Optical branching device.

0001The invention relates to an optical branching element, in particular for fiber optic applications, which is used to distribute light guided in an optical waveguide to at least two further optical waveguides.

0002Such branching members are known, for example from the articles by HF Ma_Hlein et al .: Interference filter as fiber directional coupler for WDM, Elektronic Letters 16, 1980, pages 584 - 585 and E. Miyanchi et al .: Compact wavelength multiplexer using optical-fiber pieces, Optics Letters S, 1980, pages 321- 322.

0003In arrangements of this type, the end faces of optical fiber pieces are fastened at the smallest possible distance from one another in such a way that through partially transparent or wavelength-dependent mirror faces, the obliquely polished fiber end faces

0004are applied, a distribution of the light from one fiber to several fibers is made possible. In each of the arrangements mentioned, a material is used to adjust the optical refractive index between the fiber ends. Since dielectric mirror layers are used in each of the arrangements mentioned, it is difficult or impossible to weld fiber ends and other optical components. A simple, known optical index adjustment consists in filling the space between the fiber ends with a transparent plastic, for example epoxy resin. This is quite a solution for test samples; however, it is doubtful whether such an arrangement has the long-term stability required for a commercially used arrangement. Because light intensities that usually occur in fiber-optic systems are very high: for a gradient fiber with a core diameter of 50_/um and a light output of 2mW, which is guided in the fiber core, results in an intensity of 100 W / cm. This intensity is very high. Therefore, the question arises whether an organic putty (eg epoxy resin) with permanent lighting with such a high light intensity is not subject to an aging process which, for example, leads to clouding or absorption of the initially transparent putty.

0005The invention is therefore based on the object of improving such optical branching elements in such a way that an adaptation of the optical refractive indices is avoided and that good long-term stability can be achieved.

0006This object is achieved by the features specified in the characterizing part of patent claim 1.

0007Appropriate embodiments are compiled in the subclaims.

0008An advantage of the invention is that the optical branching element can be designed as a maintenance-free optical component.

0009The invention is explained in more detail below on the basis of exemplary embodiments with reference to the schematic drawings. Show it<ul id="ul0001" list-style="none"><li>FIG. 1 to 4 embodiments of the invention</li><li>FIG. 5 is a schematic diagram for explaining FIG. 4 and 5</li></ul>

0010The structure of such a branching element is shown in FIG. 1 explained. The light (arrows) arriving, for example, in an optical fiber 10 with a core 16 and a jacket 17 is to be divided up between the optical fibers 20 and 30. The end face 13 of the optical fiber 10 is at an angle of 90⁰ to the optical axis 11, the light emerging from the optical fiber 10 into the space 40 is not refracted, but instead continues in the direction of the optical axis 11 and strikes an optical layer 24, for example a dielectric, semi-transparent mirror. Part of the light is reflected at the reflection angle and coupled into optical fiber 30. In the exemplary embodiment shown, three optical fibers 10, 20 30 are arranged in such a way that their optical axes 11, 21, 31 have a common point 22 which lies on the layer 24. The other part of the light is on the layer 24 at an angle β = sin^-1 (<maths id="math0001" num=""><img file="EP0053324A2_D0001.tif" /></maths> sin a) broken into the optical fiber 20. n_K is the effective refractive index of the fiber core 26, α the angle of incidence and reflection of the light. For an angle δ the following must apply: δ = 900 - β.

0011The data used in the exemplary embodiment are:<ul id="ul0002" list-style="none"><li>α = 45 °; n_K = 1.46; β = sin^-1<maths id="math0002" num=""><img file="EP0053324A2_D0002.tif" /></maths> sin 450 = 29 °; δ = 90 ° - ß = 61 °;</li></ul>

0012For the angle of incidence α, another angle can preferably be chosen between 20 ° and 45 °. ß and δ result accordingly.

0013The type of dielectric layer 24 determines the division of the light into optical fiber 20 and optical fiber 30. Depending on the system requirements, it is possible to choose a division ratio of 1: 1 or different; Furthermore, it is possible to choose the distribution independent of the wavelength for the wavelengths involved or depending on the wavelength; in the wavelength-dependent case, for example, the layer 24 is designed such that light of the wavelengths λ₁ and λ₂ arrives through the optical fiber 10, light of the wavelength λ₁ with high coupling efficiency in optical fiber 20 and light with wavelength λ₂ is fed into optical fiber 30 with a high coupling efficiency. Such an arrangement takes advantage of the dichroic behavior of layer 24 and can be used, for example, in an optical demultiplexer. The direction of the light flow is at least partially reversible. For example, a component is useful for so-called duplex operation (using a line in two directions), in which the light arriving in optical fiber 10 has a wavelength λ₁, for example, is completely fed into optical fiber 30, while light arriving in optical fiber 20 (against the direction of the arrow) with wavelength λ₂, is fed as completely as possible into optical fiber 10 (against the direction of the arrow). Such a component enables two-way communication between two participants.

0014In addition to the avoidance of cement material, a further considerable advantage of the embodiment according to the invention over the known solutions mentioned at the outset is that layers 24 with more favorable properties can be used. If an angle a is used between the surface normal of the layer 24 and the fiber axis 11 of the incoming light in known solutions, the light falls on the layer 24 from the glass side (refractive index ≈ 1.5). In the branching device according to the invention, however, the light falls from the glass side at an angle β onto the layer 24. From the optics of thin layers it is known that the edge steepness (optical filter effect) of a dielectric beam splitter layer in the wavelength range can be made greater, the less the angle of incidence of the light on the layer differs from the vertical striking. In the event of deviations from perpendicular impingement, the different polarization directions of the light are reflected or transmitted with different reflectivities and different wavelength dependencies. Likewise, with a divergent light beam, the edge steepness deteriorates the more the angle of incidence deviates from the vertical incidence. If an optical kit layer is omitted, the angle β is important for an optical filter effect, at the same angle a as in the known cemented solutions; while in the cemented case the angle α is the determining angle. If the angle α is reduced to 20 °, for example, the angle β = 13.5. With such an arrangement, beam splitter layers with excellent edge steepness can be used, the solution according to the invention therefore enables the separation of closely adjacent wavelengths for wavelength division multiplexing.

0015All types of optical fibers can be used as optical fibers, for example single-wave fibers, multimode fibers with a step-shaped refractive index profile or gradient index profile.

0016The branching device according to the invention is lossy since the light in a room 40 travels a distance in a medium without wave guidance, for example air. In this area without guidance, the light beam expands due to diffraction and before beam divergence (multimode fiber), and part of the light is no longer coupled into the cores of the fibers. To reduce these losses, it is possible to partially remove the sheaths of the fibers, preferably of optical fiber 10 and optical fiber 30, for example by etching. This reduces the light paths in the medium without wave guidance and thus reduces losses.

0017To protect the surfaces from damage or precipitation of moisture, it is possible to encapsulate the fiber ends airtight.

0018The FIG. 3 and 4 show further exemplary embodiments, the functioning of which is initially based on FIG. 5 is explained in more detail. FIG. 5 shows in an exemplary diagram the optical coupling losses V, in dB, as a function of a standardized distance s / d. According to the in FIG. 5 indicated coupling arrangements s means the distance (optical path length) between two end faces of coaxially arranged optical fibers, which have a core and a cladding. The curves 51 and 52 shown relate to exemplary glass fiber optical waveguides, the sheath of which each has an outer diameter of approximately 130_/um and their core diameter d is approximately 50_/um amounts to. Curve 51 shows the coupling losses, depending on the distance s, for the case of flat end surfaces, between which there is air as the coupling medium. According to curve 51, the coupling losses V increase if the distance s (optical path length) increases. In the coupling arrangement on which curve 52 is based, the end faces of the optical waveguides are fused into spherical lenses, the radius of curvature of which is approximately equal to half the outer diameter of the jacket. The curve 52 shows a clear minimum of the coupling losses V, which is approximately at the normalized distance s / d = 2.4.

0019In an optical branching element according to FIG. 1, it is not possible for design reasons to make the optical path length between the flat end faces 13 and 33 smaller than that corresponding to the standardized distance s / d = 1.2. According to FIG. 5, curve 51 results in a coupling loss V of approximately 0.9 dB for this value. If, on the other hand, the end faces are fused into spherical lenses, it is advisable, according to curve 52, to choose a normalized distance s / d = 2.4 at which the coupling loss is only 0.6 dB. This results in a doubled optical path length, which is shown in FIG. 3 built branching the mechanical construction and the optical adjustment significantly simplified. It is also possible to choose the angle 2a between the axes 11 and 31 to be less than 90 °. Such an arrangement has the further advantage that the interference filters used act more selectively, because with interference filters it is expedient to choose the angle of incidence of the light as small as possible.

0020According to FIG. 4, it is also possible to further reduce the angle 2a by reducing the outer diameter of the cladding of the optical waveguides in the region of the lenses 131 and 331, for example by etching.

0021Optical waveguides with fused lenses can also be used analogously in a branching element according to FIG. 2nd

0022To further reduce coupling losses, it is expedient to coat the end faces and / or lenses delimiting the space 40 with a so-called anti-reflective coating for the light used.

0023In a further exemplary embodiment, not shown, at least one optical fiber delimiting the space 40 has been replaced by at least one photodetector, for example a photodiode. With such an arrangement it is possible, for example, to measure and / or regulate the intensity of the light coupled into an optical fiber.