ABSTRACT
An all-fiber 4-core Yb-doped laser with a cavity formed by fiber Bragg gratings directly inscribed in each core with femtosecond laser pulses and 4% Fresnel reflection from the output fiber end face is demonstrated. It has been shown that the diameter of the active fiber winding significantly affects the power distribution between the cores, since it affects both the pump power distribution and the cross-coupling between the cores. In particular, with an active fiber winding diameter of 21 cm, the cores behave independently, and the power is distributed almost evenly over all cores. With a winding diameter of 6.5 cm, the lasing is achieved almost exclusively from one core, and a mechanism of that radiation concentration based on bending induced stress in an active multicore fiber is proposed which explains the experimental data. By analyzing the optical and radio-frequency spectra of the output laser radiation, additional details of the 4-core fiber lasing are revealed. In particular, a narrowband (several longitudinal modes) lasing with periodic linear sweeping of central wavelength in time is observed and characterized in the multicore fiber laser, for the first time to our knowledge. It is shown that crosstalk of longitudinal modes arising from different cores is greatly enhanced in the case of a strongly bent fiber.
ABSTRACT
Specially designed composite heavily Er3+-doped fiber in combination with unique point-by-point inscription technology by femtosecond pulses at 1,026 nm enables formation of distributed-feedback (DFB) laser with ultra-short cavity length of 5.3 mm whose parameters are comparable and even better than those for conventional Er3+-doped fiber DFB lasers having much longer cavity. The composite fiber was fabricated by melting rare-earth doped phosphate glass in silica tube. The ultra-short DFB laser generates single-polarization single-frequency radiation at 1,550 nm with narrow linewidth (3.5 kHz) and 0.5 mW output power at 600 mW 980-nm pumping. The same fiber with conventional CW UV (244 nm) inscription technology using phase mask enables fabrication of 40-mm long DFB laser with > 18 mW output power at 3.3% pump conversion, which is a record efficiency for Er3+-doped fiber DFB lasers. The developed technologies form an advanced platform for Er3+-doped fiber DFB lasers operating around 1.55 µm with excellent output characteristics and unique practical features, in particular, the ultra-short DFB lasers are attractive for sensing applications.
ABSTRACT
Random fiber lasers operating via the Rayleigh scattering (RS) feedback attract now a great deal of attention as they generate a high-quality unidirectional laser beam with the efficiency and performance comparable and even exceeding those of fiber lasers with conventional cavities. Similar to other random lasers, both amplification and random scattering are distributed here along the laser medium being usually represented by a kilometers-long passive fiber with Raman gain. However, it is hardly possible to utilize normal gain in conventional active fibers as they are usually short and RS is negligible. Here we report on the first demonstration of the RS-based random lasing in an active fiber. This became possible due to the implementation of a new Bi-doped fiber with an increased amplification length and RS coefficient. The realized Bi-fiber random laser generates in a specific spectral region (1.42 µm) exhibiting unique features, in particular, a much narrower linewidth than that in conventional cavity of the same length, in agreement with the developed theory. Lasers of this type have a great potential for applications as Bi-doped fibers with different host compositions enable laser operation in an extremely broad range of wavelengths, 1.15-1.78 µm.
