Photonic generation of broadband linearly chirped microwave waveform based on a low-loss silicon on-chip spectral shaper.

A silicon on-chip spectral shaper based on a Sagnac loop incorporating a chirped multi-mode waveguide Bragg grating (WBG) for linearly chirped microwave waveform generation is fabricated and demonstrated. The transmission spectrum of the spectral shaper displays low insertion loss characteristic due to the application of edge coupling taper and multi-mode waveguide based grating. An up-chirped microwave waveform with bandwidth as large as 44 GHz is generated by mapping the spectrum profile of the spectral shaper to the temporal domain through a dispersion fiber. The instantaneous frequency of the generated signal shows good linearity benefiting from the weak modulation strength in the multi-mode WBG. The low insertion loss performance as well as the low dispersion value required in our design presents feasibility in further integration with on-chip dispersion.