Frequency-modulation spectroscopy with a pulsed dye laser: experimental investigations of sensitivity and useful features

Abstract

Systematic investigations of signal-to-noise ratios obtained in frequency-modulation (FM) spectroscopic measurements utilizing pulsed tunable dye lasers are reported which show that the noise becomes smaller as the modulation frequency is raised relative to the laser linewidth. Thus by raising our modulation frequency and narrowing our laser linewidth, we are able to detect ~0.1% absorption in spite of ~50% shot-to-shot fluctuations in the dye laser intensity. In addition several useful aspects of FM spectroscopy using pulsed dye lasers are demonstrated. The broad tuning range, ~5 cm⁻¹, over which continuous scans may be made is demonstrated by recording absorption spectra of I₂, Br₂, NO₂, and Na. The sensitivity of the technique to broad absorption features is shown by absorption spectra of the above species at atmospheric pressure. The Na was in a flame showing the potential usefulness of the technique in combustion diagnostics applications. Finally the nanosecond time resolution is demonstrated by observing the absorption of a transient population in the Na 3p state which has a lifetime of 16 nsec.