FR2490045A1 - Bidirectional fibre=optic telecommunications system - has divergent laser focussed on annular receiver formed by fibre=optic core - Google Patents

Abstract

The device is for use with radiant energy systems such as fibre optic cables comprising an energy source (5) such as a semiconductor laser emitting a divergent beam coupled through an anamorphic lens system (9) to a receiver which is typically the outer annular zone (5) at a two-way fibre optic (f). The fibre optic core (fi) is connected in normal fashion to a diode detector (D). The anamorphic lens system (9) comprises an optical collimator (L1) converting the source beam into a parallel beam, a deviator (DE) transforming the parallel beam into a divergent annular beam, and a refocussing optic (L2) focussing the beam as an annular beam on the annular- zone of the receiver. The collimator and refocussing optics are Fresnel lenses. The deviator may be of moulded plastic, an optical system of symmetrical revolution or such as a Fresnel prism. The arrangement may be reversed with the source and detector interchanged.

Description

 The invention relates to a device for transmitting and receiving radiant energy.

 Such devices are used in particular in optical fiber connection systems.

 These systems are implemented to provide optical telecommunications in many fields. By way of non-limiting examples, mention may be made of telephone or telex transmissions, telematics or cable television.

 For transmissions over long distances, optical fibers with very low linear losses are used. Particular attention must also be paid to the couplings between fibers on the one hand, and on the other hand, between the fibers and the optoelectronic organs for transmitting and detecting the radiant energy transmitted by these optical fibers. These couplings must be maximum so as to limit losses as much as possible.

Finally, a method of transmitting data described in the patent application filed on June 13, 1980 by the applicant and entitled: "SYS-
FIBER OPTIC BIDIRECTIONAL BINDING TEMPERATURE "allows bidirectional links to be established using only a single optical fiber, for which purpose a first unidirectional link in a first sense is established by transmitting a light vibration in a conventional manner. by the core of the optical fiber, and a second unidirectional link, in the opposite direction, is established by transmitting a light vibration by another region of the optical fiber.

 It is known in fact that an optical fiber comprises a core and various sheaths or supports surrounding this core, optical refractive indices different from that of the heart, the refractive indices of these various regions being further different from each other. It follows that one can capture and guide light vibrations in one or more regions of the optical fiber in addition to its core.

 To obtain an optimum coupling, it is necessary for the source emitting the latter light vibrations to be of a shape adapted to that of the capture zone, that is to say to an annular zone. Finally, there must be no crosstalk between the different light vibrations that propagate simultaneously in the optical fiber.

 To provide a solution to this problem, it has been proposed in the aforementioned patent application an opto-electronic device comprising a light-emitting diode of suitable structure.

 This light-emitting diode is a diode whose active emission zone is annular. It consists of a substrate and epitaxial layers comprising an emissive active layer, a first recess having been made in the substrate to allow positioning within it of the optical fiber used for telecommunications and having been made in the epitaxial layers allowing access of a second optical fiber or intermediate fiber and its optical coupling with the first. This second optical fiber is itself coupled to a photodiode of conventional type.

 Although this device offers a good solution to the aforementioned problems, it uses an optoelectronic component, the light-emitting diode, of unconventional design that it is necessary to develop especially for this particular application. It may be preferred to use only optoelectronic components commonly available on the market, produced in large quantities and therefore cheap.

 The invention proposes to meet the needs that have just been mentioned.

 The invention therefore relates to a radiant energy transmitting-receiving device comprising at least one source emitting a divergent beam of radiant energy and whose active emission zone is inscribed in a circle; the diverging beam being intended to be picked up by an annular radiant energy receiving zone; device mainly characterized in that it comprises anamorphosing optical means for optically conjugating the ring-shaped reception zone with the emission zone inscribed in a circle and in that these means comprise a collimating optical element intended to transform the beam divergent emitted in a parallel beam, a deflecting optical element for transforming the parallel beam into a divergent beam inscribed between two cones and a convergent optical element for refocusing the diverging beam in the annular radiant energy receiving zone.

 The invention also relates to a bidirectional fiber optic links system implementing this device.

The invention will be better understood and other advantages will appear with the aid of the description which follows with reference to the appended figures:
FIG. 1 schematically illustrates a system of bidirectional links by a single optical fiber implementing the device of the invention;
FIG. 2 is an optical diagram highlighting the operation of the rays in the device;
- Figures 3 to 5 are detailed figures of the device of the invention according to several embodiments.

 Since the radiant energy transmitting-receiving device according to the invention is intended to be implemented in the context of optical fiber data transmission, it is useful to recall the structure of an optical fiber. One of the most commonly used methods for obtaining an optical fiber is the so-called "MCVD" method, the "Modified chemical vapor-phase deposition" which can be translated as "modified method of deposition in the gas phase". Such a fiber is mainly composed of a core surrounded by an optical cladding, a support tube and a protective sheath.

There are two types of fibers:
optical fibers of the so-called multimode type, most often with a gradient of index, for which the core diameter is typically 50 μFun and the outer diameter of the optical cladding is 70 mm.

 fibers of the so-called single-mode type, most often with index jump, for which the core diameter is between 5 and 10 films and the outer diameter of the optical cladding is about 40 μm.

 The outer diameter of the support tube, usually pure silica, is about 125 and the outer diameter of the protective sheath, silicone for example, is of the order of 3001li.

 In a conventional manner, the support tube is made of pure silica of refractive index ns. If nl, n2 and n3 are respectively the respective refractive indices of the optical cladding core and the protective cladding, the following relation is satisfied in the case of conventional index jump optical fibers: n1> n5> n2 > n4.

 In a conventional system of connections using an optical fiber is injected, for example with the aid of a laser source, light modulated by information to be transmitted in the core of the fiber. Due to the presence of the different sheaths and support tube, different indices than the core and responding to the above relationship, this light is "trapped" in the core of the fiber which acts as a light guide. It is also possible to guide light in one of the sheaths or support tube surrounding the core, since the optical refractive indices of these sheaths or support tube are different from that of the heart and different from each other. This process is described in the aforementioned patent application.

 It is therefore possible by this method to transmit information in the form of light vibrations modulated in a first direction, using the core of the fiber, and in a second direction, using for example the support tube. It is therefore possible to obtain bidirectional links between two stations using only one optical fiber, without using directional couplers or by using radiant energy emitters of different wavelengths. Each station has a radiant energy transmitting / receiving device.

The optical fiber used can be between either a type of fiber "MCVD" currently available on the market or optical fiber optimized for the intended application. In the latter case, it is necessary to make the following choices during manufacture:
- systematic choice of a low optical attenuation linear support tube, this is the case of a pure silica support tube.

 - Intermediate doping of the optical cladding so as to accentuate the difference of index at the interface support-optical cladding.

 As has been recalled previously, it is necessary in all cases for which connections are envisaged over long distances that the logical losses are the lowest possible. It is therefore necessary, among other things, to optimize the optical coupling between the optical fibers for connection and the device for transmitting and receiving radiant energy.

 The invention aims to meet the needs mentioned and proposes an optical device for transmitting-receiving radiant energy minimizing the coupling losses in the two directions of transmission, avoiding any crosstalk between these two directions of transmission and not requiring the development electronic components.

 A radiant energy transmitting-receiving device according to the invention satisfying this condition will now be described.

 Figure 1 schematically illustrates such a device in the particular context of the bidirectional transmission method which has been recalled.

 The optical fiber f is a connecting fiber providing these bidirectional transmissions symbolized by arrows. It comprises, as has been recalled, a core 1 surrounded by an optical cladding 2, a support tube 3 and a protective sheath 4.

 The transmissions in a first direction are ensured by guiding light vibrations in the core 1. The optimum coupling of the connection fiber with a detector D, for example a photodiode, can be obtained conventionally, without difficulty, because is to couple between them two circular areas. In the present case an intermediate fiber is used, with a cross section at least equal to the circular cross-section of the radiant zone, ie the core 1 of the optical fiber f. The only condition to be satisfied is that the active zone 7 of the photodiode D, mounted on a support 8, must simply have a diameter greater than the outside diameter of the intermediate fiber f 1.

 The coupling of the source S with the support tube 3 of the connecting fiber, to ensure transmissions in the second direction, is more difficult to achieve.

Indeed, the capture zone 5 is of annular shape, it is necessary that the source S is adapted to this configuration while leaving the free passage to the intermediate fiber fi. On the other hand, the most commonly used radiant energy sources are of the semiconductor laser type. These lasers have an active face in the form of an emissive ribbon and are characterized by a very divergent emission, in an elliptical cone. The source may still be constituted by a light emitting diode.

 The device of the invention comprises anamorphosing optical means 9 optically conjugating the circular transmission zone 6 to the annular zone 5. The set of elements 5, 9 and f have a common axis of symmetry A. The optical means 9 must be placed at a distance d from the exit face of the fiber F sufficient for the curvature of the fiber fi to remain within suitable limits.

 Typically the maximum curvature of the fiber fi, to ensure correct optical transmission, is of the order of 1 cm which leads to a value of the same order of magnitude. By way of illustration, the outside diameter of the annular zone 5 is of the order of 195 μm for an inside diameter in the range 40-70 μm. The acceptance angle of this zone corresponds to a numerical aperture of 0.25.

 FIG. 2 is an optical diagram illustrating in more detail the path of the rays between the circular emitting zone 6 of the source S and the annular sensing zone 5 of the optical fiber f. This figure illustrates in particular the march of the rays inside the anamorphosing optical means 9.

These anamorphosing optical means consist of:
a collimation optics L1 making it possible to obtain a parallel beam;
an ED deflector transforming a cylindrical incident beam into a divergent beam between two cones
and a refocusing optic L2 converging the incident beam into a ring of inner and outer diameters substantially equal to the inside and outside diameters of the annular zone 5, respectively.

 It should also be understood that the device of the invention can be made completely symmetrical. In this case, by inverting the directions of propagation in the fiber f relative to that indicated in FIG. 1, the annular zone becomes a radiant energy source in the form of a crown and the circular zone corresponding to the core a zone of caption. It suffices to reverse the respective roles of the source S and the detector D and to interchange these components.

 The optical coupling between the source S and the optical fiber can be achieved in this case according to the method described in the French patent application No. 79 22 286, filed September 13, 1979. The device obtained by the method described in this patent application comprises a plano-convex lens bonded to the input face of an optical fiber made of a glass of refractive index greater than that of the optical fiber and obtained by contacting the end of the optical fiber with a drop of molten glass. We then obtain a spherical cap whose parameters are controllable.

 The different elements of the anamorphosing optical means 9 can be made each in different ways according to several embodiments of the device of the invention.

 For L1 refocusing collimation optics L2, it is possible to use, for example, autofocusing lenses or barrel lenses, conventional microoptics, or even Fresnel lenses. These optics must have a numerical aperture at least equal to that of the fiber, and focal lengths optimized for the coupling of the source S with a fiber f of a given type. The refocusing optics L2 should allow the passage of the middle intermediate fiber fi.

 The autofocusing lenses, as is known, can be constituted by index gradient cylindrical glass rods bounded by two plane faces, the optical refraction index having a parabolic distribution along a radial axis and passing through a maximum in the center of the bar. A light beam entering through one of the planar faces, at an angle of incidence less than the maximum acceptance angle, propagates inside the bar along a sinusoldal path because of the index variation. Instead of glass, other refractive materials can be used.

 This type of lens is similar to a common spherical lens whose characteristics it has, but has many advantages.

Among these, it is possible in particular to easily obtain lenses with a very short focal length (less than one millimeter) of small diameter (up to 0.5 mm) and above all the focal length is simply determined by the length of the bar.

 Indeed, we can define a period, commonly called "pitch" according to the English terminology, which corresponds to the period of the sinusoldale function. By cutting the bar at lengths corresponding to predetermined values of period or fraction of period (for example a quarter of a period) one can obtain lenses of different kinds, for example collimator.

FIG. 3 illustrates an embodiment of the anamorphosing optical means 9 using such lenses. The lens L1 consists of a first index gradient bar. The diameter Q> of this bar is typically of the order of 1 to 2 mm. The distance d1 separating the entry face of the lens
L1 of the source S is of the order of 1 mm, d1 is also the focal length of the lens T 1. To be collimated, the lens L1 must have a length d2 equal to a quarter period. This period depends in particular on the material constituting the lens and the precise evolution of the refractive index along a radial axis. It can be obtained from tables or by calculation. Such lenses are proposed as components standardized in the trade, in particular under the registered trademark "SEL
FOC "The distance d3 is not critical, the beam emerging from the lens
L1 being a beam of parallel rays.

 The lens L2 is also a gradient index bar whose diameter may be equal to that of the bar constituting the lens L1. The distance separating the deflector DE from the lens L2 depends on the divergence of the emergent beam, that is to say the nature of the deflector DE whose examples will be described below. The distance d6 separating the fiber f from the lens L2 depends essentially on the maximum curvature that can take the fiber fi, as previously reported. The length d5 of the lens L2 will be determined as a function of the value of d6, so that the emergent beam converges in a ring of substantially the same surface as that of the annular zone 5. On the other hand the angle d The incidence of the convergent beam must be less than the acceptance angle of this zone. All the elements have the axis of common symmetry axis A.

 According to another variant, it is possible to use bar lenses.

These consist of cylinders made of glass or another refractive material having a convex face and another plane. An exemplary embodiment is described in the article by HASHIMOTO and NOSU: "Low-loss interference filters" published in the Dutch publication "Optical Communication Conference Conference Proceedings" (17-19 September 1979), pages 11.5-1 to 11.5 -4.

 In this variant, the coupling between the first lens L1 and the source S can be achieved by means of an additional intermediate optical fiber. This coupling is illustrated schematically in FIG. 4. The coupling between the source S and this intermediate fiber wire can be made according to the method of the aforementioned French patent application No. 79 22 886. This type of coupling can also be implemented in the context of the first variant described with reference to FIG.

 As other alternatives, optics of conventional design may be used, possibly doublets limiting losses, or even FRESNEL lenses.

As regards the latter type of lens, an interesting method of obtaining by photolithography can be implemented. This process is described in the article by D'AURA et al: "Photolithographic Fabrication of
Thin Film Lenses "published in the Dutch magazine" Optics Communications ", volume 5, no. 4, July 1972, pages 232 to 235. The lens obtained by this process is in the form of a thin film having on one of these faces an embossed profile made in a surface layer approximating that of a FRESNEL lens with conventional design, this profile is obtained by conventional photolithography techniques according to a method comprising a number of successive stages of etching etching of the layer superficial.

 With regard to the DE deflector, it can also be realized in different ways. Its role is to transform the collimated beam emerging from the lens L1 into a divergent beam inscribed inside a double cone. In all cases, it is an optical element with symmetry of revolution introducing a phase shift increasing linearly when deviating from the axis of symmetry to the periphery.

 A first exemplary embodiment is illustrated schematically in relation to FIG. 5. The deflection element is made in the form of a concave conical prism with a very large angle at the apex, which causes the angle α formed by an edge of the cone with the normal to the axis of symmetry A, is very small, typically of the order of 0.5. The beam emerging from the lens L1 (not shown) is a collimated beam inscribed in a circle C1 and centered on the axis of symmetry A. It emerges from the prism in the form of a divergent beam comprised in a double cone, inscribed between the two circles c2 and c3 centered on the axis A. The divergence angle ss, a function of the index of refraction of the material used will be for the aforementioned value of the order of 1.

 The prism may be glass, but this choice leads to machining difficulties, or preferably plastic molded using a polishing mold obtained. It can still be obtained by the lithography process which comes beech recalled. In this case, the conical profile will be obtained by depositing a surface layer on a thin film, followed by successive masking-etching operations so as to approximate the cone of wide angle at the top. The film can be replaced by a refractive material blade of constant thickness.

 The prism of FIG. 5 can be replaced by a prism with a FRESNEL structure, ie a sawtooth structure, all the teeth having an equal height. In this case the prism can be obtained by molding or again by the living process of being recalled.

 Other elements of revolution introducing a progressive phase shift along a radial axis can be used, possibly in combination with other optical elements such as reflecting mirrors, for example. It should be noted that this progressive variation does not have to be imperatively linear.

 Among those that may be mentioned, as a variant not represented, a rotationally symmetrical network of which only diffraction orders different from the zero order are used.

 There is of course a loss of part of the radiant energy and the efficiency is lower than that obtainable by the prism of FIG. 5. However, this loss can be minimized by a suitable geometry of the grating, which favors the transmission. energy in the orders retained and decreases that transmitted by the zero order. The zero order can be obscured by appropriate masking, but in any case, it can not introduce crosstalk with the energy captured by the intermediate fiber fi, and hence with the modulated radiant energy transmitted by the core 1 of the fiber f (FIG. 1), since this zero order can not penetrate into this fiber which in fact conceals the core 1 of the fiber 1.

 The invention is not limited to the exemplary embodiments which have just been described by way of illustration. The device of the invention allows an optimum coupling between, on the one hand, the transmission fiber and on the other hand, the radiant energy transmitting-receiving members, without introducing a crosstalk between the two transmission paths and using only technologies and materials well known in optics. In addition, it does not require specific optoelectronic components.

Claims (10)

 1. Radiant energy transmitting-receiving device7 comprising at least one source (S) emitting a divergent beam of radiant energy and whose active emission zone (6) is inscribed in a circle; the divergent beam being intended to be picked up by a radiant energy receiving zone (5) of annular shape, characterized in that it comprises anamorphosing optical means (9) intended to optically conjugate the receiving zone (5) with a shape annular with the emission zone (6) inscribed in a circle and in that these means comprise a collimating optical element (L1) intended to transform the divergent beam emitted into a parallel beam, a deviating optical element (DE) intended to transform the beam parallel to a divergent beam inscribed between two cones and a convergent optical element tL;!) - for refocusing the diverging beam in the annular radiant energy receiving zone.  2. Device according to claim 1, characterized in that the collimating optical element (L1) and the convergent optical element (L2) are each constituted by an autofocusing lens and in that this lens is in the form of a cylindrical bar made of refractive index gradient material, bounded by two plane faces.  3. Device according to claim 1, characterized in that the collimating optical element (L1) and the convergent optical element (L2) are FRESNEL lenses.  4. Device according to claim 1, characterized in that the collimating optical element (L1) and the convergent optical element (L2) each consist of a lens in the form of a cylindrical bar of refracting material limited by a convex face and a flat face.  5. Device according to claim 1, characterized in that the deflecting element (DE) is constituted by a cylindrical bar made of refracting material delimited by a flat face and a face having the shape of a concave cone of the same axis of symmetry. than the bar.  6. Device according to claim 5, characterized in that the deflector element (DE) is molded plastic.  7. Device according to claim 1, characterized in that the deflecting element (DE) is constituted by a blade of uniform thickness of refracting material on which has been disposed a layer of refracting material of variable thickness, so as to forming in the layer a bowl having the profile of a concave cone.  8. Device according to claim 1, characterized in that the deflecting element (DE) is a symmetrical optical network revolution.  9. Device according to claim 1 characterized in that the deflecting element (DE) is a FRESNEL structure prism symmetry of revolution.  10. Bidirectional fiber optic link system between a first station and a second station; the optical connecting fiber (f) being of the type comprising at least a first region (1) of material of a first optical refractive index (n1), a second region (2) surrounding the first region, made of a material of a second optical refractive index (n2), and a third region (3) surrounding the second region, made of material of a third optical refractive index (n5); the second optical refractive index (n2) being smaller than the first (n1) and third (nS) optical refractive indexes; system in which bidirectional links between the first station and the second station are provided by establishing a first unidirectional link in a first direction obtained by optical sensing and picking in the first region (1) of radiant energy and by establishment of a second unidirectional link in the second direction, obtained by optical pick-up and guidance in the third region (3) of radiant energy, characterized in that each station comprises a radiant energy transmitting-receiving device according to the any one of claims 1 to 9, the annular receiving zone (S) being constituted by the section of the third region (3) and in that the device further comprises a radiant energy detector (D) coupled to the first region (1) using an intermediate optical fiber