Many types of optical fibre sensor are based on either sensitive measurement of optical attenuation or on spectral measurements involving absorption at specific wavelengths or a shift in wavelength, for example, chemical analysis based on spectroscopy, or strain and temperature sensors based on fibre Bragg gratings. Measurements are normally performed under steady-state conditions. Here we propose a technique for optical sensors based on analysis of the transient output of fibre lasers containing an intra-cavity cell or sensor element which affects the attenuation or spectral characteristics of the cavity. We report a detailed theoretical and experimental study of the dynamics of erbium fibre ring lasers. Time domain analysis of the transient involves measurement of parameters such as build-up time and characteristics of the relaxation oscillations which are dependent on the cavity parameters. Spectral domain analysis involves monitoring the detailed optical spectrum during the build-up period. Due to the multiple circulations of light within the cavity during this period, monitoring the spectral evolution may provide high-sensitivity spectroscopic data on absorption lines of gases within an intra-cavity cell. A key challenge, however, is the experimental capture of mode evolution which requires high resolution spectra to be collected during the build-up period.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.