CA2064309A1 - Optical fibre incorporating a bragg lattice and its applications - Google Patents
Optical fibre incorporating a bragg lattice and its applicationsInfo
- Publication number
- CA2064309A1 CA2064309A1 CA002064309A CA2064309A CA2064309A1 CA 2064309 A1 CA2064309 A1 CA 2064309A1 CA 002064309 A CA002064309 A CA 002064309A CA 2064309 A CA2064309 A CA 2064309A CA 2064309 A1 CA2064309 A1 CA 2064309A1
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- Prior art keywords
- fibre
- optical
- axis
- strata
- optical system
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
- G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
- G02B6/29317—Light guides of the optical fibre type
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
- G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
- G02B6/29316—Light guides comprising a diffractive element, e.g. grating in or on the light guide such that diffracted light is confined in the light guide
- G02B6/29323—Coupling to or out of the diffractive element through the lateral surface of the light guide
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/34—Optical coupling means utilising prism or grating
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/42—Coupling light guides with opto-electronic elements
- G02B6/4201—Packages, e.g. shape, construction, internal or external details
- G02B6/4204—Packages, e.g. shape, construction, internal or external details the coupling comprising intermediate optical elements, e.g. lenses, holograms
- G02B6/4206—Optical features
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
- Investigating Or Analysing Materials By Optical Means (AREA)
- Optical Transform (AREA)
- Light Guides In General And Applications Therefor (AREA)
Abstract
ABSTRACT
The invention relates to an optical fibre (1) including, over a part of its length, variations of refractive index which are disposed in plane, parallel and periodic strata (5), of period I, which are inclined at an angle .alpha. in relation to the axis (4) of the fibre. The normal to the strata forms a non-zero angle with the axis (4) of the fibre (1).
It also relates to the application of this optical fibre as a selective extractor of flux, a light injector, a coupler, a sensor and a polariser.
The invention relates to an optical fibre (1) including, over a part of its length, variations of refractive index which are disposed in plane, parallel and periodic strata (5), of period I, which are inclined at an angle .alpha. in relation to the axis (4) of the fibre. The normal to the strata forms a non-zero angle with the axis (4) of the fibre (1).
It also relates to the application of this optical fibre as a selective extractor of flux, a light injector, a coupler, a sensor and a polariser.
Description
20~Q3~
The invention relates to an optical device comprising at least one optical fibre having a Bragg ].attice or grating and its applications as selective flux extractor, light injector, fibre optic coupler, fibre-optic sensor and fibre-optic polariser.
In order to avoid any ambiguity it is stated that the term "optical fibre" includes any light~conducting optical component in the form of a filament comprising a part forming a waveguide and referred to as the "core", at least partially surrounded by a sheath, it being possible for the core to extend coaxially with 'he sheath or to be offset or out of line, and it being possible for the sections of this core and o~ this sheath independently to be both circular and parallelepipedal, or of other shapes (for example in the form of a truncated circle, as in the patent US-A-4,867,522).
In the text which follows, reference will nevertheless essentially be made, for the sake of clarity, to substantially cylindrical optical fibres having a core and a sheath which are both of circular cross-section, extending concentrically, in the axis of the fibre under consideration.
Progress in optical fibres and in their applications has given rise to the development of components based on fibres fulfilling the same functions or at least functions which are analogous to those customarily fulfilled by conventional optical components.
Other fibre-optic components are more specific, and it is difficult to find their counterparts in conventional optics.
The invention relates to an optical device comprising at least one optical fibre having a Bragg ].attice or grating and its applications as selective flux extractor, light injector, fibre optic coupler, fibre-optic sensor and fibre-optic polariser.
In order to avoid any ambiguity it is stated that the term "optical fibre" includes any light~conducting optical component in the form of a filament comprising a part forming a waveguide and referred to as the "core", at least partially surrounded by a sheath, it being possible for the core to extend coaxially with 'he sheath or to be offset or out of line, and it being possible for the sections of this core and o~ this sheath independently to be both circular and parallelepipedal, or of other shapes (for example in the form of a truncated circle, as in the patent US-A-4,867,522).
In the text which follows, reference will nevertheless essentially be made, for the sake of clarity, to substantially cylindrical optical fibres having a core and a sheath which are both of circular cross-section, extending concentrically, in the axis of the fibre under consideration.
Progress in optical fibres and in their applications has given rise to the development of components based on fibres fulfilling the same functions or at least functions which are analogous to those customarily fulfilled by conventional optical components.
Other fibre-optic components are more specific, and it is difficult to find their counterparts in conventional optics.
2~30~
For this reason, optical fibres having a Bragg lattice have been developed.
Such fibres are described in the American patents US-A-4,725,110, US-A-4,806,012 and US-A-4,807,950.
According to the technique described in these documents, a fibre, of glass or of silica doped with germanium, is treated in such a manner as to constitute within the fibre a Bragg grating which is permanent or definitively modified.
This lattice is composed of plane and parallel strata of constant index. The index varies from one stratum to the next, and its variations are periodic, of period I, the strata accordingly not being in this case ephemeral, and th~ls not disappearing when the fibre is no longer illuminated.
It is shown, in these documents, that the luminous flux reflected by such a Bragg lattice has characteristics which are a function of its period.
The period is determined by the conditions of manufacture of the Bragg lattice.
In order to be permanent, this Bragg lattice is constructed by subjecting an optical fibre to the interference lattice produced by the intersection of two ultraviolet beams originating, in the particular case concerned, from one and the same source, the high intensity of the light waves applied and their application conditions (wavelength, angles of propagation, etc.) permitting the creation of a fibre core exhibiting a refractive index which is definitively modified by impressing thereon or by etching thereon (at least) one Bragg lattice in o ~
strata, almost irreversible periodic perturbations of the refractive index then being formed in the material constituting the core of this fibre.
The various strata of the fibre are subjected to variable illuminations which involve variable modifications of index. As the distribution of the intensity within the interference fringes is sinusoidal, of period I, they produce variations of index of the same period within the fibre.
The period of the Bragg grating is accordingly determined by the conditions of production of the interference (angle, wavelength of the beams, etc.).
It is emphasised, in these documents of the prior art, that th2 fibres are especially intended to be utilised as a strain gauge.
The object of the present invention is the design of a novel optical component which is simple and capable of very widely varying applications.
According to the invention, the normal to the strata in question of the fibre will form a non-zero angle with the axis thereof, thus in contrast to the strata of the fibres of the prior art presented hereinabove, in the case of which the normal is parallel to the axis.
Preferably, said strata will in fact be inclined in relation to the axis of the fibre.
According to a preferred embodiment, the fibre will be monomode. Nevertheless, it can also be multimode.
2~3~
Preferably the (each) fibre will include permanent strata forming the Bragg grating. But said strata could also be temporary or ephemeral, the strata appearing as long as the fibre is illuminated by an optical flux and disappearing when the fibre is no longer illuminated. Optical devices having a fibre with temporary strata are disclosed in US-A- 4 867 522 and in FR-91 03592 (of March 23, 1991) included by reference.
The variations of the refractive index of the fibre will preferably be sinusoidal in the direction if its axis.
Furthermore, according to another important fsature, the optical device of the invention will preferably comprise an optical system having an inner reflecting surface, advantageously concave or with a convergent action, suitable for surroun~ing at least partially said fibre, especially where said fibre is provided with said strata. Preferably said reflecting optical system will b~ cylindrical, the fibre(s) being then advantageously, at the part of its (their) length where it (they) comprise(s) said variations of index, situated at a focus of the optical system.
Such features were until the present time very difficult to obtain. By virtue of them, it will be possible, as a function of the orientation of the strata, to extract or to inject a light flux into the fibre~ Furthermore, this will be capable of being utilised for numerous applications.
Moreover, it is for this reason that the subject of the invention is also the application of such an optical fibre especially as a selective extractor of flux at a given wavelength 29~Q~
(~9), as light injector, as coupler of two optical fibres, each one being of the aforementioned type, as sensor and as polariser.
In the "coupler" application, said coupler may especially include a cylindrical optical system having an elliptical cross-section, each one of the fibres then being preferably situated,in that part of its length where it includes variations of index, at one of the foci of this optical system.
In another embodiment of such a coupler, the fibres may be in contact with one another in that part of their length where they include variations of refractive index. Furthermore, the angle ~1 of inclination of the strata of the first fibre will preferably be equal to the angle ~2 f inclination of the strata of the second fibre, the fibres being oriented about their axis in such a manner that the flux emerging from the first fibre enters the second. In the "sensor" application ~wavelength ~8) ~
which sensor is intended for the measurement of a parameter P, this sensor will advantageously include a reflecting cylindrical optical system having a circular arc or a circular cross-section, surrounding at least one optical fibre of the afore-mentioned type.
The relative positions of the optical system and of the fibre will be in each case a function of the parameter P, at least one of these positions preferably being such that the optical fibre will be situated at the centre of the optical system, in that part of its length where it includes variations of index.
For this reason, optical fibres having a Bragg lattice have been developed.
Such fibres are described in the American patents US-A-4,725,110, US-A-4,806,012 and US-A-4,807,950.
According to the technique described in these documents, a fibre, of glass or of silica doped with germanium, is treated in such a manner as to constitute within the fibre a Bragg grating which is permanent or definitively modified.
This lattice is composed of plane and parallel strata of constant index. The index varies from one stratum to the next, and its variations are periodic, of period I, the strata accordingly not being in this case ephemeral, and th~ls not disappearing when the fibre is no longer illuminated.
It is shown, in these documents, that the luminous flux reflected by such a Bragg lattice has characteristics which are a function of its period.
The period is determined by the conditions of manufacture of the Bragg lattice.
In order to be permanent, this Bragg lattice is constructed by subjecting an optical fibre to the interference lattice produced by the intersection of two ultraviolet beams originating, in the particular case concerned, from one and the same source, the high intensity of the light waves applied and their application conditions (wavelength, angles of propagation, etc.) permitting the creation of a fibre core exhibiting a refractive index which is definitively modified by impressing thereon or by etching thereon (at least) one Bragg lattice in o ~
strata, almost irreversible periodic perturbations of the refractive index then being formed in the material constituting the core of this fibre.
The various strata of the fibre are subjected to variable illuminations which involve variable modifications of index. As the distribution of the intensity within the interference fringes is sinusoidal, of period I, they produce variations of index of the same period within the fibre.
The period of the Bragg grating is accordingly determined by the conditions of production of the interference (angle, wavelength of the beams, etc.).
It is emphasised, in these documents of the prior art, that th2 fibres are especially intended to be utilised as a strain gauge.
The object of the present invention is the design of a novel optical component which is simple and capable of very widely varying applications.
According to the invention, the normal to the strata in question of the fibre will form a non-zero angle with the axis thereof, thus in contrast to the strata of the fibres of the prior art presented hereinabove, in the case of which the normal is parallel to the axis.
Preferably, said strata will in fact be inclined in relation to the axis of the fibre.
According to a preferred embodiment, the fibre will be monomode. Nevertheless, it can also be multimode.
2~3~
Preferably the (each) fibre will include permanent strata forming the Bragg grating. But said strata could also be temporary or ephemeral, the strata appearing as long as the fibre is illuminated by an optical flux and disappearing when the fibre is no longer illuminated. Optical devices having a fibre with temporary strata are disclosed in US-A- 4 867 522 and in FR-91 03592 (of March 23, 1991) included by reference.
The variations of the refractive index of the fibre will preferably be sinusoidal in the direction if its axis.
Furthermore, according to another important fsature, the optical device of the invention will preferably comprise an optical system having an inner reflecting surface, advantageously concave or with a convergent action, suitable for surroun~ing at least partially said fibre, especially where said fibre is provided with said strata. Preferably said reflecting optical system will b~ cylindrical, the fibre(s) being then advantageously, at the part of its (their) length where it (they) comprise(s) said variations of index, situated at a focus of the optical system.
Such features were until the present time very difficult to obtain. By virtue of them, it will be possible, as a function of the orientation of the strata, to extract or to inject a light flux into the fibre~ Furthermore, this will be capable of being utilised for numerous applications.
Moreover, it is for this reason that the subject of the invention is also the application of such an optical fibre especially as a selective extractor of flux at a given wavelength 29~Q~
(~9), as light injector, as coupler of two optical fibres, each one being of the aforementioned type, as sensor and as polariser.
In the "coupler" application, said coupler may especially include a cylindrical optical system having an elliptical cross-section, each one of the fibres then being preferably situated,in that part of its length where it includes variations of index, at one of the foci of this optical system.
In another embodiment of such a coupler, the fibres may be in contact with one another in that part of their length where they include variations of refractive index. Furthermore, the angle ~1 of inclination of the strata of the first fibre will preferably be equal to the angle ~2 f inclination of the strata of the second fibre, the fibres being oriented about their axis in such a manner that the flux emerging from the first fibre enters the second. In the "sensor" application ~wavelength ~8) ~
which sensor is intended for the measurement of a parameter P, this sensor will advantageously include a reflecting cylindrical optical system having a circular arc or a circular cross-section, surrounding at least one optical fibre of the afore-mentioned type.
The relative positions of the optical system and of the fibre will be in each case a function of the parameter P, at least one of these positions preferably being such that the optical fibre will be situated at the centre of the optical system, in that part of its length where it includes variations of index.
3 ~ ~
The invention will be described in detail with reference to the Figures, in which:
Figure 1 is a diagrammatic representation of the fibre of the invention.
Figure 2 is the representation of an extractor according to the invention.
Figure 3 is the diagrammatic representation of a fibre-optic injector according to the invention.
Figure 4 is the representation of a fibre-optic coupler according to the invention in a first embodiment.
Figure 5 is the representation of a fibre-optic coupler according to the invention, in a second embodiment.
Figures 6A and 6B are diagrams illustrating the operation of a fibre according to the invention, placed at the centre of a catadioptric optical system.
Figures 7A and 7B are the representation of a fibre-optic sensor according to the invention, in a first embodiment.
Figures 8A and 8B are a representation of a fibre-optic sensor according to the invention, in a second embsdiment.
Figure 9 is a representation of a sensor according to the invention, in a third embodiment.
~igure 10 is the representation of a fibre-optic sensor according to the invention, in a fourth embodiment.
Figure 11 is the representation of a fibre-optic sensor according to the invention, in a fifth embodiment.
Figure 12 is the representation of a fibre-optic polariser according to the invention.
2~3~9 The subject of the invention concerns an optical fibre 1.
Conventionally, the latter has a sheath 2, a core 3, and an axis 4. According to the invention, this optical fibre exhibits variations of refractive index which are disposed in plane, parallel and periodic strata 5, of period I in the direction of the axis. The plane strata 5 are inclined at an angle ~ in relation to the axis 4 of the fibre. Moreover, they are preferably permanently etched or impressed in the core andtor in the sheath.
Two arrangements may be created.
In the first, the optical fibre 1 includes a Bragg diffraction grating 6, in which the strata are inclined in relation to its axis 4 at an angle different from 90.
In the second situation, the optical fibre 1 includes a Bragg diffraction grating 6, in which the strata are parallel to its axis 4, that is to say that the normal to the strata is perpendicular to the axis of the fibre.
In the first situation, this Bragg lattice 6 generates a deflection of an incident luminous flux in a direction inclined in relation to the direction normal to the plane of the strata 5.
Thus, by exploiting this property, difficult to apply in optical fibres, it is possible to extract, or to inject, a luminous flux info a fibre and to exploit this property for the construction of a large number of devices or of components.
As shown in Figure 1, a luminous flux propagating in the fibre 1 is partially deflected by an angle ~ by reflection on the .
2 ~
Bragg lattice 6. When this angle ~ is sufficiently large, that part of the luminous flux which is initially guided by the fibre 1, deflected according to this angle, is extracted from the fibre 1.
Conversely, for the same characteristics of the Bragg lattice 6 a light beam which appears at an angle of incidence ~
at the location of this lattice is coupled into the optical fibre 1.
The Bragg lattice 6 is formed by subjecting an optical fibre, preferably constructed of silica doped with germanium, to a luminous flux at a wavelength situated in the ultraviolet, the intensity of which varies in the direction of the axis 4 of the fibre, in a manner analogous to the variations of the target refractive index. This luminous flux is easily obtained by forming, on the fibre 1, interference fringes which are inclined in relation to its axis 4 and of period I. To this end, there is produced on the fibre 1 the interference of two light beams, at the wavelength ~p, which are inclined in relation to one another at an angle 2~. It is known that this gives rectilinear interference fringes of inter-fringe spacing : I =
2nsin 9 When the Bragg lattice 6 has been formed in the fibre 1, a luminous flux 7 coupl~d into the optical fibre 1 is partially diffracted by the Bragg lattice 6 and produces a light beam 8 , ' 2 ~
inclined at an angle ~ in relation to the axis of the fibre 1 (Figure 2).
As shown in Figure 3, conversely a parallel light beam 9, produced from a source 10, for example via a collimating lens 11, incident on the Bragg lattice 6 at the angle ~ will produce a luminous flux 12 coupled into the fibre 1.
In both cases, involving extraction or involving injection, the angle of incidence ~ is linked to the period I of the Bragg lattice 6 and to the wavelength ~8 Of the signal luminous flux.
The chromatic pass band of the device ~9 for a given angle ~ is inversely proportional to the number of strata of the Bragg lattice. Thus, the characteristics of the Bragg lattice can determine, for a given angle ~, ~q and ~
By combining extraction and injection, it is possible to construct a fibre-optic coupler at the wavelength ~5 including two optical fibres 20, 21 having sheaths 22 and 23, respectively, cores 24 and 25, respectively, and axes 26 and 27, respectively.
An optical system 28 having an inner reflecting surface, normally concave and adapted for surroundiny the fi~res, connects said fibres 20 and 21 at least over a part of their length in which they exhibit the Bragg lattices 29 and 30, respectively.
~;Thus, a luminous flux 31 is guided within the fibre 20 and is at least partially extracted by the Bragg lattice 29 from the fihre 20 at the angle ~t . The extracted beam, which is taken up by the optical system 28, is incident on the fibre 21, which is -the conjugate of the fibre 20 at the angle ~2 . The orientation of the Bragg lattice 30 of the fibre 21 is such that this 2~3~
reflected beam produces the flux 31 which is guided within the fibre 21.
In the embodiment represented in Figure 4, the optical system 28 is a cylindrical mirror having an ellipsoidal cross-section, the fibres 20 and 21 having their axes 26, 27 coincident with the focal axes of this reflector.
Figure 5 shows another embodiment of a coupler with two fibres 40 and 41~ respectively. The axes of these fibres 42, 43 are parallel in that part of their length where they each include a Bragg cluster 44 and 45 respectively. The incident flux 46, which is guided within the fibre 40, is partially extracted at the angle ~ by the Bragg lattice 44. Having r~gard to the parallelism of the axes 42, 43, this flux extracted from the fibre 40 is incident on the fibre 41 at the angle ~. The strata of the Bragg lattice 45 of the fibre 41 are parallel to those of the lattice 44 of the fibre 40. Thus, this luminous flux, which is incident on the fibre 41, is guided within the latter, producing the flux 47~
The yield of this coupler, that is to say the ratio between the incident flux 46, which initially coupled to the fibre 40, and the emergent flux 47 coupled, at the exit, to the fibre 41 is improved when the cores of the fibres 40 and 41 are brought close to one another at the location of the Bragg lattices 44 and 45. This may be achieved by partially abrading the sheaths of each one of the fibres 40 and 41 in the zone of the Bragg lattices 44, 45.
20~43~9 Beneficial effects are produced when a fibre according to the invention, 50, is placed at the centre of a catadioptric optical system having always an inner reflecting surface 51 and presently being a cylinder with a circular cross-section. In this case, an incident beam 52, which is partially extracted by the Bragg lattice 54, is convergently reflected on itself by the catadioptric system 51 and produces an emergent flux 53 coupled to the fibre 50, the direction of propagation of which is opposite to that of the incident flux 52. This device may be employed for the construction of numerous sensors. In this type of device, for example those shown in Figures 6, 7 and 8 the reflecting element is advantageously based on microspheres.
The first type of sensor is shown in Figures 7A and 7B. The optical fibre 60 including a Bragg lattice 61 is placed at the centre of a cylindrical reflector having a cross-section in the form of a circular arc 62. The azimuthal orientation o~ this reflector about the axis 63 of the fibre 60 is controlled by any external parameter P between two positions represented in Figure 7A and in Figure 7B respectively.
In the position represented in Figure 7A, the incident luminous flux 64 is partially extracted by the Bragg lattice 61, re~lected by the reflector 62 and coupled upon return by the Bragg lattice 61; this gives rise to the back-guided flux 65.
On th~ other hand, in the p~sition represented in Figure 7B, the reflector 62 is in a position such that it does not reflect the flux extracted by the Bragg lattice 61.
20~3~9 The remote measurement of the back-reflected flux 65 accordingly permits the establishment of the position of the reflector 62 about the axis 63 and thus permits access to the parameter P which controls this position.
Another sensor is shown in Figures 8A and 8B. In this case, the fibre 70 has its axis coincident with a cylindrical mirror 71 having a circular cross-section. This cylindrical mirror is capable of being displaced by the effect of an external parameter P parallel to its axis. When, as represented in Figure 8A, the reflector 71 is at the location of the Bragg lattice 72, the incident flux 73, which is partially extracted by the lattice 72, is reflected by the reflector 71 and gives rise to the back-guided flux 74.
On the other hand, as represented in Figure 8B, the reflector 71 is longitudinally offset in relation to the Bragg lattice 72. The incident flux 73 does not then produce any back-guided flux such as the flux 74.
The remote measurement of the reflected flux 74 permits the establishment of the position of the reflector 71 in relation to ; 20 the Bragg lattice 72 and thus permits access to the parameter P
which controls this position. It is also thus possible to establish intermediate positions of the reflector 71 and thus to maXe an analog measurement of parameter P which controls this position.
Figure 9 shows a rotational sensor. A fibre 80 including a Bragg lattice is placed on the axis of a cylindrical sleeve 81.
This sleeve includes an inner reflecting zone 82 and a non-2 ~
reflecting zone 83 which are separated by a helix 84. According to the orientation of the sleeve 81 about its axis 85, a large or smaller part of the Bragg lattice of the fibre 80 sees the reflecting part 82 of the sleeve 81. Thus, the flux 87 refl2cted by the sleeve 81 is coupled upon return to the fibre 80 from the incident flux 86 and is dependent upon the orientation of the sleeve 81 about its axis. The remote measurement of this reflected flux 87 permits access to the angular position of the slaeve 81 and thus to the value of the parameter P which controls the same.
Figure 10 shows a sensor constructed by placing a fibre 90 including a Bragg lattice 91 on the axis of a sleeve 92. This ; cylindrical sleeve is a catadioptric system composed of a rotary mirror, having the same axis as the fibre. It is created by the 15 rotation of a line broken down into sections at right angles, the elements of which are inclined at 45 in relation to the axis.
It may be constituted by a block of transparent material, pierced by a cylindrical channel receiving the fibre; in the latter case, an index liquid is advantageously employed in order to ensure the continuity of index between the fibre and the block of material. The reflection on its faces is obtained by the effect of a metallic or dielectric deposit or by total reflection.
It may also be hollow and covered with metallic or dielectric reflective layers.
The incident beam 93, coupled into the fibrs 90, is partially extracted by the Bragg lattice 91 and then again ':' 2 0 ~
coupled into this fibre after reflection on the sleeve 92. It is thus pos~ible, by remotely measuring the reflected flux 94, to gain access either to the position of the sleeve 92 in relation to the Bragg lattice 91 or to its angular position about its axis when its surface is partially reflected. The measurement of the reflected flux 94 accordingly permits access to a parameter P which controls one or the other of these positions.
The sensor represented in Figure 11 is an all-or-nothing sensor. It is composed of a fibre 100 surrounded by a reflective ; cylinder 101. The parameter P which it is desired to measure controls the relative positions of the axis 102 of the optical fibre 100 in relation to the axis 103 of the cylinder 101. In one of the positions, referred to as active positions, these axes 102 and 103 respectively are coincident. On the other hand, in the non-active position these axes are different.
The incident flux 104 oupled to the optical fibre 100 is reflected by the cylinder 101 when it is in the active position and generates the reflected flux 105 which is also coupled to the fibre 100. On the other hand, in the non-active position the flux 104 produces upon return, in the fibre, only a reflected flux 105 which is very small in relation to the incident flux 104.
The remote measurement of the reflected flux 105 thus permits access to the position of the reflective cylinder 101 and thus to a parameter P which controls the same.
2~3~
An important benefit of the fibres incorporating a Bragg lattice according to the invention is that the Bragg lattices ~
can be determined so as to take off only a part of thP incident flux coupled to the optical fibre 1. It is thus possible to place on one and the same fibre a large number of sensors which are recognised in relation to one another either by the wavelength ~ on which they act or by their distance in relation to the measuring centre.
In the second situation represented in Figure 12, the strata 1 0 are parallel to the axis 4 of the fibre 1. These strata have a polarising effect on the signal wave guided by the fibre.
Finally, it will be understood that in an alternative embodiment, the optical system with its inner reflecting surface could possibly be located directly against the sheath of the fibres, for example by means of a metallization or a plating with metal of the external surface of said sheath.
The invention will be described in detail with reference to the Figures, in which:
Figure 1 is a diagrammatic representation of the fibre of the invention.
Figure 2 is the representation of an extractor according to the invention.
Figure 3 is the diagrammatic representation of a fibre-optic injector according to the invention.
Figure 4 is the representation of a fibre-optic coupler according to the invention in a first embodiment.
Figure 5 is the representation of a fibre-optic coupler according to the invention, in a second embodiment.
Figures 6A and 6B are diagrams illustrating the operation of a fibre according to the invention, placed at the centre of a catadioptric optical system.
Figures 7A and 7B are the representation of a fibre-optic sensor according to the invention, in a first embodiment.
Figures 8A and 8B are a representation of a fibre-optic sensor according to the invention, in a second embsdiment.
Figure 9 is a representation of a sensor according to the invention, in a third embodiment.
~igure 10 is the representation of a fibre-optic sensor according to the invention, in a fourth embodiment.
Figure 11 is the representation of a fibre-optic sensor according to the invention, in a fifth embodiment.
Figure 12 is the representation of a fibre-optic polariser according to the invention.
2~3~9 The subject of the invention concerns an optical fibre 1.
Conventionally, the latter has a sheath 2, a core 3, and an axis 4. According to the invention, this optical fibre exhibits variations of refractive index which are disposed in plane, parallel and periodic strata 5, of period I in the direction of the axis. The plane strata 5 are inclined at an angle ~ in relation to the axis 4 of the fibre. Moreover, they are preferably permanently etched or impressed in the core andtor in the sheath.
Two arrangements may be created.
In the first, the optical fibre 1 includes a Bragg diffraction grating 6, in which the strata are inclined in relation to its axis 4 at an angle different from 90.
In the second situation, the optical fibre 1 includes a Bragg diffraction grating 6, in which the strata are parallel to its axis 4, that is to say that the normal to the strata is perpendicular to the axis of the fibre.
In the first situation, this Bragg lattice 6 generates a deflection of an incident luminous flux in a direction inclined in relation to the direction normal to the plane of the strata 5.
Thus, by exploiting this property, difficult to apply in optical fibres, it is possible to extract, or to inject, a luminous flux info a fibre and to exploit this property for the construction of a large number of devices or of components.
As shown in Figure 1, a luminous flux propagating in the fibre 1 is partially deflected by an angle ~ by reflection on the .
2 ~
Bragg lattice 6. When this angle ~ is sufficiently large, that part of the luminous flux which is initially guided by the fibre 1, deflected according to this angle, is extracted from the fibre 1.
Conversely, for the same characteristics of the Bragg lattice 6 a light beam which appears at an angle of incidence ~
at the location of this lattice is coupled into the optical fibre 1.
The Bragg lattice 6 is formed by subjecting an optical fibre, preferably constructed of silica doped with germanium, to a luminous flux at a wavelength situated in the ultraviolet, the intensity of which varies in the direction of the axis 4 of the fibre, in a manner analogous to the variations of the target refractive index. This luminous flux is easily obtained by forming, on the fibre 1, interference fringes which are inclined in relation to its axis 4 and of period I. To this end, there is produced on the fibre 1 the interference of two light beams, at the wavelength ~p, which are inclined in relation to one another at an angle 2~. It is known that this gives rectilinear interference fringes of inter-fringe spacing : I =
2nsin 9 When the Bragg lattice 6 has been formed in the fibre 1, a luminous flux 7 coupl~d into the optical fibre 1 is partially diffracted by the Bragg lattice 6 and produces a light beam 8 , ' 2 ~
inclined at an angle ~ in relation to the axis of the fibre 1 (Figure 2).
As shown in Figure 3, conversely a parallel light beam 9, produced from a source 10, for example via a collimating lens 11, incident on the Bragg lattice 6 at the angle ~ will produce a luminous flux 12 coupled into the fibre 1.
In both cases, involving extraction or involving injection, the angle of incidence ~ is linked to the period I of the Bragg lattice 6 and to the wavelength ~8 Of the signal luminous flux.
The chromatic pass band of the device ~9 for a given angle ~ is inversely proportional to the number of strata of the Bragg lattice. Thus, the characteristics of the Bragg lattice can determine, for a given angle ~, ~q and ~
By combining extraction and injection, it is possible to construct a fibre-optic coupler at the wavelength ~5 including two optical fibres 20, 21 having sheaths 22 and 23, respectively, cores 24 and 25, respectively, and axes 26 and 27, respectively.
An optical system 28 having an inner reflecting surface, normally concave and adapted for surroundiny the fi~res, connects said fibres 20 and 21 at least over a part of their length in which they exhibit the Bragg lattices 29 and 30, respectively.
~;Thus, a luminous flux 31 is guided within the fibre 20 and is at least partially extracted by the Bragg lattice 29 from the fihre 20 at the angle ~t . The extracted beam, which is taken up by the optical system 28, is incident on the fibre 21, which is -the conjugate of the fibre 20 at the angle ~2 . The orientation of the Bragg lattice 30 of the fibre 21 is such that this 2~3~
reflected beam produces the flux 31 which is guided within the fibre 21.
In the embodiment represented in Figure 4, the optical system 28 is a cylindrical mirror having an ellipsoidal cross-section, the fibres 20 and 21 having their axes 26, 27 coincident with the focal axes of this reflector.
Figure 5 shows another embodiment of a coupler with two fibres 40 and 41~ respectively. The axes of these fibres 42, 43 are parallel in that part of their length where they each include a Bragg cluster 44 and 45 respectively. The incident flux 46, which is guided within the fibre 40, is partially extracted at the angle ~ by the Bragg lattice 44. Having r~gard to the parallelism of the axes 42, 43, this flux extracted from the fibre 40 is incident on the fibre 41 at the angle ~. The strata of the Bragg lattice 45 of the fibre 41 are parallel to those of the lattice 44 of the fibre 40. Thus, this luminous flux, which is incident on the fibre 41, is guided within the latter, producing the flux 47~
The yield of this coupler, that is to say the ratio between the incident flux 46, which initially coupled to the fibre 40, and the emergent flux 47 coupled, at the exit, to the fibre 41 is improved when the cores of the fibres 40 and 41 are brought close to one another at the location of the Bragg lattices 44 and 45. This may be achieved by partially abrading the sheaths of each one of the fibres 40 and 41 in the zone of the Bragg lattices 44, 45.
20~43~9 Beneficial effects are produced when a fibre according to the invention, 50, is placed at the centre of a catadioptric optical system having always an inner reflecting surface 51 and presently being a cylinder with a circular cross-section. In this case, an incident beam 52, which is partially extracted by the Bragg lattice 54, is convergently reflected on itself by the catadioptric system 51 and produces an emergent flux 53 coupled to the fibre 50, the direction of propagation of which is opposite to that of the incident flux 52. This device may be employed for the construction of numerous sensors. In this type of device, for example those shown in Figures 6, 7 and 8 the reflecting element is advantageously based on microspheres.
The first type of sensor is shown in Figures 7A and 7B. The optical fibre 60 including a Bragg lattice 61 is placed at the centre of a cylindrical reflector having a cross-section in the form of a circular arc 62. The azimuthal orientation o~ this reflector about the axis 63 of the fibre 60 is controlled by any external parameter P between two positions represented in Figure 7A and in Figure 7B respectively.
In the position represented in Figure 7A, the incident luminous flux 64 is partially extracted by the Bragg lattice 61, re~lected by the reflector 62 and coupled upon return by the Bragg lattice 61; this gives rise to the back-guided flux 65.
On th~ other hand, in the p~sition represented in Figure 7B, the reflector 62 is in a position such that it does not reflect the flux extracted by the Bragg lattice 61.
20~3~9 The remote measurement of the back-reflected flux 65 accordingly permits the establishment of the position of the reflector 62 about the axis 63 and thus permits access to the parameter P which controls this position.
Another sensor is shown in Figures 8A and 8B. In this case, the fibre 70 has its axis coincident with a cylindrical mirror 71 having a circular cross-section. This cylindrical mirror is capable of being displaced by the effect of an external parameter P parallel to its axis. When, as represented in Figure 8A, the reflector 71 is at the location of the Bragg lattice 72, the incident flux 73, which is partially extracted by the lattice 72, is reflected by the reflector 71 and gives rise to the back-guided flux 74.
On the other hand, as represented in Figure 8B, the reflector 71 is longitudinally offset in relation to the Bragg lattice 72. The incident flux 73 does not then produce any back-guided flux such as the flux 74.
The remote measurement of the reflected flux 74 permits the establishment of the position of the reflector 71 in relation to ; 20 the Bragg lattice 72 and thus permits access to the parameter P
which controls this position. It is also thus possible to establish intermediate positions of the reflector 71 and thus to maXe an analog measurement of parameter P which controls this position.
Figure 9 shows a rotational sensor. A fibre 80 including a Bragg lattice is placed on the axis of a cylindrical sleeve 81.
This sleeve includes an inner reflecting zone 82 and a non-2 ~
reflecting zone 83 which are separated by a helix 84. According to the orientation of the sleeve 81 about its axis 85, a large or smaller part of the Bragg lattice of the fibre 80 sees the reflecting part 82 of the sleeve 81. Thus, the flux 87 refl2cted by the sleeve 81 is coupled upon return to the fibre 80 from the incident flux 86 and is dependent upon the orientation of the sleeve 81 about its axis. The remote measurement of this reflected flux 87 permits access to the angular position of the slaeve 81 and thus to the value of the parameter P which controls the same.
Figure 10 shows a sensor constructed by placing a fibre 90 including a Bragg lattice 91 on the axis of a sleeve 92. This ; cylindrical sleeve is a catadioptric system composed of a rotary mirror, having the same axis as the fibre. It is created by the 15 rotation of a line broken down into sections at right angles, the elements of which are inclined at 45 in relation to the axis.
It may be constituted by a block of transparent material, pierced by a cylindrical channel receiving the fibre; in the latter case, an index liquid is advantageously employed in order to ensure the continuity of index between the fibre and the block of material. The reflection on its faces is obtained by the effect of a metallic or dielectric deposit or by total reflection.
It may also be hollow and covered with metallic or dielectric reflective layers.
The incident beam 93, coupled into the fibrs 90, is partially extracted by the Bragg lattice 91 and then again ':' 2 0 ~
coupled into this fibre after reflection on the sleeve 92. It is thus pos~ible, by remotely measuring the reflected flux 94, to gain access either to the position of the sleeve 92 in relation to the Bragg lattice 91 or to its angular position about its axis when its surface is partially reflected. The measurement of the reflected flux 94 accordingly permits access to a parameter P which controls one or the other of these positions.
The sensor represented in Figure 11 is an all-or-nothing sensor. It is composed of a fibre 100 surrounded by a reflective ; cylinder 101. The parameter P which it is desired to measure controls the relative positions of the axis 102 of the optical fibre 100 in relation to the axis 103 of the cylinder 101. In one of the positions, referred to as active positions, these axes 102 and 103 respectively are coincident. On the other hand, in the non-active position these axes are different.
The incident flux 104 oupled to the optical fibre 100 is reflected by the cylinder 101 when it is in the active position and generates the reflected flux 105 which is also coupled to the fibre 100. On the other hand, in the non-active position the flux 104 produces upon return, in the fibre, only a reflected flux 105 which is very small in relation to the incident flux 104.
The remote measurement of the reflected flux 105 thus permits access to the position of the reflective cylinder 101 and thus to a parameter P which controls the same.
2~3~
An important benefit of the fibres incorporating a Bragg lattice according to the invention is that the Bragg lattices ~
can be determined so as to take off only a part of thP incident flux coupled to the optical fibre 1. It is thus possible to place on one and the same fibre a large number of sensors which are recognised in relation to one another either by the wavelength ~ on which they act or by their distance in relation to the measuring centre.
In the second situation represented in Figure 12, the strata 1 0 are parallel to the axis 4 of the fibre 1. These strata have a polarising effect on the signal wave guided by the fibre.
Finally, it will be understood that in an alternative embodiment, the optical system with its inner reflecting surface could possibly be located directly against the sheath of the fibres, for example by means of a metallization or a plating with metal of the external surface of said sheath.
Claims (14)
1. Optical device comprising at least one optical fibre having a sheath, a core, an axis, and a length in the direction of its axis, said fibre including, at least over a part of its length, variations of refractive index disposed in strata which are parallel to one another and periodic, the normal to said strata forming an on-zero angle with the axis of the fibre.
2. Optical device according to claim 1 further comprising an optical reflecting system having an inner reflecting surface adapted for at least partially surrounding said fibre, especially where said fibre is provided with said strata.
3. Device according to claim 2 wherein said reflecting optical system is cylindrical, said fibre being, in that part of its length where it includes variations of index, situated at one focus of said optical system.
4. Device according to claim 2 comprising two optical fibres, said reflecting optical system having an ellipsoidal section and each of said fibres being at that part of its length where it is provided with said variations of refractive index, located at one of the foci of said optical system.
5. Device according to claim 4 in which the angle of inclination of the strata of the first fibre is substantially equal to the angle of inclination of those of the second fibre, said fibres being oriented about their respective axes in such a manner that the flux emerging from the first fibre enters the second.
6. Device according to claim 3 in which said two fibres are in contact with one another in that part of their length where they include variations of index, the sheaths of each one of said fibres being partially abraded in their zones of contact in such a manner as to bring their cores close.
7. Device according to claim 2, characterised in that it is intended for the measurement of a parameter (P) and said reflecting optical system has a section in the form of a circle or a circular arc, the relative positions of said optical system and of the fibre being a function of said parameter (P), at least one of these positions corresponding to that where said optical fibre is situated, in that part of its length where it includes said variations of index, at the centre of the optical system.
8. Device according to claim 7, in which the optical system is a sleeve placed about the optical fibre and being displaced longitudinally as a function of said parameter (P) to be measured.
9. Device according to claim 8, in which the sleeve is cylindrical and has a circular cross-section, said strata being inclined in relation to the axis of the optical fibre.
10. Device according to claim 8, in which the sleeve is of circular cross-section with a generatrix in the form of a line broken down into sections forming a succession of catadioptric elements.
11. Device according to claim 2, in which said optical system is a cylindrical mirror having a cross-section in the form of the circular arc which is moveable in rotation about the axis of said optical fibre.
12. Device according to claim 2, in which the optical system is a partial cylindrical mirror located between a generatrix and a helix.
13. Device forming a polarizer at a specified wavelength comprising an optical fibre having a sheath, a core, an axis, and a length in the direction of this axis, said fibre exhibiting, at least over a part of its length, variations of refractive index disposed in strata which are parallel to one another and periodic, the normal to said strata forming a non-zero angle with the axis of the fibre.
14. Polariser according to claim 13, in which the strata of the optical fibre are parallel to the axis of said fibre.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR9103902A FR2674639A1 (en) | 1991-03-29 | 1991-03-29 | BRAGG NETWORK OPTIC FIBER AND ITS APPLICATIONS. |
| FR9103902 | 1991-03-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2064309A1 true CA2064309A1 (en) | 1992-09-30 |
Family
ID=9411315
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002064309A Abandoned CA2064309A1 (en) | 1991-03-29 | 1992-03-27 | Optical fibre incorporating a bragg lattice and its applications |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US5307437A (en) |
| EP (1) | EP0506546A3 (en) |
| JP (1) | JPH05142423A (en) |
| CA (1) | CA2064309A1 (en) |
| FR (1) | FR2674639A1 (en) |
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-
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- 1991-03-29 FR FR9103902A patent/FR2674639A1/en active Granted
-
1992
- 1992-03-24 EP EP19920400795 patent/EP0506546A3/en not_active Ceased
- 1992-03-26 US US07/858,009 patent/US5307437A/en not_active Expired - Fee Related
- 1992-03-27 CA CA002064309A patent/CA2064309A1/en not_active Abandoned
- 1992-03-30 JP JP4102535A patent/JPH05142423A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPH05142423A (en) | 1993-06-11 |
| FR2674639B1 (en) | 1994-07-13 |
| US5307437A (en) | 1994-04-26 |
| EP0506546A3 (en) | 1993-01-20 |
| FR2674639A1 (en) | 1992-10-02 |
| EP0506546A2 (en) | 1992-09-30 |
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