This paper presents the results from the first monolithically integrated photonic heterodyne system that allows the two optical sources to be mutually phase locked by locking to an external optical reference. High-spectral-purity signals of up to 50 GHz have been demonstrated from this first fabricated device, where the tuning range was limited by losses in the input waveguide. Successful phase locking was accomplished through short signal propagation delay of less than 2 ns achieved by monolithic integration and custom-made fast loop electronics. The approach can be extended to generate signals at >1 THz.
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