Pushing CRDS to the parts per trillion (ppt) level requires very precise scanning and fitting of the target absorption line. The wavelength of the available near-IR laser diodes is directly tuned by changing its drive current and temperature. However, the exact relationship between current and wavelength varies from laser to laser. Moreover, because this relationship can change over time, the wavelength cannot be known from the drive current alone. Additionally, it is necessary to know the absolute value of the laser wavelength to a precision that is a few orders of magnitude narrower than the spectral linewidth. Commercial wavemeters, however, do not offer this precision. This is the main reason that older, competitive cavity-based instruments try to lock to a single wavelength rather than scan across the entire absorption line.
At Picarro we solved this problem by developing and patenting our own wavelength monitor. It can measure absolute laser wavelength to a precision more than 1000 times narrower than the observed Doppler-broadened linewidth for small gas phase molecules. We then use this in a novel way - specifically, we lock the laser to the wavemeter, which we then actively tune to known wavelengths. The result is higher spectral precision than in any commercial spectrometer - laser-based or otherwise. This spectral precision is the key to the ultra-precise fitting of lineshapes and line heights necessary to reach parts per trillion concentration sensitivity.