Fabry-Perot Interferometer Cavity Research Articles

Dual Fiber-Optic Microphone Based on Coherent Synthesis Technology

This paper proposes a dual fiber-optic microphone (DFOM) based on coherent synthesis technology, in which the sound pickup module mainly consists of two optical fibers and a reflection film, constituting two identical Fabry-Perot (FP) interferometric cavities as the acoustic sensing structure. The use of dual fiber-optic can increase the light intensity of the reflected light received by the photodetector (PD), while offsetting signal distortion caused by factors such as fiber length and temperature variations, and reducing signal interference, thereby improving the purity and accuracy of the signal. The DFOM exhibits strong fault tolerance, in case one of the optical fibers fails, the other one can still perform normal sound collection, thus enhancing the reliability of the microphone. Acoustic tests show that the sound pressure sensitivity of this DFOM at a frequency of 250 Hz is 64.8 mV/Pa. The signal-to-noise ratio (SNR) is as high as 62.45 dB under the action of a 6 kHz sound pressure signal. It maintains a linear sound pressure response and a flat frequency response in the range of 10 Hz to 20 kHz. These results indicate that the DFOM has high sensitivity and a wide frequency response range, making it suitable for acoustic detection within the audible range.

Metal-coated high-temperature strain optical fiber sensor based on cascaded air-bubble FPI-FBG structure.

Metal coatings can protect the fragile optical fiber sensors and extend their life in harsh environments. However, simultaneous high-temperature strain sensing in a metal-coated optical fiber remains relatively unexplored. In this study, a nickel-coated fiber Bragg grating (FBG) cascaded with an air bubble cavity Fabry-Perot interferometer (FPI) fiber optic sensor was developed for simultaneous high temperature and strain sensing. The sensor was successfully tested at 545 °C for 0-1000 µɛ, and the characteristic matrix was used to decouple temperature and strain. The metal layer allows easy attachment to metal surfaces that operate at high temperatures, enabling sensor-object integration. As a result, the metal-coated cascaded optical fiber sensor has the potential to be used in real-world structural health monitoring.

Reflective epoxy resin/chitosan/PAA composite-functionalized fiber-optic interferometric probe sensor for sensitive heavy metal ion detection.

A highly sensitive label-free chemical sensing platform for the detection of various metal ions is demonstrated. The chemical sensor was derived from a single-mode fiber that is inserted into the ceramic tube with epoxy resin (ER) on the end face for reflecting light and forms the Fabry-Perot (F-P) interferometric cavity. Multilayer chitosan (CS)/polyacrylic acid (PAA) were coated on the surface of the epoxy resin and act as the sensitive film. Based on the analysis of the sensing principle and the F-P cavity structure, the parameters were numerically simulated and experimentally evaluated, which enables ease of fabrication and real-time modulation of the cavity length. The sensitivity of sensing Ni2+, Zn2+, and Na+ reached 9.95 × 10-4 nm ppb-1, 2.31 × 10-4 nm ppb-1, and 4 × 10-4 nm ppb-1, respectively, and the sensing results were theoretically analyzed by the Langmuir adsorption model, which corresponds to the surface atom percentage results obtained by SEM and EDS measurements for sensing three types of metal ions. The proposed ER/CS/PAA multilayer film-coated F-P sensor can be employed as a probe, which features label-free, highly sensitivity, real-time monitoring, ease of measurement, stability, and therefore provides a remarkable analytical platform for chemical applications.

Large-scale surface-micromachined optical ultrasound transducer (SMOUT) array for photoacoustic computed tomography.

This paper reports a new 2D surface-micromachined optical ultrasound transducer (SMOUT) array consisting of 350 × 350 elements with highly uniform optical and acoustic performances. Each SMOUT element consists of a vacuum-sealed Fabry-Perot (F-P) interferometric cavity formed by two parallel partially reflective distributed Bragg reflectors (DBRs). Optical mapping in the 4 cm × 4 cm center region of the SMOUT array shows that the optical resonance wavelength (ORW) of > 94% of the elements falls within a narrow range of ≤ 10 nm. The center frequency, acoustic bandwidth and noise equivalent pressure (NEP) of the elements are determined to be 5 MHz, 5 MHz, and 20.7 Pa (with 16 times of signal averaging) or 172.5 Pa (without averaging) over a bandwidth of 10 MHz, respectively. The temperature and temporal stability of the SMOUT elements is also tested, which shows there is little variation in their ORW under large ambient temperature fluctuation and during continuous water immersion. To demonstrate its imaging capability, 2D and 3D PACT based on the SMOUT array is also conducted within a 3 cm × 3 cm field of view (FOV) at a depth of 3cm with no interrogation wavelength tuning. These results show that the SMOUT array could overcome some of the major limitations in existing ultrasound transducer arrays for PACT and provide a promising solution for achieving high-speed 3D imaging.

High Sensitive Ammonia Detection in Water With Fabry-Perot Interferometers

This letter presents the development and characterization of a Fabry-Perot interferometer (FPI) for the detection of dissolved ammonia. The FPI cavity is obtained by the alignment of two single mode fibers (SMFs), where a mixture of ultraviolet (UV) curable resin and Oxazine 170 perchlorate is positioned in the gap between both SMFs. The sensor operation principle is based on the chemical reaction between the dissolved ammonia and the Oxazine 170 perchlorate. Such reaction leads to the refractive index variation in the FPI cavity, which results in a wavelength shift of the reflected spectrum. The water absorption and temperature effects in the sensor response are characterized. Such characterizations demonstrate that a good sensor performance is achieved when it is submerged in aqueous solutions for about one hour (to enable the water absorption) and at controlled temperature conditions. Then, the FPI sensor is submerged in ammonia solutions with concentrations ranging from 0 to 700 ppb (in 100 ppb steps). Results show a reversibility of the proposed sensor and a high sensitivity of 4.3 pm/ppb (4.3 nm/ppm). Thus, the proposed sensor can be used on the detection of low concentrations of ammonia (in a sub-ppm range) with one of the highest reported sensitivities of ammonia sensors, which makes it suitable for water quality monitoring.This letter presents the development and characterization of a Fabry-Perot interferometer (FPI) for the detection of dissolved ammonia. The FPI cavity is obtained by the alignment of two single mode fibers (SMFs), where a mixture of ultraviolet (UV) curable resin and Oxazine 170 perchlorate is positioned in the gap between both SMFs. The sensor operation principle is based on the chemical reaction between the dissolved ammonia and the Oxazine 170 perchlorate. Such reaction leads to the refractive index variation in the FPI cavity, which results in a wavelength shift of the reflected spectrum. The water absorption and temperature effects in the sensor response are characterized. Such characterizations demonstrate that a good sensor performance is achieved when it is submerged in aqueous solutions for about one hour (to enable the water absorption) and at controlled temperature conditions. Then, the FPI sensor is submerged in ammonia solutions with concentrations ranging from 0 to 700 ppb (in 100 ppb steps). Results show a reversibility of the proposed sensor and a high sensitivity of 4.3 pm/ppb (4.3 nm/ppm). Thus, the proposed sensor can be used on the detection of low concentrations of ammonia (in a sub-ppm range) with one of the highest reported sensitivities of ammonia sensors, which makes it suitable for water quality monitoring.

Diamond turned micro machined metal diaphragm based Fabry Perot pressure sensor

Micro-machined Silicon (Si) diaphragm is extensively studied in Fabry Perot Interferometer (FPI) based pressure sensors, but its applications are limited to non-corrosive media due to poor corrosion resistance. Metal alloys such as Copper-Beryllium Alloy (CBA), Stainless Steel (SS) and Ni based alloys have comparable mechanical properties and much better corrosion resistance than Si. But their use as diaphragm for FPI based pressure sensor is much under reported. In this paper, design, analysis and fabrication of an FPI based pressure sensor with CBA diaphragm is discussed. Its sensing probe has two parts, a sensing head and a sleeve, welded together. The sensing head is a three-dimensional monolithic component which consists of pressure sensitive diaphragm and a step for FPI cavity. The sleeve guides the optical fiber to a fixed position such that the cleaved end of the fiber forms an FPI with the diaphragm. Both the sensing head and the sleeve are manufactured by Diamond Turning Machining (DTM) process and have CBA as material of construction. This manufacturing process ensures that flatness on interferometric side of the diaphragm and its parallelism with the fiber end are achieved within 2 µm; surface finish (Ra) on same side of the diaphragm is less than 10 nm. The sensor showed sensitivity of more than 1 µm/bar within the measurement range of 0–7 bar. DTM machined sensing probe with all essential FPI features (flatness, surface finish and parallelism) and welded diaphragm in FP pressure sensor are being reported for the first time to the best of our knowledge.

Fabry–Perot Curvature Sensor With Cavities Based on UV-Curable Resins: Design, Analysis, and Data Integration Approach

This paper presents the development of a Fabry-Perot interferometer (FPI) for curvature sensing. Ultraviolet (UV)-curable resins are used in the FPI cavity. In this case, two configurations for the FPI curvature sensor are tested, one with the UV-curable resin in between two standard single mode fibers (SMFs), namely SMF-FPI configuration and the other using a SMF and a perfluorinated polymer optical fiber (POF) with the resin between them, referred as the POF-FPI configuration. Analytical simulations were performed for both configurations in order to evaluate FPIs' wavelength shift and power variation as function of the bending angle, where such analyses are, then, confirmed by the experimental results with higher sensitivity as function of the wavelength shift for the POF-FPI and, higher sensitivity of SMF-FPI for the optical power variation. Thereafter, a data fusion for the FPI's wavelength shift and optical power variation was proposed using the Kalman filter, where the angle response after the data fusion is compared with the ones using only wavelength shift or optical power variation for both FPI configurations. The results show error decrease in all analyzed cases with up to sixfold reduction of the root mean squared error (RMSE).

An Inline Fiber Curvature Sensor Based on Eccentric Core Fiber and Off-Axis Air Cavity Fabry-Pérot Interferometer

In this paper, we present a highly sensitive inline fiber curvature sensor based on an eccentric core fiber (ECF) and off-axis air cavity Fabry-Perot interferometer (OACFPI). This ECF-based bending sensor is fabricated by fusion-splicing a section of ECF between a single-mode fiber and an off-axis air cavity fiber. Due to the location of the core in the ECF, a bend applied to the ECF-based OACFPI leads to elongation or shortening of the optical path length in the air cavity, which makes the system suitable for bending measurement. To analyze bending characteristics, three ECFs with different eccentric core locations are investigated separately. In addition, a swept source is employed for measurement. The experimental results demonstrate that, the bending sensitivities of the sensors at the two opposite most-sensitive directions are 0.241 nm/ $\text{m}^{\mathbf {-1}}$ (sensor 1), 0.131 nm/ $\text{m}^{\mathbf {-1}}$ (sensor 2), and 0.0807 nm/ $\text{m}^{\mathrm {\mathbf {-1 }}}$ (sensor 3), and −0.245 nm/ $\text{m}^{\mathbf {-1}}$ (sensor 1), −0.156 nm/ $\text{m}^{\mathbf {-1}}$ (sensor 2), and −0.077 nm/ $\text{m}^{\mathbf {-1}}$ (sensor 3). The temperature sensitivities of the sensors are 0.84 pm/°C, 0.92 pm/°C, and 0.94 pm/°C, respectively. The proposed sensors are compact and versatile, and are expected to have potential applications in biomedical sensing.

In-fiber Fabry Perot interferometer with narrow interference fringes for enhanced sensitivity in elastic wave detection

Abstract In this work, we have demonstrated using a pair of in-fiber Fabry-Perot interferometer (FPI) with matched Bragg wavelength and free spectral range (FSR) for elastic wave detection based on matched filter interrogation configuration. The cavity lengths of the FPI structures are deliberately made long to achieve small FSR to enhance their sensitivity to elastic wave pressure. It is found that the sensitivity of sensor to elastic wave pressure is linearly proportional to the FPI cavity length. This finding is in agreement with the theoretical analysis.

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.

High-sensitivity hydraulic pressure sensor based on Fabry-Perot interferometer filled with polydimethylsiloxane film.

A high-sensitivity hydraulic pressure sensor is proposed, which consists of a Fabry-Perot interferometer (FPI) filled with a polymer film of polydimethylsiloxane (PDMS). The FPI structure is fabricated by splicing a section of hollow core fiber (HCF) to the end-face of a lead-in single mode fiber (SMF). Then, the PDMS is filled into the HCF which acts as a light reflector and a diaphragm to detect external pressure variation. As a result, the length of the FPI cavity and the thickness of the PDMS are 137.8 µm and 33.8 µm, respectively. Experimental results indicate that the sensor's wavelength exhibits a linear response to the hydraulic pressure, which function is described as y = -7.35 × 10-3x + 1536.395. Here, x and y represent the hydraulic pressure and the wavelength, respectively. The pressure sensitivity is up to -7.35 nm/kPa. Besides, a temperature compensation method based on a fiber Bragg grating is proposed to eliminate the influence of temperature. Experiments show that the scheme can effectively eliminate the influence of temperature and achieve accurate measurement of hydraulic pressure.

Extrinsic Fabry-Perot interferometric cavity-based fiber-optic spectrum equalization filter for the Gaussian spectrum of superluminescent diodes.

To flatten the Gaussian spectrum of superluminescent diodes (SLDs), a fiber-optic spectrum equalization filter based on an extrinsic Fabry-Perot interferometric (EFPI) cavity is proposed. By the proper usage of a low-finesse EFPI cavity and the delicate design of a splitting and recombining fiber-optic path, the Gaussian spectrum of an SLD can be effectively flattened, which is verified by a numerical simulation and an experiment. An SLD with a center wavelength of 1568 nm and a 3-dB spectral width of 98 nm is spectrum-equalized to obtain a spectrum flatness of 0.27 dB in the wavelength range 1544.8-1605.6 nm. When the SLD is with a max power of 3.38 mW, the output power after the fiber-optic spectrum equalization filter can reach 178.06 μW.

All-Optical Demodulation Fiber Acoustic Sensor With Real-Time Controllable Sensitivity Based on Optical Vernier Effect

In the fiber sensing research area, a pair of sharp contradictions is highlighted: high sensitivity and large dynamic range. In addition, the question of how to enable user-editable features for fiber optic sensors is gaining more and more attention. This letter presents a fiber acoustic sensor with real-time controllable sensitivity based on the optical vernier effect, which is possibly a solution to such a problem. The optical vernier structure with amplified and real-time tunable detection sensitivity is formed by cascading a Sagnac interferometer and a laboratory-made tunable Fabry–Perot interferometer (FPI). Sound pressure and frequency of the acoustic signal can be all-optically demodulated simultaneously by spectral reading and spectral scanning method, respectively. Experimental results show a real-time controllable acoustic sensitivity (1×, 5×, and 10× are demonstrated) and a maximum sensitivity is as high as 37.1 nm/Pa (10× of single FPI) within relatively large sound pressure range of 62.2–92.4 dB. By adjusting the FPI cavity length in real time, sensitivity can be controlled to meet the needs of different users or occasions. The proposed sensor has merits of controllable sensitivity in real time, high sensitivity, and large dynamic range, enabling the applications related but not limited to acoustic sensing.

All-Fiber Fabry–Perot Interferometer for Liquid Refractive Index Measurement

We propose and design an all-fiber Fabry–Perot interferometer (FPI) for liquid refractive index (RI) measurements. The sensor comprises a short section of a single-mode fiber (SMF) fused between the end sections of two longer SMFs with a large intentional lateral offset. The open FPI cavity is obtained by adjusting the lateral offset. Experimental results show that the liquid to be measured can freely enter and leave the cavity without residual bubbles. The RI sensitivity of this sensor is 1,013.8 nm/RIU in the RI range from 1.3464 to 1.3777 with good linearity. Only cleaving and fusion splicing are used in its making; thus the fabrication of the sensor is relatively simple and cost effective. This sensor has potential applications in the chemical and biomedical domains.

Continuous tunable cavity Fabry-Perot interferometer by using potassium dideuterium phosphate with two ring electrodes.

In order to perform hyperspectral remote sensing, we present a continuous tunable cavity Fabry-Perot interferometer (FPI) by using potassium dideuterium phosphate (DKDP) with two ring electrodes. DKDP has the good performances of high transmittance in the ultraviolet band and large aperture of 20mm (the maximum aperture can be 100mm). Since the resonant frequency of an FPI can be continuously varied with the refractive index change of DKDP caused by the electro-optic effect, the influence of moving parts on resonant frequency can be eliminated. Digital holographic interferometry based on a Mach-Zehnder interferometer is employed to measure the refractive index modulation of DKDP. The parameters of FPI are characterized by using an experimental setup with frequency locking and temperature control technologies. Taking the temperature-measuring high-spectral-resolution lidar based on Rayleigh-Brillouin scattering as an example, a continuous tunable cavity FPI with the full width at half-maximum of 200MHz and free spectral range of 11.12GHz is realized. The results are in good agreement with the designed parameters.

Negative axicon tip-based fiber optic interferometer cavity sensor for volatile gas sensing.

In this research work we demonstrated negative axicon optical fiber tip filled with Polydimethylsiloxane (PDMS) as a sensor platform for volatile organic gases detection at room temperature. The response of the sensor was measured with various Volatile Organic Compounds (VOCs) such as Chloroform, Hexane, Isopropanol, Acetone, Toluene and Methanol in the concentration ranging from 5 to 200 ppm. The corresponding sensitivity and limit of detection (LOD) of the developed sensor for the measured VOCs were observed between the order of around 23.7 to 3.2 pm/ppm and 0.84 to 6.10 ppm, respectively. The response and recovery time of sensor were found between the order of 30 to 57 seconds and 8 to 25 seconds respectively for the measured VOCs. Thermal stability of the developed sensor was also studied at 30-70 °C with intervals of 10°C. The principle of sensing is based on change in the length of the Fabry-Perot Interferometric (FPI) cavity in the presence of varied concentrations of VOCs, which results in changes in the shift in wavelength of an interference pattern attributed to the change in PDMS filling the cavity length (swelling). The experimentally observed trends in the relative swelling of PDMS with VOCs are found in agreement with the theoretically calculated values obtained from the Hansen solubility parameter (HSP). The developed gas sensor has the potential to fulfill the demands of industrial applications.

Development of a High Spectral Resolution Lidar for day-time measurements of aerosol extinction

Lidar technique is the most performing way to obtain the atmospheric vertical profile of aerosol optical properties with high space-time resolution. With elastic scattering lidars, the retrieval of aerosol optical properties (as the extinction profile) is realizable only with assumptions on aerosol extinction-to-backscatter ratio or with Raman measurement achievable in night-time. In order to overcome these problems, the High Spectral Resolution Lidar (HSRL) technique has been examined. In this paper we present an innovative prototype of High Spectral Resolution Lidar realized at Physics Department of University “Federico II” of Naples for the LISA (LIdar for Space study of the Atmosphere) project in the framework of the China-Italy international cooperation between CNISM and BRIT. The prototype which represents a first step of a spaceborne HSRL, is based on a laser source at 1064nm and 532nm with high spectral resolution ability at 532nm. The separation between the molecular and the aerosol components was obtained through the use of a confocal Fabry-Perot interferometer (CFPI) cavity.

A IR-Femtosecond Laser Hybrid Sensor to Measure the Thermal Expansion and Thermo-Optical Coefficient of Silica-Based FBG at High Temperatures.

In this paper, a hybrid sensor was fabricated using a IR-femtosecond laser to measure the thermal expansion and thermo-optical coefficient of silica-based fiber Bragg gratings (FBGs). The hybrid sensor was composed of an inline fiber Fabry-Perot interferometer (FFPI) cavity and a type-II FBG. Experiment results showed that the type-II FBG had three high reflectivity resonances in the wavelength ranging from 1100 to 1600 nm, showing the peaks in 1.1, 1.3 and 1.5 μm, respectively. The thermal expansion and thermo-optical coefficient (1.3 μm, 1.5 μm) of silica-based FBG, under temperatures ranging from 30 to 1100 °C, had been simultaneously calculated by measuring the wavelength of the type-II FBG and FFPI cavity length.

Temperature-insensitive fiber optic Fabry-Perot interferometer based on special air cavity for transverse load and strain measurements.

We experimentally demonstrate transverse load and strain sensing based on a fiber optic Fabry-Perot interferometer (FPI) with special air cavity, which was created by fusion splicing single mode fiber (SMF), hollow core fiber (HCF) and several electrical arc discharges. The cavity height of this structure is higher than the cladding diameter of SMF so that it can sense transverse load with high sensitivity. The transverse load sensitivity of this air cavity FPI sensor is 1.31 nm∕N and about 5 times more sensitive compared to the current fiber tip interferometer (0.2526 nm∕N). Meanwhile, this sensor also can measure strain and the strain sensitivity of 3.29 pm∕με is achieved. In addition, the low temperature sensitivity (1.08 pm/°C) of the sensor can reduce the temperature-induced measurement error. This novel air cavity FPI can be developed and used as high-sensitivity transverse load and strain sensor with temperature-insensitive.

Temperature insensitive one-dimensional bending vector sensor based on eccentric-core fiber and air cavity Fabry-Perot interferometer

A temperature insensitive directional bending sensor based on an eccentric-core fiber (ECF) cascaded with an air-cavity Fabry–Perot (F-P) interferometer is presented and demonstrated. The ECF-based air cavity F-P interferometer is fabricated by fuse splicing a piece of hollow-core fiber (HCF) in between an ECF and a multi-mode fiber (MMF). The bending sensitivities of the sensor at the two opposite most sensitive directions are 79.5 pm/m−1 and −81.5 pm/m−1, respectively. The temperature sensitivity of the proposed structure is as low as 1 pm °C−1.