Very-Narrow-Line Semiconductor Laser and Optical Clocks Based on Spectral Hole Burning Frequency Standards

摘要

Single-frequency diode lasers were stabilized to 500Hz precision at 1.5 microns and to 20Hz at 793nm on 10-ms time scales using narrow spectral holes in absorption lines of Er and Tm doped cryogenic crystals as frequency references and without vibrational isolation of either the laser or frequency reference. Kilohertz stability for 100 seconds is provided by these techniques, and that performance is extendable to longer time scales with further development. Miniaturized lasers and 2 mm-sized reference crystals will provide compact portable packages with a closed cycle cryocooler. The achieved frequency stabilization provides ideal lasers for high-resolution spectroscopy, real time optical signal processing based on spectral holography, and other applications requiring ultra-narrow-band light sources or coherent detection. Feedback for stabilizing the frequency of external cavity diode lasers was derived from Pound-Drever-Hall frequency modulation probing of the spectral hole and used to control diode laser current and grating tilt. Stabilization to spectral holes is applicable to DFB lasers and other lasers and to other transducers like acousto- optic or electro-optic frequency shifters. Studies of frequency reference materials provided crystals that are programmable long-term references with spectral holes persisting up beyond liquid nitrogen temperatures. Crystal frequency references are insensitive to vibration and could be mass-produced.