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.

We report our first results on broadband electrical feedback applied to tunable lead-salt diode lasers. We use a heterodyne technique in combination with electronic frequency discrimination to reduce the diode-laser linewidth. The loop band width is about 10 MHz. We present results on high-frequency modulation to study the frequency response of the diode laser. From this experiment we are able to estimate the carrier tuning rate of the used lead-salt diode laser. Preliminary results in linewidth reduction by fast electrical feedback promise the possibility to apply the fast electrical feedback technique to lead-salt diode lasers.

We present an optically stabilized lead-salt diode-laser system which is the nucleus of a very-high-resolution instrument for sub-Doppler molecular spectroscopy in the mid-infrared. By application of external optical feedback, we have narrowed the diode-laser linewidth by two orders of magnitude, yielding a spectral width of less than 200 kHz. The diode- laser frequency is stabilized and controlled via the external reflector by variable-frequency offset-locking the diode-laser to a CO laser frequency. This substantial improvement in the spectral properties enabled us to perform a Lamb-dip experiment on a carbonyl sulfide (OCS) absorption line near 1985 cm−1. We were able to detect a saturated dispersion signal at low pressure (5 Pa) with a signal-to-noise ratio of several thousand. The present paper describes the unique features of the optically stabilized tunable diode-laser system and its use as a spectroscopic tool for sub-Doppler applications.

Optical stabilization of tunable lead-salt diode lasers (TDL) in the mid-infrared is presented for the first time. By introducing an external feedback mirror both the linewidth is substantially narrowed and the frequency of the TDL is stabilized and controlled via this mirror in a frequency-offset locked scheme. We achieved narrowing of the linewidth by 1–2 orders of magnitude or better. The TDL is offset-locked to a CO gas laser by a heterodyne technique: the beatnote between the two lasers is used to control the length of the external resonator. By this scheme we gain the capability of absolute frequency measurements with sub-Doppler accuracy. The improved spectral properties of the diode laser provide a new tool for high-resolution molecular spectroscopy in the mid-infrared.