A monochromator for isolating or recombining a narrowband optical signal from a broadband signal. A cylinder with a continuous outer wall and two planar end surfaces is composed of a dispersive medium. The cylinder forms a multi-reflective cavity, wherein a broadband optical signal entering the cavity through an end surface at an angle of incidence to the outer wall is repeatedly totally internally reflected off the outer wall. The outer wall includes exit points for permitting egress of spectral components. For recombination, narrowband optical signals are coupled into the body at different locations on the outer wall and propagated in substantially helical paths that converge at an end surface. The propagation path within the dispersive medium can thus be lengthened in a relatively compact space. The monochromator has low insertion loss, and does not require fine adjustment or particularly careful handling.