Abstract

The antiresonant ring (ARR), first introduced by Siegman, has found many applications, most recently in colliding-pulse mode-locked lasers.¹ Here we introduce the use of the ARR as a detection arrangement for performing ultrafast excite-probe induced-absorption experiments, where it offers many advantages over current techniques. Specifically, the sample cell is placed off-center in the ARR, where an excitation beam interacts with only one of the two (probe) beams counterpropagating in the ring. On return to the beam splitter, the full probe intensity and any dc effects cancel out, and only the signal light due to transient effects leaks out of the ring as signal. This optical subtraction is more accurate than electronic subtraction. Also, by deliberately allowing some leakage from the ring, optical heterodyne detection (OHD) is possible, yielding both the absorption and refractive-index transients independently without constraining the beam polarizations (as in OHD polarization spectroscopy). By choosing the best leakage value, maximum signal-to-noise ratios are achieved in the presence of laser shot-to-shot jitter.² In preliminary experiments using 1% leakage, we have improved the signal-to-noise ratio by a factor of five. This results agree with theory, which predicts even better values if less leakage is used.

© 1990 Optical Society of America