A single fibre-optic down-lead Faraday current sensor

We demonstrate a single-down-lead common-mode rejection scheme for a bulk optic Faraday current sensor that can eliminate optical noise induced by fibre-link vibration. The new configuration is based upon the creation of two temporally delayed signals at the sensor level. One signal is encoded with the Faraday signal and the optical noise. The other signal contains the noise component only. By using a pulsed laser diode at the emitting point and electric delays at the receiving point, the two signals are combined differentially and time gated to eliminate the optical noise. As the delays at the sensor level cannot be too large due to the finite size of bulk optics configurations, it is neccessary for the laser, photodetector and electronics to work with nanosecond pulses. Methods to alleviate the constraints raised by the needs to process nanosecond pulses are discussed.