A simple method for fabricating phase-shifted fibre Bragg gratings with flexible choice of centre wavelength

A simple technique for fabricating phase-shifted fibre Bragg gratings (PSFBGs) without the use of a phase-shifted phase mask is presented. Two, 3-mm long, standard fibre Bragg gratings (FBGs) were inscribed sequentially in singlemode fibre at the same Bragg wavelength such that the FBGs physically overlapped by one grating period. This induces a spectral-hole in the middle of the reflection spectrum of a standard FBG, equivalent to a π-phase shifted FBG. The flexibility of the technique in writing PSFBGs at any choice of wavelength is demonstrated. The results show that PSFBG devices produced by this method are highly reproducible and the process is fas

Fabrication and application of short and novel structure in-fibre Bragg gratings

This thesis presents details on progress made in the fabrication and application of short and novel structure fibre Bragg gratings. The basic theoretical concepts of in-fibre Bragg gratings and photosensitive mechanisms are introduced together with an overview of fabrication methods and applications presented to date. The fabrication of fibre Bragg gratings using a quadrupled Nd:YAG laser is presented and some of the issues of grating fabrication using a fabrication using a phasemask are investigated, including the variation of the separation of the fibre and phasemask, and other alignment issues. A new apodisation technique is presented, enabling the production of gratings with a wide range of spectral profiles. The technique is used to investigate the design and fabrication of length limited fibre Bragg gratings for use in telecommunication systems as filters. Application to devices designed for use in WDM systems is presented. The use of fibre Bragg gratings as high spatial resolution distributed sensors is investigated. Grating sensing arrays comprising very short apodised gratings are demonstrated and Chirped Moiré gratings are implemented as distributed sensors achieving high spatial resolution with miniature point sensing sub-elements. A novel grating sensing element designed to imitate an interferometer is also presented. Finally, the behaviour of gratings fabricated in Boron-Germania-co-doped fibre is investigated, revealing atypical behaviour of the Bragg wavelength during ageing

Long continuously chirped fibre Bragg gratings for compensation of linear- and 3rd order-dispersion

For the first time long broadband chirped fibre Bragg gratings with a dispersion profile designed to compensate 3rd order-dispersion are presented. These results demonstrate how the increased demands for dispersion compensation at very high bit-rates can be met using chirped fibre Bragg grating

The use of fibre Bragg gratings for ultrasonic Lamb wave detection and source location

There is an increasing interest in the use of ultrasonic Lamb waves for materials characterisation and the detection of structural defects. Fibre Bragg gratings offer some significant advantages over more traditional transducers such as piezoelectrics for ultrasound detection and are also dual purpose, since the same sensors can also be used for strain mapping.. We describe the use of fibre Bragg gratings to detect ultrasound signals and thereafter to determine both their direction of propagation and source location. The main aspects that will be concentrated upon will be – optimising FBG interrogation techniques; maximising the efficiency of strain transfer from the sample to the FBG sensors; describing the directional response of FBGs; how this allows them to be configured into rosettes to determine the direction of the received ultrasound signal and, finally, how an array of 2 or more rosettes can be used to determine the source of the ultrasound

Strain sensing characterization of polymer optical fibre Bragg gratings

Static tensile strain is applied to polymer optical fibre Bragg gratings for the strain sensing characterization. Experimental results indicate that the strain coefficient of polymer fiber Bragg gratings is larger that that of silica fibre Bragg gratings. It is also demonstrated the large strain sensing range with good reproducibility, reversibility and repeatability is achieved. As a result, the study shows the great potential of polymer fibre Bragg gratings in the fibre strain sensing applications

Fibre Bragg gratings in polymer optical fibre for applications in sensing

This thesis presents the potential sensing applications of fibre Bragg gratings in polymer optical fibres. Fibre Bragg gratings are fabricated in different kinds of polymer optical fibres, including Poly methyl methacrylate (PMMA) and TOPAS cyclic olefin copolymer based microstructured polymer optical fibres and PMMA based step-index photosensitive polymer optical fibre, using the 325nm continuous wave ultraviolet laser and phase mask technique. The thermal response of fabricated microstructured polymer optical fibre Bragg gratings has been characterized. The PMMA based single mode microstructured polymer optical fibre Bragg gratings exhibit negative non-linear Bragg wavelength shift with temperature, including a quasi-linear region. The thermal sensitivity of such Bragg gratings in the linear region is up to -97pm/°C. A permanent shift in the grating wavelength at room temperature is observed when such gratings are heated above a threshold temperature which can be extended by annealing the fibre before grating inscription. The largest positive Bragg wavelength shift with temperature in transmission is observed in TOPAS based few moded microstructured polymer optical fibre Bragg gratings and the measured temperature sensitivity is 250±0.5pm/°C. Gluing method is developed to maintain stable optical coupling between PMMA based single mode step index polymer optical fibre Bragg gratings and single mode step index silica optical fibre. Being benefit from this success, polymer optical fibre Bragg gratings are able to be characterised for their temperature, humidity and strain sensitivity, which are -48.2±1pm/°C, 38.3±0.5pm per %RH and 1.33±0.04 pm/µ??respectively. These sensitivities have been utilised to achieve several applications. The strain sensitivity of step index polymer optical fibre Bragg grating devices has been exploited in the potential application of the strain condition monitoring of heavy textiles and when being attached to textile specimens with certain type of adhesives. These polymer fibre Bragg grating devices show better strain transfer and lower structure reinforcement than silica optical fibre Bragg grating devices. The humidity sensitivity of step index polymer optical fibre Bragg grating devices is applied to detecting water in jet fuel and is proved to be able to measure water content of less than 20 ppm in Jet fuel. A simultaneous temperature and humidity sensor is also made by attaching a polymer fibre Bragg grating to a silica optical fibre Bragg grating and it shows better humidity measurement accuracy than that of electronic competitors

Dynamic characterisation of a damaged composite structure with stiffeners employing fibre bragg gratings

One of the key issues in composite structures for aircraft applications is the early detection and localisation of damage. Often service induced damage does not involve visible plastic deformation, but internal matrix related damage, like transverse cracks and delaminations. Their detection imposes costly maintenance techniques. Vibration based damage identification methods are promising as an alternative for the time consuming and costly Non-Destructive Testing methods currently available. These methods also offer the potential to be used in a real-time health monitoring system. The measured change of the dynamic properties is employed to identify damage such as delaminations.\ud Earlier performed research [1] showed that the Modal Strain Energy Damage Index algorithm [2] is a suitable method to identify impact induced damage in a fibre reinforced composite plate structure with stiffeners using laser vibrometer measurements. The damage identification algorithm requires the computation of the second derivative of the displacement mode shapes.\ud The goal is to extent this research by applying fibre Bragg gratings since they can be valuable. Firstly, optical fibre sensors are suitable for integration, which is required in a Structural Health Monitoring environment. Secondly, measured strain mode shapes could be advantageous with respect to the numerical errors induced by the computation of second derivatives of the displacement mode shapes.\ud Before applying the damage identification algorithm, it is a challenge to accurately extract the dynamic properties. The dynamic properties of a damaged composite T-shaped stiffener section, shown in figure 1, are investigated in this work using fibre Bragg gratings

Intrinsic, multiplexable sensors for electric field strength using structural slow light in phase-shifted fibre Bragg gratings

In this paper we demonstrate through simulation the potential for phase-shifted fibre Bragg gratings incorporating structural slow light to enable intrinsic reflection-mode point sensors for electric field or voltage. It is shown that lo-bi FBGs incorporating multiple phase shifts yield large enhancements in group index (group delay) at resonance, thus amplifying and localizing time-dependent non-reciprocal effects. A relative, multiplexable measurement of electric field by comparison of the phase unbalance between linear modes on and off resonance is proposed, yielding static resolutions of 24 V and 18 mV respectively in unpoled (dc Kerr effect) and poled (Pockels effect) fibres

Non-linear temperature response of Bragg gratings in doped and un-doped holey polymer optical fibre

We present measurements on the non-linear temperature response of fibre Bragg gratings recorded in pure and trans-4-stilbenemethanol-doped polymethyl methacrylate (PMMA) holey fibres