Interrogation of fibre optic sensors

The aim of the research work described in this thesis was to investigate the interrogation of fibre optic sensors using "off the shelf optical components and equipment developed mainly for the telecommunications industry. This provides a cost effective way of bringing fibre optic sensor systems to within the price range of their electro-mechanical counterparts. The research work focuses on the use of an arrayed waveguide grating, an acousto-optic tuneable filter and low-coherence interferometry to measure dynamic strain and displacement using fibre Bragg grating and interferometric sensors. Based on the intrinsic properties of arrayed waveguide gratings and acousto-optic tuneable filters used in conjunction with interferometry, fibre Bragg gratings and interferometric sensors a number of novel fibre optic sensor interrogation systems have been realised. Special single mode fibre, namely, high-birefringence fibre has been employed to implement a dual-beam interrogating interferometer. The first interrogation scheme is based on an optical channel monitor, which is an arrayed waveguide grating with integral photo-detectors providing a number of amplified electrical outputs. It is used to interrogate fibre Bragg grating and interferometric sensors. Using the properties of polarisation maintainability in high-birefringent fibre an interrogating interferometer was realised by winding a length of the fibre around a piezoelectric modulator generating a low-frequency carrier signal. The system was used to interrogate both fibre Bragg grating and interferometric sensors. Finally, the use of an acousto-optic tuneable filter is employed to interrogate fibre Bragg gratings. The device is used to generate a very high frequency carrier signal at the output of an optical interferometer

Interrogation of fibre Bragg grating sensors using an arrayed waveguide grating

We experimentally investigate the use of an arrayed waveguide grating (AWG) to interrogate fibre Bragg grating (FBG) sensors. A broadband light source is used to illuminate the FBG sensors. Reflected spectral information is directed to the AWG containing integral photodetectors providing 40 electrical outputs. Three methods are described to interrogate FBG sensors. The first technique makes use of the wavelength-dependent transmission profile of an AWG channel passband, giving a usable range of 500 με and a dynamic strain resolution of 96 nε Hz-1/2 at 13 Hz. The second approach utilizes wide gratings larger than the channel spacing of the AWG; by monitoring the intensity present in several neighbouring AWG channels an improved range of 1890 με was achieved. The third method improves the dynamic range by utilizing a heterodyne approach based on interferometric wavelength shift detection, providing an improved dynamic strain resolution of 17 nε Hz -1/2 at 30 Hz. © 2005 IOP Publishing Ltd

Interrogation of fibre Bragg grating sensors using an arrayed waveguide grating

We experimentally investigate the use of an arrayed waveguide grating (AWG) to interrogate fibre Bragg grating (FBG) sensors. A broadband light source is used to illuminate the FBG sensors. Reflected spectral information is directed to the AWG containing integral photodetectors providing 40 electrical outputs. Three methods are described to interrogate FBG sensors. The first technique makes use of the wavelength-dependent transmission profile of an AWG channel passband, giving a usable range of 500 με and a dynamic strain resolution of 96 nε Hz-1/2 at 13 Hz. The second approach utilizes wide gratings larger than the channel spacing of the AWG; by monitoring the intensity present in several neighbouring AWG channels an improved range of 1890 με was achieved. The third method improves the dynamic range by utilizing a heterodyne approach based on interferometric wavelength shift detection, providing an improved dynamic strain resolution of 17 nε Hz -1/2 at 30 Hz. © 2005 IOP Publishing Ltd

High birefringence fibre interrogating interferometer for optical sensing applications

The use of high birefringence fiber interrogating interferometer for optical sensing applications was discussed. The method is of low cost and permits simple adjustment of the optical path difference and has much lower sensitivity to environmental perturbation. The polarization-maintaining (PM) fiber interferometer adopted a heterodyne approach using interferometric wavelength shift detection. The study showed that the inclusion of power amplifier driving a multi-element piezoelectric stack will enable the bandwidth to be pushed up into the kHz regime

Fibre Bragg grating sensor interrogation using an acousto-optic tunable filter and low-coherence interferometry

We describe how an acousto-optic tunable filter can be used to both demultiplex the signals from multiple fibre Bragg grating sensors and simultaneously provide wide bandwidth signal demodulation in a system using interferometric wavelength shift detection. In an experimental demonstration, the approach provided a noise limited strain resolution of 24.9 n epsilon Hz(-1/ 2) at 15 Hz

Extended range interrogation of wavelength division multiplexed fibre Bragg grating sensors using an arrayed waveguide grating

A technique for interrogating multiplexed fibre Bragg grating (FBG) sensors using an arrayed waveguide grating (AWG) is described. The approach considerably extends the sensing range from that achieved previously, while providing a strain resolution of 17nevHz at 30 Hz

Interferometric sensor interrogation using an arrayed waveguide grating

We experimentally investigate the use of an arrayed waveguide grating (AWG) to interrogate interferometric sensors. A single broad-band light source is used to illuminate the system. Reflected spectral information is directed to an AWG with integral photodetectors providing 40 electrical outputs. We show that using the dual-wavelength technique we can measure the length of a Fabry-Perot cavity by determining the optical phase changes of the scanned interferometric pattern, which produced a maximum unambiguous range of 1440 mum with an active sensor and a maximum unambiguous range of 300 mum with the introduction of a second processing interferometer, which allows the sensor to be passive

Ammonia vapour in the oral cavity as a diagnostic marker for infection with Helicobacter pylori: Preliminary 'proof of principle' pharmacological investigations

Most current non-invasive tests for Helicobacter pylori depend on the conversion of labelled (13C or 14C) urea to labelled carbon dioxide (13CO2 or 14CO2) and ammonium (NH4+) by the enzyme urease, with the labelled CO2 detected in exhaled air. Despite suggestions going back over a number of years, the alternative possibility of using NH4+ (in the form of gaseous ammonia [NH3]) as the test parameter has received little or no attention. However, this approach is now being explored using a chemiresistive sensor detecting sub-parts per million concentrations of NH3. An in vitro 'glass stomach' (containing various volumes of hydrochloric acid [HCl] and ammonium chloride [NH4Cl]) was used to evaluate the means of increasing 'gastric' pH to that of the NH4+-->NH3 transition that occurs significantly at pH 9.24. This 'stomach' also was used to study mechanisms by which NH3 may be expelled in a pulse (as a surrogate belch), either by the in situ production of CO2 or through an exogenous source. On the basis of the protocols developed, H. pylori-negative subjects were tested before and after ingestion of 10 mg NH4Cl (as a surrogate for bacteria-produced NH4,), and H. pylori-positive subjects were tested without taking urea or NH4Cl. 'Intragastric' pH in the in vitro 'glass stomach' could be increased above pH 9.24 by adding a mixture of 15-30 mL magnesium hydroxide mixture (or the proprietary equivalent) and 50 mL water, and the resulting NH3 expelled by adding 100 mL CO2-saturated cold water (sparkling water). In vivo, NH3 levels in the oral cavity of H. pylori-negative subjects were increased after ingestion of 10 mg NH4Cl; however, levels in the oral cavity of a small number of H. pylori-positive subjects were two- to threefold higher after magnesium hydroxide and sparkling water. On the basis of in vitro studies, an in vivo protocol was developed to increase gastric pH above that required for the NH4+-->NH3 transition, and a mechanism established to release the NH3 into the oral cavity. Preliminary in vivo data confirm the chemiresistive sensor is sufficiently sensitive to NH3 to distinguish H. pylori-negative subjects who have taken 10 mg NH4Cl from those who have not, and clearly distinguish H. pylori-negative subjects from H. pylori-positive subjects. Ingestion of urea or other labelled tracers is not required, nor is belching; and the sensor takes less than two minutes to reach a maximum response. The data provide good evidence that the chemiresistive detection of NH3 has considerable potential as a rapid, point-of-care diagnostic test for H. pylori infection