Abstract

A simple and effective laser frequency stabilization technique uses a high finesse optical cavity (5-MHz FWHM) as the frequency discriminator and an external ADP phase modulator to supply the necessary high-speed laser frequency correction. This is supplemented by a lightweight laser PZT for mid- and low-frequency corrections. The fast frequency servo has a unity gain frequency near 2 MHz, while its low-frequency gain rolls off below 50 kHz where the PZT loop becomes dominant. The FM noise spectrum of our single-mode argonion laser output extends up to 300 kHz but is dominated by water-induced fluctuation of 1.8-MHz rms frequency excursion at 600 Hz. In closed loop operation, the high-frequency noise is suppressed down to the measurement noise level, i.e., the frequency servo error point shows an apparent noise level well below the shot noise level. This is the key achievement toward obtaining an ultranarrow laser linewidth. The present residual linewidth of 70-kHz rms is due to insufficient gain at low frequencies to suppress the water-induced FM noise at 600 Hz. We believe this can be readily corrected by increasing the low-frequency gain with a region of steeper gain slope vs frequency in the PZT channel. Alternatively, adding an acoustooptic modulator as a fast frequency shifter¹ will certainly provide the needed bandwidth. An argon laser linewidth of just a few kilohertz is expected.

© 1986 Optical Society of America