Photonic Generation of Linear-Frequency-Modulated Waveforms With Improved Time-Bandwidth Product Based on Polarization Modulation

Polarization modulation of two phase-correlated, orthogonally polarized wavelengths by a parabolic waveform is a promising way to generate linear-frequency-modulated (LFM) signals, but the time-bandwidth product (TBWP) of the generated LFM signal is intrinsically limited by the achievable modulation index of the polarization modulator (PolM). In this paper, an approach to increase the TBWP of the LFM signal generated by polarization modulation is proposed and comprehensively studied by splitting the electrical parabolic waveforms into N pieces with identical amplitude. Applying the split parabolic signal to the PolM, the total equivalent phase shift would be boosted by N/2 times. As a result, the bandwidth as well as the TBWP of the generated LFM signal is increased by N/2 times. An experiment is carried out. As compared to the scheme using an unsplit parabolic signal, the TBWP is improved by more than 500 times. The relationships between the bandwidth, the time duration, and the TBWP of the generated signal with the parameters of the electrical waveform generator are discussed.