Hello everyone I recently read a publication (attached) in Nature which talks about microwave-to-optical conversion. The setup consists of resonator depicted by a circle. It is mentioned that this resonator has non-linear electromagnetic response. Up-conversion takes place by three-wave mixing in resonator. The resonator is excited by microwaves (say 100 GHz) channeled through the rectangular waveguide. The resonator is also excited by an optical signal (say 500 THz) through the input optical fiber placed on the opposite side. Some questions: In which port of the device, do we get the output or the up-converted signal, is it not the output optical fiber? If it is the output optical fiber as shown in the paper, will it not have the same signal? I assumed this because only the original input optical signal can couple back into the optical fiber waveguide. What is the use of the input optical signal? Don't we want an optical signal at the end? What is meant by non-linear response in the resonator and what is three wave mixing in the resonator?