Wide range tunable photonic frequency synthesiser with low and non-scaling phase noise

Frequency synthesis is central to communications, sensing, navigation and spectroscopy, requiring low phase noise and wide frequency tunability from microwave to millimetre-wave bands. However, conventional electronic frequency synthesis suffers from phase noise that scales quadratically with output frequency and from intrinsically limited tuning bandwidth, leading to bulky, power-hungry architectures at high frequencies. Many photonic approaches achieve low phase noise with ultra-stable lasers and optical frequency combs, but typically lack wide tunability and impose size, weight and power constraints. Here we demonstrate a photonic frequency synthesiser based on an optical harmonic locking that overcomes these limitations by locking widely spaced narrow-linewidth lasers using only low-frequency, low-bandwidth electronics. The approach enables continuous, referenced frequency generation with Hz-level resolution from 0.375 to 170 GHz, while exhibiting non-scaling phase noise over offset frequencies from 200kHz to 10MHz. We record a phase noise of −146 dBrad2/Hz at 10MHz offset, and a fractional frequency instability of 3×10−13 at 1s, resulting in a reduction of timing jitter with increasing carrier frequency 11fs at 170GHz, outperforming state-of-the-art commercial signal generators. We further demonstrate 1024-QAM transmission at a 150GHz carrier, highlighting the potential of photonic synthesis for next-generation high-frequency communication and sensing systems. The architecture is compatible with integrated photonic implementations, promising potential chip-scale deployment.