DE4412254A1 - Optical coupling element and method for its production - Google Patents

Abstract

Optical connectors between active and/or passive waveguide structures and with optical fibres and the free space for coupling and uncoupling light play an important role in the area of integrated optics. An optical coupling member (1) which can be used in integrated optics, which can be produced by the technology conventional in this field and which can, if necessary, also deflect the passing light, forms with a waveguide (3) a structural component which is integrated monolithically on a substrate (2) and consists of planar thin layers (6i). A cylinder lens (4) there lies between the end surface of the waveguide (3) and a Fresnel zone lens (5) which is formed by planar steps disposed in an approximately "cinema-type" arrangement.

Description

The invention relates to an optical coupling element, which on an end face of a Light guide is arranged and has a Fresnel lens with structured zones, and on processes for its production.

In the field of integrated optics, optical connecting means intervene active and / or passive waveguide structures and with optical fibers and the free space for the purpose of coupling and decoupling light plays an important role to. Taper structures are technologically difficult to manufacture and are suitable for Solving the various coupling problems more or less well. Of the in the integrated optics commonly used integrated optical lenses and special Lattice structures are the last mentioned for coupling in and / or out coupling light preferred over such lenses, even if it achieves the achievable Coupling efficiency is not very high.

The invention is based on the prior art known from DE 37 35 032 A1 out. Thereafter, one is to be applied at the end of an optical waveguide Coupling optics to form the optically imaging component as a Fresnel lens, especially as one photochemically generated on the end face of the optical fiber Structure of concentric ring zones. However, this is a Coupling optics at the end of an optical fiber, both of which have a diameter of have about 125 microns.

Microlenses can now also be manufactured using thin-film technology. So it was on the occasion of a conference in Neuchatel, CH, from September 13 to 15, 1993 (cf. IEE Conference Volume No. 379, pages 54 to 59: "Thin film deposition; an alternative technique for the fabrication of binary optics with high efficiency "- E. Pawlowski) via the design and manufacture of Fresnel zone lenses are reported. With these The structured zones are concentric with rotationally symmetrical lenses Rings on a substrate surface, i. H. the main direction of propagation of light runs perpendicular to the substrate surface, with a kinoform profile that passes Light collimates / focuses. Other related details  can be found in "Optical Engineering" Feb. 1994, Vol. 33 No. 2, pages 647 to 652 E. Pawlowski et al: "Diffractive microlenses with antireflection coatings fabricated by thin film deposition ".

The invention is concerned with the technical problem for the coupling of light in optical waveguides as well as for coupling out the light, which is an optical Waveguide leaves and propagates in an optical fiber or in free space is intended to create such optical connection means, which can be found in the integrated optics use and have them produced using the usual technologies and, if necessary, that can also deflect passing light.

For an optical coupling element of the type mentioned, there is Solution according to the invention in that the optical coupling element with an optical Waveguide a monolithic integrated on a substrate, made of planar Forms thin layers existing component in which a cylindrical lens with vertical facet to the surface of the substrate between the end face of the Waveguide and the Fresnel zone lens, and the Fresnel zone lens with a is formed by planar steps approximate kinoform profile.

For the production of such a coupling element, as well as for its preferred Forms of training that will be discussed in more detail below provided according to the invention to use processes for the formation of the planar Run thin layers as strips with vertical edge zones. Are suitable for this Deposition techniques, especially ion beam sputtering (ion beam sputtering Deposition - IBSD -) or chemical vapor deposition organometallic compound (Metal Organic Chemical Vapor Deposition - MOCVD -), or special epitaxies, especially molecular beam epitaxy (Molecular Beam Epitaxy - MBE -, especially Metal-Organic Molecular Beam Epitaxy - MOMBE -), which is also otherwise in the field of integrated optics have proven. In addition, the production of Waveguide structures made of polymers gain importance.

The optical formed on the substrate as a structured planar thin layer With its axis, the waveguide gives light that can be coupled in as well as out the main direction of light propagation. If the surface of the substrate as x / z plane and the direction of the normal at this plane denoted by y, and runs the axis of the optical waveguide in the z direction, must be from the optical  Coupling element for collimating or for focusing the passing light both in x as well as be deflected in the y direction. For collimating / focusing in the The cylindrical lens, whose facet is designed as a cylinder jacket, ensures the x direction runs perpendicular to the substrate surface. Passing light is in the x direction additionally distracted if the facet is asymmetrical.

The function of collimating / focusing the passing light in the y direction and if necessary, the Fresnel zone lens takes over. Your one-dimensional Kinoform profile is approximated by planar steps parallel to the substrate surface, so that collimation / focusing in the direction of propagation at one symmetrical kinoform profile solely depending on the distance of the incident Light beam from the substrate surface. Its effect corresponds to that Fresnel zone lens with a symmetrical kinoform profile, making it an irregular one Structure of a grid parallel to the substrate surface. In neighboring stages the optical path lengths for passing light differ as a result different refractive indices. An asymmetrical, one-dimensional kinoform profile causes the passing light to be deflected in the y direction, in addition to Collimation / focusing.

Forms of the invention mainly relate to the Fresnel zone lens in the optical coupling element, but then also with regard to it Interaction with other elements, i.e. H. with the cylindrical lens and the optical waveguide in the monolithically integrated component.

The Fresnel zone lens must definitely consist of two, if possible several Materials with different refractive indices n can be constructed. It is advantageous that in a Fresnel zone lens made from a small number of materials different refractive indices, their approximate one-dimensional Kinoform profile consists of geometrically separated steps. With two Materials and only one geometric paragraph result in three different ones optical path lengths, the first with the refractive index n 1 over the entire length of the Fresnel zone lens, the second with the refractive index n 1 in one of the two paragraphs and the refractive index n₂ in the other of the two paragraphs and the third optical path length with refractive index n₂ over the entire length of the Fresnel zone lens. With three materials and a stepped step between two materials already results five different optical path lengths, e.g. B. with n₁; n₁ / n₂, n₂; n₂ / n₃; n₃. With two Materials, the number of different optical path lengths increases by  one with another paragraph. However, it is for everyone's manufacture such a geometrically different stage, a separate process step with its own mask required so that this form of training of the invention then to give preference is when the materials are not just the different refractive indices must have, but as small as possible for the passing light, should also show the same possible absorption coefficient, on a small number, e.g. B. two or three limited.

If such restrictions are not critical, and a simpler, only a mask for the manufacturing process for geometrically equally long paths for If the Fresnel zone lens is preferred, the invention offers an alternative embodiment such that their approximate kinoform profile consists of phase steps. For this will only one layer package is required, in which each planar thin layer with one certain refractive index a planar thin film with another certain Refractive index is adjacent.

In both of the aforementioned forms of training the invention is for collimating / focusing light the structure of the planar thin layers in the Fresnel zone lens is mirror-symmetrical to its parallel to the substrate surface Center line. The closer to the kinoform profile, the better the higher the Number of levels is.

Regarding the length of the Fresnel zone lens, it is from a technological point of view It is particularly advantageous to provide the 10 μm range for this purpose, it being true that L = [K / (K + 1)] [λ / Δn], with L = length in µm, K = number of geometrical Steps separated from each other, λ = wavelength of the passing light in µm and Δn = difference in the refractive indices of the materials of two planar adjacent stages. Is this refractive index difference less than z. B. 0.1 for a light wavelength of z. B. 1 micron, the length L is correspondingly greater. in the Generally such length dimensions are in integrated optical components uncritical, both towards the 1 µm range, in which structures with sufficient Accuracy and sufficient mechanical stability can be generated as also in the direction of the 100 µm range, which is the leading structure for light waves various types must be provided anyway.

In any case, with an optical coupling element according to the invention light to be coupled collimates, focused to be coupled. That is, that  The wave field in the optical waveguide is more compact than beyond the Fresnel zone lens. Accordingly, in a further development of a coupling element according to the invention provided that the optical waveguide, the cylindrical lens and the Corresponding Fresnel zone lens in the heights of the planar thin layers forming it relationships

H _W H _Z and / or H _Z H _F

differ, the indices W, Z and F for waveguides, cylindrical lenses and Fresnel zone lens stand. This ensures that none unwanted asymmetries of the wave field of the coupled or decoupled light arise.

Another step, an optical coupling element according to the technical teaching of To create invention results from a combination of cylindrical lens and Fresnel zone lens in a single element. This is just the To provide Fresnel zone lens with cylindrical jacket-shaped facets, d. H. the function the cylindrical lens is also taken over by the Fresnel zone lens designed in this way.

For the manufacture of optical coupling elements, that is already further above essential procedure according to the invention, namely the formation of the planar Thin layers are called strips with vertical edge zones. For this, z. B. Processes common with "SAE - Selective Area Epitaxy -" and "SSG - Surface Selective Growth - "are referred to and chosen for different materials integrated optics, especially based on indium phosphide - InP - and Gallium arsenide - GaAs - are suitable. In the teaching of the technical invention To this extent, action can be assumed to be that in this area available specialist knowledge and skills is available.

With regard to physical training, this requirement can also apply to one further method according to the invention for producing an optical coupling element be assumed. For realizing that approximated by planar steps Kinoform profile of the Fresnel zone lens can, however, be a particularly advantageous one Measure are used. This technical teaching is that a planar thin layer made of a polymer at least for the Fresnel zone lens, optionally also used for the cylindrical lens and / or the optical waveguide and for the functional determination of layers with different refractive indices in the Fresnel zone lens there the thin layer from the front surface or from the Side surfaces forth in strips parallel to the surface of the substrate  different light energies is irradiated. The polymers that can be used for this do not change their absorption coefficient and also have sufficient ones Long-term stability of their optical properties.

There is an advantageous one for the two production methods according to the invention Embodiment in that components consisting of homogeneous thin layers, like the waveguide and / or the cylindrical lens on the one hand, and from a multilayer Package of planar thin films with different refractive indices Components such as the Fresnel zone lens, on the other hand, are separated in time be generated.

In the drawing, embodiments of the invention are shown schematically. Here demonstrate:

Fig. 1 shows an optical coupling member in a perspective view, and three, the operation of this coupling member are explanatory views showing the wave fields passing light;

Fig. 2 and 3 in cross-section the structure of Fresnelzonenlinsen with geometrically offset with steps or phase levels;

Fig. 4 is an optical coupling member united with a cylinder element and Fresnelzonenlinse, in perspective view;

Fig. 5 is a diagram for illustrating a manufacturing method of an optical coupling element, with representations of structuring seven states in the course of production and

FIGS. 6 and 7 show two diagrams of the course of refractive index on the composition of the materials from which a Fresnelzonenlinse of the invention is constructed in accordance with.

In the example shown in Fig. 1 configuration of an optical coupling element 1 are located on a common substrate 2, an optical waveguide 3, a cylindrical lens 4 and a Fresnelzonenlinse. 5 The three spatial directions in which the optical coupling element i extends are denoted by x, y, z. Thereafter, the surface of the substrate 2 is an x / z plane, on which planar thin layers 6 i (i = 1, 2,..., K) are arranged. The waveguide 3 and the cylindrical lens 4 are made of the same material as the middle planar thin layer 6 in the Fresnel zone lens 4 , the z. B. has the refractive index n₁. The waveguide 3 and the cylindrical lens 4 lie close to one another or merge directly into one another. At this point labeled A and in the waveguide 3 , single-mode light propagates in the sketched form of the wave field. The cylindrical lens 4 is formed with a facet standing vertically on the surface of the substrate 2 . The light passing there is collimated / focused in the x direction, as a comparison of the shapes of the wave fields at points A and B shows.

Between the cylindrical lens 4 and the Fresnel zone lens 5 there is generally a medium with a refractive index by which the radius of curvature of the cylindrical lens 4 is determined. This medium therefore has no optical influence on the refractive power of the Fresnel zone lens 5 . Passing light is collimated / focused in the y direction in the Fresnel zone lens 5 due to different refractive indices n in the planar thin layers 6 i. If only pure bundling is to be brought about for light that passes here and does not change its main direction of propagation, the geometrical arrangement of the planar thin layers 6 i results in a mirror-symmetrical structure with an assignment in pairs with respect to their thickness and their refractive indices n _i . At the point denoted by C, the front surface of the optical coupling element 1 , the light coupled in or out has a wave field in the form outlined, ie light passing through the optical coupling element 1 is opposite the point B in the y direction and opposite the point A collimated / focused in the x direction.

In FIGS. 2 and 3 Fresnelzonenlinsen 5 are shown in cross-section, both of which have an approximated by planar stepped kinoform profile.

The planar steps in the Fresnel zone lens 5 shown in FIG. 2 are made of two materials with different refractive indices n₁, n₂, which are also geometrically separated. In the embodiment shown, two paragraphs are provided, so that different optical path lengths result at different altitudes. In the step at the level of the axis, the refractive index n 1 and the entire length in the axial direction give a first optical path length, in the adjacent steps the refractive index n 1 over 2/3 of the total length and the refractive index n 2 over 1/3 of the total length a second optical path length, then the refractive index n₁ over 1/3 of the total length and the refractive index n₂ over 2/3 of the total length the third optical path length and finally the refractive index n₂ over the total length of the Fresnel zone lens 5 before the fourth optical path length. The steps are also designed with different heights so that passing light experiences collimation / focusing in the drawing plane. With steps constructed mirror-symmetrically to the axis of the Fresnel zone lens 5 , pure collimation / focusing takes place. If an additional deflection of the passing light is desired, the structure can be implemented asymmetrically.

Each planar stage is manufactured with the appropriate masking to train. The number of steps geometrically separated within a material can be reduced to two and enlarged to four and more, as well as the number of Materials with different refractive indices n₁, n₂, n₃,. . . without losing sight of the principle of this cinema shape profile changes a bit.

Referring to FIG. 3, the approximate kinoform profile of phase levels. These are, in each case over the entire length of the Fresnel zone lens 5 in the axial direction, from planar layers with different refractive indices n₁, n₂, n₃, n.,. . . formed, which also differ in their thickness. For this form of training as well as for FIG. 2, the fact that a mirror-symmetrical structure is used for pure bundling of passing light and an asymmetrical structure for an additional deflection is used for the planar layers.

The more planar, geometric, the better the approximation to the kinoform profile separated or phase stages are provided.

The optical coupling element 1 shown in FIG. 4 serves to illustrate further features of the invention.

The optical waveguide 3 consists of a planar layer with the refractive index n 1 and has the height H _W. In planar layers with a refractive index n₂ above and below the actual wave-guiding layer, the wave field also extends. Accordingly, beyond the end face of the optical waveguide 3, the height H _Z for the cylindrical lens is to be made larger than H _W. From a manufacturing point of view, it is advisable to fully include the heights of the two planar thin layers with the refractive index n₂ above and below the optical waveguide 3 in the height H _Z. If this happens only in part, H _F , the height of the Fresnel lens, can also be somewhat larger than H _Z , otherwise there are no serious functional differences for the same dimensions for H _Z and H _F.

Furthermore, Fig. 4 it can be seen that the cylindrical lens and the Fresnelzonenlinse may be combined in a single element 7. The structure of the planar layers having different refractive indices n _i corresponding to the FIG. 2 or FIG. 3, the formation of the facet of the cylindrical lens, as shown in Fig. 1 and the accompanying explanations.

The production of an optical coupling element can be carried out in a simple manner. . Figure 5 shows seven states:

• 1. The cleaned substrate is provided with two planar layers, the cladding layer (Cladding) below the waveguide and the core layer of the Waveguide;
• 2. A resist material for structuring the waveguide is applied and hardened. For this, the pattern of a first mask is placed in the resist material transfer;
• 3. The wave-guiding structures are formed, for. B. by ion beam etching (Ion Beam Etching - IBE);
• 4. Similar to state # 2, structuring the waveguiding layers with a second Mask, etching the contour of the cylindrical lens;
• 5. Another photoresist step, in preparation for the formation of the Fresnel zone lens;
• 6. Training the Fresnel zone lens z. B. by means of ion beam sputtering (IBSB) of SiO _x layers and removal of the photoresist from state # 5;
• 7. Coating (cladding) of the optical coupling element and the core layer of the Waveguide.

FIGS. 6 and 7 show n for different materials the course of the refractive index, respectively at a certain wavelength λ. With SiO _x - cf. Fig. 6 - the refractive index is between about 1.5 and 3.4 depending on the oxygen concentration, which, for example, at will. B. can be adjusted in ion beam sputtering. For Al _x Ga _1-x As material - cf. Fig. 7 - depending on the composition of the ternary material, refractive indices between about 2.8 and 3.4 can be achieved.

Further details can be found in a report on the occasion of the "International Symposium on Integrated Optics ", April 11-15, 1994, Lindau, DE, by Pawlowski, E .; "Integrated planar Fresnel zone lenses for beam forming and coupling ".

Claims (9)

1. Optical coupling member, which is arranged on an end face of a light guide and has a Fresnel lens with structured zones, characterized in that the optical coupling member ( 1 ) with an optical waveguide ( 3 ) a monolithic on a substrate ( 2 ) integrated from planar Forms thin layers ( 6 i-i = 1, 2,... K -) in which a cylindrical lens ( 4 ) with a facet perpendicular to the surface of the substrate ( 2 ) between the end face of the waveguide ( 3 ) and the Fresnel zone lens ( 5 ), and the Fresnel zone lens ( 5 ) is formed with a kinoform profile approximated by planar steps. 2. Optical coupling element according to claim 1, characterized in that in the Fresnel zone lens ( 5 ), which is made up of a small number of materials with different refractive indices (n₁, n₂,...), Their approximate Kinoformprofil consists of geometrically separated steps . 3. Optical coupling element according to claim 1, characterized in that in the Fresnel zone lens ( 5 ), which is constructed from a larger number of materials with different refractive indices (n₁, n₂, n₃, n₄,...), Their approximate kinoform profile from phase steps consists. 4. Optical coupling element according to claim 2 or 3, characterized in that the Fresnel zone lens ( 5 ) is formed with a length (L) in the 10 µm range, where: L = [K / (K + 1)] [λ / Δn] with
λ = wavelength of the passing light
Δn = difference in the refractive indices of the materials of two planar neighboring stages
K = number of geometrically different levels. 5. Optical coupling element according to one of claims 1 to 4, characterized in that the optical waveguide ( 3 ), the cylindrical lens ( 4 ) and the Fresnel zone lens ( 5 ) at the heights (H) of the planar thin layers ( 6 i) forming them according to the relationship H _W H _Z and / or H _Z H _F , with
H _W = height of the optical waveguide ( 3 ),
H _Z = height of the cylindrical lens ( 4 ),
H _F = height of the Fresnel zone lens ( 5 ). 6. Optical coupling element according to one of claims 1 to 4, characterized in that the cylindrical lens ( 4 ) and the Fresnel zone lens ( 5 ) are combined in a single element ( 7 ). 7. A method for producing an optical coupling element according to one of claims 1 to 6, characterized in that processes are used which lead to the formation of the planar thin layers ( 6 i) as strips with vertical edge zones. 8. A method for producing an optical coupling element according to one of claims 1 to 6, characterized in that a planar thin layer ( 6 ) made of a polymer at least for the Fresnel zone lens ( 5 ), optionally also for the cylindrical lens ( 4 ) and / or the optical Waveguide ( 3 ) is used and for the functional determination of layers with different refractive indices in the Fresnel zone lens ( 5 ) the thin layer ( 6 ) is irradiated with different light energies from the front surface or from the side surfaces in strips parallel to the surface of the substrate ( 2 ) . 9. The method according to claim 7 or 8, characterized in that consisting of a homogeneous thin layer ( 6 ) components, such as the optical waveguide ( 3 ) and / or the cylindrical lens ( 4 ) on the one hand, and from a multilayer package of planar thin layers ( 6 i ) components with different refractive indices, such as the Fresnel zone lens ( 5 ) on the other hand, are generated separately in time.