A first signal is generated from an input signal by a first delay (70), and by low pass filtering (74). A second signal is generated from the input signal and from a second, longer delayed (72), version of the input signal, such that in response to a pulse on the input signal, the second signal has a sequence of two pulses, coinciding respectively with leading and trailing edges of a corresponding pulse on the first signal. If the signals are electrical, they can drive I and Q inputs of an IQ modulator (84, 86). If generated optically, they can be combined directly to produce the encoded optical output signal. By using such delays and filtering to produce these signals, a CAPS-3 encoded optical signal can be simulated, to obtain its chromatic dispersion tolerance advantages with less complex hardware and less power consumption.