Systems, devices, and methods for additive manufacturing as disclosed allow for improved optical access to a build platform. In at least some embodiments a multiplexing optic of an additive manufacturing device is configured to multiplex an arbitrary number of optical paths to a build platform along a substantially common optical axis by dividing a theoretical input aperture of the multiplexing optic into a plurality of sub-apertures. Each sub-aperture can independently receive and direct an optical path to the build platform. An optical path can be a light path from a light source or an optical process monitoring path from an optical process monitoring system or optical process monitoring device. In some embodiments, an optical path can enter the multiplexing optic off-axis and/or off-angle with respect to an optical axis of the multiplexing optic. The multiplexing optic can include one or more lens elements and/or one or more mirror elements.