STABILIZATION AND PRECISE CALIBRATION OF A CONTINUOUS-WAVE DIFFERENCE-FREQUENCY SPECTROMETER BY USE OF A SIMPLE TRANSFER CAVITY

A novel, simple, and inexpensive calibration scheme for a continuous-w ave difference frequency spectrometer is presented, based on the stabi lization of an open transfer cavity by locking onto the output of a po larization stabilized HeNe laser. High frequency, acoustic fluctuation s of the transfer cavity length are compensated with a piezoelectric t ransducer mounted mirror, while long term drift in cavity length is co ntrolled by thermal feedback. A single mode Ar+ laser, used with a sin gle mode ring dye laser in the difference frequency generation of 2-4 mum light, is then locked onto a suitable fringe of this stable cavity , achieving a very small long term drift and furthermore reducing the free running Ar+ linewidth to about 1 MHz. The dye laser scan provides tunability in the difference frequency mixing process, and is calibra ted by marker fringes with the same stable cavity. Due to the absolute stability of the marker cavity, precise frequency determination of ne ar infrared molecular transitions is achieved via interpolation betwee n these marker fringes. It is shown theoretically that the residual er ror of this scheme due to the dispersion of air in the transfer cavity is quite small, and experimentally that a frequency precision on the order of 1 MHz per hour is routinely obtained with respect to molecula r transitions.