We present and experimentally demonstrate a traveling wave photodetector adapted to the detection of optically carried microwave signals. The synchronous propagation of the detected modulated laser beam with the photogenerated microwave signal is obtained with a diffraction grating. This concept provides a large detection volume which could be suitable for the optical generation of high power/large dynamic range microwave signals. The operating principle of the device is detailed and experimentally demonstrated, over a 0–5 GHz bandwidth, using a coplanar metalsemiconductor-metal waveguide on a GaAs molecular beam epitaxially grown layer. ©1998 American Institute of Physics.