GB2204948A

GB2204948A - Temperature sensor

Info

Publication number: GB2204948A
Application number: GB08811119A
Authority: GB
Inventors: Meinolph Kaufmann; Lothar Schultheis
Original assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1987-05-14
Filing date: 1988-05-11
Publication date: 1988-11-23
Also published as: GB8811119D0; DE3815260A1; JPS6459020A

Description

DESCRIPTION..Temperature Measuring Instrument BACKGROUND OF THE INVENTION

871045 220 49 4 8 FieLd of the invention

The invention retates to a temperature measuring instrument with a Light source with a given spectraL bandwidth, at Least one gLass fiber, a FabryPerot irkterference etement which has a given inner medium and a temperature-dependent transmission and reftection behavior, and a Light detector. Discussion of Background

There are a Large number of processes and corresponding instruments avaiLabLe today for measu ring a temperature. The seLection of a particutar process and in- strument is determined by the demands of the appLication.

So-caLled fiberoptic temperature measuring instruments have gained great importance because they are insensitive to etectricaL and magnetic interference, are reLativeLy versatiLe and compact. A common principLe of such measuring instruments is based on the temperature dependency of a parameter, such as for exampLe the thickness of an interference Layer, being measured.

For this the great sensitivity of an interferometric measurement can be utiLized. A Fabry-Perot interfer- ence etement presents itseLf as a possibLe soLution in this context.

For exampLe. an arrangement for measuring a tempeCbture by mea"ns of a Fabry-Perot interference eLement is known from the pubtication "Fiber optic coLour sensors based on Fabry-Perot-Interferometry" by E. R. Cox et at. IEE 221, 122, 1983. In. this arrangement,, shifts of a pass frequency are defined which are caused by a temperaturedependent change of the thickness of the Fabry- Perot interference element. To change the thickness, from the point of view of a good temperature resolution, a material with a Large expansion coefficient should be used. Amongst other things, the publication presents an arrangement in which the space of the Fabry-Perot inter- ference element, which ties between two boundary surfaces acting as reflectors, is fitted with a polymer, the temperature expansion of which causes the thickness of the Fabry-Perot interference element to change.

The disadvantage of such an arrangement is its expensive construction. In particular, the reflector lenses used are expensive and must be mounted very pre cisely. A further disadvantage is the measurement of the frequency shift which entails considerable effort. - SUMMARY OF THE INVENTION

Accordingly, one object of this invention is to provide a novel temperature measuring instrument with a Light source, at Least one glass fiber, a Fabry-Perot interference element which has a given inner medium and a temperature-dependent transmission and reftection behavior, and a Light detector, which temperature measuring instrument is of simple and compact design.

The object is achieved according to the invention in that the inner medium has a Large refrective index in relation to an outer medium surrounding the Fabry-Perot 1 interference eLement.

This resuLts in the advantage that the Fabry-Perot interference eLement has an additionaL coLLimating effect on the incident Light, so that it is not necessary to pLace Lenses between the gLass fibers and the Fabry-Perot interference eLement.

A particuLarLy preferred embodiment of the temperature measuring instrument according to the invention empLays a semiconductor materiaL as the inner medium.

Besides a reLativeLy Large refractive index, semiconductor materiaLs have an aLmost Linear dependency between Ahe refractive index change and the temperature. Moreover, as inorganic materiaLs, they have a reLativeLy high temperature stabiLity and thus permit a Large temperature measurement range.

Further preferred embodiments of the invention are obtained from the subcLaims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more compLete appreciation of the invention and many of the attendant advantages thereof wiLL be readiLy obtained as the same becomes better understood by reference to the foLLowing detaiLed description when considered in connection with the accompanying drawings, wherein:

Fig. 1 shows a temperature measuring instrument, where the Light transmitted by the Fabry-Perot inter ference eLement is measured and Fig. 2 shows a section of such a temperature measuring instrument, where the Fabry-Perot interference 11 is - 4 eLement is mounted on an end face of the qLass fiber, Fig. 3 shows a schematic representation of the way the Fabry-Perot interence eLement according to the invention works, Fig. 4 shows a temperature measuring instrument, where the Light reftected by the Fabry-Perot interference eLement is measured and Fig. 5 shows a section of such a temperature measuring instrument where the Fabry-Perot interference eLement is mounted on an end face of a gLass fiber and Fig. 6 shows a temperature measuring instrument, in which the Fabry-Perot interference eLement consists of three separated Layers on an end face of a gLass fiber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein Like reference numeraLs designate identicaL or corresponding parts throughout the severaL views, Figure I shows a schematic view of a temperature measuring instrument according to the invention. It consists of a Light source 1, a first gLass fiber 3a, a Fabry-Perot interference eLement 4, a second gLass fiber 3b and a Light detector 2.

Fig. 2 shows a section of Fig. 1. The Fabry-Perot interference eLement 4 is Located between an end face Sa of the first fiber 3a and an end face 5b of the second gLass fiber 3b. It has the orm of a pLate, the voLume of which is fiLLed with an inner medium 6. The pLate is 9 j - 5 surrounded by an outer medium 11. The boundary surfaces between inner medium 6 and outer medium 11 act as reflectors. If necessary they can in addition be covered with refLecting Layers in order to increase the moduLation depth of the transmission behavior. The inner medium 6 shouLd have a high refractive index in reLation to the outer medium 11. The inner medium 6 is preferabLy a semiconductor materiaL.

1 It is known that for semiconductor materiaLs, the band gap shifts aLmost LinearLy with the temperature. This in turn resuLts in a change of the refractive index. TypicaL refractive indices n and refractive index changes An/AT are for exampLe.

Se-miconductor n &n/AT El/K3 Si 3.51 1.4. 10 -4 GaAs 3.47 1.5. 10 -4 Ge 4.13 2.8. 10 -4 Which semiconductor materiaL can be used as inner medium 6 depends on the frequency of the irradiated Light. The band gap of the semiconductor materiaL shouLd aLways be greater than the frequency of the Light over the whoLe desired temperature range of the temperature.

measuring instrument to ensure that the Light of the Light source I is not absorbed by the inner medium 6.

For exampLe, semiconductor materiaLs such as GaP and ALAs can be used with a GaAs/ALGaAs Laser, the wave Length of which Lies typicaLLy between 750 and 850 nm, and semiconductor materiaLs GaAs, Si, Ge and InP can be used with an InGaAsP Laser the waveLength of which is 01 -6 approximately 1.3 um.

A pLate from a semiconductor crystaL, for exampLe, may be used as the Fabry-Perot interference eLement 4. The Fabry-Perot interference element 4 is gLued to the end faces 5a, 5b of the first and second glass fibers 3a, 3b with a suitable compound material 7. Furthermore, the Fabry-Perot interference element 4 is surrounded compLeteLy by this compound materiaL, which then acts simuLtaneousLy as the outer medium 11. PoLyimide, for example, can be used as the compound material. In order to protect the Fabry-Perot interference element 4 from scattered Light, the outer medium 11 is covered with the protective Layer B. Black varnish, for example, is weLL suited for the protective Layer B. 15 The Light source 1 has a small spectraL bandwidth, preferabLy one of Less than 30 GHz. Suitable Light sources are Lasers, in particular semiconductor Lasers, and Lightemitting diodes with a subsequent narrow-band filter. Filtered thermal Light sources are also conceivable. 20 The small spectral bandwidth permits the measurement of the Light to be Limited to the measurement of its intensity in order to determine the temperature of the inner medium 6. The requirements which the spectral bandwidth of the Light source 1 must fulfill can be determined as follows: It is known that the transmission behavior for a given thickness of the Fabry-Perot interference element, a given refractive index of the inner medium and a given X 1 7 angLe of arrivaL is a periodic function of the frequency. Frequencies at which this function assumes a reLative maximum are termed pass frequencies. The spectraL bandwidth of Light source I shouLd therefore be such that it is smaLL in reLation to a bandwidth defined by the distance between two neighboring pass frequencies, i.e. in other words the spectrat bandwidth of Light source 1 is smaLt in reLation to the spectraL period of the transmission behavior. This ensures that the intensity maxima and minima of the transmission behavior contrast weLL.

The intensity can be measured in this case with., for exampLe. a simpLe photodiode. This eLegantLy avoids a compLicated, spectraL measurement of the L ight transmitted or reftected by the Fabry-Perot interference eLement 4.

The functioning of the temperature measuring instrument described above as an exempLary embodiment wiLL be expLained briefLy beLow.

The Light of the Light source I is Launched into the gLass fiber 3a and Led to the Location where a temperature is to be measured. The Light output from the end face 5a of the gtass fiber 3a is transmitted entirety or partiatLy by the Fabry-Perot interference etement 4 and Launched into the gLass fiber 3b. At the end of the gLass fiber 3b, it is absorbed by the Light detector 2. If required, evatuation eLectronics which assign the intensity vaLue measured to a corresponding temperature vaLue may be connected downstream of the Light detector 2.

The effect of the high refractive index of the inner medium 6 according to the invention can be described with the aid of the observation below.

Fig. 3 shows a schematic illustration of the FabryPerot interference element 4. A Light source 1 and a Light detector 2 are indicated in outline. The FabryPerot interference element 4 has an inner medium 6 with a refractive index n 1 and a thickness d and is surrounded by an outer medium 11 with a refractive index n 0.

A plane Light source of the frequency w arrives at an angle (1 0 of incidence, is refracted at a boundary surface 12 of the Fabry-Perot interference element 4 and penetrates at the angle cL 1 the inner medium 6.

The transmission of the Fabry-Perot interference 15 element 4, which can be written in the known form T (1 - r2) 2 r 2) 2 + 4r 2 sin 2 (X) 2 r = Reflectiveness of the boundary surface 12 is maximum, if a phase y, defined by (I) y = 2 M nj dcos(I1 c c velocity of w frequency of nj refractive i d thickness of Light in vacuo the Light ndex of the inner medium 6 the Fabry-Perot interference element 4 cil = angle of the light in the inner medium 6 futfitts the condition (I1) Y = 2m ir 1 - 9 m = positive integer (1, 2,...) If the arriving Light is not a pLane Light wave with a fixed angLe oo of incidence, but has, as is unavoidabLe in practice, a certain divergence, then the transmission maxima are bLurred in an undesirabLe manner.

The negative effect of the beam divergence can be aLmost compLeteLy eLiminated by means of the invention. For verticaLLy incident Light with a beam divergence cto, using equation M and the ruLe of refraction, a maximum phase difference 6 2 N d n n sin CL -n C 0 C, 11 no = refractive Gdex of the outer medium 11. cto = beam divergence is obtained.

The greater nj is in reLation to no, the smaLLer the undesired phase shift 8 caused by the beam divergence ao and resuLting in a bturring of the transmission maxima. The divergence of the Light output from a gLass fiber is chiefLy determined by the numeric aperture of the gtass fiber. On the other hand, the maximum toLerabLe beam divergence is in accordance with equation (III), dependent among other things on the thickness cl of the Fabry-Perot interference eLement: the greater the thickness d. the smaLLer the beam divergence 0 must be, and hence aLso the numeric aperture. if the thickness cl is, for exampte, 100 lim or more, then it is advantageous to use a monomode fiber as gLass fiber 3b. For thicknesses under 10 Um, a gLass fiber with a numeric aperture of approximateLy 0.2 is sufficient.

Due to the temperature-dependent refractive index change, phase y aLso changes and hence the transmission of the Fabry-Perot interference eLement. The greater the thickness d, the more a refractive index change affects the phase.

In principLe this resuLts in two measuring processes.

According to a first process, the thickness d of the Fabry-Perot interference eLement 4 is seLected so that the vaLues of the phase y, which correspond in a given temperature range to the maximum and minimum refractive index, aLL Lie in an intervaL which is bounded by two neighboring muLtipLes of ir. This means that a defined intensity of the Light transmitted or reftected by the Fabry-Perot interference eLement 4 corresponds exactLy to one temperature vaLue. In a measuring process of this kind, thicknesses of Less than 10 Um are preferabLy used.

According to a second process, the thickness d of the Fabry-Perot interference eLement 4 is seLected so that in a given temperature range the maximum and minimum vaLues of the phase y define an intervaL which is much greater than w. A temperature vaLue is determined in this case by counting the maxima of the transmission behavior passed through during a temperature change. The thickness d of the Fabry-Perot interference eLement 4 is preferabLy more than 100 jim.

A Fabry-Perot interference eLement 4, as is used in the exempLary embodiment described above, can be 9 1, 1 - 1 1 - manufactured in a simpLe manner by known means. For exampLe, a commerciaLty avaitabLe wafer can be tapped on a desired thickness and subsequentLy spLit into, for exampLe, 200. 200 jim 2 smaLL pLates. It shouLd be noted 5 that, as the thickness of the Fabry-Perot interference eLement 4 increases, the greater the requirement for the boundary surfaces 12 to be paraLLeL.

Fig. 6 shows a particuLarLy preferred embodiment of a temperature measuring instrument according to the invention. SpecificaLLy it reLates to a particuLarLy simpLe, compact and robust embodiment of the Fabry-Perot interference eLement 4.

For exampLe, a succession of three Layers 10a, 10b 10c are a.ppLied to an end face 5c of a gLass fiber 3d.

One Layer 10b acts as the inner medium of the Fabry-Perot interference eLement 4. Its thickness corresponds to the thickness of the Fabry-Perot interference eLement 4. The Layers 10a and 10c are refLecting Layers. They increase in a known manner the moduLation depth of the transmission behavior. ParticuLarLy suitabLe for this are Layers of metaL, e.g. siLver or chromium. They may be manufactured with known processes (e.g. by vapor or sputter deposition).

Here aLso semiconductor materiaLs are preferabLy used for the layer lOb-which acts as an inner medium. By vapor deposition of, for exampLe. Si, a poLycrystaLLine Layer 10b of a thickness of approximateLy 2 - 5 ttm is obtained. AccordingLy, the measuring process described above which works with thicknesses under 10 um is - 12 used to determine the temperature.

A succession of three Layers relates to only one special exemplary embodiment. Only the second Layer 10b of the above exemplary embodiment is essential for the invention. The other Layers 10a, 10c may optionally be omitted; or they themselves may also be built up from several subLayers.

It should be noted that the outer medium in generaL is not to be seen as something which is to be mounted on the Fabry-Perot interference element in the form of a material specially provided for this purpose. In the case of vapor-deposited Layers, for example, the core of the glass fiber acts as the outer medium.

Fig. 4 and Fig. 5 show a further preferred embodi- ment of the invention. This utilizes the fact that the reflected intensity is a complement of the transmitted intensity.

Fig. 4 shows a Light source 1 and a glass fiber 3b, a Light detector 2 and a glass fiber 3a, a glass fiber 3c at one end of which a Fabry-Perot interference element 4 is Located, and a directionaL coupler 9.

Fig. 5 shows a section of Fig. 4. As in an exempLary embodiment described previously, the Fabry-Perot interference element 4 is mounted on an end face 5c of the glass fiber 3d and entirely surrounded by an outer medium 11. The outer medium 11 acts simultaneously as a compound material between the glass fiber 3d and the Fabry-Perot interference element 4 and may be, for example, poLyimide. Of particular importance here is the screening against 11 1 1 11 scattered Light, for example by means of a protective Layer 8 of black varnish covering the outer medium 11. In order that as much reflected Light as possible is Launched into the glass fiber 3. the Fabry-Perot interference element 4 is arranged parallel to the end face 5c of the glass fiber 3d.

The above-described embodiment of the Fabry-Perot interference element 4 can be used preferably with all the characteristics described further above in conjunction with the transmission arrangement (Fig. 2). Special mention should be made of the embodiment in which the FabryPerot interference element consists of at Least one Layer applied to an end face of a glass fiber.

The temperature r.ange of a temperature measuring instrument according to the invention is dependent on the thermal stability of the materiaLs which the Fabry-Perot interference element and any adhesives or intermediate Layers used consist of. If the Fabry-Perot interference element consists of a semiconductor material, then very high temperatures can be measured if, for exampLe, Si is used. The primary Limiting factor in this case will be the thermal stability of the materials of the glass fiber.

In conclusion it can be said that a temperature measuring instrument according to the invention has,a simple and compact design and is insensitive to eLectricaL and magnetic interference.

Obviously, numerous modifications and variations of the present invention are possible in Light of the above teachings. It is therefore to be understood that within the scope of the appended cLaims, the invention may be practiced otherwise than as specificaLty described herein.

LIST OF TERMS 1 2 3a, 3b, 3c, 3d, 3e 4 Sa, 5b, Sc 6 7 8 9 10a, 10b, 10c 11 12 Light source Light detector GLass fibers Fabry-Perot interference eLement End face inner medium Compound materiaL Protective Layer DirectionaL coupter Layer outer medium Boundary surface

Claims

1.

1 - is - A temperature measuring instrument with a) a Light source (1) with a given spectraL bandwidth, b) at Least one gLass fiber (3a, 3b, 3d) c) a Fabry-Perot interference eLement (4) which has a given inner medium (6) and a temperaturedependent transmission and refLection behavior, and d) a Light detector (2).

wherein e) the inner medium (6) has a Large refractive index in retation to an outer medium (11) surrounding the Fabry-Perot interference eLement (4).

A temperature measuring instrument as ctaimed in ctaim 1, wherein the inner medium (6) is a semiconductor materiaL.

3. A temperature measuring instrument as cLaimed in cLaim 2, wherein a) the spectraL bandwidth of the Light source (11 is smaLL in reLation to a spectraL period of the transmission behavior, in particuLar wherein it is smatter than 30 GHz, wherein b) the Light detector (2) measures Light intensity transmitted or refLected by the FabryPerot interference eLement (4), and wherein c) the Fabry-Perot interference eLement is mounted 1 - 16 4.

on at Least one end face (5a, 5b, 50 of a gLass fiber (3a, 3b, 3d).

A temperature measuring instrument as cLaimed in cLaim 3, wherein a) the Light source (1) is a GaAs/ALGaAs Laser and b) the semiconductor materiat of the Fabry-Perot interference eLement (4) is GaP or ALAs.

A temperature measuring instrument as cLaimed in cLain 3, wherein a) the Light source (1) is an InGaAsP Laser and b) the semiconductor materiaL of the Fabry-Perot interference etement (4) is GaAs, Si, Ge or InP.

6. A temperature measuring instrument as cLaimed in either cLaim 4 or 5, wherein the Fabry-Perot interference etenent (4) has a thickness (d) at which in a given temperature range exactLy one temperature vaLue corresponds to a vaLue of a measured Light intensity, in particuLar wherein the Fabry-Perot interference eLement (4) has a thickness (d) of Less than 10 lim. 7. A temperature measuring instrument as cLaimed in either ctaim 4 or 5, wherein the Fabry-Perot interference eLement (4) has a thickness (d) at which in a given temperature range the transmission behavior exhibits a Large number of maxima so that a temperature vaLue is determined by the number of maxima Lying in one temperature intervaL, in particuLar wherein the Fabry-Perot interference eLement (4) has a thickness (d) of more than 100 tim.

S. A temperature measuring instrument as cLaimed in i v 41 either ct a) aim 6_or 7, wherein the Fabry-Perot i nterference eLeinent (4) is a singLe-crystaL pLate, wherein b) the singLe-crystaL pLate is mounted on at Least one end face (5a, 5b, 50 of at Least one gLass fiber (3a, 3b, 3d) with a compound materiaL (7), wherein C) the compound inateriaL (7) is poLyimide, wherein d) the outer medium (11) is poLyimide and the outer medium (11) is protected from scattered Light by a protective Layer (8), in particuLar by a bLack Layer of varnish.

9. A temperature measuring instrument as cLaimed in either cLaim 6 or 7, wherein the Fabry-Perot interference eLement (4) consists of at Least one Layer (10b) appLied to an end face (5b) of the gLass fiber (3d). 10. A temperature measuring instrument as cLaimed in either cLaim 9-, wherein the Fabry-Perot interference eLement (4) consists of a succession of three Layers, wherein the first Layer (10a) is a metaL Layer, the second Layer (10b) is a semiconductor Layer and the third Layer (100' is another metaL Layer. 11. A temperature measuring instrument as claimed in claim 1, substantially as described with reference to any Fi ure or Figures of the accompanying drawings.

Published 1988 at The Patent Office, State House, 86/71 High Holborn, London W01R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1/87.