Further power scaling of narrow-linewidth fiber lasers is critical for beam combining. Using allsolid photonic bandgap fibers with large effective mode area and strong higher-order-mode suppression is an interesting approach. Previously we demonstrated ~400 W single-frequency single-mode power at 1064 nm from a 50/400 photonic bandgap fiber amplifier, limited only by TMI. In this work, we demonstrate a TMIlimited single-mode power of 1.37 kW from a monolithic fiber amplifier with a 25/400 photonic bandgap fiber, the highest output power from a photonic bandgap fiber demonstrated to date. The spectral linewidth is broadened to ~8 GHz to suppress stimulated Brillouin scattering.
Using an ytterbium-doped fiber with a 50 μm core and 0.028 NA, a pulse energy of 4.8 mJ was achieved directly from a single-mode Q-switched fiber laser. The repetition rate was 10 kHz and the average power was 48.4 W. The slope efficiencies with regard to the absorbed and launched pump power were ~74% and ~59% respectively. The pulse width decreased with increasing pump power. The 4.8 mJ pulse had a FWHM width of ~300 ns. A shorter pulse of ~200 ns FWHM was also achieved at 2.06 mJ in another configuration. The M2 was below 1.3 at all pulse energies. This work demonstrates record pulse energy directly from a single-mode Q-switched fiber laser and the feasibility of operating such laser with high efficiencies.
Conventional models of Er/Yb co-doped fibers assume all ytterbium ions are equally involved in the energy transfer with erbium ions, governed by a singular transfer rate. This would predict output power clamping once ytterbium parasitic lasing starts, contrary to the observations that the output continued to grow albeit at a slower rate. One study explained this using elevated temperature at high powers. Our study, however, shows that elevated temperature and mode-dependent effects only play insignificant roles. A new model is developed based on the existence of isolated ytterbium ions, which can explain all the observed experimental behaviors.
We have demonstrated a new record of 302W single-mode power from an Er/Yb co-doped fiber master oscillator power amplifier (MOPA) with a record optical efficiency of 56%. This single-mode power is a new record for any lasers in this wavelength range. The previous record of single-mode power from an Er/Yb fiber laser pumped by a 9xx nm diode was 207 W at 1560 nm. The new optical efficiency of 56%, very close to the quantum-limited efficiency of 58.6%, is also a new record for Er/Yb fiber lasers. This new result is made possible mainly due to new fiber development from Nufern and off-resonant pumping of the Er/Yb fiber, which demonstrates further power-scaling potentials of Er/Yb fiber lasers pumped by widely available 9xx nm diodes. We also show that further power scaling is no longer limited by Yb3+ parasitic lasing near 1.06μm, but by fiber fuse in the Er/Yb fiber. The Yb3+ amplified spontaneous emission (ASE) was found to be negligible in all the cases we tested. Nonetheless, our numerical investigation shows that off-resonance pumping at 915 nm or 940 nm only plays a small role in the above-mentioned negligible ASE. We believe that the major cause may be the high Er3+ doping level in the Nufern Er/Yb co-doped fiber. Our results provide significant new insights and will stimulate further power scaling of Er/Yb fiber lasers and amplifiers.
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.
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