R.B. Williams

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.

Single-mode direct-ultraviolet-written channel waveguide laser in neodymium-doped silica on silicon

A waveguide laser with a neodymium-doped silica core is fabricated on a silicon substrate by a combination of flame hydrolysis deposition, solution doping, and direct UV writing. The neodymium-ion concentration is estimated to be approximately 8000 parts in 10(5). The propagation loss around 1.05 microns is < 0.8 dB/cm. Lasing in the range 1048-1056 nm and 1356 +/- 1 nm is observed. A slope efficiency of 33% for a high-reflectivity output coupler and a threshold of 4 mW of absorbed power for a 25% output coupler are measured for the 4F3/2-->4I11/2 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.

Beyond 1 kW with fiber lasers and amplifiers

We report recent progress and future prospects of high-power fiber sources in the 1 and 1.5 /spl mu/m spectral ranges, with a 103 W, 1565 nm erbium-ytterbium doped fiber laser as an example. Our emphasis is on diode stack pumping and short fibers with high Brillouin thresholds.

Assessment of waveguide thermal response by interrogation of UV written planar gratings

The technique of direct grating writing, based on direct ultraviolet writing, is demonstrated as a tool for highly accurate measurement of waveguide and material properties. Silica-on-silicon planar samples are processed using hydrogen loading and thermal locking before Bragg channel waveguides are defined in the photosensitive core layer. The refractive index is accurately probed to compare different thermal locking procedures and characterize waveguide thermal stability.

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.

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.

3.7-Watt single-frequency cw Ho:YAG ring laser end-pumped by cladding-pumped Tm-doped silica fibre laser

Acousto-optically induced unidirectional operation of a Ho:YAG ring laser pumped by a Tm-doped silica fibre laser has been demonstrated. 3.7W of single-frequency output at 2114nm for 8.8W of incident pump power at 1905nm was obtained.