Extrinsic Fabry-Perot Interferometer Research Articles

Ultra-thin DLC diaphragm-based optical fiber sensor achieving a highly sensitive and broadband acoustic response.

In the design of an extrinsic Fabry-Perot interferometer (EFPI) acoustic sensor, broadband response and high-sensitivity sensing are usually conflicting and need to be carefully balanced. Here, we present a novel, to the best of our knowledge, optical fiber acoustic sensor based on an ultra-thin diamond-like carbon (DLC) film, fabricated using the plasma-enhanced chemical vapor deposition method, and transferred by a surface-energy-assisted method. The sensor exhibits a broadband response ranging from 200 Hz to 100 kHz, maintains an average sensitivity of 457.3 mV/Pa within the range of 6 to 30 kHz, and can detect weak acoustic signals down to 3.23 µPa/Hz1/2@16.19 kHz. The combination of an ultra-thin DLC film with a relatively high Young's modulus and internal stresses results in a trade-off between high sensitivity and a broadband response. This performance demonstrates that our sensor is among the most advanced in the EFPI acoustic sensor family, with significant potential for applications such as photoacoustic spectroscopy, defect diagnosis, and bio-imaging.

InGaSn-assisted in-fiber Fabry-Perot temperature sensor with ultra-high sensitivity

In this paper, we proposed an in-fiber Fabry-Perot temperature sensor which filled with Indium gallium stannum (InGaSn) alloy. By splicing a single-mode fiber axially with hollow-core fibers (HCFs) of varying inner diameters, a cavity of the Fabry-Perot interferometer (FPI) structure is formed. Assisted by liquid metal with fluidity and high thermal expansivity, subtle temperature fluctuations are converted into variations in the interferometric cavity length which leads to a significant spectral drift. Experimental result shows that the temperature sensitivity for the proposed sensor reaches 11.3 nm/°C in the range of 30-40°C, which is almost an order of magnitude higher than that of other extrinsic FPIs. Such high sensitivity within this temperature range makes the sensor potentially applicable in temperature monitoring in the vitro cell culture and biopharmaceutical fields. Additionally, influences caused by the inner diameter of HCFs and the amount of liquid metal filled in structure are also considered in this work.

Temperature-Decoupled Single-Crystal MgO Fiber-Optic Fabry-Perot Vibration Sensor Based on MEMS Technology for Harsh Environments.

A high-temperature-resistance single-crystal magnesium oxide (MgO) extrinsic Fabry-Perot (FP) interferometer (EFPI) fiber-optic vibration sensor is proposed and experimentally demonstrated at 1000 °C. Due to the excellent thermal properties (melting point > 2800 °C) and optical properties (transmittance ≥ 90%), MgO is chosen as the ideal material to be placed in the high-temperature testing area. The combination of wet chemical etching and direct bonding is used to construct an all-MgO sensor head, which is favorable to reduce the temperature gradient inside the sensor structure and avoid sensor failure. A temperature decoupling method is proposed to eliminate the cross-sensitivity between temperature and vibration, improving the accuracy of vibration detection. The experimental results show that the sensor is stable at 20-1000 °C and 2-20 g, with a sensitivity of 0.0073 rad (20 °C). The maximum nonlinearity error of the vibration sensor measurement after temperature decoupling is 1.17%. The sensor with a high temperature resistance and outstanding dynamic performance has the potential for applications in testing aero-engines and gas turbine engines.

All-sapphire fiber-optic sensor for the simultaneous measurement of ultra-high temperature and high pressure

An all-sapphire fiber-optic extrinsic Fabry-Perot interferometric (EFPI) sensor for the simultaneous measurement of ultra-high temperature and high pressure is proposed and experimentally demonstrated. The sensor is fabricated based on all-sapphire, including a sapphire fiber, a sapphire capillary and a sapphire wafer. A femtosecond (fs) laser is employed to drill a through hole at the side wall of the sapphire capillary to allow gas flow. The sapphire fiber is inserted from one side of the sapphire capillary. The sapphire wafer is fixed at the other side of the sapphire capillary. The first Fabry-Perot (FP) cavity, composed of the end face of the sapphire fiber and the front surface of the sapphire wafer, is used for measuring pressure, while the second FP cavity, composed of the two surfaces of the sapphire wafer, is used for measuring temperature. Experimental results show that the sensor can simultaneously measure ultra-high temperature and gas pressure within the temperature range of 20 - 1400 °C and the pressure range of 0 - 5 MPa. The temperature sensitivity is 0.0033 µm/°C, and the pressure sensitivity decreases as the temperature increases, reaching 1.8016 µm/MPa and 0.3253 µm/MPa at temperatures of 20 °C and 1400 °C, respectively.

High-sensitivity fiber-tip acoustic sensor with ultrathin gold diaphragm

Miniature acoustic sensors with high sensitivity are highly desired for applications in medical photoacoustic imaging, acoustic communications and industrial nondestructive testing. However, conventional acoustic sensors based on piezoelectric, piezoresistive and capacitive detectors usually require a large element size on a millimeter to centimeter scale to achieve a high sensitivity, greatly limiting their spatial resolution and the application in space-confined sensing scenarios. Herein, by using single-crystal two-dimensional gold flakes (2DGFs) as the sensing diaphragm of an extrinsic Fabry-Perot interferometer on a fiber tip, we demonstrate a miniature optical acoustic sensor with high sensitivity. Benefiting from the ultrathin thickness (∼8 nm) and high reflectivity of the 2DGF, the fiber-tip acoustic sensor gives an acoustic pressure sensitivity of ∼300 mV/Pa in the frequency range from 100 Hz to 20 kHz. The noise-equivalent pressure of the fiber-tip acoustic sensor at the frequency of 13 kHz is as low as 62.8 µPa/Hz1/2, which is one or two orders of magnitude lower than that of reported optical acoustic sensors with the same size.

Four-wavelength laser interferometry for the demodulation of dual-cavity fiber-optic extrinsic Fabry-Perot interferometric sensors

A four-wavelength passive demodulation algorithm is proposed and experimentally demonstrated for the interrogation of the one cavity in a dual-cavity extrinsic Fabry-Perot interferometric (EFPI) sensor. The lengths of two cavities are adjusted to generate four quadrature signals for each individual cavity. Both simulation and experimental results are presented to validate the performance of this technique. The experimental results demonstrate that dynamic signals at frequencies of 100 Hz, 200 Hz, and 300 Hz with varying amplitude are successfully extracted from a dual-cavity EFPI sensor with initial lengths of 93.4803 µm and 94.0091 µm. The technique shows the potential application to measure dynamic signals in dual-cavity fiber-optic EFPI sensors.

Stability-enhanced low-frequency fiber optic hydrophone based on an extrinsic Fabry-Perot interferometer

Fiber optic hydrophones (FOHs) offer the notable advantage of electromagnetic interference resistance. Nevertheless, overcoming the challenge of sustaining stable, high-performance operation in intricate underwater settings at a low cost remains a considerable obstacle for them. To circumvent the restrictions noted above, we employed a miniaturized FOH, utilizing an easily fabricated extrinsic Fabry-Perot interferometer (EFPI) which is made up of a composite chromium-aluminum (Cr-Al) membrane and fiber. The linear demodulation also suppresses the drift issue in the output spectrum. The average sound pressure sensitivity of the sensor, according to experimental findings, is around -139.15 dB re 1 V/µPa, while the equivalent noise sound pressure at 1 kHz is 51.52 dB re 1 µPa/Hz1/2. This sensor has a lot of potential because of features like sensitive low-frequency response and noise performance.

Crosstalk suppression of extrinsic Fabry-Perot interferometric sensor array based on five-step phase shift demodulation scheme using multiwavelength averaging.

A five-step phase shift demodulation scheme based on a multiwavelength averaging method is proposed to suppress crosstalk within an extrinsic Fabry-Perot interferometric (EFPI) sensor array. The paper focuses on a two-element sensing system based on the EFPI sensors to investigate the crosstalk in the EFPI sensor array. A detailed theoretical analysis of crosstalk suppression using the proposed demodulation method is presented. Numerical simulations and experiments are put forward to demonstrate the effectiveness of the proposed demodulation scheme in suppressing crosstalk under varying parameters. The results of the multiwavelength demodulation scheme indicate superior crosstalk suppression capability in contrast to the conventional five-step phase shift demodulation scheme based on a single-wavelength demodulation method. Furthermore, the paper reveals the enhanced crosstalk suppression capability of the proposed demodulation scheme when the cavity length difference between elements is not equal to zero. It alleviates the requirement for consistent cavity length among different elements in the sensing array. The proposed demodulation scheme exhibits excellent crosstalk suppression capabilities in optical multiplexing arrays by decreasing the dependency on extinction ratio and could be potentially used in the large-scale optical hydrophone array system.

Chemical Classification by Monitoring Liquid Evaporation Using Extrinsic Fabry-Perot Interferometer With Microwave Photonics

Identification of liquids is essential in chemical analysis, safety, environmental protection, quality control, and research. A novel liquid identification system based on Microwave Photonics (MWP) measured time transient evaporation signals is investigated. An extrinsic Fabry-Perot Interferometer (EFPI) based optical probe using single-mode fiber (SMF) is proposed to monitor evaporation of different liquids. The MWP system is used to measure the optical path changes during liquid evaporation due to its high sensitivity, selectivity, and Signal-to-Noise Ratio (SNR). The measured S21 continuous wave (CW) time Magnitude and Phase signals were processed to extract features such as histogram and Fast Fourier Transform (FFT) peaks. Using features extracted from droplet evaporation time transient events, machine learning classification accurately identified chemicals in each liquid with an accuracy rate of over 99%, employing three algorithms: Decision Trees, Support Vector Machine (SVM), and K-nearest neighbors (KNN). The classification results demonstrate accurate liquid identification based on evaporation measurements by the MWP system.

All-sapphire-based optical fiber pressure sensor with an ultra-wide pressure range based on femtosecond laser micromachining and direct bonding

An all-sapphire extrinsic Fabry-Perot interferometer (EFPI) optical fiber pressure sensor with ultra-wide pressure range and high temperature resistance is proposed and experimentally demonstrated. The sensor is fabricated by direct bonding three sapphire wafers, including the sapphire substrate, the sapphire wafer with a through hole, and the sapphire pressure-sensitive diaphragm. A femtosecond (fs) laser is used to inscribe a through hole in the center of the sapphire wafer and roughen the outer surface of the sapphire pressure-sensitive diaphragm. By using original polished surfaces of sapphire wafers with low surface roughness as reflective surfaces of the Fabry-Perot (FP) cavity, the high-quality interference signal can be obtained, thereby improving the measurement accuracy of the sensor. The optical cavity length (OCL) of the proposed sensor changes linearly with the applied pressure in the wide range of 0 - 50 MPa at room temperature, and the pressure sensitivity is 0.0921 µm/MPa. The pressure measurement accuracy reaches 0.31%FS (full scale). High temperature experiments show that the sensor can work stably at 1000 ℃.

Three-wavelength phase demodulation technique for extrinsic Fabry-Perot interferometric sensors

A three-wavelength quadrature phase compensation method for recovering dynamic signals in extrinsic Fabry–Perot interferometers (EFPIs) is proposed and experimentally demonstrated. The influence of the direct current component is eliminated by using three symmetrical interferometric signals, and two orthogonal signals are obtained by the phase compensation algorithm to extract the phase change. A sinewave with a frequency of 1000 Hz was applied on an EFPI sensor. The cavity length of the EFPI sensor is 643.0299 μm. The demodulated signal is a sinewave with a frequency of 1000 Hz. The demodulated frequency shows good agreement with the frequency of the applied signal. This technique is suitable for measuring EFPI sensor cavity length changes under dynamic signals.

Five-Step Phase-Shift-Based Multiwavelength Averaging for Extrinsic Fabry–Perot Interferometric Sensors

In order to address issues such as low demodulation accuracy, large demodulation errors, small dynamic range, and complex algorithms for the extrinsic Fabry–Perot interferometric (EFPI) sensor, a five-step phase-shift algorithm based on a multiwavelength (MW)-averaging method is proposed to improve demodulation speed, noise stability, dynamic range, and noise suppression. The proposed demodulation method utilizes white-light interferometry (WLI) technology to acquire the reflection spectra and extract the five-step phase-shift signals at Ns consecutive operating points. The demodulation results of Ns sets of five-step phase-shift signals are averaged to obtain the average demodulated phase and cavity length variation. Theoretical analysis demonstrates the significant effects of the MW demodulation method on demodulation parameter errors and noise suppression. Particularly, when the demodulation parameter θ = π/2 rad, the method exhibits excellent stability against demodulation parameter error-induced instability. Moreover, it greatly improves noise suppression and reduces noise fluctuations. Numerical simulations are conducted to validate the performance of the proposed demodulation method. Compared with the traditional single-wavelength (SW) five-step phase-shifting demodulation method, the MW demodulation method exhibits stronger noise- and harmonic-suppression capabilities as the number of averaging wavelengths Ns increases. The harmonic distortion of the MW demodulation method with Ns = 128 is 20 dB lower than that of the SW demodulation method, and the noise is 15 dB lower. Furthermore, the proposed method effectively suppresses the influence of demodulation parameter errors on signal demodulation. This proposed demodulation method has the potential for fast real-time dynamic demodulation. It has great significance and application in the field of weak signal detection in fiber-optic sensors with interferometer structures and has enormous advantages in noise-suppression in complex environments.

High-Stability Polarization-Based Interrogation Method With Cross-Correlation for Optical Fiber Interferometric Acoustic Sensors

In this work, we propose and demonstrate a high-stability polarization-based interrogation method using cross-correlation for optical fiber interferometric acoustic sensing. Specifically, an extrinsic Fabry-Perot interferometric (EFPI) sensor based on polyphenylene sulfide (PPS) diaphragm is firstly used for signal detection, and then a cross-correlation interrogation system is formed with a birefringent crystal based on polarization low-coherence interference technology. Finally, the direct current (DC) component of the interference signal can be acquired in real-time with the help of the birefringent crystals block of known length. The system does not require an orthogonal relationship between the interference signals. Moreover, it can still work when the initial cavity length of the sensor drifts due to the influence of the environment. Theoretical simulations and a series of experiments have been carried out to verify the feasibility of the proposed scheme. And experimental results show that the sensor's initial cavity length shifts from 90.082 μm to 88.451μm or 91.692 μm under 1kHz acoustic signal, the maximum relative demodulation error is 0.4539 nm, and the signal-to-noise ratio(SNR)can reach 80 dB. Compared with the reported schemes, the proposed interrogation scheme simplifies the system configuration, and has properties of low cost, strong robustness, and high-frequency, so it has great potential in practical engineering applications.

Design of long-distance multichannel system based on passive optical fiber audio transmission system

To meet sound transmission requirements in complex work environments, this paper constructs a passive audio pickup sensor based on an extrinsic Fabry-Perot interferometer (EFPI) and develops a one-way four-channel passive audio parallel transmission system based on a passive bidirectional audio over fiber(PB-AOF) transmission system and wavelength division multiplexing technology. The system realizes optical fiber audio transmission over a distance of 20 km. The experimental results show that the signal-to-noise ratio (SNR) is greater than 64 dB, and the highest sensitivity of the four channels in the proposed system is 1.06 V/Pa. Thus, the proposed long-distance terminal passive audio communication network is ideally suited for applications in certain fields.

Improved Optical Path Structure for Symmetric Demodulation Method in EFPI Fiber Optic Acoustic Sensors Using Wavelength Division Multiplexing.

This paper presents a novel improvement in the optical path structure of a three-wavelength symmetric demodulation method applied to extrinsic Fabry-Perot interferometer (EFPI) fiber optic acoustic sensors. The traditional approach of using couplers to construct the phase difference in the symmetric demodulation method is replaced with a new approach that combines the symmetric demodulation algorithm with wavelength division multiplexing (WDM) technology. This improvement addresses the issue of a suboptimal coupler split ratio and phase difference, which can affect the accuracy and performance of the symmetric demodulation method. In an anechoic chamber test environment, the symmetric demodulation algorithm implemented with the WDM optical path structure achieved a signal-to-noise ratio (SNR) of 75.5 dB (1 kHz), a sensitivity of 1104.9 mV/Pa (1 kHz), and a linear fitting coefficient of 0.9946. In contrast, the symmetric demodulation algorithm implemented with the traditional coupler-based optical path structure achieved an SNR of 65.1 dB (1 kHz), a sensitivity of 891.75 mV/Pa (1 kHz), and a linear fitting coefficient of 0.9905. The test results clearly indicate that the improved optical path structure based on WDM technology outperforms the traditional coupler-based optical path structure in terms of sensitivity, SNR, and linearity.

Advances in Fiber-Optic Extrinsic Fabry–Perot Interferometric Physical and Mechanical Sensors: A Review

Fabry–Perot interferometers (FPIs) have found a multitude of scientific and industrial applications ranging from gravitational wave detection, high-resolution spectroscopy, and optical filters to quantum optomechanics. Integrated with optical fiber waveguide technology, the fiber-optic FPIs have emerged as a unique candidate for high-sensitivity sensing and have undergone tremendous growth and advancement in the past two decades with their successful applications in an expansive range of fields. The extrinsic cavity-based devices, i.e., the fiber-optic extrinsic FPIs (EFPIs), enable great flexibility in the design of the sensitive Fabry–Perot cavity combined with state-of-the-art micromachining and conventional mechanical fabrication, leading to the development of a diverse array of EFPI sensors targeting at different physical quantities. Here, we summarize the recent progress of fiber-optic EFPI sensors, providing an overview of different physical and mechanical sensors based on the FPI principle, with a special focus on displacement-related quantities, such as strain, force, tilt, vibration and acceleration, pressure, and acoustic. The working principle and signal demodulation methods are shown in brief. Perspectives on further advancement of EFPI sensing technologies are also discussed.

Ultrahigh-Resolution and Large-Dynamic-Range Temperature Sensor Based on Fiber-Optic EFPI Cavity

A fiber-optic extrinsic Fabry–Perot interferometric (EFPI) temperature sensor with extremely high resolution and large dynamic range is proposed and demonstrated. The sensor uses a thermally expanded core (TEC) fiber as the first reflective surface of the interference cavity to focus the beam to improve the finesse of the interference fringes, which enables extremely high cavity length resolution and large dynamic range with a multivalley averaging method based on dynamic estimation of the fringe order. Using a borosilicate capillary as the temperature-sensitizing material, a cavity length sensitivity of 79.25 nm/°C as well as a good linearity were obtained in the temperature range of 10 °C–100 °C. A temperature resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.5\times 10^{-{5}}\,\,^{\circ }\text{C}$ </tex-math></inline-formula> was demonstrated. The extremely high-temperature resolution with a large dynamic range has an important application value in the monitoring of weak temperature changes in deep sea and underground fluids.

An All-Sapphire Fiber Diaphragm-Free Extrinsic Fabry–Perot Interferometric Sensor for the Measurement of Gas Pressure at Ultrahigh Temperature

An all-sapphire fiber extrinsic Fabry–Perot interferometer (EFPI) sensor for the measurement of gas pressure at ultrahigh temperature is proposed and experimentally demonstrated. The sensor head is composed of two segments of sapphire fiber and a sapphire capillary. By using the femtosecond (fs) laser micromachining, a through hole is inscribed in the sapphire capillary wall to allow the gas flow. This EFPI is used to measure the pressure in the temperature range of 25 °C–1400 °C by monitoring the optical cavity length (OCL) of the interferometer. Experiment results show that the sensor exhibits a good linearity within a pressure range from 0 to 5 MPa at a fixed temperature. The pressure sensitivity of the sensor decreases as the temperature increases, and sensitivities are 1.018 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.188 \mu \text{m}$ </tex-math></inline-formula> /MPa at 25 °C and 1400 °C, respectively. The proposed all-sapphire fiber pressure sensor has the properties of high-temperature resistance, intrinsic safety, small volume, and large measuring range, which can be used in high-temperature and harsh environments.

An Accurate Demodulation Method for Fiber-Optic Interferometric Sensors Based on the Expanded Free Spectral Range

We propose and demonstrate an accurate demodulation method for fiber-optic interferometric sensors (FOISs) by utilizing an expanded free spectral range (EFSR). Unlike conventional demodulation schemes, this method selects a pair of resonant wavelengths (peaks or dips) with an interval of multiple free spectral ranges (FSRs) to determine the optical path difference (OPD) of the FOIS. This demodulation method significantly improves the measurement accuracy of the FOIS while maintaining a fast processing speed. The feasibility of the proposed method is experimentally demonstrated by our recently developed fiber-optic piezometer using a 178- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -gap air extrinsic Fabry–Perot interferometer (EFPI). Liquid level measurement in the range of 0–90 cm with a step of 15 cm is conducted with this method, and the experimental results demonstrate improved performance in terms of linearity, standard error (SE), and resolution. The measurement resolution increases significantly from 8.57 to 1.48 cm when the EFSR orders vary from 1, 4, 8, 12, 16, to 20. To further validate the proposed method, the measurement in the range of 0–24 cm with a step of 4 cm is performed for comparison, and the resolution is exponentially improved from a failed state to as high as 0.41 cm. Furthermore, five sets of historical data are chosen to establish regression models, and the results are consistent with the rest 14 sets of measured data. The proposed demodulation method is precise, rapid, and versatile, which would be useful in the further development and application of FOISs.

A high-resolution joint demodulation algorithm for the fiber-optic low-finesse extrinsic Fabry-Perot interferometric sensor

In order to improve the resolution of the demodulation algorithm of the fiber-optic low-finesse extrinsic Fabry-Perot interferometric (EFPI) sensor, this paper presents a joint demodulation algorithm based on the synchronous squeezing wavelet transform phase extraction method and the weighted cosine similarity approach. This algorithm combines the advantages of the synchronous squeezing wavelet transform phase extraction method which can achieve high resolution in dynamic measurement, with the weighted cosine similarity method which can reduce signal interference to obtain high resolution. In the experiment, the joint demodulation algorithm realizes a high-resolution measurement of the EFPI sensor, reaching 0.020 nm and the greatest fluctuation 0.053 nm with an initial cavity length of 60 μm. The resolution of the joint demodulation algorithm has shown promising result at the sub-nanometer level.