Spectral dynamics of multimode Nd3+- and Yb3+-doped fibre lasers with intracavity absorption

Diode-pumped Nd3+- and Yb3+-doped multimode fibre lasers with external cavi ties allow sensitive measurements of intracavity absorption in the spectral ranges of 0.9-0.945 mu m and 1.03-1.15 mu m. The sensitivity is characteri zed by the effective absorption path length and is derived from the spectra l dynamics of the lasers with intracavity atmospheric absorption, and calib rating the emission spectra with HITRAN database. The highest sensitivity i s achieved with a cw Nd3+-doped fibre laser; it corresponds to a 140 km eff ective path length of absorption. Since the achieved sensitivity decreases only weakly upon the pump rate, it seems to be restricted by the joint acti on of Rayleigh scattering and nonlinear mode coupling. Both multimode fibre lasers may be made the base of sensitive portable multicomponent gas detec tors. (C) 2000 Elsevier Science B.V. All rights reserved.