Intensity Calibration

A foundation of spectroscopy in the mm/submm is that it is possible to use the sharpness of spectral features to separate them from wider system variations in power. However, this ordinarily sacrifices knowledge of absolute power and often modifies lineshapes as well. In the heterodyne systems we directly measure the output of the IF detector to provide a DC coupled measure of power. In systems that use square law InSb detectors, the DC component is measured before high gain is used for the AC signal channel. To produce the absolute intensities, the spectrum is normalized by dividing by the channel that preserves the DC power levels.

In all of our experiments, we reduce power so as to improve the linearity of the system and to enhance the intensity calibration. We also work in the extreme Doppler limit to provide known linewidths and lineshapes. While these are at some sacrifice in sensitivity, the limiting factor in our experiments is more likely to be systematic error associated with non-linearity and this is a good trade.

We use the intensity residuals of our temperature calibration fit as a measure of the accuracy of the intensity calibration. We have found that by calibrating the response of the IF amplifier – diode detector combination and by attenuating the probe power by ~100, that excellent (~1%) intensity calibration could be obtained. Similar accuracy is obtained with the InSb square law detector systems.