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

Abstract

Mode distortion induced by stimulated Raman scattering (SRS) has become a new obstacle for the further development of high-power fiber lasers with high beam quality. Here, an approach for effective suppression of the SRS-induced mode distortion in high-power fiber amplifiers has been demonstrated experimentally by adjusting the seed power (output power of seed source) and forward feedback coefficient of the rear port in the seed source. It is shown that the threshold power of the SRS-induced mode distortion can be increased significantly by reducing the seed power or the forward feedback coefficient. Moreover, it has also been found that the threshold power is extremely sensitive to the forward feedback power value from the rear port. The influence of the seed power on the threshold power can be attributed to the fact that the seed power plays an important role in the effective length of the gain fiber in the amplifier. The influence of the forward feedback coefficient on the threshold power can be attributed to the enhanced SRS configuration because the end surface of the rear port together with the fiber in the amplifier constitutes a half-opening cavity. This suppression approach will be very helpful to further develop the high-power fiber amplifiers with high beam quality.

3.8 kW high brightness MOPA fiber laser

“…In order to improve the beam quality of the seed, the coiling diameter of YDF is controlled within 10 cm. The reason why the 6x1 forward pump combiner utilized in the seed has no signal input port is to avoid self-excited oscillation in the seed fiber oscillator to depress the SRS-induced mode distortion [9]. There are a pair of high-power FBGs (fiber Bragg gratings) engraved on the 20/400-μm double-clad passive fiber matched with the active fibers.…”

Wavelength selective bending sensitive multi-ring fiber for suppressing stimulated Raman scattering

“…For example, long-period gratings can be utilized to attenuate the scattered beam, 10 and a large-mode-area fiber or short fiber can reduce the signal light intensity to weaken the SRS. [11][12][13][14] Meanwhile, fiber designs have been considered to suppress SRS, such as the resonant ring filter fibers. In 2005, 15 Zenteno et al first proposed a dual-hole-assisted fiber design, which was effectively immune to SRS and suppressed the Raman gain by more than two orders of magnitude compared with that using the single-mode fibers.…”

“…Various techniques to suppress SRS have been developed. For example, long-period gratings can be utilized to attenuate the scattered beam, 10 and a large-mode-area fiber or short fiber can reduce the signal light intensity to weaken the SRS 11 – 14 Meanwhile, fiber designs have been considered to suppress SRS, such as the resonant ring filter fibers.…”

Fabrication of a Counter-Directional Pump/Signal Combiner with Built-In Mode Field Adapter

“…TMI and nonlinear effects are the bottlenecks for the further improvement of fiber laser power [16,17]. With regard to the fiber, the nonlinearity suppression (preferring large core size) and TMI mitigation (preferring relatively small core size) could hardly be simultaneously optimized, owing to the contradicted core size requirements [18].…”