ABSTRACT
The mode tuning method and its application to a mid-infrared cavity ring-down spectroscopy experiment
J. Remy, C.H.J.M. Groothuis, W.W. Stoffels, G.M.W. Kroesen, Technische Universiteit Eindhoven, Applied Physics Department, den Dolech 2, P.O Box 513, 5600 MB Eindhoven, The Netherlands - EU
Cavity ring-down spectroscopy (CRDS) is of particular relevance in the study of the optical absorption of dust particles in a radio-frequency silane plasma. Although CRDS has historically primarily made use of pulsed lasers as light sources, the experimental design is based on a set of 0.1 mW CW multimode lead-salt laser diodes emitting in the mid-infrared wavelength range. In order to chop the laser beam and reach similar effects to pulsed light sources, a new method called �mode tuning� has been chosen. Based on a small detuning of the incoming laser wavelength, mode tuning uses the ring-down cavity mode structure as well as the optical properties of the laser diode itself in order to control the ring-down effect. We will present the theoretical optical approach on which mode tuning relies. We will furthermore show that mode tuning is an efficient and easy way of preventing the photons from entering the cavity. Controlled detuning from the laser diode not only proves to be a non-intrusive way of chopping the infrared beam; it also proves to be a very flexible tool for the widely used CRDS technique.
