Simultaneous ultrahigh harmonic detection wavelength modulation spectroscopy for resolving congested spectra

Abstract

Wavelength modulation spectroscopy (WMS) with simultaneous detection at high harmonics (up to and including N = 11) is reported for the first time. A Vertical Cavity Surface Emitting Laser (VCSEL) is used to probe atmospheric oxygen using a multi-pass optical cell. The laser frequency is modulated at a low modulation index while synchronous detection is performed simultaneously at all harmonics up to the 11th. These higher harmonic signals allow for better resolution of congested spectra. Experimental results are used to detect and resolve absorption features in the A-band region of oxygen. The high harmonic signals are used to distinguish between stronger rotational-vibrational absorption lines in oxygen and weaker absorption lines formed by low-density isotopic oxygen. This detection method also allows for the resolution of overlap between these weaker isotopic spectra. Higher harmonic signals resolve additional structure, which does not appear at direct absorption measurements, or even in lower harmonic signals (N < 3). Since harmonic signal power decreases rapidly with detection order (N), the technique employed clearly shows that the commonly used signal-to-noise power ratio, while important, is not the only criterion for a good measurement. We examine the effects of optical pathlength saturation for these weak isotopic lines by measuring the effect of an optically thick path (at fixed density) on the signal.