Doppler-Free Spectroscopy Based on Phase Conjugation by Degenerate Four-Wave Mixing in Hollow Cathode Discharge

Optical phase conjugation in the hollow cathode lamp by degnerate four-wave mixing is a novel analytical laser spectroscopic method in which a laser beam is generated as the signal beam. With the use of a demountable hollow cathode discharge cell and a continuous-wave dye laser, the signal beam is clearly visible to the naked eye when 1 µg of sodium is placed inside the cathode cavity. Since the signal beam is a time-reversed replica of the probe laser beam, highly efficient optical signal detection can be performed with minimum optical background noise. Since the signal is Doppler-free with negligible Lorentzian broadening (30 Torr discharge pressure), it provides excellent spectral resolution. Sodium D 2 hyperfine lines of 3s 2 S 1/2 ( F' = 2) → 3p 2 P 3/2 ( F = 3) and 3s 2 S 1/2 ( F' = 1) → 3p 2 P 3/2 ( F = 0) were resolved. In addition to excellent spectral resolution and ease of signal detection, the technique also offers unique properties of optical phase conjugation.