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

Abstract

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.