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 inductance modulation circuit, for use as part of a 1/f noise reduction scheme for SQUID magnetometers. We modulate the effective inductance of a photolithographically patterned coil via its mutual inductance with three concentric superconducting washers directly above the coil. The superconducting loops formed by each of these washers can be broken by sending laser light onto a constricted meander section of each of these washers. With the laser off, the superconducting washer loops diamagnetically shield the coil, and the coil has a low effective inductance. With the laser on, the meanders are driven normal and the superconducting washer loops are open, greatly reducing the shielding. The coil then has a high effective inductance. In a series of computational runs, we have optimized the design of this circuit in terms of its inductance modulation depth and its avoidance of flux jumping incidents. The free parameters 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 size of the overlaps between washers.<>