Active feedback control of photonic molecule microcombs

Abstract

Frequency combs generate coherent and evenly spaced spectral lines. This enables applications within optical communication, for wavelength division multiplexing; metrology, for measuring time and frequency; spectroscopy, for probing a sample at multiple frequencies at the same time, and more. One novel implementation of these devices consists of two coupled resonators in a planar integrated Si3N4 platform. The device, called a photonic molecule, generates a coherent frequency comb by four wave mixing from a single input laser. A frequency comb generated by this device has been actively stabilized by thermal control of the pump to resonator detuning.

The feedback system measures the generated comb power and changes the temperature of the resonators, via integrated heaters, to thermo-optically alter the location of the longitudinal modes of the coupled resonators. This changes the detuning such that the converted power remains constant. The control system is evaluated by measuring the stability and phase noise of the repetition rate of the comb. Long term stability is found to increase five-fold with the controller enabled and phase noise is improved by 5 dB for offset frequencies within the integrating bandwidth of the controller compared to a free-running comb in the same state.

These results enable the use of photonic molecules for spectroscopy by increasing the stability of the comb as well as for optical communication by reducing phase noise. The results also make the photonic molecule easier to work with as it becomes less sensitive to environmental effects.