Utilisation of thermal annealing to record multiplexed FBG sensors in multimode microstructured polymer optical fibre

Fibre Bragg gratings have been UV inscribed in multimode microstructured polymer optical fibre in both the 1550nm and 800nm spectral regions. Thermally annealing the fibre at 80°C has been shown to shrink the fibre length and as a result a permanent negative Bragg wavelength shift is observed. The blue shift can be tuned between 0-16nm in the 1550nm spectral region and 0-6nm in the 800nm spectral region, depending on the duration the heat is applied before a saturation level is reached and the fibre stops shrinking in the region of 2 hours. Exploiting this, wavelength division multiplexed sensors have been UV inscribed in both the 1550nm and 800nm regions using a single phase mask for each wavelength region. The 800nm sensor takes advantage of the lower attenuation of poly (methyl methacrylate) of 2dB/m compared to 100dB/m at 1550nm.

Fibre optic sensors for high speed hypervelocity impact studies and low velocity drop tests

The initial aim of this project was to develop a non-contact fibre optic based displacement sensor to operate in the harsh environment of a 'Light Gas Gun' (LGG), which can 'fire' small particles at velocities ranging from 1-8.4 km/s. The LGG is used extensively for research in aerospace to analyze the effects of high speed impacts on materials. Ideally the measurement should be made close to the centre of the impact to minimise corruption of the data from edge effects and survive the impact. A further requirement is that it should operate at a stand-off distance of ~ 8cm. For these reasons we chose to develop a pseudo con-focal intensity sensor, which demonstrated resolution comparable with conventional PVDF sensors combined with high survivability and low cost. A second sensor was developed based on 'Fibre Bragg Gratings' (FBG) which although requiring contact with the target the low weight and very small contact area had minimal effect on the dynamics of the target. The FBG was mounted either on the surface of the target or tangentially between a fixed location. The output signals from the FBG were interrogated in time by a new method. Measurements were made on composite and aluminium plates in the LGG and on low speed drop tests. The particle momentum for the drop tests was chosen to be similar to that of the particles used in the LGG.

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.

The application of fibre Bragg grating networks as strain sensors and as phased array antenna true time delay elements

The fabrication of in-fibre Bragg gratings, and the application of arrays of such gratings as strain sensors and as true time delay elements for the control of phased array antennas is reported. Chirped period Bragg gratings were produced using the fibre deformation fabrication technique, with chirps of between 2.9nm and 17.3nm achieved. Arrays of 5mm and 2mm long uniform period Bragg gratings were fabricated using the inscription method, for use as true time delay elements,dissimilar wavefronts and their spectral characteristics recorded. The uniform period grating arrays were used to create minimum time delays of 9.09ps, 19.02ps and 31ps; making them suitable for controlling phased array antennas operating at RF frequencies of up to 3GHz, with 10° phase resolution. Four 4mm long chirped gratings were produced using the dissimilar wavefronts fabrication method, having chirps of 7nm, 12nm, 20nm and 30nm, and were used to create time delays of between 0.3ps and 59ps. Hence they are suitable for controlling phased array antennas at RF frequencies of up to 48GHz. The application of in fibre Bragg gratings as strain sensors within smart structure materials was investigated, with their sensitivity to applied strain and compression measured for both embedded and surface mounted uniform period and fibre Fabry-Perot filter gratings. A fibre Bragg grating sensor demultiplexing scheme based on a liquid crystal filled Fabry-Perot etalon tuneable transmission filter was proposed, successfully constructed and fully characterised. Three characteristics of the LCFP etalon were found to pose operational limitations to its application in a Bragg grating sensor system; most significantly, the resonance peak wavelength was highly (-2,77nm/°C) temperature dependent. Several methods for minimising this temperature sensitivity were investigated, but enjoyed only limited success. It was therefore concluded that this type (E7 filled) of LCFP etalon is unsuitable for use as a Bragg grating sensor demultiplexing element.

Embedded fibre Bragg grating array sensors in aluminium alloy matrix by ultrasonic consolidation

Fibre Bragg Grating (FBG) array sensors have been successfully embedded in aluminium alloy matrix by ultrasonic consolidation (UC) technique. The temperature and loading responses of the embedded FBG arrays have been systematically characterised. The embedded grating sensors exhibit an average temperature sensitivity of ~36pm/°C, which is three times higher than that of normal FBGs, and a loading responsivity of ~0.1nm/kg within the dynamic range from 0kg to 3kg. This initial experiment clearly demonstrates that FBG array sensors can be embedded in metal matrix together with other passive and active fibres to fabricate smart materials to monitor the operation and health of engineering structures.

The spectral characteristics of femtosecond laser inscribed long period grating bend sensors written into a photonic crystal fibre

A series of LPGs was inscribed in photonic crystal fibre by a low repetition femtosecond laser system. When subjected to bending they were found to be spectrally sensitive to bend orientation and displayed a strong polarisation dependence.

Fibre optics sensors in tear electrolyte analysis:towards a novel point of care potassium sensor

The diagnosis of ocular disease is increasingly important in optometric practice and there is a need for cost effective point of care assays to assist in that. Although tears are a potentially valuable source of diagnostic information difficulties associated with sample collection and limited sample size together with sample storage and transport have proved major limitations. Progressive developments in electronics and fibre optics together with innovation in sensing technology mean that the construction of inexpensive point of care fibre optic sensing devices is now possible. Tear electrolytes are an obvious family of target analytes, not least to complement the availability of devices that make the routine measurement of tear osmolarity possible in the clinic. In this paper we describe the design, fabrication and calibration of a fibre-optic based electrolyte sensor for the quantification of potassium in tears using the ex vivo contact lens as the sample source. The technology is generic and the same principles can be used in the development of calcium and magnesium sensors. An important objective of this sensor technology development is to provide information at the point of routine optometric examination, which would provide supportive evidence of tear abnormality.

Demountable connection for polymer optical fiber grating sensors

The authors fabricated a demountable Ferrule connector/Physical contact connection between silica fiber and a polymer optical fiber (POF) containing a fiber Bragg grating. The use of a connector for POF grating sensors eliminates the limitations of ultraviolet glued connections and increases the ease with which the devices can be applied to real-world measurement tasks.

Investigation into time response of polymer fiber Bragg grating based humidity sensors

In this work we experimentally investigate the response time of humidity sensors based on polymer optical fiber Bragg gratings. By the use of etching with acetone we can control the poly (methyl methacrylate) based fiber 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 12 minutes for humidity decrease and 7 minutes for humidity increase, has been achieved by using a polymer optical fiber Bragg grating with a reduced diameter of 135 microns.

Photonic crystal fiber Bragg grating based sensors : opportunities for applications in healthcare

We review the state-of-the-art in photonic crystal fiber (PCF) and microstructured polymer optical fiber (mPOF) based mechanical sensing. We first introduce how the unique properties of PCF can benefit Bragg grating based temperature insensitive pressure and transverse load sensing. Then we describe how the latest developments in mPOF Bragg grating technology can enhance optical fiber pressure sensing. Finally we explain how the integration of specialty fiber sensor technology with bio-compatible polymer based micro-technology provides great opportunities for fiber sensors in the field of healthcare.

Nonlinear response in polymer optical fibre Bragg grating based sensors

PMMA based polymer optical fibre Bragg gratings have been used for humidity, temperature and concentration sensing. Due to the water affinity of PMMA, the characteristic wavelength of the grating is largely modulated by the water content in the fibre. The rate of water transportation between fibre and surrounding depends on the permeability coefficient for PMMA, which is a function of surrounding temperature and humidity. This leads to increased water content with increasing humidity and temperature. Consequently the wavelength of the grating shows a nonlinear change over varying humidity and temperature. This nonlinearity needs to be calibrated prior to sensor application.

Strain response of POF sensors

With regard to polymer fibre Bragg gratings, we investigate one of the consequences of the visco-elastic nature of the constituent polymer: hysteresis in the response of wavelength shift vs sensor elongation. We show that when a grating sensor is directly bonded to a substrate, the hysteresis is reduced by a factor of 10 from the case where the sensor is freely suspended between two supports.

Distributive tactile sensing using fibre Bragg grating sensors

Two distributive tactile sensing systems are presented, based on fibre Bragg grating sensors. The first is a onedimensional metal strip with an array of 4 sensors, which is capable of detecting the magnitude and position of a contacting load. This system is compared experimentally with a similar system using resistive strain gauges. The second is a two-dimensional steel plate with 9 sensors which is able to distinguish the position and shape of a contacting load. This system is compared with a similar system using 16 infrared displacement sensors. Each system uses neural networks to process the sensor data to give information concerning the type of contact.

Acoustic emission detection using optical fibre sensors for aerospace applications

Structural Health Monitoring (SHM) ensures the structural health and safety of critical structures covering a wide range of application areas. This thesis presents novel, low-cost and good-performance fibre Bragg grating (FBG) based systems for detection of Acoustic Emission (AE) in aircraft structures, which is a part of SHM. Importantly a key aim, during the design of these systems, was to produce systems that were sufficiently small to install in an aircraft for lifetime monitoring. Two important techniques for monitoring high frequency AE that were developed as a part of this research were, Quadrature recombination technique and Active tracking technique. Active tracking technique was used extensively and was further developed to overcome the limitations that were observed while testing it at several test facilities and with different optical fibre sensors. This system was able to eliminate any low frequency spectrum shift due to environmental perturbation and keeps the sensor always working at optimum operation point. This is highly desirable in harsh industrial and operationally active environments. Experimental work carried out in the laboratory has proved that such systems can be used for high frequency detection and have capability to detect up to 600 kHz. However, the range of frequency depends upon the requirement and design of the interrogation system as the system can be altered accordingly for different applications. Several optical fibre configurations for wavelength detection were designed during the course of this work along with industrial partners. Fibre Bragg grating Fabry-Perot (FBG-FP) sensors have shown higher sensitivity and usability than the uniform FBGs to be used with such system. This was shown experimentally. The author is certain that further research will lead to development of a commercially marketable product and the use of active tracking systems can be extended in areas of healthcare, civil infrastructure monitoring etc. where it can be deployed. Finally, the AE detection system has been developed to aerospace requirements and was tested at NDT & Testing Technology test facility based at Airbus, Filton, UK on A350 testing panels.

Influence of mounting on the hysteresis of polymer fiber Bragg grating strain sensors

Fiber Bragg grating sensors recorded in poly(methyl methacrylate) fiber often exhibit hysteresis in the response of Bragg wavelength to strain, particularly when exposed to high levels of strain. We show that, when such a fiber grating sensor is bonded directly to a substrate, the hysteresis is reduced by more than 12 times, compared to the case where the sensor is suspended freely between two supports. © 2013 Optical Society of America.