High-power widely tunable Tm fibre lasers in cladding-pumped and core-pumped cavity configurations

Abstract

Cladding-pumped Tm-doped fibre lasers operating in eye-safe 2-µm spectral region have attracted growing interest in recent years owing to their numerous applications in areas such as lidar and medicine. In contrast to conventional 'bulk' solid-state lasers, fibre-based sources benefit from a geometry that allows relatively simple thermal management and hence offer the prospect of higher output power and improved beam quality.

A particular attraction of Tm-doped fibre lasers is the very broad transition linewidth offering the prospect of wide tunability over the ~1700-2100 nm regime. Direct pumping of double-clad Tm-doped silica fibre lasers with diode lasers at ~790 nm [1] and, more recently, Tm-doped silica fibre lasers sensitized by co-doping with Yb at 975 nm [2] has been demonstrated with output powers up to 30 W (limited by available pump power) and 75 W (limited by fibre damage) respectively. In the latter, the slope efficiency was rather low (~ 26%), due, in part, to the relatively high quantum defect heating (~51%), hence suggesting that power scaling may prove rather difficult via this scheme.

An alternative, and more promising, approach for power scaling of Tm fibre lasers is to pump directly into the upper level manifold with an Er fibre laser at 1.55-1.62 µm. This has the attraction of a very high Stokes efficiency (~0.75-0.85) and hence low quantum defect heating, opening up the prospect of very high lasing efficiencies. In addition, the good beam quality available from high-power Er-doped fibre lasers allows direct pumping into the Tm-doped fibre core leading to the possibility of much shorter device lengths. This is critically important for extending the tuning range of Tm fibre lasers to shorter wavelengths. In this paper we report highly efficient operation of Tm-doped silica fibre lasers, pumped by a high-power cladding=pumped Er,Yb fibre laser at 1565nm, in both cladding-pumped and core-pumped resonator configurations. In the cladding-pumped Tm fibre laser we obtained up to 19 W of output at 1991 nm with a slope efficiency of 69% with respect to absorbed pump power, and in the core-pumped laser we obtained a maximum output power of 12.1W at 1860nm limited by available pump power.

Using tunable cavity configurations the cladding-pumped and core-pumped Tm fibre lasers could be tuned from 1861 to 2061nm and from 1725 to 1973nm at multi-watt power levels respectively. To the best of our knowledge this represents the broadest tuning range and shortest operating wavelength from a high-power Tm-doped fibre laser reported to date.

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Identifiers

ORCID for J.K. Sahu: orcid.org/0000-0003-3560-6152

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