Experimental study of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers

Abstract

The experimental investigation of mode distortion induced by stimulated Raman scattering (SRS) in a high-power fiber amplifier, which includes the evolutions of optical spectra, spatial beam profiles, and time-frequency characteristics, has been carried out in detail. Temporal-frequency characteristics have been studied for the first time, to the best of our knowledge, by using a low-speed camera and high-speed photodiode traces, which revealed that temporal-frequency characteristics of SRS-induced mode distortion are different from traditional dynamic mode instability (MI). The experimental results show that the output beam profile remains stable before the mode distortion occurs and fluctuates obviously after the onset of SRS-induced MI but on a time scale of seconds, which is much lower than that of Yb-gain-induced MI featuring millisecond-level beam profile fluctuation. It also shows that the mode distortion became measurable in company with the onset of inter-mode four-wave mixing (IM-FWM) when the ratio of Raman light reaches 3%; further, the beam quality factor M 2 degrades gradually from 1.4 to 2.1 as the ratio of Raman light increases. The mode distortion is accompanied by an obvious temperature increase of the output passive fiber, which further confirms that the mode distortion originates from SRS. The cause of the mode distortion induced by SRS has been explained in the context of core-pumped SRS effect, and the investigation on the accompanying IM-FWM effect indicates that the main content of the SRS-induced high-order mode is the LP21 mode.

Structural origin of thermally induced refractive index changes in Yb3+/Al3+/P5+/F−‐co‐doped silica glass

“…Yb 3+ ‐doped silica fiber has been an ideal gain medium in high‐energy and high‐power fiber laser applications 1, 2 . The extreme increase in power density in the core leads to a strong nonlinear effect and even damage the fiber end facets in the conventional double‐clad fibers 3, 4 .…”

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Yb 3+ -doped silica fiber has been an ideal gain medium in high-energy and high-power fiber laser applications. 1,2 The extreme increase in power density in the core leads to a strong nonlinear effect and even damage the fiber end facets in the conventional double-clad fibers. 3,4 To avoid these harmful effects, large-mode-area (LMA) designs have been performed owing to their large core diameter and effective mode area.

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SRS-Induced Spatial-Spectral Distortion and Its Mitigation Strategy in High-Power Fiber Amplifiers

“…It is well known that the SRS effect results in the transfer of the signal power to Stokes light and the broadening of the linewidth asymmetrically [2]. Recently, a new phenomenon, named SRS-induced mode distortion, has also been observed in high power fiber lasers [3][4][5], which renders the single mode beam into a multi-mode operation and deteriorates the spatial brightness severely. The phenomenon was first observed by Hejaz et al [3], and Chu et al [4] studied the corresponding temporal-frequency characteristics, which revealed that SRS-induced mode distortion fluctuates on a time scale of seconds.…”

Effective suppression of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers

“…The key characteristic of SRS-induced mode distortion was reported as follows: when SRS emerges, the SRS-induced mode distortion appears immediately, and when the SRS effect is suppressed, the SRS-induced mode distortion disappears immediately [31,34] . In addition, the SRS-induced mode distortion could cause time-domain fluctuations in the output power, and the fluctuation frequency is just several hertz [35] .…”