A photonic integrated device having a substrate layer, epitaxial layers formed on said substrate layer and a guiding layer formed by one of the epitaxial layers. An optical waveguide is formed within the guiding layer. In the waveguide are a splitter (4) having at least two outputs, each output being transmitted to a Mach-Zehnder modulator (6, 8), the phase of the output of at least one modulator being shiftable, the signals being recombined to provide an optical phase shift key (PSK) output (14) wherein a trench is etched into the substrate layer between each pair of modulators to isolate the modulators from one another.