Photonic generation of linearly chirped microwave waveform with a large time-bandwidth product using a silicon-based on-chip spectral shaper

Photonic generation of a linearly chirped microwave waveform with a large time-bandwidth product (TBWP) using a silicon-based on-chip spectral shaper is reported. The on-chip optical spectral shaper has a Mach-Zehnder interferometer (MZI) structure in which two linearly chirped waveguide gratings (LC-WBGs) with opposite chirp rates are incorporated into the two arms. The LC-WBGs are fabricated on two rib waveguides by linearly varying the rib widths along the gratings to generate linear chirps. By controlling the length difference between the two arms in the MZI, the spectral response of the shaper can be controlled to have a symmetrical, linearly increasing or linearly decreasing free spectral range (FSR), which is needed for the generation of a linearly chirped microwave waveform based on spectral-shaping and wavelength-to-time (SS-WTT) mapping. The proposed device is fabricated using a CMOS compatible process with 193-nm deep ultraviolet lithography. The use of the spectral shaper to generate a linearly chirped microwave waveform with a chirp rate of 1.54 GHz/ns and a TBWP of 615 is demonstrated.