Generation and Amplification of Third-Order Orbital Angular Momentum Modes in a Low-Loss All-Fiber System

We theoretically and experimentally proposed the generation method of third-order orbital angular momentum (OAM) modes with reduced losses by cascading two adiabatic mode selective couplers (MSCs), in which the LP
₁₁
mode produced as an excitation mode was utilized to generate a higher-order mode (LP
₃₁
). The minimum excess loss and insertion loss of cascaded MSCs were 3.7 dB and 5.3 dB, and had been greatly reduced compared to a single MSC. Then, the third-order OAM mode turned from LP
₃₁
mode was amplified in an all-fiber system, by which maximum mode gain after amplification can be achieved up to 23.2 dB. This generation and amplification system has promising applications in the long-distance mode division multiplexing technology.