Single-frequency fiber lasers have been developed in the 1.1-µm spectral region (based on ytterbium active ions) and in the 1.5-µm region (based on erbium active ions), but, recently, scientists in Denmark developed what they believe is the first single-frequency, distributed feedback fiber laser based on thulium active ions, lasing at 1.7 µm. They predict that the laser, which they expect to lase at 1.7 to 2.1 µm, will find applications in high-resolution spectroscopy, coherent lidar and optical frequency mixing.The thulium laser was created by pumping thulium ions from the 3H6 ground level to the 3F4 level with 790-nm light from a Ti:sapphire laser. The ions subsequently relaxed to the 3H4 upper laser level.The researchers, from Technical University of Denmark in Kongens Lyngby and from Koheras A/S in BirkerØd, faced a number of obstacles in creating a distributed feedback thulium laser. For one thing, a 1.7-µm thulium laser is essentially a three-level system, lasing from the 3H4 level to the ground 3H6 level (see figure), so fully half the thulium ions must be excited to obtain a population inversion. Another difficulty is the relatively low (~50 percent) quantum efficiency that results from pumping to the 3F4 level. From the 3F4 level, the ions decay nonradiatively to the 3H5 level and then to the 3H4 upper laser level. While in the 3H5 level, however, the ions can absorb a 790-nm pump photon and jump to the 1G4 level, preventing their joining the population inversion. These ions' presence in the 1G4 level is evidenced by a faint blue glow from the fiber as they spontaneously decay back to the ground level. Yet a further difficulty is the high probability of nonradiative (phonon) decay from the 3H4 upper laser level. Despite these obstacles, the researchers observed laser threshold when they pumped a 4.7-cm length of thulium-doped silica fiber with 59 mW of 790-nm light from a Ti:sapphire laser. They generated up to 1 mW of laser output at 1735 nm when they boosted the launched pump power to 590 mW. They forced the laser to oscillate at a single frequency by writing a Bragg grating into the fiber's core with ultraviolet radiation and a phase mask. The grating period was 1196 nm, with a phase shift at the center of the resonator. Because this grating favored one longitudinal mode over the others, that mode was the first to reach threshold, and it depleted the homogeneously broadened population inversion before any other mode could reach threshold. The ultraviolet technique the scientists used to write the grating induced a birefringence into the grating that forced the laser into a single polarization as well as a single frequency. By applying a strain to the grating with a piezoelectric actuator, they tuned the laser's output from 1734 nm to 1736 nm without disturbing the polarized, single-frequency oscillation. In postpublication results, the researchers have reported a tenfold increase in efficiency, obtaining 2.5 mW of laser output from 120 mW of pump power launched into the fiber. They also observed lasing at 1984 nm, producing 4 mW of output from 130 mW of pump power launched into the fiber.