Sensitivity improvement by optimized optical switching and curve fitting in a cavity ring-down spectrometer.

We presented methods for the improvement of the sensitivity of a cavity ring-down spectrometer other than modifying the cavity length and the mirrors. As for the light switching, a fast driving scheme was proposed to address the slow switching speed of the boost optical amplifier, which makes it have only half of the switching time of that for the common acoustic-optical modulators and electro-optical modulators, as well as have higher extinction ratios. This effectively suppressed the distortions of the ring-down signals. We further adopted a realistic non-exponential curve-fitting method, taking into account the switching speed and the delayed triggering of the optical switch. These methods help accurately determine the ring-down time constants, which in turn reduced the Allan variance of the measurement results and increased the sensitivity. We performed tests at different repetition rates and all of them revealed more than 30% sensitivity improvement. At a rate of 16 kHz, we increased the minimal detectable absorption of 9.1×10−11  cm−1 to 5.7×10−11  cm−1. The effectiveness of these upgrades could benefit many spectroscopic applications of the cavity ring-down spectroscopy, especially for frontier research that requires sensitive measurement and high-quality spectral data.