Generation of Stepped Frequency Microwave Waveforms in the Temporal-Frequency Synthetic Dimension With Analog Predistortion

A novel approach to the generation of stepped frequency microwave waveforms (SFMW) in the temporal-frequency synthetic dimension with analog predistortion is proposed and experimentally demonstrated. The system consists of two coupled fiber-optic loops with an acoustic-optic modulator (AOM) incorporated in each of the two loops. When a light wave is passing through an AOM, the amplitude of the light wave is altered and its frequency is shifted, depending on the applied driving signal. The dynamic amplitude modulation and frequency shift to the optical pulses recirculating in the system implemented by the AOMs establish a temporal-frequency synthetic dimension. For SFMW generation, a seed optical pulse with a fixed carrier frequency is injected into the system. The frequency and amplitude of the optical pulse after each recirculation are determined by the frequencies and strengths of the driving signals applied to the AOMs. By applying two pre-designed driving signals to the AOMs, a stepped frequency pulse burst with the designated magnitude profile is generated after the seed pulse recirculates in the two loops for multiple round trips. By beating the pulse burst with the same optical carrier, an SFMW can be generated. The proposed approach is evaluated experimentally. Two SFMWs with predesigned spectral magnitudes having stepped frequencies from 5.44 to 7.24 GHz and 7.12 to 8.92 GHz are generated. The control of the spectral magnitude can be employed for spectrum distortion compensation. For a microwave transmitter frontend with an uneven magnitude response of up to 3.24 dB, transmitted microwave waveforms with flattened spectra having unevenness of less than 1.11 dB are achieved.