This paper discusses the microwave and millimeter wave fundamentals and applications of high-temperature superconductors. First, a high-temperature superconducting filter subsystem for a base station is described as an example of microwave devices. In the USA, over 1, 000 systems are already working as filter subsystems in 2 G mobile telephone base stations. In Japan, a field trial was carried out for a filter subsystem of an IMT-2000 base station. As the result, the effectiveness of the superconducting filter subsystem was clarified. Therefore, the demand for superconducting filter subsystems for base stations will increase in the future. A SIS mixer and superconducting calorimeter were described as examples of millimeter and sub-millimeter wave devices. Using a Nb/AlOx/Nb SIS junction and X-ray calorimeter, which utilized micromachine technology, the resolution of such devices was quite high compared to that of conventional devices.
A new type of optical-microwave conversion system has been developed as an interface between optical systems and high-speed electric circuits. The microwave was generated by mixing two optical beams in a photodiode and radiated from a rod antenna, which was placed beside a high-Tc grain boundary Josephson junction detector. Since frequencies of radiated microwaves correspond to the frequency differences between the two optical beams, the wavelength information of the beams is reflected in the current-voltage and differential resistance-voltage properties of the Josephson junction. System operation up to 20 GHz was demonstrated by changing the wavelength of the two beams and the temperature of the Josephson junction. The relation between the frequency differences and measured Shapiro step intervals suggests that the system can be utilized for a highly sensitive optical-microwave conversion.