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This paper reports on recent advances n the optical generation of continuous-wave THz signal by photomixing in ultra high-frequency 1.55μm travelling-wave photodetectors (TWPD). We discuss the fundamental physical effects determining the frequency dependent quantum efficiency and saturation output current of TWPD. An extended equivalent circuit for a TWPD is presented covering electrical wave propagation effects at frequencies above 1 THz. The power penalty in TWPD due to velocity mismatch is theoretically investigated. Experimentally, ultra-wideband (0.06-1 THz) optical heterodyne photomixing employing a 1.55μm TWPD coupled to a rectangular WR10 waveguide is presented as well as photonic emitter modules employing TWPDs integrated with planar antenna structures.