Abstract

We are investigating the problems of transferring the intrinsic stability of a thoughtfully designed and isolated stable interferometer into an equivalent frequency stability of the laser locked onto its fringe.¹ Tests with two lasers independently locked to adjacent orders show locking precision below 50 mHz (1 × 10⁻¹⁶) for times ~1–100 s, degrading toward shorter times as t^−1/2 due to finite SNR. Toward longer times performance degrades as t⁺¹ due to changes in the systematic offsets. Locking to various orders gives an estimate that the locking accuracy is <2 Hz (4 × 10⁻¹⁵). Reductions in lock inaccuracy seem possible using harmonic detection.² Experiments to measure drift of the Zerodur etalon use a ¹²⁹l₂-stabilized laser as a reference. Unfortunately its SNR is too low to give useful information quickly: the cavity’s 2-Hz/s drift just matches the reference laser’s random noise at ~40 s. Comparison over longer times (~days), taken with precise thermal controls operating, is consistent with the idea that the observed 75-Hz noise amplitudes near 24 h are due to remaining thermal problems, and the 1 % uncertainty in the observed drift rate is due to limitations of the iodine-stabilized reference laser. We are considering use of the powerful method of modulation transfer spectroscopy with an external l₂ cell,³ in combination with five massive copper shielding walls with independent stabilization and hence thermal gradient control.

© 1987 Optical Society of America