This research work focuses on the design and characterisation of indium phosphide photonic integrated circuits for continuous-wave, high-purity millimetre-wave (mm-wave) and terahertz (THz) signal generation. Each source investigated in this work is based on a photonic oscillator (PO) consisting of monolithically integrated semiconductor lasers, and a broad bandwidth photodiode to convert the heterodyne signal from the optical to the electrical domain. The resulting photocurrent contains a component at the desired mm-wave or THz frequency corresponding to the frequency difference between the two lasers. In this thesis, the specifics of three POs are discussed, and dedicated laser phase-locking solutions are investigated and implemented, resulting in a high-purity mm-wave photonic synthesiser being realised and a novel THz PO being proposed and constructed. The former is a compact mm-wave photonic synthesiser consisting of two lasers monolithically integrated with fast photodiodes. High-quality, low-phase-noise signal above 100 GHz is demonstrated through optical injection locking, allowing the synthesised signal to be finely tuned across a 30 GHz span. Furthermore, the phase stabilisation scheme based on optical phase lock loop (OPLL) was constructed and discussed. The latter is a broadly tuneable THz signal generator based on a photonic integrated circuit developed using a generic fabrication foundry approach. The implementation of the photonic chip with twin OPLL enables the two optical lines to combine at the output and create a high-purity, continuously tuneable optical heterodyne signal, which can be data modulated. Furthermore, OPLL operation principles are investigated, leading to the establishment of design guidelines and a definition of the trade-offs present in OPLLs. The integrated POs discussed in this work could be an answer to the need for tuneable, portable, cost- and energy-efficient THz sources that can operate at room temperature. Photonic-enabled emitters have the potential to overcome the limitations of conventional emitters, thereby accelerating the development of coherent THz technology and its applications in spectroscopy, sensing, security and short-range broadband wireless communications.