Fiber-optic Fabry-Perot Cavity Research Articles

All-SiC fiber-optic sensor for pressure and temperature dual-mode sensing in harsh environments

This paper presents a fiber-optic Fabry–Perot (FP) sensor with an all-silicon carbide (SiC) structure for pressure and temperature dual-mode sensing in harsh environments. The all-SiC sensing diaphragm and vacuum FP cavity were formed by direct bonding of a 6 H-SiC (N-doped) diaphragm with a 6 H-SiC (V-doped) substrate. The variation in length of the FP cavity was used to measure the external pressure, while the principle of temperature sensing was due to the temperature dependence of the refractive index and thermal expansion of the SiC substrate. The pressure and temperature characteristics of the sensor were calibrated from 25 °C up to 700 °C over a pressure range of 0–1.0 MPa. The pressure sensor achieved high sensitivity (174.3 nm/MPa) and linearity (R2 = 0.999 at 700 °C), the spectral sensitivity of the temperature sensing being approximately 32.12 pm/°C. Moreover, the temperature-pressure cross-sensitivity was as little as 4.98 °C/MPa. The dual-mode sensing capability of the all-SiC fiber-optic sensor demonstrated great promise for applications in extremely high-temperature environments.

Compact optical fiber photoacoustic gas sensor with integrated multi-pass cell

Optical fiber acoustic sensors with miniature size and high sensitivity are attractive to develop compact photoacoustic spectroscopy. Here, a compact photoacoustic gas sensor was demonstrated by utilizing a diaphragm-based fiber-optic Fabry-Perot cavity as both the acoustic sensor and the multipass cell. A nanoscale graphite film was used as the flexible diaphragm to increase the acoustic sensitivity of the Fabry-Perot cavity and the cavity inner surface was coated with highly-reflective Au film to form a multipass cell for amplification of the photoacoustic signal. With a laser power of 20 mW at 1532.8 nm, the sensor demonstrated a low detection limit of ∼ 50 ppb for C2H2 gas with an integration time of ∼ 100 s. The optical fiber photoacoustic gas sensor with a millimeter-scale diameter and ppb-level detection limit is promising for trace gas sensing in various areas including industrial process and environmental monitoring.

Integrated and Robust Fabry–Perot Humidity Sensor Based on Metal–Organic Framework onto Fiber-Optic Facet

An integrated and robust fiber-optic Fabry-Perot (FP) humidity sensor based on the Metal-Organic Framework (MOF) material FIR-53 is presented and characterized for the first time, along with its implementation method. Noticeably, the FIR-53 crystal was chosen as the moisture-sensitive material, due to its large porosity and chemical tailorability. By connecting the single-mode fiber (SMF) with FIR-53 using Polydimethylsiloxane (PDMS) rationally, our sensor, whose FP cavity was formed by FIR-53, monitors the spectrum peak shift caused by humidity change. The experiments revealed that the proposed sensor had a sensitivity of 315 pm/ %RH in the range of 10% to 90% RH, while also the good linearity, reversibility, long-time stability, and anti-high temperature interference resistance. To summarize, the proposed sensor has the advantages of easy fabrication, low cost, and is highly sensitive to humidity, which is going to have significant potential in the field of humidity sensing and promote the research and application of new fiber-optic sensors for humidity.

Label-free and selective cholesterol detection based on multilayer functional structure coated fiber fabry-perot interferometer probe

A reflective fiber-optic Fabry-Perot cavity probe sensor is proposed to selectively measure cholesterol concentration by insert single mode fiber into ceramic tube and immobilize epoxy resin (ER)/graphene oxide (GO)/beta-cyclodextrin (β-CD) multi-layer film onto end face of ceramic tube. EDC/NHS activated GO is selected to form chemical binding with β-CD, and β-CD is the sensitive materials to bind with cholesterol molecules. With multi-layer film assisted, the sensitivity of sensor to cholesterol concentration can reach 3.92 nm/mM and the limit of detection reaches 3.48 μ M. In addition, 4 mM hemoglobin, glucose and ascorbic acid are doped into a set cholesterol sample and verified the highly selectivity of sensing cholesterol. Furthermore, the reproducibility was proved by measure the spectrum of four sensors with same fabrication process, and the reusability was also proved by repeated measurements. Overall, the sensor features with high mechanical strength, ease of fabrication, real-time monitoring, low cost and ease for measurement that given by probe structure. Therefore, the sensor provides a remarkable analytical platform for biosensing applications.

Compact fiber Fabry–Perot sensors filled with PNIPAM hydrogel for highly sensitive relative humidity measurement

A fiber-optic Fabry–Perot (FP) based relative humidity (RH) sensor was proposed and experimentally investigated by using Poly(N-isopropylacrylamide) (PNIPAM) hydrogel to connect two ends of single-mode optical fiber (SMF) to formulate a PNIPAM FP cavity. The water vapor will be absorbed by the PNIPAM hydrogel, which will alter both the refractive index (RI) and dimension of the PNIPAM FP cavity, resulting in the change of output resonance intensity in the reflective spectrum. The measure RH sensitivity is 1.634 nm/%RH within RH range from 45 to 75 % with good linearity of 0.9897 and excellent repeatability. The influence of PNIPAM hydrogel concentration on the RH sensitivity has been investigated - the higher the concentration of PNIPAM hydrogel, the higher the RH sensitivity of the sensor. The developed sensor can effectively monitor human respiratory with response and recovery time of 2.29 s and 1.90 s, respectively.

High-speed, large dynamic range spectral domain interrogation of fiber-optic Fabry-Perot interferometric sensors.

We report high-speed, large dynamic range spectral domain interrogation of fiber-optic Fabry-Perot (FP) interferometric sensors. An optical interrogation system employing a piezoelectric FP tunable filter and an array of fiber-Bragg gratings for wavelength referencing is developed to acquire the reflection spectrum of FP sensors at a high interrogation speed with a wide wavelength range. A 98 nm wavelength interrogation range was obtained at the resonance frequency of ∼110kHz of the FP tunable filter. At this frequency, the resolution of the FP cavity length measurement was 1.8 nm. To examine the performance of the proposed high-speed spectral domain interrogation scheme, two diaphragm-based fiber-tip FP sensors (a pressure sensor and acoustic sensor) were interrogated. The pressure measurement results show that the high-speed spectral domain interrogation method has the advantages of being robust to light intensity fluctuations and having a much larger dynamic range compared with the conventional intensity-based interrogation method. Moreover, owing to its capability of measuring the absolute FP cavity length, the proposed interrogation system mitigates the sensitivity drift that intensity-based interrogation often suffers from. The acoustic measurement results demonstrate that the high-speed spectral domain interrogation method is capable of high-frequency acoustic measurements of up to 20 kHz. This work will benefit many applications that require high-speed interrogation of fiber-optic FP interferometric sensors.

Compact fiber-optic Fabry-Perot cavity based on sandwich structure adopting direct bonding of quartz glass.

A compact fiber-optic Fabry-Perot (F-P) cavity for a sensor is designed based on a sandwich structure, adopting direct bonding of quartz glass. The reflective F-P cavity is manufactured by a fiber optic with a quartz glass ferrule and the sandwich structure with an air cavity, which is achieved by direct bonding of quartz glass. This fabrication process includes plasma surface activation, hydrophilic pre-bonding, high-temperature annealing, and dicing. The cross section of the bonding interface tested by a scanning electron microscope indicates that the sandwich structure is well bonded, and the air cavity is not deformed. Experiments show that the quality factor of the F-P cavity is 2711. Tensile strength testing shows that the bond strength exceeds 35 MPa. The advantage of direct bonding of quartz glass is that high consistency and mass production of the cavity can be realized. Moreover, the cavity is free of problems caused by the mismatch of thermal expansion coefficients between different materials. Therefore, the F-P cavity can be made into a sensor, which is promising in detecting air pressure, acoustic and high temperature.

All-SiC Fiber-Optic Sensor Based on Direct Wafer Bonding for High Temperature Pressure Sensing

This paper presents an all-SiC fiber-optic Fabry-Perot (FP) pressure sensor based on the hydrophilic direct bonding technology for the applications in the harsh environment. The operating principle, fabrication, interface characteristics, and pressure response test of the proposed all-SiC pressure sensor are discussed. The FP cavity is formed by hermetically direct bonding of two-layer SiC wafers, including a thinned SiC diaphragm and a SiC wafer with an etched cavity. White light interference is used for the detection and demodulation of the sensor pressure signals. Experimental results demonstrate the sensing capabilities for the pressure range up to 800 kPa. The all-SiC structure without any intermediate layer can avoid the sensor failure caused by the thermal expansion coefficient mismatch and therefore has a great potential for pressure measurement in high temperature environments.

A tunable fiber-optic Fabry–Perot cavity formed between a silica microsphere and a target

In this paper, a tunable fiber-optic Fabry–Perot (FP) cavity is proposed and demonstrated experimentally. The microsphere formed by melting the end of a single mode fiber(SMF) is used as one of the reflective surfaces of the FP cavity. Compared with the interference in FP cavities formed by a pair of parallel interfaces, the FP interference generated between a convex surface and a perpendicular plane can result in a higher fringe contrast when the length of the cavity is much larger. Utilizing this characteristic, a large-scale displacement response up to 3.3 mm can be achieved with the highest sensitivity of 37 pm/μm. This method has the advantages of low cost, a simple fabrication process and high tolerance to the reflectivity of the measured object.

Fiber-optic Fabry-Perot pressure sensor based on sapphire direct bonding for high-temperature applications.

In this study, a fiber-optic Fabry-Perot (FP) high-temperature pressure sensor based on sapphire direct bonding is proposed and experimentally demonstrated. The sensor is fabricated by direct bonding of two-layer sapphire wafers, including a pressure diaphragm wafer and a cavity-etched wafer. The sensor is composed of a sensor head that contains a vacuum-sealed cavity arranged as an FP cavity and a multimode optical fiber. The external pressure can be measured by detecting the change in FP cavity length in the sensor. Experimental results demonstrate the sensing capabilities for pressures from 20kPa to 700kPa up to 800°C.

Diaphragm-Free Fiber-Optic Fabry-Perot Interferometric Gas Pressure Sensor for High Temperature Application.

A diaphragm-free fiber-optic Fabry-Perot (FP) interferometric gas pressure sensor is designed and experimentally verified in this paper. The FP cavity was fabricated by inserting a well-cut fiber Bragg grating (FBG) and hollow silica tube (HST) from both sides into a silica casing. The FP cavity length between the ends of the SMF and HST changes with the gas density. Using temperature decoupling method to improve the accuracy of the pressure sensor in high temperature environments. An experimental system for measuring the pressure under different temperatures was established to verify the performance of the sensor. The pressure sensitivity of the FP gas pressure sensor is 4.28 nm/MPa with a high linear pressure response over the range of 0.1–0.7 MPa, and the temperature sensitivity is 14.8 pm/°C under the range of 20–800 °C. The sensor has less than 1.5% non-linearity at different temperatures by using temperature decoupling method. The simple fabrication and low-cost will help sensor to maintain the excellent features required by pressure measurement in high temperature applications.

Microbubble-based fiber-optic Fabry-Perot pressure sensor for high-temperature application.

Using arc discharge technology, we fabricated a fiber-optic Fabry-Perot (FP) pressure sensor with a very low temperature coefficient based on a microbubble that can be applied in a high-temperature environment. The thin-walled microbubble can be fabricated by heating the gas-pressurized hollow silica tube (HST) using a commercial fusion splicer. Then, the well-cut single-mode fiber (SMF) was inserted into the microbubble, and they were fused together. Thus, the FP cavity can be formed between the end of the SMF and the inner surface of the microbubble. The diameter of the microbubble can be up to 360μm with the thickness of the wall being approximately 0.5μm. Experimental results show that such a sensor has a linear sensitivity of approximately -6.382 nm/MPa, -5.912 nm/MPa at 20°C, and 600°C within the pressure range of 1MPa. Due to the thermal expansion coefficient of the SMF being slightly larger than that of silica, we can fuse the SMF and the HST with different lengths; thus, the sensor has a very low temperature coefficient of approximately 0.17pm/°C.

An arc tangent function demodulation method of fiber-optic Fabry-Perot high-temperature pressure sensor

A new demodulation algorithm of the fiber-optic Fabry-Perot cavity length based on the phase generated carrier (PGC) is proposed in this paper, which can be applied in the high-temperature pressure sensor. This new algorithm based on arc tangent function outputs two orthogonal signals by utilizing an optical system, which is designed based on the field-programmable gate array (FPGA) to overcome the range limit of the original PGC arc tangent function demodulation algorithm. The simulation and analysis are also carried on. According to the analysis of demodulation speed and precision, the simulation of different numbers of sampling points, and measurement results of the pressure sensor, the arc tangent function demodulation method has good demodulation results: 1 MHz processing speed of single data and less than 1% error showing practical feasibility in the fiber-optic Fabry-Perot cavity length demodulation of the Fabry-Perot high-temperature pressure sensor.

Intrinsic Fabry-Pérot Interferometer Based on Concave Well on Fiber End

miniature fiber-optic Fabry-Perot cavity interferometerA miniature fiber-optic Fabry-Perot cavity interferometer is presented. The interferometer is fabricated at the fiber tip by wet chemical etching using hydrofluoric acid. A concave well is created on the fiber end face. Interference happens between the reflections from the fiber core and the end of the cladding. We demonstrate applications of the interferometric sensor for temperature and gas refractive index sensing.