Extrinsic Fabry-Perot Interferometer Research Articles

Utilization of extrinsic Fabry-Perot interferometers with spectral interferometric interrogation for microdisplacement measurement

The paper presents a number of signal processing approaches for the spectral interferometric interrogation of extrinsic Fabry-Perot interferometers (EFPIs). The analysis of attainable microdisplacement resolution is performed and the analytical equations describing the dependence of resolution on parameters of the interrogation setup are derived. The efficiency of the proposed signal processing approaches and the validity of analytical derivations are supported by experiments. The proposed approaches allow the interrogation of up to four multiplexed sensors with attained resolution between 30 pm and 80 pm, up to three times improvement of microdisplacement resolution of a single sensor by means of using the reference interferometer and noise-compensating approach, and ability to register signals with frequencies up to 1 ​kHz in the case of 1 ​Hz spectrum acquisition rate. The proposed approaches can be used for various applications, including biomedical, industrial inspection, and others, amongst the microdisplacement measurement.

Report of High Temperature Measurements with a Fabry-Perot Extensometer

Fabry-Perot (FP) sensors like other Fiber Optic (FO) sensors may be of particular interest for in pile experiments in MTR with little room available thanks to their compact size. Light weight also reduces gamma heating hence limiting the thermal effect. Different physical parameters such as temperature, strain, displacement, vibration, pressure, or refractive index may be sensed through the measurement of the optical path length difference in the cavity. We have developed a Fabry-Perot extensometer able to operate at high temperature (up to 400°C), under a high level of radiation (neutron and gamma flux). The measurement based on interferometry is largely insensitive to radiation induced attenuation (RIA) thanks to the wavelength encoding of the useful signal, but for such high fluence as encountered in a reactor core, a special rad-hard fiber is needed. Operating in the wavelength domain around 1ím remains preferable to minimize the impact of irradiation. Moreover, fast neutron radiation is expected to change the structure of the fiber and possibly others materials in the transducer. Then, we revised the basic scheme of Extrinsic Fabry-Perot Interferometer (EFPI) so that the effects of compaction remain limited. Tests under mixed neutron and gamma irradiation permitted to verify the general behavior and particularly the low drift with radiation induced compaction (RIC). Also, two types of tests have been conducted to verify the accuracy at high temperature. The first ones are “measurements” of thermal dilatation of materials: the sensor is fixed on a sample and knowing its thermal expansion, it is possible to predict the measurement expected from the optical sensor when the temperature is increased from low to high temperature. The comparison between the predicted and experimental outputs informs on how the sensor is accurate. The second types are tests on a tensile test bench operating at high temperature. The Fabry-Perot measurements are compared, in the elastic domain, with the expected strain given by the Young modulus of the material, and also on a larger strain domain, with the measurements of a high temperature axial extensometer. Both types of tests are presented and commented.

A Dual-Cavity Fabry–Perot Interferometric Fiber-Optic Sensor for the Simultaneous Measurement of High-Temperature and High-Gas-Pressure

A fiber optic dual-cavity Fabry-Perot interferometer(DFPI) for simultaneous high- temperature and high-gas-pressure measurements is proposed and experimentally demonstrated. The proposed sensing structure consists of an extrinsic Fabry-Perot interferometer (EFPI) with a short piece of hollow core fiber (HCF) for gas pressure sensing, and an intrinsic Fabry-Perot interferometer (IFPI) made of photonic crystal fiber (PCF) for temperature sensing. The composite interference signal is filtered in the frequency domain to obtain two independent interference spectra, and then is demodulated by using white light interferometry (WLI). The experimental results show that the sensor exhibits good linearity over a temperature range from 40 to 1000°C with a cavity-length temperature sensitivity of 25.3 nm/°C, and a pressure range from 0 to 10 MPa with a cavity-length pressure sensitivity of 1460 nm/MPa at 40°C. This proposed sensor are light-weight and small, and it has a compact structure and a high temperature resistance.

Microelectromechanical system-based, high-finesse, optical fiber Fabry–Perot interferometric pressure sensors

A high-finesse, optical fiber, extrinsic Fabry–Perot interferometric (EFPI) pressure sensor based on a microelectromechanical system (MEMS) technique is proposed and experimentally demonstrated. The essential element in the pressure sensor is the high-finesse EFPI cavity that consists of a Pyrex glass wafer, a micromachined silicon wafer, and highly reflective dielectric films. Another Pyrex glass is used for fixing an optical fiber collimator, which allows the realization of the alignment of the incident light. Experimental results show that the proposed sensor exhibits a pressure sensitivity of 1.598 μm/MPa and a high-pressure sensing resolution of 0.002% of the full scale. This sensor is expected to benefit many applications that require high-accuracy pressure measurements, and especially atmospheric pressure applications.

Fiber-optic, extrinsic Fabry-Perot interferometric dual-cavity sensor interrogated by a dual-segment, low-coherence Fizeau interferometer for simultaneous measurements of pressure and temperature.

A fiber-optic, extrinsic Fabry-Perot interferometric (EFPI), dual-cavity sensor made of sapphire was fabricated and interrogated by a dual-segment, low-coherence Fizeau interferometer to achieve simultaneous pressure and temperature measurements. The fiber-optic EFPI, dual-cavity sensor had an initial basal cavity length of 680 µm and an vacuum cavity length of 80 µm and was experimentally tested based on temperature and pressure measurements. It was demonstrated that simultaneous pressure and temperature measurement could be achieved in the respective pressure and temperature ranges of 0.1-3 MPa and 20-350 °C.

Probing the Theoretical Ultimate Limit of Coaxial Cable Sensing: Measuring Nanometer-Scale Displacements

Open-ended coaxial probes have been widely explored and commercialized for dielectric spectroscopy at microwave frequencies over the past decades. The working principle of the probes relies on the dependence of the fringing field on the complex permittivity of the material in front of the open end. In this article, inspired by the optical fiber extrinsic Fabry–Perot interferometer, we propose a novel sensing scheme based on an open-ended hollow coaxial cable resonator (OE-HCCR). The resonator is constructed using two reflectors along the coaxial line. The first reflector is a metal post at the RF input end, shorting the inner and outer conductors. The second reflector is the open end, where a metal plate is placed parallel and near the open end. The resonance frequency of the open-ended coaxial resonator depends strongly on the gap distance between the metal plate and the open end of the coaxial cable, due to the modulation of the phase of the reflection coefficient that characterizes the open end. Thus, by correlating the resonance frequency to the gap distance between the metal plate and the open end (or the movement of the metal plate), the OE-HCCR can be used as a displacement sensor device. Importantly, the displacement measurement resolution of the OE-HCCR is three orders of magnitude greater than that of the existing coaxial-cable-based displacement sensors within a certain dynamic range (~0.11 mm), affording a resolution that is comparable to fiber-optic sensors. The mathematical model of the OE-HCCR is discussed in detail, followed by a proof of concept for displacement measurements. The novel OE-HCCR device is suitable for sensing applications in harsh environments and will advance the field of physical/chemical sensing.

Simultaneous measurement of shrinkage and coefficient of thermal expansion of mortar based on EFPI sensors with nanometer resolution

Sliding and displacement instruments based on extrinsic Fabry-Perot interferometer (EFPI) are proposed to simultaneously monitor shrinkage and the coefficient of thermal expansion (CTE) of mortar specimens at early stages. Mortar specimens with three water cement ratios are tested. The effect of surface roughness on the shrinkage of mortar specimen is considered by using grooved stainless steel molds. The shrinkage from displacement sensors are compared with that from traditional linear variable differential transformer (LVDT). The CTE of mortar specimens is also measured with EFPI displacement sensor. Results indicate that both EFPI sliding and displacement sensors are capable of measuring shrinkage of mortar beam with nanometer resolution, which is higher than micrometer resolution from LVDT. The shrinkage of mortar is dependent on water cement ratio, curing age and surface roughness. The evolution of the CTE of mortar specimens over curing time is successfully measured with EFPI displacement sensor.

A novel method for simultaneous measurement of temperature and strain based on EFPI/FBG

A modified intensity demodulation method for fiber optic Fabry–Perot cavity and fiber Bragg grating combined sensing system is proposed, through which the cross-sensing error can be reduced effectively and high sensing speed can be realized at the same time. The system is based on an Extrinsic Fabry–Perot Interferometer for strain sensing, and a fiber Bragg grating for temperature sensing. Three-wavelength digital phase demodulation method and passive linear filtering demodulation method are adopted for the demodulation of F–P cavity length and fiber Bragg grating central wavelength, respectively. Meanwhile temperature varied from 25 °C to 105 °C under constant pressure, the demodulation accuracy of strain after correction is ±0.6με, which is improved by 2 times than the direct demodulation result. The demodulation accuracy of temperature is 1.4 °C after correction, with pressure ranging from 0 to 10 MPa at constant temperature. Temperature demodulation error is decreased to 1/3 of its original value. Strain sensing accuracy after correction was ±1με. Experimental studies show that the proposed sensing system has a more compact size and higher measurement accuracy than the previous ones.

Miniature on-fiber extrinsic Fabry-Perot interferometric vibration sensors based on micro-cantilever beam

Abstract An on-fiber extrinsic Fabry–Perot interferometric (EFPI) vibration sensor based on micro-cantilever beam is proposed and experimentally demonstrated. The micro-cantilever beam, with a cantilever length of 80μm and a cantilever thickness of 5μm, is created perpendicular to the fiber axis by using the femtosecond laser micro-machining technique. The on-fiber vibration sensor has same diameter with that of the single mode fiber. An acceleration sensitivity of 11.1 mV/g@300 Hz in the range of 0.5-5g is demonstrated experimentally. This on-fiber and micro-cantilever beam design allows for the sensor to be smaller size and higher temperature resistance.

Variations of magnetic field measurement with an extrinsic Fabry-Perot interferometer by double-beam technique

The magnetic field variations were measured by a designed optical fiber magnetic field sensor based on the extrinsic Fabry-Perot interferometric sensor with both single and double beam techniques. Behavior of a giant magnetostrictive material Terfenol-D rod was examined at room temperature, under magnetic field variations and varying applied force. In both techniques, as the force increased, the full width half maximum of the peaks decreased; therefore, a larger number of bright and dark fringes occurred with the magnetic field. The displacements increase from 9.24 ± 0.03 to 13.20 ± 0.03 μm for the single beam for 8.65, 9.75, 11.47 and 13.35 N. For the double beam, the displacements are found to be 3.69 ± 0.02, 3.69 ± 0.02, and 4.51 ± 0.02 μm for 8.65, 9.75, and 11.47, respectively. While Terfenol-D is under different forces, the orientations of magnetic domains are investigated. In summary, the magnetic field could be measured precisely at both short and long ranges by these techniques.

Hearing the Sounds of Aquatic Life Using Optical Fiber Microtip-Based Hydrophone

For marine bioacoustics applications, the sounds by aquatic animals must be recorded with a high precision and minimal noise in order to extract valuable information. For such marine applications, we propose and demonstrate an optical fiber microtip-based hydrophone. The hydrophone design consists of an extrinsic Fabry-Perot interferometer, which utilizes fiber microtip and processed aluminum foil as two partially reflecting mirrors. The packaged optical fiber hydrophone of dimensions 1.4 cm × 1.4 cm × 2 cm, with a microtip diameter of 31.23 μm and natural rubber diaphragm of 6 mm diameter, shows a high sensitivity of 8.94 nm/mPa or -41.42-dB re 1 nm/μPa at 1 kHz. The hydrophone shows an excellent signal-to-noise low as 0.388 μPa/√Hz at 1 kHz. The linear working range of ratio, and the noise-limited minimum detectable pressure is as the hydrophone is 0-3950 Hz. The experimental results show that the proposed optical fiber hydrophone can very accurately detect the click trains produced by dolphins for echolocation. Furthermore, depending on the applications, the performance parameters of the proposed hydrophone can be tailored by changing the diaphragm thickness and diameter, thus opening a new platform to study the marine environment.

Sensitivity-Enhanced Extrinsic Fabry-Perot Interferometric Fiber-Optic Microcavity Strain Sensor.

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.

Common-path dual-wavelength quadrature phase demodulation of EFPI sensors using a broadly tunable MG-Y laser

A common-path dual-wavelength phase demodulation technique for extrinsic Fabry-Perot interferometric (EFPI) sensors is proposed on the basis of a broadly tunable modulated grating Y-branch (MG-Y) laser. It can address the three main concerns of existing dual-wavelength phase interrogation methods: the imbalances and disturbances caused by two optical paths utilizing two lasers or two photodetectors, the restrictions between two operating wavelengths and the cavity length of EFPI, and the difficulty in eliminating the direct current (DC) component of the interferometric fringe. Dual-wavelength phase interrogation is achieved in a common optical path through high-speed wavelength switching. Taking advantage of the MG-Y laser's full spectrum scanning ability (1527 ∼ 1567 nm), initial cavity length and DC component can be directly measured by white light interferometry. Two quadrature wavelengths are then selected to perform high speed phase demodulation scheme. Three polyethylene terephthalate (PET) diaphragm based EFPI acoustic sensors with cavity lengths of 127.954 µm, 148.366 µm and 497.300 µm, are used to demonstrate the effectiveness.

Extrinsic Fabry-Perot interferometric cavity-based fiber-optic spectrum equalization filter for the Gaussian spectrum of superluminescent diodes.

To flatten the Gaussian spectrum of superluminescent diodes (SLDs), a fiber-optic spectrum equalization filter based on an extrinsic Fabry-Perot interferometric (EFPI) cavity is proposed. By the proper usage of a low-finesse EFPI cavity and the delicate design of a splitting and recombining fiber-optic path, the Gaussian spectrum of an SLD can be effectively flattened, which is verified by a numerical simulation and an experiment. An SLD with a center wavelength of 1568 nm and a 3-dB spectral width of 98 nm is spectrum-equalized to obtain a spectrum flatness of 0.27 dB in the wavelength range 1544.8-1605.6 nm. When the SLD is with a max power of 3.38 mW, the output power after the fiber-optic spectrum equalization filter can reach 178.06 μW.

Low-cost fiber optic extrinsic Fabry–Perot interferometer based on a polyethylene diaphragm for vibration detection

A low cost, compact-extrinsic Fabry–Perot interferometer (EFPI) based on a polyethylene diaphragm for vibration detection is proposed. A fiber optic Fabry–Perot accelerometer (FOFPA) with a diameter of 10.0 mm is fabricated and a silica gel is adhered to the middle of polyethylene diaphragm as a mass load to form a compact structure. Theoretical simulation, modal analysis, and experimental tests are carried out to evaluate the performance of this FOFPA. Experimental results show that the measured sensitivity and acceleration resolution are 135 mV/g and 24.4 μg/Hz1∕2, respectively. The sensor has a flat frequency response in the range of 20 Hz–1.5 kHz and is expected to be used for micro vibration monitoring.

Dual-Parameter Optical Fiber Sensor for Temperature and Pressure Discrimination Featuring Cascaded Tapered-FBG and Ball-EFPI

A dual parameter optical fiber sensor for temperature and pressure discrimination has been proposed and experimentally realized. A tapered fiber Bragg grating (tapered-FBG) and an extrinsic Fabry–Perot interferometer with a ball shape (ball-EFPI) are the two main sensing structures. The taper is fabricated by elongating it during an arc-discharge provided by the fusion splicer. Then, the femtosecond laser is used to inscribe a first-order Bragg grating in the taper region, adopting the point-by-point direct-writing technique. The ball-EFPI is fabricated using the chemical etching and arc-discharge enlargement method. The test temperature range is 40 °–400°, and the test pressure range is 0–2 MPa on the basis of ambient pressure. Experimental results show that the tapered-FBG has good temperature response while the ball-EFPI is sensitive to pressure. By analyzing the sensitivity matrix, the temperature and pressure discrimination can be achieved using the proposed hybrid sensor.

Trace Ammonia Detection Based on Near-Infrared Fiber-Optic Cantilever-Enhanced Photoacoustic Spectroscopy

A trace ammonia (NH3) detection system based on the near-infrared fiber-optic cantilever-enhanced photoacoustic spectroscopy (CEPAS) is proposed. A fiber-optic extrinsic Fabry-Perot interferometer (EFPI) based cantilever microphone has been designed to detect the photoacoustic pressure signal. The microphone has many advantages, such as small size and high sensitivity. A near-infrared tunable erbium-doped fiber laser (EDFL) amplified by an erbium-doped fiber amplifier (EDFA) is used as a photoacoustic excitation light source. To improve the sensitivity, the photoacoustic signal is enhanced by a photoacoustic cell with a resonant frequency of 1624 Hz. When the wavelength modulation spectroscopy (WMS) technique is applied, the weak photoacoustic signal is detected by the second-harmonic detection technique. Trace NH3 measurement experiments demonstrate that the designed fiber-optic CEPAS system has a linear response to concentrations in the range of 0 ppm – 20 ppm at the wavelength of 1522.448 nm. Moreover, the detection limit is estimated to be 3.2 ppb for a lock-in integration time of 30 s.

Three-wavelength passive demodulation technique for the interrogation of EFPI sensors with arbitrary cavity length.

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.

A diaphragm-free fiber Fabry-Perot gas pressure sensor.

A diaphragm-free fiber gas pressure sensor based on the extrinsic Fabry-Perot interferometer (EFPI) is proposed and experimentally demonstrated. The sensor is fabricated by fusion splicing a sandwich structure of a single mode fiber, a hollow core fiber (HCF), and a coreless fiber. A femtosecond (fs) laser is employed to drill a micro-channel on the side wall of the HCF to allow gas entering/exiting the HCF. The refractive index of the gas has a linear relationship with the gas pressure. Thus, the gas pressure can be detected by investigating the optical path difference of the EFPI. The experimental results show that the proposed sensor exhibits a high gas pressure sensitivity of ∼1.80 µm/MPa from vacuum to 10 MPa at room temperature. Pressure responses under high temperatures are also investigated. The proposed sensor has properties of robust structure, miniature size, large measuring range, and high sensitivity.

Fast and simple interrogation of extrinsic Fabry–Perot interferometer sensor based on dual-wavelength intensity ratio

A fast and simple demodulation method for the variation of Fabry–Perot (FP) cavity length is proposed and demonstrated. Taking advantages of differentially detecting optical power intensities of dual-wavelength, a reliable linear correlation between the variation of FP cavity length and differential intensity ratio of dual-wavelength can be obtained. Besides, an extrinsic Fabry–Perot interferometer (EFPI) sensor with a length of 196.8 μm was used to demonstrate performance of the method, and the time for each experimental operation is within 100 ms by the demodulation method and an integrable tunable laser assembly (ITLA). The demodulation resolution of 0.726 nm was achieved for the change of cavity length in a strain sensing experiment. By selecting two wavelengths at different positions in the system, the linear ranges of FP cavity length variation are 180 nm, 150 nm, and 130 nm responding to the sensitivities of −0.01099 nm−1, −0.01485 nm−1, and −0.02253 nm−1, respectively. It shows a high flexibility between the linear range and the sensitivity. Consequently, the different requirements of the EFPI sensor system can be easily fulfilled through appropriate selections of two different wavelengths. Moreover, the advantages of simplicity, fast speed, low cost can be fully revealed in the demodulation of large-scale production EFPI sensors.