FR2767928A1 - Coupling for optical fiber and transmitting or receiving source - Google Patents

Abstract

A coupling attaching an optical fiber (1) to a transmitting or receiving source (2) consists of a base (8) with a hole (13) for one end (6) of the fiber and a housing (9) carrying the source (2). The base is made from a translucent material, with the fiber attached to its hole and the base to the housing by a photopolymerisable adhesive (P1, P2, P3, P4) such as Loctite or Vitralit. The housing (9) has an adapter unit (10) between the source and the end of the optical fiber which incorporates at least one focusing lens (12). The housing and adapter unit can be made in one piece from a translucent material, and the source (2) is an opto-electronics component such as a laser diode, an electroluminescent diode or a photodiode.

Description

Device for coupling an optical fiber to a source, and method for fixing a base of such a device
The invention relates to the field of devices for coupling optical fibers to emitting or receiving sources, capable of cooperating with such fibers. It relates more particularly to a device of the aforementioned type and comprising a body carrying the source, and a base comprising an opening for housing one end of the fiber.

Usually, the fiber, once introduced into the opening, is stuck in the base. Then, the base itself is assembled to the body by welding or by gluing, the end of the fiber being opposite the source. This type of assembly then requires two operations on an optical bench, which can be long and costly.

The object of the invention is in particular to overcome this drawback.

To this end, it offers a connection device of the aforementioned type, but in which the base is made of a translucent material. Thus, the fiber can be bonded to the base, and the base itself can be bonded to the body by a photopolymerizable adhesive.

Advantageously, the body comprises an adaptation block between the source and the end of the optical fiber. This block includes at least one focusing lens so as to reduce the angle of incidence of the interaction beam at the input of the fiber.

Preferably, the base is produced by molding in polymethyl methacrylate.

Thus, the base is fixed to the body according to a method which the present invention further proposes, and which consists in providing the following steps - placing the body and the base on an optical bench, - placing the base substantially in contact with the body, lodge the end of the optical fiber in the opening, opposite the source, - apply a light-curing glue to one face of the base facing the body, as well as in the opening of the base, by capillarity, - optimize the spatial position and / or the angular position of the fiber end relative to the source to obtain maximum cooperation between the source and the fiber, and - emit light radiation towards the base from '' a lamp with selected radiation characteristics, for rigidly fixing the fiber in the base and the base to the body.

In a substantially different embodiment, the body and the adaptation block can be made in one piece from a translucent material, so that they can be joined to the base by gluing with a photopolymerizable adhesive.

Other characteristics and advantages of the invention will appear on examining the detailed description below, and the appended drawings in which - FIG. 1 is a simplified optical diagram of a device for coupling an optical fiber to a transmitting or receiving source, intended to cooperate with the fiber, and - FIG. 2 is a view in longitudinal section of the coupling device according to the invention, and the base of which is made of a translucent material.

The drawings essentially contain elements of a certain character. They can therefore not only serve to better understand the description, but also contribute to the definition of the invention, if necessary.

Firstly, reference is made to FIG. 1 which schematically illustrates the principle of a device for coupling an optical fiber 1 (index jump or radial index gradient) to a transmitting or receiving source 2 which , in the example described, is an optoelectronic component connected to a power supply 4.

In emission, the source 2 can then be a solid-state semiconductor laser diode or a light-emitting diode. In reception, it can be a photodiode. In the context of, for example, optical telecommunications, the minimum optical attenuation of the fibers is generally obtained for wavelengths between 0.8 and 1.55 μm. Semiconductor materials with equivalent band gap widths, such as, for example, ternary or quaternary alloys such as GaInAs or GaInAsP on InP substrates, can then be used to manufacture such sources.

For example, in the case where the source 2 is a laser diode or a light-emitting diode, the latter emits an optical beam 3, conical at the output of the diode and of substantially elliptical section. In certain applications, the emission cone is such that its opening angles can be around 40 in the Y direction shown in FIG. 1 and 15 in the X direction. These angular values are much greater than the angles of acceptance limits of conventional optical fibers. In fact, for the fiber 1 to be able to act as a waveguide, the angle of entry of the beam into the fiber must be close to ten degrees at most.

This is why provision is traditionally made to place on the optical path between the source 2 and the fiber 1, an adaptation block 10 which, in the example illustrated in FIG. 1, comprises two convex focusing lenses L1 and L2. If f and f 'are the focal distances of the optical lenses L1 and L2 respectively (with f'> f), by placing the source 2 on the object focal plane of the lens L1 and the aperture 6 of the fiber 1 on the plane focal image of the lens L2, the incidence of the beam at the input of the fiber is reduced by a ratio substantially equivalent to the ratio of the focal lengths f '/ f.

Generally, focal ratios f ′ / f close to 5 are chosen, which means that the magnification associated with such an optical system corresponds to a value close to 5.

Under these conditions, a greater proportion of light energy can be guided. In addition, the beam 3 is thus centered around the core 5 of the fiber. Generally the fibers are of substantially circular section, and their core has a diameter of about 10pu. As the sources are generally micrometric, it can be seen that with a magnification close to 5, the image of the source 2 is of greater size or, at best, equal to the diameter of the core 5 of the fiber. Coupling between a source and an optical fiber then requires an optimization of the position of the end 6 of the fiber relative to the source 2. Above all, after having optimized this position, the source 2, the adaptation block comprising the lenses L1 and L2, and the end 5 of the fiber must be rigidly fixed with respect to each other. Indeed, a lateral displacement of the source 2, for example, would cause an approximately five times greater displacement of its image relative to the core 5, and consequent energy losses at the entry of the fiber.

It is then customary to use a coupling device of the type shown in FIG. 2. The source 2, in the example described, is connected by its pins 7 to a power supply (not shown in FIG. 2). The coupling device then comprises a body 9 carrying the source 2. This body is, in the example shown in Figure 2, made of a turned or molded metallic material. It is substantially cylindrical in shape and has an axial bore to enclose an adaptation block 10 between the source 2 and the end 6 of the fiber 1. This block 10 comprises a focusing lens 12. In fact, the two lenses L1 and L2 in FIG. 1 are advantageously replaced here by a single biconvex lens with two different curvatures, and possibly with an aspherical surface to limit optical aberrations. Such a lens can be produced by molding in polymethyl methacrylate for example. Its optical function is substantially equivalent to the assembly of FIG. 1. The associated magnification is still close to 5. The source 2 and the block 10 are then housed in the body 9.

To keep the end 6 of the optical fiber 1 stationary, a base 8 (or ferrule) is generally used. This type of base is known to those skilled in the art, in particular under the name of pig tail "(English term). Usually, this base 8 is made of a metallic or ceramic material and, in the example described, is in the form of a substantially cylindrical pellet, of diameter equivalent to the external diameter of the body 9. In the center of the pellet and on the face opposite to the body 9, the base 8 comprises a cylindrical hollow rod 11 for housing the fiber 1. This rod extends perpendicularly from the center of the patch. On the opposite face of the block 9, the base 8 has an axial bore 13 through which the end 6 of the fiber appears. the bore 13, extending into the hollow rod 11, forms an opening in the base 8 to receive the end 6 of the fiber, facing the source 2.

Traditionally, this metal base 8 is welded to the body 9, or even glued to it. A few weld points are then applied to the external periphery of the joint between the base 8 and the body 9, or glue on the face of the patch facing the body 9, on the points P1 and P2 of FIG. 2 However, a conventional adhesive on metal polymerizes for a particularly long period, and fixing the base by welding is relatively expensive.

Furthermore, the base 8 and the body 9, before assembling them, are generally arranged on an optical bench, and in particular on plates which are movable in translation along the axes X, Y and Z, and possibly in rotation around these axes. In the case, for example, where the source 2 is emitting, a device on the optical bench (notably comprising a photodiode at the output of the fiber) makes it possible to measure the intensity of the signal that the fiber receives. The position of the end 6 of the fiber on the Z axis is then optimized, and the position on the X and Y axes of the base 8 relative to the body 9. Once the maximum of the measured signal is reached, the fiber is fixed by gluing on points P3 and P4, and the base 8 is fixed to the body 9 by welding or gluing. Obviously, the fiber 1, the base 8 and the body 9 must remain stationary until the polymerization of the adhesives used is completed.

The present invention then comes to improve things since it provides a base 8 of substantially identical general shape, but made of a translucent material.

Indeed, the use of such a base in combination with the use of a light-curing adhesive, may prove to be particularly judicious for fixing both the fiber 1 in the base 8 and the base itself. to the body 9.

As before, the fiber, the base and the body are arranged on the plates of an optical bench, and the signal received for example by the fiber can be measured in order to optimize the position of the end 6. The fiber is then introduced into the opening of the base 8. Then, a photopolymerizable adhesive is applied to the openings of the surfaces P1, P2, P3 and P4, which generally has the advantage of being particularly fluid. One can then use for example glues of the "Loctite" or "Vitralit" (registered trademarks) type, known per se, which polymerize when they are exposed to light radiation, and in particular to ultra-violet radiation. Thus, the glue can penetrate by capillarity and bathe all these surfaces while remaining perfectly liquid and allowing the mobility of the parts.

In the example described, the adhesive applied in P3 and P4 can rise by capillary action to the bore 13, which will make it possible to fix the fiber over practically the entire length of the rod 11. In addition, the adhesive applied to the points P1 and P2 extend over a substantial surface of the base 8 substantially in contact with the body 9.

The positions along the X, Y and Z axes of the end 6 are then adjusted so that they substantially coincide with the image of the source 2. Once these positions have been optimized, light is emitted towards the base 8 at from an ultra-violet lamp for example. The adhesive then polymerizes in just a few tens of seconds, and the position of the end 6 of the fiber along the three axes at once,
X, Y and Z, remains optimal compared to source 2.

The choice of the translucent material in which the base 8 must be made depends of course on the adhesive used.

For an adhesive sensitive to radiation in the visible wavelength range, it is possible, for example, to use styrene, silica, polycarbonate or polymethyl methacrylate. According to a preferred embodiment, common photosensitive and polymerizing adhesives are used when they are exposed to W radiation. Polymethyl methacrylate in particular is relatively poorly absorbent in this wavelength range. The translucent base of the device according to the invention is then produced by molding in this material.

Of course, the invention is not limited to the embodiment described above by way of example, it extends to other variants.

Thus, it will be understood for example that, conversely, the base 8 can be made of any material, and the body 9 of a translucent material so as to be bonded to the base with a photopolymerizable adhesive. In this embodiment, the adapter block 10 and the body 9 are made in one piece from a translucent material such as, for example, polymethyl methacrylate.

Furthermore, the general shape of the base can be different. One can imagine, in fact, a base comprising two elements between which the fiber is immobilized. By applying light-curing glue between these elements and on the face of the body opposite the fiber, the assembly is stiffened.

Claims (10)

 1. Device for coupling an optical fiber (1) to a  transmitting or receiving source (2), of the type comprising a  5 base (8) having an opening (13) suitable for housing a  end (6) of the fiber, and a body (9) carrying the source  (2), the end (6) of the fiber facing the source  (2) for their cooperation,  characterized in that the base (8) is made of a translucent material so that the fiber (1) can be  stuck in the opening of the base (8), and the base (8)  glued to the body (9), by a light-curing adhesive (P1,  P2, P3, P4).  comprising at least one focusing lens (12).  source (2) and end (6) of the optical fiber (1), and  that the body (9) has an adaptation block (10) between the 15 2. Device according to claim 1, characterized in that  in one piece in a translucent material.  that the adapter block (10) and the body (9) are made 20 3. Device according to claim 2, characterized in that  4. Device according to one of claims 2 and 3, characterized in that the adaptation block (10) comprises at  minus two convex focusing lenses (L1, L2).  5. Device according to one of the preceding claims,  characterized in that the base (8) has a patch and a rod (11) projecting substantially from the center  of the pellet, and cylindrically hollow to accommodate the end  mite (6) of optical fiber.  6. Device according to claim 5, characterized in that the patch has on a face opposite the body (9),  an axial bore (13) extending into the rod (11) to  form said opening. 7. Device according to one of the preceding claims, characterized in that the source (2) is an optoelectronic component among a laser diode, a light-emitting diode and a photodiode. 8. Device according to one of the preceding claims, characterized in that the base (8) is made of a translucent material from styrene, silica, polymethyl methacrylate and polycarbonate. 9. Device according to claim 8, characterized in that the base (8) is produced by molding in polymethyl methacrylate. 10. Method for fixing a base (8) of a device according to one of the preceding claims, characterized in that it provides the steps consisting in - placing the body (9) and the base (8) on an optical bench, - place the base (8) substantially in contact with the body (9), accommodate the end (6) of the optical fiber (1) in said opening (11), facing the source (2 ), - apply a light-curing adhesive on one face (P1, P2) of the base facing the body, as well as in the opening (P3, P4) of the base, by capillary action, - optimize the spatial position and / or the angular position of the end (6) of the fiber relative to the source (2) in order to obtain maximum cooperation between the source and the fiber, and - emitting light radiation towards the base (8) from 'a lamp with selected radiation characteristics, for rigidly fixing the fiber (1) in the base (8) and the base (8) to the body (9).