Optical sensing arrangements

Claim 1

An optical sensing arrangement in which two discrete optical signals of different wavelengths are transmitted simultaneously along each of a plurality of discrete optical fibre paths of different lengths which are terminated in respective sensors adapted for sensing conditions such as temperature and pressure at appropriate locations and optical time domain reflectometer means for detecting discrete optical signals of different wavelengths reflected back along each of the optical fibres of different lengths in response to the transmission of the aforesaid discrete optical signals of different wavelengths along each of said fibres in accordance with variations in conditions at the aforesaid locations, said optical time domain reflectometer means including filter means for filtering out the different wavelengths of the reflected optical signals in each of the optical fibres to enable one of the reflected optical signals at one wavelength to be used as an intensity reference for the reflected signal at the other wavelength and consequential measurement of the conditions being sensed.