Elimination of residual amplitude modulation in TDLS-WMS

Tunable diode laser spectroscopy with wavelength modulation (TDLS-WMS) has long been plagued by the problem of residual amplitude modulation (RAM) that limits the sensitivity in 1st harmonic detection. To circumvent the problem 2f-detection has been the common electronic signal recovery approach. However, in such systems, absorption lines cannot be recovered directly, and they require calibration. To date, no method has been devised to eliminate the RAM at the optical level. We present, for the first time, a fiber-optic technique that eliminates the concentration-independent RAM component and allows 1f calibration-free recovery of gas absorption lines. The technique is stable and can be readily modified for a real-time and automated system.