We report the first demonstration of Raman amplification in a fiber of a single Bessel-like higher order mode using a multimode pump source. We amplify the LP-mode with a 559-µm effective mode area at a signal wavelength of 1115 nm in a pure-silica-core step-index fiber. A maximum of 18 dB average power gain is achieved in a 9-m long gain fiber, with output pulse energy of 115 µJ. The Raman pump source comprises a pulsed 1060 nm ytterbium-doped fiber amplifier with V-value ~30, which is matched to the Raman gain fiber. The pump depletion as averaged over the signal pulses reaches 36.7%. The conversion of power from the multimode pump into the signal mode demonstrates the potential for efficient brightness enhancement with low amplification-induced signal mode purity degradation.

We demonstrate a fiber Raman laser directly pumped by two spectrally combined multimode diode lasers at ~950 nm and ~976 nm. The emission wavelength becomes 1020 nm, which corresponds to the 1 st Stokes of 976 nm. The pump separation of 270 cm-1 is both much smaller than the Raman peak shift of 440 cm-1 and much larger than the Raman linewidth of around 100 cm-1 , which is unfavorable for efficient Raman conversion. Nevertheless, the addition of the pump at 950 nm triples the output power compared to pumping only at 976 nm. We reach 23 W of output power with slope efficiency of 51% and beam quality M 2 of 5.2 in a germanosilicate graded-index fiber.

We numerically simulate and optimize a high-power fiber Raman amplifier cladding pumped by spectrally combined diode lasers at wavelengths from ∼0.9 to ∼1 μm in the continuous-wave regime. This amplified a signal at the first-Stokes wavelength of 1024 nm. We found that it was possible to add pumps over an increasingly wide wavelength span up to ∼90 nm, while still maintaining an incremental conversion efficiency higher than 60%, even though the Raman linewidth is only ∼15 nm. We investigated the dependence on the power of individual diode lasers and on the wavelength spacing and found that the total conversion efficiency reaches ∼70% with realistic pump sources based on state-of-the-art diode lasers. We believe this study shows the potential for high-power fiber Raman lasers pumped by spectrally combined multiwavelength diode and fiber laser sources.

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