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A temperature-compensated MZI-based optical sensor using few-mode concentric multilayer-core fiber (CMCF) was designed, and surrounding refractive index (SRI) sensing via interference spectra was successfully proposed and experimentally demonstrated. This sensor with sandwich structure generated lots of resonance dips in the interference spectra, which had differentiated sensitivities to SRI and temperature. The homemade CMCF only supported few mode groups, and variability of the resonance drift could be influenced by different mode excitation and sensitivity. For the two selected dips of this proposed sensor, the SRI sensitivities experimentally achieved were −76.88 and −50.51 nm per refractive index unit (RIU), and the temperature ones were 80.2 and 50.1 pm/°C, respectively. The experimental mechanism and results have been analyzed in brief. The standard matrix inversion method could be used for simultaneous two parameter determination. Therefore, the SRI detection could show good accuracy and sensitivity by temperature compensation.

In the chain and rope hoist systems of large cranes, instant and monitoring of the strain caused by different loads is of great importance. Strain is a size that cannot be measured directly and whose effect can be measured. Strain can be measured point by point with the help of electronic sensors. However, increasing the number of measuring points causes complexity in electronic systems. Thanks to distributed fiber sensors, this problem has been overcome and standard SMF has been used as tens of thousands of measuring points. In this study, a model of the pulley system used in large cranes is created and the strain occurring at a certain point of a SMF was experimentally measured using the BOTDA technique. SNR was improved by using the wavelet noise reduction technique in the processing of the received signals. In order to monitor the measured strain, a user-friendly interface was designed.

Based on the large evanescent field of microfibers, a novel fiber sensor composed of a microfiber knot resonator and a fiber Bragg grating is proposed for the simultaneous measurement of temperature and refractive index of solutions. Due to the different responses of the microfiber knot resonator and fiber Bragg grating to temperature and refractive index variations, the fiber sensor can measure temperature and refractive index simultaneously by utilizing the sensitivity matrix. The experimental measurement results show that the sensitivities of the microfiber knot resonator and the fiber Bragg grating to temperature are 12.1 pm/°C and 11.6 pm/°C, respectively. The sensitivity of the microfiber knot resonator to the refractive index of solutions is 315 nm/RIU. It is indicated that microfiber knot resonators have a high sensitivity to the refractive index variation of solutions. Considering its small size, low cost, and high stability, the sensor proposed has a prospect in the fields of chemistry and biology.

A cascaded fiber sensor for measuring different curvature ranges based on the cladding mode resonance in double-cladding fiber is proposed. It is formed by double-cladding fiber and single-mode fiber. Based on the evanescent wave coupling principle, the cladding mode resonance phenomenon in double-cladding fiber is illustrated. The transmission spectrum with different length are also analyzed. The experimental results show that the resonance wavelength changes with the length of double-cladding fiber. When the fiber length is 22 mm, the transmission spectrum is blue-shifted as the curvature increases in minor range of 0–0.204 m−1, the corresponding curvature sensitivity can be up to −19376.48 pm/m−1. When the fiber length is 20 mm, the corresponding curvature sensitivity can be up to −4161.73 pm/m−1 as the curvature increases in large range of 0–1.06 m−1. The proposed curvature sensor has the characteristics of simple structure and high sensitivity, so it has a good application prospects in large bridges, industrial buildings and other curvature measurement situations.

We have demonstrated a distributed vibration sensor based on phase-sensitive optical time-domain reflectometer (φ-OTDR) system exhibiting immunity to the laser phase noise. Two laser sources with different linewidth and phase noise levels are used in the φ-OTDR system, respectively. Based on the phase noise power spectrum density of both lasers, the laser phase is almost unchanged during an extremely short period of time, hence, the impact of phase noise can be suppressed effectively through phase difference between the Rayleigh scattered light from two adjacent sections of the fiber which define the gauge length. Based on the phase difference method, the external vibration can be located accurately at 41.01 km by the φ-OTDR system incorporating these two lasers. Meanwhile, the average signal-to-noise ratio (SNR) of the retrieved vibration signal by using Laser I is found to be ~37.7 dB, which is comparable to that of ~37.5 dB by using Laser II although the linewidth and the phase noise level of the two lasers are distinct. The obtained results indicate that the phase difference method can enhance the performance of φ-OTDR system with laser phase-noise immunity for distributed vibration sensing, showing potential application in oil-gas pipeline monitoring, perimeter security, and other fields.

First, the temperature effect on the output polarization degree of the fiber Lyot depolarizer was analyzed in this paper theoretically, which shows that when the environment temperature changes from −40℃ to 80℃, the degree of the output light polarization (DOP) of the fiber Lyot depolarizer will increase until about 70℃, then DOP begin to decrease. The further numerical simulations indicate that this result was caused together by thermally induced group birefringence and thermal coefficient of expansion of the polarization fiber. Using our made fiber Lyot depolarizer (DOP≤0.3% when the environment temperature is 20℃), DOP of the fiber Lyot depolarizer was studied at the condition of a variable temperature experimentally, the results are in agreement with the related theoretically results basically, at last, the difference between theoretical and experimental results is discussed, the possible methods to reduce the influence of the temperature on the output DOP of fiber Lyot depolarizer is presented in this paper. It is a good guiding significance to improve the depolarization performance of the fiber Lyot depolarizer in the fields of fiber sensor, fiber gyroscope, fiber Raman amplifier and so on as we believe.

We present the first report on a compact, temperature-insensitive, multi-axial mechanical force sensor based on a single-core antiresonant hollow-core fiber (ARHCF). Single-core antiresonant fibers are inherently few-moded in a short length and show characteristic multimode interference pattern in their transmission spectrum. We report here a simple technique that enhances the interaction between the interfering modes in these fibers, giving rise to up to four-fold increase in the peak-to-peak amplitude of the interference pattern. The enhanced interference pattern is shown to be responsive to external mechanical forces, like longitudinal and transverse strain and curvature, with distinguishable linear responses. Transverse and longitudinal mechanical forces affect different attributes of the interference pattern, making the proposed sensor suitable for their simultaneous sensing. The temperature sensitivity of the sensor is found to be 3.3 pm/°C suggesting negligible thermal crosstalk while measuring the effect of mechanical forces. The sensor has a compact configuration and is inherently insensitive to polarization of light used. National Research Foundation (NRF) Submitted/Accepted version This work was partly supported by National Research Foundation, Singapore, through Quantum Engineering Programme (QEP-P4).

We discuss the novel concept of harmonics of the Vernier effect for optical fiber sensors as a tool to break the limits of conventional optical Vernier effect currently used. The new effect provides enhancements scalable with the harmonic order.

The paper presents an experimental investigation of frequency modulation of different types of single-frequency lasers. Dependences of frequency modulation coefficients on modulating frequency were obtained experimentally and parasitic intensity modulation was estimated. The work is of an applied nature, and can be useful for interferometric fiber optic sensors engineering.

There is an optical interference noise in the conventional Raman-based fiber-optics distributed sensing, which results in a poor temperature resolution performance. In addition, the traditional whole-fiber demodulation principle complicates the operation steps of the system. In this paper, a novel dynamic difference attenuation recognition (DDAR) principle is operated in the DDP scheme (dual demodulation principle) and the SDP scheme (self-demodulation principle) respectively. It not only helps to eliminate the optical interference noise, but also omits the whole-fiber calibration process. In this experiment, a temperature resolution of 0.30 &deg, C (17.0 km) is achieved through using the DDP scheme based on the DDAR principle, and the measurement time can be shortened to 1.5 s. Meanwhile, a temperature resolution of 0.18 &deg, C (17.0 km) is obtained for the SDP scheme under the DDAR principle. The SNR of DDP and DSP schemes can be optimized to 12.82 dB and 13.32 dB by the proposed DDAR technology. Furthermore, the temperature resolution performance under a large temperature measurement range (0&ndash, 1000 &deg, C) is theoretically analyzed. The results indicate that the temperature responsivity for DDP and SDP schemes are parabolic and linear type respectively, which causes the temperature resolution of the two schemes to show a different trend with the change of temperature. The proposed DDAR method also can improve the temperature resolution in such a large temperature measurement range.

The method of a spatial resolution improvement for common phi-OTDR system is proposed. The suggested method allows to decrease the size of acquisition points in the fiber sensor due to the special arrangement of weak Fiber Bragg Gratings (wFBG). The scheme of phi-OTDR doesn’t need any changes, moreover it does not possess the limitation of an acousto-optic modulator switching frequency.

We propose and demonstrate a hybrid fiber-based sensor combining a multimode interference (MMI) structure and a surface plasmon resonance (SPR) structure for simultaneous measurement of temperature and refractive index (RI) of a liquid sample. We configure the MMI structure by connecting a single-mode fiber, a no-core fiber, and a single-mode fiber sequentially. We set up the SPR structure by coating a gold film with a thickness of 50 nm on the surface of the no-core fiber. We measure the sensitivity of RI and the temperature of the MMI and SPR structure, respectively. Then we obtain the coefficient matrix to simultaneously measure the temperature and RI of a liquid sample and obtain the highest RI sensitivity of 2061.6 nm/RIU and temperature sensitivity of 37.9 pm/°C. We verify the feasibility of the sensor in liquid alcohol. The testing results indicate that the proposed sensor and testing method are feasible, accurate, and convenient.

In this paper, an all fiber optic sensor based on a Mach-Zehnder interferometer (MZI) has been proposed for the simultaneous measurement of underwater pressure and temperature, utilizing a dual-mode fiber (DMF) which has been specially designed, and supporting only the LP01 and LP02 modes propagating in the fiber. In this design, an in-line MZI sensor that was constructed by splicing a DMF between two pieces of single mode fibers, shows a critical wavelength (CWL) which exists in the transmission spectrum of the LP01-LP02 mode interference. Since the two peaks, located closest to the CWL (and from both lower and higher wavelengths), shift in opposite directions and show different sensitivities under temperature and water pressure variations, the DMF-MZI sensor is capable of measuring both the water pressure and the temperature simultaneously. The CWL-based interference spectrum is stable with the variation of underwater salinity or impurities seen around the fiber surface and independent of the polarization states of the transmission light. As a result, in the operation of the DMF-MZI sensor, underwater pressure and temperature sensitivities increase significantly, when the peak wavelengths are close to that of the CWL. A theoretical analysis has been developed and used to predict that the sensitivities of this specific DMF-MZI structure which can be further improved by increasing the physical length of the DMF and by adjusting the position of the first left/right peak to be closer to the critical wavelength. This co-located, multi-parameter all-fiber sensor developed in this way and showing relatively high sensitivity is easy to implement in the underwater environment. It does not require a complex shell design and the peaks nearest to a CWL are convenient, allowing easy identification and detection, thereby providing a large measurement range to satisfy the requirements of practical marine and fresh water measurements.

This paper firstly and experimentally demonstrates an in-fiber axial micro-strain sensing head, combined with a Mach-Zehnder interferometer (MZI) based on the concentric multilayer elliptical-core fiber (CMECF). This MZI with a high extinction ratio (about 15 dB) is successfully achieved with a CMECF-single mode fiber-CMECF (CSC) structure. The MZI sensor theory and the resonance demodulation technology are systematically described in this paper. In this CSC structure, two sections of the CMECF have a role as the mode generator and coupler, respectively. LP01 and LP11 even, which have similar excitation coefficients, are two dominated propagating mode groups supported in the CMECF. On account of the distinct dual-mode property, a good stability of this sensor is realized. The detected resonance in the MZI shifts as the axial micro-strain variated due to the strong interaction between higher order modes. High sensitivity of ∼1.78 pm/με is experimentally achieved within the range of 0 με–1250 με, meanwhile, the intensity fluctuation is below 0.38 dB.

A novel fiber sensor composed by two single mode fibers and long period fiber grating based on a photonic crystal fiber prepared by periodic discharge heating has been experimentally investigated t...

In this study, we demonstrate the design and fabrication of a tapered fiber sensor with a sensing application based on a new (high-sensitivity) package. We study the effect of various geometric parameters such as the uniform segment diameter, the uniform segment length, and the incident angle of the optical wave on the tapered fiber sensor sensitivity for evanescent sensing. Our study shows that high sensitivity of the tapered fiber can be realized by optimizing its parameters. In addition, we design a novel packaging of a tapered-fiber sensor, which can efficiently ensure a uniform stress and reduce the cross sensitivity caused by stress in liquid detection using a tapered fiber sensor. We report the experimental results obtained using this tapered fiber sensor for refractive-index measurements. As the refractive index of liquids varies from 1.417 to 1.423, the measurement results show that the refractive-index sensitivity of the tapered fiber sensor is as high as 4860 nm/RIU.

In this paper, a new optical fiber sensor for relative humidity measurements is presented and characterized. The sensor is based on a SnO2 sputtering deposition on a microstructured optical fiber (MOF) low-finesse Fabry-Pérot (FP) sensing head. The feasibility of the device as a breathing sensor is also experimentally demonstrated. The interrogation of the sensing head is carried out by monitoring the Fast Fourier Transform phase variations of the FP interference frequency. This method substitutes the necessity of tracking the optical spectrum peaks or valleys, which can be a handicap when noise or multiple contributions are present: therefore, it is low-sensitive to noise and to artifacts signal amplitude. The sensor shows a linear behavior in a wide relative humidity range (20%–90% relative humidity) in which the sensitivity is 0.14 rad/%; the maximum observed instability is 0.007 rad, whereas the highest hysteresis is 5% RH. The cross correlation with temperature is also considered and a method to lower its influence is proposed. For human breathing measurement, the registered rising and recovery times are 370 ms and 380 ms respectively. The authors are grateful to A. Ortigosa, D. Erro, Dr. M. Bravo and Dr. R.A. Perez-Herrera. We also thank the Spanish Government projects TEC2013-47264-C2-2-R, TEC 2016-76021-C2-1-R, TEC2016-78047-R, TEC2016-79367-C2-2-R, Innocampus and the Cost Action MP 1401, as well as to the AEI/FEDER Funds.

A transverse load fiber sensor based on Mach-Zehnder interferometer constructed by a Bowknot-type taper between a single mode fiber (SMF) and a polarization maintaining fiber (PMF) was proposed. Due to the polarization maintaining fiber’s birefringence, intensities of the two peaks which are corresponding to the fast and slow axis modes changed with the transverse load applied on the PMF. The experimental results showed that the structure with a 2 cm-long PMF has the sensitivities of 104.52 and −102.94 dB/(N/mm) for the fast and slow axis spectral dip wavelengths of 1485 and 1545 nm in the interference pattern, respectively, which are almost 7 times higher than that of the current similar existing transverse load sensor.

Respiration and heartbeat monitoring during sleep is essential in the assessment of personal health. However, conventional monitoring systems are complex, expensive, and uncomfortable. In this Letter, a mat embedded with a macrobending optical fiber sensor is proposed for non-contact vital signs monitoring during sleep based on vibration sensing. Exploiting the whispering gallery mode, a small-core fiber is adopted and pre-bent to enhance sensitivity to subtle vibrations. The mat sensing system can simultaneously detect and track respiration and ballistocardiography. With high vibration sensitivity, high reliability, and good stability, the mat provides a non-contact and low-cost alternative for long-term monitoring of vital signs, especially for daily home use.

A novel refractive index (RI) sensor based on an asymmetrical Mach–Zehnder interferometer (MZI) with two different step-like tapers is proposed. The step-like taper is fabricated by fusion splicing two half tapers with an appropriate offset. By further applying offset and discharging to the last fabricated step-like taper of MZI, influence of taper parameters on interference spectrum is investigated using only one device. This simple technique provides an on-line method to sweep parameters of step-like tapers and speeds up the optimization process of interference spectrum, meanwhile. In RI sensing experiment, the sensor has a high sensitivity of −185.79 nm/RIU (refractive index unit) in the RI range of 1.3333–1.3673.

We demonstrated the refractive index sensing characteristics of a miniature fiber sensor composited by silica-hollow-tube (SHT) and Au-micro-sphere (AmS). The high sensitivity is obtained due to the evanescent field effect existing in the SHT with the inner diameter of ∼2.3 μm and the surface plasmon resonance effect excited on the surface of AmS with the diameter of ∼2 μm. Experimental results indicate that this sensor can continuously measure the glucose concentration in range of 0–60% with a good linearity. The high detection sensitivity up to 8.33 μmol/L (47.33 mW/RIU) enables its ability in determining the glucose concentration in either blood or body fluids. Furthermore, the tiny structure is promise to be integrated into the microchip or other injectable structures, and monitor the glucose concentration in real-time.

Performance parameter of a Bragg fiber waveguide based resonant sensor in presence of a defect layer in cladding regions is theoretically studied. The Bragg fiber waveguide consists of a liquid-core surrounded by alternate high and low refractive indices materials in cladding regions. Reflectivity of the proposed waveguide based resonant sensor is formulated using transfer matrix method for a non-homogeneous multilayer cylindrical system. The waveguide shows a band gap region with a narrow defect mode in the band gap region under the considered wavelength range. Instead of taking a whole band gap as a sensing signal, here the defect peak is taken as the sensing signal. It is observed that the intensity of defect mode is more sensitive for core refractive index than the intensity of traditional band gap region (lobe). This study shows that the higher sensitivity can be achieved by creating the defect at a position in cladding region where the intensity of transmitted light lies between 40% and 90%. Presence of a defect layer is able to increase the detection accuracy of the sensor and, hence increase the overall performance of this sensor.

This article describes experimental tests to determine PM fibers Panda style responses to a thermal source with different initial temperature. The aim of this study was to determine the sensitivity of a polarization maintaining fiber to the radiating heat, and to upgrade the space configuration and time response when using the 635 nm light. The sensitivity of the polarization maintaining fiber during excitation of both polarization modes is the principle of this sensor function. This excitation is caused by temperature change and by absorption of thermal radiation. This mechanism is used as an indicator for detection of temperature field disturbance. This article also provides links to previously published results and compares them to the results in this article.

We present in this paper the first demonstration of a Brillouin distributed sensor integrating a dual-frequency dual-polarization solid-state laser. This system allows the measurement of the Brillouin frequency shift at frequencies of hundreds of MHz which permits to benefit from balanced detection technique and highly sensitive electronics. The measurement results of a distributed Brillouin spectrum of two welded fibers are presented. The measured frequency shift as function of temperature is in agreement with expected values.

We design a scheme that the spectral performance of a sensor can be improved by introducing a Brillouin slow light into it. The experimental results show that spectral sensitivity is enhanced by about 1.145 time.

A multimode interference-based fiber cavity sensor was proposed. It was a single-mode–multimode–single-mode cascaded structure with an inner air cavity. The sensitivities of microcavity-based fiber sensor were theoretically analyzed. The sensitivities of temperature and axial strain were formula derived. A microcavity model was established with COMSOL software to study the sensitivities affecting factors. The transmission modes in cavity wall were analyzed, and the effective refractive indexes of them were simulated. With these simulation results, a conclusion could be obtained that the effective refractive indexes were depended on the mode order and the cavity diameter. The temperature and axial strain sensitivities were decided by the effective refractive indexes, and the size of the cavity was the key factor. The larger the size of the microcavity and the higher of the transmission modes, the higher the sensitivity of the sensor could be obtained. It can be a theoretical guidance to design and fabricate fiber cavity sensors.

Wavefront shaping (WFS) techniques have been used as a powerful tool to control light propagation in complex media, including multimode fibers. In this paper, we propose a new application of WFS for multimode fiber-based sensors. The use of a single multimode fiber alone, without any special fabrication, as a sensor based on the light intensity variations is not an easy task. The twist effect on multimode fiber is used as an example herein. Experimental results show that light intensity through the multimode fiber shows no direct relationship with the twist angle, but the correlation coefficient (CC) of speckle patterns does. Moreover, if WFS is applied to transform the spatially seemingly random light pattern at the exit of the multimode fiber into an optical focus. The focal pattern correlation and intensity both can serve to gauge the twist angle, with doubled measurement range and allowance of using a fast point detector to provide the feedback. With further development, WFS may find potentials to facilitate the development of multimode fiber-based sensors in a variety of scenarios.

We proposed and demonstrated a complex sensor, which combined a long period fiber grating and two fiber Bragg gratings. We used the proposed fiber sensor to detect the surrounding temperature, water level and refractive index of an unknown liquid. The measured parameter range of the temperature, water level and refractive index were 2 0-45°C, 40–140 cm 1.335-1.375, respectively. The measured results have linearly trend with high sensitivity.

We report in this paper the simulation analysis on the simultaneous deployment of Brillouin gain and Brillouin loss in the Golay coded phase-shift pulse Brillouin optical time domain analysis (PSP-BOTDA) fiber optic sensor for improving the sensing distance and at the same time obtaining high spatial resolution measurement. In this technique, the Golay coded pump pulse is alternately frequency modulated for the purpose of generating Stokes and Anti-Stokes scattering. The technique also modulates the coded pump with the return-to-zero (RZ) formats. Simulation analysis has revealed that the simultaneous use of Brillouin gain- and loss and the pulse coding technique in the PSP-BOTDA has improved the dynamic sensing range and the signal-to-noise improvement ratio (SNIR) of the sensor. By performing simulation over 500 m of fiber, we have also successfully demonstrated 10-cm of high spatial resolution measurement with the use of 1ns of coded pulse duration.

Brillouin scattering properties of silica fibers at 850-nm wavelength were experimentally investigated using a Brillouin optical time-domain analysis scheme. In the single-mode 780-HP fiber, the Brillouin frequency shift (BFS) and full width at-half maximum of the Brillouin gain spectrum were found to be ~19795 and 90 MHz, respectively. The estimated peak Brillouin gain is 0.32 (mW) -1 . The temperature and strain dependences of BFS were determined as well. Distributed sensing capability was demonstrated at a minimum spatial resolution of 1.5 m and a fiber length of 100 m. As a proof-of-principle, the temperature and strain dependences of the BFS in SFM-28 fiber have been determined at 850- and 1550-nm wavelengths, suggesting its possible use for temperature/strain discrimination.

We report distributed temperature measurements based on Raman scattering performed during steady state $\gamma $ -ray irradiation at a dose rate of 1 kGy( ${\rm SiO}_{2}$ )/h and up to a total ionizing dose (TID) of $\sim 0.1\ \hbox{MGy}$ . We characterize on-line the evolution of the performances of a single-ended Raman distributed temperature sensor (RDTS) during the $\gamma $ -ray exposure of different classes of commercial multimode fibers (MMFs) acting as the sensing element. RDTS is influenced by the radiation-induced attenuation (RIA) phenomena leading to both large errors in the temperature measurements and a diminution of the useful sensing length. The amplitude of the radiation-induced temperature error strongly depends on the fiber choice and on the irradiation conditions. For the single-ended RDTS operation in the targeted Cigeo application the selection of a radiation tolerant sensing fiber will be mandatory, but not sufficient, to overcome the expected severe ambient conditions around radioactive wastes. For efficient temperature sensing up to an accumulated dose of 0.1 MGy, pre-irradiation of the selected radiation resistant (RR) fibers appears also necessary to improve the sensor performances.

An in-line fiber sensor for simultaneous measurement of refractive index (RI) and temperature is proposed and demonstrated. The sensor head is composed of an asymmetric Mach–Zehnder interferometer (MZI), which is combining the single mode-multimode-single mode (SMS) fiber structure and a peanut-shape structure. The transmission dips of the sensor will shift as the ambient RI or temperature variation due to the phase changing. The different wavelength transmission dips formed by the cladding modes interfering with the core mode have different sensitivity responses, so the simultaneous measurement of the RI and temperature can be achieved by monitoring the wavelength shifts of the two transmission dips. The experimental results show that the sensing sensitivities of RI and temperature are −86.7434 nm/RIU and 0.0590 nm/°C in the RI range from 1.3105 to 1.3517 and temperature range from 25 °C to 85 °C, respectively. The novel sensor processes easy fabrication, low cost, and high sensitivity, making it offers high potential applications in physical, biological and chemical sensing.

A compact refractive index sensor is proposed and investigated based on multimode fiber-micro tapered single-mode fiber-multimode fiber (MTSM) structure. The micro tapered single-mode fiber excites the high-order cladding mode and induces a compact size of 2.2 mm. The influence of multimode fiber (MMF) for the interference fringe is studied numerically and verified experimentally. The experimental and simulated results show that the appropriate MMF length is conducive to form a clear interference fringe. At the same time, the results of refractive index test show that the maximal sensitivity reaches 1879.87 nm/RIU under external refractive index of 1.437. Due to the advantages of small size and high sensitivity, the novel fiber sensor has the potential in the application of refractive index sensing.

Based on the large evanescent field of microfibers, a novel fiber sensor using a microfiber knot resonator embedded in a Hi-Bi fiber loop mirror is proposed for simultaneous measurement of the temperature and refractive index of solutions. Due to the different responses of the microfiber knot resonator and fiber loop mirror to temperature and refractive index variations, the fiber sensor can measure temperature and refractive index simultaneously by utilizing the sensitivity matrix. The experimental measurements show that the sensitivities of the microfiber knot resonator and the fiber loop mirror to temperature are 5.6 pm/°C and −464.1 pm/°C, respectively. The sensitivity of the microfiber knot resonator to the refractive index of dilute solutions is 116 nm/RIU. The sensor has a higher sensitivity to refractive index variations than LPGs and has potential for use in the field of biochemistry.

This paper presents a description of engineering solutions and physical diagram of a device meant to measure spatially resolved time form of optical signals with detection accuracy up to 0.1 ns and a scalable number of monitoring points. It has been demonstrated that the number of simultaneously detected optical signals within the limit is up to one million. There are no moving mechanical units in the device design. A method for reducing the total length of the optical fiber based on the principle of the cascade optical delay is proposed for the multichannel sensor of diversification. The number of measured signals reaches a million. The capabilities and constraints of the described engineering solution for measuring the pulse form and monitoring the diversification of the multicomponent optical signals have been demonstrated. The principle of calculating a multichannel fiber device for recording optical pulses is presented and shown that the fiber length is up to 15 km for the detection system of 128 optical signals 100 ns long each. Generally, the number of measured signals may reach a million.

We observed that the sensitivity of a fiber sensor is changed by introducing fast and slow light. In this fiber sensor configuration, fast and slow light are generated by a double Lorenz fiber Bragg grating (DL-FBG) and the probe pulse through DL-FBG will have different delays by varying the input laser wavelength. The maximum delay is 180 ps (slow light) and the maximum advance is 90 ps (fast light). Experimental result shows that the fiber sensor sensitivity obtains enhancement by introducing slow light, while the sensitivity of fiber sensor decreases when combining with the fast light.

A new type of fiber interferometer based on two short thin-core fiber insertions with a section of standard fiber in between is proposed. The thin-core fiber insertions excite cladding modes and provide energy exchange between different fiber modes. We have simulated and measured transmission through a single thin-core fiber section. The standard fiber section is used to accumulate phase differences between core and cladding modes propagating through the structure. Transmission spectra are measured for various lengths of the interferometer. Separate control of thin-core and standard fiber insertions adds flexibility in obtaining spectra desirable for sensing applications.

An optical fiber temperature and strain fiber sensor based on the few-mode fiber (FMF) and the fiber Bragg grating (FBG) is proposed and demonstrated. The sensor unit is fabricated by a length of FMF offset splicing with one section of single-mode fiber (SMF). The temperature and strain are measured simultaneously because of the different sensitivities of the spectrum dips. The interference with the highest extinction ratio of 23 dB can be achieved. The temperature sensitivities of -34.3 pm ∕ ° C and 10.7 pm ∕ ° C and strain sensitivities of -2 pm ∕ μ e and 0.67 pm ∕ μ e are achieved, respectively. The proposed sensor may exhibit the great potential in fields of the dual-parameter measurement due to its compact structure, simple configuration.

Until now, a high-efficiency demodulation method for fiber Bragg grating (FBG) sensors has been a challenge. In this Letter, by employing multi-peak FBGs, an FBG sensor with a partial wavelength scan is proposed and initially demonstrated. By demodulating a near-symmetrical multi-peak FBG and an asymmetrical multi-peak FBG in the strain experiment, sensor sensitivities of 1.02 pm/μe and 1.01 pm/μe are measured for the interrogation system, respectively. The average demodulation deviations for the two sensors are 1.81% and 0.4%, respectively. The proposed method is expected to realize high-efficiency and low-cost FBG interrogators.

An approach to eliminating the effect of polarization pulling in Golay-coded Brillouin optical time domain analysis (BOTDA) fiber sensor by employing polarization division multiplexing (PDM) pulse coding is presented. Three different schemes, including polarization switching, polarization scrambling, and PDM pulse coding are implemented and compared. Experimental results indicate that sensing stability is enhanced by ~2.5 times using the proposed scheme.

International audience; This paper reports the testing results of radiation resistant fiber Bragg grating (FBG) in random air-line (RAL) fibers in comparison with FBGs in other radiation-hardened fibers. FBGs in RAL fibers were fabricated by 80 fs ultrafast laser pulse using a phase mask approach. The fiber Bragg gratings tests were carried out in the core region of a 6 MW MIT research reactor (MITR) at a steady temperature above 600°C and an average fast neutron (>1 MeV) flux >1.2 × 10$^{14}$ n/cm$^2$/s. Fifty five-day tests of FBG sensors showed less than 5 dB reduction in FBG peak strength after over 1 × 10$^{20}$ n/cm$^2$ of accumulated fast neutron dose. The radiation-induced compaction of FBG sensors produced less than 5.5 nm FBG wavelength shift toward shorter wavelength. To test temporal responses of FBG sensors, a number of reactor anomaly events were artificially created to abruptly change reactor power, temperature, and neutron flux over short periods of time. The thermal sensitivity and temporal responses of FBGs were determined at different accumulated doses of neutron flux. Results presented in this paper reveal that temperature-stable Type-II FBGs fabricated in radiation-hardened fibers can survive harsh in-pile conditions. Despite large parameter drift induced by strong nuclear radiation, further engineering and innovation on both optical fibers and fiber devices could lead to useful fiber sensors for various in-pile measurements to improve safety and efficiency of existing and next generation nuclear reactors.

Dual cladding modes fiber interferometer is based on the interferences between the core mode and two cladding modes. Here its principle and application for dual parameters measurement are presented.

We present a novel two-dimensional fiber-optic inclinometer with high sensitivity by crisscrossing two etched chirped fiber Bragg gratings (CFBG) arrays. Each array is composed of two symmetrically-arranged CFBGs. By etching away most of the claddings of the CFBGs to expose the evanescent wave, the reflection spectra are highly sensitive to the surrounding index change. When we immerse only part of the CFBG in liquid, the effective index difference induces a superposition peak in the refection spectrum. By interrogating the peak wavelengths of the CFBGs, we can deduce the tilt angle and direction simultaneously. The inclinometer has a resolution of 0.003° in tilt angle measurement and 0.00187 rad in tilt direction measurement. Due to the unique sensing mechanism, the sensor is temperature insensitive. This sensor can be useful in long term continuous monitoring of inclination or in real-time feedback control of tilt angles, especially in harsh environments with violent temperature variation.