Photonic Generation of Linearly Chirped Microwave Waveforms Using a Silicon-Based On-Chip Spectral Shaper Incorporating Two Linearly Chirped Waveguide Bragg Gratings

Photonic generation of linearly chirped microwave waveforms using a silicon-based on-chip spectral shaper is proposed and demonstrated. The on-chip optical spectral shaper has a Mach-Zehnder interferometer (MZI) structure, in which two identical 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 produce linear chirps. By adding an offset waveguide to one arm of the MZI and controlling the length of the offset waveguide, the spectral response of the shaper can be controlled to have a symmetrical, linearly increasing, or linearly decreasing free spectral range, which is needed for the generation of a linearly chirped microwave waveform based on spectral-shaping and wavelength-to-time mapping. The proposed device is fabricated using a CMOS-compatible process with 193-nm deep ultraviolet lithography. A theoretical analysis on the chirped microwave waveform generation is performed, which is verified by an experiment. Two linearly chirped microwave waveforms with symmetrical and uniform chirp profiles are experimentally generated using two fabricated spectral shapers with different lengths of the offset waveguides.