Abstract

We investigate a fibre laser based on a single mode silica fibre, doped with 1000 ppm Tm³⁺. The fibre laser is excited by a laser diode (70 mW at 780 nm), and it includes an intracavity absorption cell filled with air or sample gases. The fibre laser has about 1 mW cw output power, and it can be tuned from 1.7 to 2 μm by moving the aspheric lens, which couples the fibre to the output mirror through the absorption cell. For analysis of the laser emission spectrum we built a special rapid scan Fourier spectrometer with 0.015 cm⁻¹ spectral resolution. With repetitive pulsed operation of the fibre laser, the Fourier spectrometer provides 10 μs time resolution, which is achieved by synchronization of scanning process and laser pulses. The emission spectrum of the fibre laser is typically 4 nm wide. Depending on spectral position and gas composition, the emission spectrum shows narrow-band absorption lines, for example of H₂O and CO₂, as shown in Fig. 1. The strength of the absorption lines corresponds to 10 km effective absorption path length, a value which is confirmed by comparison with HITRAN data bank, and also by time resolved measurements of the evolution of the laser emission spectra. Due to strong water vapour absorption near 1.85 μm, which is illustrated in Fig 2, the thulium doped fibre laser is especially suitable to make a water vapour detector of highest sensitivity. For this purpose, the detection limit can be reduced by modulation of the fibre laser combined with detection by photoacoustic absorption cell.

© 2000 IEEE