Abstract: A method and apparatus for conducting laser absorption spectroscopy uses a frequency tunable laser aligned to pass a laser beam through a sample volume and to an optical detector. A two-step modulation is used to generate frequency components that may be used to both lock the tunable laser to the absorbance signal, and generate an output signal proportional to the absorbance signal. The method includes frequency modulating the tunable laser at a first frequency while further modulating the tunable laser with a triangular waveform having a second frequency, and generating a feedback control signal based upon an antisymmetrical signal demodulated from the optical detector using a first predetermined harmonic of the second frequency. The tunable laser is locked to the absorbance signal with the feedback control signal. An output signal proportional to the absorbance signal is generated using a symmetrical signal demodulated using a second predetermined harmonic of the second or triangular waveform frequency.

Abstract: A method and apparatus using a frequency tunable laser to generate a beam which passes through a sample cell and onto an optical detector wherein the tunable laser is modulated at a predetermined frequency which has the undesired effect of producing a periodic interference fringe near an absorption induced signal. An additional level of modulation is applied to the laser beam by a triangular waveform having a predetermined peak-to-peak amplitude and frequency. The triangular waveform peak-to-peak amplitude produces a frequency swing of the laser beam so as to generate from the periodic interference fringe, a signal having reduced components near the absorption induced signal. The signal at the optical detector is demodulated by a lock-in amplifier tuned to a predetermined harmonic of the frequency of the triangular waveform.