We report on experimental setup and characterization of a broadband fiber-optic thulium-doped source of amplified spontaneous emission, which generates radiation in a spectral region around 2 micrometer wavelengths. We present a broadband source based on core-pumped thulium-doped fiber fabricated in house using the modified chemical vapor deposition method and solution doping method, pumped by erbium-doped fiber laser at 1566 nm. The source in a backward configuration with respect to the pump operates in a single-ended configuration achieved using a simple all-fiber geometry and produces radiation with an output power of up to 350 mW. The output spectrum is combined from two local emission peaks of the Tm-doped fiber, at around 1850 nm and at around 1950 nm, with total 3-dB width of more than 140 nm and output power of 130 mW.
Fiber lasers with fiber Bragg grating (FBG) mirrors inscribed directly into the active fiber, hereinafter referred to as monolithic fiber lasers, are of great interest thanks to simpler resonator design and thanks to elimination of fiber splices between the active fiber and passive fibers with FBGs that could lead to breakdown. In order to build such monolithic fiber laser configuration, different methods for FBG inscription were reported in literature, including phase mask inscription using UV light or fs lasers in deep UV light, visible and IR spectrum, and by point-by-point technique with IR fs radiation, for details see [1, 2] and references herein.
We report to our knowledge first application of monolithic fiber laser with FBGs inscribed by so-called plane-by-plane method using femtosecond laser pulses operating in visible region [3, 4]. We used erbium and ytterbium co-doped double-clad fibre fabricated in-house. The fibre was drawn from preforms manufactured by modified chemical vapour deposition method and solution-doping of erbium and ytterbium ions with phosphorous oxide. Since the inner cladding cross-section of the fibre had a circular shape, a tailored coiling had to be applied in order to improve pump absorption [5, 6]. Indeed, the peak absorption at 976 nm increased from 3.5 dB of the standard coil to 17 dB for a tailored coil of 5 m long fibre sample. The high reflectivity mirror reflects more than 99 % and the reflectance of the low-reflective FBGs varied from 10 % to 50 % in the various samples. The laser characteristics were measured using 976 nm pump laser diode with the pump wavelength stabilized by volume Bragg grating. We report 30 % efficiency of the experimental fibre laser setup. The relatively low laser efficiency can be attributed to non-optimal absorption due to circular cross-section of the inner cladding and to non-optimized erbium-to-ytterbium concentration
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