Fabry-Perot diaphragm fiber-optic sensor

The general theory of a diaphragm fiber-optic sensor (DFOS) is proposed. We use a critical test to determine if a DFOS is based on Fabry-Perot interference or intensity modulation. By use of the critical test, this is the first design, to the best of our knowledge, of a purely Fabry-Perot DFOS, fabricated with microelectromechanical system technology, and characterized as an audible microphone and ultrasonic hydrophone with orders of improvement in signal-to-noise ratio.     

Micro-air-gap based intrinsic Fabry-Perot interferometric fiber-optic sensor

A simple intrinsic Fabry-Perot interferometric (IFPI) sensor is developed. The sensor is fabricated by two micro air gaps as reflective mirrors in a fiber to form a Fabry-Perot cavity. Theoretical and experimental studies of the sensor are described. Experimental results show that high resolution and high sensitivity can be achieved. Two structures of micro-air-gap-based IFPI sensors offer more applications than other IFPI sensors.     

Embedded micromachined fiber-optic Fabry-Perot pressure sensors in aerodynamics applications

Small size, high bandwidth pressure sensors are required for instrumentation of probes and test models in aerodynamic studies of complex unsteady flows. Optical-fiber pressure sensors promise potential advantages of small size and low cost in comparison with their electrical counterparts. We describe miniature Fabry-Perot cavity pressure sensors constructed by micromachining techniques in a turbine test application. The sensor bodies are 500 /spl mu/m squared, 300 /spl mu/m deep with a /spl sim/2 /spl mu/m-thick copper diaphragm electroplated on one face. The sensor cavity is formed between the diaphragm and the cleaved end of a single mode fiber sealed to the sensor by epoxy. Each sensor is addressed interferometrically in reflection by three wavelengths simultaneously, giving an unambiguous phase determination; a pressure sensitivity of 1.6 radbar/sup -1/ was measured, with a typical range of vacuum to 600 kPa. Five sensors were embedded in the trailing edge of a nozzle guide vane installed upstream of a rotor in a full-scale turbine stage transient test facility. Pressure signals in the trailing edge flow show marked structure at the 8 kHz blade passing frequency. To our knowledge, this is the first report of sensors located at the trailing edge of a normal-sized turbine blade.     

Phase-demodulation error of a fiber-optic Fabry-Perot sensor with complex reflection coefficients

The influence of reflector losses attracts little discussion in standard treatments of the Fabry-Perot interferometer yet may be an important factor contributing to errors in phase-stepped demodulation of fiber optic Fabry-Perot (FFP) sensors. We describe a general transfer function for FFP sensors with complex reflection coefficients and estimate systematic phase errors that arise when the asymmetry of the reflected fringe system is neglected, as is common in the literature. The measured asymmetric response of higher-finesse metal-dielectric FFP constructions corroborates a model that predicts systematic phase errors of 0.06 rad in three-step demodulation of a low-finesse FFP sensor (R = 0.05) with internal reflector losses of 25%.     

Extrinsic fiber-optic Fabry-Perot interferometer sensor for refractive index measurement of optical glass

A simple fiber-optic sensor based on Fabry-Perot interference for refractive index measurement of optical glass is investigated both theoretically and experimentally. A broadband light source is coupled into an extrinsic fiber Fabry-Perot cavity formed by the surfaces of a sensing fiber end and the measured sample. The interference signals from the cavity are reflected back into the same fiber. The refractive index of the sample can be obtained by measuring the contrast of the interference fringes. The experimental data meet with the theoretical values very well. The proposed technique is a new method for glass refractive index measurement with a simple, solid, and compact structure.     

Multiplexed,white-light interferometric fiber-optic sensor matrix with a long-cavity,Fabry-Perot resonator

A long-cavity, single-mode, fiber-optic Fabry-Perot resonator has been used to develop white-light fiberoptic multiplexed Michelson interferometers (MMIs). The MMI matrix is constructed and demonstrated. We also analyzed the capacity of the long-cavity Fabry-Perot resonator multiplexing technique. The experimental results of a 2 x 2 fiber-optic interferometric sensor matrix are given, and the effects of polarization are discussed.     

腔长损耗对非本征F-P腔光纤传感器的影响

根据光学法布里-珀罗(F-P)腔基本原理,运用单模光纤能量散射模型分析了腔长损耗对非本征F-P腔输出光强度的影响.实验过程中所用的F-P腔由单模光纤端面和弹性硅片组成.实验结果表明,非本征F-P腔输出光强幅度随腔长增大逐渐衰减,其输出特性曲线中不同部分相同周期的线性工作区间却对应不同的测量量程和灵敏度,所以强度型光纤F-P腔传感器初始腔长应基于传感器的性能指标进行选取.     

Wide-range fiber-optic strain sensor

A wide-range strain sensor which utilizes optical fiber as the transducing element is reported. This device differs from the well-known microbend type sensor in that a roller chain is used to impose constant curvature bends on the fiber, rather than a corrugated plate which imposes sinusoidal bends in the microbend sensors. This change also leads to a wide range of sensitivity adjustment and a linear calibration curve.     

Cross-talk fiber-optic temperature sensor

The temperature sensitivity of cross talk between closely spaced cores in a common cladding is calculated and compared with measurements. A periodic variation in core contrast is observed when one core is illuminated and the temperature is changed. The variation in light distribution, which is ascribable to a change in coupling between the cores, agrees with theoretical predictions. It is shown that cross talk can be made to be a sensitive, predictable function of temperature or by proper selection of materials, wavelength, and fiber geometry essentially temperature independent.     

A fiber-optic pressure-gradient sensor

Translated from Izmeritel'naya Tekhnika, No. 11, p. 36, November, 1991.     

GaAs-based fiber-optic pressure sensor

A new sensor developed for measurement of hydrostatic pressure up to at least 100 MPa at a standard range of ambient temperatures is described. The sensor exploits the displacement of the optical absorption edge occurring in semiconductors under the influence of hydrostatic stress as a result of pressure-induced energy shifting of conduction band extrema. The sensing element is composed of an intrinsically pure GaAs single crystal configured in the form of a microprism located at the sensor tip, and attached to two multimode (50/125 μm) optical fibers designed to deliver input light to the sensor and to output a pressure-modulated light signal to the outside of a pressure region. Characterization of the sensor has been performed for pressures up to 100 MPa and for temperatures ranging from 5 to 50°C. A procedure has been proposed involving the use of two sensors (active and compensating) to minimize temperature drift through appropriate signal processing     

Extrinsic optical-fiber ultrasound sensor using a thin polymer film as a low-finesse Fabry-Perot interferometer

Theoretical and experimental aspects of an extrinsic optical-fiber ultrasound sensor are described. The sensor is based on a thin transparent polymer film acting as a low-finesse Fabry-Perot cavity that is mounted at the end of a multimode optical fiber. Performance was found to be comparable with that of a piezoelectric polyvinylidene dinuoride-membrane (PVDP) hydrophone with a sensitivity of 61 mV/MPa, an acoustic noise floor of 2.3 KPa over a 25-MHz bandwidth, and a frequency response to 25 MHz. The wideband-sensitive response and design flexibility of the concept suggests that it may find application as an alternative to piezoelectric devices for the detection and measurement of ultrasound.     

Neural processing-type fiber-optic strain sensor

A neural-processing-type strain sensor insensitive to thermal variation is presented and calibration of the device through modulation of the processing system's internal parameters is described. The sensor exploits the variation of the far-field polarization pattern in a single-mode birefringent fiber under the influence of longitudinal strain. A temperature-compensating fiber element is built in, making the sensor assembly immune to thermal variation. Sampling of the sensor output and parallel distributed processing of the samples are integrated within the sensor. The processor manages both a training function and a generalization function. The training function modulates a small-size linear network built into the system. In the working phase, the generalization function is used to recover measurement information. If the sensor is thermally compensated, the network gives a reading of the measurand with an error not exceeding 0.1%. Applicability of the processing system to bimodal sensor output is also described     

White-light interferometric fiber-optic pressure sensor

A novel configuration for a fiber-optic white-light interferometric sensor is presented which allows for absolute measurements of hydrostatic pressure with an improved operation range. The performance of two fibers (York bow-tie 800 and especially designed elliptical-core side-hole fiber) used as sensing elements was experimentally studied. The sensor itself was composed of two equal lengths of the fiber spliced at 90°. This structure assures temperature compensation and enables application of a Wollaston prism as a receiving interferometer. A step delay line made of crystalline quartz was used to increase the operation range of the sensor     

Two-dimensional ultrasound receive array using an angle-tuned Fabry-Perot polymer film sensor for transducer field characterization and transmission ultrasound imaging

A 2-D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically induced changes in the optical thickness of a thin polymer film acting as a Fabry-Perot sensing interferometer (FPI). By illuminating the sensor with a large-area laser beam and mechanically scanning a photodiode across the reflected output beam, while using a novel angle-tuned phase bias control system to optimally set the FPI working point, a notional 2-D ultrasound array was synthesized. To demonstrate the concept, 1-D and 2-D ultrasound field distributions produced by planar 3.5-MHz and focused 5-MHz PZT ultrasound transducers were mapped. The system was also evaluated by performing transmission ultrasound imaging of a spatially calibrated target. The "array" aperture, defined by the dimensions of the incident optical field, was elliptical, of dimensions 16 x 12 mm and spatially sampled in steps of 0.1 mm or 0.2 mm. Element sizes, defined by the photodiode aperture, of 0.8, 0.4, and 0.2 mm were variously used for these experiments. Two types of sensor were evaluated. One was a discrete 75-microm-thick polyethylene terephthalate FPI bonded to a polymer backing stub which had a wideband peak noise-equivalent pressure of 6.5 kPa and an acoustic bandwidth 12 MHz. The other was a 40-microm Parylene film FPI which was directly vacuum-deposited onto a glass backing stub and had an NEP of 8 kPa and an acoustic bandwidth of 17.5 MHz. It is considered that this approach offers an alternative to piezoelectric ultrasound arrays for transducer field characterization, transmission medical and industrial ultrasound imaging, biomedical photoacoustic imaging, and ultrasonic nondestructive testing.     

Polymer MEMS-based Fabry-Perot shear stress sensor

This paper reports a novel optical fiber-based micro-shear stress sensor utilizing a flexible membrane and double SU-8 resist structures as a moving micro-mirror, together with an optical fiber as a micro-Fabry-Perot interferometer. This sensor can be employed in air or liquid environments with high sensitivity because of its waterproof design. Through UV lithography processes on thick SU-8 resist, the roughness of the reflection surface has approached 7 nm (Ra value), suitable for optical applications. A single-mode optical fiber is employed for detecting the displacement of the floating element induced by shear stress. Tests have been carried out successfully and a detection possibility of 10 nm-displacement of the floating element and a displacement sensitivity of 0.128 nm/nm (spectrum shift/floating element displacement) has been demonstrated. The temperature coefficient of this fiber-optic sensor has been characterized to be 3.4 nm/K linearly from 25 to 48/spl deg/C. Fluid tests have also been performed by placing the sensor inside the inner wall of a precisely machined rectangular channel and the result shows a sensitivity of 0.4 nm/ml/min (spectrum shift/flow rate), corresponding to a shear stress resolution of 0.65 Pa/nm (shear stress/spectrum shift). The minimum detectable shear stress is thereby estimated as 0.065 Pa from the reading resolution of the spectrometer of 0.1 nm, comparable to its counterparts with resolutions from 0.1-1 Pa.     

Analysis of a high-speed fiber-optic spectrometer for fiber-optic sensor signal processing

A novel high-speed fiber-optic spectrometer has been demonstrated in our previous work. The high-speed spectrum measurement is enabled by translating the spectral-domain signal into a time-domain signal through a dispersion element. We present a mathematical model that accurately describes the relationship between the optical spectrum to be measured and the dispersed time-domain signal. Based on the model, the effects of the key parameters on the performance of the spectrometer are investigated in detail using numerical simulation. The analysis is useful for the design and application of such spectrometers.     

Axial and angular displacement fiber-optic sensor

An intensity-based fiber-optic sensor for measuring axial and angular displacement has been designed and tested in a controlled laboratory environment. In addition, a mathematical model allowing the simultaneous calculation of the three desired parameters needed to characterize the tilt and the position of a surface under investigation is described. Preliminary tests show good agreement between the theory and the experimental results and show the sensor's potential for application in the manufacturing industry for position and vibration control. The sensor shows significant improvement in angular range over previously reported methods. An axial displacement range of 2 mm, with an accuracy of 40 mum, and an angular displacement range of 40 mrad, with an accuracy of 0.5 mrad, are demonstrated. Suggestions for further improvement of the range and the sensitivity of the sensor are also described.     

Inline fiber-optic polarization analyzers for sensor application

Theoretical and experimental investigations of a new type of inline fiber-optic polarization analyzer are described in this paper. The discussed devices are based on a polarimeter, as well as a Sagnac interferometer configuration, and give the possibility to detect the full polarization changes. Detection of the polarization state as well as its degree in real time by the systems containing a standard single-mode fiber and application of appropriate modulation technique is a new property of the arrangement. The comparison of the described systems based on experimental device investigation is also presented in this paper.