A CRYOGENIC SCANNING LASER MICROSCOPE FOR INVESTIGATION OF DYNAMICAL STATES IN LONG JOSEPHSON-JUNCTIONS

The first local oscillators based on moving magnetic flux quanta in lo ng Josephson junctions are being developed for superconducting integra ted quasi-optical SIS receivers. In order to further refine these osci llators one has to understand the complex dynamics of these devices. S ince the local tunnel current is one of the most important internal ju nction parameters which together with the boundary conditions determin e the dynamics, it is of vital importance to experimentally determine the current density throughout the entire junction with high spatial r esolution. Here we report on measurements on different oscillator samp les, performed with a novel Cryogenic Scanning Laser Microscope (CSLM) having a spatial resolution of less than +/- 2.5 mu m over a 500 mu m x 50 mu m wide scanning area. in the temperature range 2 K - 300 K. E ven though the dynamical states are extremely sensitive to external no ise this microscope enables us to make stable in-situ measurements on operating Josephson junctions. Recent results are presented and discus sed.