Laser Development
Example of a poor spatial profile.
Example of a fibre laser spatial profile.
Intense laser pulses with megawatt peak powers are desirable for a large number of applications including both laserwire and laser plasma wavefield acceleration. To achieve these higher energy pulses a solid-state laser system consisting of an oscillator followed by single or multi-pass amplifiers containing macroscopic crystals doped with a lasing ion are used. Fibre lasers however, use a fibre as a waveguide with the core doped with the lasing ion. Because of the waveguide nature of the fibre, a fibre laser can produce a near diffraction-limited output at high power. With the large surface area to volume ratio, very little if any active cooling is required and thermal effects that could disort the laser beam are avoided. Fibres typically have a diameter of tens of microns and therefore nonlinearities in the fibre can become evident at very high pulse energies.
To avoid this, laser pulses from an oscillator are stretched out temporally to lower the peak power and hence the degree of nonlinearities. The pulses can then be amplified to an even higher level in one or many amplifiers before being compressed again afterwards.
Fibre lasers can also be pumped by continuous-wave diode lasers which can be carried along the fibre in either the core or preferably the cladding. Diode lasers as well as having an electrical to light efficiency as high as 40% also have a narrow bandwidth that can be chosen to overlap well with the absorption of the lasing ion allowing a high efficiency of conversion of pump power to laser output. High slope efficiencies of 82% have been achieved. This high efficiency leads to less heat loading of the laser and lower running costs.
Recent results are presented in (1).
3m fibre laser amplifier wound on 10cm square spindle.