Universal linear components are provided. In general, a P input and Q output wave combiner is connected to a Q input and R output wave mode synthesizer via Q amplitude and/or phase modulators. The wave combiner and wave mode synthesizer are both linear, reciprocal and lossless. The wave combiner and wave mode synthesizer can be implemented using waveguide technology. This device can provide any desired linear transformation of spatial modes between its inputs and its outputs. This capability can be generalized to any linear transformation by using representation converters to convert other quantities to spatial mode patterns. The wave combiner and wave mode synthesizer are also useful separately, and can enable applications including self-adjusting mode coupling, optimal multi-mode communication, and add-drop capability in a multi-mode system. Control of the wave combiner and wave mode synthesizer can be implemented with single-variable optimizations.