Optical sensor with spun birefringent sensing fiber

An optical fiber current sensor having an opto-electronic module part for detecting a magneto-optic phase shift induced in a sensing fiber by a magnetic field of the current, and a sensor head including the sensing fiber, wherein the sensing fiber is a spun highly-birefringent fiber having a length L=∫ ds defined by the line integral along the space curve described by the sensing fiber,

• wherein a sensing length is defined as L′=L for a transmission optical sensor and as L′=2L for a sensing fiber being terminated by a reflector and
• L
• ′
• >
• L
• EB
• ⁢
• λ
• Δλ
• with
• L
• EB
• =
• L
• LB
• 1
• +
• x
• 2
• -
• x
• ≈
• 2
• ⁢
• xL
• LB
• and x being the spinning ratio of the spun highly-birefringent fiber,
• λ
• Δλ
• the ratio of center wavelength λ over the spectral width Δλ of the light in the sensing fiber, and LLB being the linear beat length which is a length section of equivalent unspun highly-birefringent fiber that produces a birefringent phase shift of 2π, and
• wherein
• D
• =
• a
• 2
• +
• b
• 2
• 2
• <
• D
• max
• ,
• with Dmax being 50 cm or smaller and Dmax being a length characterising the available volume for the sensor head, and a and b being the two largest edge lengths of a cuboid of minimal size enclosing the sensing fiber, wherein
• the magneto-optic phase shift accumulated in the fiber is reduced by having at least one of the following features:
• (i) one or more sections of the sensing fiber forming loops enclosing the current but being counter-wound with respect to other sections of the sensing fiber forming loops enclosing the current,
• (ii) one or more sections of the sensing fiber being at least partly magnetically shielded, and
• (iii) one or more sections of the sensing fiber forming loops not enclosing the current.