with the invention fig1 shows a planar optical circuit on which two functional units 2 3 are disposed one functional unit 2 is at a first temperature t 1 and the other functional unit 3 is at a second temperature t 2 the two functional units 2 3 are formed on a common carrier substrate by way of example the first functional unit 2 is an optical waveguide switch that has a mach zehnder interferometer in one of whose arms optical signals are phase shifted as a result of a temperature change depending on the phase shift the signals in the two arms are cancelled out or added switches such as these are known for example from international patent disclosure wo 01 42848 in order to adjust the temperature in the first arm a metal strip is provided to which power is supplied depending on the desired phase angle and the metal strip emits this power as thermal energy into the layer located underneath it a waveguide switch is accordingly a functional unit to which thermal energy is supplied other examples of such functional units are light emitting diodes or laser diodes the other functional unit 3 is for example an optical filter in an optical filter it is important for the filter to be kept at a constant temperature since otherwise the filter characteristics change the two functional units which are for example in the form of an optical waveguide switch 2 and an optical filter 3 form for example components of an add drop multiplexer which.