An optical multiplexing device spatially disburses collimated light from a fiber optic waveguide into individual wavelength bands, or multiplexes such individual wavelength bands to a common fiber optic waveguide or other destination. The optical multiplexing device has application for dense channel wavelength division multiplexing (WDM) systems for fiber optic telecommunications, as well as compact optical instrument design. Multiple wavelength light traveling in a fiber optic waveguide is separated into multiple narrow spectral bands directed to individual fiber optic carriers or detectors. An optical block has an optical port for passing the aforesaid multiple wavelength collimated light, and multiple ports arrayed in spaced relation to each other along a multiport surface of the optical block. A continuous, variable thickness, multicavity interference filter (22) extends on the multiport surface (20) of the optical block over the aforesaid multiple ports. At each of the multiple ports the continuous interference filter transmits a different sub-range of the multiple wavelength collimated light passed by the optical port, and reflects other wavelengths. Multicolor light passed to the optical block from the optical port is directed to a first one of the multiple ports on an opposite surface of the optical block. The wavelength sub-range which is 'in-band' of such first one of the multiple ports is transmitted through that port by the local portion of the continuous, variable thickness interference filter (22) there, and all other wavelengths are reflected. The light not transmitted through the first port (16) is reflected to strike a second port, at which a second (different) wavelength band is transmitted and all other light again reflected. The reflected optical signals thus cascades in a 'multiple-bounce' sequence down the optical block (10) of the multiplexing device, sequentially removing each channel of the multiplexed signal. In reverse operation, individual channels are combined in the optical block and transmitted through the optical port.