Collimator optical system for effecting elliptical-to-circular conversion

A collimator optical system for effecting elliptical-to-circular cross section is disclosed which comprises two lenses in a slab form whose refractive index distribution in one plane is such that the refractive index decreases gradually from the center outward in the direction of their thickness. The two lenses are spaced on the optical axis in such a way that the direction of refractive index distribution in one lens crossing the direction of refractive index distribution in the other lens at right angles.