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We propose and experimentally demonstrate a slit beam shaping method for femtosecond laser Point-by-Point (PbP) inscription of highly localized Fiber Bragg Gratings (FBGs). The influence of slit width on the shape and area of Refractive Index Modulation Region (RIMR) induced by a single femtosecond laser pulse was investigated. The shape of RIMR can be changed from a spot to a line with an enlarged RIMR, and hence enhances the coupling strength of core mode and cladding modes. The RIMRs were precisely assembled along the fiber core, producing highly localized FBGs with a spectral comb of pronounced Cladding Mode Resonances (CMRs) intensity of more than 30 dB, a wide wavelength span of 240 nm and a low insertion loss of 0.3 dB. Note that the total processing time for fabricating such a highly localized FBG only requires ∼3.7 s. Subsequently, by including tilted angle on the slit, highly localized tilted FBGs were fabricated and these FBGs show adjustable envelope on the CMRs. Moreover, we investigated the surrounding Refractive Index (RI) response and thermal characteristics of the fabricated highly localized FBGs, which exhibit a sensitivity of 510.87 nm/RIU in a wide RI measurement range and excellent high temperature resistance at 1000°C. Therefore, such highly localized FBGs could potentially be used for multi-parameter sensing in many extreme environments. [ABSTRACT FROM AUTHOR]

A three-parameter sensor based on a point-by-point fiber Bragg grating (PBP-FBG) is proposed. The fabricated sensor is 40 mm long. The spectrum loss at 1336.15 nm is selected to measure the surrounding liquid level. The sensitivity for liquid level sensing is −0.046 dB/mm. The temperature is measured using the Bragg dip shifts with sensitivity of 10.06 pm/ °C. The cut-off mode shift is used to measure the surrounding refractive index with sensitivity of 535.14 nm/RIU. As a benefit of femtosecond laser direct inscription fabrication, the grating length is controllable and the liquid level sensing range is expandable. This sensor provides a simple, reliable method for accurate liquid level, temperature and refractive index measurements in hazardous environments. [ABSTRACT FROM AUTHOR]

An effective method, reducing the material volume of the air cavity by asymmetrical tapering fiber, is demonstrated to enhance the sensitivity of the strain sensor dramatically. High-sensitive strain sensors are fabricated through this method that splicing and tapering fiber with arc discharge at the misaligned center of the air cavity. The strain sensitivity of the sensor is measured to be ∼ 41.81 pm/μϵ at a resonant wavelength ∼ 1560 nm ranging from 0 to 500 μϵ. In addition, the experimental results show that the asymmetrical tapered-fiber enables both the material volume to smaller and the wall thickness to thinner, resulting in strain sensitivity higher. The proposed sensors have the advantages of fabricated repeatability, high sensitivity, and compact size, benefiting its practical sensing applications. [ABSTRACT FROM AUTHOR]

High-quality helical long-period fiber gratings (HLPFGs) with a coupling strength greater than 30 dB and an insertion loss less than 0.5 dB are formed by applying CO2 laser to expose an optical fiber that is simultaneously translated and rotated to relieve the residual stress of the fiber along a helical pathway, rather than twisting the fiber under a high-temperature condition. The mode coupling characteristics of this type of HLPFG are investigated for the first time. Experimental results demonstrate that the helical refractive-index modulation of the HLPFGs can generate light with orbital angular momentum (OAM) modes, where the chirality of the OAM modes is insensitive to the polarization state of the input light. The HLPFGs provide excellent torsion characteristics, and the resonant wavelengths of OAM modes can be linearly tuned when applying torsions. [ABSTRACT FROM AUTHOR]

An ultra-compact cascaded Fabry Perot (F-P) interferometer in fiber, which exhibits a high-visibility and enhanced detection limit, has been developed by chemical-assisted femtosecond laser micromachining. The reflection spectrum of the cascaded F-P interferometer shows stronger and narrower resonance than single F-P cavity. The liquid refractive index (RI) measurement demonstrates that the wavelength sensitivity and the dip intensity sensitivity are ∼888 nm/RIU and −882 dB/nm/RIU, respectively, and the obtained detection limit is 4.22 × 10−5 RIU in RI range 1.325–1.330, which is higher than that of the conventional single F-P cavity in optical fibers based on wavelength modulation (10−4 RIU). Moreover, the gas pressure measurement demonstrates that the wavelength sensitivity is 2.86 nm/MPa and the detection limit reaches 1.311 × 10−2  MPa in pressure range 1.5–2.0 MPa. The proposed in-fiber cascaded F-P interferometer will find potential applications in chemical and biological sensors, biomedical probes, and low-cost care products. [ABSTRACT FROM AUTHOR]

Conventional grating-inscription technology usually inscribes only one fiber Bragg grating (FBG) in the core of an optical fiber. A novel femtosecond laser point-by-point technology was demonstrated to parallel-inscribe multiple FBGs, i.e., so-called parallel-integrated FBGs (PI-FBGs), with either different or the same reflection wavelengths in the core of a standard single-mode fiber. Single- or multiwavelength PI-FBGs exhibited one or multiple peaks in the reflection spectrum, respectively. The length of PI-FBGs can be shortened to hundreds of micrometers to increase the spatial resolution of the FBG-based sensors. Our ultrashort PI-FBGs with a length of 500 μm could be used to realize ultrahigh temperature sensing with a high spatial resolution of less than 1 mm, a linear sensitivity of 15.0 pm/°C, and a large measurement range of up to 1100 °C, which overcomes the shortcomings of conventional FBG-based sensors with a low spatial resolution of more than 1 cm and a small measurement range of up to 300 °C. Moreover, the grating parameters, such as reflectivity, reflection wavelength, polarization-dependent loss, and full width at half maximum, can be improved to achieve desired PI-FBGs by changing the spatial distribution and grating number of the PI-FBGs. The PI-FBGs are insensitive to bending, which reduces bend-induced measurement errors. Therefore, our PI-FBGs will find potential applications in the fields of optical fiber sensors, communications, and lasers. [ABSTRACT FROM AUTHOR]

A Fabry–Perot interferometer (FPI) composed of a miniature fiber collimator is proposed and demonstrated for high-sensitivity bend measurements. The device consists of a quarter-pitch graded index fiber (GIF) spliced with a section of silica tube, which is interposed between single-mode fibers. The divergence angle of the included miniature fiber collimator was 0.058 rad, which was found to enhance the sensitivity of the proposed FPI to –3.353 dB/degree over the applied angle range. In comparison, the divergence angle without the collimator was 0.130 rad and FPI sensitivity was –0.618 dB/degree over the applied angle range. The fiber bend sensors demonstrated in this paper are compact, robust, and highly sensitive, and hence, are promising in various biomedical applications. [ABSTRACT FROM AUTHOR]

We demonstrated femtosecond laser inscription of fiber Bragg gratings (FBGs) in a twin-core few-mode fiber (TC-FMF) for directional bend sensing. An FBG was selectively inscribed in one core of the TC-FMF by using an 800 nm femtosecond laser through a phase mask. Three resonance peaks at the wavelengths of 1549.05, 1547.65, and 1546.08 nm were observed in the reflection spectrum of the TC-FM FBG, and were generated by the LP01 mode resonance, LP01–LP11 mode cross-coupling resonance, and LP11 mode resonance, respectively. Moreover, the TC-FM FBG exhibited the capability of directional bend sensing and achieved a maximum bend sensitivity of −37.41 pm/m−1. Hence, the proposed TC-FM FBG directional bend sensors could further be developed as promising solutions for detecting vectorial seismic or acoustic waves and 3-D shape sensing. [ABSTRACT FROM PUBLISHER]

We demonstrate a novel polyvinyl chloride (PVC) diaphragm-based fiber-tip Fabry–Perot interferometer for gas-pressure measurements with ultrahigh sensitivity. The PVC diaphragm has been coated to the end facet of a well-cut standard single-mode fiber by use of a plastic welder. An ultrahigh-pressure sensitivity of ∼65.5 nm/MPa at 1565 nm and a low-temperature cross sensitivity of ∼–5.5 kPa/°C have been experimentally demonstrated. The proposed sensor has advantages of high pressure sensitivity, miniature size, low cost, and easy fabrication. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate an automatic arc discharge technology for inscribing high-quality helically twisted long period fiber gratings (H-LPFGs) with greatly improved inscription efficiency for single mode fibers. The proposed technology has been developed by implementing an embedded program in a commercial fusion splicer, which employs an ultraprecision motorized translation stage while the tensioning mass required by conventional inscribing technology is eliminated. More significantly, the arc-induced H-LPFGs have been reported to have potential usage as sensors in temperature, refractive index, twist stress, and strain. [ABSTRACT FROM PUBLISHER]

The degeneration of Ag coatings on the polished surfaces of side-polished fibers (SPFs) with different surface topographies, and its inhibition by Au plating were studied by the time-dependent red shift of the resonance wavelength of SPF surface plasmon resonance (SPR) sensors. The surface topography determines the Ag-particle distribution, and affects the optical performance. The resonance wavelength of rough and smooth samples demonstrates a red shift over time. However, the service lifetime of an Ag-SPR sensor with a rough surface is less than 48 h because an abundance of coating flakes destroys the integrity of the Ag coating, and accelerates oxidation. Degeneration does not decrease its sensitivity. The smooth surface retards the degeneration rate. A thin Au plating decreases the resonance bandwidth for the rough surface, and provides a resonance wavelength near the middle of the ideal range between 533 and 620 nm. The position of the resonance wavelength can be adjusted by changing the proportions of Ag and Au in the compound coating. The Au plating retards the rate of Ag degeneration, where the maximum degeneration rate of a pure Ag coating is 3.5 times greater that of an Ag–Au compound coating between 96 and 192 h. [ABSTRACT FROM PUBLISHER]

A bidirectional bend sensor based on U-shaped Fabry-Perot interferometer (UFPI) is experimentally demonstrated. The UFPI is constructed by eccentrically splicing a section of microfiber between two cleaved standard single mode fiber end faces serving as mirrors. A preliminary theoretic analysis of bending response of UFPI is presented. The bend and temperature sensing properties are measured. The theoretical and experimental investigation demonstrates the bending direction discrimination and the monotonous sensing characteristic in a single direction along or opposite to the cavity opening direction. The proposed bend sensor with more compact (∼105 μm in length) and easier fabrication presents a high contrast of the interferometer fringes and low temperature sensitivity of 1.21 × 10 −3 nm/°C. [ABSTRACT FROM PUBLISHER]

The influence of surface topography of side-polished fiber on resonance wavelengths and the full-width-at-half-maximum (FWHM) of surface plasmon resonance was evaluated in this work, based on the power spectrum density, wavelet, and finite-difference time-domain methods. The abrasive size determined the surface topography with various spatial period components. Coarse abrasives induced obvious low-frequency waviness features and a blue shift in the resonance wavelength. Fine abrasives introduced middle frequency microwaviness (0.5–1.0 μ m), which led to a blue or red shift, depending on the special period extent between 0.5 and 0.75 μ m. All waviness components broadened the FWHM because of the superimposed effect and the introduction of a high-order coupling model. High-frequency roughness components were able to shift the resonance peaks toward shorter wavelengths. Larger coupling energy tended to decrease the FWH, while high-order coupled modes tended to broaden the FWHM. We established a roughness model with Maxwell–Garnett theory incorporated into fractal dimensions. Experimental results demonstrated the feasibility of such a model. Small abrasive particles were able to narrow the FWHM, which is beneficial for identification of resonance wavelengths and evaluation of the surface plasmon resonance effect. [ABSTRACT FROM PUBLISHER]

We report a new fiber optic sensor for temperature measurement at a temperature range of up to 900  °C with excellent stability and repeatability. The sensing head is comprised of a short hollow-core fiber segment spliced between two single-mode fibers using a commercial splicer. Optical power from the lead-in fiber is partly coupled to silica cladding of the hollow-core fiber with the remainder propagating in air core by short distance. The two beams are then re-coupled to the core of the lead-out fiber, owing to the offset splicing joint. Due to the effective index difference between the two beams, an interference pattern in the transmission spectrum is obtained. The distinct thermo-optic coefficients between silica and air result in a high-temperature sensitivity of 41 pm/ °C, and the enclosed structure ensures its immunity to the external refractive index. [ABSTRACT FROM PUBLISHER]

We demonstrate an ultrasensitive temperature sensor based on a unique fiber Fabry–Pérot interferometer (FPI) that was created by means of splicing a mercury-filled silica tube with a single-mode fiber. The air cavity of the fiber FPI was formed by the surface of the mercury column and the endface of the single-mode fiber. Moreover, the mercury-based fiber FPI exhibits an ultrahigh temperature-sensitivity of up to −41.9 nm/°C, which is about one order of magnitude higher than that of the ordinary FPI-based fiber-tip sensors reported so far. Hence, such a mercury-based fiber FPI temperature sensor could be used for highly sensitive ambient temperature sensing applications. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrated the fabrication of asymmetric long-period fiber gratings (LPFGs) in thin core fiber by use of focused CO 2 laser beam. The proposed device exhibits a high extinction ratio of over 25 dB at the resonant wavelength and a narrowed 3-dB bandwidth of only 8.7 nm, which is nearly one order of magnitude smaller than that of LPFGs in conventional single-mode fibers. It also exhibits a high polarization-dependent loss of over 20 dB at resonant wavelength. The temperature and external refractive index (RI) sensitivity of the proposed structure are measured to be 46 pm/°C, within a temperature range from 25 °C to 100 °C, and 1047.3 nm/RIU, within the RI range from 1.400 to 1.440, respectively. The temperature induced error is ∼8% for RI measurement. Such long LPFGs may find potential applications of highly sensitive RI sensors in the fields of chemical and biomedical sensing. [ABSTRACT FROM PUBLISHER]

We demonstrate a high-sensitivity gas pressure sensor by use of an in-fiber Fabry–Pérot interferometer (FPI) based on hollow-core photonic bandgap fiber (HC-PBF) with a side-opened channel. The FPI was constructed by splicing a thin piece of HC-PBF between two stander single-mode fibers. Then, a side-opened channel was drilled through the hollow core of the HC-PBF by use of a femtosecond laser. Such an FPI with a side-opened channel greatly enhanced the gas pressure sensitivity up to 4.24 nm/MPa, which is two orders of magnitude higher than that of FPI with an enclosed cavity. In addition, the effects of cavity length on the gas pressure sensing performance were also studied. A shorter cavity gives rise to broader measurement range while offering larger detection limit, and vice versa. The structure size is tens of micrometers, which makes it possible to develop an ultracompact high-sensitivity gas pressure sensor. [ABSTRACT FROM PUBLISHER]

We demonstrate a simple, robust, and highly sensitive temperature sensor based on the resonant excitation of whispering gallery modes in a coated single-mode fiber (SMF) loop that was created by means of bending a standard coated SMF into a loop with a radius of 5 mm. Clear interference fringes were observed in the transmission spectrum of the coated-SMF loop. The resonant wavelength exhibits a linear temperature response at the temperature range from −60° to 10 °C, and a quadratic temperature response at the temperature range from 10° to 140 °C. Moreover, the coated-SMF loop exhibits an ultrahigh sensitivity of up to −5.22 nm/°C at 120 °C, and the sensitivity (i.e., −4.01 nm/°C at 80 °C) is two orders of magnitude higher than that of an uncoated-SMF loop (i.e., 0.04 nm/°C at 80 °C). Hence, such a coated-SMF loop could be used to develop a temperature sensor with a large measurement range from −60° to 140 °C. [ABSTRACT FROM PUBLISHER]

An integrated surface plasmon resonance (SPR) based on an Au-film-coated, side-opened suspended core fiber is proposed and simulated to develop a highly sensitive, real-time refractive index sensor. The inner surface of the side-opened hole is supposed to be uniformly deposited with a thin Au film for realizing the SPR operation. Such a sensor not only ensures a fast response time, but also brings up a designing flexibility for SPR biosensing applications. Two potential sensing mechanisms, i.e., monitoring the transmission loss spectrum peak shift and measuring the transmission power change at a fixed wavelength are utilized and analyzed to achieve a better sensitivity. Moreover, our calculation results show that this novel sensor could cover a wide index detection range with an optimized resolution of up to 2.3e−5 refractive index unit. [ABSTRACT FROM PUBLISHER]

We proposed and demonstrated a novel scheme for simultaneous measurement of temperature and refractive index (RI) by cascading a liquid-filled photonics crystal fiber (PCF) to a long period fiber grating (LPFG) written in a single-mode fiber. The liquid-filled PCF showed a high-temperature sensitivity of 1.695 nm/°C, which is two orders of magnitude higher than that of the LPFG (0.0642 nm/°C). The LPFG exhibited a blue shift with the increased RI. In contrast, liquid-filled PCF showed a total immunity to the surrounding RI. So, the temperature was solely measured by the liquid-filled PCF, and the RI information was extracted from the total response of the LPFG with the temperature effect having been compensated by the liquid-filled PCF. [ABSTRACT FROM PUBLISHER]