GB2130719A

GB2130719A - Fibre-optic thermometer

Info

Publication number: GB2130719A
Application number: GB08329203A
Authority: GB
Inventors: Massimo Brenci; Roberto Falcial; Anna Maria Scheggi
Original assignee: Consiglio Nazionale delle Richerche CNR
Current assignee: Consiglio Nazionale delle Richerche CNR
Priority date: 1982-11-18
Filing date: 1983-11-02
Publication date: 1984-06-06
Also published as: JPS59160729A; IT1158799B; DE3341048A1; IT8284155A0; GB8329203D0; GB2130719B; FR2536535A1; FR2536535B1

Abstract

A fibre-optic thermometer comprises a light source (1), an optical fibre (5) coupled with the light source and having a core covered by a cladding, and a temperature sensor (6). The temperature sensor (6) consists of a portion of the optical fibre (5), where the cladding is replaced by a covering having a refractive index (nl) not less than that of the cladding (nm) over a given temperature measurement range and variable as a function of temperature. The end of the fibre remote from the light source (1) is provided with a reflecting surface. <IMAGE>

Description

SPECIFICATION Fibre-optic thermometer This invention relates to fibre-optic thermometers.

Known fibre-optic thermometers are based principaily on the use of a sensor applied to an optical fibre and having a behaviour which varies in a unique manner as the temperature varies.

A comparison between the light intensity at the input of the sensor and the corresponding intensity at its output enables the temperature of a body to which the sensor is applied to be correctly determined.

Fibre-optic thermometers are particularly suitabie for remote measurements in hostile and inaccessible environments, and have the advantage over thermocouples of being constructed of a dielectric material and thus being insensitive to electromagnetic fields.

A known fibre-optic thermometer (U.S. Patent Specification No. 4,176,552) comprises two optical unclad fibres, namely an input and output fibre, placed in a capillary partially filled with a liquid. As the reflective index of the liquid is greater than the refractive index of the fibres, when light originating from a source reaches the end of the input fibre, it leaves the termination and couples with the output fibre. As the liquid used has a coefficient of thermal expansion which depends on temperature, the volume and thus the level of the liquid varies with the temperature to produce a greater or lesser coupling between the input fibre and the output fibre.In order to increase sensitivity, it has also been proposed to use a plurality of input optical fibres and a plurality of output optical fibres, so that a smaller space available for the liquid within the capillary induces greater variations in level (and thus in the coupling between the fibres) for equal volume variations.

One drawback of this known fibre-optic thermometer is that because its operation is based on level variations, precise vertical positioning is necessary. A further drawback is that the limited coupling between the fibres, which takes place only for refracted light, leads to limited sensitivity, and in order to increase sensitivity it is necessary to use a plurality of fibres consequently increasing size. A yet further drawback is that any bending of the fibres leads to attenuation, which further reduces sensitivity, by introducing use related errors.

The present invention seeks to provide a fibreoptic thermometer which does not suffer from these drawbacks and, in particular, has a relatively wide temperature measuring range, has a realtively high sensitivity even when using only one fibre, can be miniaturised more easily, is practically insensitive to sensor position and generally to the method of use, is of reliable operation, even at low power, and is of relatively low cost.

Although the present invention is primarily directed to any novel integer or step, or combination of integers or steps, herein disclosed and/or as shown in the accompanying drawings, nevertheless, according to one particular aspect of the present invention to which, however, the invention is in no way restricted, there is provided a fibre-optic thermometer comprising: a light source; an optical fibre coupled with said source and having a core covered by cladding; and a temperature sensor, coupled with said optical fibre, the temperature sensor consisting of a portion of the optical fibre where the cladding is replaced by a covering having a refractive index not less than that of the cladding over a given temperature measurement range and variable as a function of temperature, the end of the optical fibre remote from the light source being provided with a reflecting surface.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a diagrammatic view of one embodiment of a fibre-optic thermometer according to the present invention; Figure 2 is an enlarged longitudinal section through a temperature sensor at an end of an optical fibre of the fibre-optic thermometer of Figure 1; Figure 3 shows graphically the relationship between sensor response and temperature when an optical fibre core covering the temperature sensor is glycerine; Figure 4 shows graphically response curves for different substances forming the core covering at the temperature sensor; Figure 5 shows a modification of the temperature sensor of Figure 2; Figure 6 shows a further modification of the temperature sensor of Figure 2; and Figure 7 shows another embodiment of a fibreoptic thermometer according to the present invention.

Throughout the drawings like parts have been designated by the same reference numerals.

Referring first to Figure 1, a fibre-optic thermometer according to the present invention comprises a light source 1 , for example an LED, modulated by a modulator 2. A beam splitter 3 is disposed facing the source 1. A part of the light beam 4 emitted by the source 1 passes through the beam splitter 3 and enters an optical fibre 5 having a temperature sensor 6 at its end. The optical fibre 5 is preferably of the step index type, for example with a silica core 7 of diameter 200-600 ,um, and a plastics cladding 8 (Figure 2).The sensor 6 is preferably formed from a glass or teflon capillary tube 9 1-2 mm in diameter, into which the end of the fibre 5 is inserted after removing part of the cladding 8, and into which a material 10, which will be referred to hereinafter as a liquid (e.g. glycerine), having a refractive index n1 greater than the refractive index nm of the cladding 8 is introduced. A reflecting surface 11 is provided at the output end of the core 7 of the optical fibre 5.

A reference channel consisting of a reference detector 12 followed by an amplifier 1 3 and a filter 14 are disposed in the path of the part of the beam 4 divered by the beam splitter 3.

Beyond the beam splitter 3 opposite to the detector 12 there is a measurement channel consisting of a detector 1 5 followed by an amplifier 1 6 and a filter 17. The outputs of the two filters 14, 1 7 pass to a divider circuit 1 8 and then to a processor-display unit 1 9.

The fibre-optic thermometer of Figures 1 and 2 operates in the following manner: a part of the light beam 4 emitted by the source 1 is focused into the optical fibre 5, and a part thereof is diverted by the beam splitter 3 to the reference detector 12. An electrical signal generated by the detector 12 is amplified by the amplifier 13, filtered by the filter 14 and then fed to the divider circuit 18. The part of the light beam which is conveyed by the optical fibre 5 undergoes attenuation in the sensor 6, the attenuation being a function of the refractive index n, of the liquid 10.

In particular, the light energy transmitted by the optical fibre 5 reaches the sensor 6, and here, because the refractive index n1 of the liquid 10 is greater than the refractive index nm of the cladding 8 and because the angle of acceptance of the optical fibre at the liquid 10 is consequently smaller, part of this light energy is refracted outside the core 7, and this corresponds to an energy attenuation in the passage through that portion of the core 7 surrounded by the liquid 10.

On reaching the reflecting surface 11, the attenuated light beam is refelected and again traverses that portion of the core 7 surrounded by the liquid 10, where the light energy undergoes further attenuation. As the refractive index n, of the liquid 10 varies with temperature, the angle of acceptance of the optical fibre 5 and thus the attenuation of the light energy also vary with temperature.The reflected beam, attenuated with respect to the beam emitted by the source 1, retraverses the reserve path in the optical fibre 5, and on encountering the beam splitter 3 is diverted towards the detector 1 5. The electrical signal generated by the detector 1 5 is amplified by the amplifier 16, filtered by the filter 17 and fed to the divider circuit 1 8. In the divider circuit 18, the ratio of the signal from the measurement channel to the signal from the reference channel is determined in order to nullify fluctuations of the source 1, and the resultant signal is processed and displayed by the processor-display unit 1 9.

Figure 3 shows graphically the response of the sensor 6 as a function of temperature when the liquid 10 is pure glycerine, which has a refractive index n, which decreases with increasing temperature.

The abscissa axis shows temperature (in OC), and the ordinate axis shows voltage measured at the output of the amplifier 1 6. This voltage is proportional to the light intensity received by the detector 1 5. As can be seen, the curve decreases until a temperature of about 520C is reached, at which the refractive index nm of the liquid 10 coincides with the refractive index nm of the core 7. For temperatures between 520C and about 700 C, the curve rises with temperature. It is normally preferable to use the instrument within this second temperature range, in which nm < nm < n1, in that the greater slope of the curve allows high response sensitivity to be obtained, although over a limited temperature measurement range.This range can be widened by replacing the sensor 6 with others using different liquids 10.

Figure 4 shows, for example, four curves representing the response for four different liquids.

Curve a relates to dilute glycerine, curve b to pure glycerine and corresponding to curve b of Figure 3, and curves c and d relate to two different types of mineral oil. In this manner it is possible to cover a very wide temperature measurement range by simply replacing the sensor 6.

In some cases it is possible to use the fibreoptic thermometer in a temperature range in which the curve decreases with increasing temperature (nm < nm < n,), by accepting lower sensitivity in favour of wider temperature measurement range.

In a modification of the sensor 6 shown in Figure 5, a constriction in the core 7 enables the response of the sensor 6 to be modified, so widening the temperature measurement range. In this respect, the constriction of the core 7 of the optical fibre 5 corresponds to two tapers which are encountered twice by the light beam (on the outward path before encountering the reflecting surface 11, and on the return path). A reduction in the angle of acceptance of the optical fibre 5 corresponds to each taper, leading to greater attenuation. With reference to Figure 3, the presence of the constriction modifies the response curve in the manner shown by a dashed line e, resulting in an obviously wider temperature measurement range.

Figure 6 shows another modification of the sensor 6 in which the core 7 of the optical fibre 5 comprises a single taper, and in this case the reduction in the fibre diameter can be utilised in order to miniaturise the sensor 6.

Figure 7 illustrates another embodiment of a fibre-optic thermometer according to the present invention which, compared with the fibre-optic thermometer shown in Figure 1, is insensitive to additional attenuation produced by any creasing of the optical fibre.

In this embodiment, the fibre-optic thermometer comprises two identical light beams 4, 4' emitted by the modulated source 1, and after passing through two beam splitters 3, 3' these two beams are focused into two optical fibres 5, 5' which are rigidly connected to each other. The optical fibre 5 is analogous to the fibre 5 of the fibre-optic thermometer of Figure 1 , whereas the optical fibre 5' is without a temperature sensor, although possessing a reflecting surface at its end.

Thus the measurement and reference channels are separate in this fibre-optic thermometer but as the optical fibres are rigid with each other any creasing influences both in the same manner, and thus nullifies the effect on their ratio.

The fibre-optic thermometers according to the present invention and described above can be used in various applications, such as monitoring high voltage electrical apparatus (lines, transformers etc.) or electronic apparatus (microwave ovens etc.). They are also suitable for medical applications (hyperthermia), in which, inter alia, small size is required. In this latter case in particular, using a miniaturised sensor it is possible to make localised temperature measurements.

Claims

1. A fibre-optic thermometer comprising: a light source; an optical fibre coupled with said source and having a core covered by cladding; and a temperature sensor, coupled with said optical fibre, the temperature sensor consisting of a portion of the optical fibre where the cladding is replaced by a covering having a refractive index not less than that of the cladding over a given temperature measurement range and variable as a function of temperature, the end of the optical fibre remote from the light source being provided with a reflecting surface.

2. A thermometer as claimed in claim 1 in which the refractive index of the covering lies between the refractive index of the cladding and the refractive index of the core.

3. A thermometer as claimed in claim 1 or 2 in which the covering is a liquid.

4. A thermometer as claimed in claim 3 in which the covering is glycerine.

5. A thermometer as claimed in any preceding claim in which the sensor is applied to the end of the optical fibre remote from the light source.

6. A thermometer as claimed in any preceding claim in which the core has at least one taper in a position corresponding to the sensor.

7. A thermometer as claimed in any preceding claim including a further optical fibre rigidly connected to the first-mentioned optical fibre and provided at its end remote from the light source with a reflecting surface.

8. A fibre-optic thermometer substantially as herein described with reference to and as shown in the accompanying drawings.

9. Any novel integer or step, or combination of integers or steps, hereinbefore described, irrespective of whether the present claim is within the scope of, or relates to the same or a different invention from that of, the preceding claims.