All-fiber tunable laser in the 2 μm region, designed for CO₂ detection

A stable and tunable thulium-doped "all-fiber" laser offering a narrow linewidth has been created specifically to act as a compact and simple laser source for gaseous CO₂ detection. This has been done through a careful design to match the laser output wavelengths to the CO₂ absorption lines at 1.875 and 1.997 μm, respectively. A sustainable output power of 11 mW over a tuning range of 7 nm has been obtained by using a combination of a high-reflective fiber Bragg grating with a low-reflective broadband mirror, fabricated at the end of the fiber through silver film deposition. The tuning was achieved using the relaxation-compression mechanism of the fiber Bragg grating, which formed an integral part of the laser resonant cavity. A fiber Bragg grating at 1.548 μmwas utilized as a wavelength reference to monitor the tuning of the laser output over the 2 μm wavelength range with a simple and inexpensive interrogator, to avoid the use of an expensive optical spectrum analyzer and to facilitate "in-the-field" operation. This "all-fiber" laser resonator has been shown to be superior in terms of laser tuning range, output power, and linewidth compared to that created with a fiber Bragg grating pair, which was limited by the nonuniform strain transfer to both fiber Bragg gratings.