Linewidth compression of tunable Tm-doped fiber laser and its hyperspectral absorption application

Abstract

Tunable diode laser absorption spectroscopy (TDLAS) is a widely used technology for measuring absorption spectrum. However, the measurement efficiency of TDLAS is greatly limited by the narrow tuning range of conventional tunable laser diode. Exploiting a wideband, narrow linewidth tuning laser source, hyperspectral absorption spectroscopy possesses the ability to provide the overall absorption information over a continuous waveband in a single scan, which would significantly improve the data volume and diagnostic capability of TDLAS. With profound and strong absorption lines of water and carbon dioxide, the 2 μm waveband is an ideal candidate for water and carbon dioxide related absorption spectrum. An absorption line recognition threshold of 0.07 nm is derived for the absorption spectrum measurement of water around 2 μm through theoretical analysis. Utilizing the wideband emission spectrum of Tm-doped fiber, a wideband tunable, narrow linewidth fiber laser operating at 2 μm is built by combining a tunable FP filter with a fiber saturable absorber. The tunable FP filter is responsible for the wavelength control of the laser system, with which a 60 nm wideband tuning range from 1840 nm to 1900 nm is achieved. With a section of Tm-Ho codoped fiber as the fiber saturable absorber which is used for linewidth compression, a static linewidth of 0.05 nm is attained. This wideband tunable, narrow linewidth fiber laser is tested for the hyperspectral absorption spectrum measurement of water around 2 μm. Drived with a 0–10 V triangle wave at a repetition rate of 50 Hz, the output spectrum of the laser spans over a wavelength range of about 30 nm from 1856 nm to 1886 nm. The laser beam propagates about 50 cm through an open air, and then enters into the detectors for direct measurement. The 35 absorption lines of water are recognized after processing the data. Within the 1870–1880 nm range, comparisons with the theoretical absorption spectra at different laser linewidths, derived from the HITRAN2012 absorption database, show that the measured data cannot effectively distinguish two absorption lines adjacent to the strong absorption line at 1873 nm and 1877 nm. And, the measured results can be best fitted to a laser linewidth of about 0.08 nm, demonstrating that in the dynamic scanning process, the linewidth of the laser is expanded beyond the absorption line recognition threshold. Thus, when operating in a fast wideband scanning mode, the laser system should further compress its linewidth.