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• PDMS is introduced to fiber-optic Fabry-Perot interferometer for sensitization. • The sensor has a wide range of temperature measurement from −30 °C to 85 °C and has high temperature sensitivity. • The sensor's specifications are tested to prove its excellent performance. • The sensor is simple to make and has the potential for mass production. In this paper, a highly sensitive temperature sensor based on polymer-coated Fabry-Perot interferometer (FPI) is proposed and demonstrated. The FPI temperature sensor consists of the end face of single-mode fiber (SMF) and the Polydimethylsiloxane (PDMS) cap on the end face. By varying the number of coatings, the thickness of the PDMS cap can be flexibly controlled to obtain FPI sensors with different cavity lengths. The temperature sensitivity is studied experimentally from −30 °C to 85 °C, and it is found that the sensor shows different sensitivity in different temperature intervals, up to 0.698 nm/°C. Compared to similarly constructed sensors, the sensor in this paper has a large temperature range while maintaining a high sensitivity. In addition, the performance of the sensor is tested by experiments and the results show that the sensor possesses the advantages of good stability, fast response time and good repeatability. [ABSTRACT FROM AUTHOR]

A rigorous analysis of pulse propagation in planar photonic crystal (PhC) coupled cavity waveguides (CCWs) of finite length is reported. Conventional PhC waveguides, formed by a single line defect, are used at both interfaces of the CCW. An adiabatic taper based on progressively varying the radii of the spacing defects between cavities is used to achieve flat transmission bands with respect to the butt coupling case. The influence on the main parameters of the propagated pulse such as group delay, full width at half maximum and pulse attenuation are investigated for both the adiabatic and butt coupling cases. Furthermore, the Fabry-Perot formula has been used for modeling the pulse propagation along the CCW of finite length, which permits to analyze a large range of parameters avoiding the huge computation time requirements of finite-difference time-domain simulations. [ABSTRACT FROM AUTHOR]

We present two methods based on the analysis of Fabry-Pérot interference for a detailed characterization of a 90° corner in a two-dimensional photonic crystal waveguide fabricated in a thin Si membrane. These methods are a means of identifying the critical waveguide elements in the process of improving photonic crystal devices. The effects of the elements forming the photonic crystal waveguide are identified and quantified by means of a stage-by-stage analysis. By Fourier transforming the transmission spectra we observe the amount of light that is back reflected inside the waveguide and based on the fringe contrast of the Fabry-Pérot modulation we calculate the loss contribution of each waveguide element, such as the tapers and the 90° corner. [ABSTRACT FROM AUTHOR]

We present a Fabry–Pérot resonator technique for room temperature optical loss measurements on mid-infrared (λ∼2–4 μm) lasers. The quality of optical waveguides for λ≈2.3 μm InGaAsSb/AlGaAsSb/GaSb interband lasers and a λ≈3.7 μm strain-compensated InGaAs/InAlAs/InP quantum cascade laser have been estimated using this method. The optical losses for these lasers lie in the range 15–25 cm-1 for interband lasers and 4–5 cm-1 (transverse electric polarization) and 21–23 cm-1 [transverse magnetic (TM) polarization] for the quantum cascade laser. The considerably higher losses for TM polarization in the case of quantum cascade laser are explained by intersubband absorption in the active layers. The method may be applied to structures with only a minimum amount of device processing, facilitating rapid progress in development of mid infrared laser designs in new materials systems. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have developed a technique for photothermal displacement spectroscopy that is potentially orders of magnitude more sensitive than conventional methods. We use a single Fabry-Perot resonator to enhance both the intensity of the pump beam and the sensitivity of the probe beam. The result is an enhancement of the response of the instrument by a factor proportional to the square of the finesse of the cavity over conventional interferometric measurements. In this paper we present a description of the technique, and we discuss how the properties of thin films can be deduced from the photothermal response. As an example of the technique, we report a measurement of the thermal properties of a multilayer dielectric mirror similar to those used in interferometric gravitational wave detectors. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Optical interferometric sensors have acquired significant importance in metrology and information technology, especially in terms of their potential application in launching size, selectivity, sensitivity, resolution, spectral tuning ranges, efficiency, and cost. However, these demands are often contradictory and counteract one another, and are thus difficult to simultaneously fulfill during their interaction. This review focuses on a detailed comparison of seven different strongly miniaturized sensor concepts investigating the limits of these demands. For the visible and near-infrared spectral range, seven optical sensors were reviewed based on the following methodologies: classical optical transmission and reflection gratings, arrayed waveguide gratings, static Fabry–Pérot (FP) filter arrays, MEMS tunable FP interferometers, MEMS tunable photonic crystals, plasmonic filters, and fiber tip sensors. The comparison between the selected concepts concentrates on (i) the minimum space required for a particular spectral range, (ii) resolution, (iii) the integration in optical fiber technology, (iv) tunability to save space, (v) efficiency in using available light, (vi) multiplexing, (vii) miniaturization limits, and (viii) the potential of nanoimprint for cost reduction. Technologies for enhancing efficiency to obtain more available light and their applicability to the different methodologies were studied.

In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand.

A continuous-wave Nd:KGd(WO4)2 single-longitudinal-mode laser is demonstrated with Fabry–Perot etalons in a simple linear cavity. The thermal lens effect is dramatically lowered by propagating the laser beam along the 'athermal' direction inside the laser crystal, which is very beneficial to removing the heat generated in the mode selection process. The maximum single-longitudinal-mode output power obtained is 64.8 mW at incident pump power of 4.7 W, corresponding to an optical conversion efficiency of 1.3% and a slope efficiency of 1.7%. [ABSTRACT FROM AUTHOR]

Presents information on a study which analyzed the neodymium:titanium:lithium niobate Fabry-Perot waveguide laser model using Fourier transform beam propagation method. Research design and methodology; Results and discussion; Conclusions.

Investigates the dispersion relations of n for zinc (Zn) magnesium (Mg) selenium (Se) and Zn-cadmium-Se semiconductor alloys below their respective band gaps. Measurements using prism coupler and the reflectivity method; Analysis of Fabry-Perot oscillations of the reflectivity spectra; Conclusion.

It is generally assumed that horizontal wind velocities are independent of height above the F 1 region (> 300 km) due to the large molecular viscosity of the upper thermosphere. This assumption is used to compare two completely different methods of thermospheric neutral wind observation, using two distinct locations in the high-latitude Northern Hemisphere. The measurements are from ground-based Fabry–Perot interferometers (FPI) and from in situ accelerometer measurements onboard the challenging mini-satellite payload (CHAMP) satellite, which was in a near-polar orbit. The University College London (UCL) Kiruna Esrange Optical Platform Site (KEOPS) FPI is located in the vicinity of the auroral oval at the ESRANGE site near Kiruna, Sweden (67.8 ∘ N, 20.4 ∘ E). The UCL Longyearbyen FPI is a polar cap site, located at the Kjell Henriksen Observatory on Svalbard (78.1 ∘ N, 16.0 ∘ E). The comparison is carried out in a statistical sense, comparing a longer time series obtained during night-time hours in the winter months (DOY 300–65) with overflights of the CHAMP satellite between 2001 and 2007 over the observational sites, within ±2 ∘ latitude (±230 km horizontal range). The FPI is assumed to measure the line-of-sight winds at a height of ∼240 km, i.e. the peak emission height of the atomic oxygen 630.0 nm emission. The cross-track winds are derived from state-of-the-art precision accelerometer measurements at altitudes between ∼450 km (in 2001) and ∼350 km (in 2007), i.e. 100–200 km above the FPI wind observations. We show that CHAMP wind values at high latitudes are typically 1.5 to 2 times larger than FPI winds. In addition to testing the consistency of the different measurement approaches, the study aims to clarify the effects of viscosity on the height dependence of thermospheric winds. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

An optical pressure sensor working on the principle of Fabry–Perot Interferometer (FPI) is designed for pressures range of 1 bar absolute. The sensor is based on MEMS fabrication technology involving micromachined silicon diaphragm and anodic bonding with glass wafer. The fabrication results in trapped gas in the sealed reference cavity. The pressure of gas is likely to change with the movement of diaphragm in response to applied pressure. To find the deflection of diaphragm in presence of 'deflection-dependent reference pressure', a finite element (FE) based model of sensor is developed. The model includes structural as well as fluidic parts for diaphragm and the trapped gas respectively. The reference pressure as well as diaphragm deflection are evaluated at various applied pressures. The FE simulations are carried out for sensor models of various cavity lengths. It is established that the span of the sensor gets reduced (suppressed) for small cavity lengths. Two types of sensors are fabricated and tested. Close agreement is found between theoretically predicted and experimentally observed values. [ABSTRACT FROM AUTHOR]

• Demodulation method of Fabry-Perot sensor by cascading a traditional Mach-Zehnder interferometer is proposed. • The temperature sensitivity of the cascaded sensor is −107.2 pm/°C, while the sensitivity of the single FPI is 1.2 pm/°C. • The sensitivity is greatly improved 89 times by employing the Vernier-effect. • The proposed method is suitable for remote sensing. A novel demodulation method of Fabry-Perot sensor by cascading a traditional Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated, which enhances the temperature sensitivity greatly by the Vernier-effect. The Fabry-Perot interferometer (FPI) fabricated by a segment of hollow-core fiber (HCF) is used for sensing. The MZI is used as demodulation device that can enhance the sensitivity. The two interferometers have very similar free spectral range (FSR), which results in the Vernier-effect. Experimental results show that, the temperature sensitivity of the cascaded sensor is −107.2 pm/°C, while the sensitivity of the FPI is 1.2 pm/°C. The sensitivity amplification factor is about 89. In addition, the FPI used for sensing and the MZI used for demodulating can be separated for a long distance, so the proposed method is suitable for remote sensing. [ABSTRACT FROM AUTHOR]

A fiber-optic carbon monoxide (CO)-sensing system with high sensitivity was proposed by combining Fabry-Perot interferometer (FPI) sensor with sensing material Au/ZnO. The sensitivity is 814 pm/% within the CO concentration range of 0% to 4%. The FPI sensor was fabricated by inserting single-mode fiber into a partially polymer-filled glass capillary to form an air cavity; the length of the air cavity is 76.83 μm. The temperature sensitivity of the FPI is -2.7 nm/°C, which provides better conditions for measurement of CO. The catalyst Au/ZnO was prepared by coprecipitation, which could catalyze the oxidation of CO at ambient temperature and humidity. X-ray diffraction and scanning electron microscope analysis were employed to reveal the crystalline structure and morphology of the catalyst. To eliminate the influence of ambient temperature change on the accuracy of the concentration test, a fiber Bragg grating with central wavelength of 1550.24 nm was introduced in the experimental setup for the temperature compensation. [ABSTRACT FROM AUTHOR]

A cost effective folded Fabry-Perot (F-P) interferometer is developed using a He-Ne laser source tuned by acousto-optic modulators (AOM). The two AOMs are employed to expand the frequency tuning range of the tracking He-Ne laser. The configuration and diffraction efficiency of each AOM is fully investigated to maintain balanced efficiency along the whole frequency tuning range. The optical phase modulated Pound-Drever- Hall frequency locking technique is employed in this system to minimize the locking error of the F-P cavity resonance. The displacement of the target corner cube reflector is converted to the tracking frequency change and measured by beating with an iodine-stabilized laser. A self-developed xPC target system is employed to accomplish the frequency locking, data acquisition, and air refractive index compensation in the overall system. A displacement comparison experiment between the F-P interferometer and a commercial interferometer shows the residual error is within ±4 nm in the range of 500 nm. [ABSTRACT FROM AUTHOR]

Using Fabry‐Perot interferometers at five midlatitude stations (Boulder, Palmer, Millstone Hill, Mount John, and Kelan) in both hemispheres, we examine the interhemispheric and seasonal variations of midlatitude thermospheric dynamics. We also use the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) to simulate the seasonal changes of winds and the effects from Sub‐Auroral Polarization Streams. The observations and TIEGCM simulations show a clear seasonal variation with more westward and equatorward summer winds. The TIEGCM runs overestimate the westward zonal winds and underestimate the electron densities in the northern summer. We believe that the underestimated TIEGCM electron density leads to a weak ion drag effect in the model, and strong westward zonal winds. TIEGCM overestimates the Sub‐Auroral Polarization Stream effects on neutral winds in most cases, probably because the empirical Sub‐Auroral Polarization Stream model used by the TIEGCM applies an unrealistic persistent electric field for a long period of time (over 3 hr) due to the low temporal resolution of the Kp index. Key Points: Summer zonal (meridional) winds are more westward (equatorward). The seasonal changes in the zonal winds are largerThe TIEGCM simulated zonal winds are more westward in the northern midlatitudes during the June solstice than the measured windsThe SAPS‐enabled TIEGCM overestimates the SAPS effects in most cases due to unrealistic electric field [ABSTRACT FROM AUTHOR]

We propose an asymmetrical Fabry‐Perot interferometer (AFPI) based on fiber Bragg gating (FBG) with low reflectivity and a segment of single‐mode fiber (SMF). The AFPI occurs in the narrow bandwidth of FBG. The theoretical analysis of measurement for refractive index (RI) and temperature is explained in detail, and experimental results are consistent with the theory. As the temperature and RI of environment change, the peak wavelength and the extinction ratio of the interferometer show different trends, and the measurement of RI and temperature simultaneously can be achieved. The fiber sensor has the RI sensitivity of 33.55 dB/RIU in the range from 1.333 to 1.413 and temperature sensitivity is 9 pm/°C in the range from 10°C to 70°C. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Active spectroscopy plasma diagnostics (CXRS, charge eXchange recombination spectroscopy) and plans for its use in the ITER tokamak are described. A technique for the wavelength calibration of a multichannel polychromator spectrometer using a Fabry–Perot etalon is presented. This idea of the wavelength calibration of the spectrometer was originally used for the CXRS diagnostics in the JET tokamak. Using this technique, a high-resolution tri-band polychromator spectrometer, which was created for the CXRS diagnostics at the ITER facility, was calibrated. This polychromator spectrometer allows simultaneous measurements in three spectral ranges: 469–477, 529–538, and 650–661 nm. The obtained error of the calibration for various wavelengths was evaluated, which was on average 0.003 nm. The resulting calibration accuracy for different wavelengths is enough to meet the requirements for the CXRS diagnostics in the ITER tokamak with a margin. [ABSTRACT FROM AUTHOR]

DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO) is a future Japanese space gravitational-wave antenna. The most important objective of DECIGO, among various sciences to be aimed at, is to detect gravitational waves coming from the inflation of the universe. DECIGO consists of four clusters of spacecraft, and each cluster consists of three spacecraft with three Fabry–Perot Michelson interferometers. As a pathfinder mission of DECIGO, B-DECIGO will be launched, hopefully in the 2020s, to demonstrate technologies necessary for DECIGO as well as to lead to fruitful multimessenger astronomy. B-DECIGO is a small-scale or simpler version of DECIGO with the sensitivity slightly worse than that of DECIGO, yet good enough to provide frequent detection of gravitational waves. [ABSTRACT FROM AUTHOR]

The ability to sense force in surgery is in high demand in many applications such as force feedback in surgical robots and remote palpation (e.g., tumor detection in endoscopic surgery). In addition, recording and analyzing surgical data is of substantial value in terms of evidence-based medicine. However, force sensing in surgery remains challenging because of the specific requirements of surgical instruments, namely, they must be small, bio-compatible, sterilizable, and tolerant to noise. In this study, we propose a modular optic force sensor using a Fabry–Perot interferometer that can be used on surgical devices. The the proposed sensor can be implemented like a strain gauge, which is widely used in industrial applications but not compatible with surgery. The proposed sensor includes two key elements, a fiber-optic pressure sensor using a Fabry–Perot interferometer that was previously developed by one of the authors and a structure that includes a carbide pin that contacts the pressure sensor along the long axis. These two elements are fixed in a guide channel fabricated in a 3 × 2 × 0.5 mm sensor housing. The experimental results are promising, revealing a linear relationship between the output and the applied load while showing a linear temperature characteristic that suggests temperature compensation will be needed in use. [ABSTRACT FROM AUTHOR]

Wavelength interrogation has been widely used for fiber-optic sensors based on Fabry‐Perot interferometers (FPIs) because of its high measurement resolution. However, phase ambiguity often limits the measurement range to be less than one free-spectral range (FSR) of the FPI. Here, we investigate a simple but effective method that can eliminate the phase ambiguity of a silicon FPI temperature sensor using a fiber Bragg grating (FBG) fabricated on the lead-in fiber close to the silicon FPI. The reflection spectrum of the sensor shows sinusoidal fringes from the FPI and a single peak from the FBG. Assuming the FBG and the silicon FPI have similar temperatures, the FBG peak can be used merely as a marker to identify an FPI fringe peak without ambiguity even when it shifts by more than an FSR. Although the temperature resolution of the FBG is lower than that of the silicon FPI, the measurement range of the silicon FPI can be greatly improved without sacrificing the measurement resolution. Using a temperature sensor based on a 75-μm-thick silicon FPI, we demonstrated temperature measurement over a range of −60 to 140 °C (corresponding to a spectral shift of about five FSRs) with a resolution of 0.0033 °C. The measurement range with unambiguous peak recognition is expected to be larger than demonstrated in the experiment and only limited by the spectral bandwidth of the sensor system and the capability of materials used for the sensor construction. [ABSTRACT FROM AUTHOR]

Context. Herbig–Haro (HH) flows exhibit a large variety of morphological and kinematical structures such as bow shocks, Mach disks, and deflection shocks. Both proper motion (PM) and radial velocity investigations are essential to understand the physical nature of such structures. Aims. We investigate the kinematics and PM of spectrally separated structures in the FS Tau B HH flow. Collating these data makes it possible to understand the origin of these structures and to explain the unusual behavior of the jet. Besides, the study of emission profiles in the associated reflection nebulae allows us to consider the source of the outflow both from edge-on and pole-on points of view. Methods. We present the data obtained with the 6 m telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences using the SCORPIO multimode focal reducer with a scanning Fabry–Perot interferometer. Two epochs of the observations of the FS Tau B region in Hα emission (2001 and 2012) allowed us to measure the PM of the spectrally separated inner structures of the jet. Results. In addition to already known emission structures in the FS Tau B system, we discover new features in the extended part of the jet and in the counter-jet. Moreover, we reveal a new HH knot in the HH 276 independent outflow system and point out its presumable source. In the terminal working surface of the jet, structures with different radial velocities have PMs of the same value. This result can be interpreted as the direct observation of bow-shock and Mach disk regions. A bar-like structure, located southwest from the source demonstrates zero PM and can be considered as one more example of deflection shock. An analysis of Hα profiles in the reflection nebulae R1 and R3 indicates the uniqueness of this object, which can be studied in pole-on and edge-on directions simultaneously. [ABSTRACT FROM AUTHOR]

• A novel composite cavity fiber tip applied for high temperature sensing is proposed. • Coherent mixing of the three reflected beams gives rise to the interference pattern. • Maximum temperature sensitivity of 14.31 pm/°C was obtained for the silica cavity. • The composite cavity fiber tip is independent of external strain and RI. A simple composite cavity fiber tip (CCFT) Fabry-Perot interferometer (FPI) is proposed and experimentally demonstrated. The composite cavity is composed of an air cavity and a silica cavity. The air cavity is an elliptical air hole embedded in the SMF. The silica cavity is a short section of SMF cascaded to the air cavity. To avoid the influence of other external physical quantities (refractive index, humidity, etc.) on the CCFT FPI, the CCFT was successfully encapsulated by a tapered-shaped silica capillary. The packaged CCFT FPIs were applied for high temperature sensing in the range of 100–800 °C. Temperature sensitivity of 14.31 pm/°C was obtained for the silica cavity. In addition, the composite cavity can be fabricated to be very small in size (dozens of micrometers) with extremely low-cost, thus the CCFT FPI can be a competitive candidate for local point temperature sensing. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Aeolus, launched on August 22nd in 2018, is the first ever satellite to directly observe wind information from the surface up to 30 km on a global scale. An airborne prototype called ALADIN Airborne Demonstrator (A2D) was developed at the German Aerospace Centre (DLR) for validating the Aeolus measurement principle based on realistic atmospheric signals. To obtain accurate wind retrievals, the A2D uses a measured Rayleigh response calibration (MRRC) to calibrate its Rayleigh channel signals. However, the atmospheric and instrumental variability currently limit the reliability and repeatability of this MRRC. Thus, a procedure for a simulated Rayleigh response calibration (SRRC) is developed and presented in this paper to resolve these limitations of the A2D Rayleigh channel MRRC. The transmission functions of the A2D Rayleigh channel interferometer, consisting of the double-edge Fabry-Perot interferometers (FPIs), are firstly characterised and optimized based on measurements performed during different airborne and ground-based campaigns. The optimized FPI transmission function is then combined with the molecular Rayleigh backscatter spectrum to derive an accurate A2D SRRC which can finally be implemented in the A2D wind retrieval. Using dropsonde data as a reference, a statistical analysis based on data from a flight campaign in 2016 reveals a bias and a standard deviation of line-of-sight (LOS) wind speeds derived from an SRRC of only 0.05 m s-1 and 2.52 m s-1, respectively. Compared to the result derived from a MRRC with a bias of 0.23 m s-1 and a standard deviation of 2.20 m s-1, the accuracy improved while the precision is considered to be at the same level. Furthermore, it is shown that SRRC allows the simulation of receiver responses over the whole altitude range from the aircraft down to sea level, thus overcoming limitations due to continuous ground elevation during the performance of airborne instrument response calibrations. [ABSTRACT FROM AUTHOR]

The paper presents a FPI sensor by splicing a photonic crystal fiber (PCF) to a well-cleaved single-mode fiber (SMF) and a multimode fiber (MMF), preparing a SMF–PCF–MMF structure, and the section of PCF was the cavity. Temperature and NaCl solution concentration responses were investigated experimentally. The temperature sensitivity is ∼12.3 pm/°C and concentration sensitivity is ∼0.01 dB/%. The proposed sensor could offer such features as low cross-sensitivity, good temperature stability, temperature and concentration linearity are both over 0.996, making it attractive for liquid concentration and temperature measurement. [ABSTRACT FROM AUTHOR]

• The humidity response time of the sensor reaches less than 1s. • A drawing-curing method was used to fabricate the sensor simply and cheaply. • The repeated experimental results show that the sensor has good stability and repeatability in humidity sensing. A Fabry-Perot interferometer (FPI) based on a polymer micro-rod on the end-face of fiber core was fabricated for humidity sensing. The whole fabrication process by using the simple and low-cost drawing-curing method takes less than one minute. The diameter and height of the polymer micro-rod are only 9.2 μm and 11.3 μm, respectively. Because of its ultra-compact structure, the rise time and recover time of the sensor have been improved to 291 ms and 886 ms when the relative humidity of the environment changes between 61% and 76%. The proposed humidity sensor is expected to have important applications in bio-pharmaceutics, chemical industry, and environmental science, owing to its simple fabrication process, high sensitivity (659.67 pm/%RH), fast response and good stability. [ABSTRACT FROM AUTHOR]

One of the main tools for high resolution remote sensing and photogrammetry is the lightweight hyperspectral frame camera, that is used in several application areas such as precision agriculture, forestry, and environmental monitoring. Among these types of sensors, the Rikola (which is based on a Fabry–Perot interferometer (FPI) and produced by Senop) is one of the latest innovations. Due to its internal geometry, there are several issues to be addressed for the appropriate definition and estimation of the inner orientation parameters (IOPs). The main problems concern the possibility to change every time the sequence of the bands and to assess the reliability of the IOPs. This work focuses the attention on the assessment of the IOPs definition for each sensor, considering the impact of environmental conditions (e.g., different time, exposure, brightness) and different configurations of the FPI camera, in order to rebuild an undistorted hypercube for image processing and object estimation. The aim of this work is to understand if the IOPs are stable over the time and if and which bands can be used as reference for the calculation of the inner parameters for each sensor, considering different environmental configurations and surveys, from terrestrial to aerial applications. Preliminary performed tests showed that the focal length percentage variation among the bands of different experiments is around 1%. [ABSTRACT FROM AUTHOR]

A study on orientation of hyperspectral band cubes acquired with frame camera is presented in this paper. The camera technology is based on a tuneable Fabry-Perot Interferometer (FPI) and captures cubes of images sequentially using two sensors. However, the bands are not recorded at the same instant, which results different exterior orientation parameters (EOPs) for each image band. A technique based on polynomial model is assessed, which determines the EOPs within the hypercube from few sample bands, since a large number of bands are generated. Experiments were performed to assess the feasibility of using the polynomial technique. An analysis of the UAV trajectory was performed and the results of the polynomial technique were compared with those obtained by a conventional bundle adjustment. The trials showed that the results of both techniques were comparable, indicating that the time-dependent polynomial model can be used to estimate the EOPs of all spectral bands, without requiring a bundle adjustment including all bands. The accuracy of the block adjustment was analysed based on the discrepancies obtained from independent checkpoints. The root mean square error (RMSE) was calculated and showed an accuracy of approximately 1 GSD in planimetry and 1.5 GSD in altimetry. This accurate result is important because the proposed technique can significantly reduce the processing workload. [ABSTRACT FROM AUTHOR]

Context. Several astronomical instruments, for both nighttime and solar use, rely on tunable Fabry–Perot interferometers (FPIs). Knowing the exact shape of the etalons' cavity is crucial for assessing the overall instrumental transmission profile and its possible variations during the tuning process. Aims. We aim to define and test a technique to accurately measure the cavity defects of air-spaced FPIs, including distortions due to the spectral tuning process that are typical of astronomical observations. We further aim to develop a correction technique to maintain the shape of the cavity as constant as possible during the spectral scan. These are necessary steps to optimize the spectral transmission profile of a two-dimensional spectrograph (polarimeter) using one or more FPIs in series, and to ensure that the spectral transmission profile remains constant during typical observing conditions. Methods. We devised a generalization of the techniques developed for the so-called phase-shifting interferometry to the case of FPI. This measuring technique is applicable to any given FPI that can be tuned via changing the cavity spacing (z-axis), and can be used for any etalon regardless of the coating' reflectivity. The major strength of our method is the ability to fully characterize the cavity during a spectral scan, allowing for the determination of scan-dependent modifications of the plates. We have applied the measuring technique to three 50 mm diameter interferometers, with cavity gaps ranging between 600 μm and 3 mm, coated for use in the visible range. Results. The technique developed in this paper allows us to accurately and reliably measure the cavity defects of air-spaced FPIs, and of their evolution during the entire spectral scan. Our main, and unexpected, result is that the relative tilt between the two FPI plates varies significantly during the spectral scan, and can dominate the cavity defects; in particular, we observe that the tilt component at the extremes of the scan is sensibly larger than that at the center of the scan. Exploiting the capability of the electronic controllers to set the reference plane at any given spectral step, we then develop a correction technique that allows the minimization of the tilt during a complete spectral scan. The correction remains highly stable over long periods, well beyond the typical duration of astronomical observations. [ABSTRACT FROM AUTHOR]

The mode evolution of an InGaN blue laser diode with operating current and diode temperature was investigated. The spectra of a 445-nm InGaN blue laser diode with operating currents and diode temperatures was recorded using an ultrahigh resolution spectrometer and a scanning Fabry-Perot interferometer. The blue laser diode can deliver a mW-level single frequency beam with about 20 MHz linewidth at nearly above the lasing threshold. And the temperature-dependent wavelength drift was estimated as 0.31 nm for single longitudinal mode operation region. The spectra showed the number of the longitudinal modes increased with raising operating current. Analysis of the mode evolution of a blue laser diode with operating current and diode temperature is important when how to choose the laser diode for specific applications. [ABSTRACT FROM AUTHOR]

A sensitivity-improved ultrasonic fiber optic sensor is proposed and demonstrated for ultrasound wave (UW) imaging of mural models. The sensor contains a Fabry-Perot (F-P) cavity at one end of a single-mode fiber (SMF) and a gold-coated ceramic ferrule. Because the gold film is 120 nm thick, the sensor has extremely high ultrasonic sensitivity. Compared to similar sensors in the literature, by changing its package structure, removing the acoustic lens and using a thinner film in the interference cavity, the signal-to-noise ratio (SNR) of the proposed sensor can reach 68.18 dB. Using the sideband filtering method, the echo wave signal can be extracted, the cross sectional image of hollows in the mural model can be obtained by 1D scanning, and 3D images can be reconstructed by 2D scanning. We calculated the thicknesses of the three hollows in the ground layer to be 0.1986 cm, 0.6752 cm, and 0.3873 cm, and the thickest part of the efflorescence area to be 2.857 cm. The ultrasonic sensor helps archaeological conservators to detect information on size of defects in the mural ground layer. [ABSTRACT FROM AUTHOR]

The amplitude and phase method is successfully applied to analytically solve a dielectric Fabry–Perot etalon of arbitrary thickness. The transmittivity of the system as a function of wavelength obtained in this way is shown to be equivalent to the results given by the conventional matrix theory or the multiple beam interference method. An asset of the amplitude and phase formalism is that in addition to describing the overall behaviour of the system, the wave properties are displayed as the field propagates through the optical system. [ABSTRACT FROM AUTHOR]

Abstract We propose 2D single-shot comb interferometry with the capability of zooming along the axial axis for profilometry and tomography. A spatial phase modulator (SPM) based a diffraction grating is utilized to realize the zooming operation with a fine scale in the proposed system. The measurable range of the fine scale can be varied from 200 μ m to 1.2 mm by controlling the diffraction orders of the SPM, allowing a magnification tool for surface profilometry. An optical-frequency comb-interferometry-based Fabry–Perot etalon is used to extend the measurement range of proposed system with a coarse scale up to 10.8 mm. In both the coarse and fine scales, the tomography resolution is maintained at approximately 2. 5 μ m. [ABSTRACT FROM AUTHOR]

Abstract A fast and simple demodulation method for the variation of Fabry–Perot (FP) cavity length is proposed and demonstrated. Taking advantages of differentially detecting optical power intensities of dual-wavelength, a reliable linear correlation between the variation of FP cavity length and differential intensity ratio of dual-wavelength can be obtained. Besides, an extrinsic Fabry–Perot interferometer (EFPI) sensor with a length of 196.8 μ m was used to demonstrate performance of the method, and the time for each experimental operation is within 100 ms by the demodulation method and an integrable tunable laser assembly (ITLA). The demodulation resolution of 0.726 nm was achieved for the change of cavity length in a strain sensing experiment. By selecting two wavelengths at different positions in the system, the linear ranges of FP cavity length variation are 180 nm, 150 nm, and 130 nm responding to the sensitivities of −0.01099 nm−1, −0.01485 nm−1, and −0.02253 nm−1, respectively. It shows a high flexibility between the linear range and the sensitivity. Consequently, the different requirements of the EFPI sensor system can be easily fulfilled through appropriate selections of two different wavelengths. Moreover, the advantages of simplicity, fast speed, low cost can be fully revealed in the demodulation of large-scale production EFPI sensors. Highlights • A fast and simple demodulation method for the variation of Fabry–Perot (FP) cavity length is proposed and demonstrated. • The entire interrogation system has the advantages of low demodulation cost, and simple fabrication. • Utilizing demodulation method and an integrable tunable laser assembly (ITLA) in an EFPI sensors system. [ABSTRACT FROM AUTHOR]

• In this work, a kind of ultrasensitive refractive index (RI) sensor based on parallel-connected dual Fabry-Perot interferometers (FPIs) with Vernier effect is investigated. • The open groove of the open cavity FPI provides a convenient, efficient and economical method for liquid RI detection. • The proposed sensing structure features ultra-high sensitivity, good stability, quick and accurate measurement, which can be potentially used for chemical and biological liquid RI detection. A kind of ultrasensitive refractive index sensor based on parallel-connected dual Fabry-Perot interferometers (FPIs) with Vernier effect is proposed and demonstrated. The open and closed cavity FPIs are used as the sensing and reference units, respectively. The Vernier effect is realized by superimposing the reflective comb spectra of the dual FPIs, which have slightly different free spectral ranges. The open groove of the open cavity FPI provides a convenient, efficient and economical method for filling in or draining out the cavity. The stability, repeatability and temperature response of the structures are investigated. Experimental results indicate that the proposed sensing structure features high sensitivity, good stability, quick and accurate measurement. [ABSTRACT FROM AUTHOR]

Electron density fluctuation has been measured based on the collect forward scattering method using a single-chord microwave interferometer system on the Keda Torus eXperiment (KTX) reversed field pinch. The interferometer system is a heterodyne one with two terahertz solid-state sources with frequency around 650 GHz, and the transmitted beam and forward scattering beam are received simultaneously by a Schottky planar diode mixer with sensitivity about 500 mV/mW. Constrained by the beamwidth and geometry limitation, the measurable wave vector range is $k_{w}\le 5 {\mathrm {cm}}^{-1}$ with a resolution $\Delta k\approx 2.07\,{\mathrm {cm}}^{-1}$. MHD activities are detected via this forward scattering technique. The results are consistent with other diagnostics, such as the chord averaged electron density, the edge magnetic probe, and high-speed charge-coupled device (CCD) camera. [ABSTRACT FROM AUTHOR]

We have proposed and experimentally demonstrated a compact temperature sensor based on cascaded fiber Sagnac interferometer (FSI) and fiber Fabry-Perot interferometer (FFPI) with Vernier effect. The FSI is composed of a certain length of panda polarization-maintaining fiber and a 3-dB four-port coupler, whereas FFPI consists of two collimators and a wave plate. Both are used for temperature sensing, but where FSI is more sensitive to temperature than FFPI, the free spectral ranges of the two interferometers differ by 0.12 nm. The experimental results show that the temperature sensitivity is increased from -1.7 nm/°C (single FSI) and 0.008 nm/°C (single FPI) to 10.28 nm/°C (cascade configuration), which is basically consistent with the theoretical analysis (10.82 nm/°C). [ABSTRACT FROM AUTHOR]

The high‐latitude ionosphere‐thermosphere system is strongly affected by the magnetospheric energy input during magnetospheric substorms. In this study, we investigate the response of the upper thermospheric winds to four substorm events by using the Fabry‐Perot interferometer at Tromsø, Norway, the International Monitor for Auroral Geomagnetic Effects magnetometers, the EISCAT radar, and an all‐sky camera. The upper thermospheric winds had distinct responses to substorm phases. During the growth phase, westward acceleration of the wind was observed in the premidnight sector within the eastward electrojet region. We suggest that the westward acceleration of the neutral wind is caused by the ion drag force associated with the large‐scale westward plasma convection within the eastward electrojet. During the expansion phase, the zonal wind had a prompt response to the intensification of the westward electrojet (WEJ) overhead Tromsø. The zonal wind was accelerated eastward, which is likely to be associated with the eastward plasma convection within the substorm current wedge. During the expansion and recovery phases, the meridional wind was frequently accelerated to the southward direction, when the majority of the substorm WEJ current was located on the poleward side of Tromsø. We suggest that this meridional wind acceleration is related to a pressure gradient produced by Joule heating within the substorm WEJ region. In addition, strong atmospheric gravity waves during the expansion and the recovery phases were observed. Key Points: Ion drag plays an important role in acceleration of the zonal wind during substormsEastward acceleration of the zonal wind is associated with intensification of westward electrojet during the expansion and recovery phasesObserved meridional wind acceleration may be produced by pressure gradient caused by the Joule heating [ABSTRACT FROM AUTHOR]

The influence of an optical fringe patterns on the viscoelastic properties exhibited by Au nanofluids samples was evaluated. A sensitive interferometric technique for analyzing optofluidic effects in different samples was implemented. The assistance of the plasmonic characteristics in Au nanostructures allows measuring particular mechano-optical effects at 532 nm wavelength by a Fabry-Perot interferometer to explore multiaxial strains. A representative volume of the sample was studied to determine the stability and maximum viscoelastic properties exhibited by the nanostructures. A vectorial two-wave configuration allows controlling the maximum strain induced in the sample. The oscillating nature of the colloid was examined by using interferometric optical signals reflected from a remnant drop pending at the end of an optical fiber. Nanosecond pulses were used to induce inelastic optofluidic effects. The mechanical parameters were approximated by a nonlinear second order system activated by a Dirac delta functions. [ABSTRACT FROM AUTHOR]

We report a sensitive and mechanically robust fiber-optic refractometer with temperature compensation based on a Fabry–Perot interferometer formed by two chirped fiber Bragg gratings on a seven-core multicore fiber. A small part of the fiber cladding at the sensing region is etched to expose the outer cores to the environment. While the optical modes supported by the outer cores are affected by both temperature and refractive index (RI) changes of the surrounding liquid, the optical mode in the central core of the fiber is affected by the temperature changes only. The wavelength shift due to the RI change is obtained by subtracting the wavelength shift of the central core from that of one of the outer cores so that the temperature effect is removed. The demodulation can also remove the errors caused by the wavelength drift of laser. Due to the narrow spectral features of the sensor, the sensor shows a low RI detection limit of ${2.4} \times {10}^{-{6}}$ at a background RI of 1.316 at a wavelength of ~1550 nm. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability. [ABSTRACT FROM AUTHOR]

Purpose As an emerging measurement technique with the merit of easy alignment and high sensitivity, laser self-mixing interferometry (SMI) has wide applications in the detection of physical quantities. Considering that the characteristics of lasers have a determining influence on sensors' performance, the authors have established an open cavity solid-state laser (SSL) with more adjustment flexibility to act as a laser source.Design/methodology/approach The fundamental structure of a SSL has been presented with an Nd:YAG rod severing as an active material and a birefringent filter inserted in the resonator as a mode selecting element. The power stability has been tested by a power meter, while the mode pattern has been inspected with a scanning Fabry–Perot interferometer, and the linewidth has been observed through a wavelength meter. A loudspeaker driven by a function generator is located in the extracavity to introduce phase modulation for SMI signal observation.Findings The established Nd:YAG SSL operates in a single longitudinal mode with the power stability of 0.2 mW and the linewidth less than 10 MHz. The SMI phenomenon occurs in the SSL, and the SMI signal obtained shows a fine signal-to-noise ratio of about 30 dB.Originality/value To the authors knowledge, SMI sensors using SSLs, especially in open cavity type, have rarely been reported, and they can find significant applications in designing high performance SMI sensors and instruments. [ABSTRACT FROM AUTHOR]

Abstract In this paper we revisit the theory of selective reflection from a dilute vapor cell. A self-consistent theory is developed for reflection spectrum for Fabry–Perot interferometer. Formulas for single and multiple reflections are obtained. We also obtain the effective refractive index for a single selective reflection. The results of the paper are in a good agreement with existing experimental results. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we present a high-sensitivity homogenous fused-silica low pressure sensor. It was constructed from the commercially available standard silica fiber, fused-silica diaphragm and ferrule through CO 2 laser all-welding. A Fabry–Perot (FP) cavity was fully sealed between the ultra-thin fused-silica diaphragm and the ferrule end face, and the ultra-thin diaphragm acts as a pressure transducer. We proposed the laser heat conduction welding for the fused-silica diaphragm, and the laser deep penetration point welding for the fiber and the ferrule. So the monolithic structure fused-silica FP interference optical fiber sensor was achieved. The sensor has a vent hole between the fiber and the ferrule forming during the laser deep penetration point welding, which can effectively reduce temperature dependence. The sensor exhibits excellent stability and repeatability, extreme low temperature dependence with 0.025nm/ o C that is 1/76 of the theoretical value, a sensitivity of 5.15nm/kPa, and a resolution of 4.7Pa. With a monolithic fused-silica configuration, the adhesive-free bonding high-sensitivity fused-silica sensor is expected to be suitable for low pressure measurements in extreme harsh environments. Highlights • Adhesive-free bonding homogenous fused-silica FPIOF sensor in harsh environments by CO 2 laser welding was investigated. • Laser deep penetration point welding during the fiber and the fused-silica ferrule was present. • Laser heat conduction welding for the ultra-thin fused-silica diaphragm was proposed. • The FP cavity of sensor has a vent hole employed to reduce effectively temperature dependence. • Monolithic structure sensor exhibits good sensitivity, stability, repeatability, and extreme low temperature dependence. [ABSTRACT FROM AUTHOR]

A highly sensitive demodulation method for the low-finesse Fabry–Perot interferometer (FPI) sensor is proposed and experimentally demonstrated. To generate the Vernier effect, the sampling interval of a tunable laser source is set close to the free spectral range of the FPI sensor. In order to improve the demodulation precision, the spline fitting process was applied to the sampling data. The dip wavelength of the fitted curve was chosen to measure the spectral shift. In the strain test experiment, the sensitivity of $0.01836~nm/{\mu \epsilon }$ was obtained. A controllable sensitivity is achieved by changing the sampling interval. It is convenient to balance the sensitivity and the measuring time. The method can be used to enhance the sensitivity of the preexistent low-finesse FPI sensors by changing an optical source in the sensing system. There is no need to make any changes to the FPI sensors and sensing nodes. Furthermore, the method can also be applied for other two-beam interferometry sensors. [ABSTRACT FROM AUTHOR]

Abstract A flexible force sensor based on the spheroidal Fabry-Perot micro-cavity is proposed. Through theoretical analysis and simulation, we find out that the relationship between cavity length and force sensitivity is negatively correlated. Then two Intrinsic Fabry-Perot Interferometers with cavity lengths of 59.5 um and 65.3 um are fabricated by chemical etching. The force and temperature experiments are implemented, and the results show that the force sensor has good linearity and stability. The force sensitivity and temperature sensitivity of the flexible Fabry-Perot force sensors are 106 pm/N , 4.3 pm/℃ and 104 pm/N , 4.2 pm/℃ respectively. In addition, the flexible Fabry-Perot force sensor has advantages of lightweight, high flexibility, compact structure and low force-temperature cross sensitivity, which is expected to be used in the robot finger force sensing, the pulse vibration detecting and the surgical instruments clamping force monitoring in the future. [ABSTRACT FROM AUTHOR]

Abstract Compared with dual-channel Fabry-Perot interferometer (DFPI), dual-channel Mach-Zehnder interferometer (DMZI) is a new frequency discriminator for Doppler wind lidar based on double-edge detection. The double-edge frequency discrimination systems based on DFPI and DMZI in the wind lidar system are designed, respectively, and the detection principle is analyzed theoretically. Under certain simulation conditions, performances such as line-of-sight (LOS) wind velocity measurement sensitivities, signal-to-noise ratios (SNR) and LOS wind velocity measurement errors of both DFPI and DMZI systems are simulated and compared based on the U.S. standard atmospheric model. The simulation results show that for 1064 nm aerosol system, the LOS wind velocity measurement sensitivity of the DMZI system is lower than that of DFPI system and the error is higher than that of DFPI system with the same dynamic wind velocity measurement range. Nevertheless, for 355 nm atmospheric molecular system, DMZI system provides a factor of 1.33 times lower error in the LOS wind velocity than that of DFPI with an altitude of 30 km and the LOS wind velocity of 200 m/s. Furthermore, in the LOS wind velocity range of −200 m/s∼ +200 m/s, the measurement error of DMZI system hardly increases with the increasing of wind speed. Therefore, the results of this study not only provide guidance for the selection of discriminators, but also provide theoretical support for the application of DMZI in the double-edge detection of atmospheric molecular lidar. [ABSTRACT FROM AUTHOR]