Abstract: An optical delay line includes first and second hollow front surface retroreflectors. A translator is coupled to one of the first and second retroreflectors for adjusting the distance between the retroreflectors while maintaining the optical relationship between them. An entrance is provided for introducing a light beam into the delay line so that the light beam is reflected between the first and second retroreflectors a plurality of times; and an exit is provided to couple the beam out of the delay line.