Effect of recycled light in two-beam interferometry

We developed a laser interferometer allowing parts per million resolution to be achieved in fringe division for the measurement of the Si (220) lattice spacing by combined x-ray and optical interferometry. With the aim of obtaining 10-9 measurement uncertainty, we pursued the study of a troublesome noise in interferometer operation and eventually identified its origin in a minute interferometer optical feedback. We investigated mathematically the feedback effect and verified predictions experimentally. Imperfect isolation and light recycling having been identified and subsequently experimentally proved, we took remedial steps in order to improve measurement capabilities. This investigation has brought into light and has made it possible to quantify an interferometer noise up to now overlooked