L.J. Cooper

Efficient single-mode operation of a cladding-pumped ytterbium-doped helical-core fiber laser

A novel approach to achieving robust single-spatial-mode operation of cladding-pumped fiber lasers with multimode cores is reported. The approach is based on the use of a fiber geometry in which the core has a helical trajectory within the inner cladding to suppress laser oscillation on higher-order modes. In a preliminary proof-of-principle study, efficient single-mode operation of a cladding-pumped ytterbium-doped helical-core fiber laser with a 30 microm diameter core and a numerical aperture of 0.087 has been demonstrated. The laser yielded 60.4 W of output at 1043 nm in a beam with M2 < 1.4 for 92.6 W launched pump power from a diode stack at 976 nm. The slope efficiency at pump powers well above threshold was approximately 84%, which compares favorably with the slope efficiencies achievable with conventional straight-core Yb-doped double-clad fiber lasers.

High-power Yb-doped multicore ribbon fiber laser

A highly elongated double-clad ribbon fiber that comprises a pure-silica inner cladding with transverse dimensions of approximately 1.4 mm by 0.23 mm with a linear array of ten ytterbium-doped cores has been fabricated and operated in a simple laser configuration pumped by two diode stacks. The fiber laser yielded 320 W of output power at a center wavelength of 1045 nm in a combined beam with beam propagation factors of approximately 2 (perpendicular to the array) and approximately 150 (parallel to the array) for 576 W of launched pump power. The slope efficiency with respect to absorbed pump power was 62%. The prospects for further power scaling and improved beam quality and efficiency are discussed.

Efficient single-axial-mode operation of a Ho:YAG ring laser pumped by a Tm-doped silica fiber laser.

Efficient single-frequency operation of a Ho:YAG ring laser at room temperature with a traveling-wave TeO2 acousto-optic modulator to enforce unidirectional operation is reported. By use of a 2-at. % Ho3+-doped 10-mm-long Ho:YAG rod, end pumped by a cladding-pumped tunable Tm-doped silica fiber operating at 1.9 microm, the Ho:YAG ring laser yielded 3.7 W of single-frequency output at 2.1 microm in a diffraction-limited TEM00 beam with M2 < 1.1 for an incident pump power of 8.8 W. The rf power required for unidirectional operation was 0.3 W and corresponded to an increase in cavity loss for the lasing direction (due to diffraction) of only 0.5%. The prospects for further improvement in efficiency are discussed.

A cladding pumped neodymium-doped fiber laser tunable from 932 nm to 953 nm

A cladding pumped neodymium-doped fiber laser tunable from 932 nm to 953 nm produced a single-mode laser output with maximum power 705 mW at 944 nm. An effective design was used to avoid competing four level transition.

Recent advances in high power fiber lasers

We will review recent progress on cladding-pumped fiber devices with output powers up to the kilowatt level and broad wavelength tunability in the 1 - 2 µm wavelength regime where silica fibers work well.

Helical-core Yb-doped fibre laser

In this paper we report a new approach for achieving robust single-mode operation in a large-core fibre laser or amplifier by employing a core, which has a helical trajectory within the cladding material. As a confirmation of the benefits of the helical-core geometry, an Yb-doped fibre with a straight core of the same diameter and numerical aperture was tested, which yielded a multimode output beam with beam propagation factor, M/sup 2/=5.5 and /spl sim/50% of the fibre output rejected from the polariser.

Geometrical factor modification of helical-core fiber radiation loss formula

A strict consideration of the geometry of helical-core fibers leads to a modification of a previously published formula for radiation loss in such fibers. We derive an expression for the radiation loss of the fundamental mode, and find that it differs from the previous expression by a simple modification factor, which depends on the geometric parameters of the helix (pitch and offset). The derived expression predicts a loss which is of the order of 40% lower for typical helix parameters.

High-power two-micron fiber lasers

A double-clad thulium-doped silica fibre laser, pumped by two beam-shaped diode-bars operating at 787-790nm, has yielded 14W of single-mode output at ~2µm, with a slope efficiency with respect to launched power of 46%. Using a tunable laser configuration, comprising an external cavity with a diffraction grating, a maximum output power of 10.5W at 1921nm has been obtained, and wavelength tuning at multi-watt power levels over 215nm from 1855 to 2070nm has been demonstrated. The prospects for further improvement in performance will be discussed. In addition, a simple approach for further scaling of power levels from fibre lasers in the two-micron regime, based on intracavity wavelength combining of multiple fibre lasers is described. In a preliminary demonstration, four cladding-pumped thulium-doped silica fibre lasers are wavelength-combined yielding an output power >14W. The prospects for further increase in output power via this approach will be discussed.

Power scaling concepts for fiber lasers

The use of a two-mirror beam shaper to re-format the output from a diode-stack to allow efficient cladding-pumping of fiber lasers is described. In addition, the use of a helical-core geometry to suppress oscillation on higher-order modes in large-core double-clad fiber lasers is discussed. In preliminary experiments we have obtained ∼90W of output in a beam with M2□5 from a conventional straight-core Yb-doped double-clad fiber with V=11 for ∼150W of launched pump power at ∼938nm from a beam-shaped diode-stack, and ∼ 24.3W of output in a beam with M2□3 from a Yb-doped helical-core fiber for ∼50W of launched pump power. The prospects for further improvement in performance and beam quality are considered.

Novel concepts for high-power fibre lasers

High power operation of an ytterbium-doped multi-core ribbon fiber laser and a cladding pumped ytterbium-doped helical core fiber laser is reported. 320W laser output at 1045nm was demonstrated with a 10-core ytterbium doped ribbon fibre. Efficient single-mode operation of a cladding-pumped ytterbium-doped helical-core fiber laser with a 30 μm diameter core and a numerical aperture of 0.087 have also been demonstrated. The laser yielded 60.4 W of output at 1043 nm in a beam with M2 1.4 for 92.6 W launched pump power from a diode stack at 976 nm. Further more, the recently progress of a cladding-pumped ytterbium-doped helical-core superfluorescent fiber source is also reported. The highly efficient operation of the helical core fiber ASE source yielded 107W of amplified spontaneous emission output spanning the wavelength range from 1030-1160nm with a bandwidth (FWHM) of 37nm.