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.

Realisation of single polarisation state of 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 45°tilted fibre Bragg gratings (45° TFBGs). The degree of polarisation of 99.94% of the laser was demonstrated with good stability.

In-fibre twist sensor based on a fibre Bragg grating with 81º tilted structure

We report a strong polarization dependent coupling behavior of fiber Bragg gratings with excessively tilted structures up to 81 . This unique property has been utilized to implement a novel twist sensor, showing high torsion sensitivity. The twist induced light coupling interchange between the two birefringence modes makes it possible to interrogate such a sensor using low-cost intensity demodulation technique.

Realisation of an effective dual-parameter sensor employing a single fibre Bragg grating structure

A novel technology for simultaneous and independent measurement of dual parameters is proposed and experimented. The length of a single fibre Bragg grating (FBG) is divided into two parts. The temperature variation and another measurand can be measured independently and simultaneously, and the thermal effect can be erased with great ease.

Implementation of optical chemsensors based on HF-etched fibre Bragg grating structures

High-sensitivity optical chemsensors have been implemented by exploiting fibre Bragg grating structures UV-inscribed in D-shape, single-mode and multimode fibres and post-sensitized by hydrofluoric acid (HF) etching treatment. We have demonstrated that the Bragg grating structures which are intrinsically insensitive to chemicals can be sensitized by effective etching. All etched devices possess refractive index sensing capability that offers an encoding function to chemical concentrations. Most etched devices have been used to measure the concentrations of sugar solutions, showing a potential capability of detecting concentration changes as small as 0.1–0.5%.

Remotely tuneable optical filter based on polymer fibre Bragg grating

We propose a remotely tuneable optical Bragg grating filter written in polymer optical fibre (POF). Fibre optical pumping in the fibre's absorption bands increases the fibre temperature, which causes a negative wavelength change of the POF Bragg grating. By choosing a proper pumping wavelength remote tuning of the optical filter can be readily realized without changing the gain of the optical signal.

Optical fibre Bragg grating recorded in TOPAS cyclic olefin copolymer

A report is presented on the inscription of a fibre Bragg grating into a microstructured polymer optical fibre fabricated from TOPAS cyclic olefin copolymer. This material offers two important advantages over poly (methyl methacrylate), which up to now has formed the basis for polymer fibre Bragg gratings: TOPAS has a much lower water affinity and has useful properties for biosensing. The grating had a Bragg wavelength of 1569 nm and a temperature sensitivity of -36.5±0.3 pm/°C.

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.

Improved time response for polymer fibre Bragg grating based humidity sensors

In this work we experimentally investigate the response time of humidity sensors based on polymer optical fibre (POF) Bragg gratings. By the use of etching with acetone we can control the diameter of POF based on poly (methyl methacrylate) in order to reduce the diffusion time of water into the polymer and hence speed up the relative wavelength change caused by humidity variations. A much improved response time of 11 minutes has been achieved by using a POF FBG with a reduced diameter of 135 microns.

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.

Tactile sensing with fibre Bragg gratings and neural networks

Fibre Bragg grating sensors are usually expensive to interrogate, and part of this thesis describes a low cost interrogation system for a group of such devices which can be indefinitely scaled up for larger numbers of sensors without requiring an increasingly broadband light source. It incorporates inherent temperature correction and also uses fewer photodiodes than the number or sensors it interrogates, using neural networks to interpret the photodiode data. A novel sensing arrangement using an FBG grating encapsulated in a silicone polymer is presented. This sensor is capable of distinguishing between different surface profiles with ridges 0.5 to 1mm deep and 2mm pitch and either triangular, semicircular or square in profile. Early experiments using neural networks to distinguish between these profiles are also presented. The potential applications for tactile sensing systems incorporating fibre Bragg gratings and neural networks are explored.

Fibre Bragg grating strain sensors

The fabrication of in-fibre Bragg gratings (FBGs) and their application as sensors is reported. The strain and temperature characteristic results for a number of chirped and uniform gratings written into three different host fibres are presented. The static and dynamic temperature response of a commercially available temperature compensated grating is reported. A five sensor wavelength division multiplexed fibre Bragg grating strain measurement system with an interrogation rate of 25 Hz and resolution of 10 was constructed. The results from this system are presented. A novel chirped FBG interrogation method was implemented in both the 1.3 and 1.5 m telecommunication windows. Several single and dual strain sensor systems, employing this method, were constructed and the results obtained from each are reported and discussed. These systems are particularly suitable for the measurement of large strain. The results from a system measuring up to 12 m and with a potential measurement range of 30 m are reported. This technique is also shown to give an obtainable resolution of 20 over a measurement range of 5 000 for a dual sensor system. These systems are simple, robust, passive and easy to implement. They offer low cost, high speed and, in the case of multiple sensors, truly simultaneous interrogation. These advantages make this technique ideal for strain sensing in SMART structures. Systems based on this method have been installed in the masts of four superyachts. A system, based on this technique, is currently being developed for the measurement of acoustic waves in carbon composite panels. The results from an alternative method for interrogating uniform FBG sensors are also discussed. Interrogation of the gratings was facilitated by a specifically written asymmetric grating which had a 15 nm long linearly sloped spectral edge. This technique was employed to interrogate a single sensor over a measurement range of 6 m and two sensors over a range of 4.5 me. The results obtained indicated achievable resolutions of 47 and 38 respectively.