Development of a line-narrowed and frequency-stabilized TDL system for sub-Doppler molecular spectroscopy

Abstract The spectral resolution of TDL spectrometers is generally limited by the finite spectral width of the TDL radiation. Usually, the spectral purity of lead-salt diode lasers is sufficient to perform Doppler-limited studies but is inadequate to resolve sub-Doppler spectroscopic features like saturated-absorption signals. The application of external optical feedback allowed us to reduce the linewidth of tunable diode lasers by two orders of magnitude down to 200 kHz. In a frequency-offset locking scheme we stabilized and controlled the frequency of the narrowed line. This technique enabled us to use the TDL as saturating and probing laser in a saturation experiment: we observed a nonlinear absorption signal (Lamb dip) in an OCS absorption line with very high resolution. The current performance of our TDL system is reported and the resolution and accuracy limits are discussed.