Matthew J. Gander

Remotely addressed optical fibre curvature sensor using multicore photonic crystal fibre

We demonstrate an all-fibre curvature sensor that uses two-core photonic crystal fibre (PCF) as the sensing element. The PCF acts as a two-beam interferometer in which phase difference is a function of curvature in the plane containing the cores. A broadband source illuminates both cores, and the spectrum at a single point in the far-field interferogram is recorded. Applying a three-wavelength phase recovery algorithm to the data provides an unambiguous measurement of the interferometer phase, and hence curvature.

Two-dimensional bend sensing with a single, multi-core optical fibre

Measurement of two-dimensional bending in a structural element using intrinsic optical fibre strain gauges would normally require three sensors to be attached to, or embedded within, the structure. The same measurement can now be made using a single multi-core optical fibre, reducing deployment cost and increasing practicality. Fabrication of a novel three-core photonic crystal fibre is described. The ability of the fibre sensor to measure bend in two dimensions is demonstrated in the laboratory using interferometric interrogation at a single wavelength. Deployment of the sensor to measure the deformation of a bridge undergoing loading trials is described.

Embedded micromachined fiber-optic Fabry-Perot pressure sensors in aerodynamics applications

Small size, high bandwidth pressure sensors are required for instrumentation of probes and test models in aerodynamic studies of complex unsteady flows. Optical-fiber pressure sensors promise potential advantages of small size and low cost in comparison with their electrical counterparts. We describe miniature Fabry-Perot cavity pressure sensors constructed by micromachining techniques in a turbine test application. The sensor bodies are 500 /spl mu/m squared, 300 /spl mu/m deep with a /spl sim/2 /spl mu/m-thick copper diaphragm electroplated on one face. The sensor cavity is formed between the diaphragm and the cleaved end of a single mode fiber sealed to the sensor by epoxy. Each sensor is addressed interferometrically in reflection by three wavelengths simultaneously, giving an unambiguous phase determination; a pressure sensitivity of 1.6 radbar/sup -1/ was measured, with a typical range of vacuum to 600 kPa. Five sensors were embedded in the trailing edge of a nozzle guide vane installed upstream of a rotor in a full-scale turbine stage transient test facility. Pressure signals in the trailing edge flow show marked structure at the 8 kHz blade passing frequency. To our knowledge, this is the first report of sensors located at the trailing edge of a normal-sized turbine blade.

Laser-machined fibers as Fabry-Perot pressure sensors.

Cavities have been laser ablated in the ends of single-mode optical fibers and sealed by aluminized polycarbonate diaphragms to produce Fabry-Perot pressure sensors. Both conventional fibers and novel, multicore fibers were used, demonstrating the possibility of producing compact arrays of sensors and multiple sensors on an individual fiber 125 microm in diameter. This high spatial resolution can be combined with high temporal resolution by simultaneously interrogating the sensors by using separate laser sources at three wavelengths. Shock tube tests showed a sensor response time of 3 micros to a step increase in pressure.

Two-axis bend measurement using multicore optical fibre

Abstract We describe the first use of a four-core optical fibre to measure bending about two orthogonal axes simultaneously. Individual cores in the fibre act as independent strain gauges where local curvature determines the difference in strain between cores. The multicore sensing element, interrogated in reflection, generates a two-dimensional far-field interferogram. The component of fibre curvature in the plane of the two cores shifts the corresponding fringes in the interferogram. Bend angle is then calculated using phase values derived from Fourier analysis of the far-field interferogram. This technique achieved a bend angle resolution better than 120 μrad.

Embedded micromachined fibre optic Fabry-Perot pressure sensors in aerodynamics applications

Small size, high bandwidth pressure sensors are required for instrumentation of probes and test models in aerodynamic studies of complex unsteady flows. Optical fibre pressure sensors promise potential advantages of small size and low cost in comparison with their electrical counterparts. We describe miniature Fabry-Perot cavity pressure sensors constructed by micromachining techniques in a turbine test application. The sensor bodies are 500 /spl mu/m square, 300 /spl mu/m deep with a /spl sim/2 /spl mu/m thick copper diaphragm electroplated on one face. The sensor cavity is formed between the diaphragm and the cleaved end of a singlemode fibre sealed to the sensor by epoxy. Each sensor is addressed interferometrically in reflection by 3 wavelengths simultaneously, giving an unambiguous phase determination; a pressure sensitivity of /spl sim/1.8 rad bar/sup -1/ was measured, with a typical range of vacuum to 600 kPa. Five sensors were embedded in the trailing edge of a nozzle guide vane installed upstream of a rotor in a full-scale turbine stage transient test facility. Pressure signals in the trailing edge flow show marked structure at the 10 kHz blade passing frequency. To our knowledge, this is the first report of sensors located at the trailing edge of a normal-sized turbine blade.

Blast-pressure measurement with a high-bandwidth fibre optic pressure sensor

A sensor to measure rapidly changing pressures in an explosive air blast requires high bandwidth and high spatial resolution. For such an application a low-cost, electrically isolated sensor is particularly attractive. We describe an optical fibre-based pressure sensor that meets these requirements. The sensor is subjected to an experimental explosive-blast measurement test and the results are discussed with respect to the sensor performance and compared with a simple blast-wave model and conventional pressure transducers.

Measurement of the wavelength dependence of beam divergence for photonic crystal fiber

We report measurements of the wavelength dependence of beam divergence for single-mode photonic crystal fiber. These measurements confirm predictions of strongly wavelength-dependent beam divergence, consistent with the effective-index model for the photonic crystal cladding material.

Measurement of bending in two dimensions using multicore optical fibre

We describe the use of a four-core optical fibre as the basis of a sensor capable of measuring the angle through which the fibre is bent in two dimensions. The intended application of the sensor is in measuring the shape of flexible structures.

Channeled spectrum interrogation of an all-fibre broadband interferometric differential strain sensor

Optical fibre interferometric strain sensors embedded into structures offer a very accurate and robust method for shape measurement [1]. Many schemes have been demonstrated in which strain and/or temperature in a structure are inferred from monochromatic optical phase delay [2].