Extrinsic Fabry-Perot Interferometric Optical Fiber Research Articles

Fiber-optic extrinsic Fabry–Perot interferometer sensors with quadrature phase three wavelength demodulation

Fiber-optic extrinsic Fabry–Perot interferometer sensors with quadrature phase three wavelength demodulation

Real-Time Measurement of Refractive Index Using 3D-Printed Optofluidic Fiber Sensor

This work describes a 3D-printed optofluidic fiber sensor to measure refractive index in real time, combining a microfluidic system with an optical fiber extrinsic Fabry-Perot interferometer. The microfluidic chip platform was developed for this purpose through 3D printing. The Fabry-Perot cavity was incorporated in the microfluidic chip perpendicularly to the sample flow, which was of approximately 3.7 µL/s. The optofluidic fiber sensor platform coupled with a low-cost optical power meter detector was characterized using different concentrations of glucose solutions. In the linear regression analysis, the optical power shift was correlated with the refractive index and a sensitivity of -86.6 dB/RIU (r2 = 0.996) was obtained. Good results were obtained in terms of stability with a maximum standard deviation of 0.03 dB and a sensor resolution of 5.2 × 10-4 RIU. The feasibility of the optofluidic fiber sensor for dynamic analyses of refractive index with low sample usage was confirmed through real-time measurements.

High-Precision Large-Range Optical Fiber Interferometric Piezometer and Its Wideband Interferometry for Water Pressure Measurement

A high-precision large-range fiber-optic interferometric piezometer is designed, packaged, and demonstrated for pore water pressure measurement. The piezometer is an optical fiber extrinsic Fabry–Perot interferometer (EFPI), which comprises a fiber ferrule connector/flat contact (FC/FC) connector and an aluminum foil pasted on a thin metal plate. The EFPI forms a sealed chamber with only the outer plane of the aluminum alloy plate being subjected to the water pressure through another hydraulic chamber with a standard filter stone. The deformation of the metal plate is analyzed and simulated for parameter optimization by using the small deflection theory. By adopting the conventional wavelength tracking method, the theoretical simulation and experimental results show a high measurement accuracy of the cm level but a limited measurement range. To resolve the problem, several wideband interferometry (WBI) methods, including free spectral range (FSR)-based, spatial frequency, interference order, and fitted FSR, are employed to analyze the spectrum of the EFPI for comparison. The fitted FSR method has demonstrated its superiority in applications that require simultaneous high resolution and large measurement range. A gas pressure test verifies that the proposed piezometer can work in a wide range of up to 0.5 MPa that is equivalent to a water depth of 50 m. In addition, the proposed piezometer offers several advantages, such as cost-effectiveness, robustness, repeatability, and stability, which further benefits its practical application.

Humidity sensing and temperature response performance of polymer gel cold-spliced optical fiber Fabry-Perot interferometer

An extrinsic Fabry-Perot Interferometric (EFPI) fiber humidity probe has been proposed. This composite EFPI fiber humidity sensor combines the characteristics of various fiber structures, such as the cascaded spliced fibers, coating film on the fiber end-face and in-line fixed Fabry-Perot cavity in capillary. The polymethyl methacrylate (PMMA) was filled in between the air gap between single mode fiber and muti-modes fiber as the humidity sensitive material to establish the EFPI optical fiber humidity sensor. The best sensitivity has been experimentally demonstrated with 0.4172 nm/%RH in the relative humidity range of 10%RH-70%RH, where the corresponding fitting coefficient is 0.99939, and the response time is less than 14 min. Furthermore, the wavelength temperature sensitivity based on wavelength modulation is 0.06074 nm/℃, and the power responded sensitivity is 0.056 dBm/℃ in the temperature range of 20℃-65℃. The experimental results indicate that the proposed fiber probe can simultaneously measure humidity and temperature, or in other way, it will significantly improve the accurate measurement for humidity by analyzing and compensating the influencing factors from the environmental temperature.

A mechanical amplifier based high-finesse fiber-optic Fabry–Perot interferometric sensor for the measurement of static magnetic field

A high-finesse fiber-optic extrinsic Fabry–Perot interferometer (EFPI)-based sensor for the measurement of static magnetic fields is proposed and experimentally demonstrated. The high-finesse EFPI is composed of two aligned fiber ends, each of which has a micro-lens mirror. The high-finesse EFPI is mounted on a specially designed mechanical amplification structure which is driven by a TbDyFe rod. Under the applied bias magnetic field, the TbDyFe rod works in the linear region, and its length changes with the change of the magnetic field strength. The mechanical amplification structure amplifies the elongation of the TbDyFe rod in the magnetic field, thus changing the optical cavity length of the high-finesse fiber-optic EFPI. The magnetic field strength can be obtained by measuring the spectral resonance wavelength. Experimental results show that the wavelength sensitivity with the applied magnetic field is 0.218 nm μT−1, the magnetic field resolution is 4.59 nT, and the magnification of the mechanical amplifier is 5.66 times.

High-Sensitivity Temperature Sensor Based on Photosensitive Polymer-Filled Silica Capillary Tube

A highly sensitive fiber-optic extrinsic Fabry-Perot interferometer (EFPI) is designed and demonstrated experimentally. The EFPI is composed of two sections of single mode fiber (SMF), a segment of silica capillary tube (SCT), an air cavity, and cured polymer which is filled in the SCT. The sensing part of structure of proposed EFPI is the air cavity, and the cured polymer has a modulating effect on the air cavity. Due to the high thermal expansion coefficient (TEC) of the polymer, the sensor exhibits a high sensitivity to temperature. Experimental results show that the temperature sensitivity of the proposed sensor reaches −2.88 nm/°C with a perfect linearity of 99.88%. Therefore, the proposed sensor with high-sensitivity, ease of fabrication and low cost is favorable to be used in high-accuracy temperature measurement.

Probing Changes in Pressure With Subpascal Resolution Using an Optical Fiber Fabry–Perot Interferometer

This article reports a novel optical fiber extrinsic Fabry–Perot interferometer (EFPI) for probing pressure changes with subpascal (~0.1 Pa, $\sim 1.45\times 10^{-5}$ psi) resolution. The principal idea of the sensor is to employ a traditional C-shaped Bourdon tube as a mechanical transducer that is coupled to a highly sensitive EFPI device. The cavity of the EFPI device is formed between a gold-coated mirror, mounted to a concave section of the Bourdon tube, and the endface of an optical fiber mounted at the base section of the Bourdon tube. Based on this design, the pressure-induced deflections of the Bourdon tube are directly correlated with the changes in cavity length of the EFPI, which can be determined by analyzing the interference signals. An experiment based on probing hydrostatic pressure changes induced by the addition of single water droplets to a test chamber was performed to quantify the measurement resolution of the proposed sensor because an apparatus for producing exceedingly small, stable, and reproducible pressure changes does not exist. Compared with conventional optical fiber pressure sensors, the proposed pressure sensor requires a simple fabrication process and can be used to measure pressure changes with high sensitivity ( $\sim 23.5~\mu \text{m}$ /kPa, cavity length change/pressure change). Moreover, the sensitivity and resolution of the pressure sensor can be flexibly adjusted using Bourdon tubes designed for different dynamic ranges (e.g., 0–70 MPa, 0–10152 psi). It is believed that the proposed novel pressure sensor has the potential to find a wide range of applications that require precise instrumentation.

Microwave Device Inspired by Fiber-Optic Extrinsic Fabry-Perot Interferometer: A Novel Ultra-Sensitive Sensing Platform

The fiber-optic extrinsic Fabry-Perot interferometer (EFPI) is one of the simplest sensing configurations and is widely used in various applications. Inspired by the EFPI, we report a novel and universal ultra-sensitive microwave sensing platform based on an open-ended hollow coaxial cable resonator. Two highly-reflective microwave reflectors were fabricated in a coaxial cable to form a microwave Fabry-Perot etalon. Although the operating wavelength of the proposed device is increased by five orders of magnitude compared to the fiber-optic EFPI (e.g., from 1500 nm to 150 mm), the resolution regarding the "pseudo cavity length" of the proposed device is as high as 0.6 nanometer, which is comparable to that of the EFPI. The resolution can be further increased by high-precision machining of the device. Due to its low cost, high sensitivity, all-metal structure, robustness, and ease of signal demodulation, it is envisioned that the proposed device will revolutionize the sensing field and enable many important sensing applications that take place in harsh environments.

Quadrature Operating Point Stabilizing Technique for Fiber-Optic Fabry–Perot Sensors Using Vernier-Tuned Distributed Bragg Reflectors Laser

Drift of Quadrature operating point (Q-point) due to variations in ambient temperature restricts the demodulation accuracy of fiber-optic extrinsic Fabry-Perot interferometer (EFPI) sensors. To overcome this challenge, in this paper, we propose and demonstrate a self-stabilizing Q-point system based on Vernier-tuned distributed Bragg reflectors (VT-DBR) laser, the laser wavelength is locked to a point with the maximum slope on the interference spectrum of fiber-optic EFPI sensor. Taking advantage of large-tuning range (40nm) and fast wavelength switching capability (<; 20ns), we develop a robust EFPI acoustic sensor system with stable Q-point operation. When the EFPI sensor is subject to ambient temperature variations, we use an FPGA to implement the fast laser wavelength switching of the laser and automatic Q-point locking that ensure Q-point stability. The operating point drift from Q-point is obtained by dc voltage output changes. Experimental results indicate that stabilizing Q-point of the EFPI sensor is effectively realized during the temperature changes between 27-32°C. Without the stabilization method, the deviation is up to 85.5% of dc voltage output at operating point from Q-point value. With the stabilization method, the deviation is less than 0.68%. This self-stabilizing Q-point method based on VT-DBR laser has a strong ability to resist ambient temperature variations, and provides a novel solution to Q-point drift of fiber-optic EFPI sensors.

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.

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.

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.

A Highly Precise Demodulation Method for Fiber Fabry-Perot Cavity Through Spectrum Reconstruction

A highly precise demodulation method based on spectrum sampling and reconstruction for optical fiber extrinsic Fabry–Perot interferometer (EFPI) sensor is proposed and experimentally evaluated. A tunable laser and an optical power meter were used to sample the response of the EFPI sensor discretely. The full spectral response of the EFPI sensor is obtained through a nonlinear least squares fitting between the measured data and the theoretical model. An EFPI sensor was experimentally tested to verify the performance of the proposed demodulation method. The demodulation fluctuation range of the cavity length was within 0.84 nm, during 50 continuous measurements. The demodulation resolution of 4.59 nm was achieved for the change of cavity length around $112~\mu \text{m}$ in a strain sensing experiment. Furthermore, the maximum measurement fluctuation of the cavity length just increased from 0.84 to 1.27 nm, when the number of sampling points decreased to one-third by increasing the sampling step from 0.4 to 1.2 nm. This demodulation method may be applied to a wide range of interference spectrum demodulation applications for the high precision and simplicity.

Five-step phase-shifting white-light interferometry for the measurement of fiber optic extrinsic Fabry-Perot interferometers.

Five-step phase-shifting white-light interferometry is presented for interrogating the absolute cavity length of the fiber optic extrinsic Fabry-Perot interferometer (EFPI). It combines ideas of phase-shifting interferometry and white-light interferometry (WLI) to extend the measurement range of fiber optic WLI. Five sub-interferograms intercepted from the white-light optical spectrum are used to recover the optical path difference (OPD) of the EFPI. This method is demonstrated to interrogate a wider range of OPD. The experimental results show that the measurement resolution ranges from 0.5μm to 5μm with cavity length ranges from 16μm to 12,402μm, and it has a great advantage in measuring EFPIs with short cavity lengths.

Probing changes in tilt angle with 20 nanoradian resolution using an extrinsic Fabry-Perot interferometer-based optical fiber inclinometer.

In this paper, we introduce and demonstrate a novel optical fiber extrinsic Fabry-Perot interferometer (EFPI) for tilt measurements with 20 nrad resolution. Compared with in-line optical fiber inclinometers, an extrinsic sensing structure is used in the inclinometer reported herein. Our design greatly improves on the tilt angle resolution, the temperature stability, and the mechanical robustness of inclinometers with advanced designs. An EFPI cavity, which is formed between endfaces of a suspended rectangular mass block and a fixed optical fiber, is packaged inside a rectangular container box with an oscillation dampening mechanism. Importantly, the two reflectors of the EFPI sensor remain parallel while the cavity length of the EFPI sensor meters a change in tilt. According to the Fabry-Perot principle, the change in the cavity length can be determined, and the tilt angle of the inclinometer can be calculated. The sensor design and the measurement principle are discussed. An experiment based on measuring the tilt angle of a simply-supported 70-cm beam induced by a small load is presented to verify the resolution of our prototype inclinometer. The experimental results demonstrate significantly higher resolution (ca. 20 nrad) compared to commercial devices. The temperature cross-talk of the inclinometer was also investigated in a separate experiment and found to be 0.0041 μrad /°C. Our inclinometer was also employed for monitoring the daily periodic variations in the tilt angle of a windowsill in a cement building caused by local temperature changes during a five-day period. The multi-day study demonstrated excellent stability and practicability for the novel device. The significant inclinometer improvements in differential tilt angle resolution, temperature compensation, and mechanical robustness also provide unique opportunities for investigating spatial-temporal modulations of gravitational fields.

A Novel High-Performance Beam-Supported Membrane Structure with Enhanced Design Flexibility for Partial Discharge Detection.

A novel beam-supported membrane (BSM) structure for the fiber optic extrinsic Fabry-Perot interferometer (EFPI) sensors showing an enhanced performance and an improved resistance to the temperature change was proposed for detecting partial discharges (PDs). The fundamental frequency, sensitivity, linear range, and flatness of the BSM structure were investigated by employing the finite element simulations. Compared with the intact membrane (IM) structure commonly used by EFPI sensors, BSM structure provides extra geometrical parameters to define the fundamental frequency when the diameter of the whole membrane and its thickness is determined, resulting in an enhanced design flexibility of the sensor structure. According to the simulation results, it is noted that BSM structure not only shows a much higher sensitivity (increased by almost four times for some cases), and a wider working range of fundamental frequency to choose, but also an improved linear range, making the system development much easier. In addition, BSM structure presents a better flatness than its IM counterpart, providing an increased signal-to-noise ratio (SNR). A further improvement of performance is thought to be possible with a step-forward structural optimization. The BSM structure shows a great potential to design the EFPI sensors, as well as others for detecting the acoustic signals.

温度自补偿高灵敏度非本征光纤珐珀横向负载传感器

温度自补偿高灵敏度非本征光纤珐珀横向负载传感器

UV Adhesive Diaphragm-Based FPI Sensor for Very-Low-Frequency Acoustic Sensing

In this paper, we propose and demonstrate a fiber-optic extrinsic Fabry–Perot interferometer (FPI) sensor based on an ultraviolet (UV) adhesive diaphragm for very-low-frequency acoustic sensing. The sensing diaphragm is formed by the surface tension of UV adhesive solution and exhibits a good acoustic response from 1 Hz to 20 kHz. The sensor has a sensitivity of 57.3 mV/Pa at 1000 Hz and a flat frequency response ranging from 1 to 2000 Hz with a small fluctuation of about ±1.5 dB. The good response of infrasound and rather simple fabrication process make it a wonderful candidate for very-low-frequency acoustic sensing.

Temperature-insensitive refractive index sensor based on an optical fiber extrinsic Fabry–Perot interferometer processed by a femtosecond laser

An optical fiber extrinsic Fabry–Perot interferometer (EFPI) is designed and fabricated for refractive index (RI) sensing. To test the RI of liquid, the following two different methods are adopted: the wavelength tracking method and the Fourier-transform white-light interferometry (FTWLI). The sensitivities of sensors with cavity lengths of 288.1 and 358.5 μm are 702.312 nm/RIU and 396.362 μm/RIU, respectively, by the two methods. Our work provides a new kind of RI sensor with the advantages of high sensitivity, mechanical robustness, and low cross sensitivity to temperature. Also, we provide a new method to deal with gold film with a femtosecond laser.

Gas Refractometer Based on Optical Fiber Extrinsic Fabry—Perot Interferometer With Open Cavity

In this letter, we proposed a gas refractometer based on optical fiber Fabry–Perot interferometer with an open cavity. The Fabry–Perot cavity is a short section capillary tube spliced between a single-mode fiber and a side-hole fiber (SHF). The unique cladding of the SHF enables gas entering or leaving the Fabry–Perot cavity freely. The measurement of pressure-induced refractive index change of nitrogen gas has been demonstrated by monitoring the wavelength shift of the interference pattern. The robust and microsize architecture make the interferometer a good candidate for gas sensing applications.