Photoacoustic spectroscopy with a laser diode based excitation and a self-mixing detection scheme

Photoacoustic spectroscopy (PAS) is a powerful method for spectroscopic measurements in turbid materials. It enables volumetric measurements inside a sample despite multiple scattering of light. Furthermore it combines a high optical contrast with a low scattering coefficient of acoustic waves. In order to make PAS suitable for industrial inline measurements some improvements concerning robustness, size and cost of the system are introduced. Instead of a bulky and expensive Nd:YAG/Dye laser combination a NIR laser diode is used for photoacoustic excitation. Different modulation techniques for efficient ultrasound generation in the sample are applied. An interferometric detection system based on the self-mixing effect in a laser diode is used for displacement measurements of the sample surface. The detection part needs no fine optical adjustment and makes a physical contact with the sample, as compared to the common transducer approach, unnecessary. In order to estimate particular absorber concentrations in the sample the local fluence is calculated by means of Monte Carlo simulations at the excitation wavelength.