Directional coupler modulator with high linearity

Analog optical fiber communication requires linear modulators with low third order intermodulation and harmonic distortions in order to realize sufficient dynamic range. We report here a novel design of the directional coupler modulator which has used passive serial feedforward mechanism to effectively compensate nonlinear distortions. Low frequency testing has demonstrated that, for an optical modulation depth of 30% per channel, the third order intermodulation distortion is more than 30 dB lower than that of the conventional directional coupler or Mach-Zehnder modulators. Harmonic distortions are reduced substantially at the same time. Operation at rf and microwave frequencies is limited by the transit time effect.