A Microwave Frequency Comb with Measurement Capability up to 100 GHz

When the laser light of an OFC (optical frequency comb) shines on a photodiode, the latter's output current (or voltage) will contain information about the different modes from the OFC. These generated pulse current, after Fourier transformation, could be regarded as a new frequency comb in the microwave frequency band with the frequency interval the same with the original OFC. The cutoff bandwidth of the photodiode determines the upper limit of this new frequency comb. We used two waveguide integrated photodiodes, XPDV3120R and XPDV412xR produced by Finisar Company, as the photoelectric conversion elements required for the experiments. Since the operating limit of our best spectrum analyzers could only reach to 50 GHz, we designed a beat frequency measurement architecture to evaluate properties of the microwave frequency comb within a specific frequency range (> 50 GHz). In our experiments, the microwave frequency comb with measurement capability up to 100 GHz was verified to be feasible, and its higher frequency resolution than that of those methods by using traditional microwave frequency signal generators & counters was also a noteworthy advantage. Besides, some components used in the beat frequency measurement architecture are of waveguide form and related rigid connections might cause troublesome heat dissipation problems. They were finally solved by using some handpicked thermal pads and heat sinks.