An active optical-waveguide branch which has the maximum possible switching efficiency and minimum length. The waveguide includes a single main waveguide branched into two element arms or paths such that optical radiation passing through the main waveguide will be normally divided equally into each branched arm. Electrodes are formed between the branching arms and on the outer side of each arm. The electrodes are connected electrically so that the potential between the center electrode and each of the outer electrodes may be either negative or positive with each outer electrode having the same potential with respect to the center electrode. The optical radiation will choose one arm or the other depending on the polarity of the potential applied. The electrode separation is preferably a decreasing function of the length, z, of the electrode along the length of the arms. That is, for a constant voltage on the electrodes, the mode synchronism is a function of the length, z.