An optical time delay apparatus comprises: a multi-wavelength optical source; a diffractive element set imparting a wavelength-dependent delay on signals routed from the source to a 1.times.N optical switch; and N diffractive element sets routing signals from the 1.times.N switch to an output port. The optical propagation delay between the source and the output port varies according to the operational state of the source and the 1.times.N switch. A photodetector may receive the time-delayed signal at the output port.