We report the strong microwave Josephson radiation from an array of high-Tc junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUID's geometry which guaranteed the oscillation of all junctions at the same frequency. For a five-junctions array, we observed the coherent output power of about 13 pW at receiving frequency fREC22GHz without an external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the selfradiation in coherent state less than 1 GHz by the adjustment of the external flux. The phase differences between adjacent junctions with different IcRn products could be controlled by an external small magnetic field. Submillimeter-wave detector response of the five-junction array was also studied experimentally at frequency f478 GHz.

We have investigated the Josephson microwave self-radiation and the linewidth from different types of YBa2Cu3Oy(YBCO) grain boundary junctions: natural grain boundary junctions, step-edge junctions and bicrystal junctions. The Josephson self-rediation was directly observed using a total power radiometer receiver with receiving frequencies fREC=1.7-72 GHz. All junctions exhibited microwave self-radiation peaks with intensity of order of 10-12-10-14 W. For step-edge and bicrystal junction, they appeared at a voltage related to the Josephson frequency-voltage relation, V=n(h/2e)f, while for natural grain boundary junctions, the above relation did not hold, suggesting a Josephson medium property. For all types of junctions the observed Josephson linewidth deviated from the theoretical RSJ values due to the extra noise source in the grain boundary junction. The Josephson linewidth decreased with increasing the receiving frequency for all type of junctions. The reduction of Josephson linewidth at higher frequencies indicates that the critical current fluctuations due to a critical current spread at small bias voltages and a crystalline disorientation at the junction boundary generate an additional noise in grain boundary junctions.