Frequency modulation spectroscopy with tunable diode lasers

Abstract

Frequency modulation spectroscopy with injection modulated lead-salt diode lasers offers the prospect of quantum-noise-limited detection of numerous atmospheric trace gas species. This sensitivity limit would allow detection of absorptions <10−7 in the mid-IR region and consequently detection of numerous atmospheric trace gases at the ppt level. The technique uses sample absorption to convert some of the laser frequency modulation (FM) into amplitude modulation (AM). The laser injection modulation and subsequent AM detection are usually accomplished in the radio frequency (rf) region where the diode laser excess noise is low. Unfortunately, a number of technical problems arise when the technique is used in the laboratory. These include incidental amplitude modulation of the diode laser and FM to AM conversion by external etalons, both of which shift some of the low-frequency laser noise into the rf region. We review several approaches to FM spectroscopy with diode lasers, discuss some of the experimental difficulties in implementing the technique and methods of overcoming the sensitivity limitations imposed by residual amplitude modulation. Finally, we discuss our most recent laboratory results and the prospects for sensitive monitoring instruments using FM spectroscopy.