PhD defence of Weixuan Lin – Novel components and strategies to reduce nonlinear impairments of high-power fiber lasers
High-power fiber lasers (HPFL) based on Ytterbium-doped fibers are widespread in metal processing for cutting, drilling, and welding. However, the power scaling of HPFLs is limited by factors of stimulated Raman scattering (SRS) and transverse mode instability (TMI). This thesis has investigated novel components and strategies mitigate these detrimental effects.
First, we demonstrate an HPFL with chirped and tilted fiber Bragg grating (CTFBG) inserted inside the gain medium for an improved SRS threshold of 1.8 kW. In comparison, the insertion of a CTFBG after the gain medium only increases SRS threshold of the HPFL from 1.54 kW to 1.64 kW. Furthermore, a broadened SRS-rejection band is achieved by the design of a cascade CTFBG. The intra-gain medium insertion of the cascade CTFBG further enhances the SRS threshold to 2 kW.
Second, we demonstrate an experimental optimization on master-oscillator power-amplifier lasers with an SRS filter inserted in between the X m long oscillator and the (40-X) m long amplifier stage, where X takes values of 10, 20, 30 and 40 m. Results show that the insertion of an SRS filter when X=30 m leads to the largest output power of 2 kW. When an SRS filter is inserted after the oscillator with X≤30 m, SRS threshold is increased but at the cost of reducing the power conversion efficiency (PCE). The insertion of an SRS filter when X>30 m results in a significant decrease in PCE that constrains the power scaling.
Third, we present the first pump recycler for kW HPFL. The pump recycler is a pump-and-signal (6+1)×1 tapered fiber bundle with pump fibers being spliced in pairs. 78% of the cladding pump light entering the pump recycler can be recirculated back to the cladding. Compared to a 40 m long HPFL that has PCE=73% and SRS threshold=1.54 kW, the pump-recycled 20 m HPFL has PCE =70.5% and SRS threshold of 2.78 kW, showing an 80% improvement in SRS threshold.
Last, we demonstrate a forward-pump HPFL with record high power of 3 kW. SRS and TMI are simultaneously suppressed thanks to the combination of the 78% pump recycler and the pumping wavelength at 915 nm. Besides, the gain fiber lengths of the oscillator and amplifier are experimentally redistributed. It shows a 19% increase in the TMI threshold by shortening the length of the oscillator from 20 m to 5 m.