FR2536535A1 - FIBER OPTIC THERMOMETER - Google Patents

Abstract

THE FIBER OPTIC THERMOMETER CONSISTS OF A LIGHT SOURCE1, AN OPTICAL FIBER5 COUPLED WITH THE SOURCE AND CONSISTS OF A HEART7 COVERED WITH A SHEATH8 AND A TEMPERATURE SENSOR6, COUPLED WITH FIBER5. THE TEMPERATURE SENSOR6 CONSISTS OF A PORTION OF OPTICAL FIBER5 IN WHICH THE SHEATH8 IS REPLACED BY A COVER10 HAVING A REFRACTION INDEX N NOT LOWER THAN THAT OF THE SHEATH N OVER THE ENTIRE MEASUREMENT RANGE OF THE INSTRUMENT AND VARIABLE ACCORDING TO OF THE TEMPERATURE, THE END OF THE FIBER DISTANT FROM THE LIGHT SOURCE1 IS PROVIDED WITH A REFLECTIVE SURFACE.

Description

-2536535

TRE Rfl OPTICAL FIBER OMET

  The invention relates to an optical fiber thermometer.

  Fiber optic thermometers are known They are

  mainly based on the use of an applied sensor

  to an optical fiber and having a behavior that varies unequivocally when the temperature varies. A comparison between the light intensity at the input of the sensor and the intensity corresponding to its output allows

  to correctly determine the body temperature on the-

which sensor is applied.

  Fiber optic sensors are particularly suitable for

  remote measures in hostile environments

  difficult to access and present on the thermo-

  normal couples the advantage of being built in a material

  dielectric, thus being insensitive to electromagnetic fields

ticks.

  A known optical fiber thermometer (US-PS 4 176 552) comprises two uncoated optical fibers, namely an input fiber and an output fiber, placed in a capillary

  partially filled with a liquid Like the refractive index

  of the liquid is larger than the refractive index of

  the fiber, the light that comes from a source and reaches the ex-

  the end of the corresponding fiber, leaves the termination and couples with the other fiber As the liquid used has a coefficient of expansion which depends on the temperature, the volume and therefore the level of the fluid varies with the temperature to produce a coupling more or less

  between the input fiber and the output fiber In order to increase

  the sensitivity of the instrument, it has also been proposed to use a plurality of input optical fibers and a plurality of output optical fibers, so that the reduction

  space for the liquid inside the capillary pro-

  Vary greater variations in the level, and thus in the coupling between the fibers (for equal volume variations) 2- A disadvantage of this known thermometer is that, its operation being based on level variations, precise positioning ( vertical) from the end of the probe

is necessary.

  Another advantage of this known thermometer is that the limited coupling between the fibers, which only intervenes for the refracted light, leads to a limited sensitivity, and that to increase it, it is necessary to use more

  fibers, which increases the dimensions of the

  Another disadvantage is that

  curvature of the fiber leads to alterations, which reduces

  feel the sensitivity of the instrument, introducing

  errors from e -a way it is used.

  The object of the present invention and to provide a

  optical fiber thermometer that does not present the incon-

  The above-mentioned advantages for the known instruments, which in particular have a wider measuring range, have a high sensitivity but with a single fiber, are more easily miniaturizable, are practically indifferent to the position of the sensor and in general to the how it is used is reliable in operation, even at low power,

is of a low cost.

  All these objectives are achieved by the fiber optic thermometer according to the invention, which comprises a source

  an optical fiber coupled to the source and consis-

  with a sheath covered with a sheath and a sensor

  temperature, coupled with fiber The thermometer is typical

  characterized in that the temperature sensor consists of a portion of optical fiber in which the sheath is replaced by a cover having a refractive index (n 1), 3-

  not less than that of the sheath (n M) throughout the gamma

  me measuring the instrument and variable depending on the temperature, the end of the fiber remote from the source

  illuminated being provided with a reflective surface.

  The invention is illustrated hereinafter with reference to

  FIG. 3 is a diagrammatic view of an optical fiber thermometer according to the invention, is an enlarged longitudinal section through FIG.

  the temperature sensor applied at the end

  of the optical fiber, shows the response curve of the sensor in

  depending on the temperature when the

  the heart of the optical fiber to the sensor is in the form of glycerine, Figure 4 Figure 5

  shows the response curves for different

  rents substances that can form the cover of the heart to the sensor, shows a variant of the temperature sensor in the same view as Figure 2, Figure 6 shows a further variant, Figure 7 shows a different embodiment of the fiber thermometer optical, in a view

corresponding to Figure 1.

  As can be seen in the figures, the thermometer

  according to the invention comprises a light source 1, for example

  example an LED, modulated by a modulator 2 A beam divider 3 is arranged facing the source 1 On the extension

  of the light beam 4 emitted by the LED 1, beyond the divider -

  of radius 3, is disposed an optical fiber 5, at the end of which is applied a temperature sensor generally indicated by 6 The fiber is preferably index jump type, for example with a silica core 7 a

  diameter from 200 to 600 μm, and a plastic sheath 8

  6 is preferably formed of a glass or teflon capillary 9 with a diameter of 1 to 2 mm, in which the end of the fiber 5 is inserted after removal of the sheath.

  8, and in which a material 10 is introduced, preferably

  a liquid (eg glycerin) having a

  refractive index N 1 greater than the refractive index

n M of the sheath 8.

  A reflective surface 11 is disposed at the end.

  output of the core 7 of the fiber 5.

  A reference detector 12, followed by an amplifier 13 and a filter 14 is disposed on the path of the beam

  deviated from the radius divider 3.

  Beyond the radius divider 3-in the opposite direction

  at the detector 12 is another detector 15 followed by an amplifier 16 and a filter 17. The outputs of the two filters 14 and 17 pass into a divider 18 and then

  in a processing and display unit 19.

  The thermometer according to the invention operates as follows: the light beam 4 emitted by the modulated source 1 is focused in the optical fiber 5 and a part is deflected by the beam divider 3 towards the detector

  reference 12 The electrical signal generated by the

  12 is amplified at 13, filtered at 14 and will feed the divider 18 The ray that is sent by the fiber 5 undergoes in the sensor 6 an attenuation which is function

  of the refractive index N 1 of the liquid 10.

  In particular, the light energy transmitted by the fiber 5 reaches the sensor 6 and there, since the refractive index N 1 of the liquid 10 is greater than the refractive index N of the sheath 8 and the angle of acceptance of the fiber to the liquid 10 is accordingly

  smaller part of the light energy is refracted.

  on the outside outside the heart 7, which corresponds to

  The attenuated light beam is reflected at the attenuation of the energy in the passage-through this portion of the core 7 surrounded by the liquid. Upon reaching the reflecting surface 11, the attenuated light ray is reflected and again passes through the portion of the core 7 surrounded by the liquid 10. , where he

  undergoes a new attenuation Like the refractive index

  N 1 of the liquid 10 varies with the temperature, the angle

  acceptance of the fiber 5 and thus the attenuation of the energy

  The luminous radius also varies with the temperature. The reflected ray, attenuated with respect to the rays emitted by

  LED 1, goes back into fiber 5 and, meeting again with

  caliber divider 3 D is deflected towards the

  detector 15 The electrical signal generated by this

  deny is amplified in 16, filtered in 17 and will feed the

  divider 18 In this one, the ratio of the signal of the

  The measurement (detector 12 amplifier 13 filter 14) is taken to cancel the fluctuations of source 1, and the resulting signal is processed and displayed.

in 19.

  FIG. 3 shows the response curve of the sensor 6 as a function of the temperature when the liquid is pure glycerin, which has a refractive index N 1 which

  decreases with increasing temperature.

  The abscissa shows the temperature in O C, and the ordinate shows the voltage measured at the output of the amplifier

  16 in cascade with the detector 15 This voltage is proportional

  the light intensity received by the detector 15.

  As can be seen, the curve decreases until a

  temperature of about 520 C, at which the refractive index

  N 1 of the liquid 10 coincides with the refractive index n of the core. For temperatures between 520 ° C.

  and about 700 C, the curve climbs with the temperature

  It is normally preferable to use an instru-

  within the second temperature range, wherein n ,, <N 1 <nn in that the greater slope of the curve allows a high sensitivity of response, albeit over a limited range of measurement O This range may be enlarged by replacing the sensor 6 with other sensors using different liquids Figure 4 shows by

  example four curves representing the answer for different

  The curve a relates to glycerol

  dilute, curve b to pure glycerine

  corresponds to the curve b of Figure 3, and the curves c and

  d refer to two different types of mineral oil.

  In this way it is possible to cover a very large

  measurement range by simply replacing the probe.

  In some cases, it is also possible to use

  read the instument in a temperature range where the curve decreases when the temperature increases (nm <: nd N), accepting a smaller sensitivity to the profit

a wider range of measures.

  In the variant shown in Figure 5, a foreigner

  in the heart 7 allows a modified response of the sensor 6, thus widening the temperature range that can be covered In this respect, the strangulation of the heart

  7 of the fiber 5 corresponds to two cones which are encountered

  very twice by the light beam (on the exit path before meeting the reflecting surface 11, and on the return path) to each cone corresponds

  a reduction in the acceptance angle of the fiber 5,

  In view of the diagrams of FIG. 3, the presence of the throttle modifies the response curve in the manner shown by the dashed line, having as obvious result

a wider measurement range.

253-6535

  FIG. 6 shows a probe end in which the core 7 of the fiber 5 comprises a single cone, and in this case the reduction in the diameter of the fiber can be used

to miniaturize the sensor 6.

  The diagram in Figure 7 shows a thermometer at

  optical fiber which, in comparison with the thermometer

  diagrammatically in Figure 1, is insensitive to

  nuations resulting from a possible wrinkling in the fiber. In this embodiment, the thermometer comprises two identical light rays 4, 4 'emitted by the modulated source 1, and after passing through two dividers of radius 3, 3', these two rays are focused in two fibers

  optics 5, 5 'which are rigid relative to each other.

  The fiber 5 is similar to the fiber 5 of the thermometer of FIG. 1, while the fiber 5 'does not include a temperature sensor, although having the reflecting surface at its end.

  of reference are separated, but since the fibers are

  With respect to each other, any wrinkling influences both in the same way and thus nullifies the effect

  of this type of error by their report.

  This thermometer according to the invention can be used

  in many applications, such as monitoring

  high-voltage electrical appliances (lines, trans-

  trainers, etc.) or electronic devices (microwave ovens

  waves etc.) It is also suitable for medical applications (hyperthermia), in which, among other things, reduced sizes are required. In this last case in particular, the use of a construction with a miniaturized probe makes it possible to make localized temperature measurements. .

Claims (6)

  1 Fiber optic thermometer comprising a light source (1), an optical fiber (5) coupled to the source and consisting of a core (7) covered with a sheath (8) and a temperature sensor (6), coupled with the fiber (5), characterized in that the temperature sensor (6) consists of a portion of optical fiber (5) in which the sheath (8) is replaced by a cover (10) having a refractive index (n 1) not less than that of the sheath (nm) throughout the measurement range   of the instrument and variable according to the temperature   ture, the end of the fiber remote from the light source   thin (1) being provided with a reflective surface (11).   Thermometer according to claim 1, characterized in that the refractive index (n 1) of the medium forming   a cover (10) is included between the index of   fraction (nm) of sheath (8) and refractive index m (nn) of the heart (7). not   Thermometer according to claims 1 and 2, characterized   characterized in that the medium forming a blanket (10) is a liquid.   Thermometer according to claims 1 and 3, characterized   characterized in that the medium forming the cover (10) is glycerine.   Thermometer according to one of claims 1 to 4,   characterized in that the sensor (6) is placed at the end   the fiber (5) distant from the light source (1).   Thermometer according to claims 1 and 5, characterized   in that the core (7) of the optical fiber (5)   comprises at least one cone in a corresponding position to the sensor (6).   Thermometer according to claims 1 and 5, characterized   characterized in that it comprises two fibers (5,5 ') rigid relative to each other, one of them being provided at its end with a sensor (6) and the other with 'having no sensor.