Residual Amplitude Modulation Method Implemented at the Phase Quadrature Frequency of a 1650-nm Laser Diode for Line Shape Recovery of Methane

This paper reports the extraction of concentration and pressure of methane using a 1650-nm laser diode operated at its phase quadrature frequency (f_q) 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 with the phasor decomposition (PD) method for small values of the phase difference ψ between the laser intensity modulation and frequency modulation (FM). For the laser diode used in this paper, ψ turned 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 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 (SNR) 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 f_q, 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 SNR ratio. The time resolution of measurements is currently 10 s. The prospect of using the RAM method with a 2004-nm vertical cavity surface emitting laser is also explored.