Tunable diode-laser heterodyne spectrometer for remote observations near 8 μm

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Abstract
A diode-laser based ultrahigh resolution IR heterodyne spectrometer for laboratory and field use has been developed for operation between 7.5 and 8.5 μm. The local oscillator is a PbSe tunable diode laser kept continuously at operating temperatures of 12–60 K using a closed-cycle cooler. The laser output frequency is controlled and stabilized using a high-precision diode current supply, constant temperature controller, and a shock isolator mounted between the refrigerator cold tip and the diode mount. The system largely employs reflecting optics to minimize losses from internal reflection and absorption and to eliminate chromatic effects. Spectral analysis of the diode-laser output between 0 and 1 GHz reveals excess noise at many diode current settings, which limits the IR spectral regions over which useful heterodyne operation can be achieved. Observations have been made of atmospheric N2O, O3, and CH4 between 1170 and 1200 cm−1 using both a single-frequency swept IF channel and a 64-channel rf spectral line receiver with a total IF coverage of 1600 MHz.