Optical waveguide and process for its manufacture

The invention relates to an optical waveguide according to the preamble of patent claim 1.

Waveguides of the type mentioned are generally known.

The invention has for its object to show how, in the case of a waveguide of the type mentioned, the refractive index difference between the core and the cladding can be increased given fixed refractive indices for the core and cladding.

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

The refractive index of the cladding is effectively reduced by the cavity according to the invention in the cladding of the waveguide and the difference in refractive index between cladding and core is thereby effectively increased. As a result, the numerical aperture of the waveguide can be greatly increased and the radiation losses of the waveguide in the case of a curved course can be considerably reduced.

The increased numerical aperture enables couplings between the waveguide according to the invention and a semiconductor laser without optical lenses. The reduced radiation losses with a curved course of the waveguide enable sharply curved waveguide curves with a small space requirement, which previously could only be achieved with the aid of reflectors which could only be produced with great effort and with the necessary precision.

Although a cavity according to the invention can adjoin the core of the waveguide, preferred configurations of the waveguide according to the invention are designed such that the cavity is arranged at a radial distance from the core relative to the axis (claim 2).

A cavity according to the invention can certainly extend over the entire axial length of the core of the waveguide, but preferred embodiments of the waveguide according to the invention are designed such that the cavity extends along the axis only over a fraction of the axial length of the core (claim 3).

Particularly in the case of waveguides with a straight axis, it can be preferably and advantageously set up in such a way that two or more cavities are distributed over the circumference of the core surrounding the axis (claim 4).

A waveguide according to the invention with a greatly increased numerical aperture is advantageously designed in such a way that at least one cavity is arranged at an axial end of the core (claim 5). The optical radiation is to be coupled in via this axial end of the core. It is particularly advantageous if two or more are distributed at the axial end of the core over the circumference of the core surrounding the axis. With the help of these measures, the light acceptance from a light-emitting diode in the waveguide can be increased considerably.

To reduce the radiation losses in the case of a curved waveguide, in which the core runs at least in sections along a curved axis, it is particularly advantageous if a cavity is arranged in the jacket only on one side of the axis remote from a center of curvature of the curved axis (claim 6) .

A cavity can be evacuated, but is preferably filled with a fluid, for example a gas. According to a preferred embodiment of the waveguide according to the invention, at least one cavity is filled or can be filled with an optically active medium, for example an optically nonlinear fluid (claim 7). In this case, an optical switch can advantageously be implemented with the waveguide according to the invention.

The waveguide according to the invention can advantageously also be used for spectrophotometric analyzes. For this purpose, at least one cavity in the waveguide according to the invention can be filled with a fluid to be analyzed spectrophotometrically (claim 8), which preferably absorbs. The long optical path in the evanescent field of the waveguide according to the invention allows high detection sensitivity.

A hermetic seal of a cavity of the waveguide according to the invention (claim 9) prevents deterioration due to aging effects.

Although the invention is applicable to all waveguides of the type mentioned at the outset, it is preferably and advantageously applied to a planar waveguide integrated on the surface of a substrate. Planar waveguides are widely used in micro-optics. In conventional planar waveguides, the difference in refractive index between the core and the cladding is of the order of magnitude about 1%. The numerical aperture of this conventional planar waveguide is therefore usually very small and is at most 0.30, as a result of which the coupling efficiency between such a planar waveguide and a laser diode is generally considerably limited. In addition, sharp curvatures are prohibited in curves with these planar waveguides, since otherwise considerable radiation losses would have to be accepted.

With a planar waveguide according to the invention, these restrictions can advantageously be removed. A particular advantage of the planar waveguide according to the invention is the considerable increase in the numerical aperture due to the lowering of the refractive index of the cladding. If several cavities are arranged at one axial end of the core of the planar waveguide, the light acceptance from a light-emitting diode can be increased considerably, as a result of which lensless couplers for optically coupling a light-emitting or laser diode and a waveguide can be realized. Beam converters can also be implemented. The outside arrangement of one or more cavities in a curvature of the planar waveguide considerably reduces its radiation losses. In the case of planar waveguides in particular, it can also be implemented as a switch or used for spectral photometric analysis.

Preferred and advantageous refinements of the planar waveguide according to the invention emerge from claims 11 to 18.

A preferred method for producing a planar waveguide according to the invention is generally apparent from claim 19. The embodiment of this method according to the invention specified in claim 20 has the advantage that a plurality of cavities distributed around the core can be produced. Further preferred and advantageous refinements of the method according to the invention emerge from claims 21 to 23.

The invention is explained in more detail in the following description using the figures as an example. Show it:<dl id="dl0001"><dt>Figure 1</dt><dd>3 shows a cross section perpendicular to the axis along the section line II in FIG. 2 through an exemplary embodiment of a planar waveguide according to the invention which has two cavities,</dd><dt>Figure 2</dt><dd>2 shows an axial longitudinal section along the section line II-II in FIG. 1 through the exemplary embodiment according to FIG. 1,</dd><dt>Figure 3</dt><dd>3 shows an axial longitudinal section through another exemplary embodiment of a planar waveguide according to the invention, which differs from the example according to FIGS. 1 and 2 in that the axis has a curvature and only one cavity is present,</dd><dt>Figure 4</dt><dd>3 shows a cross section perpendicular to the axis through a further exemplary embodiment of a planar waveguide according to the invention, which differs from the examples according to FIGS. 1 to 3 in that there are four cavities,</dd><dt>Figure 5</dt><dd>3 shows a cross section through a planar waveguide structure in which three parallel cores are surrounded by a common cladding and which serves as an output stage for producing a plurality of planar waveguides according to the invention,</dd><dt>Figure 6</dt><dd>the structure of Figure 5 in the same representation after creating recesses in a recess layer and</dd><dt>Figure 7</dt><dd>in the same representation as in FIGS. 5 and 6, the final stage of the waveguide structure, which consists of three planar waveguides according to the invention, each of which corresponds to the waveguide according to FIGS. 1 and 2, and</dd><dt>Figure 8</dt><dd>in the same representation as in FIG. 6, a waveguide structure consisting of three planar waveguides according to the invention, each of which corresponds to the waveguide according to FIG. 4.</dd></dl>

The figures are schematic and not to scale.

In the exemplary embodiment shown in FIGS. 1 and 2, the planar waveguide, generally designated 1, is applied to the surface 20 of a substrate 2 and consists of the jacket 12, which is applied to the surface 20 of the substrate 2 and consists of solid material, and of which is completely made of Sheath 12 surrounded and core 11.

The material of the core 11 has a refractive index n1 and the material of the cladding 12 has a refractive index n2 which is smaller than the refractive index n1 of the core 11, so that a difference in the refractive index between the core 11 and the cladding 12 <maths id="math0001" num=""><math display="inline"><mrow><mtext>Δn = n1-n2&gt; 0</mtext></mrow></math><img file="EP0831343A2_D0001.tif" /></maths> consists.

The core 11 extends along an axis 10, along which an optical radiation (not shown) is guided in the core 11 and which is perpendicular to the plane of the drawing in FIG. The cross section of the core 11 perpendicular to the axis 10 is essentially rectangular, ie the circumference 110 of the core 11 surrounding the axis 10 describes a rectangle. .

According to the invention, at least one cavity 13 is formed in the solid material of the jacket 12. In the example according to FIGS. 1 and 2, two cavities 13 are specifically formed in the jacket 12, which are arranged opposite opposite sides of the rectangle 113 and 115 of the circumference 110 of the core 11.

Each of the two cavities 13 is arranged at a distance a from the core 11, which can be the same or different for both cavities 13. A cavity 13 could also directly adjoin the core 11, so that the distance a of this cavity 13 would be zero.

In the example according to FIGS. 1 and 2, each of the two cavities 13 does not extend along the axis 10 over the entire axial length l of the core 11 delimited by end or axial ends 111 and 112, although this would be possible, but only over one Fraction l / z of this axial length l (see FIG. 2), z being any number greater than 1. z can be different or the same for different cavities. In the example according to FIGS. 1 and 2, both cavities 13 are chosen to have the same axial length, so that z&gt; 1 is the same for both cavities 13.

The position of the cavities 13, which are shorter than the length l of the core 11, in relation to the core 11 can in principle be chosen as desired.

If the waveguide 1 is to have the highest possible aperture at one of the two axial ends 111 and 112 of the core 11, for example at the end 111 for the optical radiation to be coupled into the core 11 via this end 111 and to be guided in the core 11, at least one cavity is formed 13 arranged at this axial end 111. In this case, it is expedient if two or more cavities 13 are arranged at the axial end 111, as can be seen from FIG. 2. The same applies to the other end 112.

If the axis 10 and thus the core 11 do not run in a straight line, as in the case of the exemplary embodiment according to FIGS. 1 and 2, but instead have a curvature K as in the exemplary embodiment according to FIG. 3, it is favorable for reducing radiation losses in the region of the curvature K. to arrange a cavity 13 in the region of the curvature K, in which case it is again expedient to arrange the cavity 13 only on the outside 102 of the curvature K, where that of the outside 102 is the side of the curvature K facing away from a center of curvature 100 of the curvature K.

A cavity 13 can be hermetically sealed in the jacket or have an opening to the outside. In FIG. 2, for example and without restricting the generality, the cavity 13 on the left of the core 11 is shown as open, the cavity 13 on the right of the core 11, on the other hand, is hermetically sealed, the open cavity 13, for example, an opening 132 in which the end 111 of the core 11 containing end face 110 of the waveguide 1. A hermetic seal of a cavity 13 prevents deterioration due to aging effects.

A vacuum or preferably a fluid 130 (see FIG. 4) can be located in a cavity 13. Since the cavities 13 serve to effectively lower the refractive index n2 predetermined by the solid material of the jacket 12, care must be taken that a fluid 130 has a lower refractive index n3 than the refractive index n2.

The fluid 130 can be an active medium, for example an optically non-linear fluid, as a result of which an optical switch can be implemented with the waveguide 1 according to the invention. The fluid 130 can also be a fluid to be analyzed spectrometrically, whereby a waveguide 1 according to the invention is suitable for spectral photometric analyzes, the long optical path in the evanescent field of the waveguide 1 permitting high detection sensitivity.

The exemplary embodiment according to FIG. 4 differs from the examples according to FIGS. 1 to 3 in that a cavity 13 is arranged opposite all four rectangular sides 113, 114, 115 and 116 of the circumference 110 of the core 11, so that the core 11 is four Cavities 13 is surrounded.

To produce a planar waveguide 1 according to FIGS. 1 to 4, it is advantageous if this planar waveguide 1 is designed as shown in FIGS. 7 and / or 8, ie so that a strip-like layer 31, which extends along the axis 10 parallel to the surface 20, forms the core 11 of the waveguide 1 and consists of a material having the refractive index n1 of the core 11, is embedded in a layer stack 32 which forms the cladding 12 of the waveguide 1 and consists of several layers each of a solid material having a refractive index n2 of the cladding 12.

In the examples according to FIGS. 7 and 8, there is the advantageous special feature that not only a strip-like layer 31 forming a core 11, but also its several, for example three mutually parallel strip-like layers 31, which each form a core 11, is embedded in the layer stack 32 are, which forms the jacket 12. In this way, three waveguides 1 are realized on the surface 20 of the substrate 2, the cores 11 of which are surrounded by a common jacket 12.

In the example according to FIG. 7, the layer stack 32 forming the common jacket 12 consists of layers 321, 322 and 323, and in the example according to FIG. 8 from layers 320 to 325.

The refractive index n2 of the different layers 321 to 323 or 320 to 325 of the layer stack 32 can differ, it is then only important that the maximum refractive index n2 of these layers 321 to 323 or 320 to 325 is smaller than the refractive index n1 of a core 11.

It is essential that at least one of the layers 321 to 323 or 320 to 325 of the layer stack 32 is a recess layer, in which at least one recess 131 defining a cavity 13 is formed, which is from a side facing away from the surface 20 of the substrate 2 further layer of the layer stack 32 applied to the recess layer is covered and thereby forms a cavity 13 according to the invention.

In the example according to FIG. 7, only the layer 322 in the layer stack 32 is a recess layer, in which recesses 131 are formed, which are covered by the layer 323 applied to the recess layer 322 on the side facing away from the surface 20 of the substrate 2, and each have a cavity Define 13. In this case, it is specifically designed so that in the recess layer 322, two strips 131, each assigned to this strip-like layer 31, are formed per strip-like layer 31. In this way, each waveguide 1 according to FIG. 7 is a waveguide 1 according to FIGS. 1 and 2, in which two cavities 13 are distributed over the circumference 110 of the core 11.

In the example according to FIG. 8, the layers 320, 322 and 324 of the layer stack 32 are three recess layers which are separate from one another and in which recesses 131 are formed. The recesses 131 of the recess layer 320 are covered by the layer 321, which is applied to this recess layer 320 on the side facing away from the surface 20 of the substrate 2, and each of these recesses 131 forms a cavity 13 according to the invention. Correspondingly, the recesses 131 of the recess layers 322 and 324 are covered by the layer 323 and 325, and each of these recesses 131 likewise forms a cavity 13 according to the invention.

It is specifically designed so that a recess 131 is formed under each strip-like layer 31 in the recess layer 320 and a recess 131 is formed above each strip-like layer 31 in the recess layer 324, and that, as in the example according to FIG. 7, in the recess layer 322 for each strip-like layer 31 there are two recesses 131 assigned to this strip-like layer 31, which are arranged on both sides of this strip-like layer 31. In this way, each waveguide 1 according to FIG. 8 is a waveguide according to FIG. 4, in which four cavities 13 are distributed in the cladding 12 over the circumference 110 of the core 11.

Embodiments in which three cavities 13 are distributed over the circumference 110 of the core 11 can be obtained, for example, with reference to the example according to FIG. 8, in that either the layer 320 or the layer 324 is omitted or has no cutouts 131. An embodiment different from the example according to FIGS. 1 and 2, in which two cavities 13 are distributed over the circumference 110 of the core 11, can be obtained in that, in the example according to FIG. 8, the layer 322 has no recesses 131.

Other distributions of cavities 13 over the circumference 110 of a core 11 can be obtained by a different number and / or arrangement of recess layers and / or a different assignment of cavities 13 to this core 11.

In particular with regard to a type of manufacture of a waveguide 1 according to the invention described below, the deeper in a recess layer the recess 131 defining a cavity 13 is the more favorable. The recess 131 defining a cavity expediently extends over the entire thickness of the relevant recess layer, ie over the thickness d0 of the recess layer 320, the thickness d2 of the recess layer 322 or the thickness d4 of the recess layer 324.

An advantageous method for producing the example according to FIG. 7 will be explained in more detail with reference to FIGS. 5 to 7. This process begins with the output stage shown in FIG. 5.

To produce the output stage according to FIG. 5, the layer 321 is first applied to the surface 20 of the substrate 2, on which in turn a layer made of the material of the core 11 is applied. This layer is structured, for example by means of a conventional lithography process, in such a way that the strip-like layers 31 shown, separated from one another, are formed, each of which defines a core 11 and an axis parallel to the surface 20 of the substrate 2 and determining the longitudinal direction of the strip-like layer 31 10 has. The layer 322 is applied to the resulting strip-like layers 31 and the layer 321 exposed between these layers 31 in such a way that the strip-like layers 31 are completely covered.

The layer 322 is a recess layer, in which recesses 131 defining cavities 13 are produced in such a way that each strip-like layer 31 is assigned two recesses 131, which are arranged on both sides of this assigned strip-like layer 31. Thereafter, the manufacturing stage shown in Figure 6 was created.

The recesses 131 can be produced by a conventional photolithographic process, for example an etching process.

Finally, a layer 323 is applied to the recess layer 322 having the recesses 131 in such a way that the recesses 131 are covered, but a recess 131 is not completely filled with layer material, but instead a cavity 13 remains in this recess 131 under the covering layer 323 .

A similar procedure is used to produce the example according to FIG. 8. Here, the recess layer 320 is first applied to the surface 20 of the substrate 2, in which recesses 131 are produced after the application, each of which is assigned to a core 11 to be produced later in such a way that this core 11 lies above the recess 131 assigned to it Cutout layer 320 is arranged.

The layer 321 is applied to the recess layer 320 in such a way that the recesses 131 in the recess layer 320 are covered, but a recess 131 is not completely filled with layer material of this layer 321, but a cavity 13 remains under this layer 321 in each recess 131 .

After the layer 321 has been applied, the layer structure shown in FIG. 7, consisting of the layers 321, 322 and the strip-like layers 31 embedded between these layers 321 and 322, in which the layer 322 is a recess layer which has the cutouts 131 as well as in the case of FIG Example according to Figure 7 has, as described above.

The layer 323 is applied to the layer 322 of this layer structure in such a way that, although the recesses 131 are covered in the recess layer 322, a recess 131 is not completely filled with layer material of this layer 323, but a cavity 13 under this layer in each recess 131 323 remains.

A further recess layer 324 is applied to the covering layer 323, in which a recess 131 associated with each strip-like layer 31 is produced above each strip-like layer 31. After the cutouts 131 have been produced in the further cutout layer 324, a layer 325 is applied to this cutout layer 324 in such a way that, although the cutouts 131 in the cutout layer 324 are covered, a cutout 131 is not completely filled with layer material of this layer 325, but in each Recess 131 a cavity 13 remains under this layer 325.

The final product shown in FIG. 8 is then available.

To apply a layer to a recess layer having a recess 131 in such a way that the recess 131 is covered in the recess layer, but a cavity 13 remains in the recess 131 under the covering layer, the covering layer, for example layer 321, 323 or 325, advantageously deposited from a gas phase 5 indicated in FIG. 6 on the recess layer, in the example of layer 320, 322 or 324. Such a method can ensure that a recess 131 is not added with the layer material of the layer to be deposited on the relevant recess layer.

Advantageously, not only are the layers to be deposited on the recess layers, but each layer of the layer stack 32 is deposited from a gas phase 5.

It is particularly advantageous if a layer of the layer stack 32 to be deposited from a gas phase 5 is deposited by means of flame hydrolysis. Flame hydrolysis has changed in an application filed on the same day with the official file number ..... (GR 96 P 2173 DE) described method for producing a capillary proved to be particularly effective, in which a groove formed in one surface and defining the capillary is covered with a layer such that a cavity forming the capillary in the groove under the covering layer remains. The described there with respect to the groove applies in the same way to the recesses 131 described here, so that in the application .... (GR 96 P 2173 DE) described method can be applied in the same way in the method described here. As there, recesses 131 with a width of 2 to 100 μm can also be covered here.

Accordingly, it is advantageous to use a material based on SiO for each layer of the layer stack 32 to be deposited by flame hydrolysis or otherwise from a gas phase₂, preferably more than 60 mol% of SiO₂ is to be used, preferably a material based on SiO₂that with at least one substance from substance group B₂O₃, P₂O₅, GeO₂, Al₂O₃, Ga₂O₃, TiO₂, ZrO₂, ZnO, MgO, As₂O₃, Sb₂O₅, Bi₂O₃ and / or SnO₂ is endowed. Such layers are deposited as porous layers. If such a deposited layer has a sufficient thickness of, for example, more than 10 μm and a high gas viscosity, it can be sintered to a clear glass after careful deposition at a temperature of over 900 ° C. after careful deposition so that covered recesses 131 are not filled , with deeper recesses 131 being cheaper than less deep ones. It is also favorable to separate a layer from the gas phase 5 by means of a particle stream, which is preferably directed at a flat angle of, for example, 25 ° to 30 ° to the surface 20 of the substrate 2.

For layers of the layer stack 32 made of material based on SiO₂ the substrate 2 advantageously consists of Si, Al₂O₃ and / or a material based on SiO₂, since these materials are thermomechanically compatible with the material of the layer stack 32.