The sensor was comprised of three fibre types: a photonic crystal fibre (PCF) which carried the incident light from a broadband light emitting diode to the sensor and guided the reflected light back to an optical spectrum analyser; a short section of hollow core optical fibre (HOF) fused onto the end of the PCF; and a short length of single-mode fibre (SMF) which was fused onto that (see diagram below).

Sensors could be made with lengths as short as 580 μm, this being the combined lengths of the HOF and SMF sections. And since single-mode fibre is typically 125 μm in diameter when stripped down to the cladding layer, these sensors can be made very small.

Both the HOF and SMF sections act as Fabry-Pérot cavities in which light reflects back and forth producing interference patterns in the spectrum of light reflected back down the PCF and recorded by the optical spectrum analyser. The interference pattern from each cavity was seen as a series of consecutive low and high intensity peaks as a function of wavelength. Careful analysis of the combined interference patterns revealed the effect of temperature change on the sensor.

A demonstration was conducted in which a series of spectra were recorded as the sensor temperature was varied between 50° and 1000° C; in steps of 50°. The sensor was sensitive to changes in temperature in two ways: by the thermo-optic effect, which is caused by a change in refractive index with temperature; and by thermal expansion, whereby the length of the Fabry-Pérot cavity changes with temperature. Both effects cause a change in the optical path length taken by the light resonating in the cavities.

The authors did not specifically address their reasons for using the 70 μm length HOF, referring to it as an "auxiliary" Fabry-Pérot cavity; the actual sensing element being the SMF. As an experimental device, however, it allowed the researchers to discern the comparative magnitudes of the thermo-optic and thermal expansion effects. As a working device, it is possible that the HOF would be used to improve the visibility of the light interference produced in the SMF optical cavity, by increasing the reflection at the front surface to the cavity.

The appeal of this sensor is in its construction, which relies on standard fusion splicing technology, and in the fact that it uses only optical fibre with no additional parts, making it well suited to high temperature applications. As a very small sensor, it could also enable temperature measurements to be made with pinpoint spatial accuracy. One might conceive of a design in which an array of such sensors are spread across a surface, enabling multiple temperature measurements to be made within a compact area.

More information:

† Hae Young Choi, Kwan Seob Park, Seong Jun Park, Un-Chul Paek, Byeong Ha Lee, Eun Seo Choi (2008). Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry-Perot interferometer Optics Letters, 33 (21) (2008).

http://www.opticsinfobase.org/abstract.cfm?URI=ol-33-21-2455 

Source: Stuart Watson, www.opticalfutures.com