An optical transmitter for an optical communication system includes a light source that outputs optical signals having a plurality of wavelengths, and a wavelength control unit. The wavelength control unit receives an optical signal from the light source, resonates an optical signal having a first wavelength, modulates the optical signal of the first wavelength with a first transmission data signal to obtain an intensity modulated optical signal, and outputs the intensity modulated optical signal. The wavelength control unit may be integrally formed on a semiconductor substrate in which a high thermal conductivity material is used. Alternatively, a trench that intercepts external heat may be formed in a boundary surface of the wavelength control unit, and may be filled with a low thermal conductivity material.