FR2504691A1 - Optical fibre coupler used between light transmitter and receiver - where fibre is subjected to ion exchange so its numerical aperture varies along fibre and coupling losses are reduced - Google Patents

Abstract

The coupler consists of an optical waveguide fibre possessing both longitudinal and transverse variations in its optical index, so that its numerical aperture varies from one end of the fibre to the other end. At one end, the numerical aperture matches that of the transmitter, whereas at the other end of the fibre the numerical aperture corresponds with that of the receiver. The result is a redn. in the loss of light when coupling a transmitter to a receiver. The fibre is pref. conical and subjected to torsional stress. The fibre is pref. made by immersing an optical fibre in a bath so ion exchange occurs and the optical index of the fibre is reduced at its surface. Used esp. to couple an LED to a transmission fibre in telecommunications.

Description

 The present invention relates to an optical coupling device with digital aperture variation and a method of manufacturing this device.

It applies in particular to the optical coupling of a transmitting photodiode, such as a light-emitting diode, and of a receiving optical fiber.

 We know that the light power from a light source and injected into an optical fiber depends on the ratio of the surface of the source to the surface of the fiber, the radiation pattern of the source and the numerical aperture of the fiber.

où No désigne l'indice optique du milieu dans lequel est placée la fibre. (Lorsque ce milieu est de l'air, t est très voisin de 1).Recall that the numerical aperture of an optical fiber having a core of optical index N1 and a sheath of optical index N2, is the sine of an angle ec, maximum angle of incidence of a light ray on a end of the optical fiber so that it propagates this light ray. We also recall that this numerical aperture is equal to

where No indicates the optical index of the medium in which the fiber is placed. (When this medium is air, t is very close to 1).

 Thus, a laser diode emitting photodiode, which has a small source surface and a small radiation angle, does it cause, with coupling optics known as hemispherical, semi-cylindrical, cylindrical, or cylindrical lenses with fiber Selfoc type, a loss of coupling, compared to the light power emitted by this laser diode, of approximately 3dB for an optical fiber with digital aperture of the order of 0.15 to 0.2. (Without coupling optics, this loss is worth approximately 10dB>.

 An emitting photodiode of the light-emitting diode type, which has a source surface identical to that of a laser diode but a very open radiation angle (of the order of 1800), injects into a conventional optical fiber only about 1% of the light energy emitted by this light-emitting diode, thus causing a loss of 20 dB relative to the light power emitted by said diode.

 It is therefore advantageous, particularly for light-emitting diodes, to increase the digital aperture of the coupled optical fibers, that is to say to give the latter an angle ec, defined above and called acceptance angle, more important, so as to obtain the maximum coupled light power.

However, the techniques for manufacturing optical fibers, the attenuation and bandwidth of which are of interest for telecommunications greatly limit the digital opening of the fibers produced: for example, this digital opening varies approximately between 0.1 and 0, 2 for techniques such as flame hydrolysis and the CVD method In addition, known coupling optics, such as those listed above, make it possible to adapt the size of a light source to that of an optical fiber but do not make it possible to artificially increase the digital aperture of this fiber.

 The present invention aims to remedy these drawbacks and it relates to an optical coupling device with digital aperture variation and a method of manufacturing this device.

 The invention firstly relates to an optical coupling device interposable between a light emitter and receiver, characterized in that it comprises an optical fiber having variations in transverse and longitudinal optical index such as its digital aperture , then varying from one end to the other, has values at these extremes adapted to the light emitter and receiver, thus making it possible to reduce the losses of light energy occurring when said emitter and said receiver are coupled.

 By light emitter is meant any direct or indirect light source: it may be light guiding means, such as one or more optical fibers, or light producing means, such as a light emitting diode or a laser diode. By light receiver is meant any means for capturing light and then converting or transmitting it: it can again be light guiding means, such as one or more optical fibers, or a photo-detector (converting light into current electrical) such as a receiving photodiode.

 According to a preferred characteristic of the device which is the subject of the invention, said optical fiber has transverse and longitudinal gradients of optical index.

 According to a particular characteristic of the device which is the subject of the invention, said optical fiber is of revolution and widens from one end to the other. Said optical fiber can then be of frustoconical shape. It can also have a twist along its axis so as to be a gradient of longitudinal index. Such an optical fiber, of revolution and flaring from one end to the other, can be applied to the coupling of another optical fiber to a plurality of other optical fibers.

The present invention further relates to a method of manufacturing the optical coupling device also subject of the invention. This process is characterized in that it consists in - moving an optical fiber in a bath containing
suitable ions, by diffusion in said op fiber
tick, to lower the optical index of it by
surface, and simultaneously, to vary the concentration of said ions in
said bath.

 According to a particular characteristic of the process which is the subject of the invention, said optical fiber being made of a glass based on ions of a first type and doped with ions of a second type, said ions contained in said bath are said ions of the first type and the concentration of the latter is varied in said bath by injecting ions of the second type into it. Said ions of the first type are, for example, Na + ions and said ions of the second type are, for example, Ag + ions.

Other characteristics and advantages of the device and of the manufacturing method which are the subject of the invention will appear better on reading the following description of several examples of embodiments given by way of non-limiting indication, with reference to the appended drawings in which - the Figure 1 is a schematic representation of a
particular embodiment of the
optical coupling object of the invention, interposed in
be an emitting photodiode and an optical fiber
receiver, associated with a graph showing the indi
this device optics at different points of
this one - figure 2 is a schematic representation of a
frustoconical device in accordance with the invention
tion and making it possible to couple an optical fiber to
several other optical fibers - Figure 3 is an explanatory diagram of a method
obtaining transverse and longitudi gradients
Optical index rating for the device shown
in Figure 2; and - Figure 4 is a schematic representation of a
device for implementing the object process
of the invention.

 In Figure 1, there is shown schematically a particular embodiment of the optical coupling device object of the invention. This device comprises an optical fiber 1 provided for optically coupling a light emitter 2 to a light receiver 3. The light emitter 2 is for example a light-emitting diode 4 and the light receiver 3 is for example another optical fiber 5 transmission, of the type used in telecommunications and whose numerical aperture is of the order of 0.1 to 0.2, for example 0.17. The optical fiber 1 follows for example the shape of a cylinder of radius R and of length L. It has an axis x'x originating from a point O located at the center of one of its faces which is opposite the light-emitting diode 4. Its other face 7, facing the other optical fiber 5, has a point A at its center, the distance from O to A being equal to B. The length L is for example of the order of 10 to 20 cm and the radius R of the fiber 1 is for example of the order of 50 to 60 sun.

The optical fiber 1 is designed (in a manner which will be indicated below) so that its digital aperture varies from its end 6 to the other 7 and more precisely, in the present case (coupling of the light-emitting diode 4 to the transmission optical fiber 5), so that this digital aperture progressively decreases from end 6 to end 7 of the optical fiber 1 and has values adapted to the troluminescent diode 4 at these ends 6 and 7 respectively (value of on the order of 0.5 for example), and on the transmission optical fiber 5 (value of 0.17 with the example chosen). In other words, the optical fiber 1 is designed to have a gradient of transverse optical index and a longitudinal optical index gradient such that, by designating by x the abscissa of a point of the optical fiber 1 and by r the distance from this point to the axis x'x, the optical index n of optical fiber 1, also noted n (x, r), decreases when, at x fixed x, the distance r augm ente, and increases when, at fixed distance r, the abscissa x increases (with the chosen orientation of the axis x'x: from the diode 4 to the other optical fiber 5). For r = 0 (on l 'axis x'x) this index n retains for example a value n1. Around the perimeter of facing 6
1 of the light-emitting diode 4 (periphery corresponding to x = 0 and r = R) this index has a value n2 less than nl and on the periphery of face 7 opposite the other optical fiber 5 (periphery corresponding to x = L and r = R) this index has a value n3 between n2 and n.

 In FIG. 1, the variations of the index n as a function of x and of r have also been represented graphically: for fixed x (0 <x <L), the index n has been plotted as a function of the distance r. The shape of the curve obtained is always the same: n decreases when r increases; but moreover the difference n (x, 0) -n (x, r) = nl-n (x, r) decreases when x increases, for a fixed value r (0 <r <R).

 The device which is the subject of the invention makes it possible to reduce the losses of light energy during the coupling of the light-emitting diode 4 and the optical fiber 5 of transmission; it makes it possible to recover the light energy lost on coupling in the prior art for injecting it into the transmission fiber 5. In fact, any light ray entering the optical fiber 1 undergoes two deviations during its propagation in this fiber : a deviation due to the longitudinal gradient of optical index and a deviation due to the transverse gradient of optical index both causing a bringing together of said light ray, of the axis x'x of the optical fiber 1; this light ray straightens "during its propagation in this fiber and its angle of emergence from it is less than its angle of incidence, hence an optical adaptation of the diode 4 to the optical fiber 5 of transmission thanks with the double index gradient.

 In Figure 2, there is shown schematically a device according to the invention and comprising an optical fiber 8 having the shape of a truncated cone of revolution of axis X'X, having a face 11 whose periphery has the radius R1 and another face 12 whose periphery has a radius R2 greater than R1. The reduction in cross section of the optical fiber 8 going from the face 12 to the face 11 makes it possible to compensate for the straightening of light rays propagating in the fiber 8, straightening due to the longitudinal gradient of optical index. This optical fiber 8 can then be used to couple several other optical fibers 10, for example seven fibers 10, to another optical fiber 9. The latter is placed opposite the face 11 and the other fibers 10 are placed opposite the the other face 12. A possible arrangement (hexagonal arrangement) of these seven fibers 10 is also shown in section.

 The arrangement thus obtained makes it possible to inject light beams propagating in these seven fibers 10, in said other fiber 9. In a conventional cone, the light rays coming from the seven fibers 10 would deviate from the axis X'X; the longitu dinal gradient straightens these rays and makes it possible to preserve the digital aperture of the beam resulting from the seven partial beams coming from the fibers 10. (Said arrangement would also make it possible to inject part of a light beam propagating in the other fiber 9 , in each of the other seven optical fibers 10).

In FIG. 2, the optical index profiles have also been shown at the two ends or faces 11 and 12 of the optical fiber 8. On the face 11 facing the other optical fiber 9, the index decreases by one value nl on the X'X axis at a value n2 at distance R. On side 12 opposite the other fi
1 optical bres s 10, the index decreases from the value nl on the axis X'X to a value n2 at the distance R2, n2 being less than n2.

 The frustoconical optical fiber 8 with double gradient is achievable by the method making it possible to obtain the fiber 1 of FIG. 1, method which will be explained below. But there is, in the particular case of this frustoconical fiber 8, a ratio, calculable by those skilled in the art, to be observed between the variation in numerical aperture and the variation in section of the fiber 8. For example a radius R2 equal to three times the radius R1, the numerical aperture of the face 12 corresponding to R2 is close to seven times that of the face ll corresponding to R1.

 However, another method of obtaining the double gradient for the tapered optical fiber 8 can be used. This other method is explained using FIG. 3. It consists in applying a twist to a frustoconical optical fiber 8 with a transverse gradient of optical index. This twist establishes stresses which vary from one end to the other of the fiber 8. Furthermore, these stresses are zero on the axis X'X and maximum at the surface of the fiber 8. However, it is found experimentally that 'a constraint induces a variation in optical index. It is therefore understood that the constraints obtained will create the longitudinal gradient of desired optical index.

(The optical index is equal to nl on the X'X axis and the rest. It goes from a value nO to the value n2, with n0 <n2 <n1, on the periphery of the face 11 corresponding to Rl and at the value n2, with nO <n2, <n2, on the periphery of the face 12 corresponding to R2. (R1 being less than R2, the constraints on the face ll are stronger than the constraints on the face 12). FIG. 3, the variations of the optical index as a function of the distance to the axis X'X are shown in solid lines before application of the constraints and in dotted lines after this application.

 We note, however, that this method using constraints can give for certain optical fibers less good results than the method which we will now expose in connection with FIG. 4. Indeed, the constraints to be applied to obtain a good adaptation (c that is to say a coupling in which the losses of light energy are low) may, for certain materials usable for the manufacture of the frustoconical fiber 8, be such that the fiber 8 does not resist them and breaks.

 In Figure 4, there is shown schematically a device for implementing the method of the invention. This process makes it possible to manufacture the double gradient optical fibers 1 and 8 shown in FIGS. 1 and 2.

 An optical fiber with a double gradient of optical index can be obtained from a glass fiber F for example of the order of 10 to 20 cm in length and 50 to 60 μm in diameter and without any gradient of index.

This glass is for example a soda-lime glass doped with Ag + ions. Furthermore, the implementation device essentially comprises a tank 13 filled with a bath B containing Na + ions.

 According to the process which is the subject of the invention, several hundred fibers F (rather than one, in order to have a high production yield) are maintained and lowered by mechanical lowering means 14, which can be produced by those skilled in the art, in bath B. Simultaneously, the relative concentration of bath B in Na ions is varied by injecting Ag + - ions into said bath B.

In fact, ion diffusion studies in soda-lime glasses have made it possible, by introducing Ag + ions into them, to increase the optical index on the order of 0.1 to 0.2 over several tens of microns thick. The replacement of Na + by Ag + thus having the drawback of increasing the surface index, the opposite reaction (reduction of index) can be produced by starting from a glass doped with Ag + and by diffusing Na ions therein.
Na + ions therefore diffuse in fibers
F thus lowering the optical index of the glass in the area where they diffuse. Due to the combination of the downward movement and the variation in concentration of the bath B in Na + ions by injection of Ag ions, we therefore create in the fibers F a transverse gradient of optical index and a longitudinal gradient of optical index: fibers F are thus obtained whose numerical aperture varies from one end to the other.

In order to obtain determined digital openings at the two ends of the fibers F or a determined ratio between these digital openings, it suffices to adjust suitably (what a person skilled in the art can do using the known laws of diffusion or laws of
FICK) the speed of descent of the fibers F into the bath
B, the temperature thereof and the speed of injection of the Ag ions into this bath B. The fibers F thus treated are then covered with a sheath made of a material with an optical index lower than the optical index in surface, and which protects the F fibers.

Of course, glasses based on ions (other than Na) making it possible by diffusion to lower the optical index on the surface and doped with ions other than Ag + could be used. In addition, other methods of manufacturing optical fibers with a double gradient of optical index could be used: double crucible method, so-called MCVD method, flame hydrolysis, phase separation method
SPL, fluorinated glass projection method, methods adaptable by a person skilled in the art to the envisaged manufacture.

The device which is the subject of the invention makes it possible to reduce the losses of light energy occurring when a light emitter and receiver are coupled. It allows in particular to gain approximately 10dB in the case of the coupling of a light-emitting diode
te to an optical fiber and around 3dB in the case of the coupling of a laser diode to such a fiber.

Claims (9)

 1. Optical coupling device interposable between a light emitter (2) and a receiver (3), characterized in that it comprises an optical fiber (1, 8) having variations in transverse and longitudinal optical index such as its numerical aperture, then varying from one end (6, 12) to the other (7, 11), has at these ends (6, 12 and 7, 11) values adapted to the transmitter (2) and to the receiver (3) of light, thus making it possible to reduce the losses of light energy occurring when said transmitter (2) transmitter (2) and said receiver (3) are coupled.  2. Device according to claim 1, ca acterized in that said optical fiber (1, 8) has transverse and longitudinal gradients of optical index.  3. Device according to any one of the re-vendications 1 and 2, characterized in that said optical fiber (8) is of revolution and widens from one end (11) to the other (12).  4. Device according to claim 3, characterized in that said optical fiber (8) is of frustoconical shape.  5. Device according to any one of claims 3 and 4, characterized in that said optical fiber (8) has a twist along its axis (x'X).  6. A method of manufacturing the device according to any one of claims 1 to 5, characterized in that it consists of - moving an optical fiber (F) in a bath (B)  containing suitable ions, by diffusion in said  optical fiber (F), to lower the optical index by  this on the surface, and simultaneously - to vary the concentration of said ions in  said bath (B).  7. Method according to claim 6, characterized in that, said optical fiber (F) being made of a glass based on ions of a first type and doped with ions of a second type, said ions contained in said bath (B) are said ions of the first type and in that the concentration of the latter is varied in said bath (B) by injecting ions of the second type into it.  8. Application of the device according to any one of claims 1 to 5 to the optical coupling of a transmitting photodiode (4) and a fiber. receiving optics (5).  9. Application of the device according to any one of claims 3 to 5 to the coupling of another optical fiber (9) to a plurality of other optical fibers (10).