Theoretical and numerical treatment of modal instability in high-power core and cladding-pumped Raman fiber amplifiers

Abstract

Raman fiber lasers have been proposed as potential candidates for scaling beyond the power limitations imposed on near diffraction-limited rare-earth doped fiber lasers. One limitation is the modal instability (MI) and we explore the physics of this phenomenon in Raman fiber amplifiers (RFAs). By utilizing the conservation of number of photons and conservation of energy in the absence of loss, the 3 × 3 governing system of nonlinear equations describing the pump and the signal modal content are decoupled and solved analytically for cladding-pumped RFAs. By comparing the extracted signal at MI threshold for the same step index-fiber, it is found that the MI threshold is independent of the length of the amplifier or whether the amplifier is co-pumped or counter-pumped; dictated by the integrated heat load along the length of fiber. We extend our treatment to gain-tailored RFAs and show that this approach is of limited utility in suppressing MI. Finally, we formulate the physics of MI in core-pumped RFAs where both pump and signal interferences participate in writing the time-dependent index of refraction grating.

Impact of Stimulated Raman Scattering on the Transverse Mode Instability Threshold

“…Indeed, commercial fibers have comparatively lower pump absorption due to their smaller mode field, so longer pieces of gain fiber are needed to ensure pump depletion, which might intensify the stimulated Raman scattering (SRS) effect. It has been demonstrated that Raman conversion could cause TMI in Raman fiber lasers [11]. In previous research [12], we found that as the length of the beam delivery fiber after the fiber amplifier increased, the TMI effect was enhanced along with the Raman content in the laser output.…”

“…4, shortening the length of the active and passive fibers could be effective to mitigate the SRS effect, but this might limit the application of fibers for lasers requiring delivery at long distances. It should be mentioned that, unlike the laser system presented in the paper, the TMI threshold is independent of the fiber length in Raman fiber lasers [11]. According to previous research [12], the counter-pumped scheme applied to fiber lasers could also suppress SRS and thus increase the TMI threshold.…”

Gaussian-Shaped Gain-Dopant Distributed Fiber for High Output Power Fiber Amplifier

“…The energy coupling between different modes on the time scale of millisecond will lead to a sudden degradation in beam quality [10][11][12][13][14]. The TMI threshold will be improved by reducing the core V value [15], increasing the inner cladding diameter [16], employing new fiber structures [17], and controlling the gain dopant distribution in fiber core [18][19][20], etc.…”

Towards the Enhancement of the TMI Threshold in Monolithic High-Power Fiber System by Controlling the Pump Distribution and the Seed Power

“…Then the system-based ones referred to shifting pump or signal wavelength [39], [47], changing the linewidth of the signal laser or pump laser [48], [49], controlling the excitation of mode content at injection end of the amplifier [22], adapting the cooling condition [20], [49], and so on. The recent research even disclosed that TMI was related to the occurrence of the nonlinear effects such as the stimulated Brillouin scattering (SBS) [50] and the stimulated Raman scattering (SRS) [51], [52], which provided new cognitions to suppress the TMI in terms of optimizing the system configuration to prevent the nonlinear effects. Furthermore, some latest in-depth research targeting the physical origin of the phase shift between the refractive index grating and the modes interference pattern has proven that the active control of the phase shift by the pump modulation was feasible to mitigate the TMI [30], [53].…”