An optical system including first and second optical elements for guiding light therein. For each optical element, the propagating light enters or exits the optical element through a coupling surface of the optical element. The coupling surfaces of the optical elements face, and align with, each other so that light propagating in one of the optical elements exits the optical element through the coupling surface of the optical element and enters the other optical element through the coupling surface of the other optical element. The coupling surfaces are separated from each other and define a region therebetween. The region is filled with a coolant, the coolant substantially surrounding at least one of the first and second optical elements.