Intensity-interrogated hot-wire anemometer based on chirp effect of a fiber Bragg grating.

An intensity-interrogated optical fiber hot-wire anemometer based on the chirp effect of fiber Bragg grating (FBG) is presented. The FBG is coated with a silver film and heated optically by a 1480 nm laser beam, which is coupled into the fiber cladding by a long-period grating (LPG) and absorbed by the silver film to convert to thermal heat. Due to the gradual decrease of laser power along the length of the FBG, a temperature gradient is formed that induces a chirp effect to the FBG. Bandwidth of the FBG's reflection spectrum is therefore broadened that increases its reflected light power. The chirp rate of the FBG reduces with airflow velocity since the temperature gradient is weakened under the cooling effect of the airflow, resulting in a certain relationship between the reflected power of the FBG and airflow velocity. In the experiment, by detecting the reflected power of the FBG, airflow velocity measurement is achieved successfully with a high sensitivity up to -28.60 µW/(m·s-1) at airflow velocity of 0.1 m/s and a dynamic response time of under one second. The measurement range is up to 0 to 11 m/s. The intensity interrogation scheme of the FBG hot-wire anemometer reduces its cost greatly and makes it a promising solution for airflow velocity measurement in practical applications.

Ring-core few-mode fiber and DPP-BOTDA-based distributed large-curvature sensing eligible for shape reconstruction.

This study proposes a distributed large-curvature sensor based on ring-core few-mode fiber (RC-FMF) and differential pulse-pair Brillouin optical time-domain analysis (DPP-BOTDA). The RC-FMF is adhered to a thin steel substrate and an asymmetric hump shape is reconstructed using the Frenet-Serret algorithm. The proposed curvature sensor demonstrates a larger curvature-sensing range, excellent tolerance to bending-induced optical loss, and increased Brillouin gain coefficient. The proposed sensor also demonstrates longer sensing distance and continuous absolute measurement compared to other sensors. The proposed model can be applied to the end tracking of soft robotics and structural health monitoring of civil infrastructures.

Optical Fiber Interferometric Humidity Sensor by Using Hollow Core Fiber Interacting with Gelatin Film.

An optical fiber Fabry-Perot interferometer (FPI) is constructed for relative humidity measurement by fusion splicing a short hollow core fiber (HCF) to the end of a single-mode fiber and coating the tip of the HCF with a layer of gelatin. The thickness of the gelatin film changes with ambient humidity level and modulates cavity length of the FPI. Humidity measurement is therefore realized by measuring the wavelength shift of the interreference fringe. RH sensitivity of 0.192 nm/%RH is achieved within a measurement range of 20-80%RH. Dynamic measurement shows a response and recovery time of 240 and 350 ms, respectively. Sensor performance testing shows good repeatability and stability at room temperature but also reveals slight dependence of the RH sensitivity on environmental temperature. Therefore, a fiber Bragg grating is cascaded to the FPI sensing probe to monitor temperature simultaneously with temperature sensitivity of 10 pm/°C.

Distributed refractive index sensing based on bending-induced multimodal interference and Rayleigh backscattering spectrum.

A distributed refractive index (RI) sensor based on high-performance optical frequency domain reflectometry was developed by bending a piece of standard single-mode fiber to excite sets of higher-order modes that penetrate the surrounding medium. External variations in RI modifies the profiles of the sets of excited higher-order modes, which are then partially coupled back into the fiber core and interfere with the fundamental mode. Accordingly, the fundamental mode carries the outer varied RI information, and RI sensing can be achieved by monitoring the wavelength shift of the local Rayleigh backscattered spectra. In the experiment, an RI sensitivity of 39.08 nm/RIU was achieved by bending a single-mode fiber to a radius of 4 mm. Additionally, the proposed sensor maintains its buffer coating intact, which boosts its practicability and application adaptability.

Bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber.

We developed a bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber (Er-FMF) and differential pulse-width pair Brillouin optical time-domain analysis. With Er ion amplification compensating for bending-induced optical loss, radii in the ∼0.3 to 2.02 cm range could be monitored correctly. The corresponding Brillouin frequency shift variations were in the range of 91.7 to 9 MHz, which agrees well with theoretical calculations. The sensitivity of the Er-FMF device increased inversely with the bending radius, with the optimal sensitivity of 292.7 MHz/cm obtained at a radius of 0.3 cm. To the best of our knowledge, this is the smallest radius of curvature detected and highest sensitivity reported to date, indicating potential applications in distributed sharp-bend sensing, such as intelligent robotics and structural health monitoring.

Investigation of the effect of gold coating of gold-coated fiber on distributed strain measurement by differential pulse pair Brillouin optical-time analysis.

Gold-coated fiber (GCF) shows the potential to sense strain at high temperature owing to the hermeticity of gold coating that prevents hydrogen penetration. Nevertheless, there are trivial details of the gold coating of GCF that need to be addressed before using GCF to measure strain at high temperature. In this study, we thoroughly investigate the effect of the gold coating of GCF on strain measurement both at room temperature and high temperature up to 700°C with differential pulse pair Brillouin optical-time analysis (DPP-BOTDA). Owing to the inhomogeneity of gold coating induced by the manufacturing process, it is necessary to select the GCF with the gold coating of better homogeneity via DPP-BOTDA. Due to the residual stress that solidified in the GCF during the cooling process of coating, the GCF would first undergo plastic deformation and then elastic deformation in the strain measurement. After one-time strain measurement to remove the residual stress of GCF, the standard deviation of the strain coefficients at room temperature and high temperature are $ \pm {0.5}\% $±0.5% and $ \pm {1.3}\% $±1.3%, respectively, which is mainly due to the nonuniform thickness of the gold coating and the nonuniformity of silica fiber at high temperature.

Vernier effect of two cascaded in-fiber Mach-Zehnder interferometers based on a spherical-shaped structure.

The Vernier effect of two cascaded in-fiber Mach-Zehnder interferometers (MZIs) based on a spherical-shaped structure has been investigated. The envelope based on the Vernier effect is actually formed by a frequency component of the superimposed spectrum, and the frequency value is determined by the subtraction between the optical path differences of two cascaded MZIs. A method based on band-pass filtering is put forward to extract the envelope efficiently; strain and curvature measurements are carried out to verify the validity of the method. The results show that the strain and curvature sensitivities are enhanced to -8.47 pm/µÎµ and -33.70 nm/m-1 with magnification factors of 5.4 and -5.4, respectively. The detection limit of the sensors with the Vernier effect is also discussed.

[All-Fiber Mach-Zehnder Interferometer Based on Lateral-Offset and Peanut Shape Structure].

A new in-fiber Mach-Zehnder interferometer (MZI) based on lateral-offset and peanut shape structure is proposed and demonstrated for the measure of curvature and liquid level. The sensor consists of lateral-offset structure and peanut shape structure. A section of single mode fiber (SMF) is spliced between them. A part of core mode in the single mode fiber is excited to cladding modes by lateral-offset. The cladding modes are re-coupled to the core mode by peanut-shape structure and get interference with the core mode. A high-quality interference spectrum with a fringe visibility of about 12 dB is observed. The effective refractive indices of cladding mode would change with the external environment parameters, which further bring about a shift of the interference fringes. The liquid level or curvature can be measured by record the shift of the valley, because the shift of the valley shows a linear dependence with the variation of the liquid level or curvature theoretically. In the water level experiment, the water level changes from 1.00 to 5.00 cm and the wavelength valley shows a red shift. The sensitivity of the MZI with a length of 6.10 cm is -0.68 nm·cm-1. In the curvature experiment, the curvature changes from 0.3 to 1.2 and the wavelength valley shows a blue shift. The sensitivity of the MZI with a length of 2.10 cm is 22.47 nm·m. The lateral-offset structure and peanut shape structure are spliced to fabricate the MZI. The sensitivity of the MZI is high, especially in the curvature measurement, it is higher than that of other fiber curvature sensors. Moreover the MZI presented in this paper has advantages of low cost and easy fabrication, which can be a potential application in the liquid level and curvature measurement.

Miniature temperature sensor with germania-core optical fiber.

A miniature all-fiber temperature sensor is demonstrated by using a Michelson interferometer formed with a short length of Germania-core, silica-cladding optical fiber (Ge-fiber) fusion-spliced to a conventional single-mode fiber (SMF). Thanks to the large differential refractive index of the Ge-fiber sensing element, a reasonably small free spectral range (FSR) of 18.6 nm is achieved even with an as short as 0.9 mm Ge-fiber that may help us increase the measurement accuracy especially in point sensing applications and, at the same time, keep large measurement temperature range without overlapping reading problem. Experimental results show that high sensitivity of 89.0 pm/°C is achieved and the highest measurement temperature is up to 500°C.

All-fiber multiwavelength thulium-doped laser assisted by four-wave mixing in highly germania-doped fiber.

An all-fiber multiwavelength Tm-doped laser assisted by four-wave mixing (FWM) in highly Germania-doped highly nonlinear fiber (HG-HNLF) has been experimentally demonstrated. Benefiting from the high nonlinearity of the HG-HNLF, intensity-dependent gain caused by FWM is introduced into the laser cavity to mitigate the gain competition in Tm-doped fiber. Thanks to a 50-m HG-HNLF, 9, 22, and 36 lasing lines with considering 10-dB, 20-dB, and 30-dB bandwidth, respectively is obtained at room temperature with wavelength spacing of 0.86 nm. More than 30-nm broad-band lasing can be obtained. The stability of the proposed fiber laser has also been studied. Repeat measurements show the power fluctuations and wavelength drifts of the lasing lines are less than 1.6 dB and 0.05 nm, respectively. The laser performances without the assistance of HG-HNLF have fewer center wavelengths lasing, which indicates that FWM in HG-HNLF plays an important role for the multiwavelength laser operation.

Intensity-modulated relative humidity sensing with polyvinyl alcohol coating and optical fiber gratings.

A relative humidity (RH) sensor in reflection mode is proposed and experimentally demonstrated by using a polyvinyl alcohol (PVA)-coated tilted-fiber Bragg grating (TFBG) cascaded by a reflection-band-matched chirped-fiber Bragg grating (CFBG). The sensing principle is based on the RH-dependent refractive index of the PVA coating, which modulates the transmission function of the TFBG. The CFBG is properly designed to reflect a broadband of light spectrally suited at the cladding mode resonance region of the TFBG, thus the reflected optical signal passes through and is modulated by the TFBG again. As a result, RH measurements with enhanced sensitivity of ∼1.80 µW/%RH are realized and demodulated in the range from 20% RH to 85% RH.

Photonic crystal fiber interferometric pH sensor based on polyvinyl alcohol/polyacrylic acid hydrogel coating.

We present a simple photonic crystal fiber interferometer (PCFI) that operates in reflection mode for pH measurement. The sensor is made by coating polyvinyl alcohol/polyacrylic acid (PVA/PAA) hydrogel onto the surface of the PCFI, constructed by splicing a stub of PCF at the distal end of a single-mode fiber with its free end airhole collapsed. The experimental results demonstrate a high average sensitivity of 0.9 nm/pH unit for the 11 wt.% PVA/PAA coated sensor in the pH range from 2.5 to 6.5. The sensor also displays high repeatability and stability and low cross-sensitivity to temperature. Fast, reversible rise and fall times of 12 s and 18 s, respectively, are achieved for the sensor time response.

Optical fiber axial micro-displacement sensor based on Mach-Zehnder interferometer.

A Mach-Zehnder interferometer (MZI) based fiber axial micro-displacement sensor was proposed. The MZI was constructed by a bowknot-type taper (BTT) combining with a fiber core-offset between two single mode fibers (SMFs). The axial micro-displacement of the core offset is correlated with the MZI transmission spectrum and varied with the interferometer arm length. For the arm length L of 12, 18, 24 and 30 mm, the proposed sensors showed high sensitivity of -0.362 dB/µm, -0.385 dB/µm, -0.332 dB/µm and -0.235dB/µm, and temperature errors of -0.056 dB/°C, -0.036 dB/°C, -0.044 dB/°C, -0.048 dB/°C, respectively. The theoretical simulations of the energy distributions were also given. The obtained sensitivity of -0.385 dB/µm is about 150 times high than that of the current similar existing axial micro-displacement sensor.

Hollow fiber-based Fabry-Perot cavity for liquid surface tension measurement.

A Fabry-Perot (FP) cavity, formed by a single-mode fiber (SMF) fusion spliced with a section of hollow fiber, was proposed and experimentally used as a sensor for liquid surface tension measurement. Due to the FP cavity length changing with different liquids, the surface tension can be measured by monitoring the interference wavelength spacing. In our experiment, five kinds of liquids were measured, and the relationship between wavelength spacing and liquid surface tension was obtained. The experimental results show that the achieved sensitivity to liquid surface tension is 30.96 nm/(N/m), and the maximum error is 5.43%.

High-sensitivity temperature sensor based on a droplet-like fiber circle.

A low-cost yet high-sensitivity temperature fiber sensor is proposed and demonstrated in this paper. A single-mode fiber with coating is simply bent in a droplet-like circle with a radius of several millimeters. The strong bending induces mode interferences between the silica core mode and the excited modes propagating in the polymer coating. Many resonant dips were observed in the transmission spectra and are found to shift to a shorter wavelength with the increase of environmental temperature. Our linear fitting result of the experimental data shows that the proposed sensor presents high temperature sensitivity up to -3.102 nm/°C, which is even comparable with sensors based on selective liquid-filled photonic crystal fibers. Such high temperature sensitivity results from the large thermo-optical coefficient difference between the silica core and the polymer coating. The influence of a circle radius to the sensitivities is also discussed.

Liquid seal for temperature sensing with fiber-optic refractometers.

Liquid sealing is an effective method to convert a fiber-optic refractometer into a simple and highly sensitive temperature sensor. A refractometer based on the thin-core fiber modal interferometer is sealed in a capillary tube filled with Cargille oil. Due to the thermo-optic effect of the sealing liquid, the high refractive-index sensitivity refractometer is subsequently sensitive to the ambient temperature. It is found that the liquid-sealed sensor produces a highest sensitivity of -2.30 nm/°C, which is over 250 times higher than its intrinsic sensitivity before sealing and significantly higher than that of a grating-based fiber sensors. The sensing mechanisms, including the incidental temperature-induced strain effect, are analyzed in detail both theoretically and experimentally. The liquid sealing technique is easy and low cost, and makes the sensor robust and insensitive to the surrounding refractive index. It can be applied to other fiber-optic refractometers for temperature sensing.

Magnetic field sensor using tilted fiber grating interacting with magnetic fluid.

A novel magnetic field sensor using tilted fiber Bragg grating (TFBG) interacting with magnetic fluid is proposed and experimentally demonstrated. The TFBG is surrounded by magnetic fluid whose complex refractive index changes with external magnetic field. The guiding properties of cladding modes excited by the TFBG are therefore modulated by the external magnetic field. As a result, the magnetic field strength measurement is successfully achieved within a range up to 196 Gauss by monitoring extinction ratio of cladding mode resonance. Furthermore, temperature variation can be obtained simultaneously from the wavelength shift of the TFBG transmission spectrum.

Polarization-dependent curvature sensor based on an in-fiber Mach-Zehnder interferometer with a difference arithmetic demodulation method.

A curvature sensor based on a polarization-dependent in-fiber Mach-Zehnder interferometer (MZI) is proposed. The MZI is fabricated by core-offset fusion splicing one section of polarization maintaining fiber (PMF) between two single mode fibers (SMFs). Two independent interference patterns corresponding to the two orthogonal polarization modes for the PMF are obtained. The couple efficiency between the core mode and the cladding mode decreased with the increasing of the bending on the MZI part. The curvature variation on the MZI part can be obtained by detecting the fringe visibility of the interference patterns. A difference arithmetic demodulation method is used to reduce the effects of the light source power fluctuations and temperature cross-sensitivity. Experimental results show that maximal sensitivity of -0.882 dB/m(-1) is obtained under a measurement range of 0.1 to 0.35 m(-1) for the curvature sensor. With the use of difference arithmetic demodulation method, the temperature-curvature cross-sensitivity and light source power fluctuations effects on the proposed sensor are decreased by 94% and 91%, respectively.

Acoustic vibration sensor based on nonadiabatic tapered fibers.

A simple and low-cost vibration sensor based on single-mode nonadiabatic fiber tapers is proposed and demonstrated. The environmental vibrations can be detected by demodulating the transmission loss of the nonadiabatic fiber taper. Theoretical simulations show that the transmission loss is related to the microbending of the fiber taper induced by vibrations. Unlike interferometric sensors, this vibration sensor does not need any feedback loop to control the quadrature point to obtain a stable performance. In addition, it has no requirement for the coherence of the light source and is insensitive to temperature changes. Experimental results show that this sensing system has a wide frequency response range from a few hertz to tens of kilohertz with the maximal signal to noise ratio up to 73 dB.

Magneto-optical fiber sensor based on magnetic fluid.

A novel magnetic field fiber sensor based on magnetic fluid is proposed. The sensor is configured as a Sagnac interferometer structure with a magnetic fluid film and a section of polarization maintaining fiber inserted into the fiber loop to produce a sinusoidal interference spectrum for measurement. The output interference spectrum is shifted as the change of the applied magnetic field strength with a sensitivity of 16.7 pm/Oe and a resolution of 0.60 Oe. The output optical power is varied with the change of the applied magnetic field strength with a sensitivity of 0.3998 dB/Oe.