Perot Sensor Research Articles

Surface stress sensor using MEMS-based Fabry–Perot interferometer for label-free biosensing

We have recently developed a surface stress sensor based on a microelectromechanical system Fabry–Perot interferometer integrated with a photodiode, which utilizes the nonlinear optical transmittance change in the Fabry–Perot interference to enhance the sensitivity of the surface stress. The novel signal transducing technique from the surface stress caused by molecular adsorption to the electrical readout is carried out in three steps, namely, mechanical deflection, transmittance change, and photocurrent change. Parylene-C was used as a deformable membrane in the sensor because of its high optical transmittance, low Young's modulus, and negligible residual stress, which provides a flat freestanding membrane. The theoretical minimum detectable surface stress of the proposed sensor was predicted to be −1μN/m, which is two orders of magnitude greater than that of the piezoresistor-embedded surface stress sensor. The Fabry–Perot surface stress sensor was fabricated using a semiconductor fabrication process and sacrificial release process. An encapsulating structure for the Fabry–Perot cavity that uses a dry film resist by a roll-to-roll technique enabled us to use wet condition for detection of biomolecules. Amino-methyl-functionalized parylene was coated on the parylene-C membrane for immobilization of the biomolecules via electrostatic coupling between biomolecules and the amino group. The photocurrent was demonstrated to shift by 23.7nA at a 3-V reverse bias voltage with immobilized antibodies. This Fabry–Perot sensor allows the use of a universal biochemical sensing platform in a label-free manner.

Simultaneous measurement of absolute strain and differential strain based on fiber Bragg grating Fabry–Perot sensor

We present a fiber Bragg grating Fabry–Perot (FBG-FP) sensor using the fast Fourier transform (FFT) demodulation for measuring the absolute strain and differential strain simultaneously. The amplitude and phase characteristics of Fourier transform spectrum have been studied. The relation between the amplitude of Fourier spectrum and the differential strain has been presented. We fabricate the fiber grating FP cavity sensor, and carry out the experiment on the measurement of absolute strain and differential strain. Experimental results verify the demodulation method, and show that this sensor has a good accuracy in the scope of measurement. The demodulating method can expand the number of multiplexed sensors combining with wavelength division multiplexing and time division multiplexing.

Design of fiber Bragg grating-based Fabry–Perot sensor for simultaneous measurement of humidity and temperature

A new sensor for simultaneous measurement of humidity and temperature is proposed. The sensor consists of Fabry–Perot cavity formed by two identical uniform fiber Bragg gratings. To make the cavity serves as humidity sensor, moisture sensitive polymer, which is polyimide, is coated on the FBG and on the cavity with different thickness. When the sensor is exposed to the relative humidity change, the polyimide will expand and stretch the fiber and induces strain on the FBG and on the cavity. The induced strain alters the grating period, cavity length and effective refractive index of fiber. The simulation results show that the humidity sensitivity and thermal sensitivity are 1.92pm/%RH and 8.87pm/°C, respectively, for polyimide coating thickness of 10μm on the FBG and 15μm on the cavity.

Signal processing of white-light interferometric low-finesse fiber-optic Fabry–Perot sensors

Signal processing for low-finesse fiber-optic Fabry-Perot sensors based on white-light interferometry is investigated. The problem is demonstrated as analogous to the parameter estimation of a noisy, real, discrete harmonic of finite length. The Cramer-Rao bounds for the estimators are given, and three algorithms are evaluated and proven to approach the bounds. A long-standing problem with these types of sensors is the unpredictable jumps in the phase estimation. Emphasis is made on the property and mechanism of the "total phase" estimator in reducing the estimation error, and a varying phase term in the total phase is identified to be responsible for the unwanted demodulation jumps. The theories are verified by simulation and experiment. A solution to reducing the probability of jump is demonstrated.

Long-distance fiber sensor system based on the second-order Raman pump and amplification

We demonstrate a novel technique to realize a long-distance fiber sensor system based on the second-order Raman pump and amplification. With the second-order Raman pump and amplification, a gain spectrum around 1582-nm is achieved. It serves as not only the optical source for the remote fiber sensor but also the filter to intensity modulated the wavelength shift of the fiber sensor. A long-distance fiber strain sensor system is demonstrated by adopting a fiber Fabry–Perot sensor. Experimental results show that the wavelength shift of the sensor in response to the strain is simultaneously intensity modulated by the steep slope with a high sensitivity, while it is temperature insensitive. With the side gain spectrum served as a quasi-linear filter, the sensor can also be quasi-linearly intensity-modulated.

Miniature all-fiber Fabry–Perot sensor for simultaneous measurement of pressure and temperature

This article presents a miniature, high-sensitivity, all-silica Fabry-Perot fiber-optic sensor suitable for simultaneous measurements of pressure and temperature. The proposed sensor diameter does not exceed 125 μm and consists of two low-finesse Fabry-Perot resonators created at the tip of an optical fiber. The first resonator is embodied in the form of a short air cavity positioned at the tip of the fiber. This resonator utilizes a thin silica diaphragm to achieve the sensor's pressure response. The second resonator exploits the refractive index dependence of silica fiber in order to provide the proposed sensor's temperature measurement function. Both resonators have substantially different lengths that permit straightforward spectrally resolved signal processing and unambiguous determination of the applied pressure and temperature.

Chitosan based fiber-optic Fabry–Perot humidity sensor

A relative humidity fiber sensor based on Fabry–Perot interferometry configuration is presented. The proposed fiber sensor is functionalized with a thin layer of a moisture-sensitive natural polymer chitosan to form a low fineness Fabry–Perot sensor. The sensing scheme used in this work is based on the swelling effect of chitosan sensing film (degree of swelling varies as a function of relative humidity) which will induce optical path modulation when relative humidity is changed. As observed, the proposed sensor exhibits a sensitivity of 0.13nm/%RH for relative humidity ranging from 20%RH to 95%RH with a RH uncertainty of ±1.68%RH and fast response time of 380ms.

Resolution and sensitivity enhancements in strong grating based fiber Fabry–Perot interferometric sensor system utilizing multiple reflection beams

We investigate an asymmetric intensive fiber Bragg grating (FBG) defined Fabry–Perot (F–P) sensor system decoded by a multiple-path-matched Michelson interferometer. The interrogation of higher order reflection beams cannot only solve the problem of the degraded resolution induced by the spectral mismatch of the FBGs, but also amplify the effect of the fiber strain on the phase of the light. We demonstrate multiple reflection beams in the F–P cavity based on the concept of the FBG effective length for constructing respective interrogation interferometers, and present a cost function with optimized system parameters to improve noise properties. The performances of interrogating the second, third and fourth order reflection beams are compared in a strain sensing experiment arrangement. Under the condition of the same optical path length mismatch, the interrogation of the fourth order reflection beam can achieve 9.8dB sensitivity enhancement and 3dB resolution promotion compared with the result using the second order reflection beam.

Hollow-core photonic crystal fiber Fabry–Perot sensor for magnetic field measurement based on magnetic fluid

Based on the characteristic of magnetic-controlling refractive index, the magnetic fluid filled in hollow-core photonic crystal fiber (HC-PCF) can be used as the sensitive medium in the cavity of a fiber Fabry–Perot (F–P) magnetic field sensor. The structure and the sensor principle are introduced. The theoretical simulations of the mode distribution of the HC-PCF filled with the magnetic fluid and the sensor output spectra are discussed in detail. The sensor multiplexing capability is indicated as well. Magnetic field measurement sensitivity is about 33 pm/Oe based on the proposed sensor.

Spectral-mismatch-induced resolution limit of interferometric fiber Fabry–Perot sensor system

The mismatch of fiber Bragg gratings (FBGs) in spectral profiles can lead to a severe degraded resolution of the constructed fiber Fabry–Perot (FFP) sensor system through its effect on the fringe visibility. The variation of visibility induced by spectral mismatch and the corresponding phase resolution limit are analyzed theoretically and experimentally. Theoretical analyses are based on the approximation of Gaussian profiles to the reflection spectra of weak FBGs, especially with consideration of side lobes. The investigation provides an insight into the evolution of the fringe visibility caused by spectral mismatch, and shows good agreement with experimental results. An optimum phase resolution of about 55 μrad/Hz 1/2 above 100 Hz is achieved for a nearly 4 m-long FFP sensor by matching spectral profiles of the gratings and balancing path length differences of the tandem interferometers.

Integration of miniature Fabry–Perot fiber optic sensor with FBG for the measurement of temperature and strain

A sensor has been fabricated by the integration of a fiber Bragg gating sensor (FBGs) with a fiber Fabry–Perot (F-P) sensor fabricated by etching method. In the integrated sensor, the FBG was used to measure temperature, while the fiber Fabry–Perot interferometer sensor (FFPIs) was used for strain measurement. Wavelength decoding for FBG and peak tracking for FFPI was employed for demodulation, respectively. The result showed that the temperature and strain sensitivity for the integrated sensor is ~ 2.7 pm/ μεand ~ 9.3 pm/°C, respectively.

Structural health monitoring: a Canadian perspective

This paper presents an overview of structural-health-monitoring technologies for constructed structures, based on six field demonstration projects in Canada. The principle of fibre-optic sensors is discussed, including the fibre Bragg grating, Fabry–Perot and long-gauge sensors together with their positive and negative aspects. Site applications of these technologies include a cable-stayed pedestrian bridge, prestressed concrete girder bridges, structural beams in a cemetery, a hollow bronze sculpture and substructures of a steel-plate girder bridge.

High-resolution absolute frequency referenced fiber optic sensor for quasi-static strain sensing

We present a quasi-static fiber optic strain sensing system capable of resolving signals below nanostrain from 20 mHz. A telecom-grade distributed feedback CW diode laser is locked to a fiber Fabry-Perot sensor, transferring the detected signals onto the laser. An H(13)C(14)N absorption line is then used as a frequency reference to extract accurate low-frequency strain signals from the locked system.

High performance chitosan diaphragm-based fiber-optic acoustic sensor

A miniature fiber-optic Fabry–Perot is fabricated directly on the tip of a single mode fiber. The active sensing element is a thin linear polysaccharide (chitosan) diaphragm with thickness of only 1.5 μm. The chitosan diaphragm exhibits excellent physicochemical properties that include membrane-forming ability, good adhesion, insensitive to background pressure and high mechanical strength. Together with fiber-optic Fabry–Perot interferometric's inherent advantages, the chitosan diaphragm-based Fabry–Perot sensor becomes a wonderful candidate for dynamic pressure measurement such as acoustic pressure. A chitosan diaphragm-based miniature Fabry–Perot system for acoustic pressure measurement is presented which exhibits high sensitivity of 0.002 V/mPa at 1 kHz with frequency response from range 20 Hz to 20 kHz.

Stabilization of optical Fabry–Perot sensor by active feedback control of diode laser

This paper presents the design and characterization of a fiber Fabry–Perot interferometer (FFPI) acoustic wave detector with its Q point being stabilized actively. The relationship between the reflectivity of the F–P cavity facets and cavity length was theoretically analyzed, and high visibility of 100% was realized by optimized design of the F–P cavity. To prevent the drifting of the Q point, a new stabilization method by actively feedback controlling of the diode laser is proposed and demonstrated, indicating the method is simple and easy operating. Measurement shows that good tracing of Q point was effectively realized.

A novel fiber optics based method to measure very low strains in large scale infrastructures

An optic fiber strain measurement sensing mechanism has been developed that can measure extremely low strains, i.e. in the order of 0–25microstrains, with a resolution close to 0.1microstrain. It is an alternate to traditional foil gauges as well as the more recent Fabry–Perot sensors for some applications. It is particularly suited to measure very low strains in large scale infrastructures where portions of the infrastructure can be used to mount extended lengths of the fiber optic, thereby providing high resolution. Examples of such applications are the measurement of small strains or vibrations in large structures such as rocket test stands, buildings, oil rigs, bridges, dams, etc. The sensing system includes all off-the-shelf components and the focus of this paper is to provide details on how to develop the system, calibrate it and then use it for specific applications.

Stabilized fiber-optic extrinsic Fabry–Perot sensor system for acoustic emission measurement

A fiber-optic Fabry–Perot (F–P) acoustic emission (AE) sensor system based on an improved double wavelength stabilization technique is described. Without stabilization, the sensor system drifts out of quadrature due to the presence of low-frequency dynamic strains. The stabilization is achieved by using a dense wavelength division multiplexer (DWDM) to generate two quadrature phase-shifted output signals. An optimum model of double wavelength stabilization is established. The filtering wavelengths of DWDM are calculated using an optimization design method. The performance of the developed sensor system was verified by two preliminary tests. One was a test on the stabilization of operating point of fiber-optic F–P sensor, and other was the detection of simulated AE signals generated by the impact and pencil lead breaking. The test results demonstrate that simulated AE signals are successfully detected using this stabilized sensor system, which solves the fade-out problem.

Fabrication of intrinsic fibre Fabry–Perot sensors in silica fibres using hydrofluoric acid etching

When a cleaved optical fibre is immersed in hydrofluoric acid, the compositional variation within the core and the cladding results in the core-region being etched at a faster rate. This paper presents details of the experimental procedures that were used to etch SM 800 optical fibres and the characterization of the micro-cavities using a 3D surface profiler, optical and scanning electron microscopy. Two such acid-etched fibres were fusion spliced to create an intrinsic Fabry–Perot cavity. These strain sensors were surface-mounted on to fibre reinforced composite test specimens and subjected to quasi-static and cyclic loading. The intrinsic fibre Fabry–Perot sensors were interrogated using low-coherence interferometry. The output from the intrinsic Fabry–Perot sensors showed excellent correlation with surface-mounted electrical resistance strain gauges during quasi-static and cyclic loading.

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.

Structural integrity monitoring of reinforced concrete structures. Part 1: evaluation of protection systems for extrinsic fibre Fabry–Perot sensors

This paper reports on the use of extrinsic fibre Fabry–Perot sensors for strain monitoring of reinforced concrete columns and beams. The use of optical sensors in the concrete and on the rebar and column surfaces was made possible by the development of appropriate protection systems for the optical sensors. Two sensor protection systems were evaluated. The first involved the use of stainless steel tubing with flanges for the embedded sensors. In the second, the optical sensor was sandwiched between a four-layered prepreg that was either flat or had the shape of the rebar. A minimum of six Fabry–Perot sensors were interrogated on each concrete structure using a 16-channel optical channel selector unit and an optical spectrum analyser. Experimental results showed that, irrespective of the protection systems, the strain data from the Fabry–Perot sensors were in agreement with those obtained from the electrical resistance strain gauges. All the optical fibre sensors survived the structure manufacturing process and the majority of them survived up to the failure of the columns and beams. These experiments demonstrated that the sensor protection systems deployed in conjunction with the embedment methodology offer a reliable method for interrogating the mechanical integrity of concrete structures.