We report on a newly designed and fabricated ytterbium-doped large mode area fiber with an extremely low NA (~0.04) and related systematic investigations on fiber parameters that crucially influence the mode instability threshold. The fiber is used to demonstrate a narrow linewidth, continuous wave, single mode fiber laser amplifier emitting a maximum output power of 3 kW at a wavelength of 1070 nm without reaching the mode-instability threshold. A high slope efficiency of 90 %, excellent beam quality, high temporal stability, and an ASE suppression of 70 dB could be reached with a signal linewidth of only 170 pm.

Narrow linewidth, single mode 3 kW average power from a directly diode pumped ytterbium-doped low NA fiber amplifier

Fabrication of high aluminium containing rare-earth doped fiber without core–clad interface defects

We report on the preparation and optical characteristics of an Yb(3+)-doped large core silica fiber with the active core prepared from nanoporous silica rod by the glass phase-separation technology. The measurements show that the fiber has an Yb(3+) concentration of 9811 ppm by weight, a low background attenuation of 0.02 dB/m, and absorption from Yb(3+) about 5.5 dB/m at 976 nm. The laser performance presents a high slope efficiency of 72.8% for laser emission at 1071 nm and a low laser threshold of 3 W within only 2.3 m fiber length. It is suggested that the glass phase-separation technology shows great potential for realizing active fibers with larger core and complex fiber designs.

Yb 3+ -doped large core silica fiber for fiber laser prepared by glass phase-separation technology

Recent advances in high-power fiber lasers have imposed several pairs of higher requirements on fiber materials, such as larger fiber core/higher homogeneity and higher doping level/lower numerical...

3D Nanoporous Silica Rods for Extra-Large-Core High-Power Fiber Lasers

In this thesis the optimisation and peak power scaling of pulsed holmium-doped fibre lasers were investigated with the aim of demonstrating a fibre gain medium that is able to address the requirements of applications that currently rely on bulk crystalline Ho:YAG or Ho:YLF solutions. Conventional fibre processing techniques such as cleaving, end-capping and component fabrication were improved upon using CO2 laser processing. The resulting components and processes are also characterised under high power operating conditions and have enabled subsequent experimentation and demonstrations. Holmium-doped silica fibres were fabricated and characterised with the aim of reducing impurity contaminations, improving composition and achieving efficient operation at 2.1 μm. These fibres were characterised passively using transmission spectroscopy and actively in a laser configuration. The most efficient of these compositions operated with a 77% slope efficiency in a core-pumped laser up to average powers of 5 W and was then processed into a double-clad geometry. The cladding-pumped fibre was operated at 70 W output power with a slope efficiency of 67% and represents one of the highest power and most efficient cladding-pumped holmium-doped fibres demonstrated to-date. Small-signal amplifiers utilising both thulium-doped and holmium-doped silica fibres were demonstrated. These amplifiers offered a broad wavelength coverage spanning 490 nm at 15 dB gain from 1660 nm – 2150 nm. This remarkably broad wavelength coverage is attractive for a large number of disciplines looking to exploit this previously difficult-to-reach wavelength range. In addition to these devices, the average power and peak power scaling of 2 μm fibre sources was investigated. A thulium-doped fibre laser operating at 1950 nm with >170 W of output power, a tuneable holmium-doped fibre laser producing >15 W over the wavelength span from 2040 nm – 2171 nm and a pulsed holmium-doped fibre amplifier with >100 kW peak power at 2090 nm are reported. Finally we review the requirements for efficient scaling of mid-infrared optical parametric oscillators and analyse the non-linear effects that arise when attempting to scale the peak power in silica fibres in the 2 μm spectral region. We implement a range of strategies to reduce the onset of nonlinear effects and demonstrate a holmium-doped fibre amplifier with peak power levels exceeding 36 kW in a 5 ns pulse with a spectral width of

Development of components and fibres for the power scaling of pulsed holmium-doped fibre sources

The mechanism involved during solution doping process has been systematically investigated by correlating the soot characteristics and solution parameters with the amount of rare earth (RE) incorporated in the core of optical fiber. Experiments show that the amount of RE incorporation may be controlled with better precision by adjusting Al ion concentration in the soaking solution. A model has been developed on the basis of cooperative adsorption mechanism correlating different parameters in the overall process. Theoretical estimation shows good agreement with the experimental results and can be used to predict the extent of RE incorporation for any composition if the soot layer characteristics are known.

P Ray

Papers

The mechanism of rare earth incorporation in solution doping process.