  Multi-plane light conversion (MPLC) has recently been developed as a
versatile tool for manipulating spatial distributions of the optical field
through repeated phase modulations. An MPLC Device consists of a series of
phase masks separated by free-space propagation. It can convert one orthogonal
set of beams into another orthogonal set through unitary transformation, which
is useful for a number of applications. In telecommunication, for example,
mode-division multiplexing (MDM) is a promising technology that will enable
continued scaling of capacity by employing spatial modes of a single fiber.
MPLC has shown great potential in MDM devices with ultra-wide bandwidth, low
insertion loss (IL), low mode-dependent loss (MDL), and low crosstalk. The
fundamentals of design, simulation, fabrication, and characterization of
practical MPLC mode (de)multiplexers will be discussed in this tutorial.
