Scalable Hermite–Gaussian mode-demultiplexing hybrids

We propose a Hermite-Gaussian (HG) mode-demulti-plexing hybrid (MDH) for coherent detection of mode-division multiplexed signals. The MDH, which performs multiple functionalities, including demultiplexing, local oscillator splitting, and optical 90-deg mixing, is realized based on the multi-plane light conversion technique. An isosceles right triangle output layout is employed to reduce the number of phase masks to fewer than the number of modes, significantly simplifying the construction of the MDH. A 10-Hermite-Gaussian (HG) mode MDH with only five phase masks is demonstrated by numerical simulation, achieving an insertion loss (IL) and mode dependent loss as low as ${-}{2.3}$-2.3 and 1.7 dB, respectively. The IL was further reduced to ${-}{1.5}\;{\rm dB}$-1.5dB through optimization of MDH parameters, such as the beam waists of the input HG modes and the output spots.