Intrinsic Fabry-Perot Interferometer Research Articles

Erbium-doped fiber laser based on FBGs-FP fabricated by femtosecond laser for temperature and strain sensing

In this paper, an erbium-doped fiber laser (EDFL) system based on intrinsic Fabry-Perot interferometer (IFPI) structure is proposed for simultaneous measurement of temperature and strain. IFPI with a central wavelength of 1546.2 nm, an interference fringe interval of 1.8 nm is composed of two FBGs inscribed by femtosecond laser and used as the frequency selective device in fiber laser. The signal-to-noise ratio (SNR) and 3-dB linewidth of 1546.2 nm fiber laser are 27.735 dB and 0.21 nm, respectively. For 1546.2 nm-laser stability, the maximum fluctuation of laser peak wavelength and output power is 0.08 nm and 0.655 dBm, respectively. Experimental results show that the proposed EDFL sensing system exhibits a temperature sensitivity of 9.16 pm/℃, the linearity R2 is 0.99965 within the temperature range of 100 °C-800 °C and a strain sensitivity of 7.45 pm/με, the linearity R2 is 0.99985 within the strain range of 0 με–1050 με at 800 °C. The stability of both maximum temperature and maximum strain was monitored within 30 min, and the maximum fluctuation of laser peak wavelength and output power was 0.12 nm and 0.372 dBm, 0.05 nm and 0.667 dBm, respectively.

Ultrasonic Vibration Sensing of Intrinsic Fabry-Perot Interferometer Sensor Array Based on Local Spectral Analysis

This paper presents a low-cost integrated intrinsic Fabry-Perot interferometer (IFPI) sensor array demodulation algorithm based on local spectral analysis, which exploits multiple local spectra instead of the entire spectrum to achieve ultrasonic vibration measurement well beyond the sweeping period of the tunable laser source. By balancing the demodulation range and the demodulation resolution of IFPIs, this paper divides the entire spectrum produced by a distributed feedback (DFB) laser into 152 local spectra by a sliding window for IFPIs demodulation respectively, thereby implementing up to 160 kHz vibration measurement. To reduce the load on the data acquisition system and simplifies the interrogation system, this paper adopts a nonlinear tuning error compensation method based on an in-line IFPI sensor. In addition, chirp Z-transform is used in this paper to replace the traditional zero-padding-based fast Fourier transform to achieve the local fineness of demodulated optical path difference (OPD) spectrum, so as to reduce the waste of computation caused by zero-padding.

CO2Laser Tapering of Intrinsic Fabry–Perot Interferometers for Sensing

We present a highly reproducible method of fabricating a tapered intrinsic Fabry–Perot interferometer (IFPI) device with 5–6 <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> diameter at the taper waist. A femtosecond laser was applied to inscribe an IFPI with 3-cm cavity length in a single-mode fiber. A CO2 laser-heated tapering process enabled by digitally controlled mirrors and a precision motorized fiber feed system was used to create a stable heating zone with the desired temperature profile for tapering the fiber IFPI cavity. The well-engineered tapering process produced tapered IFPI devices with insertion loss less than 0.3 dB at 1550 nm. A strong evanescent field exposed by the taper Section was explored for refractive index (RI) sensing. Using swept optical frequency domination reflectometry (OFDR), the tapered IFPI fiber sensor achieved a minimal RI sensing 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">$2\times 10^{-{5}}$ </tex-math></inline-formula> . This article demonstrates an integrated laser fabrication technique to produce tapered fiber optic devices for sensing applications.

A High-Temperature Multipoint Hydrogen Sensor Using an Intrinsic Fabry–Perot Interferometer in Optical Fiber

This paper presents a multiplexable fiber optic chemical sensor with the capability of monitoring hydrogen gas concentration at high temperatures up to 750 °C. The Pd-nanoparticle infused TiO2 films coated on intrinsic Fabry–Perot interferometer (IFPI) array were used as sensory films. Strains induced upon exposure to hydrogen with varied concentrations can be monitored by IFPI sensors. The fiber sensor shows a repetitive and reversible response when exposed to a low level (1–6%) of hydrogen gas. Uniform sensory behavior across all the sensing cavities is demonstrated and reported in this paper.

High-Density Weak in-Fiber Micro-Cavity Array for Distributed High-Temperature Sensing With Millimeter Spatial Resolution

High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼−55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000 °C, showing a temperature sensitivity of −2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.

Super-resolution demodulation for fiber sensor arrays based on the MUSIC algorithm.

This paper studies the use of MUltiple SIgnal Classification (MUSIC) as a super-resolution algorithm to improve demodulation results for intrinsic Fabry-Perot interferometer (IFPI) sensor arrays. Through distinction between noise and signal subspaces in an observation matrix, this paper shows that a 38-fold improvement in the full width at half maximum (FWHM) estimation of IFPI optical path differences (OPD) can be achieved using this algorithm. Based on this improved method, this paper demonstrates that a tunable laser with a 1.3-nm tuning range can achieve the same sensor demodulation performance as a tunable laser with a 50-nm tuning range if a conventional Fourier transform-based algorithm is used. This paper presents a new approach to analyzing optical signals produced by multiple multiplexed interferometers with similar OPDs with potential applications for both single-mode and multiple-mode devices.

Distributed Acoustic Sensing Based on Coherent Microwave Photonics Interferometry.

A microwave photonics method has been developed for measuring distributed acoustic signals. This method uses microwave-modulated low coherence light as a probe to interrogate distributed in-fiber interferometers, which are used to measure acoustic-induced strain. By sweeping the microwave frequency at a constant rate, the acoustic signals are encoded into the complex microwave spectrum. The microwave spectrum is transformed into the joint time–frequency domain and further processed to obtain the distributed acoustic signals. The method is first evaluated using an intrinsic Fabry Perot interferometer (IFPI). Acoustic signals of frequency up to 15.6 kHz were detected. The method was further demonstrated using an array of in-fiber weak reflectors and an external Michelson interferometer. Two piezoceramic cylinders (PCCs) driven at frequencies of 1700 Hz and 3430 Hz were used as acoustic sources. The experiment results show that the sensing system can locate multiple acoustic sources. The system resolves 20 nε when the spatial resolution is 5 cm. The recovered acoustic signals match the excitation signals in frequency, amplitude, and phase, indicating an excellent potential for distributed acoustic sensing (DAS).

Optical Fiber Fabry–Perot Interferometer Based Spirometer: Design and Performance Evaluation

Spirometry enables the diagnosis and monitoring of multiple respiratory diseases, such as asthma and chronic obstructive pulmonary disease (COPD). In this paper, we present an optical fiber-based device to evaluate the pulmonary capacity of individuals through spirometry. The proposed system consists of an optical fiber containing an intrinsic Fabry–Perot interferometer (FPI) micro-cavity attached to a 3D printed structure that converts the air flow into strain variations to the optical fiber, modulating the FPI spectral response. Besides providing the value of the flow, its direction is also determined, which enables a differentiation between inhale and exhale cycles of breathing. A simulation study was conducted to predict the system behavior with the air flow. The preliminary tests, performed with the FPI-based spirometer led to average values of forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC) parameters of 4.40 L and 6.46 L, respectively, with an FEV1/FVC index (used as an airway function index) of 68.5%. An average value of 5.35 L/s was found for the peak expiratory flow (PEF). A comparison between the spirometry tests using the presented FPI system and a commercial electronic device showed that the proposed system is suitable to act as a reliable spirometer.

A Composite Fabry-Perot Interferometric Sensor with the Dual-Cavity Structure for Simultaneous Measurement of High Temperature and Strain.

In this paper, an optical fiber composite Fabry-Perot interferometric (CFPI) sensor capable of simultaneous measurement of high temperature and strain is presented. The CFPI sensor consists of a silica-cavity intrinsic Fabry-Perot interferometer (IFPI) cascading an air-cavity extrinsic Fabry-Perot interferometer (EFPI). The IFPI is constructed at the end of the transmission single-mode fiber (SMF) by splicing a short piece of photonic crystal fiber (PCF) to SMF and then the IFPI is inserted into a quartz capillary with a reflective surface to form a single-ended sliding EFPI. In such a configuration, the IFPI is only sensitive to temperature and the EFPI is sensitive to strain, which allows the achieving of temperature-compensated strain measurement. The experimental results show that the proposed sensor has good high-temperature resistance up to 1000 °C. Strain measurement under high temperatures is demonstrated for high-temperature suitability and stable strain response. Featuring intrinsic safety, compact structure and small size, the proposed CFPI sensor may find important applications in the high-temperature harsh environment.

Flexible gauge length intrinsic fiber-optic strain sensor using broadband interferometry [Invited

Monitoring strain is important in precision engineering applications that require maintaining the precise alignments of structures over time, such as those found in machine tools and metrology frames. We present a fiber-optic strain measurement technique based upon broadband interferometry that is variously configurable in terms of gauge length and sensitivity. This is achieved by the use of an unbalanced interrogation interferometer configuration that alleviates the cavity length limit imposed by the temporal coherence of the system. We also demonstrate that dispersion in fiber sensors based on intrinsic Fabry-Perot interferometers must be considered to optimize performance. The possibility of multisensor interrogation using the same optical system is also reported.

Phase Demodulation by Frequency Chirping in Coherence Microwave Photonic Interferometry

This article presents a signal processing method to demodulate the optical interference phase of cascaded individual optical fiber intrinsic Fabry-Perot interferometric (IFPI) sensors in a coherent microwave-photonic interferometry (CMPI) distributed sensing system. The new method utilizes the chirp effect of electro-optic modulator (EOM) to create a quasi-quadrature optical interference phase shift between two adjacent pulses which correspond to two adjacent reflection points in the time domain. The phase shift can be controlled by adjusting the bias voltage that is applied to the EOM. The interference phase is calculated by elliptically fitting the phase shift. The interference phase change is proportional to the optical path difference (OPD) change of the interferometer, and the sign can be used to differentiate the increase or decrease of the OPD. The method is demonstrated for distributed strain sensing, showing good linearity, high resolution and large dynamic range.

Multiplexable high-temperature stable and low-loss intrinsic Fabry-Perot in-fiber sensors through nanograting engineering.

This paper presents a method of using femtosecond laser inscribed nanograting as low-loss- and high-temperature-stable in-fiber reflectors. By introducing a pair of nanograting inside the core of a single-mode optical fiber, an intrinsic Fabry-Perot interferometer can be created for high-temperature sensing applications. The morphology of the nanograting inscribed in fiber cores was engineered by tuning the fabrication conditions to achieve a high fringe visibility of 0.49 and low insertion loss of 0.002 dB per sensor. Using a white light interferometry demodulation algorithm, we demonstrate the temperature sensitivity, cross-talk, and spatial multiplexability of sensor arrays. Both the sensor performance and stability were studied from room temperature to 1000°C with cyclic heating and cooling. Our results demonstrate a femtosecond direct laser writing technique capable of producing highly multiplexable in-fiber intrinsic Fabry-Perot interferometer sensor devices with high fringe contrast, high sensitivity, and low-loss for application in harsh environmental conditions.

Multiplexable intrinsic Fabry-Perot interferometric fiber sensors for multipoint hydrogen gas monitoring.

This Letter presents an approach to produce multiplexable optical fiber chemical sensor using an intrinsic Fabry-Perot interferometer (IFPI) array via the femtosecond laser direct writing technique. Using the hydrogen-sensitive palladium (Pd) alloy as a functional sensory material, Pd alloy coated IFPI devices can reproducibly and reversibly measure hydrogen concentrations with a detection limit of 0.25% at room temperature. Seven IFPI sensors were fabricated in one fiber and performed simultaneous temperature and hydrogen measurements at seven different locations. This Letter demonstrates a simple yet effective approach to fabricate multiplexable fiber optical chemical sensors for use in harsh environments.

Laser-assisted embedding of all-glass optical fiber sensors into bulk ceramics for high-temperature applications

We develop a laser-assisted sensor embedding process to embed all-glass optical fiber sensors into bulk ceramics for high-temperature applications. A specially designed two-step microchannel was fabricated on an Al2O3 substrate for sensor embedment using a picosecond (ps) laser. An optical fiber Intrinsic Fabry-Perot Interferometer (IFPI) sensor was embedded at the bottom of the microchannel and covered by Al2O3 slurry which was subsequently sintered by a CO2 laser. The sensor spectrum was in-situ monitored during the laser sintering process to ensure the survival of the sensor and optimize the laser sintering parameters. By testing in furnace through high temperature, the embedded optical fiber shows improved stability after CO2 laser sealing, resulting in the linear temperature response of the embedded optical fiber IFPI sensor. To improve the embedded IFPI sensor for thermal strain measurement, a dummy fiber was co-embedded with the sensing fiber to improve the mechanical bonding between the sensing fiber and the ceramic substrate so that the thermal strain of the ceramic substrate can apply on the sensing fiber. The response sensitivity, measurement repeatability and high-temperature long-term stability of the embedded optical fiber IFPI sensor were evaluated in this work.

Information integrated glass module fabricated by integrated additive and subtractive manufacturing.

In this Letter, we report a novel integrated additive and subtractive manufacturing (IASM) method to fabricate an information integrated glass module. After a certain number of glass layers are 3D printed and sintered by direct ${{\rm CO}_2}$CO2 laser irradiation, a microchannel will be fabricated on top of the printed glass by integrated picosecond laser, for intrinsic Fabry-Perot interferometer (IFPI) optical fiber sensor embedment. Then, the glass 3D printing process continues for the realization of bonding between optical fiber and printed glass. Temperature sensing up to 1000°C was demonstrated using the fabricated information integrated module. In addition, the long-term stability of the glass module at 1000°C was conducted. Enhanced sensor structure robustness and harsh temperature sensing capability make this glass module attractive for harsh environment structural health monitoring.

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.

Highly sensitive fiber optic temperature and strain sensor based on an intrinsic Fabry-Perot interferometer fabricated by a femtosecond laser.

In this Letter, we report on a simple, but highly sensitive sensor based on two intrinsic Fabry-Perot interferometers (FPIs) inscribed in a standard single-mode optical fiber. A brief theoretical study on the Vernier effect is presented, in which a simulation of the sensitivity magnification factor dependence on the FPI's length is performed. Based on the simulation results, the FPIs were fabricated using a custom micromachining setup that integrates a near-infrared femtosecond laser and a motorized XYZ platform. Using the Vernier effect, sensitivities of 145pm/με and 927pm/°C were obtained for strain and temperature, respectively. The sensor's performance combined with its versatile and customizable configuration allows real-time and in situ strain and temperature monitoring under harsh environments.

Sapphire derived fiber based Fabry-Perot interferometer with an etched micro air cavity for strain measurement at high temperatures.

A sapphire derived fiber (SDF) based Fabry-Perot interferometer (FPI) with an etched micro air cavity for strain measurement at high temperatures is proposed. The FPI is formed by splicing a section of SDF between an etched single mode fiber (ESMF) and a capillary. The SDF's core containing 51.3mol.% aluminum provides the intrinsic Fabry-Perot interferometer cavity with an enhanced fringe contrast through the narrow etched air cavity reflector. Because the different Poisson effects of the cladding and the core have different deformations under axial stress, the transverse strain imposed from the cladding to the core was introduced to the additive model. The strain sensitivity of the FPI was theoretically analyzed and experimentally demonstrated at room temperature. A thermal annealing process was performed to study the stability in high temperatures and to release the residual stress during the sensor's fabrication. The strain calibration was carried out subsequently from 20℃ to 1000℃. Benefiting from the doping in the core and diffusion in the cladding of the high temperature resistant material Al2O3, the proposed sensor was proved to operate well in 950℃ and was also characteristized by a sensitivity of 1.19 pm/µɛ and 1.06 pm/µɛ in the process of loading and unloading strain separately.

Recent developments in novel silica-based optical fibers

We have summarized our recent work in the area of novel silica-based optical fibers, which can be classified into two types: silica optical fiber doped with special elements including Bi, Al, and Ce, and micro-structured multi-core fibers. For element-doped optical fiber, the Bi/Al co-doped silica fibers could exhibit a fluorescence spectrum covering the wavelength range between 1000 and 1400 nm with a full width at half maximum (FWHM) of about 150 nm, which enables its use in fiber amplifiers and laser systems. The Ce-doped fiber’s center wavelengths of excitation and emission are about 340 and 430 nm, respectively. The sapphire-derived fiber (SDF) with high alumina dopant concentration in the core can form mullite through heating and cooling with arc-discharge treatment. This SDF can be further developed for an intrinsic Fabry-Perot interferometric that can withstand 1200 °C, which allows it to be used in high-temperature sensing applications. Owing to the strong evanescent field, micro-structured multi-core fiber can be used in a wide range of applications in biological fiber optic sensing, chemical measurement, and interference-related devices. Coaxial-core optical fiber is another novel kind of silica-based optical fiber that has two coaxial waveguide cores and can be used for optical trapping and micro-particle manipulation by generating a highly focused conical optical field. The recent developments of these novel fibers are discussed.

Simultaneous Measurements of Temperature and Pressure With a Dual-Cavity Fabry–Perot Sensor

A fiber-optic dual-cavity Fabry-Perot interferometric sensor for the simultaneous measurement of temperature and pressure is proposed and experimentally demonstrated. The sensor cascades a fiber-optic intrinsic Fabry-Perot interferometer (IFPI) and an extrinsic Fabry-Perot interferometer (EFPI). The IFPI is used as the temperature sensor and the EFPI is used as the pressure sensor. The pressure sensor is based on a diaphragm-free structure and can work stably at high temperature. Two optical path differences are interrogated simultaneously by using the fast Fourier transform-based white-light interferometry and the peak-to-peak method. Experimental results show that the proposed sensor exhibits a temperature sensitivity of 29.63 nm/°C within the temperature range of 40 °C-1100 °C and a pressure sensitivity of 1465.8 nm/MPa within the pressure range of 0-10 MPa at the room temperature.