Active optical fibre

A laser system, including:

• a pump laser or pump laser array;
• an active optical fibre coupled to the pump laser or pump laser array, the active optical fibre including:
• a core comprised of at least a first material with at least a first refractive index;
• an inner cladding comprised of at least a second material with at least a second refractive index substantially surrounding the core, whereby the core and the inner cladding form an area configured to propagate pump radiation from the pump laser or pump laser array when in use;
• an outer cladding comprised of at least a third material with at least a third refractive index substantially surrounding the inner cladding, the third refractive index being smaller than the second refractive index, whereby the outer cladding confines pump radiation from the pump laser or pump laser array to the core and the inner cladding;
• and
• a coating comprised of a thermally conductive metal, graphite or thermally conductive other material substantially surrounding the outer cladding,
• wherein the inner cladding has a symmetry breaking shape configured to reduce impact of spatial hole-burning on absorption of the pump radiation as the pump radiation propagates through a length of the active optical fibre as compared to a circular cladding,
• wherein the thermally conductive metal, graphite, or thermally conductive other material of the coating supports a reduced temperature increase between the area and an outer surface of the outer cladding via the coating and the outer cladding and supports an increased operating temperature of the area configured to propagate pump radiation, wherein thickness of the coating is between 0.1-30 μm, and wherein the coating supports a minimised size, weight, and cooling of the active optical fibre so as to minimise size and weight of the laser system.