A wavelength separating element is provided for separating a converted beam from a fundamental beam in an NLFC device, wherein the converted beam has a wavelength different from a wavelength of the fundamental beam. The wavelength separating element includes a first mirror surface and a second mirror surface opposite to the first mirror surface. The first and second mirror surfaces may have a high reflectivity of the converted beam relative to a reflectivity of the fundamental beam, and the first and second mirror surfaces are configured such that the fundamental and converted beams undergo multiple reflections between the first mirror surface and the second mirror surface to separate the converted beam from the fundamental beam. The fundamental and converted beams undergo at least three reflections at the first and second mirror surfaces, and/or undergo at least two reflections at one of the first mirror surface or the second mirror surface.