Raman amplification of optical beam carrying orbital angular momentum in a multimode step-index fiber

We experimentally demonstrate 15 dB of Raman amplification of 1115 nm pulses in an orbital angular momentum mode (OAMM) with charge l 2, S 1 in 5 m of step-index 25 μm-diameter-core fiber. The total output reaches 4.5 kW of peak power and 68.5 μJ of energy in ∼15 ns pulses at 4 kHz repetition rate. An Yb-doped fiber source pumps the Raman amplifier at 1060 nm with 60 ns pulses. Using a spatial light modulator for modal decomposition, we measure 83% purity for the amplified target OAMM of selected polarization. To the best of our knowledge, this is the first time high energy, peak power, gain, and purity are achieved in a fiber Raman amplifier for a single OAMM.