Power-scalable wavelength-agile fibre laser source at two-microns

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Power-scalable wavelength-agile fibre laser source at two-microns
J. M. O. Daniel, M Tokurakawa, W. A. Clarkson
Optoelectronics Research Centre, University of Southampton, Highfield, SO17 1BJ, UK.
Laser sources operating in the two-micron wavelength band have a multitude of applications ranging from laser
surgery through to remote sensing and materials processing. The broad emission bandwidth of thulium-doped
silica fibre sources has allowed the realisation of laser devices operating at wavelengths from ~1725 nm to over
2100 nm. However, tunable Tm fibre lasers reported to date have provided limited coverage of the emission
band and have relied on rather cumbersome mechanical arrangements for wavelength tuning. As a result the
utility of these devices has been rather limited. Here we present preliminary results for a simple wavelength-agile
fibre laser architecture that offers the prospect of rapid wavelength tunability over the entire emission band
without the need for mechanical adjustment and has the potential to be scaled to very high power levels.
Our approach is based on the use of a dual Tm fibre gain stage laser architecture with a common external
feedback cavity incorporating an acousto-optic tunable-filter (AOTF) to provide electronically-tunable
wavelength-dependent feedback. The use of an AOTF for wavelength discrimination in a fibre laser is attractive
as it provides a very fast means for wavelength tuning or switching between pre-selected wavelengths without
any moving parts. Wavelength tuning across the entire emission band with a single fibre gain stage is extremely
difficult due to the combination of varying quasi-three-level character as a function of wavelength and gain
saturation due to short wavelength amplified spontaneous emission. As a result, the shorter wavelengths in the
emission band are easiest to access with a short (or lightly-doped) Tm fibre and intense pumping, whereas longer
wavelengths can be accessed using a longer (or more highly-doped) Tm fibre. These two regimes of operation
can be realised using core and cladding-pumped fibre laser architectures respectively. To test the validity of this
approach and verify the advantages of using an AOTF, core-pumped and cladding-pumped Tm fibre lasers
employing the cavity configurations shown in figure 1(a) were evaluated. Feedback for lasing in both cases was
provide by the 4% Fresnel reflection from a perpendicularly-cleaved facet at the output end of the fibre and by
an external cavity containing an AOTF and a broadband high reflectivity mirror at the opposite end of the fibre.
A single-clad Tm-doped fibre with a 10µm diameter single-mode core and a relatively low Tm concentration (~
0.2 wt.%) was employed in the core-pumped laser. Pump light was provided by an Er,Yb fibre laser operating at
1565 nm. In contrast, in the cladding-pumped laser, a double-clad Tm fibre with core and inner-cladding
diameters of 10 µm and 110 µm respectively was used. This fibre had a relatively high Tm concentration (~4
wt.%) to promote the two-for-one cross-relaxation process and was pumped by a fibre-coupled diode source at
795 nm. The results for wavelength tuning for both lasers are shown in figure 1(b). The core-pumped laser
could be tuned from ~1700 nm to 1950 nm and the cladding-pumped laser could laser could be tuned from
~1880 nm to ~2050 nm. A slightly longer operating wavelength range extending to >2100 nm was expected for
the cladding-pumped laser. The shorter tuning range was attributed to higher core propagation loss at long
wavelengths due OH contamination. In both cases, the maximum output power achieved (~1 W) was limited by
available pump power and thus much higher power levels should be readily achievable with higher power pump
sources. The emission linewidth was measured to be >12 GHz (i.e. ~0.12 nm @ 1700nm) and the lasing
wavelength could be switched in a timescale ~20 µs limited by the switching speed of the AOTF. It is worth
noting that the spectral power density achievable in the two-micron band via this approach is >10
3
times larger
than for commercial supercontinuum sources. The use of a simple spectral combination scheme with the core-
pumped and cladding-pumped lasers and a common external feedback cavity in conjunction with a lower loss
double-clad Tm fibre should allow wavelength selection across the ~1700 nm to >2100 nm band with the
potential for scalability to the multi-tens-of-watts and beyond. This source will be unique in terms of
wavelength agility and spectral power density in the two-micron band and will doubtless find use in a range of
applications.
(a) (b)
Fig. 1
(a) Core-pumped and cladding-pumped laser set-ups. (b) Laser output power versus wavelength for core-pumped
(blue) and cladding-pumped (red) tunable laser sources.
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0.2
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1
Power (a.u.)
Wavelength (nm)
Core Pumped Cladding pumped