A multimode variable directional coupler has been fabricated by piling up films. It uses an electrostrictive element. The output powers can be continuously controlled over a wide range. The output powers from the two output ports of the coupler were equalized at a stress of 16.8 kg/cm2, and the loss due to the stress was small (0.19 dB).

The propagation constant, the effective index, the effective guide thickness and the refractive index has been measured by using the method of a variable spacing of a high-index film and the waveguide surface. The spacing can be varied by the electrostrictive element.

A new fabrication method of a star coupler has been described. It is fabricated by stacking polymer films and then sticking them by a heat press. Using this method, the star coupler can be produced very easily and has square core cross sections. The 5-port multimode star coupler made on an experimental basis has an output power variation of 0.9 dB and an insertion loss of 3 dB.