High sensitivity temperature sensor based on a side-hole fiber.

This paper proposes a temperature sensor based on a side-hole fiber (SHF). The sensor is formed by single-mode fiber (SMF)-coreless fiber (CLF)-SHF-CLF-SMF fusion splicing. The SHF adopts the dislocation fusion splicing method to ensure that one air hole is exposed. Two different interferences form a superposition, making the response more sensitive. The experiment shows that the sensitivity during heating and cooling is 1.587 nm/°C and 1.681 nm/°C, respectively, in the temperature range of 25-45°C. The sensor has high temperature sensitivity, exhibits easy processing, is smaller in size, and has important research value for temperature monitoring in daily life and industrial production.

Highly efficient free-space fiber coupler with 45° tilted fiber grating to access remotely placed optical fiber sensors.

In this work, a 45° tilted fiber grating (TFG) is used as a waveguide coupler for the development of a portable interrogation system to access remotely placed optical fiber sensors. The TFG is directly connected to a remote fiber sensor and serves as a highly efficient light coupler between the portable interrogation unit and the sensor. Variation of strain and temperatures are measured with a standard fiber Bragg grating (FBG) sensor, which serves as a remotely placed optical sensor. A light beam from the interrogation unit is coupled into the TFG by a system of lenses, mirrors and optical collimator and acted as the input of the FBG. Reflected light from the FBG sensor is coupled back to the interrogation unit via the same TFG. The TFG is being used as a receiver and transmitter of light and constituent the key part of the system to connect "light source to the optical sensor" and "optical sensor to detector." A successful demonstration of the developed system for strain and temperature sensing applications have been presented and discussed. Signal to noise ratio of the reflected light from the sensors was greater than ∼ 40 dB.

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%.

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.

Self-Mixing Demodulation for Coherent Phase-Sensitive OTDR System.

Phase-sensitive optical time domain reflectometry (Ф-OTDR) attracts much attention due to its capability of telling the type and position of an intrusion simultaneously. In recent decades, coherent Ф-OTDR has been demonstrated to realize long-distance detection. For coherent Ф-OTDR, there are three typical demodulation schemes in the reported studies. However, they still cannot realize real-time monitoring to satisfy practical demands. A simple and effective demodulation method based on self-mixing has been put forward to demodulate the beat signal in coherent Ф-OTDR. It not only saves a local electrical oscillator and frequency locked loop, but also demodulates the beat signal without residual frequency. Several vibrations with different frequency were separately applied at the same location of a 42.5 km fiber. The spatial resolution of 10 m and frequency response range from 8 Hz to 980 Hz have been achieved. The precise location with signal-to-noise ratio of 21.4 dB and broadband measurement demonstrate the self-mixing scheme can demodulate the coherent Ф-OTDR signal effectively.

Multiple vibrations measurement using phase-sensitive OTDR merged with Mach-Zehnder interferometer based on frequency division multiplexing.

A novel measurement scheme for multiple high-frequency vibrations has been demonstrated by combining phase-sensitive optical time domain reflectometry (Ф-OTDR) and Mach-Zehnder interferometer (MZI) based on frequency division multiplexing. The light source is directly launched into the MZI structure, while it was modulated by an acoustic optical modulator (AOM) with a frequency shift of 200 MHz for the Ф-OTDR part. The vibration frequency is obtained by demodulating the interference signal obtained by the MZI structure, while the vibration position is located by Ф-OTDR system. The spatial resolution of 10m is obtained over 3 km sensing fiber. And the detectable vibration frequency reaches up to 40 kHz. Compared to the previous schemes, this system works without dead zone in the detectable frequency range. Furthermore, the frequency spectrum mapping method has been adopted to determine multiple high-frequency vibrations simultaneously. The experimental results prove the concept and match well with the theoretical analysis.

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.

Regeneration of fiber Bragg gratings under strain.

The effect of strain on both the index modulation, Δn(mod), and average index, Δn, during grating regeneration within two types of fibers is studied. Significant tunability of the Bragg wavelength (λ(B)>48 nm) is observed during postannealing at or above the strain temperature of the glass. The main reason for the grating wavelength shift during annealing with load is the elongation of the fiber. As well, the observed Moiré interference cycling through regeneration indicates the presence of two gratings.

Label-free, disposable fiber-optic biosensors for DNA hybridization detection.

A novel and highly sensitive fiber-optic DNA sensor based on a thin-core fiber modal interferometer (TCFMI) is demonstrated by using a layer-by-layer (LbL) self-assembly technology. Poly(ethylenimine) (PEI), poly(acrylic acid) (PAA) and single-stranded DNA (ssDNA) were used for the preparation of a polyelectrolyte multilayer film for DNA detection. The film thickness was measured through a surface profilometer. The surface morphologies of (PEI/PAA)4, (PEI/PAA)4.5 and (PEI/PAA)4(PEI/DNA)1 multilayer films were characterized by atomic force microscopy. The fabricated DNA sensors were tested with different types of target ssDNA solutions with a concentration of 1 µM. The results show that the sensitivity of the TCFMI-based ssDNA sensor is 0.27 nm/matched-base at the concentration of 1 µM and can even distinguish the number of matched bases of ssDNA chains.

Temperature and strain characterization of regenerated gratings.

Both temperature and strain characterization of seed and regenerated gratings with and without post annealing is reported. The high temperature regeneration has significant impact on thermal characterization and mechanical strength of gratings while the post annealing has little effect. The observed difference is evidence of viscoelastic changes in glass structure.

Fiber optic pH sensor with self-assembled polymer multilayer nanocoatings.

A fiber-optic pH sensor based on a tilted fiber Bragg grating (TFBG) with electrostatic self-assembly multilayer sensing film is presented. The pH sensitive polymeric film, poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) was deposited on the circumference of the TFBG with the layer-by-layer (LbL) electrostatic self-assembly technique. The PDDA/PAA film exhibits a reduction in refractive index by swelling in different pH solutions. This effect results in wavelength shifts and transmission changes in the spectrum of the TFBG. The peak amplitude of the dominant spectral fringes over a certain window of the transmission spectrum, obtained by FFT analysis, has a near-linear pH sensitivity of 117 arbitrary unit (a.u.)/pH unit and an accuracy of ±1 a.u. (in the range of pH 4.66 to pH 6.02). The thickness and surface morphology of the sensing multilayer film were characterized to investigate their effects on the sensor's performance. The dynamic response of the sensor also has been studied (10 s rise time and 18 s fall time for a sensor with six bilayers of PDDA/PAA).

Bulk regeneration of optical fiber Bragg gratings.

The reliability and reproducibility of regenerated gratings for mass production is assessed through simultaneous bulk regeneration of 10 gratings. The gratings are characterized and variations are compared after each stage of fabrication, including seed (room-temperature UV fabrication), regeneration (annealing at 850°C), and postannealing (annealing at 1100°C). In terms of Bragg wavelength (λ(B)), the seed grating variation lies within Δλ(B)=0.16 nm, the regenerated grating within Δλ(B)=0.41 nm, and the postannealed grating within Δλ(B)=1.42 nm. All the results are within reasonable error, indicating that mass production is feasible. The observable spread in parameters from seed to regenerated grating is clearly systematic. The postannealed spread arises from the small tension on the fiber during postannealing and can be explained by the softening of the glass when the strain temperature of silica is reached.

Highly sensitive fiber loop ringdown strain sensor using photonic crystal fiber interferometer.

A highly sensitive strain sensor is demonstrated by introducing a photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI) in a cavity ringdown fiber loop as a sensing element. The MZI is fabricated by splicing a short length of PCF between two single-mode fibers with collapsed air holes over a short region at two splicing points, which allows coupling between core and cladding modes inside the PCF. By measuring the decay constants of the fiber ringdown loop under different applied strains, a high strain sensitivity of ~0.21 µs⁻¹/εm and a minimum detectable strain of ~3.6 µÎµ are obtained. As a benefit from the ultralow thermal dependence of PCF, the maximum temperature-induced measurement error could be reduced to ~0.24 µÎµ.

Linear and nonlinear optical properties of carbon nanotube-coated single-mode optical fiber gratings.

Single-wall carbon nanotube deposition on the cladding of optical fibers has been carried out to fabricate an all-fiber nonlinear device. Two different nanotube deposition techniques were studied. The first consisted of repeatedly immersing the optical fiber into a nanotube supension, increasing the thickness of the coating in each step. The second deposition involved wrapping a thin film of nanotubes around the optical fiber. For both cases, interaction of transmitted light through the fiber core with the external coating was assisted by the cladding mode resonances of a tilted fiber Bragg grating. Ultrafast nonlinear effects of the nanotube-coated fiber were measured by means of a pump-probe pulses experiment.

High resolution interrogation of tilted fiber grating SPR sensors from polarization properties measurement.

The generation of surface plasmon resonances (SPRs) in gold-coated weakly tilted fiber Bragg gratings (TFBGs) strongly depends on the state of polarization of the core guided light. Recently, it was demonstrated that rotating the linear state of polarization of the guided light by 90° with respect to the grating tilt allows to turn the SPR on and off. In this work, we measure the Jones matrix associated to the TFBG transmission properties in order to be able to analyze different polarization-related parameters (i.e. dependency on wavelength of polarization dependent loss and first Stokes parameter). As they contain the information about the SPR, they can be used as a robust and accurate demodulation technique for refractometry purposes. Unlike other methods reported so far, a tight control of the input state of polarization is not required. The maximum error on refractive index measurement has been determined to be ~1 10(-5) refractive index unit (RIU), 5 times better than intensity-based measurements on the same sensors.

Fiber optic pressure sensing with conforming elastomers.

A novel pressure sensing scheme based on the effect of a conforming elastomer material on the transmission spectrum of tilted fiber Bragg gratings is presented. Lateral pressure on the elastomer increases its contact angle around the circumference of the fiber and strongly perturbs the optical transmission of the grating. Using an elastomer with a Young's modulus of 20 MPa, a Poisson ratio of 0.48, and a refractive index of 1.42, the sensor reacts monotonically to pressures from 0 to 50 kPa (and linearly from 0 to 15 kPa), with a standard deviation of 0.25 kPa and maximum error of 0.5 kPa. The data are extracted from the optical transmission spectrum using Fourier analysis and we show that this technique makes the response of the sensor independent of temperature, with a maximum error of 2% between 25°C and 75°C. Finally, other pressure ranges can be reached by using conforming materials with different modulii or applying the pressure at different orientations.

Intrinsic temperature sensitivity of tilted fiber Bragg grating based surface plasmon resonance sensors.

A miniature surface plasmon resonance sensor is fabricated from a gold-coated standard optical fiber with an in-core tilted fiber Bragg grating fabricated by UV exposure. The sensor has a measured refractive index sensitivity of 571.5 nm/RIU (refractive index unit) at constant temperature. We show here that the intrinsic temperature sensitivity of this device is reduced to less than 6.3 pm/degrees C (between 23 degrees C and 59 degrees C) when measurements are referenced to a core mode reflection resonance of the grating. This residual sensitivity is essentially that of the 50 nm thick deposited gold layer but it is bigger by one order of magnitude than the expected value (0.51 pm/degrees C) for a gold-water interface.

Compact fiber-optic vector inclinometer.

What we believe to be a novel fiber-optical inclinometer based on a dual-path core-to-cladding mode coupling mechanism is demonstrated. Both the amplitude and tilt direction of bends about a 2-mm-long flexure joint in an optical fiber can be determined. The sensor head consists of a nonadiabatic abrupt taper cascaded with a weakly tilted fiber Bragg grating. Measured bend angles of up to 12 degrees are demonstrated, as well as a method to increase the sensitivity for bend angles smaller than 4 degrees.

Pressure sensor realized with polarization-maintaining photonic crystal fiber-based Sagnac interferometer.

A novel intrinsic fiber optic pressure sensor realized with a polarization-maintaining photonic crystal fiber (PM-PCF) based Sagnac interferometer is proposed and demonstrated experimentally. A large wavelength-pressure coefficient of 3.42 nm/MPa was measured using a 58.4 cm long PM-PCF as the sensing element. Owing to the inherently low bending loss and thermal dependence of the PM-PCF, the proposed pressure sensor is very compact and exhibits low temperature sensitivity.

Long-period grating fabricated by periodically tapering standard single-mode fiber.

We fabricated an asymmetric long-period grating (LPG) by periodically tapering a section of standard single-mode fiber using a resistive filament heating. The LPG exhibits large peak transmission attenuation of -30.31 dB with only 22 periods in a 1.0 cm long optical fiber and possesses unique characteristics for sensing applications. The bending and strain sensitivities are 1.74 nm m and 1.11 pm/mu epsilon, respectively. The polarization dependent loss is large, up to 11.65 dB, which is caused by an asymmetric index profile in the cross section of the tapered LPG.