Application of 940 nm high-power DFB lasers for line-broadening measurements at normal pressure using a robust and compact setup

摘要

High-power distributed-feedback (DFB) lasers for the wavelength range near 940 nm (i.e. about 10,600 cm~(-1)) were used for line-broadening measurements of individual rotational-vibrational absorption lines of water vapour at atmospheric pressure using a minimalist set-up. The laser has a maximum output power larger than 500 mW. Over the whole power range from threshold to maximum power, it operates in single mode operation with a tuning range of 4.7 nm, i.e. 50 cm~(-1), at 20℃. With an emission line-width≤2 MHz (0.66×10~(-4) cm~(-1)), the device is well suited for high-resolution spectroscopy. Instead using a sophisticated optical set-up with a multireflection cell to obtain the necessary path length for measuring rather weak absorption lines, in this work a 4" integrating sphere was used as a multi-reflection cell providing an effective absorption length of 6.7 m. The laser was placed without any optical elements in front of the entrance hole. The power-current-characteristics, the spectral features of the diode laser, and examples for the spectral tuning will be given. In the spectral range from 10,584.3 cm~(-1) to 10,670.2 cm~(-1), line broadening coefficients in the range 0.059 cm~(-1)/atm≤γH_2o-air≤ 0.112 cm~(-1)/atm were measured for 18 H_2O absorption lines from the 2v_1+v3-band and 7 H_2O lines from the 3v_1-band. The good agreement of the measured air-broadening coefficients with data from the literature for absorption lines with a J" ≤ 4 and K"_a ≤ 3 proves the applicability of this low-cost set-up. Some deviations between the literature and this work were observed for lines from the 2v_1 +v_3-band with larger J" > 4 and K"_a = 3.