Record-low quantum defect efficient high-power diffraction-limited fiber laser

Quantum defect heating in high-power fiber lasers can lead to thermally induced nonlinear instabilities. One example is transverse mode instability which sets a fundamental limit on power from diffraction-limited fiber lasers. High-power diffraction-limited Yb³⁺-doped fiber lasers have some of the lowest quantum defect (~5%.) and the highest powers among all solid-state lasers to-date. Further lowering the quantum defect through choice of pump and laser wavelengths is severely limited by amplified spontaneous emission (ASE) and associated degradation of efficiency. In this work, we demonstrate an approach to potentially lower the quantum defect to ~1% without significant compromise of efficiency. In a first demonstration, diffraction-limited ~154W at ~993nm with negligible ASE, pumped at ~976nm, was achieved with a slope efficiency of ~75% versus the launched pump power. The laser quantum defect is a record low of 1.7% for high-power (> 100W) solid-state lasers to the best of our knowledge. The output is only limited by the available pump power, providing a viable path for significant further power scaling of diffraction-limited power from a single laser! This work also significantly extends the low wavelength limits of high-power ytterbium fiber lasers and thereby enabling many new applications. The key to this approach is an Yb³⁺-doped double-clad all-solid photonic bandgap fiber which was engineered to support robust single-mode operation at large core sizes and to provide a strong ASE suppression at the longer wavelength.