Spatially-efficient optical multiplexers and optical demultiplexers include elements interrelating along orthogonal axes. A transmission block of extreme thinness has highly reflective coatings on opposed parallel surfaces. Lasers of multiplexer are on one side of transmission block with transmission axes perpendicular to transmission block surface. An associated multiplexed signal transmitting port on opposite side of transmission block has receiving axis parallel to transmission block surface on that side. Detectors of demultiplexer are on one side of transmission block with reception axes perpendicular to transmission block surface. An associated multiplexed signal receiving port on opposite side of transmission block has receiving axis parallel to transmission block surface on that side. A unitary structure performs both optical multiplexer functions and optical demultiplexer function with a single thin transmission block. Related optical signal processing methods are included.