The experimental results show that both wavelength modulation and cavity length modulation can complete the periodic acquisition of ring-down signal and repeated measurement of ring-down time. However, from the comparison, the cavity length modulation method had higher measurement accuracy and lower limit detection of concentration, while the wavelength modulation exploits a tunable laser, and its structure is relatively simple without additional mechanical structures such as PZT. Therefore, in practical application, the selection can be made between measurement performance, device complexity, and cost according to the demand.