High Frequency Modulation Spectroscopy with a Lead Salt Diode Laser
High frequency modulation (FM) spectroscopy, (1,2,3) is a promising tool for high speed and ultrasensitive atmospheric trace gas monitoring. Based on a measured wideband noise characteristics (4) of a lead-salt diode-laser emitting in the mid IR a sensitivity improvement of 2 orders of magnitude in comparison to conventional derivative (2f) spectroscopy is expected. This improvement can be achieved by moving in modulation and detection frequency space from the 1/f noise dominated region (10 kHz) into a shot noise limited frequency domain above 100 MHz. Optical densities of 10−4 were measured on a µsec timescale with a 200 kHz detection bandwidth. Using digital averaging techniques (5) a detection limit of 2.5.10−6 has been demonstrated with the actual measurement bandwidth of 1.5 kHz. Up to this sensitivity no limitations due to etalons or laser intrinsic parameters could be observed. Further bandwidth reduction is currently being investigated and requires digital background subtraction techniques as well as etalon suppression schemes.