Folded Optical Delay Lines

TLDR

The system provides a dispersionless optical delay line for filtering or storing information‑modulated light. A long optical path is folded between two 7.5‑cm mirrors, enabling 1000+ passes; beam patterns are generated by perturbing or astigmatic mirrors, and the output is isolated by angle and position. The 3000‑m folded path delivers a 10‑μs delay with diffraction losses lower than an open beam, and its loss sensitivity to mirror quality makes it useful for measuring mirror loss and for filtering or storage applications.

Abstract

A long optical path has been folded between two 7.5-cm diam spherical or aspherical mirrors to provide an output beam which can be well separated from previous reflections with 1000 or more passes between the mirrors. The 3000-m path provides 10 μsec of delay. This system can be used as a dispersionless optical delay line for use in filtering or storage of information modulated onto the light beam. The pattern of beams between the two mirrors is obtained in one of two ways. A small perturbing mirror may be inserted to give a series of offset ellipses, or one or both of the mirrors can be made astigmatic to give a Lissajous pattern of spots on each mirror. The output beam can be separated from others by discriminating in both angle and position. The diffraction losses of the system are much lower than those for an open beam because of the periodic focusing of the spherical mirrors. The extreme dependence of the loss of the delay line upon the absorption and scattering loss of the mirrors makes the system dependent upon very low loss mirrors and also makes the system a suitable method for measuring mirror loss. Block diagrams are shown for some possible filtering and storage applications.