A laser-locked cavity ring-down spectrometer employing an analog detection scheme

A system is described that employs a diode-pumped Nd:YAG continuous-wave laser source servolocked to a three-mirror optical cavity and an analog detection circuit that extracts the ring-down rate from the exponentially decaying ring-down waveform. This scheme improves on traditional cavity ring-down spectroscopy setups by increasing signal acquisition rates to tens of kilohertz and reducing measurement noise sources. For example, an absorption spectrum of a weak ${\mathrm{CO}}_{\mathrm{2}}$ transition at 1064 nm is obtained in less than 10 s at a spectral resolution of 75 kHz employing a cavity with an empty-cavity ring-down decay lifetime of 2.8 μs and a total roundtrip path length of 42 cm. The analog detection system enables laser frequency scan rates greater than 500 MHz/s. The long-term sensitivity of this system is ${\text{8.8×10}}^{\text{−12}} {\mathrm{cm}}^{\mathrm{-1}}{\mathrm{ Hz}}^{\mathrm{-1/2}}$ and the short-term sensitivity is ${\text{1.0×10}}^{\text{−12}} {\mathrm{cm}}^{\mathrm{-1}}{\mathrm{ Hz}}^{\mathrm{-1/2}}.$