NLFM microwave waveform generation with increased time bandwidth product based on polarization modulation and phase coding

A photonic method for nonlinear frequency-modulated (NLFM) waveform generation with increased time bandwidth product (TBWP) is proposed based on polarization modulation and phase coding. A lightwave generated by a laser diode is firstly injected into a phase-coherent orthogonal lightwave generator (POLG). The POLG consists of a dual-polarization binary phase shift keying (DPBPSK) modulator and a polarization controller (PC). The DPBPSK modulator is driven by a radio frequency (RF) signal via a 90° hybrid coupler. After the POLG, a pair of phase-coherent orthogonally polarized optical carriers are generated and injected into a polarization modulator (PolM) which is driven by a specially designed phase modulation signal. After another PC and a polarizer, the optical signal is detected by a photodetector and an NLFM waveform can be generated. The central frequency and bandwidth of the generated NLFM waveform can be easily adjusted by tuning the frequency of the RF signal applied to the DPBPSK modulator and the phase modulation signal applied to the PolM. The TBWP of the generated NLFM waveform can be further increased by phase coding technique. A numerical simulation is conducted to verify the proposal. The generated NLFM waveforms have a great improvement in peak-to-sidelobe ratio of the compressed pulses compared with linear frequency-modulated waveforms with the same bandwidth, duration and phase coding.