High-power fiber sources: fundamental properties

Abstract

Rare-earth (RE) doped fiber lasers form a compelling combination of the attractive laser-optical properties of rare-earth ions and optical fibers. Optically pumped rare-earth ions have many attractive laser properties such as a high efficiency (with in-band pumping it can exceed 80%) and a relative immunity to the environment. This means, for example, that even a potentially high temperature does not significantly change the spectroscopy. Rare-earth ions (when triply ionized) also have a relatively weak oscillator strength (typical value 10^-7 - 10^-5, because the normally dominant electrical dipole transitions are forbidden. As a result, a relatively high stored energy is required to reach a given level of gain. This is good for Q-switched lasers and other pulsed devices, in which the energy acts as a reservoir and counteracts distortions of the pulse shape. The downside is that a relatively large number of ions (or more accurately, concentration - length product) is required to reach a specific gain. Since concentration-quenching limits the concentration, the device has to be relatively long to reach a significant gain. In a bulk (non-waveguiding) gain medium this is impossible because of diffraction. With the development of low-loss rare-earth doped fibers, the diffraction could be counteracted so that tight beam confinement could be maintained over arbitrary lengths. Thus, gains of many tens of decibels became possible, even in glass hosts in which the emission lines are much broader, with lower peak emission cross-sections. In addition, the long length of fiber lasers (10 m is a typical value) simplifies heat-sinking and, with cladding-pumping, opens up for kW-level output power.
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ORCID for Johan Nilsson: orcid.org/0000-0003-1691-7959

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