Optics: measuring and testing – Optical pyrometers – With sample engaging rod or tube
Patent
1984-12-18
1987-12-22
McGraw, Vincent P.
Optics: measuring and testing
Optical pyrometers
With sample engaging rod or tube
356345, 374161, G01J 538, G01B 902
Patent
active
047143428
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a temperature sensor based upon an optical interferometer which is constructed from single mode optical fibre and in which the detection and measurement of temperature is based upon optical interference or fringe patterns. More particularly, it relates to such a temperature sensor based on an all fibre Michelson interferometer which can be operated remotely from sources of power or light generation and is connected to such sources by optical fibres.
The measurement of temperature in an industrial environment is usually achieved using well-established transducers such as platinum resistance thermometers, thermocouples or bi-metallic strips. Although these transducers have become industry standards it has become evident that there is a demand for new types of thermometers for special applications where electrically based transducers cannot operate for reasons of safety or the presence of noisy electrical environments. Some of the potential applications of optical fibre thermometry are in the measurement of temperature in for example jet engines, nuclear reactors, and transformers. A fibre optic thermometer could also be used in the treatment of cancer where the tumour is subjected to microwave radiation which consequently eliminates the use of any of the conventional electronic sensors.
Optical phase modulation sensors offer the highest resolution of all optical displacement sensors; for example, classical interferometers of the Michelson or Mach Zehnder type can be used to detect displacements as small as 10.sup.-14 m. Although interferometers offer tremendous resolution (or sensitivity) until recently very little effort had been devoted to developing them into general purpose displacement sensors. This is primarily because the sensitivity of an interferometer is a function of the relative alignment of the optical beams, and in a typical interferometer where beam splitters and mirrors are used to control the amplitude division and recombination of the light the sensitivity will fluctuate if the optical components are randomly disturbed. However, the recent introduction of all fibre interferometers has virtually eliminated this problem as the alignment of the fibre guided optical beams is virtually immune from mechanical disturbance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a temperature sensor based on an optical interferometer constructed with optical fibres.
The invention consists in a temperature sensor comprising an optical interferometer, conveniently of the Michelson type, constructed from single-mode optical fibre and having an input and output coupled by coupling means to signal and reference optical fibres, said signal and reference fibres being of unequal lengths to provide optical path length imbalance and having reflecting ends remote from the coupling means to reflect light beams propagating in said fibres, said signal fibre having a sensing zone at its reflecting end, and said coupling means being adapted to mix the reflected beams in the fibres to produce an interference pattern at said output, a photodetector for monitoring the interference pattern at said output, a laser light source coupled to said input, means for modulating the laser injection current to produce a moving interference pattern, and means for processing the output of the photodetector to determine the optical phase shift between the light beams propagated in the signal and reference fibres which phase shift is related to the temperature at the sensing zone of the signal fibre.
Preferably, the laser light source is coupled to the input by a single mode optical fibre and the coupling means is a single mode optical fibre directional coupler. The photodetector may also be coupled to the output by an optical fibre.
In one preferred embodiment of the invention light from a single-frequency diode laser (.lambda..apprxeq.800 nm) is coupled into the single-mode optical input fibre and is then amplitude divided by the single-mode dire
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Imai et al., "Fiber-Optic Michelson Interferometer Using an Optical Power Divider", Optics Letters, vol. 5, #10, Oct. 1980, pp. 418-420.
Glatzel et al, "Temperature Measurement Technique Using Fresnel Interference Technique", IBM Technical Disclosure, vol. 20, No. 11A, Apr. 1978, pp. 4571-4572.
Corke Michael
Jackson David A.
Kersey Alan D.
Kent Scientific and Industrial Projects Limited
McGraw Vincent P.
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