Patent ID: 7050172

Claim:
Optical sensing apparatus ( 10 ; 100 ) comprising (i) an optical fibre ( 20 ; 120 ) having a length ( 18 ; 123 ) the core of which contains a reflective grating structure, (ii) means ( 12 , 30 , 31 ; 113 , 115 ) for producing signal radiation in the core of the optical fibre, and (iii) means ( 22 , 24 , 26 , 28 ; 122 , 124 , 126 , 128 ) for obtaining a measure of the wavelength of signal radiation reflected by the grating structure, characterised in that the apparatus further comprises heating means ( 12 , 16 ; 112 , 116 ) for cyclically heating the core of said length of the optical fibre at a frequency such that parameters of the cyclical heating are influenced by the temperature and flow-rate of a fluid in which said length of optical fibre is immersed.