Laser Absorption Spectrometry - Modulated Techniques

Modulated Techniques

Modulation techniques make use of the fact that technical noise usually decreases with increasing frequency (often referred to as a 1/f noise) and improves on the signal contrast by encoding and detecting the absorption signal at a high frequency, where the noise level is low. The most common modulation techniques, wavelength modulation spectroscopy (WMS) and frequency modulation spectroscopy (FMS), achieve this by rapidly scanning the frequency of the light across the absorbing transition. Both techniques have the advantage that the demodulated signal is (ideally) zero in the absence of absorbers but they are also limited by residual amplitude modulation (RAM), either from the laser or from multiple reflections in the optical system (etalon effects). The most frequently used laser-based technique for environmental investigations and process control applications is based upon diode lasers and WMS and often referred to as tunable diode laser absorption spectroscopy (TDLAS). The typical sensitivity of WMS and FMS techniques is in the 10−5 range.

Due to their good tunability and long lifetime (> 10 000 hours), most practical laser-based AS is today performed by distributed feedback (DFB) diode lasers. This gives rise to systems that can run unattended for thousands of hours, with a minimum of maintenance.

However, since these laser are mostly developed for the telecom industry, they emit in the near infrared (NIR) region, primarily in the 700 nm – 2 μm range. With light in this wavelength region, mostly only weak overtone transitions of molecules can be addressed. This limits the sensitivity of conventional TDLAS to detection of species down to the mid or high ppm m range (part-per-million concentrations times meter interaction lengths). This is still insufficient for a large range of applications, wherefore other actions have to be taken.