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 .