Abstract:
A method for the manufacture of optical components, at least one three-dimensional optical waveguide structure being produced in a light-sensitive substrate by locally subjecting the substrate to an exposure so that a difference in refractive index between the substrate and the at least one optical waveguide structure is created. Provision is made for an exposure to occur at least twice, at different angles of incidence for the light perpendicular to a light wave propagation direction of the optical waveguide structure; the substrate surrounding what will later be the optical waveguide structure thereby experiences a diminution in refractive index, the optical waveguide structure being defined using a mask.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for the manufacture of optical components, at least one three-dimensional optical waveguide structure being produced in a light-sensitive substrate by locally subjecting the substrate to an exposure so that a difference in refractive index between the substrate and the at least one optical waveguide structure is created. The present invention also relates to an optical component. 
     BACKGROUND INFORMATION 
     Optical components having integrated optical waveguide structures are known. These optical waveguide structures possess a difference in refractive index as compared to the substrate surrounding them, so that they are suitable for guiding light waves. A conventional method is described, for example, in U.S. Pat. No. 5,136,677, in which the chalcogenide glasses are subjected locally to an exposure in order to produce the optical waveguide structure. An exposure occurs here in relatively thin substrates, in the light wave transmission direction of what will later be optical waveguide structures. It is also known from U.S. Pat. No. 5,136,677 that intersecting optical waveguide structures can be created in a substrate using two different light sources, and that the optical properties of the optical waveguide structures can be influenced using interference phenomena. 
     It is known, in order to allow the optical waveguide structures, once produced, to guide electromagnetic waves, for example light waves, that they must have a refractive index which is typically a few percent higher than the substrate surrounding the optical waveguide structure. In addition, the dimensions for the optical waveguide structures lying perpendicular to the light wave propagation direction must be selected so that they are on the order of the wavelength of the light to be guided, typically from 1 to 10 μm. Using the difference in refractive index between the optical waveguide structure and the substrate, and the dimensions of the optical waveguide structures, it is possible to establish the number of modes of light waves being transmitted for a given wavelength being guided. This defined refractive index difference, with the necessary small dimensions, can be achieved with the conventional methods for the manufacture of optical waveguide structures only at the cost of substantial losses. 
     The optical waveguide structures are usually produced in integrated optical components which are configured, for example, as amplifiers, splitters, couplers, multiplexers, or switches. For this purpose, optical fibers, for example glass fibers, which feed signals in and out are coupled to the optical waveguide structures in the optical components. The problem arises here that the glass fiber cross section must be coupled to the optical waveguide cross section. The effective cross section for common glass fibers is 5 to 10 μm. With integrated optical components, it is advisable to work with smaller cross sections, for example in order to increase the energy density in the optical waveguide structures or to spatially delimit light guidance so that the physical size of the integrated optical components can be reduced. Because of the difference in cross sections at the coupling point, attention must be paid to the numerical aperture, which describes the angular region from which an optical fiber can accept incident light. Light which is incident at a limit value greater than one corresponding to the numerical aperture cannot be guided, and is lost. On the other hand, the small cross section of the optical waveguide structures in the integrated optical components necessarily results in an increase in the numerical aperture, so that light signals sent out from the integrated optical components can be only partially transferred into the coupled glass fibers. 
     In order to mitigate this problem and the losses associated therewith, it is known to provide a so-called “taper” between the optical waveguide structures and the glass fibers, as a transitional structure. This is intended to effect a continuous transition for the effective cross sections of the glass fibers and the optical waveguide structures, and for the numerical aperture. 
     The taper can be only incompletely configured using the conventional manufacturing methods for optical waveguide structures, in which optical waveguide structures are produced in, for example, glass, polymer, or Ormocer substrates or in surface layers of silicon wafers using ion exchange, a local change in the stoichiometry of oxides or oxynitrides, or a local filling of etched or stamped valley structures. Because of the limited depth of the layer thickness, cross-sectional adaptation can be accomplished only by expanding the waveguide cross section while the depth remains the same. The refractive index is often defined by the material properties, and is therefore constant over the entire taper. The result can be that the taper becomes entirely or partially multimodal, i.e. that propagation directions which cannot be received by the adjacent optical waveguide or glass fibers become possible within it. 
     SUMMARY OF THE INVENTION 
     The method according to the present invention has an advantage that highly precise optical waveguide structures, configured in particular as tapers, with which optical waveguide structures and glass fibers can be coupled in low-loss fashion, can be created with simple technical means. Because an exposure occurs at least twice, at different angles of incidence for the light perpendicular to the light wave propagation direction of the optical waveguide structure, the substrate surrounding what will later be the optical waveguide structure experiences a diminution in refractive index, the optical waveguide structure being defined using a mask which preferably has a width which varies in the light wave propagation direction; as a result, three-dimensional optical waveguide structures can be achieved which have not only an expansion of the optical waveguide cross section but also an increasing depth. It is thereby possible, very advantageously, to create cross sections of optical waveguide structures, in the coupling region to glass fibers, with which it is possible to adapt the effective cross sections of the optical waveguide structures and the glass fibers. 
     A preferred exemplary embodiment of the present invention provides for the angles of incidence of the exposure in the light wave propagation direction to be variably adjustable. This allows a further optimization of the cross-sectional adaptation via the taper, for example by the fact that the latter has a cross section which is triangular when viewed in cross section and expands in trumpet-like manner from the optical waveguide structure toward the glass fiber. 
     In a further preferred exemplary embodiment of the present invention, provision is made for the mask to have, outside the optical waveguide structure, a transparency which varies in the light wave propagation direction. This advantageously makes it possible to vary the difference in refractive index between the optical waveguide structure and the substrate surrounding the optical waveguide structure in defined manner, so that, in particular in the case of an increase in the cross section of the optical waveguide structure, the difference in refractive index decreases in defined manner to ensure that the optical waveguide structure is monomodal in the light wave propagation direction as the cross section increases. 
     Provision is also made, in a preferred exemplary embodiment of the present invention, for the angle of incidence of the exposure to be established so as to result in buried optical waveguide structures. The result of this is that additional covering of the optical waveguide structure that is produced, to prevent external influences, is no longer necessary. The entire process for manufacturing the optical components having the optical waveguide structures is thereby simplified. 
     Provision is moreover made, in a preferred exemplary embodiment of the present invention, for the exposure to be accomplished at different angles of incidence with a varying mask, in order to produce buried optical waveguide structures. The highly advantageous result of this is that in addition to a buried optical waveguide structure, a further optimization of the cross section of the optical waveguide structure at the coupling point between the optical waveguide structure and a glass fiber is achieved. It becomes possible to arrive at rhomboidal (diamond-shaped) cross sections for the optical waveguide structures, with cross sections which increase in the light wave propagation direction. A further optimized adaptation of the effective cross sections between the optical waveguide structure and a glass fiber thereby becomes possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1   a  shows process steps for the manufacture of an optical component according to a first exemplary embodiment of the present invention. 
     FIG.  1   b  shows other process steps for the manufacture of the optical component of the first exemplary embodiment. 
     FIG. 2 shows a schematic view of the optical component including an optical waveguide structure. 
     FIG. 3 shows a schematic view of a mask for the manufacture of the optical waveguide structure. 
     FIG. 4 shows process steps for the manufacture of a buried optical waveguide structure according to the first exemplary embodiment. 
     FIG.  5   a  shows process steps for the manufacture of a buried optical waveguide structure in a second exemplary embodiment. 
     FIG.  5   b  shows other process steps for the manufacture of a buried optical waveguide structure in the second exemplary embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS.  1   a  and  1   b  respectively show a schematic sectioned depiction of an optical component  10 , with reference to which the method according to the present invention for the manufacture of an optical waveguide structure  12  will be explained. The assumption used for the explanation of the method according to the present invention is that of the simplest construction of an optical component, i.e. the existence of only one optical waveguide structure  12 . It is possible, using the method according to the present invention, to produce simultaneously a plurality of optical waveguide structures  12  having different dimensions. 
     Optical component  10  is made of a chalcogenide glass  14 . The chalcogenide glasses are generally understood to be amorphous, non-stoichiometric compounds of the chalcogens. The sulfides and selenides of arsenic (As), antimony (Sb), germanium (Ge), gallium (Ga), indium (In), bismuth (Bi), and lanthanum (La) are preferred, along with their mixtures if they form glass phases. Mixed systems made up of chalcogenides and halides, for example fluorides, chlorides, bromides, and iodides, which lead to the creation of so-called chalcohalide glasses, are also possible. 
     The glass used—in the exemplary embodiment, chalcogenide glass  14 —is equipped with a mask  16  which is made of an opaque material. Mask  16  is adapted in terms of its dimensioning, in a manner yet to be explained, to optical waveguide structure  12  that is to be created. Mask  16  can be applied onto chalcogenide glass  14  using conventional methods, for example by screen printing. 
     In a first process step (FIG.  1   a ), an exposure of chalcogenide glass  14  equipped with mask  16  occurs, a light  20  (exposure  20 ) incident to the surface of chalcogenide glass  14  at an angle of incidence α first being produced by a source (not depicted). The photoenergies of light  20  that are used lie in the absorption range of chalcogenide glass  14 . The wavelength of light  20  is thus usually in the visible range or in the near ultraviolet range. The wavelengths of light  20  can be, for example, between 200 nm and 600 nm. 
     Light  20  that is incident at angle of incidence α strikes chalcogenide glass  14  except in a region  26  that is shaded by mask  16 , and therein (with the exception of shaded region  26 ) leads to a diminution in the refractive index of chalcogenide glass  14 . Shaded region  26  thus possesses a higher refractive index than the remaining substrate of chalcogenide glass  14 . 
     The degree to which the refractive index is decreased can be adjusted via an exposure duration and/or an exposure intensity and/or the wavelength of light  20 . The exposure intensity of light  20  is, for example from approximately 1 to 100 J/cm 2 . Exposure by light  20  usually takes place at room temperature and in a standard atmosphere, so that no additional equipment outlay is required to maintain specific requisite process conditions. 
     Subsequently, in accordance with the process step shown in FIG.  1   b,  an exposure is performed with light  20  at an angle of incidence β, so that an exposure of chalcogenide glass  14  takes place with the exception of a region  28  shaded by mask  16 . Because angle of incidence β differs from angle of incidence α, shaded region  26  shown in FIG.  1   a  is partially exposed, so that a diminution in refractive index also occurs here. The region respectively shaded (i.e. unexposed) during the two exposure steps depicted in FIGS.  1   a  and  1   b  leads to the formation of optical waveguide structure  12 , since the latter has a higher refractive index than the environment, i.e. than the substrate of chalcogenide glass  14 . The dimensioning of optical waveguide structure  12 , in particular its depth, can be varied by selecting angles of incidence α and β. A further dimensioning of optical waveguide structure  12 , in particular of its width and length, can be established by dimensioning mask  16 . 
     By combining different exposures, i.e. different angles of incidence α and β, different exposure intensities, different wavelengths of light  20 , and/or different exposure durations, an optical waveguide structure  12  with any defined dimensions and any magnitude of change in refractive index as compared with the substrate of chalcogenide glass  14  can be produced. 
     FIG. 2 shows, in a schematic perspective view, a component  10  which has an integrated optical waveguide  12 . Optical waveguide  12  possesses a channel-shaped section  22  and, facing an end face  18 , a transition region hereinafter called a taper  24 . Taper  24  possesses at end face  18  a triangular cross section which transitions, becoming continuously smaller, into section  22 . If it has been manufactured, for example, using the method explained with reference to FIGS.  1   a  and  1   b,  section  22  can also have a triangular cross section. What essentially occurs, therefore, is a funnel-shaped expansion of the cross section of the entire optical waveguide structure  12 , via taper  24 , toward end face  18 . Taper  24  can be achieved by a suitable configuration of mask  16  and a suitable incidence of light  20  during the exposure. For this purpose, mask  16  possesses, when viewed in plan view, a contour which expands in trapezoidal or triangular shape toward end face  18 , so that a shaded region corresponding to what will later be taper  24  is created during exposure  20  (FIGS.  1   a,    1   b ). 
     Angles of incidence α and β of exposure  20  can be defined either using a light source mounted in suitably movable fashion, or a corresponding movement of chalcogenide glass  14  while exposure  20  remains uniformly aligned. In addition to angles of incidence α and β which are considered in FIGS.  1   a  and  1   b  to be in the plane of the drawing, a rotation into or out of the plane of the drawing can also occur here, so that optimally adapted tapers  24  can be attained. 
     Taper  24  is used to achieve the adaptation, mentioned above, of the effective cross sections of optical waveguide structure  12 , in particular its section  22 , to a glass fiber (not depicted). Taper  24  results in a continuous transition, in the light wave transmission direction marked with an arrow  30  in FIG. 2, both in the cross section of optical waveguide structure  12  and in numerical aperture. Arrow  30  is characterized as a double arrow, since optical waveguide structure  12  that is shown can serve both to receive an optical signal and to send out an optical signal of component  10 . The triangular cross-sectional area of taper  24  at end face  18  results in an optimized adaptation to a round cross section of the glass fiber (not depicted). 
     FIG. 3 shows a schematic plan view of a chalcogenide glass  14  equipped with mask  16 . Mask  16  defines on the one hand channel-shaped section  22  and taper  24  of what will later be optical waveguide structure  12 . As explained with reference to FIGS.  1   a  and  1   b,  optical waveguide structure  12  is produced by the fact that an exposure of chalcogenide glass  14  occurs in such a way that an unexposed region, forming optical waveguide structure  12 , remains. In order to ensure monomodality in the region of taper  24  over its length as viewed in light wave propagation direction  30 , a difference in refractive index between optical waveguide structure  12  and the substrate of chalcogenide glass  14  must decrease in defined fashion, in the region of taper  24 , in the direction of end face  18 . This is achieved by the fact that mask  16  is configured so that a transparency to light  20 , which must exist outside the structure of taper  24  in order to attain the previously described diminution in refractive index therein, decreases in the direction of the cross-sectional expansion of taper  24 , i.e. in the direction of end face  18 . A uniform decrease in transparency to light  20  in the direction of end face  18  results in a correspondingly uniformly lesser exposure  20  of section  22  of optical waveguide structure  12  viewed in the direction of end face  18 . This results in a correspondingly lesser diminution in refractive index in the substrate of chalcogenide glass  14  in the longitudinal extension of taper  24 , so that a difference between the refractive index of taper  24  and the refractive index of the substrate of chalcogenide glass  14  also decreases uniformly from section  22  in the direction of end face  18 . 
     FIGS. 4,  5   a  and  5   b  show the manufacture of buried optical waveguide structures  12  by exposure  20  at a variety of angles of incidence α and β. The manufacture of buried optical waveguide structures eliminates the need for a subsequent covering of the optical waveguide structures in order to protect them from external influences. The Figures which follow respectively show a schematic plan view of end face  18  of a component  10 , it being clear that using the corresponding configuration of mask  16  already explained, both the channel-shaped section  22  and taper  24  of optical waveguide structure  12  can be achieved. 
     In accordance with the exemplary embodiment shown in FIG. 4, a layer of chalcogenide glass  14  is applied onto a support substrate  32 . A mask  16  is applied directly onto chalcogenide glass  14  in such a way that a mask opening  34  remains in the region of what will later be optical waveguide structure  12 . Exposure  20  takes place first at angle of incidence α, thus resulting, because of the oblique incidence, in a tunnel-shaped section  36  which has a refractive index diminished by the exposure and extends through chalcogenide glass  14  to support substrate  32 . Exposure  20  then takes place through mask opening  34  at angle of incidence β, thus resulting again in a tunnel-shaped section  38 , also having a refractive index diminished by the exposure. Tunnel-shaped sections  36  and  38  enclose a section that is triangular when viewed in cross section, which yields optical waveguide structure  12 , since no diminution in refractive index has taken place in this region so that the latter has an elevated refractive index as compared with tunnel-shaped sections  36  and  38 . Depending on the selection of angles of incidence α and β, there results above optical waveguide  12  a region  40  belonging both to section  36  and to section  38 , which because of the exposure that is in fact doubled in this case, has experienced a diminution in refractive index. Optical waveguide structure  12  is thereby buried in chalcogenide glass  14  applied onto support substrate  32 , and is delimited by support substrate  32  and by sections  36  and  38 . 
     A further possibility for achieving a buried optical waveguide structure  12  is shown by FIGS.  5   a  and  5   b.  First, as shown in FIG.  5   a,  in a manner similar to FIG. 4, an unexposed region is produced that is triangular in shape when viewed in cross section and is delimited laterally by exposed tunnel-shaped sections  36  and  38 . 
     In a subsequent step shown in FIG.  5   b,  mask  16  according to FIG.  5   a  is removed in suitable fashion and a mask  16 ′ is applied which covers the region of chalcogenide glass  14  that was initially uncovered by mask opening  34 . As a result of subsequent exposure at angles of incidence α and β, respectively, further portions of chalcogenide glass  14  are exposed. All that remains is an unexposed region that is rhomboidal or diamond-shaped when viewed in cross section, which remains shaded both during the double-sided exposure shown in FIG.  5   a,  by mask  16 , and during the double-sided exposure shown in FIG.  5   b,  by mask  16 ′. As a result, this region possesses a refractive index that is elevated as compared to the substrate of chalcogenide glass  14 , and forms optical waveguide structure  12 . A configuration of this kind of optical waveguide structure  12 , in conjunction with the continuous cross-sectional expansion of taper  24  explained with reference to FIGS. 2 and 3, makes it possible, by suitably varying masks  16  and  16 ′ and angles of incidence α and β, to produce a taper  24  which is adapted on the one hand to buried channel-shaped sections  22  (FIGS.  2  and  3 ), and to glass fibers (not depicted). The cross-sectional expansion of taper  24  in the direction of end face  18  then extends not only laterally and depthwise, but also in the direction of the surface of chalcogenide glass  14 . This makes possible an even more exact approximation of the cross section of taper  14  to a circular cross section of a glass fiber.