Suppression of intensity modulation contributions to signals in second harmonic wavelength modulation spectroscopy

Recovery of the full 2f wavelength modulation spectroscopy (WMS) signal in isolation from the 2f residual amplitude modulation (RAM) due to nonlinear intensity modulation (IM) and distortion due to linear IM is demonstrated. The 2f RAM is eliminated using a fiber delay line, while the linear IM-induced distortion is eliminated by a phasor decomposition approach. This generic and robust two-pronged strategy removes the need to separately measure the 2f RAM in high-modulation-index calibration-free 2f WMS. It is also important for widely tunable 2f WMS using nontelecom diode lasers with highly nonlinear characteristics leading to high-2f RAM levels. © 2010 Optical Society of America.