A method for forming a pressure sensor

An optical fibre pressure sensor 1 with core 7, that may be silica, cladding 3 and birefringence structure with two holes 2, 4 extending throughout. The sensor being encased by a housing 11 with a pressure transfer element 12 and filled with non-compressible fluid 14. An optical sensor 5 provided along the fibre exhibiting intrinsic birefringence under optical pressure. The structure formed to convert pressure to anisotropic stress in the optical sensor. The sensor may be inscribed with a femtosecond laser. The fibre may have an inorganic coating to direct the laser light to the core. The cladding may be internally etched by flowing an etchant through the holes, thus increasing the holes width. The cladding may be heated during etching with a localised heat source. The structure may have third and fourth holes. The cladding may be externally etched to expose the third and fourth holes, this may be done with laser ablation. Etching fluid may be flushed before filling with non-compressible fluid. The sensor may be spliced to single mode optical fibres. The optical sensor may be configured to produce a reflection spectrum with two peaks.