Optical interferometric current sensor and method using a single mode birefringent waveguide and a pseudo-depolarizer for measuring electrical current

An interferometric current sensor for measuring an electrical current, comprising:

• a light source for generating a light beam, said light source is characterized by a spectral distribution having a central frequency equal to .nu..sub.o and a spectral frequency full-width equal to .delta..nu. at half maximum, said light beam having a coherence length L.sub.c equal to c/.delta..nu. where c is the velocity of light in vacuum, said light source being an electroluminescent diode;
• a single mode birefringent waveguide having a linear birefringence and a circular birefringence, said circular birefringence and linear birefringence having respectively given orders of magnitude, the order of magnitude of said circular birefringence being equal or higher than the order of magnitude of said linear birefringence, said waveguide having a portion for encircling said electrical current, said single mode birefringent waveguide being a spun fibre;
• a beamsplitter having a first input port optically coupled to said light source, and second and third input/output ports optically coupled to ends of said waveguide for launching counter-propagating light beams into the respective ends of the waveguide and for receiving said counter-propagating light beams therefrom, said beamsplitter with said single mode birefringent waveguide forming a Sagnac interferometer;
• pseudo-depolarizer means for reducing sensitivity to at least one of mechanical vibrations and temperature variations, said pseudo-depolarizer means being optically coupled in series with said waveguide for converting each of said counter-propagating light beams into a predetermined ratio of useful counter-propagating light signals and useless counter-propagating light signals, said pseudo-depolarizer means being a Lyot depolarizer comprising:
• a first highly birefringent fibre having a high birefringence .DELTA..beta..sub.1, a length L.sub.1, polarization axes P.sub.1 and P.sub.2, and a depolarization length L.sub.d1 defined by the following equation:
• ##EQU7## said depolarization length L.sub.d1 being greater than L.sub.c ;
• and a second highly birefringent fibre having a high birefringence .DELTA..beta..sub.2, a length L.sub.2, polarization axes P.sub.3 and P.sub.4, and a depolarization length L.sub.d2 defined by the following equation:
• ##EQU8## said L.sub.d2 being equal to or higher than said 2L.sub.d1, said first highly birefringent fibre having an end connected to an end of said second highly birefringent fibre in such a manner that said polarization axes P.sub.1 and P.sub.2 are rotated at 45.degree. from said polarization axes P.sub.3 and P.sub.4 ;
• and
• an optical detector optically coupled to the ends of said waveguide via said beamsplitter for detecting a light intensity, said light intensity resulting from an interference of useful counter-propagating light signals and useless counter-propagating light signals, whereby said light intensity is representative of said current.

A method for measuring a first electrical current, said method comprising steps of:

• a) encircling said first electrical current by means of a first loop portion of a single mode birefringent waveguide, said waveguide having a linear birefringence and a circular birefringence, said circular birefringence and linear birefringence having respectively given orders of magnitude, the order of magnitude of said circular birefringence being equal or higher than the order of magnitude of said linear birefringence;
• b) generating a light beam;
• c) splitting said light beam to launch two counter-propagating light beams into respective ends of said waveguide;
• d) reducing the sensitivity to at least one of mechanical vibrations and temperature variations by pseudo-depolarizing said two counter-propagating light beams launched into said waveguide to convert each of said counter-propagating light beams into a predetermined ratio of useful and useless light signals;
• e) interfering said useful counter-propagating light signals and useless counter-propagating light signals to produce an interfered light signal;
• and
• f) optically detecting a light intensity of said interfered light signal, whereby said light intensity is representative of said current.