Simultaneous generation of a stepped-frequency signal and microwave frequency comb based on a dual recirculating frequency shift loop.

A scheme to generate a stepped-frequency (SF) signal and microwave frequency comb (MFC) simultaneously based on a dual recirculating frequency shift loop (RFSL) is proposed and demonstrated by simulation. In a photoelectric RFSL (loop 1), the DPMZM generates two first-order optical sidebands by the RF signal and RF pulse. The two frequency components beat in a photodiode and produce a frequency shift RF pulse, one part of which feeds back to the DPMZM to replace the initial RF pulse. As the number of cycles increases, loop 1 can output an SF signal. An optical RFSL (loop 2) is driven by the one period of the SF shift signal from loop 1, generating an optical frequency comb (OFC) with a linearly decreasing frequency spacing. The OFC can be converted to multiple sets of equispaced MFCs by self-heterodyne detection and MFC with linearly decreasing frequency spacing by heterodyne detection. In a proof-of-concept simulation, loop 1 successfully generates an SF signal consisting of five sub-pulses with the frequency increasing from 6 to 10 GHz in a frequency increment of 1 GHz. The OFC output from loop 2 is converted into five sets of MFCs with different frequency bands, frequency spacings, and comb lines by self-heterodyne detection, whose power fluctuations are less than 3 dB and SNRs are greater than 20 dB. It also generates MFC with a linearly decreasing frequency spacing by heterodyne beating with an optical carrier via a balanced photodiode, with a power fluctuation of less than 1.39 dB and a signal-to-noise ratio larger than 34 dB. By adjusting the bandwidth of the EBPF and the frequency of the input signals, the parameters of the SF signal and MFCs can be tuned flexibly. By reconfiguring the driving RF signals, an RF signal with decreasing frequency and MFC with a linearly increasing frequency spacing can be generated.