On the temperature dependent characteristics of a photoacoustic water vapor detector for airborne application

A detailed study on the temperature dependent sensitivity of a wavelength modulated diode laser based photoacoustic (PA) water vapor detection system is presented. The temperature dependence of the resonance frequency, the Q-factor of the resonator, the microphone sensitivity and the response time is investigated. It is shown that the overall temperature dependent sensitivity of the system is primarily determined by the temperature dependence of the microphone sensitivity. Effort was made to improve the system’s accuracy for measurements under varying ambient temperature typical in airborne applications, while maintaining the fast response time of the PA system. For this purpose a wavelength-stabilization method is introduced, with which the wavelength instability of the laser can be decreased to be as low as 0.008 nm. Test measurements proved the feasibility of the implementation of the wavelength locking method within the framework of the CARIBIC (civil aircraft for the regular investigation of the atmosphere based on an instrument container) project.