Heterodyne spectroscopy for astrophysical applications using tunable laser sidebands at 10 microns

A heterodyne system was assembled from a microwave tunable sideband laser, a blackbody source, a gas cell, a CdHgTe photomixer and a filterbank. A measurement of line profiles of CH₃F is presented and compared with absorption measurements. System sensitivity is sufficient for high resolution measurement of fine structure lines from astronomical objects, though falling short by a factor of 12, when compared with the most sensitive astronomical heterodyne spectrometers. The usefulness of such a system for astrophysical applications is discussed.     

Optimization of CO2 laser frequency stabilization viaexternal Doppler dither modulation

An external Doppler dither modulation scheme for locking the CO ₂ laser frequency to the center of the 4.2-μm fluorescence Lamb dip of CO₂ is discussed. The optimum values for the frequency dither, fluorescence cell temperature and CO₂ absorber gas pressure depending on the experimental conditions are determined. The design of two different dither mirror arrangements is described. With optimum parameters and with τ=40 ms integration time, it is possible to lock the CO₂ laser to within ±10 kHz to line center over several minutes. Measurements of ultrahigh-resolution Lamb dip spectra of the υ₂ as R (3,3) transition of NH₃ show the superiority of external dither modulation over the conventional internal modulation scheme with respect to the spectral purity of the laser radiation