Stepwise Frequency-Shifted Optical Heterodyne for Flexible and Ultrawide Frequency Microwave Down-Conversion

A photonics-assisted method based on stepwise frequency-shifted optical heterodyne is proposed for flexible and ultrawide frequency microwave down-conversion with a low-frequency electrical local oscillator (LO). The method consists of a stepwise tunable optical carrier generator and a frequency-shifted heterodyne interferometer. A pair of coherent optical carrier with stepwise frequency spacing is modulated by the high-frequency radio frequency (RF) and the low-frequency LO, respectively. The combined optical signals heterodyne with each other and generate the desired down-converted intermediate frequency (IF) signal. The down-conversion achieves ultrawide frequency range with a low-frequency LO. Moreover, it enables flexible and configurable operation by choosing the optical carrier pairs with different frequency spacing and/or tuning the LO frequency. In the demonstration, 6.25~40-GHz RF signals are experimentally down-converted to IF signals below 1 GHz with a dc ~6.25-GHz LO.