The invention concerns an optical system. The optical system comprises an input for receiving an optical signal, a predetermined output plane, and a diffraction grating for separating the optical signal received at the input into spectral elements thereof. The grating has a diffraction surface, which is formed by a photolithography process. The diffraction surface has a first predetermined profile. The first profile is formed by a plurality of points each conducted by different equations. Consequently, each spectral component is focused on the predetermined output plane.