Abstract: In a digital optical switch which is based on the mode-selectivity principle, made up of a passive section (A) containing a first optical wave guide (1) and a second optical wave guide (2) and a switching section (B) containing a third optical wave guide (3) and a fourth optical wave guide (4), both the required asymmetry and the bringing of the optical wave guides within or outside each other's interaction region are achieved with the aid of curves (K.sub.1, K.sub.2) in respective walls of the optical wave guides. In the switching section (B) said asymmetry is reversible. Use is made of the effect that a curved wall results, in general, in a reduction of the propagation constant in the optical wave guide. Suitable curves are of circular-arc shape and sinusoidal. The switching section (B) may furthermore incorporate a short intermediate section (s-u) in which the optical wave guides (12, 13) run straight at a small mutual angle (.PHI.