Detailed calculations have been made of the optical switching behaviour in twin-guide travelling-wave laser amplifiers driven with equal current in each guide. For typical device parameters, appropriate to 1.55 μm InGaAsP devices, the analysis indicates that the switching occurs at about one quarter of the saturated power, which in practice means at about 1 mW for an unsaturated gain of 26 dB. The factors influencing this switching power are discussed and some suggestions offered as to ways of reducing the value.