Frequently Asked Questions
Fibre lasers are part of the family of solid state lasers. (What is a solid state laser?). These lasers have been used for materials processing only recently, although fibre lasers (more strictly fibre amplifiers) have been used in the telecommunications field for several years. In the fibre laser, the lasing medium is an optical fibre doped with low levels of a rare earth element, such as yttrium, erbium or thulium, which determine the wavelength of the output light. Diode lasers are used to couple infrared light into the cladding of the doped fibre and act as the pump source. This pumping action excites the dopant atoms, stimulating them to emit photons at a specific wavelength. Diffraction gratings are used as the rear mirror and output coupler, which form the resonator. It is possible to use the doped fibre, acting as the laser resonator, also as the beam delivery fibre, using appropriate beam shaping and focusing optics at its end. Such an approach will maximise the useful beam quality of the laser. However, for materials processing at high powers, it is usual to de-couple the beam delivery fibre from the laser, in case of damage.
To maintain high beam quality, the way the diode laser pumping source is coupled to the fibre is critical, especially at high powers. The easiest method is to pump the fibre through one end. However, due to constrains in diode laser design, the amount of power available for pumping using this method is limited. The most usual method employed is the so-called cladding-pumping technology. Cladding-pumping enables the use of high-power, low-cost, laser-diodes to pump the fibres. A cladding-pumped fibre has an inner core doped with the chosen rare earth element. The inner core is surrounded by an inner cladding with a lower refractive index, so that the inner core can propagate the output laser beam. The inner cladding is surrounded by an even lower-index material, the outer cladding. In this way, the inner cladding becomes a waveguide for the pump light. Because the inner cladding is relatively large, it is highly multi-mode and it is possible to launch large amounts of pump power into it, from the multi-mode diode pump sources. As the pump light propagates down the fibre, it is absorbed by the rare earth ions in the inner core of the fibre, stimulating lasing action. Because the interaction between the pump beam and the rare earth doped inner core is relatively weak, the geometry of the inner cladding is quite important. In a fibre with a circular inner cladding and a centred inner core, a large fraction of the pump light never crosses to the doped core. As a result, other geometries are often used, for example, with an off-center core, a rectangular inner cladding, or a flower-shaped inner cladding. This approach generally increases the beam quality and the energy efficiency of the laser, but also adds to the complexity of the design and therefore the cost.
The long, thin, fibre geometry is ideal to minimise thermal effects due to the pump light, and the inherently high gain of the fibre system means that high efficiencies are possible (25% is claimed for high power commercial systems)[GV1]. As the fibres can be coiled up, the footprint of the laser is significantly reduced when compared to other solid state or CO2 lasers. This combination of inherent ruggedness, small size, scalability and efficiency, makes the fibre laser an attractive choice for materials processing applications.
Single-mode fibre lasers operating as individual units have delivered around 1kW of power in laboratory conditions. Commercial units of 200-300W are available. Multi-mode fibre lasers are available which combine together many single mode lasers of low power. This approach is interesting for materials processing in that it would appear that the fibre laser is very capable of being easily scalable, whilst maintaining a reasonable beam quality. At an industrially relevant power of 4kW, for example, the beam quality of a fibre laser will be better than that of an Nd:YAG lamp pumped laser, and comparable to that of a diode pumped Nd:YAG laser or even an Yb:YAG disk laser.