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-wave difference frequency spectrometer is presented,
based on the stabilization of an open transfer cavity by locking
onto the output of a polarization stabilized HeNe laser. High
frequency, acoustic fluctuations of the transfer cavity length are
compensated with a piezoelectric transducer mounted mirror, while
long term drift in cavity length is controlled by thermal feedback.
A single mode Ar+ laser, used with a single mode ring dye laser in
the difference frequency generation of 2–4 µm 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 calibrated by
marker fringes with the same stable cavity. Due to the absolute
stability of the marker cavity, precise frequency determination of
near infrared molecular transitions is achieved via interpolation
between these marker fringes. It is shown theoretically that the
residual error 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 molecular transitions. Review of Scientific Instruments
is copyrighted by The American Institute of Physics.