Directional couplers, filters, modulators, switches, sources, etc., are being built either in ferroelectrics or in semiconductors. These devices make more likely the incorporation of integrated-optics technology into the rapidly growing field of optical communication.
E. A. J. Marcatili,
"Integrated Optics Research At Bell Laboratories", Proc. SPIE 0139, Guided Wave Optical Systems and Devices I, (13 July 1978); doi: 10.1117/12.956252; https://doi.org/10.1117/12.956252