Mode-locked fiber laser-based RF signals generation for VLBI radio telescopes

We demonstrate that a single 40-MHz repetition-rate mode-locked fiber laser can generate broadband phase calibration (PCal) signals and low-noise ~17-GHz local oscillator (LO) signals, both of which are required for the operation of the Korea VLBI Network (KVN) Yonsei Radio Telescope. The direct photodetection of a 40-MHz optical pulse train allowed the generation of flat RF comb signals with 40-MHz spacing up to the photodiode bandwidth (~50 GHz) for instrumental calibration required for multi-frequency VLBI investigations. At the same time, using a 5-stage fiber Mach-Zehnder interferometer pulse interleaver, we were able to generate the 16.64-GHz and 19.2-GHz low-noise single-tone LO signals required for downconversion of astronomical signals. With broader-bandwidth photodetectors, this system has the potential to be used for millimeter-wave signal generation, calibration, and downconversion for multi-frequency broadband radio telescopes.