Optics: measuring and testing – By polarized light examination
Patent
1993-11-24
1995-12-12
Rosenberger, Richard A.
Optics: measuring and testing
By polarized light examination
3242441, G01R 33032
Patent
active
054754895
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. The Technical Field
The present invention relates to a method of determining induced change of polarization state of light in a polarization element, fiber optic sensor devices for determining such induced change of polarization state of light, particularly linear birefringence induced by an electric voltage, an electric field, and a mechanical force, and circular birefringence induced by an electric current and a magnetic field.
A magnetic field, an electric field, or a mechanical force, or a combination of these, can induce anisotropy in the index of refraction of an optically transparent electrooptic, magnetooptic and/or photoelastic material, respectively, which can be detected by a change of polarization state of light.
Linear birefringence can be induced by an electric field in an electrooptic material by the so-called electrooptic Pockels effect and Kerr effect which can be used in the determination of electric voltage and electric field; material by the so-called magnetooptic Faraday effect which can be used in the determination of electric current and magnetic field; and material by the so-called photoelastic effect which can be used in the determination of mechanical force e.g. acceleration and pressure.
Generally, induced change of polarization state of light in a polarization element is used in fiber optic polarimetric optical sensors and the induced change can be determined by: optical transmitter waveguide means to polarizer means at the other end; by an analyzer means; and receiver waveguide means to a light detector at the other end; the optical transmitter waveguide means and the polarizer means, and the analyzed polarized light is focused by output coupling means onto the optical transmitter waveguide means, said coupling means consisting of lenses or graded index rod lenses.
Specifically for a fiber optic magnetooptic current sensor, it has been suggested that the transmission of the polarized light through the polarization element is provided by using at least one plane reflective optical surface reflecting a collimated beam.
These optical arrangements have a number of disadvantages.
First of all, a collimated beam or slightly convergent beam is used which requires auxiliary coupling lenses to obtain an optimal light energy transmission from the optical transmitter waveguide means to the optical receiver waveguide means.
Secondly, the use of conventional lenses as coupling means makes the size of the arrangement inconveniently large because the lenses must be surrounded by air in order to obtain a suitably large change of refractive index at the optical surfaces.
Also, mechanical holders for the lenses are required which complicates the mechanical construction.
Further, in the case of a voltage or electric field sensor using conventional lenses, the air present around the lenses limits the electric field that can be applied to the sensor before disruptive electric discharge occurs.
Graded index rod lenses may avoid the problem of air if coupled directly on to the optical waveguide, such as an optical fiber. Such an arrangement can be made compact and the optical interfaces can be filled with an electrically insulating optical cement. However, in order for the graded index rod lenses to collimate the beam from the transmitter, a relative small core diameter (max. 200 .mu.m) optical fiber is required which limits the amount of light that can be launched into the optical fiber by the light source, hereby limiting the signal-to-noise ratio and the sensitivity of such a device, unless compensated for by a more powerful light source.
Thus, cheap visible light emitting diodes e.g. LED's emitting light in the range 500-700 nm cannot be used because they cannot supply sufficient light through such optical fibers having that small core diameter. Instead powerful and more expensive LED's emitting light in the range 800-950 nm must be used. However, LED's and polarizers working at these wavelengths as well as graded index rod lenses are expensive.
Therefore, there is
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W. B. Spillman, Jr. and D. H. McMahon; "Multimode fiber optic sensors based on the photoelastic effect;" pp. 110-114 (no date).
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