Resolution limits of extrinsic Fabry–Perot interferometric displacement sensors utilizing wavelength scanning interrogation

A three-wavelength passive demodulation technique to interrogate extrinsic Fabry-Perot interferometric (EFPI) sensors with arbitrary cavity length has been developed. The DC component is obtained online, then the applied dynamic signal is recovered by using three new signals without the DC component. The performance of the technique is demonstrated by simulations and experiments. The demodulation technique can extract dynamic signals, regardless of whether the phase modulation is larger than 2π. Theoretically, EFPI sensors with arbitrary cavity length can be demodulated by the demodulation technique, and EFPI sensors with cavity lengths in the 22.96-1002.3 μm range are detected successfully by the same demodulator in experiments. The technique is robust with respect to the bending loss of the leading fiber. The demodulation technique provides a robust and accurate solution to measure dynamic signals for EFPI sensors. It has the properties of high frequency, a large dynamic range, and high sensitivity. The paper demonstrates this technique's potential for measuring dynamic signals.

As an important member of optical fiber sensor, fiber optic Extrinsic Fabry-Perot Interferometric (EFPI) sensor has prospects for a wide range of industrial applications due to its small size, compact configuration, good reliability and flexibility. In a white light based fiber optic EFPI sensor system, which has the advantages of large dynamic range and high resolution, spectral analysis equipment is the most important part which takes the major cost of the system. Therefore, multiplexing spectral analysis equipment to measure multiple sensors simultaneously is an effective and necessary way that can save the cost of the whole sensor system. In this paper, a scheme based on the frequency division multiplexing of EFPI optical fiber sensors is proposed. Two EFPI sensors with different gap lengths are parallely connected through the optical fiber coupler. The overlapped interference spectrum of the two EFPI sensors is measured by using an optical spectrum analyzer, which is separated into two individual interference spectrums with a Finite Impuse Response (FIR) band-pass filter. Their envelope components are extracted with Hilbert transform, and then the interference spectrums are normalized by the envelope components. The respective gap lengths of the two EFPI sensors are achieved with the cross correlation calculation. Using this frequency division spectral analysis method, a two-channel multiplexing EFPI sensor system is implemented in the laboratory. The deployment of this frequency division multiplexing technology can greatly reduce the cost of the whole system which has extensive potential applications for distributed fiber optic EFPI sensor system.

A Fourier‐transform phase comparator is developed to measure fiber optic extrinsic Fabry‐Perot interferometric sensors. The phase comparator is realized by comparing the phases of two white‐light spectra, which are obtained by scanning the wavelength of the light source. The changes of a cavity length, which are concerned in most applications, can be interrogated. A linear output is obtained, and a measurement resolution of 15 pm is experimentally achieved, which is very close to the theoretical value. The technique has high resolution and a large dynamic measurement range. In addition, the measurement is not affected by various imperfections of the sensor, light source spectrum profile, and the loss of leading fiber and connectors. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2621–2625, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23719

To perform a high temperature strain measurements, the sapphire extrinsic Fabry-Perot interferometer (EFPI) sensor head was attached to a silicon carbide rod and placed in a RF induction heater mounted on an MTS load frame. The high-temperature ceramic adhesive is designed to withstand temperatures up to 4000° F. As a basis for comparison, a gold-coated silica EFPI sensor was attached along side the sapphire EFPI. Initially, strain data was recorded at room temperature as the silicon carbide rod was slowly loaded to 4500 lbs. When the temperature was stabilized at a maximum of 2000° F, the SiC rod was again loaded to determine the effects of temperature on strain. Although the sapphire sensor was capable of measuring strain at higher temperatures, die induction heater, being poorly insulated, was not able to further heat the test material. The purpose of the second experiment was to determine the survivability and repeatability of tiie sapphire EFPI due to high-cycle fatigue loading. The long-term survivability of the sensor, in both static and fatigue loading, has been demonstrated. Although the load frame was shutoff after 200,000 cycles, one would expect to be able to fatigue load tiie sensor for millions of cycles since the components of the EFPI sensing head are free to move inside the alignment tube. This is consistent widi data obtained for similar silica opdcal EFPI sensors diat were fadgue loaded to over 10 million cycles as compared to resistive strain gages that typically can be cycled only to 60,000 at 2000°F.

Optical fiber Bragg grating (FBG) sensors have significant potential for use as embedded devices to monitor the structural integrity of engineering materials. The main drawback of the FBG strain sensor is its cross-sensitivity to temperature. This paper reports a simple scheme for multiplexing a FBG and an extrinsic Fabry-Perot interferometric (EFPI) sensor to enable the decoupling of strain from temperature. The EFPI sensor was constructed using a precision bore quartz capillary tube which housed two cleaved optical fibers. The gap between the fiber end- faces served as the Fabry-Perot cavity. Since the coefficients of thermal expansion between the optical fiber and the capillary tube were similar, the EFPI sensor has a very low sensitivity towards temperature. Therefore, when both sensors are placed close together, the EFPI sensor can act as the strain sensor, and temperature can be determined from the FBG wavelength shift after taking out the strain effect. The signal processing for the EFPI sensor was based on a channelled spectrum method using a CCD spectrometer. The same CCD spectrometer was also used to determine the wavelength shift of the FBG. The cross-talk between the EFPI and FBG sensors was evaluated. The feasibility of conducting simultaneous strain and temperature measurements was demonstrated.

In this paper we investigate the performance of a SCS-6/Ti-15-3 composite system, a carbon/SiC woven composite system, and an AS4/3501-6 composite system, subjected to long term mechanical fatigue, with an extrinsic Fabry-Perot interferometric (EFPI) fiber optic strain sensor. Both stiffness reduction and the degradation of thermal expansion coefficient (TEC) are monitored. The obtained results show that the EFPI sensor provides reliable data during long term fatigue loading up to 1 million cycles. The results suggest that the EFPI sensor is a viable means to monitor current and proposed characteristic damage metrics for various composite systems. We also monitor the TEC degradation during the thermal fatigue of celion G30-500/PMR-15 woven cross-ply composite system, and present a simple micromechanical model, that employs a shear lag approach, utilized to predict the effect of matrix cracking on the TEC of the composite. Results show that good agreement between experimental data and theoretical results is obtained, and that this parameter changed by as much as 80% over the fatigue life of the composite.

In this Letter, a long-range optical fiber displacement sensor based on an extrinsic Fabry-Perot interferometer (EFPI) built with a strongly coupled multicore fiber (SCMCF) is proposed and demonstrated. To fabricate the device, 9.2 mm of SCMCF was spliced to a conventional single-mode fiber (SMF). The sensor reflection spectrum is affected by super-mode interference in the SCMCF and the interference produced by the EFPI. Displacement of the SMF-SCMCF tip with respect to a reflecting surface produces quantifiable changes in the amplitude and period of the interference pattern in the reflection spectrum. Since the multicore fiber is an efficient light collecting area, sufficient signal intensity can be obtained for displacements of several centimeters. By analyzing the interference pattern in the Fourier domain, it was possible to measure displacements up to 50 mm with a resolution of approximately 500 nm. To our knowledge, this is the first time that a multicore fiber has been used to build a displacement sensor. The dynamic measurement range is at least seven times larger than that achieved with an EFPI built with a conventional SMF. Moreover, the SMF-SCMCF tip is robust and easy to fabricate and replicate.

In this article, we have proposed and experimentally demonstrated an optical fiber refractive index (RI) sensing interrogation system realized by extrinsic Fabry–Perot interferometer (EFPI) structure-based microwave photonic filter (MPF) technology. EFPI is considered as a sensing element, as well as a spectrum slicer of the MPF. The shift of the central frequency of the MPF’s passband caused by the change of the RI in the Fabry–Perot (FP) cavity can be monitored to demodulate the RI variation. In our experiment, four EFPI sensors with different FP cavity lengths have been applied for the RI sensing test. The experimental results show that the sensitivity can be tailored by adjusting the length of the FP cavity, and the longer the length of the FP cavity, the higher the sensitivity can be achieved. When the length of the FP cavity is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$960 ~\mu \text{m}$ </tex-math></inline-formula> , a high sensitivity of 1.2 GHz/RIU has been obtained. The proposed sensing system can transfer the changes of the interference pattern in the optical domain to the central frequency shift of the MPF’s passband in the electronic domain and thus avoid the limitation of measurement range induced by the periodic characteristics of the FP interference spectrum. The proposed sensing system also has the advantages of high sensitivity, the feasibility of multiplexing, temperature insensitivity, good repeatability and stability, simple fabrication, low cost, and so on.

Different from the previously reported cryogenic temperature measurement based on the fiber Bragg grating (FBG) sensor, we propose a novel cryogenic temperature measurement scheme with a fiber interferometric sensor. The proposed sensor is constructed by the extrinsic Fabry–Perot interferometer (EFPI) which is consisted of a ceramic ferrule, a copper sleeve and two fiber ends. Under the cryogenic surrounding, the cavity length of the EFPI will change with the thermal deformation of the copper sleeve. With liquid nitrogen, the surrounding temperature can be changed from 77K to room temperature, and as high as 2.246nm/K of the temperature sensitivity can be achieved at a temperature range from 113K-153K in the experiment, with a 12.38μm EFPI cavity. The proposed interferometric sensor will still have relatively high sensitivity at a temperature lower than 77K according to our numerical simulation, so, has good application prospect in cryogenic temperature environment.

A white-light interferometry based on the Fourier transform method is proposed to measure the optical path difference (OPD) of the high-finesse fiber optic extrinsic Fabry–Pérot interferometric (EFPI) sensor. The Fourier spectrum of the transmission spectrum signal consists of multiple frequency components because of the multiple-beam interference occurring in the high-finesse EFPI sensor. The high-order frequency component of the Fourier spectrum can be extracted by the Fourier transform method to recover the OPD in order to overcome the spectrum overlapping, which happens when the OPD of the EFPI sensor is short. In the experiment, a high-finesse fiber optic EFPI sensor with the cavity length of 120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(\mu \) </tex-math></inline-formula> m is measured, and the second-order frequency component was extracted to recover the cavity length. The standard deviation of the measurement results was 9.132 nm. The measurement range of the Fourier transform method was effectively extended to the short OPD.

This study presents an extrinsic Fabry–Perot interferometric (EFPI) fiber-optic strain sensor with a very short cavity. The sensor consists of two vertically cut standard single-mode fibers (SMFs) and a glass capillary with a length of several centimeters. The two SMFs penetrate into the glass capillary and are fixed at its two ends with the use of ultraviolet (UV) curable adhesives. Based on the use of the lengthy glass capillary sensitive element, the strain sensitivity can be greatly enhanced. Experiments showed that the microcavity EPFI strain sensor with initial cavity lengths of 20 μm, 30 μm, and 40 μm, and a capillary length of 40 mm, can yield respective cavity length–strain sensitivities of 15.928 nm/με, 25.281 nm/με, and 40.178 nm/με, while its linearity was very close to unity for strain measurements spanning a range in excess of 3500 με. Furthermore, the strain–temperature cross-sensitivity was extremely low.

In this paper, we proposed an optical fiber extrinsic FabryPerot interferometer (EFPI) sensor for distance detection of ultrasound signal. This optical fiber EFPI sensor was fabricated with a novel and simple technics. In the experiments, both traditional piezoelectric transducer (PZT) and high voltage electrode discharge were used as the ultrasound signal source. The experimental results indicate that strong ultrasound signals can be detected by the fiber EFPI senor in both cases and the maximum detection distance in the air is more than 180 cm.

Reconfigurable highly-sensitive interferometric sensor with Vernier effect and a virtually tunable reference cell

In the framework of the Joint Instrumentation Laboratory (LCI), gathering resources from SCK-CEN (Belgium) and CEA (France), we are developing an optical sensor in order to accurately measure radiation-induced elongation of material placed in the core of a Material Testing Reactor (MTR). This extensometer displays common advantages of Fibre Optic (FO) sensors: high resolution, easy remote sensing and multiplexing, and also compact size which is of particular interest for in pile experiments with little room available. In addition, light weight reduces gamma heating hence limiting the thermal effect. In accordance with the specifications, the sensor has preferably two fixing points defining a gauge length of 10 to 15 mm (as a minimum). The diameter is less than 2 mm. Intense gamma and neutron irradiation as well as high temperatures are the most difficult environment conditions to withstand. Reactor radiation produces huge losses in common optical fibre. The losses can be limited by selecting the fibres (mainly with high purity silica core), the wavelength range (800–1200nm), and a measurement based on interferometry (largely insensitive to losses in the fibre thanks to the wavelength encoding of the useful signal). Heavy neutron — mainly — and gamma flux such as in MTR, also produce compaction of silica, resulting in a significant drift and preventing the use of commercial FO sensors in such environment. Knowing this issue we revised the basic scheme of Extrinsic Fabry Perot Interferometer (EFPI) in order to limit the effects of compaction. A first sensor prototype fixed on a stainless steel support was tested in the Smirnof test facility in the BR2 MTR in Mol (Belgium). The support was subject to a constant mechanical and thermal stress, and his dimensions were not supposed to vary. This test showed a very low drift of the revisited EPFI design under high irradiation field in comparison with a commercial EFPI. This result has to be confirmed with second generation sensors with an increased robustness. The other difficulty to face is high temperature. Fibre optics with metal coating allows safe operation under temperatures up to 400°C and even higher. But differential dilatation between silica and typical metallic material produces differential elongation in the range of 0.5 10–2 i.e 5000μe for an increase in temperature of 300–400°C. Such large elongation has to be considered carefully in the sensor design and its fixing on the sample. Laboratory tests with a second generation of adapted sensors demonstrated their good accuracy: less than 1% error over a displacement of 100μm (< 1 μm error), at room temperature. The displacement measured can reach 250μm (20000μe with a gauge length of 12.5 mm) or more. We are currently implementing metal-coated fibre and we are preparing the next in pile irradiation on the BR2 reactor scheduled for mid-2011. Other applications of the sensor can be considered.

A novel sensing peak identification method for high accuracy refractive index (RI) sensing is proposed. The implementation takes the intensity of interference maximum as the characteristic to distinguish interference peaks, tracking the sensing peak continually during a RI changes, with high measurement accuracy and simple computation. To verify the effect of the method, the extrinsic Fabry–Perot interferometer (EFPI) sensor has been fabricated using the large lateral offset splicing technique. In the RI range from 1.346 to 1.388, the measurement range of the EFPI with the proposed method reaches at least 6 times larger than that of EFPI with the wavelength tracking method and the largest measurement error is −4.47 × 10−4. The EFPI refractive index (RI) sensor identified the sensing peak is believed to play an important role in RI, concentration and density sensing, etc., for superior performance.