Optical fiber feedback SQUID magnetometer

A description is given of an optical-fiber feedback superconducting quantum interference device (SQUID) magnetometer which was developed to improve electromagnetic interference characteristics. The SQUID consists of an RF SQUID control unit. Phase-locked pulse-width modulation (PWM) was used to construct a flux-locked-loop (FLL) circuit in the SQUID control unit. The operation of the optical fiber feedback SQUID is stable when a common mode voltage of AC 100 V/50 Hz is applied. It has an energy resolution of 1*10/sup -28/ J/Hz. The authors describe the measurement of an auditory evoked field from the human brain in a magnetically shielded room using the fiber feedback SQUID with a gradiometer-type pickup coil.<
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