Innovative optical design for variable optical delay lines enabling low-losses variations

Standard Variable Optical Delay Lines (VODL) made in free space optics rely on the displacement of a corner cube (CC) to adjust the optical delay. The use of a CC is very helpful as it enables the reflected beam to always be parallel to the input beam. This is particularly important as fiber coupling of free space collimated beams is extremely sensitive to angle misalignments. However, if the CC is laterally shifted or rotated during adjustment of the delay, the reflected beam will be laterally translated, which will lead to additional fiber coupling losses. Therefore, losses variations across the delay range will be degraded. In this paper, we present an innovative optical design enabling to realize VODL which are insensitive to shift or tilt of the CC. This means the coupling losses variations of the VODL will exactly match the theoretical losses of two gaussian beams coupling in function of the distance between their respective waists. This also enables to add lenses in the VODL to reach very long optical delays while keeping beam size reasonable and losses variations extremely low. We will present the working principle of this optical design with a theoretical analysis and some experimental results for VODL with different optical delays (600 ps, 1.2 ns and 12 ns) for which we achieved very low losses variations, matching the theoretical predictions down to 0.01 dB.