Diffraction-limited 976nm lasers can be used to core-pump ultrafast fiber lasers to overcome nonlinearities with significantly shortened fiber lengths or to pump ultrafast solid-state lasers for much improved beam quality. In order to make Yb fiber lasers operate efficiently as a three-level system at ~976nm, it is critical to use double-clad fibers with large core-to-cladding ratio and additional spectral filters, such as dichroic mirrors in free space or fiber Bragg gratings in all-fiber configurations, to suppress lasing at longer wavelengths. Diffraction-limited 94W at 976nm was achieved in 2008 with an efficiency of ~50% with respect to the launched pump powers at ~915nm using a rod-type PCF and multiple dichroic mirrors. However, the results from flexible fibers with the potential to be used for monolithic fiber lasers are far worse. In this work, the Yb-doped double-clad all-solid photonic bandgap fiber has a core diameter of ~25μm and a cladding diameter of ~125μm. The photonic bandgap was engineered to have its long wavelength band edge just beyond 976nm to suppress lasing at longer wavelengths. We demonstrate a record efficiency of ~54% with regard to the coupled pump power at ~915nm. Pump-limited 38W at ~977nm was achieved with a M2 of ~1.24. ASE at ~1026nm was suppressed by <30dB at all powers. This is possible due to the use of all-solid photonic bandgap fibers which provide both the necessary large core-to-cladding ratio and the additional suppression of the four-level system by strong out-of-band transmission loss.
Turghun Matniyaz, Monica T. Kalichevsky-Dong, Thomas W. Hawkins, Joshua Parsons, Guancheng Gu, Wensong Li, Max Faykus, Bradley Selee, Jonathan A. Dong, and Liang Dong, "Diffraction-limited Yb-doped double-clad all-solid photonic bandgap fiber laser at 976 nm," Proc. SPIE 10897, Fiber Lasers XVI: Technology and Systems, 108970V (Presented at SPIE LASE: February 05, 2019; Published: 7 March 2019); https://doi.org/10.1117/12.2505798.
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