Hyperspectral absorption of water around 2 μm based on a boradband tunable, narrow linewidth Tm-doped fiber laser

Abstract

The 1.8–2.0 μm waveband contains abundant absorption lines of water, which are much stronger than those in the traditional 1.3–1.5 μm waveband, exhibiting huge potentials for absorption spectrum applications of water. In the hyperspectral absorption spectrum, physical parameters of the target molecule can be derived from lots of absorption lines within a broadband scanning range, achieving the results more robust, accurate and versatile than the results from the conventional tunable diode laser absorption spectrum in which only one or two absorption lines are used. The key to hyperspectral absorption is the development of broadband tunable, narrow linewidth laser sources emitting in the wavelength range of interest. With a tunable fiber FP filter and a fiber saturable absorber, a Tm-doped fiber laser is established, featuring broadband tenability and narrow linewidth. Taking advantage of the re-absorption characteristics of Tm-doped silica fibers, a wavelength tuning range covering 60 nm from 1910–1970 nm is obtained through the appropriately designing of the active fiber length. The measured laser linewidth at steady state is smaller than 0.1 nm, which is suitable for water absorption spectrum. Hyperspectral absorption measurements of water in air and alcohol flame are conducted. In room-temperature air, more than 40 absorption lines are recognized within a tuning range of 1910–1965 nm, while, in alcohol flame, the number of detected lines reaches about 50. Comparison with the HITRAN2016 database gives a laser linewidth of about 0.06 nm which is very close to the static linewidth measured by an OSA. The temperature of the air is derived to be 298 K with a water mole fraction of about 2%, which is consistent with the measurement of the hygrothermograph. And the calculation indicates an alcohol flame temperature of about 1220 K, which is very close to the measurement result of the thermocouple.