20-km distributed temperature sensor based on spontaneous Brillouin scattering

We report on a novel experimental setup for distributed measurement of temperature, based on spontaneous Brillouin scattering in optical fiber. We have developed a mode-locked Brillouin fiber ring laser in order to generate the dual frequency source required for a heterodyne detection of the backscattered signal. This relatively simple system enables temperature measurements over 20 km with a spatial resolution of 7 m.

Experimental and theoretical studies on a distributed temperature sensor based on Brillouin scattering

Results are reported from recent research on the use of the Brillouin gain/loss mechanism for distributed sensing. A theoretical model of the interaction of the pulsed and CW beams is described and compared with experiments. Results from a system with a 51 km sensing length are presented. We finally investigate issues related to the variation within the sensing fiber of the polarizations of the two beams.

Distributed temperature change sensor, based on Rayleigh backscattering in an optical fibre

A frequency-modulated continuous-wave technique is used to detect the presence of frequency shifts in the Rayleigh-backscattered light in a single-mode optical fiber as a result of a changing temperature. The system is able to detect a rate of temperature change of 0.014 K/s, when a 20-cm length of fiber is heated. The system is also able to demonstrate a spatial resolution of better than 15 cm.

Demultiplexing of fibre Bragg grating temperature and strain sensors

We describe a demultiplexing scheme for fibre optic Bragg grating sensors in which signal recovery is achieved by locking each sensor grating to a corresponding receiver grating. As a demonstration, the technique is applied to strain and temperature sensing, achieving a resolution of 3.0 µe and 0.2°C, respectively.

High extinction ratio in-fibre polarizer based on a 45º titled fibre Bragg grating

We report an in-fiber polarizer implemented using a 45° tilted FBG. Polarization extinction ratio of 28dB at 1550nm and HMFW over 80nm has been demonstrated. When the un-polarized light passes this device, 99.5% of degree of polarization can be achieved.

Single polarisation fibre ring laser by utilising intracavity 45° tilted fibre Bragg grating

Single polarisation operation of fibre ring laser has been realised by employing an intracavity 45deg-tilted fibre Bragg grating (45deg-TFBG). The degree of polarisation up to 99.94% of the laser was demonstrated with good stability.

Implementation of optical fibre sensors with low thermal cross-sensitivity utilising ex-45° tilting grating structures

We report an implementation of optical fibre sensors based on fibre Bragg gratings with excessively tilted (>45°) structures, showing distinctive polarisation characteristics, desirable low thermal-cross-sensitivity and enhanced responsivity to surrounding-medium-refractive-index.

Bending and orientational characteristics of long period gratings written in D-shaped optical fiber

Long period gratings (LPGs) were written into a D-shaped single-mode fiber. These LPGs were subjected to a range of curvatures, and it was found that as curvature increased, there was increasingly strong coupling to certain higher order cladding modes without the usual splitting of the LPGs stopbands. A bend-induced stopband yielded a spectral sensitivity of 12.55 nm · m for curvature and 2.2 × 10-2 nm°C-1 for temperature. It was also found that the wavelength separation between adjacent bend-induced stopbands varied linearly as a function of curvature. Blue and red wavelength shifts of the stopbands were observed as the sensor was rotated around a fixed axis for a given curvature; thus, in principle, this sensor could be used to obtain bending and orientational information. The behavior of the stopbands was successfully modeled using a finite element approach.

DNA hybridisation biosensor based on dual-peak long-period grating

Using an optical biosensor based on dual-peak long-period fibre grating, we demonstrate the detection of interactions between DNA biomolecules in real-time, showing a high sensitivity and reusability function.

Microstructures made in optical fiber with femtosecond laser

Different types of microstructures including microchannels and microslots were made in optical fibers using femtosecond laser inscription and chemical etching. Integrated with UV-inscribed fiber Bragg gratings, these microstructures have miniature, robustness and high sensitivity features and have been used to implement novel devices for various sensing applications. The fiber microchannels were used to detect the refractive index change of liquid presenting sensitivities up to 7.4 nm/refractive index unit (RIU) and 166.7 dB/RIU based on wavelength and power detection, respectively. A microslot-in-fiber based liquid core waveguide as a refractometer has been proposed and the device was used to measure refractive index, and a sensitivity up to 945 nm/RIU (10-6/pm) was obtained. By filling epoxy in the microslot and subsequent UV light curing, a hybrid waveguide grating structure with polymer core and glass cladding was fabricated. The obtained device was highly thermal responsive, demonstrating a linear coefficient of 211 pm/°C.

Low thermal sensitivity grating devices based on ex-45° tilting structure capable of forward-propagating cladding modes coupling

The authors describe a detailed investigation on tilted fiber Bragg grating (TFBG) structures with tilted angles exceeding 45°. In contrast to the backward mode coupling mechanism of Bragg gratings with normal and small tilting structures, the ex-45° TFBGs facilitate the light coupling to the forward-propagating cladding modes. The authors have also theoretically and experimentally examined the mode coupling transition of TFBGs with small, medium, and large tilt angles. In particular, experiments are conducted to investigate the spectra and far-field distribution, as well as temperature, strain, and refractive-index sensitivities of ex-45° devices. It has been revealed that these ex-45° gratings exhibit ultralow thermal sensitivity. As in-fiber devices, they may be superior to conventional Bragg and long-period gratings when the low thermal cross sensitivity is required.

Characterisation and performance of a Terfenol-D coated femtosecond laser inscribed optical fibre Bragg sensor with a laser ablated microslot for the detection of static magnetic fields

We present a novel device for the characterisation of static magnetic fields through monitoring wavelength shifts of femtosecond inscribed fibre Bragg grating and micromachined slot, coated with Terfenol-D. The device was sensitive to static magnetic fields and can be used as a vectoral sensor for the detection of magnetic fields as low as 0.046 mT with a resolution of ± 0.3mT in transmission and ± 0.7mT in reflection. The use of a femtosecond laser to both inscribe the FBGs and micromachine the slot in a single stage prior to coating the device significantly simplifies the fabrication.

870nm Bragg grating in single mode TOPAS microstructured polymer optical fibre

We report the fabrication and characterization of a fiber Bragg grating (FBG) with 870 nm resonance wavelength in a single-mode TOPAS microstructured polymer optical fiber (mPOF). The grating has been UV-written with the phasemask technique using a 325 nm HeCd laser. The static tensile strain sensitivity has been measured as 0.64 pm/µstrain, and the temperature sensitivity was -60 pm/°C. This is the first 870nm FBG and the first demonstration of a negative temperature response for the TOPAS FBG, for which earlier results have indicated a positive temperature response. The relatively low material loss of the fiber at this wavelength compared to that at longer wavelengths will considerably enhance the potential utility of the TOPAS FBG.

All-fibre twist sensor system based on 45 and 81 tilted fibre gratings

We experimentally demonstrated a highly sensitive twist sensor system based on a 45° and an 81° tilted fibre grating (TFG). The 81°-TFG has a set of dual-peaks that are due to the birefringence induced by its extremely tilted structure. When the 81°-TFG subjected to twist, the coupling to the two peaks would interchange from each other, providing a mechanism to measure and monitor the twist. We have investigated the performance of the sensor system by three interrogation methods (spectral, power-measurement and voltage-measurement). The experimental results clearly show that the 81°-TFG and the 45°-TFG could be combined forming a full fibre twist sensor system capable of not just measuring the magnitude but also recognising the direction of the applied twist.

Numerical modelling of complex femtosecond laser inscribed fiber gratings - comparison wih experiment

We present experimental studies and numerical modeling based on a combination of the Bidirectional Beam Propagation Method and Finite Element Modeling that completely describes the wavelength spectra of point by point femtosecond laser inscribed fiber Bragg gratings, showing excellent agreement with experiment. We have investigated the dependence of different spectral parameters such as insertion loss, all dominant cladding and ghost modes and their shape relative to the position of the fiber Bragg grating in the core of the fiber. Our model is validated by comparing model predictions with experimental data and allows for predictive modeling of the gratings. We expand our analysis to more complicated structures, where we introduce symmetry breaking; this highlights the importance of centered gratings and how maintaining symmetry contributes to the overall spectral quality of the inscribed Bragg gratings. Finally, the numerical modeling is applied to superstructure gratings and a comparison with experimental results reveals a capability for dealing with complex grating structures that can be designed with particular wavelength characteristics.