and 30 nm/√Hz, respectively. Multiplexing is accomplished by dithering each transducer at a different frequency and using parallel phase-sensitive detectors (PSD) at the interferometer output to demultiplex the information. A single Mach-Zehnder interferometer driven by a single unmodulated laser was used. Factors affecting the performance of the multiplexing scheme, including interferometric demodulation technique, PSD performance, and nonlinear transduction mechanism, are discussed     

Latest development of fiber-optic fibry-Perot interferometric sensors

简单介绍了光纤F-P传感器的工作原理,然后根据光纤F-P传感探头的制作方法进行分类,从传感特性的角度阐述了当前F-P传感器的研究方向和现状,最后给出了其发展趋势.     

Large-scale multiplexing of interferometric fiber-optic sensorsusing TDM and DWDM

This paper describes multiplexing schemes for interferometric fiber sensors based on time-division multiplexed and dense wavelength-division multiplexing using optical add/drop multiplexers. The results of an experimental arrangement, which is based on one of the architectures, is also presented. Topics include a discussion of the noise sources in the system, dynamic range, and a characterization of the distributed feedback fiber laser source noise. We show the crosstalk levels in the experimental arrangement to be between -47 and -76 dB depending on the mechanism involved. The multiplexing schemes demonstrate the potential to address at least 192 interferometric sensors through two fibers based on a system with six wavelengths with a phase resolution less than 20 μrad/√Hz. For application to sonar arrays, our analysis has shown that hydrophones multiplexed in this type of architecture would achieve ambient acoustic noise-limited pressure resolution with an in-water dynamic range up to 135 dB at frequencies up to 10 kHz. In general, these architectures would find application in systems requiring very large numbers of sensors with a minimum of telemetry cabling required     

Self-referencing multiplexing technique for fiber-optic intensity sensors

An all-optical technique for multiplexing and self-referencing a number of intensity modulating fiber-optic sensors is described. The optical transducers are fabricated as integral parts of recirculating optical fiber loops connected in parallel between transmit and receive optical fibers. A portion of an input pulse is tapped off by each sensor loop module. Successive fractions are tapped off on each circulation around the loop and transmitted to a detector. These form pulse trains that characterize each sensor's output. The relative magnitudes of the components of the pulse train are insensitive to lead and connector losses between the sensor modules and the source and detector. Time division multiplexing of the sensor return signals is created by the delays introduced by the fiber leads between the sensor modules. This paper details the response of a single sensor loop versus theoretical performance, and a three sensor system is demonstrated.     

Demodulation of interferometric sensors using a fiber-optic passive quadrature demodulator

The passive quadrature demodulator (PQD) eliminates the phase stretcher and feedback electronics frequently used in fiber interferometric sensors by passively extracting the desired signal using two distinct interferometers which differ in phase bypi/2. A fusion technique is described to fabricate a fiber PQD which is sufficiently stable with respect to temperature, polarization, and wavelength to maintain the sensitivity of interferometric sensors constant to 0.25 dB.     

Recent progress in fiber-optic extrinsic Fabry–Perot interferometric sensors

Fiber-optic extrinsic Fabry–Perot (EFPI) sensors have been successfully used for a wide range of applications. The recent progress in EFPI sensors is reviewed in this paper. First, the optical amplification technology is adopted into the EFPI sensor system to enhance the interferometric signal considerably. Second, both spatial-frequency multiplexing and coarse-wavelength-division multiplexing technologies are demonstrated for multiplexing of a modified EFPI sensor called the Fizeau sensor. Finally, the EFPI sensor is integrated with other fiber-optic sensing elements to realize the measurement of multiple parameters, for example, the EFPI is integrated with the in-fiber Bragg-grating (FBG) or the long-period-fiber-grating (LPFG) to achieve simultaneous strain and temperature measurement. Furthermore, simultaneous measurement of displacement, transverse load, static strain, temperature, and vibration can be achieved by using the combination of EFPI/FBG/LPFG.     

General sensitivity enhancement and common-mode compensationprinciple for interferometric fiber-optic sensors

Numerical simulations are presented which show that, when an optical circuit for common-mode compensation and an electrooptic feedback network for measure and sensitivity enhancement are incorporated into any of four main types of interferometric fiber-optic sensors, measured sensitivity can be enhanced by at least an order of magnitude, while sensitivity to common-mode environmental perturbations (such as source wavelength drift or spatially uniform temperature variation) is drastically reduced     

A fiber-optic interferometric seismometer

A fiber-optic interferometric sensor has been developed which consists of a seismic mass of 520 gm supported by two rubber mandrels, each wound with a single layer of single-mode optical fiber 6.5 m long. One end of each fiber is cleaved to enhance reflection. The other ends are interconnected via a fiber-to-fiber 3-dB coupler, forming a Michelson interferometer. When the case of the sensor is displaced, the fiber around one mandrel extends in length while the other contracts. The resulting "push-pull" mechanical operation of the sensor allows both legs of the interferometer to be active, providing good common mode rejection of spurious effects, as a reference leg is not required. This, together with the fact that the light traverses each leg of a Michelson interferometer twice due to reflection, provides the sensor with four times the sensitivity of a conventionally constructed interferometric sensor. Sensitivities of 8500 rad of optical phase shift per micrometer of case displacement have been measured above the mass-spring resonance, where the sensor operates as a seismometer. Below resonance the sensor operates as an accelerometer with a measured sensitivity of 10 500 rad/g, the highest reported to date. Including both thermodynamic and demodulator noise sources (approx 10 murad/sqrt{Hz}), below resonance the sensor has a detection threshold of 1 ng/sqrt{Hz}, a 20- dB improvement over the best existing conventional low noise vibration sensors.     

集成光学干涉仪光纤陀螺

简述了集成光学干涉仪光纤陀螺（ＦＯＧ）的技术优势、发展概况和采用的技术途径，介绍了适用于不同技术条件要求的三种多功能集成光学芯片结构和相应的信息处理方法。简要地概括了目前国外ＦＯＧ的发展水平。     

Frequency division multiplexing of fiber-optic gyroscopes

The principle of operation of a 2-D frequency-division-multiplexed fiber-optic gyroscope (FDM-FOG) system is presented under the assumption that the two gyro signals do not interfere with each other. The theory is expanded to include the residual cross-interference effect, which may be minimized by proper choice of modulation depths or frequencies. A 2-D FDM-FOG system is realized with a noise equivalent rotation rate of 0.23°/h (0.25 Hz NEB)     

Side-projected fiber-optic velocity sensors

Side-projected dual-fiber velocity sensors with various fiber core separations for in vivo measurements were developed and tested. Good linearity between the flow velocity and the Doppler shift frequency is obtained with this sensor in both the forward (toward the sensor tip) and reverse (away from the sensor tip) flow directions. Experimental and numerical analysis of the data showed that the boundary layer along the side wall of the fiber sensor varies with the flow velocity     

Time-domain addressing of remote fiber-optic interferometric sensor arrays

This paper describes and analyzes a particular application of high duty-cycle time-division multiplexing to the separation and identification of signals from an interferometric sensor array. Using the method discussed here, the coherence length of the laser is no longer a severe design constraint. Also, the source phase-induced intensity noise which limits some other multiplexing methods may be overcome, leading to a higher sensitivity. The arrays of all-passive remote sensors exhibit minimal crosstalk between sensors, and have downlead insensitivity. A synthetic heterodyne demodulation technique prevents environmentally induced signal fading. Analysis includes coupling ratios for all directional couplers in the system, signal and noise spectra, minimum detectable phase shift, and the effect of ac coupling on noise and crosstalk. An experimental all-fiber implementation of a two sensor array has yielded a measured sensitivity of approximately 10 μrad/sqrt{Hz}over a range of signal frequencies, and a crosstalk level of better than 55 dB.     

Planar flexible fiber-optic acoustic sensors

The acoustic sensitivity of optical fibers is considered analytically. High sensitivity is obtained with low bulk modulus, very thick polymeric coatings. The fiber coatings play an important role in determining the fiber acoustic sensitivity. The very thick coating limit is realized by embedding optical fibers in an appropriate elastomer (polyurethane). A planar flexible fiber-optic interferometric acoustic sensor is developed by wrapping optimized single-mode fibers in a planar spiral form, and then embedding the fiber in a thin polyurethane layer. The acoustic sensitivity of the sensor is found to be high, and frequency (0.2-2.5 kHz) and static pressure independent. The sensor is found to be compatible with water, and is compared favorably to that of a planar polyvinylidene fluoride sensor of similar geometry     

Smart bridges with fiber-optic sensors

This article describes an approach to bridge improvements that uses smart structures, introduces key technologies for health monitoring systems based on fiber optics and gives an overview of two instrumented bridges. A sensing system in one application monitors general performance and health of the structure, while the system in a second application interrogates the behavior of a major structural repair.     

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.     

Historical review of microbend fiber-optic sensors

This paper traces microbend sensors from the early investigative work into initial applications to measure many different parameters, through advanced prototype development and commercial hardware. Advantages and disadvantages of microbend sensors are discussed     

光纤传感器的研究及应用

介绍了光纤传感器与其他传感器相比的优点,还介绍了传感型光纤传感器与传光型光纤传感器的基本原理.同时,文章阐述了强度调制型光纤传感器、干涉型光纤传感器、光纤光栅和光纤声发射传感器的应用.文章最后提出了我国光纤传感技术存在的问题以及发展方向.     

光纤光栅传感器复用新技术

复用技术是实现光纤光栅传感网络的关键.文章在现有的光纤布喇格光栅(FBG)复用技术的基础上,总结了近几年出现的三种新的复用技术来提高FBG的复用能力,并分析了其工作原理、性能和特点.最后,展望了这三种新的复用技术的应用前景,为光纤光栅传感网络的实用化提供了依据.     

Polarization fading in fiber interferometric sensors

Mach-Zehnder interferometer sensors fabricated with conventional nonpolarization-preserving fibers are subject to polarization fading caused by temperature variations and minor positional changes in the sensor. For such sensors, we calculate the probability of a given decrease in sensitivity and in signal-to-noise ratio due to fading assuming the polarization of the light in the signal and reference legs is uncorrelated and drifts randomly. The resultant reduction of the signal-to-noise ratio may exceed 10 dB 10 percent of file time and exceed 20 dB more than 2 percent of the time.     

光纤强度调制加速度传感器的研究

设计了一种透射式强度调制光纤加速度传感器,利用带尾纤的准直器(内置自聚焦透镜)直接作敏感质量块,采用双准直器组接收,经过加、减及比值运算,提高了光路的光耦合效率并使光源扰动及微弯损耗干扰的影响得到有效补偿,给出了模拟计算和初步实验结果.模拟计算和实验数据表明,该加速度传感器具有良好的线性度、一致性和稳定性.