Abstract:
This paper reports the extraction of concentration and pressure of methane using a 1650nm laser diode operated at its phase quadrature frequency (fq) and using the calibration-free residual amplitude modulation (RAM) method. Although the RAM method is the simplest calibration-free technique it has low signal levels compared to the Phasor Decomposition (PD) method for small values of the phase difference between the laser intensity modulation (IM) and frequency modulation (FM). For the laser diode used in this studyturned out to be 90° at a very modest modulation frequency of 125.5 kHz which is an order of magnitude lower than values reported elsewhere. The RAM signal and the FM signal are at phase quadrature at this frequency and each can be fully and simultaneously recovered along a detection axis of a lock-in amplifier free from distortion by the other. The absolute absorption profile is accurately recovered with the signal to noise ratio maximized for this laser. The gas parameters are extracted by fitting a Voigt line shape to the experimental data. These results show that when operating at fq, the PD method offers no advantage over the RAM method and is therefore redundant. In addition, the background RAM is eliminated by an automatic fiber-optic RAM nulling technique. Finally the RAM method is also implemented at high values of modulation index to increase the signal-to-noise ratio. The time resolution of measurements is currently 10s.The prospect of using the RAM method with a 2004nm VCSEL is also explored.