High-order mode waveguide amplifier with high mode extinction ratio written in an Er³⁺-doped phosphate glass

Inscription of fiber-compatible active waveguides in high-gain glass, followed by direct interconnection with few-mode fibers, is one of the most promising solutions for all-optical mode-division multiplexing. In this work, based on the femtosecond laser writing technique, we propose a general fabrication scheme for inscribing high-order mode waveguides in glass, by carefully tailoring the cross-section of the waveguides to match the mode intensity distribution via an improved multi-scan approach. Specifically, we design and fabricate two kinds of waveguides, namely, LP₀₁-mode waveguide and LP₁₁-mode waveguide in a highly Er³⁺-doped phosphate glass, enabling the insertion loss of the waveguides to be as low as 1.88 dB, and the mode extraction factor of the LP₁₁-mode waveguide up to ∼24 dB. Importantly, we have successfully achieved optical amplification of the waveguides, with an on-off gain as high as 3.52 dB. This novel high-order mode waveguide amplifier has broad application prospects in monolithic on-chip integrated photonic light sources and optical interconnection with few-mode fiber and/or silicon-based waveguide.