Phase dynamics and radiated power of dc SQUIDS

The time variation of the quantum phase difference and the radiated power of two resistively shunted junctions in a superconducting loop are analyzed using a first harmonic approximation method. It is shown explicitly that an output coupling impedance connected across the junctions plays an important role in determining the output power as well as the cutoff frequency in the radiation spectrum. The radiated power of an array of dc SQUIDs is compared with the power output of a synchronized series array in a resonant cavity. With N superconducting weak links coupled to a fixed load, the SQUID array suffers an output power saturation for large N; our analysis shows that it is very difficult to obtain radiated power higher than a few microwatts. On the other hand, a cavity-coupled large array gives an output power proportional to N with a narrow linewidth, suggesting that it could be very useful in practice.