A first transmissive diffraction grating includes a multiplexed transmission region; a second diffraction grating includes multiple demultiplexed transmission regions that are spatially displaced from one another and characterized by average corresponding grating-normal vector direction, grating wavevector magnitude, and grating wavevector direction. The demultiplexed transmission regions differ with respect to at least one of those parameters. The gratings are arranged such that each one of multiple optical signals at corresponding different wavelengths co-propagating to the multiplexed transmission region along a multiplexed beam path would: (i) be transmissively, dispersively diffracted by the first diffractive optical element; (ii) propagate directly, without any intervening reflection, between the multiplexed transmission region and a corresponding one of the demultiplexed transmission regions; (iii) be transmissively, dispersively diffracted by the second diffractive optical element; and (iv) propagate from the corresponding demultiplexed transmission region along a corresponding one of multiple demultiplexed beam paths.