Abstract

We demonstrated a single down lead common-mode rejection scheme for a Faraday current sensor that can eliminate optical noise induced by fibre-link vibration [1]. The configuration was based upon the creation of temporally delayed replicas. One replica was encoded with the Faraday signal and the optical noise, (F+Z), and the other contained the noise component only, Z. By using a pulsed laser diode and suitable optical delays at the probe level and electrical delays at the receiver ground level, the two replicas were combined differentially and time gated to eliminate the noisy signal Z. The probe used a rod made of diamagnetic Schott SF6 glass as the Faraday transducer and the sensitivity was quite low. In addition, the common mode rejection was sensitive to pulse shape alteration in the system as the two replicas were subtracted in their pulse format at the receiver point. We present an improved set-up where the diamagnetic glass is replaced by a semimagnetic Faraday crystal. Cd_0.57Mn_0.43Te with a much higher Verdet constant [2] to increase the probe sensitivity. A second improvement consists in processing the two replicas up to the DC level (i.e. they are amplified and rectified separately) and only after that subtracting them. A further development is introduced by increasing the delay between the two replicas. Light from the crystal is directed towards a single mode coupler, whose output leads are silvered and have different lengths to implement a differential delay of τ ≈ 10 ns between the pulses reflected back to the single mode coupler inputs. The light from the other coupler input is directed via a fibre optic lead to the ground receiver. Polarization controllers in the two arms maximize the Faraday effect on one arm and minimizes it on the other arm. Then, at the receiver point, the signal is split into two channels where an electrical delay τ ≈ 10 ns is introducd. The two channels are fed into two time gating amplifiers, each passing the input signal to the output during the pulsewidth of the gating signal. The gating signal is supplied by the same generator used to drive the laser and it is suitably delayed to coincide with the signal to be selected. Then the signals (F+Z) and Z are rectified and applied to a differential amplifier. The main focus of the paper is the optical and electronic processing unit, designed to perform the common mode rejection without resorting to high cost optical and electronic RF components. Pulsewidths of 4 ns have been used ensuring good separation of adjacent pulses at 10 ns. which represents a good compromise cost-performance. As a first proof of the improvements, the subtraction of signals ceased to be dependent on the variation of the electrical delay alteration, a problem experienced in the implementation [1].

© 1998 IEEE