Filip Todorov

Coherently combined power of 20 W at 2000 nm from a pair of thulium-doped fiber lasers

We experimentally demonstrated coherent beam combining of a pair of thulium-doped fiber lasers using an all-fiber Fox‐Smith resonator. We built two thulium-doped fiber lasers from PM fibers and pumped them at 793 nm. Each laser provided a power of more than 10 W at a wavelength of 2000 nm with a slope efficiency of more than 0.5. Then a compound Fox‐Smith resonator was created using the PM coupler. The obtained laser power was more than 20 W due to a constructive interference at the output of the laser, while the slope efficiency decreased to a value of 0.35. A stable CW output signal was achieved despite the fact that the individually operated lasers had the tendency to self-pulsate. (Some figures may appear in colour only in the online journal)

Self-swept holmium fiber laser near 2100 nm

Self-sweeping of laser wavelength corresponding to holmium emission near 2100 nm is reported. The sweeping occurred in ~4 nm interval with rate ~0.7 nm/s from longer towards shorter wavelengths. Origins of the selection of the sweeping direction are discussed. The laser wavelength drift with time was registered by Fourier transform infrared spectrometer. To our knowledge it is the first observation of self-swept fiber laser beyond 2000 nm.

Enhanced pump absorption efficiency in coiled and twisted double-clad thulium-doped fibers.

Results of the first experimental demonstration of the recently proposed technique for improvement of the pump absorption in double-clad fibers by their simultaneous coiling and twisting are reported. The peak absorption (14 dB) of 3-m long hexagonal thulium-doped fiber was increased by 8 dB by its simultaneous coiling and twisting. Explanation of the effect is given by numerical modelling of the pump absorption in hexagonal and panda-type double-clad fibers. Improvement of fiber laser performance was also proved. The slope efficiency increased from 19.6% of the straight fiber to 23.9% of the coiled only fiber and 29.4% of the simultaneously coiled and twisted fiber.

Monolithic Tm-Doped Fiber Laser at 1951 nm With Deep-UV Femtosecond-Induced FBG Pair

We report on a monolithic Tm-doped fiber laser emitting in the region

High-power fiber laser with a polarizing diffraction grating milled on the facet of an optical fiber

{\sim}{\rm 2}~\mu{\rm m}

A mode-locked thulium-doped fiber laser based on a nonlinear loop mirror

. A pair of fiber Bragg gratings (FBGs) was written into the Tm-doped fiber using a two-beam interferometry and a deep ultraviolet (DUV) femtosecond laser source at 266 nm. Such a monolithic setup allows a simple setup of the laser made of a single fiber only. This configuration also offers greater reliability because fusion splices inside the resonator cavity could eventually generate failure in the laser cavity. To the best of our knowledge, this is the first rare-earth-doped fiber laser with an FBG pair written by DUV femtosecond laser radiation. Laser characteristics are presented as well as compared with the fiber laser with external FBGs conventionally spliced to the Tm-doped fiber. Dependence of the laser wavelength on the pump power is investigated in detail.

Thulium-doped fibre broadband source for spectral region near 2 micrometers

We milled a sub-wavelength diffraction grating on the facet of a large mode area fiber. The diffraction grating had different reflectivities for TE and TM polarized light. It was tested in a thulium-doped fiber laser where it functioned as a low reflectivity output mirror integrated with an intracavity polarizer. Compared to the laser with a perpendicularly cleaved output fiber, the laser with diffraction grating had a slightly increased threshold power and the same slope efficiency. The beam quality factor M2 was not impaired. Polarization extinction ratios of about 20 dB that were observed at low laser powers dropped to 10 dB at high powers.

Characterization of fluorescence lifetime of Tm-doped fibers with increased quantum conversion efficiency

We have demonstrated an all-fiber mode-locked thulium-doped fiber laser with extremely long pulses, a very narrow spectrum and a high average power. The mode-locker is formed by a nonlinear loop mirror. The laser generates sinus-squared waveform with a repetition period of 82 ns and an average power between 100 and 150 mW. The wavelength is defined by the fiber Bragg grating and is 1956 nm.

Monolithic thulium-doped fiber laser with UV femtosecond-laser-induced fiber-Bragg-grating pair

Abstract We demonstrated two methods of increasing the bandwidth of a broadband light source based on amplified spontaneous emission in thulium-doped fibres. Firstly, we have shown by means of a comprehensive numerical model that the full-width at half maximum of the thulium-doped fibre based broadband source can be more than doubled by using specially tailored spectral filter placed in front of the mirror in a double-pass configuration of the amplified spontaneous emission source. The broadening can be achieved with only a small expense of the output power. Secondly, we report results of the experimental thulium-doped fibre broadband source, including fibre characteristics and performance of the thulium-doped fibre in a ring laser setup. The spectrum broadening was achieved by balancing the backward amplified spontaneous emission with back-reflected forward emission.

Monolithic Er/Yb double-clad fibre laser with FBG inscribed using the direct-write plane-by-plane fs-laser inscription method

In this work we report on the fluorescence lifetime characterization of the experimentally prepared Tm-doped silica optical fibers with increased quantum conversion efficiency (QE). Optical fibers were drawn from preforms prepared by conventional solution-doping of thulium and aluminium chlorides and by deposition of dispersed alumina nanoparticles with thulium chloride. Prepared preforms and optical fibers were characterized by means of thulium and aluminium concentrations, refractive index profiles, optical spectral attenuations (absorptions) and fluorescence lifetimes. Highly aluminium-codoped optical fiber prepared from alumina nanoparticles exhibited fluorescence lifetime of about 690 μs, which is about 40% higher compared to the conventionally prepared Tm-doped silica fiber.