Static and dynamic mode coupling in a double-pass rod-type fiber amplifier

High gain in a dual-pass rod-type fiber amplifier

“…Input and output signals can then be separated by a polarizing beam splitter (PBS). In previous work [17], we found that the onset of SMD was characterized by an unexpected polarization change, with part of the optical power emerging from the double-pass amplifier in the polarization state rejected by the final PBS. While it is known that a linear polarization input to the LP 11 modes of a fiber will rotate due to the vectorial 0740-3224/20/020451-08 Journal © 2020 Optical Society of America Research Article nature of the true eigenmodes, this polarization should also in principle be undone by the Faraday rotation.…”

“…The differences in this setup compared to that used in [17] are as follows. A telescope is used to protect the seed.…”

“…In this case, the output power continues to increase into the unstable regime. In the particular case of dual-pass, the output power peaks and then decreases with pump power along with the gradual increase in SMD, and dynamic TMI has been found to set in as pump power is increased beyond the signal power maximum [17].…”

“…The actual limitation for scaling of average power in a single-pass amplification module, despite many studies on comprehension and mitigation, remains the so-called transverse mode instability (TMI) [3][4][5][6][7][8][9][10][11][12][13][14], a low-frequency dynamic instability phenomenon. In contrast, only a few studies on the doublepass configuration at high average-power levels have appeared [15][16][17][18]. According to these publications, two instability phenomena are found to occur in dual-pass rod-type fiber amplifiers: static mode degradation (SMD) and dynamical TMI.…”

“…Because of these different origins, TMI is dynamic (0-5 kHz), while SMD is static. A more precise description of SMD can be found in recent theoretical [15] and experimental publications [17].…”