A method of fabricating a sensor probe comprises: placing the end of an optical fiber 10 in a polymerization space 2; applying a drop of hydrogel pre-gel solution 30 to the end of the fiber; exposing said pre-gel solution to an ultra-violet light source 24 to cause polymerization therein; and monitoring a light signal that has passed down said optical fiber 10 in order to monitor said polymerization.