Flexible Generation of Optically-Carried Sub-THz Beat Notes Stabilized Through an Integrated SiN Frequency Locker

A promising technique to generate on-chip THz signals relies on the synthesis of a tone out of heterodyne interference of two frequency-spaced semiconductor lasers. Still the thermal cross-talk deriving from the integration of multiple sub-THz signals may be detrimental on the long-term scale, strongly affecting the working frequencies of the circuit. In this letter, a flexible generation of a beat note at a multiple of 50 GHz and its stabilisation in the frequency domain through an integrated silicon nitride high Q resonator-based locker are reported. As a field-test, two generated beat note signals at 100 and 150 GHz, respectively, were launched into a 183.8 km long dark fibre network. To evaluate the frequency locker system robustness, an external frequency drift of -125 MHz/hour that mimicked a thermal drift (~2°C/hour) was applied to one laser. The integrated apparatus allows the stabilisation of the generated frequency tone, with a maximum variation of
$\pm ~30$
MHz around the nominal value. In principle, the adopted free-dispersion design of the SiN waveguide makes possible to place two spectral lines far enough to generate thermally stable THz-waves.