Image formation by phase coincidences in optical waveguides

A thick dielectric slab of uniform refractive index, acting as a multimode optical waveguide, can produce real optical images provided its length L is chosen so that the phase changes suffered along L by the various propagating modes differ by integer multiples of φ or of 2φ. For slab of thickness W, refractive index n, and light of wavelength λ, this condition is satisfied if L ? 4hnW²/λ, where h = 1, 2, 3, …. The formation of the images is described theoretically and demonstrated experimentally.