Extrinsic Fabry-Perot Interferometric Fiber Research Articles

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.

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.

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.

2D sound source localization technology based on diaphragm EFPI fiber microphone array

The technologies of two-dimensional sound source localization based on diaphragm extrinsic Fabry–Perot interferometer (EFPI) fiber microphone array have been studied in this paper. The fiber microphone array consists of three EFPI. Self-calibrated intensity demodulation technology with simple structure and low cost is adopted to demodulate the sound signals. Using effective sound signal extraction technology to find the starting point and end point of sound signal, combining with time differences of arrival (TDOA) positioning technology, high-precision sound source location is successfully realized. The average positioning error of this system is no larger than 4.82%. The system has the advantages of low cost, high practicability and wide application scenarios.

Two-Dimensional Displacement Sensor Based on a Dual-Cavity Fabry-Perot Interferometer

Simultaneously and independently measuring two-dimensional (2D) displacement, one-dimensional (1D) for out-of-plane measurement and 1D for in-plane measurement, is important for practical applications. Although extrinsic fiber Fabry-Perot interferometers (EFPIs) have been reported for highly precise out-of-plane displacement measurements for many years, simultaneous measurement of 2D displacement with sub-micrometer accuracy has not been realized by a monolithic EFPI as far as we know. In this paper, a monolithic dual-cavity EFPI to realize 2D displacement measurement of a target is reported. By quantitatively analyzing and experimentally calibrating the linear dependences of the in-plane displacement sensitivity and interception on the cavity length, the effect of out-of-plane displacement on the in-plane displacement measurement is removed, which is the basis of the 2D displacement detection. 2D random movement detection and the repeatability of the system are investigated experimentally. Within the range of ±7 μm for in-plane displacement, and 352 μm-358 μm for cavity length, demodulation errors better than 96 nm are achieved. The 2D displacement measurement system based on the dual-cavity EFPI with the advantages of noncontact, high accuracy and compact size is promising to be applied in the 2D acceleration measurement, 2D rotor vibration measurement, 2D precise positioning in semiconductor manufacture, thermal expansion coefficient measurement, and so on.

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.

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.

Highly precise in-plane displacement sensor based on an asymmetric fiber Fabry-Perot interferometer.

An in-plane displacement sensor based on an asymmetric extrinsic fiber Fabry-Perot interferometer (EFPI) is proposed and demonstrated. The asymmetric EFPI composed of a step-shaped external reflector and a cleaved fiber end face can be equivalent to two parallel FPIs with slightly different cavity lengths. By calculating the peak intensity difference of the two FPIs, the in-plane displacement can be demodulated with enhanced sensitivity and suppressed common mode noise. Both theoretical analyses and experimental results show that the sensitivity and the linear range of the in-plane displacement sensor are dependent on the cavity length. A displacement resolution of 5 nm and a linear range of ±7µm under the cavity length of 250 µm are achieved in the experiment. The proposed in-plane displacement sensor with a nanometric resolution and compact size can be widely used in the fields of metrology, accelerometers, and semiconductor manufacture.

Modeling of Fabry-Perot Micro Cavities Under Partial Spatial Coherence Illumination Using Multimode Optical Fibers

Extrinsic Fiber Fabry-Perot Interferometers (EFFPIs) are used for a wide range of applications such as sensors and optical filters. They are usually built using single mode (SM) fibers as they provide better performance and sensitivity when compared to multi-mode (MM) fibers. However, SM fibers are more sensitive to misalignment and MM fiber has a much higher capacity to handle wide spectral range optical power. In this work, a model is developed and experimentally examined for MM fibers based EFFPIs. The model is based on considering the MM fiber as a partial coherent source emitting a Gaussian-Schell beam. The model is used to predict the EFFPIs response and extracted parameters such as finesse, visibility and insertion loss and to study their variation with the cavity length. It can be used for both low and high reflectivity cavities. An experimental setup is formed using two Bragg-coated MM fibers having a numerical aperture (NA) of 0.22 and a core diameter of 62.5 μm, the coating has a high reflectivity above 0.98 over a wavelength range extending from 1490 nm to 1600 nm. The model output is compared with the experimental results and a good agreement is observed.

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.

Direct Non-Invasive Measuring Techniques of Nanometric Liquid Level Variations Using Extrinsic Fiber Fabry–Perot Interferometers

This article investigates two different non-contact non-invasive solutions for measuring nanometric-order liquid-surface displacements with Extrinsic Fiber Fabry-Perot interferometers. They are investigated for developing hydrostatic leveling sensor (HLS) systems targeting the detection of very slowly-evolving movements in geophysics and geotechnics. In the first technique, the sensing beam from the interferometer traverses a liquid of known refractive index and is reflected by a mirror submerged at the bottom of the HLS vessel. The liquid-level variation is thus sensed as a variation of the optical path length of the interrogating beam. The second solution, on the other hand, directly exploits the reflection of the sensing beam at the air-liquid interface in the absence of a reflective surface in the vessel. The subsequent variation of liquid level is then measured directly as the beam's optical path variation in air. The common denominator of these two techniques is an Extrinsic Fabry-Perot sensor with nanometric precision operating at a wavelength of ~1310 nm. The interrogating beam suffers from high IR absorption in water, hence the latter solution is more advantageous in terms of dynamic range. In applications where liquids other than water can be employed, the use of low optical absorption liquids such as Polydimethylsiloxanic fluids is recommended at this operating wavelength. Being more viscous and less volatile than water, these fluids can significantly improve the noise floor of HLS systems, hence contributing to a larger dynamic range, lower instrumental drift and higher signal-to-noise ratio.

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.

Dual-Wavelength DC Compensation Technique for the Demodulation of EFPI Fiber Sensors

A dual-wavelength DC compensation technique for the demodulation of extrinsic Fabry-Perot interferometric (EFPI) fiber sensors is developed. The technique can overcome two problems in the measurement: the cavity length of the EFPI and wavelengths of light sources must be matched, and the DC component of the interferometric fringe must be eliminated. The proposed demodulation technique is verified by demodulating EFPI sensors with different cavity lengths and different phase modulations. Experimental results show that the demodulation technique can interrogate an EFPI sensor with any cavity length and any phase modulation. Besides, the technique has properties of high frequency, large dynamic range, and high sensitivity.

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.

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.

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

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.

Sourceless optical fiber high temperature sensor.

A sourceless sapphire fiber extrinsic Fabry-Perot interferometer for ultrahigh temperature sensing is developed. A sapphire wafer is mounted on the tip of a sapphire fiber as the Fabry-Perot cavity. The interference signal is generated by the thermal radiation that transmits through the wafer and is guided to a spectrometer by a sapphire and then a silica fiber. The entire sensor system is compact and low cost. The sensor was experimentally tested up to 1593°C, and a resolution around 1°C was achieved.

Multiplexed Extrinsic Fiber Fabry–Perot Interferometric Sensors: Resolution Limits

Resolution investigation of multiplexed displacement sensors based on extrinsic fiber Fabry-Perot interferometers (EFPI) was carried out. The cases of serial and parallel configurations were considered, the analysis of the problems and the advantages of the both was performed. An analytical model, describing the resolution limits for the both configurations was developed. Serial and parallel multiplexing schemes have been experimentally implemented with 3 and 4 sensing elements, respectively. For the both configurations the achieved optical path difference (OPD) standard deviations were between 30 and 80 pm, which is, to the best of our knowledge, more than an order better than any other multiplexed EFPI resolution ever reported. A good correspondence between experimental results and analytical predictions was demonstrated. A mathematical apparatus, relating the attained sensors resolutions with the number of multiplexed sensors and the optical setup parameters was developed, also demonstrating good correspondence with the experimental results. The main origins of possible sensors cross-talk are described, with the supporting experiments, proving an importance of its consideration. Some recommendations, based on the theoretical analysis and experimental studies, dedicated to optimization of sensors resolution and elimination of the cross-talk influence are provided.

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.