IMPROVED DESIGN OF AN OPTICALLY SWITCHED INDUCTANCE MODULATION CIRCUIT FOR NOISE-REDUCTION IN SQUID SYSTEMS

We have modeled and optimized the design of an optically driven induct ance modulation circuit, for use as part of a 1/f noise reduction sche me for SQUID magnetometers. We modulate the effective inductance a pho tolithographically patterned coil its mutual inductance with three con centric superconducting washers directly above the coil, The supercond ucting loops formed by each of these washers can be broken by sending laser light onto a constricted meander section of each of these washer s, With the laser off, the superconducting washer loops diamagneticall y shield the coil, and the coil has a low effective inductance. With t he laser on, the meanders are driven normal and the superconducting wa sher loops are open, greatly reducing the shielding, The coil then has a high effective inductance. In a series of computational runs, we ha ve optimized the design of this circuit in terms of its inductance mod ulation depth and its avoidance of flux jumping incidents, The free pa rameters in this study were the inner diameter of the coil, the number of loops in the coil, the dimensions of the three washers,and the siz e of the overlaps between washers.