Stephen E. Staines

A polarization-maintaining fibre Bragg grating interrogation system for multi-axis strain sensing

In this paper we report the development of a FBG (fibre Bragg grating) interrogation system, that is capable of detecting independently the two orthogonally polarised signals reflected from a FBG fabricated in a PM (polarisation maintaining) fibre. The interrogation system imposes no limitation on the measurable strain. It is possible to detect a strain gradient by measuring independently the spectral broadening of the two polarised signals. The system has been used to fabricate and characterise FBGs in different PM fibres. As a demonstration of the feasibility of the interrogation system for multi-axis strain sensing, the transverse strain sensing characteristics of PM FBGs are presented.

A polarisation maintaining fibre Bragg grating interrogation system for multi-axis strain sensing

In this paper we report the development of a FBG (fibre Bragg grating) interrogation system, that is capable of detecting independently the two orthogonally polarised signals reflected from a FBG fabricated in a PM (polarisation maintaining) fibre. The interrogation system imposes no limitation on the measurable strain. It is possible to detect a strain gradient by measuring independently the spectral broadening of the two polarised signals. The system has been used to fabricate and characterise FBGs in different PM fibres. As a demonstration of the feasibility of the interrogation system for multi-axis strain sensing, the transverse strain sensing characteristics of PM FBGs are presented.

Fabrication of fiber optic long period gratings operating at the phase matching turning point using an ultraviolet laser

It is known that optical fiber long period gratings (LPGs) exhibit their highest sensitivity to environmental perturbation when the period is such that the phase matching condition is satisfied at its turning point. The reproducible fabrication of LPGs with parameters satisfying this condition requires high resolution control over the properties of the grating. The performance of an LPG fabrication system based on the point-by-point UV exposure approach is analyzed in this paper, and the control of factors influencing reproducibility, including period, duty cycle, and the environment in which the device is fabricated, is explored.

Surface strain measurement: a comparison of speckle shearing interferometry and optical fibre Bragg gratings with resistance foil strain gauges

The performance of two complementary optical strain measurement techniques, speckle shearing interferometry (shearography) and fibre Bragg grating (FBG) sensors, is compared with that of resistance foil strain gauges (RFSGs) and with theoretical predictions. The test object used for the surface strain measurements was a hydrostatically loaded ABS pipe. A multi-component shearography instrument, capable of full surface strain measurement, was used to determine the displacement gradient components, from which the surface strain components were calculated. Six surface mounted wavelength division multiplexed FBG sensors were used to measure the axial and the hoop strains. RFSGs located on the surface of the pipe, adjacent to the FBGs, were used for comparison. Reasonable agreement between theory and the axial and hoop strains determined by the different techniques was found. Issues associated with deploying and comparing the techniques are discussed.

Development and application of optical fibre strain and pressure sensors for in-flight measurements

Fibre optic based sensors are becoming increasingly viable as replacements for traditional flight test sensors. Here we present laboratory, wind tunnel and flight test results of fibre Bragg gratings (FBG) used to measure surface strain and an extrinsic fibre Fabry–Perot interferometric (EFFPI) sensor used to measure unsteady pressure. The calibrated full scale resolution and bandwidth of the FBG and EFFPI sensors were shown to be 0.29% at 2.5 kHz up to 600 μe and 0.15% at up to 10 kHz respectively up to 400 Pa. The wind tunnel tests, completed on a 30% scale model, allowed the EFFPI sensor to be developed before incorporation with the FBG system into a Bulldog aerobatic light aircraft. The aircraft was modified and certified based on Certification Standards 23 (CS-23) and flight tested with steady and dynamic manoeuvres. Aerobatic dynamic manoeuvres were performed in flight including a spin over a g-range −1g to +4g and demonstrated both the FBG and the EFFPI instruments to have sufficient resolution to analyse the wing strain and fuselage unsteady pressure characteristics. The steady manoeuvres from the EFFPI sensor matched the wind tunnel data to within experimental error while comparisons of the flight test and wind tunnel EFFPI results with a Kulite pressure sensor showed significant discrepancies between the two sets of data, greater than experimental error. This issue is discussed further in the paper.

Monitoring techniques for the manufacture of tapered optical fibers.

The use of a range of optical techniques to monitor the process of fabricating optical fiber tapers is investigated. Thermal imaging was used to optimize the alignment of the optical system; the transmission spectrum of the fiber was monitored to confirm that the tapers had the required optical properties and the strain induced in the fiber during tapering was monitored using in-line optical fiber Bragg gratings. Tapers were fabricated with diameters down to 5 μm and with waist lengths of 20 mm using single-mode SMF-28 fiber.

Fibre Bragg grating based effective soil pressure sensor for geotechnical applications

An effective-soil-pressure sensor for geotechnical applications based on Fibre Bragg Gratings is presented. The sensor simultaneous measures total soil pressure and pore pressure, allowing the calculation of the effective stress of soil. Calibration of the sensor using pressurised air demonstrated a pressure sensitivity of 2.02x10-3 ± 2.84x10-5nm/kPa and 1.87x10-3 ± 6.88x10-5nm/kPa for the total and pore pressure respectively. This corresponds to a pressure resolution of 4.95x10-1kPa and 5.46x10-1kPa for total and pore pressure using a 1pm interrogation system. Measurements undertaken in two types of soil demonstrated dependence of the total pressure sensitivity on soil density/stiffness. Pore pressure measurements agreed well with the preliminary calibration.

Submicrometer fiber-optic Fabry-Perot interferometer formed by use of the Langmuir-Blodgett technique.

The fabrication of an optical cavity at the distal end of an optical fiber has been achieved by Langmuir-Blodgett (LB) deposition of tricosanoic acid. This technique allows nanometer-scale control over the cavity length to a total thickness of ~0.5 microm . The cavity has been shown to act interferometrically and, thus, has potential sensing applications.

A fibre Bragg grating-based inclinometer system for ground movement measurement

The measurement of ground movement is an essential part of many geotechnical engineering operations. For decades, inclinometer systems have traditionally been used for this purpose to provide crucial information to engineers and researchers. However, conventional inclinometer systems have their limitations, such as high cost and poor durability. In this paper a fibre optic based inclinometer system is reported, which utilizes fibre Bragg grating sensors attached to the casing of a conventional inclinometer. The characterization of the sensor revealed good agreement with theory and conventional displacement measurements. For a casing of length 2.5 m, the minimum measureable deflection of the top of the casing was found to be 0.48 mm when the FBGs were interrogated with a 1 pm wavelength resolution system.

Field monitoring of static, dynamic, and statnamic pile loading tests using fibre Bragg grating strain sensors

Pile loading test plays an important role in the field of piling engineering. In order to gain further insight into the load transfer mechanism, strain gauges are often used to measure local strains along the piles. This paper reports a case whereby FBG strain sensors was employed in a field trial conducted on three different types of pile loading tests in a glacial till. The instrumentation systems were configured to suit the specific characteristic of each type of test. Typical test results are presented. The great potential of using FBG sensors for pile testing is shown.