A multilevel light intensity modulation system can generate a multilevel modulated optical signal from a single light source without handling multilevel electrical signals. The light from a light source 1 is branched by a Y-branch 40 into two light beams, which are phase-modulated by phase modulation sections 41, 42 with two sequences each of a bi-level electrical signal. The phase-modulated light beams are coupled by a directive coupler 43 to generate intensity-modulated output light beam. Phase modulation sections 41, 42 provide a phase difference between the branched light beams approximately of 0, (3/8)À, (5/8)À and À depending on level combinations of the two sequences each composed of a bi-level electrical signal. This enables multilevel light modulation in the light domain.
Apparatus and method for light intensity modulation and optical transmission system employing the same