High power Ho-doped fibre laser (HDFL) operating >2.1μm spectral region that exhibits excellent atmospheric transmission, can find significant advantages for applications in remote sensing, defence and as pump source for mid-IR generation. HDFLs can be cladding pumped at ~1.95μm by high brightness thulium-doped fibre lasers (TDFLs) for power scaling beyond 2.1μm . In a double clad HDF, a fluorosilicate glass cladding is needed rather than the low-index polymer commonly used in double clad fibres. This is to avoid absorption of the pump wavelength at 1.95μm by the polymer coating materials that brings additional challenges in fibre fabrication. In addition, the use of TDFL as a pump source for HDFLs will limit the overall optical to optical conversion efficiency. An alternative approach is to use Tm3+ as a sensitizer for HDFs which can be directly diode pumped at 790nm. So far, the cladding pumped Tm:Ho co-doped silica fibre laser system had been demonstrated with slope efficiencies up to 42% .
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