Optical frequency comb calibrated laser heterodyne radiometry in the near infrared

Using laser heterodyne radiometry with a resolution of 100 MHz we have observed a selection of iron lines in the solar spectrum near 1565 nm with an approximate resolving power of 2,000,000 million. With a laser frequency comb as the wavelength calibration source we have achieved absolute frequency instability on measurements of single lines at better than 13 MHz in a 10 second averaging window, and SNR of over 5000 when averaging up to a 1 hour exposure. An amplitude servo on the laser power creates a stable, repeatable background level, allowing for simple extraction of the line depth.