Multi-plane light conversion with low plane count

Multi-plane light conversion (MPLC) [1], [2] consists of a series of phase masks separated by a Fourier transform or free-space propagation and is illustrated conceptually in Fig. 1(a). Through this cascaded sequence of phase masks, and the diffraction which occurs between the masks, it is possible to implement arbitrary unitary spatial transformations. That is, any spatial input basis can be converted to any spatial output basis. A common example is to convert an array of Gaussian spots from a single-mode fibre (SMF) array at the input, to a set of fibre modes at the output, where each input Gaussian spot is mapped to an orthogonal mode of the fibre. Such a device can be used for mode-division multiplexing (MDM) in optical telecommunications as multiplexer [3] or as part of other network sub-systems [4], as well as other applications such as high-power beam combining [5]. In these applications, the specific mapping of input modes to output modes is often irrelevant. For MDM, mode coupling along the transmission fibre will quickly redistribute the input light amongst various modes regardless of how it was originally input to the fibre. For beam-combining, the property of interest is often the amount of power which can be concentrated in a given amount of space. In both instances, the primary requirement is the device be as close to lossless as possible, with both low insertion loss (IL) and low mode-dependent loss (MDL), such that no information (for MDM) or power (for beam-combining) is lost in the MPLC process.