Fig. 1. Schematic of the experimental setup.

Fig. 2. Least-squares fitting of the measured absorption signal. (a) The measured signal (black line) and standard signal (red line). (b) The residual, i.e., the difference between the two curves in (a). Optical fringes with a width of 16 pm exist in the absorption signal as well as in the residual, and are therefore the result of reflections between the PD window and the collimating lens.

Fig. 3. Measurement results of 1% CO2 concentration during a 60 s interval. Measured concentrations with and without 40 ms average are shown by the red line and black line, respectively.

Fig. 4. Allan variance of measured concentration.

Fig. 5. Measured concentration of CO2 in real breath cycles.