Optical waveguides – Optical transmission cable
Patent
1998-03-09
2000-09-19
Lee, John D.
Optical waveguides
Optical transmission cable
385 32, G02B 626
Patent
active
061224251
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an optical-fibre cable for the transmission of light, comprising at least one flexible portion and including at least one optical fibre having a core and a surrounding cladding the refractive index of which is different from that of the core.
REVIEW OF RELATED TECHNOLOGY
Optical-fibre cables are used in many applications to transmit optical signals from one location to another. For example, EP-A-0 309 666 discloses a breath analyzer in which an optical-fibre cable is provided to transmit light from an optical measuring cell to a photoelectric detector in an instrument which converts the optical signals to electric signals.
In performing signal transmission in this manner it is important that external influences do not disturb the transmission such that the optical signals are distorted in an unforeseeable manner. Such an external influence may occur if by virtue of its flexibility the cable changes its configuration while the signals are being transmitted. For example, if the cable is unsupported between a sensor and a detector, it may be acted on by forces which cause a portion of the cable to be curved or to change its curvature while the transmission is in progress. As a consequence, the transmission characteristics and, accordingly, the magnitude of a signal being transmitted may be changed in an unforeseeable manner so that the measurement will show false values.
When light is transmitted in an optical fibre, most of the light is reflected at the interface between the fibre core and the surrounding cladding, because the refractive index of the cladding is different from (lower than) that of the core. Most of the light strikes the interface at an angle of incidence (as measured from the normal to the interface at the striking point) which is sufficiently large to result in total reflection at the interface.
However, a portion of the light will strike the interface at an angle of incidence which is close to the critical angle for total reflection. If as a result of motion of the cable a reduced radius of curvature should be imposed on the optical fibre at some location along the length thereof, the angle of incidence of such light at that location may become smaller than the critical angle so that the light is radiated out of the optical fibre, resulting in a weakening of the signal that reaches the detector.
Accordingly, random changes of the configuration of the optical-fibre cable occurring while the optical signals are being transmitted may result in the intensity or magnitude of a given signal fed into the fibre being affected in a more or less random manner before it reaches the detector.
OBJECTS AND SUMMARY OF THE INVENTION
The present invention aims at solving this problem and providing a method and means for substantially eliminating influences of the above-described kind in an optical-fibre cable.
This aim is realized according to the invention by arranging the cable such that it extends along an invariable curved line over at least a portion of its length, the curved section and the angle the curved section subtends being such that a certain proportion of the intensity of a signal transmitted through the cable is lost from the curved section. Advantageously, the radius of curvature is smaller than the smallest radius of curvature that the cable exhibits or may be expected to exhibit in normal use. In most cases it is preferable for the curved section to subtend an angle or change of direction of the cable of at least 90.degree..
Accordingly, inventive solution of the above-described problem comprises providing in the flexible optical-fibre cable a fixed or invariable curved section in which all of the light which is likely to be affected by changes of curvature is caused to be radiated out of the optical fibre. Thus, what will be left is only light which will always travel the full length of the fibre, regardless of any changes of the curvature which occur while the light is being transmitted through the cable.
The curved section may ext
REFERENCES:
patent: 4697869 (1987-10-01), So et al.
patent: 4983007 (1991-01-01), James et al.
patent: 5259045 (1993-11-01), Azuma et al.
Connelly-Cushwa Michelle R.
Icor Instruments AB
Lee John D.
LandOfFree
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