Analysis of the transient response of erbium fibre lasers and application for sensors

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.