This PDF file contains the front matter associated with SPIE Proceedings Volume 11901, including the Title Page, Copyright information and Table of Content

Different from the previously reported cryogenic temperature measurement based on the fiber Bragg grating (FBG) sensor, we propose a novel cryogenic temperature measurement scheme with a fiber interferometric sensor. The proposed sensor is constructed by the extrinsic Fabry–Perot interferometer (EFPI) which is consisted of a ceramic ferrule, a copper sleeve and two fiber ends. Under the cryogenic surrounding, the cavity length of the EFPI will change with the thermal deformation of the copper sleeve. With liquid nitrogen, the surrounding temperature can be changed from 77K to room temperature, and as high as 2.246nm/K of the temperature sensitivity can be achieved at a temperature range from 113K-153K in the experiment, with a 12.38μm EFPI cavity. The proposed interferometric sensor will still have relatively high sensitivity at a temperature lower than 77K according to our numerical simulation, so, has good application prospect in cryogenic temperature environment.

The early diagnosis of myocardial infarction can significantly improve the survival rate in clinical medicine, therefore the high sensitivity detection of myocardial infarction biomarkers, such as creatine kinase (CK), lactate dehydrogenase (LDH) and cardiac troponin (cTn), is very important. In this work, a thin-wall microtubule whispering gallery modes (WGM) cavity biosensor to detect myocardial infarction marker has been achieved. The thin-wall microtubule WGM cavity is simply fabricated by tapering the silica capillary with oxyhydrogen flame. Using the self-polymerization effect of dopamine, the antibody is modified on the inner wall of the microtubule cavity to achieve specific capture of the cTnI-TnC complex protein. Moreover, by introducing the WGM microtubule cavity into the erbium-doped fiber laser cavity, the lasing wavelength can be utilized for the label-free detection of the myocardial infarction biomarker. The proposed microtubule cavity biosensor has advantages of inherent microfluidic channel, label-free detection and low detection limit, making itself a potential sensing platform in early diagnosis of heart disease.

Cardiovascular disease is a major risk to human health, which needs long-term monitoring for prevention and early diagnosis. Optical sensors present the advantage of immunity to electromagnetic field and high sensitivity, and have been growing in a variety of emerging medical applications to monitor human cardiac parameters. Most of the current optical sensors can only measure limited cardiovascular information such as the heart rate, therefore, the optics-based approach for cardiac electrophysiology has attracted the attention of more researchers. In this paper, we developed a method to evaluate the availability of our proposed anti-EMI optical sensor. The sensitivity of optical sensor based on electro-optic modulation can achieve 266.4μW/V and detect the electrocardiogram (ECG) by attached to the chest and edge of clavicle. A series of ECG signals over 1 hour were analyzed using proposed method, which is driven by the optimization of R-peak location, Lorenz plot and statistical correlation. ECG monitoring results of the optical sensors are in accordance with a standard clinical device (SOMNOtouch™ RESP) among different subjects. Moreover, both the sensors are tested in daily electromagnetic conditions, and it causes some obvious signal artifacts to the SOMNO system, but almost no effect on the optical sensors during the long-term test. We provide further grounds for such clinical applications by demonstrating, for the first time to our knowledge, optics-based device used in long-term ECG monitoring, an essential tool in modern cardiac monitoring applications.

The principle of fiber optical current transformer and the function of λ/4 wave plate are described in this paper. We propose and demonstrate three kinds of main fabrication errors of λ/4 wave plate, including fiber material error, axial angle error and length interception error. In order to reduce the errors above, an elliptical core polarization-maintaining fiber, which is relatively temperature-insensitive and has a long beat length 20 mm, is used as the wave plate fiber. And we build a set of all-fiber λ/4 wave plate fabrication platform utilizing polarization analyzer and optical microscope. Therefore, the excellent performance λ/4 wave plate (DOP<98%, PER<0.2dB) was obtained with the fabrication platform. Experimental results of DC current measurement (100A˜3000A) and temperature test (-40°C˜85°C) show that the measurement accuracy of prototype with the fabricated λ/4 wave plate is better than 0.2S, which verified that the analysis of fabrication error and optimization technique of λ/4 wave plate in this paper can improves the performance of FOCT effectively

A fiber optic current sensor utilizing orbital angular momentum (OAM) beam is proposed in this paper. The superposition principle of composite OAM beam is deduced, and the current sensing process is derived by Jones matrix. The current is measured by detecting the rotation angle of petal-like patterns formed after the composite OAM beam through the polarizer. The reflective structure of the sensor doubles the rotation angle, which improves the measuring sensitivity and the reciprocity of the system. Through simulation analysis, we verified that the rotation angle changes linearly with the increase of current, and the sensitivity of the proposed sensor is 0.1254°/A. Finally, on this basis, the angle recognition method is optimized, and the final measurement error is less than ±0.2% in the range of 50A-1500A. .

When the method of dual 90-degree rotation splices in the resonator is used to suppress the zero-bias drift caused by polarization noise of the RFOG, the asymmetry of the two 90-degree rotation splices will cause the Shupe effect error. The mathematical model of the Shupe effect caused by the asymmetry of two 90-degree rotation splices is established and simulated in this paper. The simulation results show that the value of the Shupe effect error is proportional to the asymmetric length of the two 90-degree rotation splices. The Shupe effect error can be suppressed by reducing the asymmetric length.

A novel sensitivity-enhanced all-fiber Fabry-Perot high-pressure sensor consisting of a long gauge-length and a hard-core diaphragm is proposed and demonstrated. The hard-core diaphragm can effectively improve the pressure sensitivity of the sensor and eliminate the additional reflection of its outer surface. Theoretical and experimental results show that the pressure sensitivity of the sensor is increased to about 2 times when its diaphragm thickness is decreased from 5 mm to 11 μm. In addition, the pressure and temperature sensitivity of –342.6 pm/MPa and –2.8 pm/℃ are achieved, respectively, in the pressure range of 0~20 MPa and temperature range of 13.6~300 ℃. The proposed sensor could offer some excellent features such as good high-temperature stability, low-temperature pressure cross-sensitivity, low mechanical hysteresis, making it attractive for pressure measurement under harsh environments.

In the optical parity-time-symmetry systems, at certain values of their parameters, a phase transition from real eigenvalues of the Hamiltonian to the complex ones is observed. This phase transition, which can be directly or indirectly caused by a change in the measured physical quantity, leads to a sharp change in the optical properties of the system. This can be used to improve the accuracy of various optical sensors, in particular, angular rate sensors. Present communication is devoted to the analysis of previously announced and newly developed by the authors methods for measuring angular rates based on the use of the optical parity-time-symmetry systems. In particular, various variations of laser gyroscopes based on the systems of two identical coupled ring resonators that differ from each other only in the level of losses and gain are considered. We propose a new method for measuring the angular rate, based on the use of a parity-time-symmetry system of two straight coupled waveguides with a passive ring resonator connected to it.

A metal diaphragm-based airflow sensor based on fiber-optic Fabry-Perot (F-P) interference has been proposed and experimentally demonstrated. The sensor is composed of glass sleeving, ceramic ferrule and metal diaphragm. Through data calibration, a practical airflow sensor has been fabricated. As a result of the stainless steel diaphragm and open F-P cavity, the durability of the sensor is ensured, and it can be used in poor air quality environments. Experimental results in the airflow field show that the sensor has the potential to estimate the air quantity of high-speed airflow in various air conduit

We present a distributed high sensitivity static strain sensing method based on all grating optical fiber in optical frequency domain reflectometry (OFDR).Through the use of all grating optical fiber, the backscattered light can be greatly improved, which increases the signal-to-noise ratio. A higher signal-to-noise ratio can help to obtain more accurate distance domain information, thereby obtaining better segmented optical frequency domain information for cross-correlation. Therefore, in the same cross-correlation method, our system obtains even larger range of FUT information due to the signal with higher signal-to-noise ratio. Using this system, we measured the change of strain at the end of a total 200 m FUT. The minimum measurable strain variation is 5 με and the spatial resolution of the system is 10cm. It should be noted that we use interpolation zero padding method on the basis of cross-correlation to obtain higher strain accuracy. In addition, we verified the stability of the system by fitting the results of different strains. The linearity of the fitting results is extremely high, and the R-square can reach more than 0.9. Compared with the OFDR system using the single mode fiber, the OFDR system using all grating optical fiber can provide a better sensing performance of measuring a minor strain variation under the same sensing spatial resolution. This is of great significance for the improvement of OFDR measurement applications.

Microfluidic optomechanical device are a unique optofluidics platform that can exhibit optomechanical oscillation in the 10-20 MHz, driven by radiation pressure (RP). The resonant enhancement of both mechanical and optical response in microcavity optomechanical devices allows exquisitely sensitive measurements of environment stimuli (pressure, force, sound speed change) and non-solid states of matter (freely flowing particles, viscous fluids). In this work, we experimentally investigate temperature tuning of these hollow-shell oscillators. We also demonstrate the effect of temperature on the frequency domain of optical machine oscillation resonance shift and applied it to the field of temperature sensing. Our result is a step towards optomechanical sensor in the field of temperature.

We demonstrate that incorporating Förster resonant energy transferring (FRET) mechanism to the Whispering gallery mode (WGM) microcavity can greatly enhance the sensitivity. A donor-dye doped microsphere is embedded in an acceptor dye solution, which forms a FRET-WGM sensing platform. Pumping the WGM microcavity with a pulsed laser, we obtained simultaneous lasing of donor and acceptor dyes. The gap between donor and acceptor resonant wavelengths serves as a readout for acceptor quantification. Compared with the pure WGMs sensing system, the detection limit of FRET-WGM is greatly decreased, which enables us to realize real-time monitoring and quantitative analysis of intracellular fluorescent substance at single cell level.

We propose and demonstrate a coherent anti-Stokes Raman scattering (CARS) spectroscopic fiber probe based on a tapered optical fiber. The fiber probe prepared by the fiber heating fused and tapered method ensures that the output optical power density is high enough to excite the CARS signal. We have been able to detect Raman spectra of various chemical samples. The CARS fiber probe has the potential to achieve high spatial resolution. These results pave the way for flexibility and miniaturization of CARS probes

A bent core-offset in-line fiber Mach-Zehnder interferometer (MZIs) is proposed for refractive index (RI) sensing. After simply bending, not only the RI sensitivity but also the temperature sensitivity can be enhanced as the bending radius decreases. To solve the cross-sensitivity problem, a simultaneous measurement of RI and temperature was carried out. When the bending radius is 35.64mm, the highest RI sensitivity of -44.55nm/RIU for the RI range from 1.333 to 1.373, and the highest temperature sensitivity of 0.0799nm/℃ for the temperature range from 25℃ to 60℃ were measured simultaneously.

The effect of broad-range (10 nm) self-sweeping near 2100 nm in Ho-doped fiber laser has been demonstrated experimentally. The narrowband linearly-polarized tunable radiation with average output power of more than 200 mW is obtained. The use of highly-doped holmium fiber allowed to obtain the widest sweeping range. The polarization maintaining elements and temperature stabilization of the active fiber were applied to stabilize the laser operation. The developed source can be used for spectroscopy of Nitrous oxide, having absorption lines in that spectral region.

In this paper, we propose a semiconductor optical amplifier (SOA)-fiber ring laser (FRL) for fiber Bragg grating (FBG) dynamic strain sensing system with an adaptive demodulator based on two-wave mixing (TWM) photorefractive interferometry. Any strain in the FBG is encoded as a wavelength shift of the light reflected by the FBG. The wavelength modulation is perfectly converted to intensity modulation by splitting the light into signal and pump beams and interfering the beams in an photorefractive InP:Fe crystal. The classical beam-combiner was replaced by a dynamic hologram continuously recorded in the InP:Fe crystal. The results demonstrate that TWM interferometer has the characteristics of adaptability and multiplexing. To investigate multiplexability, a three-channel SOA-fiber ring laser sensor system is presented to detect dynamic strain signals from three FBG sensors simultaneously. Experimental results prove that true multiplexing of several FBG dynamic strain sensors with a single adaptive source is feasible. This technique is expected to be suitable for the monitoring of external impact as well as acoustic emission in structures.

We demonstrate direct time-domain bandwidth measurement of 11 cm-long multimode polymer waveguides based on an optical sampling technique for the first time. The pulse shape can be recovered after propagating waveguides due to the advantages of large bandwidth (low time resolution) of optical sampling technology. A reduction in averaged bandwidth (bandwidth-length product) from 241 GHz (27 GHz·m) to 180 GHz (20 GHz·m) of straight waveguides is observed when using mode scramblers to fully stimulate the higher-order modes. The effects such as bending and crossing structure of the waveguides on the bandwidth are also investigated. The proposed method is effective for measuring the bandwidth and dispersion of meter and centimeter-long waveguides, fibers and optical devices

For the purpose of utilizing changes in speckle patterns to be observed in an output light spot from an optical fiber for sensing applications, certain load was applied onto an optical fiber in which laser beams from a laser diode were propagating, and resultant changes in the output light intensity were measured. In order to realize effective load application onto the optical fiber, load application mechanism was employed in which several ridges were intentionally provided onto opposite flat plates. A jacket-covered communication-grade multi-mode glass optical fiber was placed in the load application mechanism so that corrugated bending of the fiber was intentionally induced via load application due to the ridges. Laser beams propagated through the optical fiber was allowed to be output and projected onto a PV cell panel disposed at about 10 cm from the fiber end. The output voltage from the PV cell panel was measured as the output light intensity from the optical fiber. For the load application up to 40 kg, certain changes (reductions and increases) in the output light intensity were measured with increasing or decreasing level of load application with sufficient reproducibility. Even with load application of smaller load level, similar changes were observed. Such changes (reductions and increases) in the output light level with increasing and decreasing load application levels were caused by changes in speckle patterns contained in an output light spot from the optical fiber.

Self-sweeping lasers is the simplest type of tunable laser source and have many advantages such as broad tuning range, narrow linewidth, and linear polarization state. However, a main drawback of the laser is the absence of wavelength control. In the present work, automatic control of the tuning parameters (sweeping rate and wavelength) is demonstrated using a Tm-doped fiber self-sweeping laser as an example. The method consists of measuring the laser wavelength, fast data processing to obtain the sweeping direction and the sweeping rate, and subsequent automatic adjustment of the pump power with a feedback loop and high-resolution equipment is not required. The automatic control will provide the possibility of performing more delicate measurements in the field of gas absorption spectroscopy.

Fiber Bragg grating has been widely used as a sensor for detecting static quantities such as strain and temperature, but it is still difficult to detect ultrasonic guided waves due to the limitation of sensitivity and bandwidth. By comparing the traditional fiber Bragg grating and the phase-shifted fiber Bragg grating (PSFBG), it was found that the latter has a shorter and effective grating length and steeper slope. Thus, the PSFBG can greatly improve bandwidth and sensitivity, which is suitable for the detection of ultrasonic Lamb waves. Furthermore, a high-speed demodulator and a feedback controller were designed and integrated with the PSFBG to reduce the noise and enhance the robustness of the sensing system. The excellent performance of the PSFBG-based system is demonstrated in linear and nonlinear acousto-ultrasonic detection, and acoustic emission detection, by comparing with the PZT sensors.

Line-width enhancement factor (α) is a fundamental parameter of semiconductor lasers (SLs). In this paper, we propose a method for measuring α of SLs. The method is based on back-propagations neural network (BPNN) for all feedback regimes. MATLAB was used to carry out the numerical calculations and simulations of the BPNN. We used the training set and the test set to train the prediction model, and then used the predictive model to output the predicated value. The results of the BPNN model showed that the R2 value was 0.99994, and the results were following the requirement model. The accuracy of the method has been confirmed and tested by computer simulations, which show that the method can estimate α with a relative error less than 2.5%.

Optical fiber sensors based on Michelson interferometers (MIs) have potential applications in condition monitoring and measurement systems. We propose an optoelectronic oscillator (OEO)-based interrogation system with MI. The interrogation system has a high interrogation resolution and large measurement scale. The sinusoidal nature of the MI spectrum results in a single-passband microwave photonic filter (MPF), whose central frequency is determined by the dispersion parameters of the employed dispersive element and the free spectral range (FSR) of the MI. When the external environmental or physical factors change, the FSR of the MI varies and leads to the frequency shift o f the MPF, ultimately contributing to the frequency shift of the OEO-generated signal. We verify that the variation of temperature and strain can be demodulated by tracking the frequency of the OEO. We also employ an infinite impulse response (IIR)- MPF based on a fiber ring resonator (FRR) for fine oscillation mode selection and evaluate the interrogation resolution and the measurement accuracy of the interrogation system. Different from conventional interrogation systems tracking the wavelength shift of the MI spectrum, our scheme demodulates the sensing information in the electrical domain utilizing an OEO, providing a potential way to implement high-resolution sensing for conventional optical fiber sensors. Moreover, thanks to the wavelength-to-frequency mapping and the wide frequency tunable range of the OEO, our scheme would support large-scale sensing because it can avoid the overlap of MI periodic spectrum in wavelength demodulation.

The orthogonal phase demodulation method can effectively avoid the problem of the working interval of the optical fiber Fabry-Perot (F-P) sensor and has high accuracy. Based on the principle of the low-coherence interference, polarizers and birefringent crystals are used to construct signals with orthogonal relationships in the orthogonal phase demodulation system. Four-quadrant inverse tangent operation is used to accurately calculate the phase value. In order to verify the frequency band response of the demodulation system, the acoustic signals of frequencies in the range of 200 Hz-25 kHz are interrogated. Experimental results show that the orthogonal phase demodulation system can be appled to demodulate the signals of wide frequency band. The research has value for the promotion and application of orthogonal phase demodulation system based on birefringent crystals and polarization technology

Dynamic speckle analysis (DSA) is a non-invasive method to detect movements of the inspected objects. By illuminating the observed sample using a coherent light source, motion information can be obtained from a series of reflecting speckle patterns. Conventional DSA methods record the intensity of the speckle patterns using a frame-based imaging sensor. Here, we propose a novel implementation of DSA using the event sensor which captures the brightness changes of the dynamic speckle patterns with high temporal resolution and low latency. Our method is based on block matching algorithm in which the captured event stream is divided into many non-overlapping blocks and motion information can be computed by searching for the most likely blocks. The experiment results demonstrate the feasibility of our proposed method in different dynamic levels and this work will be beneficial for various applications such as biomedical imaging and material science.

The differential group delay of polarization mode dispersion is calculated using simple vector analysis and Fourier analysis in fibers with randomly varying birefringence. We show that under weak random birefringence conditions and assuming each dispersion component to be statistically independent and Gaussian distributed, the general formula for polarization decorrelation can be derived from the polarization coherence matrix without having to solve complex stochastic differential equations using Stratonovich integration. The polarization decorrelation length is dependent on autocorrelation length and average beat length of fiber.

Fiber-optic interferometers (FOIs) are common methods in the field of sensing. However, several issues hinder the technological advancements and the expansion of the scope of applications, including the polarization fading effect, multimodal interferences, and difficulties in resolving the phase of an optical signal. Aiming at solving these problems, we theoretically and experimentally analyze an original method based on broadband light source carried microwave interferometry. Mathematical models of broadband light source interference, broadband light source modulation, and modulated signal interference are constructed and simulated. Proved that optical interference fringes visibility is small enough to be regarded as zero when using broadband ASE light source. Thus, this method will eliminate the influence of optical interference and achieve accurate OPD measurement by interrogating single frequency microwave interference pattern. Unlike an optical signal with a frequency of hundreds of THz, the phase of a microwave signal can be easily and accurately measured. Therefore, the system provides an easy, convenient, and affordable way to achieve various physical quantities sensing with a satisfactory spatial resolution. Experiment results are in good agreement with theoretical calculations, which proves the superiority of the system in practice

A tilted long period grating (LPG) fabricated on a commercial plastic optical fiber (POF) by a simple mechanical die-press-print method was proposed for liquid-level sensing. The liquid level sensing performances of the sensor with different structural parameters were studied theoretically and experimentally. The results show that when the LPG fabricated on a POF with a diameter of 0.25mm and a tilted angle of 30° and the groove depth of 75μm, the highest sensitivity of -0.4631dB/mm was obtained in the level range of 20mm. Additionally, the influences of temperature on the liquid-level sensing performances of sensors were also studied. It shows that the sensitivity of the sensor was decreased with the increase of temperature.

In this work, we present a novel Ni@Au H₂O₂ sensor fabricated by employing both the direct laser writing lithography and selective electrodeposition techniques. The proposed sensing configuration exhibits excellent sensitivity and low detection limit of the hydrogen peroxide due to the large specific surface area of the self-supported nickel-metal mesh, as well as the remarkable electrocatalytic performance of Au nanoparticles when hydrogen peroxide is present. More specifically, the detection sensitivity of the employed structure in the concentration range of 0.05~96.55 μM is as high as 91.1 mAmM^-1 cm^-2 . Besides, we can still detect a significant amperometric response of the H₂O₂ for concentrations down to the value of 100 nM. Additionally, the interference signals produced by using various substances such as NaCl, urea, uric acid, lactose, sucrose and so on are usually less than 5% of the H₂O₂ response signal. Our sensing element possesses also outstanding flexibility and transparency properties, which renders it attractive for wearable applications

The continuous zoom multi configuration method is developed, and the correlative model is established, which overcomes the limitations of traditional methods, and realizes the expected system evaluation criteria，such as MTF, REA, RMS, etc..through the balance between value function, optimization algorithm and model parameterization

The corresponding experimental system was designed, and the direct absorption technology based on the principle of tunable laser absorption spectrum of inter-band cascade laser (ICL) was used to detect low concentration of sulfur fluoride (SO₂F₂) gas. The position of 3619 nm is chosen as the measurement spectrum line of SO₂F₂ gas, which can effectively avoid the cross interference of background components. At 35 ℃ of working temperature and 80 mA of current, the inter-band cascade laser emission wavelength can completely cover the absorption spectrum line at 3619 nm at the center of SO₂F₂ gas. The direct absorption spectrum of SO₂F₂ gas after deducting the background absorption is obtained by taking the logarithm of the ratio of the transmitted light intensity to the incident light intensity and the time-frequency conversion of the horizontal axis. The results show that the linear relationship between gas concentration and absorption intensity is good, the fitting coefficient R 2 is 0.997, and the system responsiveness is 0.782 mV/ppm. Three hundred sets of data were collected for 50 min, the relative standard deviation (RSD) was 0.25%, and the system sensitivity was 3.94 ppm. This method can provide a new method for the optical detection of SO₂F₂ gas, and then provide a reliable experimental basis for the detection of SO2F2 concentration of SF₆ decomposition product in GIS gas chamber.

In this paper, the fault detection sensitivity (FDS) analysis model for the splice fault of few-mode fiber (FMF) links under dynamic crosstalk conditions is constructed, which is based on the principle of FMF Rayleigh Backscattering. Under the condition of the cumulative effect of different modes of crosstalk, the variation trend of the loss characteristics of the fusion splicing fault and the FDS characteristics of the three-mode fiber LP01, LP11a and LP11b are analyzed, respectively. The simulation results show that as the modal crosstalk introduced by the axial misalignment of the fusion splice increases, the FDS of the LP11a and LP11b modes deteriorates higher than that of LP01 mode. At the same time, this paper build an FMF fusion fault detection system based on photon lantern and fiber circulators. The experimental results shows that the results are consistent with the simulation results, which shows the feasibility of the FDS analysis model under the cumulative effect of modal crosstalk in the FMF link

Damping vibration is one of the most common physical phenomena and an important research topic in the field of mechanical engineering. Self-mixing interferometry (SMI) is a non-destructive and non-contact optical sensing and measurement method. An SMI system commonly operates at weak or moderate feedback regime. The strong feedback regime is always avoided because of the possible instability in this regime. Recently, it has been demonstrated that if an SMI is stable in the strong feedback region, its input and output may maintain a linear relationship under proper operation conditions. In this paper, we proposed to apply an SMI system at strong feedback regime for measurement of damping vibration. The results show that an SMI system at strong feedback regime can achieve linear sensing even without need of extra SMI fringe processing, contributing to a new simple solution for measurement of damping vibration.

This paper introduces a distributed optical fiber temperature measurement system for oil well temperature profile measurement. By using reference light source to measure the loss of Raman scattering light in the optical fiber, the loss of optical signal propagation at each working wavelength in the distributed optical fiber temperature measurement system is acquired online, and the influence of optical fiber propagation loss caused by fiber optic cable stress concentration or damage on distributed optical fiber temperature measurement is corrected. A new distributed optical fiber temperature measurement system is built and temperature test is carried out. The experimental results show that the system can effectively compensate the Raman scattering light of the optical fiber and improve the adaptability and compatibility of the distributed optical fiber temperature measurement system to the optical cable. The system has the advantages of simple structure, high integration degree, low cost, wide applicability of optical cable, and practical engineering value.

In this paper, a magnetic field sensor based on Terfenol-D coated optical fibers is proposed and demonstrated. In our scheme, standard single mode fibers are coated by a thick layer of a magnetostrictive composite consisting of Terfenol-D particles dispersed in a polymer, which is then utilized as sensing elements. The magnetic field-induced strain on single mode fibers coated by the Terfenol-D was interrogated by an optical frequency-domain reflectometry (OFDR). The experimental results show that the sensitivity of the magnetostrictive fiber sensor depends on the coating characteristics including the surface of coated optical fiber. The sensitivity of the proposed sensor is 0.175 με/mT.

The model of laser return field backscattered by the atmospheric aerosols is established based on the Huygens-Fresnel diffraction integral formula in weak turbulence environment. The laser transmission properties in turbulent atmosphere provide the foundation to study the laser return field. The characteristics of intensity, complex coherence degree and the backscattering enhancement effect of laser return are analyzed under typical laser source coherence, turbulence, optical transceiver diameters and beam truncations. The laser source partial coherence reduces the coherence of laser return in turbulent environments. For the laser source beams with the same coherence degree, the laser intensities on the aerosols plane change relatively less with the increase of turbulence and detection range when the optical aperture is smaller; however, the laser return complex coherence degree is higher using larger optical transceiver in turbulent atmosphere. Moreover, the backscattering enhancement effect is mainly related to the turbulence, especially in the far field. The research is of significance to reveal the heterodyne detection process and the optimization method of coherent lidar systems.

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) is able to detect the perturbation through the phase demodulation of the Rayleigh backscattering (RBS) light using ordinary optical fiber. However, the intrinsic interference fading at certain points deteriorates the signal-to-noise ratio (SNR) of RBS light severely, reducing the reliability of vibration detection. In this work, a pulse-intensity-coding Φ-OTDR (using 8-bit Golay complementary codes) with spectrum extraction and the rotated-vector-sum (SERVS) method is proposed to deal with the interference fading as well as maintain a high SNR of the demodulated disturbance signal. We demonstrate that the SERVS method can reduce the percentage of fading points from 2.98% to 0.33% significantly within a sensing distance of 2 km. What’s more, the averaged power spectral density (PSD) level of the differential phase at all locations of the fiber with SERVS method is lower than that without SERVS method, verifying the reduction of interference-fading-induced phase noises along the fiber. Also the SERVS method achieves a SNR of 43.96 dB for the demodulated disturbance signal, about 8.6 dB higher than the single-pulse Φ-OTDR due to the correlation characteristics of Golay complementary sequence.

We present the results of modelling of pressure-driven gas flow in a 14.7 meters long nodeless Antiresonant Hollow Core Fiber (ARHCF) for predicting the gas exchange time in the ARHCF-based laser absorption spectroscopy measurement systems. The implemented physical model is based on the Navier-Stokes equations for laminar flow. The tunable diode laser absorption spectroscopy has been used for determining experimentally the ARHCF gas filling time. The obtained results confirmed the requirement for more complex geometric models to properly predict the core filling time of nodeless ARHCFs than a simple, single-channel approach, which can be used effectively for gap-less ARHCFs.