Photonic Generation of Chirped Microwave Pulses Using Superimposed Chirped Fiber Bragg Gratings

A novel approach to generating linearly chirped microwave pulses in the optical domain based on spectral shaping and linear frequency-to-time mapping is proposed and experimentally demonstrated. In the proposed system, the spectrum of a femtosecond pulse generated by a mode-locked fiber laser is spectrum-shaped by an optical filter that consists of two superimposed chirped fiber Bragg gratings (SI-CFBGs) with different chirp rates. The SI-CFBGs form a Fabry-Perot cavity with a cavity length linearly dependent on the resonance wavelength, thus a spectral response with an increased or decreased free spectral range is generated. A chirped microwave pulse with the pulse shape identical to the shaped spectrum is obtained at the output of a high-speed photodetector thanks to the frequency-to-time mapping in a dispersive device. The proposed technique is experimentally demonstrated, a linearly chirped microwave pulse with a central frequency of 15 GHz and a chirp rate of 0.0217 GHz/ps is experimentally generated.