On the pressure dependent sensitivity of a photoacoustic water vapor detector using active laser modulation control

A detailed study on the pressure dependent sensitivity of a wavelength modulated diode laser based photoacoustic (PA) water vapor detection system is presented. It is shown that the pressure dependence of sensitivity is primarily determined by the pressure dependence of the microphone’s sensitivity and the quality factor of the excited acoustic resonance of the PA cell. Effort was made to improve the system’s sensitivity for the whole pressure range (from 200 mbar to 1000 mbar) of operation typical in atmospheric research, while maintaining the inherent fast response time of the PA system. For this purpose active control of modulation based on the continuous adjustment of the unmodulated and modulated parts of the laser current in accordance with the actual gas pressure was introduced, with which a minimum detectable water vapor concentration (3σ) of 300 ppb at 200 mbar and 188 ppb at 1000 mbar was achieved. The system’s sensitivity improves slightly at the lower end of the pressure range and increases by a factor of more than two at the higher end, when compared with that of our PA system currently on board of a commercial aircraft within the framework of an atmospheric research project (CARIBIC—Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container). Test measurements proved the feasibility of the implementation of the developed modulation method within the framework of the CARIBIC project.