13 September 2012
The smallest photonic crystal fibre (PCF)-based hydrostatic pressure sensor has been demonstrated by researchers at The Hong Kong Polytechnic University. By using a hollow-core PCF rather than solid, Dr Daru Chen and his colleagues were able to reduce the length from at least several cm to 225μm. Expected to operate up to hundreds of MPa, this new sensor could replace its bulkier counterparts in, for example, the oil and electricity industries.
Optical fibre-based hydrostatic pressure sensors, which offer higher sensitivity, electromagnetic interference resistance and various other advantages over their conventional electric counterparts, have been around since the 1980s. They operate using the photoelastic effect that allows pressure to be measured through changes in the index of the fibre material. Fibre Bragg gratings or fibre Fabry–Perot interferometers can also be used, and several commercial optical fibre-based pressure sensors are currently used in the oil industry or in medical care.
PCF-based fibre sensors have started to appear over the last few years in which the design of the fibre cross section can be customised to give even higher sensitivity. Several recently reported PCF-based fibre sensors, which are mainly based on pressure-induced birefringence measurement, offer a higher sensitivity over other optical fibre-based sensors, but still need several tens of centimetres of fibre to form, for example, a PCF-based Sagnac interferometer. PCF-based fibre Bragg grating sensors can be more compact but still need several centimetres of PCF.
Chen and his colleagues have been working to create a PCF-based hydrostatic sensor that can be as easily used in practical applications as the compact electric sensors that are widely used in industry. The sensor that they have presented in their Letter in this issue is about 10 times shorter than the next smallest sensor, and over 100 times shorter than many other common designs.
To achieve this, a segment of the hollow-core photonic bandgap fibre (HC-PBF) was spliced to a singlemode fibre (SMF) which forms the sensor head. The end of the HC-PBF has a gold-coated spherical end to reflect light back to the SMF, therefore creating a micro-Michelson interferometer that detects the changes in pressure through the changes in the index in the silica of the HC-PBF.
The researchers built and tested a prototype in a sealed oil tank up to a pressure of 42 MPa and the sensor demonstrated a sensitivity of 6.5 pm/MPa, which is better than the alternative fibre Bragg grating-based pressure sensors. The maximum pressure was limited by the oil tank and pump that they used, and they expect that their sensor will actually operate at pressures up to hundreds of MPa, as do most optical fibre-based hydrostatic pressure sensors.
With initial results looking promising, Chen and his colleagues are now working on reducing the temperature cross-talk, and will be seeking a multiplexing solution which can help to lower the sensor cost by sharing the light source and measurement equipment in the sensing system. To make it as practical as possible, they have also designed a standard package for their sensor and they will be trying this out in a water hydraulic bursting test system for gas cylinders.
The researchers expect to see PCF-based pressure sensors replacing conventional optical fibre-based pressure sensors in the future. They are currently designing and fabricating new PCFs specifically for hydrostatic pressure sensing that are likely to deliver an even higher sensitivity, and they hope that an enhanced performance and reductions in fabrication cost will lead to commercial PCF-based hydrostatic pressure sensors in the next few years.
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