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Fiber gratings give abrasion alert

24 Jun 2004

A team of UK scientists develop an optical sensor for monitoring wear.

A wear sensor based upon a chirped fiber Bragg grating (CFBG) has been demonstrated by researchers at Aston University, UK. The device, which operates in-situ with a resolution of up to 120 microns, could bring cost savings to the road haulage and drilling industries. (Meas. Sci. Technol. 15 885)

The idea for the sensor came while researcher Andrew Gillooly was working at Aston University spin-off Indigo photonics (now owned by the UK sensor specialist Insensys) developing high quality CFBGs. By thinking about the grating writing process in reverse, Gillooly realised that it might be possible to configure the gratings as wear sensors.

The sensor works by monitoring the optical reflection from a single CFBG. As the CFBG wears down, the bandwidth and total power of its reflection reduces. The team's prototypes consist of 20 mm long CFBGs written in hydrogenated singlemode fiber (SMF-28).

The gratings, with a 97% reflectivity, were written by a 244 nm CW FreD laser using a phase mask fabrication technique. The CFBGs were then annealed at 90 degC for 48 h to prevent hydrogen out-diffusion from affecting their performance.

To simulate a practical sensor, a CFBG was glued to a carbon fiber rod using epoxy adhesive and its reflection monitored by either a reflectometer or a photodiode.

As the sensor undergoes abrasion the higher pitch frequencies within the grating are destroyed, causing a reduction in both the bandwidth and total power of the reflected spectrum. The team then calculates the wear (reduction in grating length) using the linear relationship between grating length and either chirp rate or reflected power.

"Though sacrificing accuracy, the advantage of using a photodiode is its potential as a low-cost solution," Gillooly told Optics.org.

By offering customers in-situ wear data, Gillooly believes their sensor would help reduce downtime of vehicles or machinery. For example, by placing the sensor in vehicle brake assemblies road haulage operators would be in a position to schedule more efficient servicing.

The oil and gas industry may also benefit from the device. "The wear in drill heads is currently measured indirectly using torque data," explained Gillooly. "Embedding our fiber sensor in the drill head would give you a direct measurement."

Currently negotiating IP issues, the team is keen to bring project partners onboard to commercialize the device. "The high level research has been done," Gillooly told Optics.org. "We now need to focus on the engineering aspects, such as how to package the device."

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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