Method for forming pressure sensor involves filling a chamber with non compressible fluid and shaping etched cladding to convert external pressure within chamber by anisotropic stress

A method for forming a pressure sensor comprising:

• providing an optical fibre comprising a core, a cladding surrounding the core, and a birefringence structure for inducing birefringence in the core, wherein the birefringence structure comprises first and second holes enclosed within the cladding and extending parallel to the core;
• encasing a portion of the optical fibre comprising the core and the birefringence structure within a chamber, wherein the chamber is defined by a housing comprising a pressure transfer element for equalising pressure between the inside and the outside of the chamber;
• providing an optical sensor along the core of the portion of optical fibre;
• internally etching the cladding by flowing an etchant through the first and second holes along a length of the portion of optical fibre which comprises the optical sensor so as to increase the width of the first and second holes;
• heating the cladding during the internal etching using a localised heat source so as to increase the internal etching rate in a region of the encased portion encompassing the optical sensor;
• and
• filling the chamber with a substantially non-compressible fluid;
• wherein the etched cladding is shaped so as to convert an external pressure provided by the non-compressible fluid within the chamber to an anisotropic stress in the optical sensor, wherein the internal etching causes the first and second holes to have a larger cross sectional area along the region of the portion encompassing the optical sensor than along another region of the portion not encompassing the optical sensor.