Reshaping periodic light pulses using cascaded uniform fiber Bragg gratings

The authors demonstrate the use of cascaded uniform fiber Bragg gratings (FBGs) for the generation of periodic optical pulses with arbitrary waveform. It is a significantly simplified structure compared to complex FBG shapes. The pulse shaping is based on splitting of the input pulses by low-reflecting FBGs into a number of replicas and their superposition with proper amplitude, time delay, and phase shift that depend on the FBG parameters. The reflection amplitude and phase of each grating are unambiguously determined by the needed pulse shape. This method was experimentally verified for converting sinusoidally phase-modulated radiation of continuous-wave laser diode into a Gaussian pulse train with a pulsewidth of 30 ps. A method for controlling the parameters of FBGs during their fabrication process is also presented. It is done by measuring the spectral interference between the reflections from the FBGs and the fiber end by an optical spectrum analyzer and performing a fast Fourier transform. The method allows correction of the FBGs until the needed parameters are obtained during the writing process, as well as at any time after that.