White-light Interferometric Sensor Research Articles

Resolution limit of the white-light interferometric sensor for absolute position measurement based on central fringe maximum identification

In this paper a simple theoretical analysis together with an experimental verification of the analysis that provides an estimation of the resolution limit of the white-light interferometric sensor for absolute position measurement based on central fringe maximum identification is presented. The resolution better than 10 pm is obtained in the white-light interferometric sensing system where the signal-to-noise ratio was greater than 80 dB.

Understanding the effects of Doppler phenomena in white light Fabry–Perot interferometers for simultaneous position and velocity measurement

In static tests, low-power (<5 mW) white light extrinsic Fabry-Perot interferometric position sensors offer high-accuracy (μm) absolute measurements of a target's position over large (cm) axial-position ranges, and since position is demodulated directly from phase in the interferogram, these sensors are robust to fluctuations in measured power levels. However, target surface dynamics distort the interferogram via Doppler shifting, introducing a bias in the demodulation process. With typical commercial off-the-shelf hardware, a broadband source centered near 1550 nm, and an otherwise typical setup, the bias may be as large as 50-100 μm for target surface velocities as low as 0.1 mm/s. In this paper, the authors derive a model for this Doppler-induced position bias, relating its magnitude to three swept-filter tuning parameters. Target velocity (magnitude and direction) is calculated using this relationship in conjunction with a phase-diversity approach, and knowledge of the target's velocity is then used to compensate exactly for the position bias. The phase-diversity approach exploits side-by-side measurement signals, transmitted through separate swept filters with distinct tuning parameters, and permits simultaneous measurement of target velocity and target position, thereby mitigating the most fundamental performance limitation that exists on dynamic white light interferometric position sensors.

Spectrally Resolved White-Light Interferometric Sensor for Absolute Position Measurement Based on Hilbert Transform

A spectrally resolved white-light interferometric sensor for absolute position measurement based on Hilbert transform is presented. In order to avoid the possible singularities in the standard method obtained during the division of the measured spectral fringes and their Hilbert transform, a novel method is developed. The method is based on the simple mathematical apparatus where the division with zero is avoided by the division of the measured channeled spectrum and its Hilbert transform with envelope of both functions. Both of the divided functions are further differentiated, squared, and summed in order to find the optical path difference in the interferometer and thus the absolute position. The proposed sensing system currently provides unambiguous measurement over a range of 200 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> with a mid range error of better than 17 nm.

Loop topology based white light interferometric fiber optic sensor network for application of perimeter security

A loop topology based white light interferometric sensor network for perimeter security has been designed and demonstrated. In the perimeter security sensing system, where fiber sensors are packaged in the suspended cable or buried cable, a bi-directional optical path interrogator is built by using Michelson or Mach-Zehnder interferometer. A practical implementation of this technique is presented by using an amplified spontaneous emission (ASE) light source and standard single mode fiber, which are common in communication industry. The sensor loop topology is completely passive and absolute length measurements can be obtained for each sensing fiber segment so that it can be used to measure quasi-distribution strain perturbation. For the long distance perimeter monitoring, this technique not only extends the multiplexing potential, but also provides a redundancy for the sensing system. One breakdown point is allowed in the sensor loop because the sensing system will still work even if the embedded sensor loop breaks somewhere.

Compact optical fiber whitelight interferometric distance sensor for arbitrary small distance measurement

A compact optical fiber distance sensor that is capable of measuring arbitrary small distance is produced by fabricating a microsized polymer core at the end of a single mode fiber. The polymer core introduces an additional reflective interface to a conventional Fabry-Perot cavity. As such, sufficient fringes are presented in the whitelight reflectance spectrum of the distance sensor so that arbitrary small distances can be demodulated using the Fourier transformation technique. The performance of the distance sensor is verified experimentally.

Package and installation of embeddable fiber optic sensors

The issues related with design of pre-embedded white light interferometric fiber optic steel, epoxy and concrete bar sensors for use in concrete structures in construction are discussed in this paper. The shapes are designed as spindle for steel and epoxy bar sensors and the simple cylinder or rectangular shape has been used in the cement-based concrete bar sensor. The manufacture procedures of the pre-embedded fiber optic sensors are described in detail. Finally, the issues on the installation of the fiber optic pre-embedded bar sensor into host structures are also considered.

Tunable Fabry–Perot-resonator-based fiber-optic white-light interferometric sensor array

A tunable Fabry-Perot-resonator-based fiber-optic white-light interferometric quasi-distributed sensing system permitting absolute length measurement in a remote reflective sensor array is proposed and demonstrated. The sensor reflective signals characteristics have been analyzed, and the relationship between light signal intensities and sensors number was given for multiplexing potential evaluation. The proposed sensing scheme will be useful for the measurement of strain distribution. An important application could be deformation sensing in smart structures. Experimentally, a four-sensor array has been demonstrated.

Development of an in-fiber white-light interferometric distance sensor for absolute measurement of arbitrary small distances

The fabrication, implementation, and evaluation of an in-fiber white-light interferometric distance sensor that is capable of measuring the absolute value of an arbitrary small distance are presented. Taking advantage of the mode-coupling effect of a long-period fiber grating, an additional cavity distance is added to the optical path difference of the distance sensor; therefore, it can generate a sufficient number of fringes for distance demodulation even if the free-space cavity distance is very small. It is experimentally verified that the distance sensor is capable of measuring small distances that are beyond the capability of a Fabry-Perot interferometric distance sensor.

Analysis of a fiber ring resonator as a multi-path light-wave circuit in white-light fiber-optic interferometric sensors

According to the phase shift response of a ring resonator, the role of a fiber-optic ring resonator as a multi-path light-wave circuit in white-light fiber-optic interferometric sensors is analyzed. Formulas related to the maximum intensity of the interference fringes and the system parameters are derived by exact mathematic deduction. The main factors affecting the multiplexing capability of the sensor array are discussed. The analysis method not only gives an explicit explanation for the enhancement of multiplexing capability but also can be applied to other structural white-light sensor systems, in which the ring resonator is used as a multi-path light-wave circuit.

Parallel multiplexing of fibre optic white light interferometric and fibre Bragg grating sensors for local and global strain measurements

A scheme, based on the parallel multiplexing of long gauge fibre optic white light interferometric (FOWLI) and fibre Bragg grating (FBG) sensors, is proposed to realize quasi-static local and global strain measurements. The demodulation of the sensor system is realized in the spectral domain. A system with two FOWLIs of ∼500 mm in gauge length and two FBGs of ∼20 mm in gauge length is built and tested. The results demonstrate that the multiplexing scheme is feasible and the low crosstalk system can be used to realize local and global strain measurements at the same time.

Simultaneous Long- and Short-Gauge Strain Measurement in Spectral Domain by a Novel Optical Fiber Sensor Unit

A novel fiber-optic sensor unit by combining fiber-optic white-light interferometry and fiber Bragg gratings is proposed for simultaneous long- and short-gauge strain measurements. A new demodulation scheme based on spectral coherence theory for fiber-optic white-light interferometric sensors is proposed. This makes it possible that two quite different types of sensors in one unit can be simultaneously demodulated in the spectral domain. An experimental sensor unit has been built and tested. The results demonstrated that the spectral demodulation of the sensor unit is readily realized and that the sensor scheme is feasible.

Twin multiplexing strain sensing array based on a low-coherence fiber optic Mach–Zehnder interferometer

A twin multiplexed white light interferometric fiber optic strain sensor array was designed and demonstrated. In this sensing system, based on a low-coherence fiber optic Mach–Zehnder interferometer, an optical path matching technique is used to demodulate each sensor. The strain sensor array consists of 2 × N sensing elements linked by N 3 dB couplers. When one of the fiber sensors has been used to measure the strain, the variation caused by the temperature can be compensated by the other sensor. The multiplexing capacity of the sensing scheme has been analyzed. The results shown that it is depend on the light source power. The maximum number of the twin fiber sensor size is 4 for light source with 35 μW, and 7 for light source with 3 mW, respectively. The experimental result with a two-coupler sensor array was conducted and demonstrated which confirm this. It is useful for temperature compensation of distributed strain measurement in smart materials or structures health monitoring.

Mode power distribution effect in white-light multimode fiber extrinsic Fabry-Perot interferometric sensor systems

Theoretical and experimental results have shown that mode power distribution (MPD) variations could significantly vary the phase of spectral fringes from multimode fiber extrinsic Fabry-Perot interferometric (MMF-EFPI) sensor systems, owing to the fact that different modes introduce different extra phase shifts resulting from the coupling of modes reflected at the second surface to the lead-in fiber end. This dependence of fringe pattern on MPD could cause measurement errors in signal demodulation methods of white-light MMF-EFPI sensors that implement the phase information of the fringes.

Sapphire-fiber-based white-light interferometric sensor for high-temperature measurements

We present a fiber-optic temperature sensor with a single-crystal sapphire fiber as the light guide and a sapphire wafer as the sensing element. Temperature is determined by measurement of the thermal dependence of the wafer's optical thickness by use of white-light interferometry. We applied digital signal processing techniques to analyze the sensor's spectrum. A prototype sensor was tested to 1600 degrees C and demonstrated excellent reproducibility. An accuracy of +/- 0.2% full scale was obtained. The sensor is simple, small, and flexible, with potential advantages of batch fabrication and easy calibration.

Nonparametric smoothing of interferometric height maps using confidence values

We use an extension of normalized convolution to smooth height maps from interferometry using confidence values. The latter are often used for dichotomous good/bad decisions only, with all bad data being discarded. To minimize loss of information, we weight each pixel individually by the inverse of its expected variance. The relation between supplied confidence values and empirical variances is found by regression. The width of the smoothing kernel—as small as possible to prevent loss of spatial resolution, as large as necessary to average out noise—is adjusted locally so as to yield a smoothed image with a prespecified uncertainty that is homogeneous throughout. In our experimental investigations using metrological data from a white light interferometric sensor, the variable-width mask leads to images with somewhat lower absolute deviation from an average image than the fixed-width masks we use for comparison.

Error analysis of temperature-compensated white-light interferometric fiber-optic strain sensor

A software-based temperature-compensated fiber-optic white-light interferometric (WLI) strain sensor is presented. Measurement error of this sensor is studied theoretically and experimentally. The results quantitatively indicate how the CCD camera pixel resolution, interference fringe pattern center determination errors, strain sensing sensitivity, sensing fiber and compensating fiber lengths affect the measurement errors. Measurement accuracy limitation for the individual strain and temperature sensors with specified highly birefringent fibers and WLI decoding system is also investigated. The main sources of the central fringe position measurement error as well as solutions for further improvements are particularly discussed. Critical factors for sensor design and potential for civil and composite structure monitoring are highlighted by comparing sensitivities, fiber length limitation, power budget and dynamic ranges with those of other fiber interferometers. The analysis provides a convenient way for sensor design to satisfy applications with different accuracy requirements.

Multiplexed fiber optic sensors matrix demodulated by a white light interferometric Mach–Zehnder interrogator

A multiplexed white light interferometric fiber optic sensors matrix system was designed and demonstrated. In this system, a Mach–Zehnder optical path interrogating technique is used to demodulate each sensor of the sensors matrix. The sensors matrix consists of M× N sensing elements linked by a 1× M star coupler. The multiplexing capacity of the sensing scheme has been analyzed and an experimental result with a 2×2 sensors matrix was presented.

Recent progress in bidirectional interrogation techniques for enhancing multiplexing capability of fiber optic white light interferometric sensors

In smart structure applications where fiber sensors are embedded within structural materials, multiple lead in/out fibers are preferred for redundancy and improving reliability. The use of only one lead/out fiber is not optimal because the breakage of a fiber at one location due to, for example, local structural damage, would cause the failure of the whole sensing system. The multiplexing and networking techniques suitable for such applications have attracted considerable research recently. In this article, based on the bidirectional interrogation technique for white light interferometric sensors arrays, a multiplexed fiber optic deformation sensor loop network suitable for smart structure applications has been designed and demonstrated. Loop-network sensor systems are based on the white light interferometric technique. Michelson and Mach–Zehnder optical path interrogators have been developed and demonstrated, respectively. For the usually used one direction interrogate sensing system, it is clear that multiplexed sensor arrays suffer from relatively large fiber segment-induced optical reflective and excess insertion loss that generally limit the total number of sensors that can be accommodated in this configuration. This loop-network bidirection interrogating technique greatly extended the multiplexing capacity of fiber optic white light interferometric sensors system. A practical implementation of this technique is presented which makes use of a popular light emitting diode, superluminescent diode, or amplified spontaneous emission optical light source and standard single mode fiber, which are commonly used in the communication industry. The sensor loop topology is completely passive and absolute length measurements can be obtained for each sensing fiber segment so that it can be used to measure quasidistribution strain or temperature. For large-scale smart structures, this technique not only extends the multiplexing potential, but also provides a redundancy for the sensing system. It means that the sensor loop permits one point breakdown, because the sensing system will still work whenever the embedded sensor loop breaks somewhere.

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.

Multiplexing of fiber-optic white light interferometric sensors using a ring resonator

A single-mode fiber-optic ring resonator is used as a multipath reference for multiplexing white light interferometric sensors. The theory of applying the resonator for multiplexing Michelson-type interferometer sensor is presented. The capacity of the technique in terms of maximum sensor number is evaluated for both a linear array and a M x N sensor matrix. Experimental results for a three-sensor linear array and 2 x 1 sensor matrix are also presented.