Scatterometer using a bidirectional ring laser

A technique for the absolute measurement of very low-level scattering was developed using the locking frequency for the two counter-circulating beams in a mode-locked ring cavity. The locking threshold is a function of the scattering coefficient at the crossing of the bidirectional pulses and is measured through the beat note characteristics of the laser. Backscattering coefficients of 0.6 x 10(-10) and 2.2 x 10(-10) were measured for BK7 samples of scratch/dig quality of 10-5 and 40-20, respectively. A beat note bandwidth of 1 Hz, as has been observed with the unstabilized laser, corresponds to a sensitivity limit of an intensity backscattering of 10(-15). Even smaller limits could be achieved by stabilization. (C) 2004 Elsevier B.V. All rights reserved.