Abstract
Mode instability with different mode excitation has been investigated by off-splicing the fusion point in a 4 kW-level monolithic fiber laser system, which reveals that the fiber systems exciting more high order mode content exhibits lower beam quality but higher mode instability threshold. The static-to-dynamic mode degradation and dynamic-only mode degradation have also been observed in the same high power fiber amplifier by varying the mode excitation, which implicates that the mode excitation plays an important role in mode characteristics in high power fiber lasers. By employing a seed with near fundamental mode beam quality, only dynamic mode degradation-mode instability sets in with negligible static beam quality degradation. Then the fusion point in the seed laser is offset spliced to excite high order mode. As the output power of the main amplifier scales, the beam quality degrades with the beam profile being static, and then the dynamic mode instability sets in, the power threshold of which is higher than that with good beam quality seed. We consider that the static mode degradation is caused by the presence of incoherent supposition of fundamental and high order mode, which leads to that the measured dynamic mode instability threshold is higher.
Highlights
Transverse mode instability (TMI), which results to that the output beam profiles become unstable, and exhibit temporal fluctuation in the range of a few kHz1,2, is currently a major limitation on the power scalability of large mode area single mode Yb-doped fiber amplifiers[3,4]
To the best of our knowledge, the static-to-dynamic mode coupling (first steady-state power transfering to high order mode (HOM) and dynamic power coupling between fundamental mode (FM) and HOM) and dynamic-only mode coupling in a same single-pass high power fiber amplifier by varying the mode excitation have been observed simultaneously for the first time, which is different from the type of the static and dynamic mode coupling reported in a double-pass rod-type fiber amplifier[26]
The mode degradation characteristics for the high beam quality system has been studied, and the beam quality factor M2 measured by 4-sigma method is employed to feature the beam quality of the system
Summary
The output power of seed laser is amplified to 100 W by three stage preamplifiers. The active fibers of the three preamplifiers are Nufern 10/125 Yb-doped fiber with pump absorption factor of 4.1 dB/m at 975 nm. To increase the TMI threshold, the counter pumping configuration has been employed in the main amplifier stage[10]. The main amplifier is made up of a counter (6 + 1) × 1 signal/pump combiner, a 18 m long Nufern 25/400 large mode area Yb-doped fiber with pump absorption factor of 0.6 dB/m at 915 nm, two cladding power strippers (CPSs) and an output quartz block holder (QBH). Wavelength-locked 915 nm laser diodes (LDs) are employed to pump the gain fiber and achieve higher TMI threshold[6], and the CPSs are utilized to remove unwanted cladding light.
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