Method of Sellctive Intracavity Absorption Using Dye Lasers

The method of the selective intracavity absorption is based on the high sensitivity of the laser intensity on the absorption of the sample placed into the resonator. The method for Nd-glass laser with inhomogeneous broadening of the gain spectrum was grounded and experimentally checued by the authors of the papers^1,2. Later demands for the spectral region expanding of the method have led to application of dye lnsers for the intracavity measurements. The capability to broadrband emission in the different spectral regions makes of dye lasers the unique instruments for intracavity spectroscopy. A number of the successful experiments on the dye lasers applications to the sensitive detection of Na, da, Ho, Pr, I₂, Br₂ small concentrations was published until now^3–6 At the same time one cannot consider the theoretical analysis of the intracavity absorption characteristics of the dye lasers, i.e, the lasers with homogeneous spectral broadeninti of active media as sufficiently complete and reliable. The theoretical estimation of the sensitivity of the method for CW laser operation was given in papers^7,8. But analytical formulae for sensitivity in reference⁷ are valid only for near-threshold levels of pumping power which does not often correspond to experimental conditions and leaves open the question about potentialities of the method at the higher pumping power (two or more times exceeding the threshold). The authors of the reference⁸ consider the problem at any pumping levels, but their approach is based on the simplified model which seem to be doubtful and requires the special foundation. In particular they suppose the existence in the active media of the number of N_j particles, which interact only with one mode j and do not interact with other laser modes, and also of the number of N_o molecules (so called reservoir) equally interacting with every mode. In the case of the homogeneous spectral broadening such dividing the molecules into types does not seem to be rightful. In addition in the both the gain and loss of all modes (except one) are assumed to be of the same values so the total number of modes is given arbitrary and not connected with resonator, active media and pumping parameters. Because of this reason one cannot calculate correctly the value of the method sensitivity or analyze it's dependence on the parameters.