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In this letter, a novel strain sensor based on four-sided modulated long period fiber grating (FM-LPFG) is introduced and demonstrated. Due to the asymmetric refractive index modulation of the CO2 laser, the cladding radius is reduced and the core deviation effect is strengthened. In addition, by simulating the equivalent strain during tension, the strain sensitive area of FM-LPFG is proposed. The measurement results indicate that the strain sensitivity of FM-LPFG is 127.5 pm/ $\mu \varepsilon $ ($0~\sim ~125~\mu \varepsilon$) and 17.7 pm/ $\mu \varepsilon $ ($125~\sim ~575~\mu \varepsilon$). The proposed FM-LPFG can be realistically applied in precision measurement. [ABSTRACT FROM AUTHOR]

An all-fiber bend sensor based on Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The fiber interferometer consists of two helical structures in single mode fiber (SMF) and is fabricated by continuous arc discharge technique. The first helical structure forms a twisted long-period fiber grating (LPFG) to excite the higher-order cladding modes and acts as a beam splitter. The LPFG arrangement is adopted to improve the bending sensitivity. The second helical structure only contains two turns and can recouple the light in the cladding back into the core. The bending characteristics of the MZI are discussed. The experimental results show that high curvature sensitivities of 24.06 nm/ $\text{m}^{-1}$ and −7.524 dB/ $\text{m}^{-1}$ are obtained in the curvature range of 0-0.422m−1. Due to its low cost, small size, simple preparation, less time consuming, it could be a competitive candidate in many bending sensing applications. [ABSTRACT FROM AUTHOR]

We demonstrate an ultra-compact optical fiber thermo-hygrometer on the base of a bilayer OA (Optical Adhesive, a kind of polymer) micro-cap covering the entire SMF (Single-Mode-Fiber) facet. In fabrication, two types of OA are dispensed to fiber facet successively and stratified distinctly forming the bilayer micro-cap, which contains a flat and 2 spherical interfaces reflecting emitted light. Hence, hybrid interference would occur. The experimental results show that the sensitivity for humidity and temperature reach −3.641deg/%RH and −4.527deg/°C when demodulating interference phase shift, and dual-parameter measurement could also be achieved. The proposed optical fiber thermo-hygrometer is integrated, robust and with high sensitivity, making it satisfy lab-on-tip measuring requirement. [ABSTRACT FROM AUTHOR]

Graphene Oxide (GO) coated silver (Ag) island system based on localized surface plasmon resonance (LSPR) phenomenon have been developed using optical fiber as a sensing probe. The Ag-island is used to excite LSPR field which is responsible for the analyte sensing through a shift in resonance wavelength. The deposition of Ag-islands is confirmed from UV-VIS spectra and Field Emission Scanning Electron Micro-graph (FESEM). The usage of GO helps to protect Ag-island from oxidation along with opening a new window for bio-sensing due to its structural compatibility with the bio-sensing system. We have experientially observed that the GO as a dielectric overlayer on Ag-island shows a refractive index sensitivity of 2217.04 nm/RIU with a resolution of $4.05 \times 10^{-4}$ RIU. [ABSTRACT FROM AUTHOR]

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 $\sim 19$ 795 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. [ABSTRACT FROM PUBLISHER]

We report on the experimental fabrication and functioning of an optical fiber sensing probe operating on the principle of a localized surface plasmon resonance (LSPR) at near infrared (NIR) wavelengths, around the 2- \mu \textm window. As indium–tin oxide exhibits an absorption peak at NIR wavelengths, we have synthesized and characterized ITO nanoparticles with a doping concentration of 9:1 of In:Sn and attached these onto the unclad core of the fiber. Corresponding to the absorption peak of the bare nanoparticles, the fiber probe displays an LSPR peak at 1950 nm, which falls in the signature band of many bacterial contaminants. The probe is further used for the sensing of analytes of refractive indices varying from 1.335 to 1.375. The results reveal that the average peak absorbance of the probe changes by 3.46 absorbance units for a unit change in refractive index. [ABSTRACT FROM PUBLISHER]

We report that the main photosensitive mechanism of poly(methyl methacrylate)-based optical fiber Bragg grating (POFBG) under ultraviolet laser micromachining is a complex process of both photodegradation and negative thermo-optic effect. We found experimentally the unique characteristics of Bragg resonance splitting and reunion during the laser micromachining process providing the evidence of photodegradation, while the mean refractive index change of POFBG was measured to be negative confirming further photodegradation of polymer fiber. The thermal-induced refractive index change of POFBG was also observed by recording the Bragg wavelength shift. Furthermore, the dynamic thermal response of the micromachined-POFBG was demonstrated under constant humidity, showing a linear and negative response of around −47.1 pm/°C. [ABSTRACT FROM PUBLISHER]

A novel ultra-sensitive liquid-level indicator using an etched chirped fiber Bragg grating is experimentally demonstrated with a sensitivity of 1.214 nm/mm for detecting tens of micro-meter liquid-level variation. The operation mechanism is based on the superposition wavelength-peak shift created by the overlap between the spectrum of the etched CFBG section immersed in the liquid and the spectrum of the rest of the grating in air. This sensor may be used to precisely measure the liquid-level variation in bio-hazardous systems or industrial containers that demand accurate interrogation of the amounts of the contents. [ABSTRACT FROM PUBLISHER]

A polymer microbubble-based Fabry–Perot fiber interferometer (FPI) for pressure and temperature measurement is proposed and demonstrated. By splicing a small segment of photonic bandgap fiber to a single-mode fiber and immersing such a fiber tip into an optical adhesive, a micro air bubble could be buried into the formed polymer diaphragm. Size of air bubble and polymer diaphragm thickness can be controlled by adjusting arc discharge intensity at the fiber splice point. In order to achieve the wide dynamic range and high-resolution measurement, a demodulation algorithm based on absolute phase analysis is adopted and present high sensitivity for simultaneous pressure and temperature sensing. This simple and reproducible fabrication method of such a sensor gives an alternative way to construct FPI-based biomedical and microfluidic sensors. [ABSTRACT FROM PUBLISHER]

A linear five-core fiber was sandwiched in between two single-mode fibers to construct an all-fiber Mach–Zehnder interferometer (MZI). The linear five-core fiber can support multiple supermodes due to a small distance between two adjacent cores. The interferences between the fundamental supermodes, between the fundamental supermodes and the high-order core modes, and between the fundamental supermodes and the low-order cladding modes are investigated. The proposed MZI can realize strain, temperature, and curvature fiber sensors. The experimental results show both the interference between the core modes and the interference between the core modes and the cladding modes have approximately equal temperature sensitivity. The interference between the core modes is insensitive to the axial strain and the bending. In contrast, the interference between the core and the cladding modes is highly sensitive to the strain and the bending perpendicular to the arranged plane of the cores, the corresponding strain and bending sensitivities are 0.8 pm/ $\mu \varepsilon $ and 10.37 nm/ \text{m}^{\mathrm {\mathbf {-1}}} . [ABSTRACT FROM PUBLISHER]

A photonic crystal fiber polarization rotator (PR) is proposed based on a novel mechanism-mode interference by two hybrid waveguide modes, rather than adjusting optical property in the light-guided part. The proposed PR is achieved by introducing two defects and an ancillary Au nanowire into the background lattice, and under the optimized parameters, the PR can offer almost a 100% polarization conversion ratio in the wavelength of 1.55-μm band (~100-nm bandwidth), compact length ~162.7 μm, low modal insertion loss of ~1.2 dB, and high coupling efficiency over 0.92 for both TE and TM mode based on the numerical simulation result of the full-vector finite-element method. [ABSTRACT FROM PUBLISHER]

We present a temperature and strain fiber sensor utilizing fiber Bragg grating (FBG) inscribed in silica-germanate few-mode fiber. The temperature and strain sensitivities of resonant wavelength formed by mode LP01 self-coupling and mode LP01-LP11 cross-coupling are investigated theoretically and characterized experimentally. Two resonant wavelengths demonstrate different temperature sensitivities of 11.5 pm/°C 10.6 pm/°C, respectively, due to different mode profile, while the strain sensitivities are quite similar with a value of 0.9 pm/με. By using FBG character matrix, the proposed fiber sensor can measure those two parameters simultaneously within a compact and cost-effective setup. [ABSTRACT FROM PUBLISHER]

In this letter, we present a structured polarization-maintaining chirped fiber Bragg grating (PM-CFBG) for temperature-independent and strain-independent twist measurement. The structured PM-CFBG, which has two transmission peaks, is created by tapering directly on the PM-CFBG. The grating transmissivity of structured PM-CFBG changes linearly with twist while it is insensitive to strain and temperature. The wavelength interval changes proportionally to the temperature, but remains the same as the strain increases. This novel sensor can be used to measure temperature, strain, and twist simultaneously. [ABSTRACT FROM PUBLISHER]

We demonstrate a miniaturized fiber device based on fiber inner air-cavity with periodical microstructures and platinum (Pt)-doped WO3 coating for hydrogen concentration sensing. The fiber inner air-cavity is formed by use of femtosecond laser micromachining together with fusion splicing technique and the line shape microstructures are inscribed by femtosecond laser beam scanning. The Pt-doped WO3 coating on the fiber surface near the cavity leads to the device sensitivity to hydrogen concentration, and the maximum wavelength shift obtained is ~2.64 nm when the hydrogen concentration is increased from 0% to 4% in volume percentage. The system is highly sensitive and miniature and exhibits a low temperature cross-sensitivity. [ABSTRACT FROM PUBLISHER]

A novel scheme for distributed Brillouin sensing based on differential pump-width pair Brillouin optical time-domain analysis (DPP-BOTDA) with balanced detection of Brillouin gain is proposed and demonstrated. Compared with standard DPP-BOTDA, the proposed method allows us to reduce the measurement time and simultaneously improve the accuracy in Brillouin frequency shift estimation. Experimental results, carried out at 10-cm spatial resolution, demonstrate the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This letter describes the manufacture and performance of a monolithic double-layer silicon photonic crystal temperature sensor. The sensor is fabricated on standard silicon wafers using oxide passivation and a combination of isotropic and anisotropic etching, and mounted on the facet of a standard single-mode optical fiber using template-assisted epoxy bonding. The double-layer configuration leads to coupling of the guided resonances in the two photonic crystals and enables sharper resonances and consequently higher temperature sensitivity and better detection limit (0.011 °C) than the single-layer counterpart. We experimentally demonstrate that the sensor has a twofold increased temperature sensitivity in terms of reflectivity change at a fixed wavelength (-0.00576/°C), and report on electromagnetic simulations explaining the enhanced sensor operation. The photonic crystal fabrication method and template-assisted bonding enable batch-fabrication of the sensors. Their small size, robust construction, and fiber interface make the sensors promising for numerous applications, including sensing in harsh environments. [ABSTRACT FROM PUBLISHER]

Distributed temperature measurements in a perfluorinated graded-index polymer optical fiber (POF) with 50- μm core diameter are reported for the first time, to the best of our knowledge. Brillouin optical frequency-domain analysis is shown to be able to resolve spatially the temperature-dependent Brillouin frequency shift profile along a 20-m POF fiber sample, at a nominal spatial resolution of 4 m. The results indicate that POFs are potentially useful for distributed temperature measurements based on stimulated Brillouin scattering. [ABSTRACT FROM PUBLISHER]

We have demonstrated an end-of-fiber polydimethylsiloxane (PDMS) cap based fiber Fabry-Pérot interferometer in this letter, which could be used for simultaneous measurement of refractive index (RI) and temperature with extremely low cross sensitivity. The RI and temperature sensitivities of the interferometer are -240.425 dB/RI unit based on extinction ratio measurement in the RI range of 1.3625-1.4150 and 385.46 pm/°C in the temperature range of 25°C-60°C, respectively. Not only does the interferometer have the characteristics of high sensitivity and low cost, but also PDMS has a good biocompatibility, so that the sensor can be used as a cell culture or bacteria growth detection probe. [ABSTRACT FROM PUBLISHER]

We propose and demonstrate a compact micromulticavity Fabry–Pérot (FP) optical fiber tip sensor for refractive-index (RI) and temperature measurement, which is fabricated by laser micromachining. The fabrication process of the sensor offers a quite simple way to fabricate compact in-line FP at a low cost. At the end of the single-mode fiber, there is a short air FP cavity, which forms a multicavity together with the fiber flat face tip. Two independent cavities exist in our sensor, and the air cavity is used for RI sensing and the glass cavity for temperature sensing, thus, avoiding the temperature and RI cross-sensitivity. The measurement results show that the in-line FPI exhibits an RI sensitivity of 1074.36 nm/RIU and a temperature sensitivity of 10.6 pm/^\circC. [ABSTRACT FROM PUBLISHER]

A novel formation method of long-period fiber gratings (LPFGs) based on periodic microchannels, which are fabricated by femtosecond laser micromachining and selective chemical etching in conventional single-mode fibers, is proposed and experimentally demonstrated. This kind of grating may be as short as only 3 mm and exhibits low-temperature sensitivity of 9.95 pm/^\circC from 30^\circC to 120^\circC. The strain sensitivities of the resonant wavelength and peak loss are -2.4~nm/m\varepsilon and -3.5~dB/m\varepsilon, respectively. High refractive-index (RI) sensitivity with one order of magnitude larger than that of the conventional LPFGs near RI=1.33 is achieved in general single-mode fibers. [ABSTRACT FROM PUBLISHER]

Compact 5-mm-long 100-μm-period long-period fiber gratings (LPFGs) are fabricated by femtosecond laser direct writing. Nearly symmetrical photomodified grating segments located at the center of a fiber core lead to well-defined transmission spectra. The LPFGs can couple the guided mode to high-order cladding modes of second-order diffraction, resulting in a high refractive index (RI) sensitivity of 5265 nm/RI units in the RI range of 1.433 ~ 1.443 and a low-temperature sensitivity of -15.52 pm/°C from 20°C to 500°C. The latter property is desired for crosstalk-free high-sensitivity RI sensors. [ABSTRACT FROM PUBLISHER]

We propose a novel quasi-real-time sensing scheme by employing a modified optical frequency domain reflectometry technique to multiplex long-period grating (LPG) sensors of transmission mode. Different sensing signals are demodulated by different beat signals through presetting different values for the optical path differences between the measuring arms and a static reference arm. The amplitude of each beat signal is associated with the transmissivity of a sensing LPG. The present multiplexing scheme has the advantages of real-time, high speed, high sensitivity, large capacity, and high resolution over an intermediate length. A measurement of fiber bending curvature is demonstrated to verify good performances of the present LPG sensing system. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate a novel refractive index (RI) sensor with the measurand infiltrated into the air channels of a single mode photonic crystal fiber (PCF). A tunable long-period grating is formed in a short length of PCF by use of the periodic mechanical pressure. Resonance shift with the change of the infiltrated liquid is observed with a sensitivity of 4.1 times 10^-6 RI unit in a refractive index range from 1.32 to 1.39. The thermal stability of the pure-silica-based PCF device shows a three times better performance compared with conventional single mode fiber. [ABSTRACT FROM PUBLISHER]

A cascaded side-polished multimode fiber sensor based on surface plasmon resonance (SPR) is proposed. The sensing area is a multimode optical fiber in which half of the core has been polished away and a 40-nm gold thin film is deposited on to the polished surface. The cascaded structure is demonstrated in the small difference variation of the refractive index between the two solutions, deionized (DI) water and phosphate buffered saline (PBS). The transmitted intensity of SPR fiber sensor in dynamic measurement between single SPR fiber structure and the cascaded structure achieved to increase the variation from 73% to 58% for DI water and 70.5% to 53% for PBS, and enhance the difference between DI water and PBS from 2.5% to 5% is obtained in the experiment. The stability and the double variation to enhance the high sensitivity of the intensity measurement is achieved to be 2 times 10-5 refractive-index units. [ABSTRACT FROM PUBLISHER]

A novel Brillouin optical time-domain reflectometry (BOTDR) system, called a double-pulse BOTDR (DP-BOTDR) system, is proposed for measuring distributed strain and temperature in a fiber with a sub-meter spatial resolution. Our experiment confirmed that the DP-BOTDR system enables us to measure the distributed Brillouin frequency shift, i.e., the distributed strain and temperature, with a spatial resolution of 20 cm. This spatial resolution is five times better than that provided by the conventional single-pulse BOTDR system. [ABSTRACT FROM PUBLISHER]

A novel digital demodulation scheme for an interferometric optical fiber sensor is developed, which uses only an interferometer with zero optical path difference, and has a larger dynamic range and a faster operation speed than that of a digital phase generated carrier scheme [ABSTRACT FROM PUBLISHER]

We report a new configuration of the fiber-optic voltage sensor based on the Bi12TiO20 crystal, which allows simultaneous measurements of both voltage and temperature. In our scheme, a quarter-ware plate, being an inherent element of voltage sensor, serves simultaneously as a phase-shifting element and as a temperature sensitive element. The sensor operates at two wavelengths (633 and 976 nm). The sensor has a linear temperature characteristic within the range of 10°C-70°C, providing the accuracy of temperature measurements of 0.3°C. As a voltage sensor, this device has a linear amplitude characteristic up to 1000 Vrms and the excellent temperature stability of 0.1% within the temperature range of 10°C.-70°C. [ABSTRACT FROM PUBLISHER]