High pressure-sensitive and stable fiber Fabry-Perot interferometer with nano-diaphragm assembled by H-O catalysis bonding.

Herein, a high pressure-sensitive and stable fiber Fabry-Perot (FP) interferometer with nano-diaphragm assembled by H-O catalysis bonding is proposed and demonstrated. In order to assemble a nano-diaphragm-based fiber FP interferometer by H-O catalysis bonding technique, a SiO2 film, introduced as a bridging layer on the nano-diaphragm, can be regarded as a solid adhesive to bridge hollow-core fiber end-face and nano-diaphragm. As thus, by depositing bonded layers on different diaphragm materials, this H-O catalysis bonding technology can be used to for assembling FP interferometer with different materials nano-diaphragms. Experimentally, Si nano-diaphragm is transferred to hollow-core fiber end-face to build a stable fiber FP interferometer without polymeric adhesive. Experimental results reveal that this Si nano-diaphragm-based fiber FP interferometer has a high (79.6 pm/kPa) pressure sensitivity and a low (17.3 pm/°C) temperature sensitivity. Besides that, different materials nano-diaphragm also can be assembled by using this H-O catalysis bonding technique, and the functional FP interferometer can be realized by using functional nano-diaphragm material. Thus, a Pd nano-diaphragm is successfully assembled to build a FP interferometer with a hydrogen concentration measurement capacity. Further investigation will focus on exploitation of multi-material nano-film patterning transfer and different nano-film integration by using this H-O catalysis bonding transfer.

Customizable miniaturized SPR instrument.

Prism-based surface Plasmon resonance (SPR) system, as one of the leading candidate concepts for scale application and commercial solution, has good stability, high-sensitivity and greater theoretical/technical maturity. Therefore, to take advantage of prism-based SPR system fully, and break up limitations of complicated and bulky traditional prism-based SPR system, optimal and compact design of optical system is an effective solution. Herein, a customizable miniaturized prism-based SPR system is developed by optical system optimization and integrated design, combining portable data acquisition and processing technology (FPGA-based multifunctional data processing). This proposed prism-based SPR system can achieve a miniaturized SPR system, thus, it also can meet the requirements of flexibility configuration and customizable performance to accommodate the various needs of different users and application scenes. Additionally, the customizable features can make it to achieve the best performance optimization and differentiation.

Optics-mechanics synergistic fiber optic sensor for hydrogen detection.

As a carbon-free energy carrier and an attractive alternative energy source, hydrogen energy has great development potential for future considerations, and it may be the ultimate answer to the global energy crisis. Due to the high combustibility of hydrogen, hydrogen sensors will be a vital component of safe use of hydrogen. Among the various sensors, the optical hydrogen sensor can meet the requirements of intrinsic safety, online detection, surrounding immunity, and lack of spark. Hence, we demonstrate a miniature optics-mechanics synergistic fiber optic hydrogen sensor by using Pd nanofilm, it has a large response range (0.5%-3.5%), high sensitivity of -0.334 nm/1% concentration and a short response time (10s)/recovery time (25s). Experimental results reveal that the proposed optics-mechanics synergistic fiber optic hydrogen sensor is reusable, durable, and low temperature sensitive. In this optics-mechanics synergistic fiber optic hydrogen sensor, nano Pd film with a large surface-to-volume ratio allows for rapid hydrogen dissociation, and Pd lattice expansion caused by Pd-hydrogen reaction is effectively transduced into optical change. This proposed sensor integrated Pd nanofilm with optical fiber by using an optics-mechanics synergistic strategy, resulting in a compact and all-optical solution for the safe measurement of hydrogen concentration, which can be used in hazardous or space-limited environments.

In-fibre micro-channel: its potential for in-fibre detection.

Micro-channels (µ-channels) in microstructure fibres can be regarded as natural in-fibre flow channels. Thus, the advent of microstructure fibres with µ-channels makes it possible to realize in-fibre integrated microfluidic devices, and microstructure fibres with µ-channels provide the possibility of creating new online monitoring systems and define new concepts. Herein, we developed a novel compact in-fibre detection platform, which is combined with a µ-channel in a new-type microstructure fibre, side-hole fibre, for in-fibre detection. The optical component of this proposed in-fibre detection platform is made of a simple cross-axial open-cavity Fabry-Perot interferometer. This miniaturized cross-axial open-cavity Fabry-Perot interferometer is formed by a 45°-angled side-hole fibre, which is fabricated by a simple end-face polishing process. For a 45°-angled fibre, the incident optical axis can be steered based on total internal reflection at the oblique fibre-air interface, and the reflected light will enter the side-hole µ-channel, and the front and rear inner µ-channel walls form the cavity; the natural in-fibre µ-channel functions as a (liquid/gas) flow channel. Experimentally, this proposed in-fibre detection platform was fabricated, and its spectral characteristics were investigated. Its relative humidity characteristics and potential application in breath sensing were calibrated by measuring the evolution of the reflection spectrum. As a whole, the proposed detection platform demonstrates the advantages of simple structure, easy fabrication without additional sensitive materials, and potential application in breath sensing or lab-in-fibre technology.

Frequency splicing code-based Brillouin optical time domain collider for fast dynamic measurement.

We propose a frequency splicing code-based Brillouin optical time domain collider (FSC-BOTDC) for fast dynamic sensing. By delicately designing the frequency splicing code (FSC), multiple collision modes with controllable characteristics are realized for probing multiple target areas with a high sampling rate. Moreover, the sensing system is simpler and more robust than the previous BOTDC. In the experiment, the FSC-BOTDC with 10-time enhanced sampling rate is implemented for single and multiple target areas measurements. Results demonstrate that tailorable measurements can be achieved by the tunable FSC. Furthermore, the FSC-BOTDC is executed to measure periodic mechanical vibrations over 7.9-km sensing range with the sampling rate of 625 Hz.

Brillouin optical time domain collider for fast dynamic sensing.

The dynamic sampling rate of Brillouin optical time domain analysis (BOTDA) is limited by fiber length. For breaking through this limit, a Brillouin optical time domain collider (BOTDC) is proposed and experimentally demonstrated. In this BOTDC, by employing frequency-hopping pump and probe waves, sensing information-crosstalk between adjacent pump pulses is avoided even though the pump pulse interval is shorter than the round-trip time of flight in the fiber. In the experiment, periodic mechanical vibrations with a 19.75 Hz fundamental frequency and a 39.49 Hz harmonic frequency are measured by a 10-frequency BOTDC with a sampling rate of 49 kHz which is 10 times higher than that in the BOTDA.

Enhanced range of the dynamic strain measurement in phase-sensitive OTDR with tunable sensitivity.

Phase-sensitive optical time domain reflectometry (Φ-OTDR) realizes quantitative measurement of the dynamic strain employing phase demodulation. Unfortunately, it is difficult to measure the large dynamic strain with the conventional Φ-OTDR due to the restriction of the unwrapping algorithm. In this work, an approach based on two-wavelength probe is proposed and demonstrated to improve the measurable range of the dynamic strain in Φ-OTDR. By utilizing the difference between the two phases acquiring with two different lasers, the large dynamic strain can be recovered. In experiments, dynamic strains with peak values from 10.32 uɛ to 24.08 uɛ are retrieved accurately, which cannot be recovered with the conventional Φ-OTDR. Moreover, the tunable sensitivity is also demonstrated through adjusting the wavelengths of the probe. With the increment of the wavelength interval from 9.06 nm to 23.06 nm, the normalized sensitivity increases from 0.4 to 1 accordingly. That agrees well with the theoretical prediction. Foreseeably, the proposed method will extend the scope of application fields for Φ-OTDR, which requires large dynamic strain recognition.

Overcoming EDFA slow transient effects in a combined Golay coding and coherent detection BOTDA sensor.

Firstly, Erbium-doped fiber amplifier (EDFA) served as pre-amplifier may distort Brillouin gain spectrum (BGS) in coded Brillouin optical time domain analysis sensor. Here, we found that the EDFA has negligible impact on the shape of Brillouin phase spectrum (BPS). Experimental results show that a ∼5.4 MHz Brillouin frequency shift error caused by EDFA has been avoided by using BPS instead of BGS. Secondly, after eliminating phase fluctuation caused by optical fiber, the combination of Golay coding and coherent detection has been realized.

Polarization push-pull effect-based gain fluctuation elimination in Golay-BOTDA.

A Brillouin gain fluctuation elimination scheme based on a hybrid polarization pulling and pushing effect (HPPP) is proposed and experimentally demonstrated in a Golay-coded Brillouin optical time domain analysis (BOTDA) fiber sensor. The analysis reveals that, due to the non-negligible probe state of polarization (SOP) deviation caused by the polarization pulling or pushing effect, the effectiveness of eliminating Brillouin gain fluctuation by using polarization switch is significantly degraded. Nevertheless, when probe Stokes and anti-Stokes components separately interact with orthogonal polarization pumps, the SOP evolution of the probe Stokes component due to the polarization pulling is totally identical to the SOP evolution of the probe anti-Stokes component caused by the polarization pushing. Based on this characteristic of the SOP evolutions, a novel HPPP method is proposed to eliminate the gain fluctuation. Experimental results demonstrate that the gain fluctuation falls to one-eighth of that of the conventional gain-only scheme by using this proposed HPPP method.

Optically functionalized microfiber Bragg grating for RH sensing.

Herein, a novel material functionalization of fiber components is proposed, which is based on optically induced film forming. The film formation of polymer around a microfiber is achieved only by injecting an ultraviolet laser through the microfiber. The evanescent field of an ultraviolet laser, propagating along the microfiber surface, provides activation energy for photopolymerization of polymer and film formation directly onto the surface of a microfiber. The thickness of the deposited film can be controlled by adjusting the irradiation time. Additionally, the potential application case demonstrates that fiber components can be endowed with a given function by this material functionalization method. Therefore, a fiber relative humidity sensor using a polymer film-functionalized microfiber Bragg grating fabricated by this functionalization technique is demonstrated. This proposed sensor has the advantages of relatively fast response, large measurement range, and high stability.

Dual Kretschmann and Otto configuration fiber surface plasmon resonance biosensor.

We present a dual-resonance fiber surface plasmon resonance (SPR) sensor for biological analysis. The sensing element was fabricated by sequentially sputtering layers of indium tin oxide (ITO) (100 nm thickness) and Au (35 nm thickness) on the surface of an optical fiber. The refractive index dispersion effect of ITO material led to resonances in the near infrared and visible wavelength regions. The refractive index of ITO is larger than the optical fiber in visible spectral area (400 to 733nm), such that the structure is a typical Kretschmann configuration surface plasmon resonance sensor. However, an Otto configuration is observed in the near infrared area (NIR) due to the ITO refractive index being smaller than the fiber core. We characterized the sensor performance by measuring bulk refractive index (RI) sensitivity in the two configurations, which were 1345 nm/RIU in the Kretschmann configuration and 1100 nm/RIU in the Otto configuration. In addition, this sensor was applied for real-time and label-free monitoring of the IgG/anti-IgG biomolecular interaction. As a robust and ultra-compact SPR sensor, which possesses wide detection range and is highly sensitive, this fiber SPR sensor can be applied for real-time biological analysis and monitoring.

Annealing properties of fiber Bragg grating UV-inscribed in boron-germanium codoped fiber.

In this work, we mainly focus on the investigation of the feasibility of production of high-temperature stable fiber Bragg grating (FBG) based on reduplicative alternate annealing and hydrogen loading. The experimental results also can demonstrate the significance of the presence of hydrogen to the thermal regeneration of FBGs. The gratings are characterized and variations are compared after each stage, including UV fabrication, annealing, and reduplicative hydrogen-preloaded annealing. In different stages, the spectral and annealing responses of FBG are, respectively, investigated, as temperature increases, the Bragg wavelength consistently shifts to longer wavelengths; nevertheless, the reflection variations are distinctly discrepant. After reduplicative alternate annealing and hydrogen loading, the thermal stability is tremendously improved, and a reborn, stable grating is formed.

Chirped fiber tip Fabry-Perot interferometer.

We demonstrate a novel fiber tip Fabry-Perot (FP) interferometer with a chirped spectral characteristic. The FP interferometer is formed by an etched chirped fiber Bragg grating (FBG) and the fiber tip broadband end-face mirror, and the etched chirped FBG is prepared by a dynamic chemical etching process, shaping the taper and reducing the size from 125 µm to just a few micrometers. Due to the spectral characteristic of the etched chirped FBG, the different resonance wavelengths correspond to different gating positions which give rise to the gradient cavity length in the chirped fiber tip FP interferometer; thus, this FP interferometer possesses a chirped interference spectrum. Subsequently, the FP interferometer is fabricated, and the temperature response is investigated by measuring the wavelength shift of the resonance dips in the reflection spectrum. Significantly, because of its special interference principle and configuration, we experimentally demonstrate that this FP interferometer can be used for the spatial localization of the external temperature perturbations.

Novel birefringence interrogation for Sagnac loop interferometer sensor with unlimited linear measurement range.

A novel demodulation method for Sagnac loop interferometer based sensor has been proposed and demonstrated, by unwrapping the phase changes with birefringence interrogation. A temperature sensor based on Sagnac loop interferometer has been used to verify the feasibility of the proposed method. Several tests with 40 °C temperature range have been accomplished with a great linearity of 0.9996 in full range. The proposed scheme is universal for all Sagnac loop interferometer based sensors and it has unlimited linear measurable range which overwhelming the conventional demodulation method with peak/dip tracing. Furthermore, the influence of the wavelength sampling interval and wavelength span on the demodulation error has been discussed in this work. The proposed interrogation method has a great significance for Sagnac loop interferometer sensor and it might greatly enhance the availability of this type of sensors in practical application.

The Longitudinal Force Measurement of CWR Tracks with Hetero-Cladding FBG Sensors: A Proof of Concept.

A new method has been proposed to accurately determine longitudinal additional force in continuous welded rail (CWR) on bridges via hetero-cladding fiber Bragg grating (HC-FBG) sensors. The HC-FBG sensor consists of two FBGs written in the same type of fiber but with different cladding diameters. The HC-FBGs have the same temperature sensitivity but different strain sensitivity because of the different areas of the cross section. The differential strain coefficient is defined as the relative wavelength differences of two FBGs with the change of applied longitudinal force. In the verification experiment in the lab, the HC-FBGs were attached on a section of rail model of which the material property is the same as that of rail on line. The temperature and differential strain sensitivity were calibrated using a universal testing machine. As shown by the test results, the linearity between the relative wavelength difference and the longitudinal additional force is greater than 0.9999. The differential strain sensitivity is 4.85 × 10-6/N. Moreover, the relative wavelength difference is not affected by the temperature change. Compared to the theoretical results, the accumulated error is controlled within 5.0%.

Dual-channel fiber surface plasmon resonance biological sensor based on a hybrid interrogation of intensity and wavelength modulation.

We investigate an intensity and wavelength modulation combined plasmon resonance-based fiber-optic sensor technology. Composed of gold nanoparticles (GNPs) and sandwich configuration of Au/indium tin oxide (ITO)/Au film, two sensing regions are fabricated separately along with unclad portions of the fiber-optic probe. It can simultaneously monitor both the light intensity from the Au NP channel and the wavelength from the Au/ITO/Au film channel with a single detector. As the refractive index (RI) of the external environment changes, the transmission intensity and resonance wavelength in the two channels are modified, which provides an interrogation of intensity and wavelength modulation. The sandwich film structure is formed using magnetron sputtering technology, and the GNPs functioning as localized surface plasmon resonators are coated on a multimode optical fiber via the layer-by-layer method. The experimental results reveal that the RI sensitivities of the two sensing channels are 334.1% RIU?1 and 1963.2??nm/RIU, respectively. Based on the above sensing design, we conduct real-time and label-free monitoring of IgG/anti-IgG and Con A/RNase B biomolecular interaction. The resonant dips excited by different sensing modes make it more attractive as a multichannel surface plasmon resonance analysis technology, which is valuable in biological and life sciences research and rapid diagnostics.

Optical Fiber Temperature and Torsion Sensor Based on Lyot-Sagnac Interferometer.

An optical fiber temperature and torsion sensor has been proposed by employing the Lyot-Sagnac interferometer, which was composed by inserting two sections of high-birefringence (HiBi) fiber into the Sagnac loop. The two inserted sections of HiBi fiber have different functions; while one section acts as the temperature sensitive region, the other can be used as reference fiber. The temperature and twist sensor based on the proposed interferometer structure have been experimentally demonstrated. The experimental results show that the envelope of the output spectrum will shift with the temperature evolution. The temperature sensitivity is calculated to be -17.99 nm/°C, which is enlarged over 12 times compared to that of the single Sagnac interferometer. Additionally, the fringe visibility of the spectrum will change due to the fiber twist, and the test results reveal that the fringe visibility and twist angle perfectly conform to a Sine relationship over a 360° twist angle. Consequently, simultaneous torsion and temperature measurement could be realized by detecting the envelope shift and fringe visibility of the spectrum.

Liquid crystal filled surface plasmon resonance thermometer.

A novel surface plasmon resonance (SPR) thermometer based on liquid crystal (LC) filled hollow fiber is demonstrated in this paper. A hollow fiber was internally coated with silver and then filled with LC. The SPR response to temperature was studied using modeling and verified experimentally. The results demonstrated that the refractive index of LC decreases with the increasing temperature and the variation can be detected by the resonance wavelength shift of the plasmon resonance. The temperature sensitivities were 4.72 nm/°C in the temperature range of 20 to 34.5 °C and 0.55 nm/°C in the temperature range of 36 to 50 °C, At the phase transition temperature between nematic and isotropic phases of the LC, the temperature sensitivity increased by one order of magnitude and a shift of more than 46 nm was observed with only a 1.5 °C temperature change. This sensor can be used for temperature monitoring and alarming, and can be extended for other physical parameter measurement.

Fiber Fabry-Perot interferometer with controllable temperature sensitivity.

We proposed a fiber taper based on the Fabry-Perot (FP) interferometer structure with controllable temperature sensitivity. The FP interferometer is formed by inserting a segment of tapered fiber tip into the capillary and subsequently splicing the other end of the capillary to a single-mode fiber (SMF), the tapered fiber endface, and the spliced face form the FP cavity. Through controlling the inserted tapered fiber length, a series of FP interferometers were made. Because the inserted taper tip has the degree of freedom along the fiber axial, when the FP interferometer is subjected to temperature variation, the thermal expansion of the fiber taper tip will resist the FP cavity length change caused by the evolution of capillary length, and we can control the temperature sensitivity by adjusting the inserted taper length. In this structure, the equivalent thermal expansion coefficient of the FP interferometer can be defined; it was used to evaluate the temperature sensitivity of the FP interferometer, which provides an effective method to eliminate the temperature effect and to enhance other measurement accuracy. We fabricated the FP interferometers and calibrated their temperature characters by measuring the wavelength shift of the resonance dips in the reflection spectrum. In a temperature range of 50°C to 150°C, the corresponding temperature sensitivities can be controlled between 0 and 1.97 pm/°C when the inserted taper is between 75 and 160 µm. Because of its controllable temperature sensitivity, ease of fabrication, and low cost, this FP interferometer can meet different temperature sensitivity requirements in various application areas, especially in the fields which need temperature insensitivity.

Optimization of long-period grating-based refractive index sensor by bent-fiber interference.

In this paper, we propose and demonstrate a novel approach to enhance the refractive index (RI) sensitivity and eliminate the temperature cross-sensitivity of a long-period grating (LPG) -based refractive index sensor by bent-fiber interference. The approach is based on a hybrid structure composed of an LPG and a bent-fiber intermodal interferometer. The bent-fiber intermodal interferometer has a simple structure, which consists of a bare fiber semi-circular bending region with a 5 mm bending radius. As the RI increases, the resonance wavelength of the LPG moves toward a shorter wavelength, while the resonance wavelength of the bent-fiber intermodal interferometer shifts to a longer wavelength. The separation of two resonance dips increases with the RI; using two resonance dips allows us to measure an RI with a higher sensitivity than if we had only used one resonance dip. However, as the temperature increases, the separation of the two resonance dips is constant. This approach can effectively enhance the RI sensitivity and eliminate temperature cross-sensitivity.