Intracavity absorption spectroscopy with thulium-doped fibre laser

Abstract For applications to intracavity laser absorption spectroscopy, we study the spectral properties of a multimode Tm 3+ -doped silica fibre laser pumped by a diode laser. In cw operation, the emission spectrum of the fibre laser is typically 4 nm wide and can be tuned from 1.70 to 1.98 μm. It shows many absorption lines of H 2 O and CO 2 due to the presence of the atmosphere in the cavity. The strength of the lines corresponds to a 7 km effective absorption path length. With repetitively pulsed operation of the fibre laser, measurements of the spectral dynamics are carried out by a specially designed rapid-scan Fourier spectrometer. Highly sensitive detection of water vapour in the laser cavity is possible when tuning the laser to the strongest absorption near 1.9 μm, and by using photoacoustic detection combined with wavelength modulation.