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Current Sensing Coil

The basic principle of FOCS and FOCTs is to measure polarization rotation due to the Faraday effect. The Faraday effect is the rotation of the polarization state of light, β, when it passes through a magnetic field, B, induced by an electrical current. The larger the electric current, the greater the magnetic field and hence the larger the polarization rotation.

The Current Sensing Coil which have a sensing cable in it and there is a fiber mirror in the end of the cable. Other end will be spliced with a 2mm or 3mm transmission PM fiber cable, and link to signal processor unit. The sensor fiber which are made by Comcore’s patents technology is a Spun Hi-Bi fiber which can convert linearly polarized light into elliptical polarized light and preserve the elliptical polarization.

Model FCA-01-CO
Operating Wavelength (nm) 1310
Bandwidth (nm) ±40
Insertion Loss (dB/meter) <0.1 (0.06 Typical)
Rated Primary Current 10mA to 2000A
Working Condition (Temperature) -40°C~+70°C
Working Condition (Humidity) ≤95%
Coil Diameter 10cm or customize
Output Fiber Type PM PANDA Fiber
Modal Field Diameter (μm) 9.0±0.5

Principle diagram:
The light signal (Figure2:(1)) be converted into two linear polarized signals, both sent though a PM fiber into the sensing fiber, the two linear polarized signals are converted into right and left elliptical polarized light signals, the two signal travel around the conductor many times, Magnetic field created by the current flowing in the conductor slows one signal and accelerates the other (Faraday effect), the signals are reflected by the mirror (Figure2:(11))and travel back through the fiber, the polarization now are reversed, the effect is doubled; Both signals make the way back to the sensing fiber which converts them back into the linear polarized signals, so the phase shift between the two signal created by Faraday effect can be measured by PD (Figure2:(3)), since both signals travel the identical path, vibration and temperature affects them equally, the highly accurate current measurement is unaffected, optical-electronics translates the optical phase shift into highly accurate current measurement.

Comcore’s sensing fiber (own patent) which are placed in Current Sensing Coil:
Integrates the function of Quarter wave plates (Figure2:(9)) and elliptical polarization maintaining (Figure2:(10)) into the whole fiber, which owns better uniformity, lower IL, (without the wave plates, Figure2:(9)),

Typical sensing fiber (spun HiBi fiber):

Elliptical polarization maintaining fiber, which must be spliced with a Quarter wave plates (Figure2:(9))