Optical switch with heat-created bubble

An optical switch has a trend (15) which, in a first state, is filled with index matching fluid so that light from a first waveguide (11) passes through the trench (15) to a second waveguide (12). In a second state, a bubble (16) is formed within the index matching fluid so that light from the first waveguide (11) is reflected at the trench (15) to a third waveguide. The bubble (16) is formed by heat so that volume of the bubble (16) is substantially less than total volume of the trench (15), resulting in pressure within the bubble (16,116,216,316) not being substantially increased by pressure exerted by sidewalls of the trench (15). The trench (15) may have a sidewall with a mirror wall. The heating system may comprises pillars extending up the trench sidewalls.