Ultra-Low Quantum-Defect Heating in Ytterbium-Doped Aluminosilicate Fibers

We theoretically investigate the quantum defect between pump and signal photons in ytterbium-doped fiber lasers and amplifiers, and find that this can be as low as 0.6%. We find that the lowest quantum defects can be achieved with a low area ratio between the pump and signal waveguide of a double-clad fiber, and with high-brightness pumping in the core being an ultimate approach. The change in achievable quantum defect is small over a relatively large range of pump wavelengths, but it is still necessary to optimize the wavelengths and match the fiber length to reach the smallest quantum defect.