Performance improvement and analysis of a 1.6 μm continuous-wave modulation laser absorption spectrometer system for CO2 sensing

In a previous study, we developed a 1.6 mu m continuous-wave (cw) modulation laser absorption spectrometer system for CO2 sensing and demonstrated the measurement of small fluctuations in CO2 corresponding to a precision of 4 parts per million (ppm) with a measurement interval of 32 s. In this paper, we present the process to achieve this highly specific measurement by introducing important points, which have not been shown in the previous study. Following the results of preliminary experiments, we added a function for speckle averaging on the optical antenna unit. We additionally came up with some ideas to avoid the influences of etalon effects and polarization dependence in optical components. Because of the new functions, we realized a calibration precision of 0.006 dB (rms), which corresponds to a CO2 concentration precision of less than 1ppm for a 2km path. We also analyzed the CO2 sensing performance after the improvements described above. The measured short time fluctuation of the differential absorption optical depth was reasonably close to that calculated using the carrier-to-noise ratio of the received signal. (C) 2011 Optical Society of America