Silicon-based on-chip electrically-tunable spectral shaper for continuously tunable linearly chirped microwave waveform generation

A silicon-based on-chip electrically-tunable spectral shaper for the generation of a tunable linearly chirped microwave waveform (LCMW) based on spectral shaping and wavelength-to-time (SS-WTT) mapping is designed, fabricated, and demonstrated. The on-chip spectral shaper has a Michelson interferometer structure with two linearly chirped waveguide Bragg gratings (LC-WBGs) incorporated in its two arms. Due to the wavelength-dependent length difference between the two arms of the interferometer, the spectral response of the spectral shaper exhibits a wavelength-dependent free spectral range, which is required for the generation of an LCMW based on SS-WTT mapping. To enable electrical tuning of the spectral response, a lateral PN junction is introduced to each of the waveguides where the LC-WBGs are inscribed. Thanks to the plasma dispersion effect, the spectral response of the spectral shaper can be tuned by changing the bias voltages applied to the PN junctions, which would lead to the tuning of the generated LCMW. A theoretical analysis on the LCMW generation is performed, which is verified by an experiment in which an electrically-tunable spectral shaper is fabricated using a CMOS-compatible process with 248-nm deep ultraviolet lithography. By independently controlling the bias voltages to the PN junctions, a continuous tuning of the generated LCMW is demonstrated.