Extrinsic Fabry-Perot Interferometer Research Articles

Real-Time Air Gap and Thickness Measurement of Continuous Caster Mold Flux by Extrinsic Fabry–Perot Interferometer

Real-Time Air Gap and Thickness Measurement of Continuous Caster Mold Flux by Extrinsic Fabry–Perot Interferometer

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.

Optofluidic Fabry-Perot interferometric sensor for the real-time measurement of refractive index

In this work, a microfluidic system combined with a fibre-optic extrinsic Fabry-Perot interferometer is proposed to measure refractive index continuously and in real time. A microfluidic platform was designed and created for this purpose through 3D printing. The Fabry-Perot cavity is an integral part of the microfluidic chip and is perpendicular to the sample flow. The light is conducted through a single mode optical fibre and the refractive index measurements were based on the optical power and wavelength shift of the reflected spectra. The developed optofluidic setup was characterised using different concentrations of glucose solutions. A sensitivity of 1102 nm/RIU was obtained when using the wavelength shift, however, when the same solution was analysed over time, the signal for wavelength shift measurements was found to be unstable. The optical power shift was correlated with the refractive index and a sensitivity of -79.6 dB/RIU was obtained, with a good linearity (r2 = 0.996). Good results were verified in terms of stability with a maximum standard deviation of 0.028 dB and a sensor resolution of 4.3×10−4 RIU. This sensor has a great potential for applications in which refractive index real-time measurements are required, such as food and beverages industry process control.

Strain optical fiber sensor with modified sensitivity based on the vernier effect

An optical fiber strain sensor with adjustable sensitivity is proposed and demonstrated. The strain sensing setup employs a fiber optic Michelson interferometer (sensing element) and an extrinsic Fabry–Perot interferometer, in which both structures excited the Vernier effect. By adjusting the cavity length of the extrinsic Fabry–Perot interferometer, it is possible to control the M-factor. The set-up offers both sensitivity enhancement and magnification mode, where the maximal sensitivity is 4.38 pm/με (M-factor = 2.3). This design improves the sensor performance and its implementation is not intricate.

Magnet-assisted hybrid EFPI/FBG sensor for internal corrosion monitoring of steel pipelines

Internal corrosion reduces the pipeline thickness and thus is one of the most predominant causes for pipeline malfunction. In this work, a highly sensitive, magnet-assisted hybrid sensor of fiber Bragg grating (FBG) and extrinsic Fabry-Perot interferometer (EFPI) is proposed, and designed for simultaneous measurement of temperature and pipeline thickness loss. The proposed sensor consists of a cylindrical magnet supported with springs, a reflection mirror fixed on the pipe for EFPI setup and a FBG for temperature measurement. The magnetic force between the magnet and the steel pipe is a function of the pipe wall thickness, which can be transferred to the cavity length (distance between the magnet and the reflection mirror) of the EFPI. The cavity length decreases with steel pipe corrosion due to the increase of the magnetic force. The proposed sensor has the sensitivity of 3.5 µm in thickness measurement and can be installed directly on the existed pipeline without interrupting its operation status.

Ultrasensitive Cryogenic Temperature Sensor Based on a Metalized Optical Fiber Fabry-Perot Interferometer

Cryogenic technologies are becoming essential in future applications in aerospace, quantum computing, cryogenic computing, and superconducting research. A highly sensitive temperature sensor is critical for such cryogenic applications. In this paper, we propose an extrinsic Fabry–Perot interferometer (EFPI) for cryogenic temperature measurement, leveraging the high thermal deformation of the copper sleeve to overcome the sensitivity drop issue in temperatures below 50 K for conventional cryogenic temperature sensors. Experimental results exhibit a temperature sensitivity of 2.836 nm/K throughout a temperature range of 5 K-50 K with a 15.82 μm long EFPI cavity, which is over three orders higher than the recently reported metal-coated FBG. The high sensitivity holds for the entire temperature range from 5 K to 295 K with excellent sensing repeatability. Our device, featuring a compact structure, and capability for multiplexing, is with great promise in cryogenic applications.

A Fiber-Optic Accelerometer Based on Extrinsic Fabry-Perot Interference for Low Frequency Micro-Vibration Measurement

A fiber-optic accelerometer based on extrinsic Fabry-Perot interferometer (EFPI) is reported. The sensor adopts flywheel diaphragm structure as a sensitive element, where the mass block is bonded in the center, while the beam of the flywheel acts as a spring. The sensitive element forms single degree of freedom spring-mass-damper system. The parameters of the accelerometer are designed based on finite element simulation (FEM), and the transfer function of the sensor is established by numerical simulation. The experiments indicate that the sensor has a wide detection frequency band with the detectable ability for micro-vibration low as 0.01 Hz, and the sensitivity is 7.02 rad/g @20 Hz, and reach an average sensitivity of 6.39 rad/g in 5–20 Hz. The research shows that the accelerometer has a good development prospect in the field of low frequency micro-vibration detection.

An all-sapphire fiber temperature sensor for high-temperature measurement

An all-sapphire high-temperature optical fiber sensor with an extrinsic Fabry–Perot interferometer (EFPI) structure is proposed and experimentally demonstrated. The EFPI structure of the sensor is composed of a sapphire ferrule with a sapphire fiber and a polished solid sapphire crystal rod. The first reflection occurs on the left end of the sapphire ferrule, and the second reflection occurs on the right end of the sapphire rod, forming double-beam interference. The interference signal is picked up by the sapphire fiber and transmitted to the white light interferometric demodulator by the multimode fiber. The picked-up signal is demodulated by the Fourier transform and interference algorithms. Experimental results show that the temperature response of the sensor is quadratic, and that the sensitivity changes linearly with a temperature range from room temperature to 1500 °C. The first-order thermal expansion coefficient and the second-order thermal expansion coefficient are 5.4575 × 10−6 °C−1 and 7.3755 × 10−9 °C−2, respectively. The thermal expansion coefficient is linearly related to temperature. Due to its all sapphire structures, this sensor solves the problem of thermal expansion coefficient mismatch, which was observed in a previous sensor made of sapphire and ceramic materials. In addition, it improves the high-temperature resistance, so it can be fully qualified for long-term high-temperature measurements above 1000 °C–1500 °C.

High Precision Optical Path Difference Compensation Method Based on Three- Parameter Cosine Fitting Method

For low fineness extrinsic Fabry-Perot inter-ferometer (EFPI) sensors, the discrete gap transform (DGT) algorithm can be usually adopted to acquire the cavity length, and the magnitude of its demodulation error is often larger than several hundred picometer. In this paper, three-parameter cosine fitting (TPCF) method is proposed to compensate for the EFPI cavity length error caused by traditional spectrum truncation, which can improve the demodulation accuracy by 3800 times of that of DGT. This method has a large cavity length demodulation range of 20 μm ∼30000 μm with a maximum error of 65 pm, which was proved by numerical simulation. The experiment shows that when the cavity length is 31 μm and 282.3 μm, the demodulation resolutions with a confidence interval of 68.27% are 15.142 pm and 27.582 pm, respectively, and its actual demodulation time is no more than 0.021 s. Compared with the traditional cavity length demodulation algorithms, the proposed algorithm has broader demodulation range, higher demodulation accuracy and resolution. Moreover, it will be unrestricted by wavelength shift for more than one period and can realize real-time fast demodulation.

Hybrid optical fiber Fabry-Perot interferometer for nano-displacement sensing

Nano-displacement sensing based on an extrinsic hybrid fiber Fabry-Perot interferometer is proposed and demonstrated. The lead-in fiber tip of such an interferometer consists of a strongly-coupled multicore fiber section fusion spliced to a single-mode fiber. The referred lead-in fiber tip is placed in front of a microscope slide, whose rear surface is coated with a high reflecting layer. The gap between the end-face of the fiber tip and the layer is composed of an air cavity in series with a glass one. Light exiting from the lead-in fiber tip is partially reflected at the fiber-air and air-glass interfaces and the reflecting layer generating three beams that are recoupled into the multicore fiber and combined with supermode interference. By making the optical path length of the air cavity slightly different from the glass one, it is possible to generate an envelope in the interference spectra with a larger period. Thus, by tracking the shift of such an envelope, displacements of 0.47 nm can be resolved. The nano-displacement sensing approach reported here is easy to implement; moreover, the sensitivity, resolution, and dynamic range can be reconfigured by an appropriate selection of the glass cavity.

Asymmetric Reflection Spectrum of Fabry-Perot Interferometer and the Application in Pressure Sensing

The performance of a fiber optic Fabry-Perot (F-P) sensor largely depends on the spectral characteristics of F-P interference, nevertheless there is little research on the spectral characteristic, such as the asymmetry shapes which are often observed in practical experiments. Here, a simple analytical formula for simulating the reflected spectrum of a F-P resonator have been provided, based on which the asymmetry spectral shape can be diversified through adjusting the refined parameters in the analytical formula. In addition, the composite film of Pt/Ge with different thickness on fiber tip has been theoretically demonstrated to change the parameters, and the F-P interferometer composed by composite film of Pt/Ge on fiber tip and gold film has been simulated to exhibit variated asymmetric reflection spectra. Meanwhile, the reflection spectrum from fiber F-P interferometer based on the composite film deposited on the end face of optical fiber has been experimentally proven to change according to the thickness variation of Ge film. Finally, a fiber based miniature extrinsic Fabry-Perot interferometer (EFPI) showing a saw tooth wave shape spectrum has been fabricated to sense static pressure through a dual-wavelength based intensity demodulation scheme, which possesses the potential to overcome the offset between sensitive and dynamic range for a traditional sensor.

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.

Four-wavelength quadrature phase demodulation technique for extrinsic Fabry-Perot interferometric sensors.

In this Letter, we report a four-wavelength quadrature phase demodulation technique for extrinsic Fabry-Perot interferometric (EFPI) sensors and dynamic signals. Four interferometric signals are obtained from four different laser wavelengths. A wavelength interval of four wavelengths is chosen according to the free spectrum range (FSR) of EFPI sensors to generate two groups of anti-phase signals and two groups of orthogonal signals. The linear fitting (LF) method is applied to two groups of anti-phase signals to eliminate the dc component and ac amplitude to obtain two normalized orthogonal signals. The differential cross multiplication (DCM) method is then used to demodulate the phase signal from these two normalized orthogonal signals. The proposed LF and DCM (LF-DCM) based four-wavelength quadrature phase demodulation overcomes the drawback of the traditional ellipse fitting (EF) and DCM (EF-DCM) based dual-wavelength demodulation method that it is not suitable for weak signal demodulation since the ellipse degenerates into a straight line, which makes the EF algorithm invalid. Moreover, it also avoids the assumption that the dc component and ac amplitude of interferometric signals are identical, which is widely used in three-wavelength demodulation. An EFPI acoustic sensor is tested to prove the four-wavelength quadrature phase demodulation and experimental results show that the proposed phase demodulation method shows advantages of large dynamic range and wide frequency band. Linearity is as high as 0.9999 and a high signal-to-noise ratio (SNR) is observed from 1 Hz to 100 kHz.

High sensitivity optical fiber magnetic field sensor based on semi fixed extrinsic Fabry-Perot interferometer

A semi fixed extrinsic magnetic field sensor (MFS) with high sensitivity based on giant magnetostrictive material (GMM) and Fabry-Perot (F-P) was proposed here. The single-mode fiber (SMF) was only fixed at both ends of the GMM rod, and the connection between the SMF and quartz capillary was canceled, which significantly improved the conversion efficiency from the variation of GMM rod to the variation of cavity length. Consequently, the sensitivity of the MFS was greatly improved. Experiment results indicate that the sensor designed can produce good interference spectrum and is very sensitive to the change of magnetic field. Through measuring the magnetic field intensity, the MFS with high sensitivity of 1.867 nm/Gs to the magnetic field was obtained. The corresponding magnetic field resolution reaches 0.01 Gs, and the sensor has excellent linearity. In addition, the problem of temperature cross sensitivity was solved by series Fiber Bragg Grating (FBG). The sensor proposed is compact in structure and simple in fabrication.

Sensitivity enhanced fiber optic hydrophone based on an extrinsic Fabry-Perot interferometer for low-frequency underwater acoustic sensing.

A miniaturized fiber optic hydrophone (FOH) based on a composite metal diaphragm with an air back cavity and a high finesse extrinsic Fabry-Perot interferometric (EFPI) scheme for low-frequency underwater acoustic sensing is proposed and experimentally demonstrated in this paper. A composite metal diaphragm is used to improve the stability of the hydrophone. A balance channel is used to equilibrate the hydrostatic pressure and maintain an air cavity, which improves the mechanical sensitivity. In addition, a white light interferometry (WLI) phase demodulation is used to demodulate the high finesse interferometer consisted of the fiber collimator end face and the diaphragm, which improves the phase sensitivity. Experimental results show that the enhanced phase sensitivity of the hydrophone is about -122.5 dB re 1 rad/µPa @ 200 Hz and the sensitivity fluctuation is below 2.5 dB between 3 Hz and 400 Hz, while the minimal detectable pressure (MDP) is 63.7 µPa/Hz1/2 @ 400 Hz. Due to its miniaturized structure and high sensitivity, the FOH may have an enormous potential in underwater target detection.

An all-fiber diaphragm-based extrinsic Fabry–Perot sensor for the measurement of pressure at ultra-low temperature

An all-fiber diaphragm-based extrinsic Fabry–Perot interferometer (EFPI) sensor for the measurement of pressure at ultra-low temperature is proposed and experimentally demonstrated. The sensor head is manufactured at the end of a single mode fiber with a micro hole and a fiber diaphragm. By femtosecond (fs) laser micromachining, sensors with different sensitivities can be fabricated with different diameter of the micro hole and different thickness of the diaphragm. The deformation of the diaphragm has a linear relationship with the applied pressure. This EFPI is used to measure the pressure at temperature of −196 °C by monitoring the cavity length of the interferometer. Experiment results show the sensor exhibits good linearity within a pressure range from 0 to 7 MPa, and the pressure sensitivities at −196 °C during the process of pressure increasing and decreasing are 111.17 and 111.22 nm MPa−1, respectively. The proposed all-fiber pressure sensor can find applications in ultra-low temperature environment.

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.

Measurement of Sapphire Wafer Thermo-Optic Coefficient Using High-Temperature Optical Fiber Sensors

The thermo-optic coefficient (TOC) of sapphire wafers at temperatures ranging from 25 &#x00B0;C to 1500 &#x00B0;C is measured by an extrinsic Fabry-Perot interferometer capable of operating at high temperatures. Sapphire wafers with two thicknesses are compared in the experiment. The TOC of the sapphire wafer is retrieved based on the measured temperature response of the optical path length of the Fabry-Perot cavity and the known thermal expansion coefficient of sapphire wafer. Experimental results demonstrate that the TOC of sapphire wafer at 1550 nm wavelength has a quadratic dependence on temperature up to 1500 &#x00B0;C. The first and second-order TOCs of the sapphire wafer are calculated as <inline-formula> <tex-math notation="LaTeX">$8.163\times 10$ </tex-math></inline-formula>^&#x2212;6 and <inline-formula> <tex-math notation="LaTeX">$1.1075\times 10$ </tex-math></inline-formula>^&#x2212;8, respectively. The temperature response and sensitivity of the sapphire-wafer-based high-temperature sensors with the same structure can be inferred using the obtained sapphire TOC. The reported approach and results significantly simplify the design and calibration procedures of the high-temperature sensors made of sapphire wafers.

Optical Interferometric Force Sensor Based on a Buckled Beam

This paper reports a novel extrinsic Fabry-Perot interferometer (EFPI)-based fiber optic sensor for force measurement. The prototype force sensor consists of two EFPIs mounted on a stainless steel rectangular frame. The primary sensing element, i.e., the first EFPI, is formed between the endface of a horizontally-placed optical fiber and a stainless steel buckled beam. The second EFPI, fashioned between a longitudinally-placed optical fiber and a silver-coated glass beam, is arranged to demonstrate the amplification mechanism of the buckled beam structure. When the sensor is subjected to a tension force, the pre-buckled beam will deflect backward, resulting in a longitudinal/axial displacement of the pre-buckled beam. The axial displacement is further transferred and amplified to a horizontal/vertical deflection at the middle of the buckled beam, leading to a relatively-significant change in the Fabry-Perot cavity length. A force sensitivity of 796 nm/N (change in cavity length/Newton) is achieved with a low-temperature dependence of 0.005 N/°C. The stability of the sensor is also investigated with a standard deviation of ± 5 nm, corresponding to a measurement resolution of ±0.0064 N. A simulation is conducted to study the axial displacement and stress distribution of the sensor when it is subjected to a tension load of 250 N. It is demonstrated that the maximum stress of the sensor is tremendously reduced attributed to the buckled design, enabling a long service life cycle of the force sensor. The robust and simple-to-manufacture force sensor has great potential in structural health monitoring, robotics control, and oil/ gas refining systems.

Sapphire optical fiber high-temperature vibration sensor.

A sapphire fiber high-temperature vibration sensor with an extrinsic Fabry-Perot interferometer (EFPI) structure is proposed and experimentally demonstrated. The vibrating diaphragm of the sensor is a supported beam structure fabricated by etching a single-side polished sapphire wafer using a femtosecond laser. The FP cavity of the sensor is composed of the sapphire fiber end face and the polished surface of the vibrating diaphragm. The interference signal of the sensor is picked up by the sapphire fiber and transmitted to a laser interferometry demodulator through a multimode fiber. Experimental results show that the acceleration response is linear in the range of 0-10 g along with an acceleration sensitivity of 20.91 nm/g. The resonance frequency of the sensor is 2700 Hz, which is consistent with the ANSYS simulation results. The sensor can also work in the temperature range from room temperature to 1500 ℃, providing a feasible method for vibration measurements in high-temperature environments.