Abstract

We studied the spectral dynamics of a broad hand neodymium-doped glass laser during extreme relaxation oscillations (contrast ratio up to 10⁸) occurring at the beginning of the lasing action. By changing the transition time of the Q-switch process used to start the laser, the amplitude of the relaxation oscillations could be accurately tuned. Intra Cavity Laser Absorption Spectroscopy (ICLAS) with this system is found to have a ’'normal" behaviour except when the intensity in the first relaxation oscillation minimum becomes too low. In t his limit, we observe a complete cancellation of spectral information accumulated by intracavity absorption during the first laser spike. Numerical modeling by integration of the multimode laser rate equations reproduces the observed behaviour of the system. This demonstrates that the “loss of memory” is elite to the average contribution of spontaneous emission in the gain medium, whose effect prevails over amplification and loss processes in the limit of small intensities (1-100 photons/mode). If nonlinear mode interaction is negligible, this effect sets the fundamental limit of the sensitivity of an ICLAS spectrometer.¹ It was observed before only in a diode laser with high internal losses², which is not a system well adapted for ICLAS measurements. Thus. to our knowledge, this is the first observation of ICLAS sensitivity saturation by spontaneous emission in a laser with low cavity losses.

© 1996 IEEE