Quasi-distributed optical fiber hydrogen leakage detecting system based on bus chain topology structure.

Micro-mirror optical fiber hydrogen sensors have the advantages of compact structure and fast demodulation speed. All-optical sensor networks consisting of micro-mirror optical fiber hydrogen sensors are essentially necessary across the hydrogen value chain. A bus chain topology structure hydrogen leakage detecting system based on micro-mirror sensors is proposed and experimentally demonstrated. A compensating optical path with constant power supply is introduced, and the power dissipation scheme is theoretically and experimentally proposed by designating the splitting ratios of the splitters array. By constructing such a network with twenty micro-mirror hydrogen sensors, the system has been experimentally verified with good repeatability and stability under different concentrations of hydrogen. By employing this bus chain topology strategy, a quasi-distributed optical fiber hydrogen leakage detection system with micro-mirror hydrogen sensors array is realized. It will provide a promising solution with high feasibility on multi-point leakage detecting in hydrogen infrastructures.

Antiresonant fiber-enhanced Raman spectroscopy gas sensing with 1†ppm sensitivity.

Antiresonant hollow-core fiber (AR-HCF) exhibits unprecedented optical performance in low transmission attenuation, broad transmission bandwidth, and single spatial mode quality. However, due to its lower numerical aperture, when utilizing the Fiber-Enhanced Raman Spectroscopy (FERS) principle for gas detection, the efficiency of AR-HCF in collecting Raman signals per unit length is significantly lower than that of hollow-core photonic crystal fiber. Nonetheless, AR-HCF effectively suppresses higher-order modes and offers bandwidth in hundreds of nanometers. By increasing the length of AR-HCF, its advantages can be effectively harnessed, leading to a considerable enhancement in the system's ability for low-concentration gas detection. We combine the nodeless antiresonant hollow-core fiber and Raman spectroscopy for enhanced Raman gas sensing in a forward scattering measurement configuration to investigate the attenuation behavior of the silica background signals. The silica background attenuation behavior enables the low baseline of the gas Raman spectroscopy and extends the integration time of the system. In addition, a convenient spatial filtering method is investigated. A multimode fiber with a suitable core diameter was employed to transmit the signal so that the fiber end face plays the role of pinhole, thus filtering the silica signal and reducing the baseline. The natural isotopes 12C16O2, 13C16O2, and 12C18O16O in ambient air can be observed using a 5-meter-long AR-HCF at 1 bar with a laser output power of 1.8 W and an integration time of 300 seconds. Limits of detection have been determined to be 0.5 ppm for 13C16O2 and 1.2 ppm for 12C16O2, which shows that the FERS with AR-HCF has remarkable potential for isotopes and multigas sensing.

Improved performance of a fiber-optic hydrogen sensor based on a controllable optical heating technology.

A novel, to the best of our knowledge, and compact fiber-optic hydrogen sensor based on light intensity demodulation and controllable optical heating technology is proposed and experimentally investigated. This system employs three photodetectors for optic signal transformation. The first PD is used to receive a little fraction of the amplified spontaneous emission (ASE) for calibration, and the second PD is utilized to detect optic signal reflected by a single mode fiber deposited with WO3-Pd2Pt-Pt composite film. The last PD is utilized to receive the optical power reflected by the short fiber Bragg grating (SFBG) with a central wavelength located in a steep wavelength range (the intensity decreases approximately linearly with the increase of the wavelength) of the ASE light source. A 980 nm laser and proportion integration differentiation (PID) controller were employed to ensure the hydrogen sensitive film working at an operating temperature of 60°C. This sensing system can display a quick response time of 0.4 s toward 10,000 ppm hydrogen in air. In addition, the detection limit of 5 ppm in air can be achieved with this sensing system. The stability of this sensor can be greatly enhanced with a controllable optical heating system, which can greatly promote its potential application in various fields.

Simultaneous distributed acoustic and temperature sensing system based on ultra-weak chirped fiber Bragg grating array.

The quasi-static temperature and dynamic acoustic signals based on ultra-weak chirped fiber Bragg grating (CFBG) array is proposed and experimentally demonstrated, which can be employed to detect distributed acoustic and temperature signals simultaneously. Distributed temperature sensing (DTS) was realized by cross-correlation operation to measure the spectral drift of each CFBG, and distributed acoustic sensing (DAS) was achieved by gauging the phase difference between adjacent CFBG. Using CFBG as the sensor unit can protect acoustic signals from the fluctuations and drifts induced by temperature change without deterioration of signal-to-noise ratio (SNR). Applying least square mean adaptive filter (AF) can enhance harmonic frequency suppression ratio and increase the SNR of system. In the proof-of-concept experiment the SNR of acoustic signal is higher than 100 dB after the digital filter and frequency response is from 2 Hz to 1.25 kHz with repetition frequency of laser pulses of 10 kHz. Temperature measurement from 30°C to 100°C is achieved with demodulation accuracy of ±0.8°C. The spatial resolution (SR) of two-parameter sensing is 5 m.

Temperature-decoupled hydrogen sensing with Pi-shifted fiber Bragg gratings and a partial palladium coating.

A novel, to the best of our knowledge, sensor architecture for palladium-coated fiber Bragg gratings is proposed and demonstrated that allows highly accurate multi-parameter sensing and decoupling of hydrogen concentration from temperature. By means of partly Pd-coated Pi-shifted FBGs (PSFBGs), the notch wavelength of the narrow transmission band and the flank wavelength of the broader reflection band experience different hydrogen and temperature sensitivities. PSFBGs were calibrated at hydrogen concentrations between 800 and 10,000 ppm and temperatures from 20 to 40°C, and a decreased hydrogen sensitivity at increased temperatures was found. Nonlinear temperature-dependent hydrogen calibration functions were therefore determined. An iterative matrix algorithm was used to decouple hydrogen concentration and temperature and to account for the nonlinear calibration functions. Achieved improvements and results have great importance for real field applications of FBG-based hydrogen sensing.

Large-capacity and long-distance distributed acoustic sensing based on an ultra-weak fiber Bragg grating array with an optimized pulsed optical power arrangement.

A large-capacity, long-distance distributed acoustic sensing (DAS) system without inline optical amplification was proposed and experimentally demonstrated using an ultra-weak fiber Bragg grating (UWFBG) array and coherent detection. The effect of the finite extinction ratio of an acousto-optic modulator and the Stokes signal of stimulated Brillouin scattering (SBS) in UWFBGs on the performance of DAS was simulated and revealed. A high extinction ratio and a balanced input pulsed optical power can improve the capacity and distance of the DAS. The dynamic acoustic signal can be well reconstructed for a serial array of 10828 near-identical UWFBG with a length of 54.14 km. An acoustic signal sensitivity of 189.54 pɛ/√Hz and a signal SNR of 40.01 dB with a spatial resolution of 5 m can be achieved at the far end.

Highly sensitive hydrogen sensor based on an in-fiber Mach-Zehnder interferometer with polymer infiltration and Pt-loaded WO3 coating.

A highly sensitive fiberized hydrogen sensor based upon Mach-Zehnder interference (MZI) is experimentally demonstrated. The hydrogen sensor consists of an MZI realized by creating an air cavity inside the core of a half-pitch graded-index fiber (GIF) by use of femtosecond laser micromachining. Thermosensitive polymer was filled into the air cavity and cured by UV illumination. Subsequently, the external surface of the polymer-filled MZI was coated with Pt-loaded tungsten trioxide (WO3). The exothermic reaction occurs as Pt-loaded WO3 contacts the target of the sensing, i.e. hydrogen in the atmosphere, which leads to a significant local temperature rise on the external surface of the coated MZI sensor. The sensor exhibits a maximum sensitivity up to -1948.68 nm/% (vol %), when the hydrogen concentration increases from 0% to 0.8% at room temperature. Moreover, the sensor exhibits a rapid rising response time (hydrogen concentration increasing) of ∼38 s and falling response time (hydrogen concentration decreasing) of ∼15 s, respectively. Thanks to its small size, strong robustness, high accuracy and repeatability, the proposed in-fiber MZI hydrogen sensor will be a promising tool for hydrogen leakage tracing in many areas, such as safety production and hydrogen medical treatment.

Tapered multicore fiber interferometer for ultra-sensitive temperature sensing with thermo-optical materials.

An in-line interferometer based on tapered multicore embedded into a flexible thermo-optical material is proposed and investigated, theoretically and experimentally. The device consists of a tapered multicore fiber spliced between two single-mode fibers covered with PDMS, with high thermo-optic coefficient. The temperature sensitivity improvement obtained from PDMS applied on a tapered multicore fiber (TMCF) interferometer has been fundamentally and experimentally verified. The experimental results show the temperature sensitivity can be improved by reducing the tapered waist diameter of TMCF. The sensor exhibits the high sensitivity of 5-25 nm/°C within the decreasing temperature range from 50 °C down to 10 °C. A sequence of simulations and corresponding experiments are performed to clarify the evolution of the interference fading and consequently build the criteria for sensor design and reachable lower limit of temperature sensing. The proposed sensor can be employed as photonic thermometer with ultra-high sensitivity for biological and deep-sea applications, particularly based on the claimed quantitative criteria.

Simultaneously distributed temperature and dynamic strain sensing based on a hybrid ultra-weak fiber grating array.

A fiber-optic sensing system based on two types of ultra-weak fiber Bragg gratings (UWFBG) for simultaneous temperature and vibration sensing was proposed. Narrowband and broadband UWFBGs are alternately written into an optical fiber with equal spacing. Distributed temperature sensing is realized by demodulating the wavelength shift of the narrowband UWFBG, while distributed vibration sensing is achieved by detecting phase variation between two adjacent broadband UWFBG interference pulses. The experimental results show that the proposed hybrid UWFBG array can perform temperature and vibration sensing simultaneously. The experimentally conducted temperature measurement ranges from 20°C to 100°C, with the measurement error less than 0.1°C. Vibration signals at different temperatures can be accurately restored, and the signal-to-noise ratio (SNR) is improved by 21.1 dB compared with a normal single-mode fiber (SMF).

Hypersensitive H2 sensor based on polymer planar Bragg gratings coated with Pt-loaded WO3-SiO2: erratum.

We present an erratum to our Letter [Opt. Lett.45, 3601 (2020)OPLEDP0146-959210.1364/OL.395341]. Labeling errors in two figures and an incorrect sentence are revised. The corrections have no influence on the conclusions of the original Letter.

Hypersensitive H2 sensor based on polymer planar Bragg gratings coated with Pt-loaded WO3-SiO2.

This Letter demonstrates a novel, to the best of our knowledge, hydrogen sensor based on a polymer planar Bragg grating coated with Pt-loaded WO3-SiO2. The reflected Bragg signal shows a distinct peak splitting correlated to substrate anisotropies originating from the injection molding process. Especially at low H2 concentrations, both sensing peaks exhibit an outstanding response to the heat generated by the exothermic reaction between hydrogen molecules and coating. Thereby, a hydrogen volume ratio of 50 ppm leads to a Bragg wavelength shift of -37pm, which yields an outstandingly low detection limit of only 5 ppm H2 in air. Thus, functionalized polymer planar Bragg gratings are eminently suitable for H2 leak detection applications.

Multiport swept-wavelength interferometer with laser phase noise mitigation employing a broadband ultra-weak FBG array.

Optical vector network analyzers (OVNAs) based on swept-wavelength interferometry are applied widely in optical metrology and sensing to measure the complex transfer functions of optical components, devices, and fibers. Phase noise from laser sweep nonlinearities degrades the measurement quality as the distance increases and limits the usage of the OVNA in characterizing systems with long impulse responses as required in space-division multiplexing links with a high mode count or in the presence of large modal differential group delay (DGD). In this Letter, we use a densely distributed broadband ultra-weak fiber Bragg grating array to directly measure the distortion due to phase noise at a 5-m increment up to 400 m and use this measured data to directly eliminate the distortion. We experimentally extend the measurement range of the swept-wavelength OVNA over 400 m and successfully characterize a 2-km six-mode multimode fiber link with an accumulated impulse response as wide as 20 ns.

Polar-groups-modified polyimide based on a fiber Bragg grating for relative humidity sensors.

In this work, the effect of the polar groups modified polyimide fiber Bragg grating relative humidity sensor was studied. Polyimide films containing different polar groups were deposited on fiber Bragg gratings by an impregnating method to form different relative humidity (RH) sensors. The experimental results show that the addition of a carboxyl group and a hydroxyl group in the synthesis could improve the humidity sensitivity (2.28 and 1.59 times, respectively) from 35% RH-95% RH. The sensor based on modified polyimide still has good linear response to humidity and temperature. In addition, the sensors containing a carboxyl group and a hydroxyl group can shorten the response time of the humidity sensor and improve the stability of the sensor at the same time.

Novel optical fiber SPR temperature sensor based on MMF-PCF-MMF structure and gold-PDMS film: erratum.

Figure 1(c) in [Y. Wang, Optics Express 26, 1910 (2018)] contains an error and is corrected in this erratum.

Dynamic phase extraction in high-SNR DAS based on UWFBGs without phase unwrapping using scalable homodyne demodulation in direct detection.

We demonstrate a Distributed Acoustic Sensor (DAS) based on Ultra-Weak Fiber Bragg Gratings (UWFBGs) using a scalable homodyne demodulation in direct detection. We show that a distributed interferometric system using delay and mixing of backscattering from consecutive identical gratings can be combined with a Phase-Generated Carrier Differentiate and Cross-Multiply (PGC-DCM) demodulation algorithm to perform dynamic measurements with high SNR, employing a simple narrowband laser and a pin photodiode. The proposed homodyne demodulation technique is suitable for real-time monitoring using distributed measurements, as it does not require computationally costly phase unwrapping common in conventional schemes and is robust against detrimental harmonic distortions, while not requiring additional mechanisms to handle division-by-zero operations. The demodulation scheme is also scalable, as it involves symmetric ordinary differentiation and integration operations suitable for processing with FPGA-based or analogue systems which, thanks to readily realizable schemes for implementing fractional order calculus, are also candidates for small-scale integration. We experimentally demonstrate the effectiveness of the technique by monitoring the dynamic response of a generic 2.5 kHz vibration applied to a PZT placed at the end of a sensing fiber comprised of a 1 km array of 200 UWFBGs each with a reflectivity of ~-43 dB written at a spacing of 5 m, with an SNR of ~34.52 dB.

Investigations of Different Ion Intercalations on the Performance of FBG Hydrogen Sensors Based on Pt/MoO3.

α-MoO3 has been used as a hydrogen sensing material due to its excellent properties and unique crystalline layer structure. However, the low repeatability of α-MoO3 based hydrogen sensor restricts its practical application. In this paper, the effect of intercalated ion species and the amount in α-MoO3 is experimentally investigated and discussed. It is concluded that the repeatability of the sensor depends on the radius of intercalated ions and amount of ionic bonds. The optimal ion species is Na+ and the optimal amount of precursor is 1 mmol.

Novel optical fiber SPR temperature sensor based on MMF-PCF-MMF structure and gold-PDMS film.

In this paper, a novel optical fiber temperature sensor based on surface plasmon resonance (SPR) is presented. The sensor consists of multimode fiber-photonic crystal fiber-multimode fiber (MMF-PCF-MMF) structure coated with gold film, whose refractive index (RI) sensitivity was found to range from 1060.78 nm/RIU to 4613.73 nm/RIU in the RI range of 1.3330-1.3904. Through simulation and experimental results, the RI sensitivity of the MMF-PCF-MMF structure is found to be higher than that of multimode fiber-single mode fiber-multimode fiber (MMF-SMF-MMF) structure. The sensing area was coated with polydimethylsiloxane (PDMS) that has a high thermal coefficient, obtaining a high temperature sensitivity of -1.551 nm/°C in the temperature range of 35-100 °C, which means it has a broad application prospect in medical, environmental monitoring and manufacturing industry.

Thin-film-based optical fiber Fabry-Perot interferometer used for humidity sensing.

A thin-film-based optical fiber Fabry-Perot interferometer that consists of ZrO2 and SiO2 porous thin films is designed and fabricated by electron beam physical vapor deposition. Since the SiO2 porous thin film has the capability of water adsorption, the proposed Fabry-Perot interferometer is appropriate to detect humidity. Experimental results show that the prepared sensor has a humidity detection range from 0.06% RH to 70% RH. A cycling test shows that the humidity sensor has a responding or recover time of 4 s and good repeatability among different humidity environments. Especially, the proposed humidity sensor is insensitive to temperature variation and suitable for the detection of low relative humidity.

High-sensitivity fiber optic hydrogen sensor in air by optimizing a self-referenced demodulating method.

Self-referenced demodulating methods of fiber optic hydrogen sensors based on WO3-Pd2Pt-Pt composite film are studied in this paper. By employing the proper baseline intensity as sensing parameters, fluctuations of the sensing signal of the hydrogen sensor can be obviously depressed, and sensitivity can be greatly improved. Experimental results show that the resolution of the hydrogen sensor can reach 3 parts per million (ppm) when the hydrogen concentration is lower than 1000 ppm. Additionally, the hydrogen sensor shows better sensitivity toward lower concentrations of hydrogen, enabling a hydrogen threshold down to 10 ppm in air at room temperature. To the best of our knowledge, this is the lowest threshold reported for an optical hydrogen sensor operated at room temperature in air. Moreover, the sensor has good repeatability during hydrogen response. This work proposes a simple and novel method to improve the performance of fiber optic hydrogen sensors, which can greatly promote their potential application in various fields.

Cascaded-Cavity Fabry-Perot Interferometric Gas Pressure Sensor based on Vernier Effect.

An extrinsic Fabry-Perot interferometer (EFPI) composed of double fiber FP cavities in a glass capillary tube to generate Vernier effect has been fabricated and employed for gas pressure sensing. A lead-in single-mode fiber (LSMF) and a reflective single-mode fiber (RSMF) were inserted into the capillary tube to form a FP cavity. Femtosecond (fs) laser was used to ablate openings on a capillary tube for gas passage to the FP cavity. A fusion hole was also drilled on the end face of a SMF by fs laser. The sensitivity of the sensor is enhanced due to Vernier effect. Experimental results show that the sensitivity was as high as 86.64 nm/MPa in the range of 0~0.6 MPa, which is 32.8 times larger than that of an open-cavity EFPI sensor without Vernier effect. The temperature cross-sensitivity of the sensor was measured to be about 5.18 KPa/°C. The proposed sensor was characterized by its high sensitivity, compact structure and ease of fabrication, and would have extensive application prospects in gas sensing fields.