James S. Barton

Two-axis bend measurement with Bragg gratings in multicore optical fiber

We describe what is to our knowledge the first use of fiber Bragg gratings written into three separate cores of a multicore fiber for two-axis curvature measurement. The gratings act as independent, but isothermal, fiber strain gauges for which local curvature determines the difference in strain between cores, permitting temperature-independent bend measurement.

Ultrafast-laser inscription of a three dimensional fan-out device for multicore fiber coupling applications

A three dimensional fan-out device has been fabricated using ultrafast laser inscription. The device allows each core of a multicore fibre to be addressed individually by a single mode fiber held in an FVA.

Strain and temperature sensitivity of a single-mode polymer optical fiber

We have measured the optical phase sensitivity of fiber based on poly(methyl methacrylate) under near-single-mode conditions at 632.8 nm wavelength. The elongation sensitivity is 131 +/- 3 x 10(5) rad m(-1) and the temperature sensitivity is -212 +/- 26 rad m(-1) K(-1). These values are somewhat larger than those for silica fiber and are consistent with the values expected on the basis of the bulk polymer properties.

Transient deformation measurement with electronic speckle pattern interferometry and a high-speed camera

To the best of our knowledge, transient deformations have been measured in real time with microsecond temporal resolution for the first time with speckle pattern interferometry. The short exposure period and high framing rate of a high-speed camera at as many as 40,500 frames per second allow low-power continuous-wave laser illumination and fiber-optic beam delivery to be used. We have applied the technique to measure both harmonic vibration and transient deformation.

Two-Axis Temperature-Insensitive Accelerometer Based on Multicore Fiber Bragg Gratings

We report a compact two-dimensional accelerometer based upon a simple fiber cantilever constructed from a short length of multicore optical fiber. Two-axis measurement is demonstrated up to 3 kHz. Differential measurement between fiber Bragg gratings written in the multicore fiber provides temperature-insensitive measurements.

Fibre optics in palladium-based hydrogen sensing

Hydrogen sensing at low concentrations in the order of tens to hundreds of parts per million (ppm) over extended periods requires highly sensitive and stable sensor technology. Palladium and its alloys are widely used in hydrogen sensing as they show a high and selective affinity for hydrogen. On exposure to hydrogen, Pd experiences a volumetric expansion, roughly proportional to the hydrogen concentration in the environment. This volumetric expansion can be measured either directly by dimensional monitoring or through monitoring of secondary physical effects. Optical sensor technology is a preferred technique for hydrogen sensing because of its inherent safety for use in a potentially explosive environment, and several optical hydrogen sensors have been reported. However, the majority have been designed to measure hydrogen concentrations of a few per cent in air, whereas the sensors described here are capable of operating in concentrations down to hundreds of ppm. Fibre Bragg gratings (FBGs) and interferometric length monitoring are shown to be able to monitor the dimensions of a palladium sensor element. Hydrogenation not only induces physical strain but also changes the electronic configuration resulting in a change in refractive index which can be monitored using a long-period grating (LPG). The paper discusses the respective characteristics of these different optical measurement techniques using Pd and Pd/Ag alloys as sensor elements for hydrogen concentrations between 100 ppm and 1%.

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.

Bend sensors with direction recognition based on long-period gratings written in D-shaped fiber

The curvature- or bend-sensing response of long-period gratings (LPGs) UV inscribed in D-shaped fiber has been investigated experimentally. Strong fiber-orientation dependence of the spectral response when such LPGs are subjected to bending at different directions has been observed and is shown to form the basis for a new class of single-device sensor with vector-sensing capability. Potential applications utilizing the linear response and unique bend-orientation characteristics of the devices are discussed.

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.

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.