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.
The large capacity handled by the IMT 2000 requires that a high-frequency band of 2 GHz be used. However, raising the frequency results in the occurrence of various problems related to the large volume of data handled, such as increased signal loss, lower signal quality and interference in adjacent frequency bands. It is hoped that substituting a high-temperature superconductor in a place of the existing dielectric filter will reduce insertion loss, noise and high Q values. The authors have applied current technologies for film, microwave and cryocooler to develop a high-temperature superconducting (HTS) filter subsystem for the base stations of mobile telecommunication systems. The new filter has a fractional bandwidth below 1.02% at 1.955 GHz and has a minimum insertion loss at 0.2 dB. A small, highly efficient and long-life pulse-tube cryocooler a with cooling capacity of 0.65 W at 70 K and 60 W compressor input power is used 60 W. The components have been integrated into a subsystem, the external dimensions of which have a total volume of 10.1 L.
Slot antenna-coupled high-Tc YBa2Cu3O7-δ(YBCO) microbridges were fabricated and their millimeter-wave detection properties in the resistive state were investigated at 94 GHz. The nonbolometric response due to vortex motion was observed in the vicinity of the critical temperature. It became dominant in the extended temperature region when the bias current was increased. The detected voltages were almost proportional to the vortex velocity in the microbridge and the maximum voltage was detected when the vortex velocity was at its maximum. We have shown that the nonbolometric response is obtained by enhancement of the vortex motion due to the high frequency current induced by the slot antenna. Keeping the bias current constant, the detected voltages were almost proportional to the input power, and a responsivity of approximately 180 V/W was obtained.
Development of microwave integrated circuits and high-Tc superconductive thin-film circuits is performed briskly, and dielectric substrate is used as the material. Circular cavity resonance and microstripline resonator methods are used for measuring the permittivity of dielectric substrates in the microwave region. This paper presents the measured results of permittivity of c-sapphire and r-sapphire substrates by these methods.
A cryogen-free measurement system has been developed to examine the flux-flow phenomena in Bi2Sr2CaCu2O8+y (Bi-2212) intrinsic Josephson junctions. The system consists of a pulse-tube cryocooler and a variable-field permanent magnet. Flux-flow measurements were carried out at 44 K at which the Bi-2212 intrinsic Josephson junction exhibited 90% of the critical current at 4.2 K. Field direction could be set parallel to the superconducting CuO2 layers of the intrinsic junctions accurately, free from pancake vortices. The current-voltage characteristics measured under magnetic fields up to 1 T revealed a broad velocity-matching step corresponding to the strongly coupled Josephson junction stack.