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

Abstract

The 1.8 μm~2 μm waveband contains abundant absorption lines of water, which are ever stronger than those in the traditional 1.3 μm~1.5 μm waveband, exhibiting huge potential for absorption spectroscopy applications of water. In hyperspectral absorption spectroscopy, physical parameters of the target molecule can be derived with lots of absorption lines within a broadband scanning range, making it more robust, accurate and versatile compared with the conventional tunable diode laser absorption spectroscopy with only exploits one or two absorption lines. The key for hyperspectral absorption is the development of broadband tunable, narrow linewidth laser sources emitting in the interested wavelength range. 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 nm to 1970 nm is obtained through proper design of the active fiber length. The measured laser linewidth at steady state is smaller than 0.1 nm, which is appropriate for water absorption spectroscopy. Hyperspectral absorption measurements of water in air and alcohol flame are conducted. In room-temperature air, more than 40 absorption lines are recognized within the tuning range from 1910 nm to 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 298K with a water mole fraction of about 2%, which is consistent with the measurements of the hygrothermograph. And, calculations indicate an alcohol flame temperature of about 1220 K, which is very close to the measurement results of the thermocouple.