Described herein are embodiments of a diffractive optical element (23) such as a grism. In one embodiment, the diffractive optical element (23) includes an input surface (31) configured to receive an input optical signal (29), a diffractive surface (33) adapted to spatially disperse the input optical beam (29) into a dispersed signal and an output surface (35) configured to output the dispersed signal from the diffractive optical element. The input surface (31) and the diffractive surface (33) are non-parallel and the diffractive surface (33) is formed in situ by a photolithographic technique.