Optical waveguides – Optical waveguide sensor
Patent
1991-09-09
1993-04-27
Lee, John D.
Optical waveguides
Optical waveguide sensor
385 13, 385 24, 356350, G02B 626
Patent
active
052069230
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Fibre optical sensor technique is a field that during the last ten years has been predicted a very expanding market. The expansion has, however, not reached the pace that was predicted. The reason for this is principally the difficulty to separate the sensor information from the varying influence from the environment, both on the sensor and on the transmission to and from the sensors. Another reason is that the single-mode technique that has the highest degree of sensitivity is complex and demands that the components used are very accurate. The multimode technique, on the other hand, can allow a more simple design and moreover, it uses more robust components. Thereby it is possible to reach such a level for the costs for the fibre optical sensors that they become economically interesting. But the multimode technique has also not experienced a break-through, mostly due to the lower sensitivity that hitherto has been achieved with this technique. Conventional multimode technique is of cause based on principles of intensity modulation of the light that is transmitted in the core of the multimode fibre. This technique is less sensitive than the phase-sensitive detection that the single-mode technique is based on.
The fibre optical sensors have potentially very great advantages, primarily due to their non-galvanic construction, that result in for instance insensibility for EMP, EMC etc. The sensitivity that can be achieved is also as good as or better than what can be achieved with other types of sensors. Another interesting characteristic of the fibre optical sensor technique is the possibility to multiplex several sensors along the same fibre optical cable and in that way implement sensor systems that is able to cover for instance large surveillance zones. The sensors of the system can be dimensioned to be sensitive to several physical quantities such as pressure and magnetism. Great tactical advantages can be achieved by this. In order to achieve this goal the sensors have to be based upon such a technique that a realization of the sensor technique is robust and functionally reliable and this at a reasonable cost in comparison with conventional technique.
SUMMARY OF THE INVENTION
The present invention relates to a method to use an optical fibre as a sensor. The detection is phase-sensitive and can be applied to both single- and multimode fibres. The invention solves the present problem.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention will be described in more detail with reference to the attached drawings where
FIG. 1 shows a laser, a fibre, in which the radiation propagates in several modes and the interference pattern after the fibre,
FIG. 2 shows an interference detector according to the invention,
FIG. 3 shows a first sensor system according to the invention,
FIG. 4 shows a second sensor system according to the invention,
FIG. 5 shows a third sensor system according to the invention,
FIG. 6 shows two variants of the sensor circuit in FIG. 5 and
FIG. 7 shows a fourth sensor system according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The suggested sensor system is based on detection of variations in the phase condition of the light that is transmitted in an optical fibre. This is a more sensitive method of measuring mechanical influence on the optical fibre than measuring the variations in the effect or the intensity of the light. A mechanical influence on the optical fibre can be translated to serve as a measuring quantity of, for instance, acoustical waves or magnet fields. It is very important that the sensor system is not sensitive to external influence from the environmental parameters, that can give rise to an increased noise level for the measuring quantity of interest. For solutions that concernes multiplexing several fiber optical sensor, this is of particular relevance. The solution according to the invention is based on a principle which among other things means that the variations in the optical fibre due to the env
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Heartney Phan Thi
Lee John D.
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