Fig. 1. Principle of PDH locking

Fig. 2. LL-CRDS experimental setup

Fig. 3. Measurement of cavity longitudinal mode

Fig. 4. (a) R(5) CO transition at 0.8 mm under different pressures; (b) residuals fitted from Voigt profile

Fig. 5. (a) Pressure shift coefficients and (b) self-broadening coefficients for 12C16O transitions with m=-J for the P(J) lines and m=J+1 for the R(J) lines

Fig. 6. H2 Q3(1) transition at 39.8 kPa fitted with different profiles. (a) Experimental data (the dot line) fitted with Galatry profile (the solid line); residuals between Galatry profile and those from (b) Voigt;(c) SDVP; (d) Rautian; (e) Galatry profiles

Fig. 7. (a) H2 Q3(1) transition measured with different pressures; (b) linear dependence of the relative line positions with corresponding sample pressures; (c) linear dependence of the integral areas with corresponding sample pressures

Fig. 8. (a) Line position comparison between this work and theoretical calculations from Ref. [61]; (b) line intensity comparison between this work and theoretical ones from Ref. [61] as well as other experimental results from Ref. [62-63]

Fig. 9. CO R(10) transition measured at the pressure range of 0.2-1.5 Pa

Fig. 10. Measured frequency of CO R(10) transition at different pressures, and the red points are corresponding to the measurements done at half year later