Abstract:
The invention relates to a module for multiplexing and/or demultiplexing optical signals, having at least one wavelength-selective filter for multiplexing or demultiplexing into the module optical signals which have been coupled in or out, light beams of at least one optical channel respectively striking a wavelength-selective filter at a specific angle of incidence and, in the process, being separated from the light beams of other optical channels or being combined therewith. According to the invention, at least one wavelength-selective filter (Fi) can be set with reference to the angle of incidence of the light beams. The invention makes available a module in the case of which the center wavelength of a filter can be set precisely on the basis of the adjustability of the angle of incidence and, moreover, a specific filter (Fi) can also be used for several wavelengths.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Increasing use is being made of wavelength multiplexing/demultiplexing techniques for the purpose of more effective utilization of the transmission capacity of optical conductors. In this case, several mutually independent optical signals of different wavelengths are transmitted over a common optical conductor. Multiplexing/demultiplexing techniques are then used to combine and separate these signals at the receiver and/or transmitter. One possibility for separating and/or combining optical signals of different wavelengths is to use narrowband optical bandpass filters. 
     Such filters mostly consist of dielectric layer systems which are mounted on suitable carrier substrates, generally glass substrates. Their transmission and/or return characteristics for a specific wavelength region are specifically set by selecting the thicknesses and refractive indices of the individual layers and their arrangement. 
     The reflectivity and/or transmissivity of optical bandpass filters of different center wavelengths is illustrated schematically and in an idealized fashion in FIG. 9 as a function of the wavelength of the light. In this case, λ 1  to λn stand for the center wavelengths of the filters F 1  to Fn, λ stands for the wavelength of the light, and R and T stand for the reflectance and transmittance. The transmittance is illustrated with unbroken lines, and the reflectance with dashed lines. It holds that R≈1−T. These filters have a particularly high transmission in a specific region about their center wavelength, but reflect light of wavelengths outside this region. They therefore act as wavelength-selective mirrors. The center wavelength is a function in this case of the angle of incidence of the light beams. 
     A multiplexing/demultiplexing arrangement having optical filters, which is known in the prior art, is illustrated in FIG.  10 . The optical signal emerging from an optical conductor F 1 , as a rule a glass fiber, of wavelengths λ 1 -λn is collimated by an imaging system L 1 , rendered parallel and coupled into a filter system. As a rule, the imaging system in this case comprises a lens, mostly a graded-index lens. The filter system comprises optical filters Fi 1 , Fi 2 , Fi 3 , which are fitted at a fixed spacing from one another onto the mutually opposite sides of a plane-parallel glass plate  8  and in a fashion offset relative to one another. The light coupled into the glass plate  8  by the imaging system L 1  at a specific angle then runs to and fro inside the glass plate in a zigzag fashion between the mutually opposite filters Fi. Light of a specific wavelength is coupled out of the beam path at each filter Fi and coupled into an optical conductor F 2 , F 3 , F 4  by the associated imaging system L 2 , L 3 , L 4 . 
     It is a disadvantage of such arrangements that a special filter of a specific center wavelength must be used respectively for each wavelength. Several different filters are required depending on the wavelength separation and bandwidth of the individual signals to be separated and/or combined, and in this case only very small fault tolerances are permissible in producing the filters. This leads, on the one hand, to an increased rejection of filters for specific wavelengths and thus to increased production costs and, on the other hand, requires high costs for storing individual special filters. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to make available a module for multiplexing and/or demultiplexing optical signals in which there is a reduction in the number of the filters of a different characteristic which are required for a multiplexing/demultiplexing arrangement, and in which it is possible to use filters with higher manufacturing tolerances. 
     It is provided in accordance therewith that at least one wavelength-selective filter can be set with reference to the angle of incidence of the light beams. The desired center wavelength of the respective filter can be set exactly in this case via the angle of incidence. This has two advantages. Firstly, individual special filters can be used as multiplexing elements for several wavelengths, the selected wavelength being set via the angle of incidence of the light beams. By using identical filters for different wavelength channels, it is possible to reduce the total number of different filters to be produced, and thus to lower the costs of production and storage. 
     Secondly, it is possible to use filters with higher tolerances with reference to the center wavelength, since the desired center wavelength can be set precisely by appropriately tilting the filter even in the case of high tolerance values. Consequently, the rejection in filter production can be substantially reduced, and costs can be saved correspondingly. 
     In a preferred refinement of the invention, the light is coupled into or out of the module by means of optical conductors, each optical conductor being assigned at least one imaging element which is arranged between the optical conductor and a filter. In this case, the optical axes of the optical conductors and the optical axes of the imaging elements assigned to the optical conductors are preferably arranged parallel to one another. Both the adjustment and the fixing of the individual components are facilitated in this way by a parallel arrangement. 
     In a first development of this refinement of the invention, in each case the imaging element is a lens which is transirradiated off-axis by the light of the assigned optical conductor. The beam deflection required for the functioning of the multiplexing/demultiplexing arrangement is achieved in this case by means of a parallel offset of the optical axis of the optical conductor and assigned imaging element. Such an arrangement is of particularly simple design and is therefore also simple in terms of production engineering and can be executed with a low outlay on adjustment. In this case, use is also made in some circumstances of more complicated, multistage lens systems, for example, to reduce imaging errors resulting from the off-axis transirradiation of the lens. 
     In a second development, in each case the imaging element is a lens which is transirradiated axially by the light of the assigned optical conductor. Imaging errors owing to off-axis transirradiation of the lens are thereby avoided. However, in order to retain the parallelism of the optical axes of the optical conductors there is then additional need for at least one optical element which can be tilted with reference to the angle of incidence of the light beam and deflects light reflected by the filter in the direction of the lens and the assigned optical conductor. The tiltable optical element is, in particular, a mirror or prism arranged in the beam path between the filter and lens. In this case, by comparison with the use of a prism, the use of a mirror has the advantage of avoiding an additional wavelength dependence on the basis of the dispersion of the glass. Mirrors, prisms and also the wavelength-selective filters should have a weak dependence on polarization. 
     The arrangements described for producing a beam deflection have the advantage that it is possible in a multichannel multiplexing/demultiplexing arrangement having cascades of optical conductors, imaging systems and tiltable filters to avoid the direct adaptation, which is very complicated in terms of production engineering, of the angular settings of the optical axes of the imaging systems and optical conductors to the tilted filter/filters. 
     Particularly compact designs are provided by arrangements in which either several filter cascades or filter and mirror/prism cascades are combined. The individual filters of a cascade can be filtered either individually or in common in this case. In the cascaded arrangements, the respective optical conductors are preferably arranged parallel to one another in accordance with the above-described beam deflecting arrangements, and can thereby easily be adjusted and fixed. 
     A first such advantageous arrangement is provided by two, mutually opposite filter cascades. The filters, which can be tilted about the beam axis, of the two cascades are mutually offset in this case, such that the beam path between the filter cascades describes a zigzag line. A specific wavelength is coupled out at each filter element in a wavelength-selective fashion and coupled into the appropriate optical conductor by the imaging system. The optical axis of the imaging system and the axis of the optical conductor are parallel to one another in this case. Beam deflection and the compensation of the tiltability of the filters are preferably achieved by an adjustable parallel offset between the optical axis of each imaging system and the respective optical conductor axis. 
     A second such advantageous arrangement is provided by the combination of a filter cascade with a mirror cascade. The elements, which can be tilted about the beam axis, of the two cascades are preferably arranged in this case offset relative to one another such that the beam path between the cascades describes a zigzag line. In this case, a specific wavelength is coupled out at each filter element and coupled into the appropriate optical conductor by the imaging system. The optical axis of the imaging system and the optical axis of the optical conductor preferably coincide in this arrangement, in order to avoid additional imaging errors. The required beam deflection and the compensation of the tiltability of the filter elements are achieved by the tiltable mirror elements. 
     A third advantageous arrangement is provided by the combination of a filter cascade and a mirror cascade, in the case of which the individual filters of a cascade are arranged one behind another. In this arrangement, an individual filter element of a cascade preferably reflects light of only one wavelength, all others being transmitted. The light reflected by a filter is deflected onto an element of the imaging system via a tiltable mirror of the mirror cascade. The optical axes of the imaging system and the optical conductor axis preferably coincide in this case. It is also preferred to provide that the light respectively deflected by a tiltable mirror is coupled out into the filter cascade essentially at right angles to the beam direction, such that the individual optical conductors are in turn arranged parallel to one another. 
     In a preferred refinement of the invention, all the optical channels are arranged on one side of the component or the filters. For this purpose, the optical channels of the first or the second optical imaging system are coupled into or out of the module via a deflecting prism, if appropriate. The arrangement of the optical channels on only one side of the module has advantages in terms of production engineering. 
     In an advantageous development of the invention, the filters and, if appropriate, mirrors or prisms are arranged on a flat platform, and this platform is inserted into a housing which has at least one light entry/exit port, the lenses and optical conductors of the first and/or second imaging system being permanently connected to the outside of the housing. In this case, the fastening and adjustment of the imaging systems and/or lens holders and optical fiber holders is preferably performed by means of laser welding technology at the fixed housing. This produces a fastening which is particularly stable in the long-term mechanically. 
     In particular, the fastening of the lenses/fiber elements is preferably performed by a means of a free active adjustment of the elements in a holder flange or a holder sleeve, and by subsequently welding these-flanges or sleeves to the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows a first arrangement of the optical components of a two-channel multiplexing/demultiplexing module according to the invention; 
     FIG. 1 b  shows a second arrangement of the optical components of a two-channel multiplexing/demultiplexing module according to the invention, with lenses and fibers which can be tilted about their axes; 
     FIG. 2 shows a third arrangement of the optical components of a two-channel multiplexing/demultiplexing module according to the invention, with the use of a deflecting prism; 
     FIG. 3 shows a fourth arrangement of the optical components of a two-channel multiplexing/demultiplexing module according to the invention, with the use of a deflecting mirror; 
     FIG. 4 shows an exemplary embodiment of a multichannel multiplexing/demultiplexing module according to the invention, having beam deflection in accordance with FIG. 1 a ; 
     FIG. 5 shows an exemplary embodiment of a multichannel multiplexing/demultiplexing module according to the invention, having beam deflection in accordance with FIG. 3; 
     FIG. 6 a  shows an exemplary embodiment of a multichannel multiplexing/demultiplexing module according to the invention, in which a filter cascade and a mirror cascade are provided; 
     FIG. 6 b  shows an exemplary embodiment of a multichannel multiplexing/demultiplexing module according to the invention, with a one-sided arrangement of all the fiber and lens elements; 
     FIG. 7 shows a schematic of the holding, adjustment and fastening of a lens and a glass fiber in a multiplexing/demultiplexing module according to the invention; 
     FIG. 8 a  shows an exemplary embodiment for the arrangement of the light entry/exit ports on a housing of a multiplexing/demultiplexing module according to the invention; 
     FIG. 8 b  shows an exemplary embodiment for the arrangement of fiber and lens holder flanges on a housing of a multiplexing/demultiplexing module according to the invention; 
     FIG. 9 shows a schematic of the transmissivity and reflectivity of optical filters of different center wavelengths, as a function of wavelength; and 
     FIG. 10 shows an arrangement of the optical components of a wavelength multiplexer/demultiplexer having optical filters in accordance with the prior art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A first arrangement of the optical components of a two-channel wavelength division multiplexing/demultiplexing module according to the invention is illustrated in FIG. 1 a . The module has a first optical conductor F 1  with an assigned lens L 1  which constitutes an optical imaging system for coupling light beams of the optical conductor F 1  into the module. The optical conductor F 1  guides light of several wavelengths λ 1  to λn which constitute different optical channels. 
     The coupled-in light is collimated by the lens L 1  and ideally imaged as parallel light onto a wavelength-selective filter Fi, which transmits a wavelength λ 1  and reflects the remaining wavelengths λ 2  to λn, and thereby separates light of wavelength λ 1  from the other wavelengths. The filter Fi is tiltably arranged, as is indicated by an arrow. Rotatable holders for the filter Fi are provided, for example, for this purpose. The adjustment can be performed by hand, but can also be automated, for example, by carrying out a computer-monitored active adjustment. 
     A desired center wavelength which is to be transmitted is set precisely by tilting the filter Fi about the beam axis. 
     The transmitted or reflected light is imaged by lenses L 2 , L 3  onto the entry port of the optical conductors F 2 , F 3 , and relayed by the latter. 
     In FIG. 1, the optical axes of the lenses L 1 , L 2 , L 3  are arranged parallel to one another and aligned with the axes of the fibers F 1 , F 2 , F 3 , each lens respectively coupling in or out the beam from a specific fiber. The light of the fiber F 1  in this case strikes the lens L 1  off-axis, and is therefore deflected by an angle which is a function of the offset of the axes of the fiber F 1  and lens L 1  relative to the optical axis of the lens. It strikes the filter Fi at an angle which is a function of the tilt position of the filter Fi. As described, the filter Fi transmits only the portion of the light of wavelength λ 1 . The transmitted light falls in turn off-axis onto the lens L 2 , is focused by the latter, coupled into the fiber F 2  and led off laterally. The light, reflected by the filter Fi, of wavelengths λ 2  to λn falls off-axis onto the lens L 3 , is coupled into the fiber F 3  by the latter and likewise led off laterally. 
     The beam deflection must be compensated given tilting of the filter Fi. This is performed by a further parallel offset of the fiber axes relative to the main axes of the lenses L 1 , L 2  and L 3 . 
     An advantage of the parallel arrangement of all the optical axes of fibers and lenses resides in the adjustment and mounting, which are particularly easy and simple to carry out, of the elements, and in the possibility of thereby designing the module more compactly. 
     The beam path of the light of wavelengths λ 1 -λn in FIG. 1 b  is similar to that in FIG. 1 a . In this arrangement, however, the optical axes of the lenses L 1 , L 2 , L 3  and the fiber axes coincide. The required beam deflection and the correction it requires because of the tiltability of the filter are produced by tilting the main lens axes and fiber axes (indicated by an arrow). 
     The advantage of such an arrangement resides in the avoidance of imaging errors owing to off-axis transirradiation of the lens, such as can occur in an arrangement in accordance with FIG. 1 a  if no complicated imaging systems are used to avoid such imaging errors. 
     A further possibility for arranging the optical axes of the imaging systems and optical conductors in parallel while still avoiding additional imaging errors, and for carrying out the compensation of the beam deflection rendered necessary by the tilting of a filter Fi resides in the use of further deflecting elements in the beam path. 
     The principle governing the use of a deflecting prism in the optical beam path is shown in FIG. 2 using the example of a two-channel demultiplexer. The beam path runs axially from the fiber F 1  through the lens L 1  to the filter Fi. Light of wavelength λ 1  is transmitted through the filter Fi and coupled into the fiber F 2  through the lens L 2 . The beam axis for light of this wavelength coincides with the fiber axes and the main lens axes of the fibers F 1  and F 2  and the lenses L 1  and L 2 , respectively before and after passage through the filter Fi. A parallel offset of the beam which occurs from the passage through the tiltable filter Fi is compensated by an offset of the lens L 2  and fiber F 2 . 
     The beam deflection for the beams, reflected by the filter Fi, of wavelengths λ 2 -λn is compensated by a deflecting prism P which can be tilted about the beam axis. After passage through the prism P, the beam is coupled into the fiber F 3  through the lens L 3 . 
     The use of a mirror S for beam deflection in the optical beam path is shown in FIG. 3 using the example of a two-channel demultiplexer. The beam path runs axially from the fiber F 1  through the lens L 1  to the filter Fi. Light of wavelength λ 1  is transmitted through the filter Fi and coupled into the fiber F 2  through the lens L 2 . The beam axis for light of this wavelength coincides with the fiber axes and the main lens axes of the fibers F 1  and F 2  and the lenses L 1  and L 2 , respectively before and after passage through the filter. A parallel offset of the beam which occurs from the passage through the tiltable filter Fi is compensated by an offset of the lens L 2  and fiber F 2 . 
     A beam deflection for the beams, reflected by the filter Fi, of wavelengths λ 2 -λn is compensated by a deflecting mirror S which can be tilted about the beam axis. After reflection at the mirror S, the beam is coupled into the fiber F 3  through the lens L 3 . The direction in which the light is coupled out is at right angles in this case to the direction in which it is coupled in. The use of a mirror as beam-deflecting element has the advantage that no further dispersive element occurs in the beam path. 
     In order to be able to separate more than two wavelengths, or to combine them in the case of reversal of the beam path, several of the arrangements shown in FIGS. 1 a ,  2  and  3  are designed in a cascade. 
     An exemplary embodiment of a multichannel multiplexer and/or demultiplexer exhibiting the beam-deflecting principle illustrated in FIG. 1 a  is shown by FIG.  4 . In the arrangement shown, the fibers F 1 , F 2 , F 3 , F 4 , F 5  are fed from two sides. Their axes are respectively parallel and offset in parallel relative to the optical axes of the coupling-in and coupling-out lenses L 1 , L 2 , L 3 , L 4 , L 5  in such a way as to produce a beam deflection which can be set individually to each of the individual tiltable filters Fi 1 , Fi 2 , Fi 3 , Fi 4 . The filters Fi 1 , Fi 3  and Fi 2 , Fi 4  are opposite one another offset in two cascades  20 ,  30 . Each of the filters Fi transmits just light of one wavelength, light of all other wavelengths being reflected. This produces a zigzag beam path between the filter cascades  20 ,  30  after the light of wavelength λ 1 -λn is coupled in from the fiber F 1 . The light of one wavelength is coupled out of this beam path at each filter Fi. 
     It may be pointed out.that each filter Fi of the two cascades  20 ,  30  can be set individually in order to be able to set the desired center wavelength exactly. In this case, it is preferred to use identical filters for several neighboring wavelengths, something which is possible by appropriate rotation of the filters with reference to the angle of incidence of the collimated beam. As indicated by the double arrow B, the lenses L 1  and assigned optical conductors are arranged capable of displacement at right angles to their optical axis in order to compensate the offset occurring upon rotation and/or tilting of a filter. 
     Exemplary embodiments having lenses centered with reference to the collimated beams are illustrated in FIGS. 5,  6   a  and  6   b  according to the beam-deflecting principles illustrated FIGS. 2 and 3. 
     Individually tiltable mirrors S 1 , S 2  are used for beam deflection and for compensating the filter tilting in the case of the exemplary embodiment illustrated in FIG.  5 . The beam path is of cruciform design in this case: the light of wavelengths λ 1  to λn is fed from one side via the fiber F 1 , the light of wavelengths λ 3 -λn is coupled into the outgoing fiber F 4  on the opposite side via the lens F 4 . The filters Fi 1  and Fi 2  respectively couple light of wavelengths λ 1  and λ 2  out of the beam path by reflection, which light is then coupled into the fibers F 2 , F 3  via the mirrors S 1 , S 2  and the lenses L 2 , L 3 , and led off at right angles to the direction in which the light is fed. 
     The exemplary embodiment illustrated in FIG. 6 a  likewise shows a design having individually tiltable mirrors for beam deflection and compensation of filter tilting. The beam path is designed in this case such that the outgoing fibers F 2 , F 3  are all situated on one side. The filters Fi 1 , Fi 2  and the mirrors S 1 , S 2  are respectively arranged in a cascade  20 ,  40 . The individual elements are offset relative to one another in this case. The result, after the light of wavelength λ 1 -λn is coupled in from the fiber F 1 , is a zigzag beam path between the two cascades  20 ,  40 . Light of one wavelength is coupled out of this beam path at each filter, coupled into a fiber through the respective lens and led off laterally. 
     A modification of the design shown in FIG. 6 a  is illustrated in FIG. 6 b . In this case, the incoming fiber F 1  is also on the side of the outgoing fibers F 2 , F 3 . The beam deflection required for this purpose is performed by a 90° prism UP. However, pentaprisms or mirrors are also possible. 
     All the exemplary embodiments shown for the optical design are, of course, not limited to the use of a specific number of incoming or outgoing fibers and the corresponding number of filters, lenses and beam-deflecting elements, but can be designed for an arbitrary number of fibers. It is also within the scope of the invention for the light of the individual wavelengths not to be led off or fed via optical conductors, but for the multiplexer/demultiplexer to be coupled directly to an optoelectronic module which, for example, couples light of the individual wavelengths in or out via a transmitting or receiving array of optoelectronic elements. 
     It can be provided, moreover, that instead of the optical conductors there are arranged on the module plugs into which the optical conductors are then plugged. 
     When the module is used as a multiplexing module, it is merely necessary to reverse the beam path in the above-named exemplary embodiments. 
     The filters Fi and any prismatic and mirror elements are arranged on a flat platform (not illustrated) which is surrounded by a housing. The layout and fastening of the individual filter and mirror/prismatic elements on the platform are performed, for example, by means of bonding, soldering or mounting or welding. As base material for the platform, use is made in this case of, for example, the materials of glass, ceramic, silicon or else metals. The filters, mirrors or prisms either themselves have a sufficiently large, flat supporting surface, or they are mounted on the platform by means of appropriate carriers. 
     Since the connection of the optical components to the platform has to be particularly stable, the coefficients of thermal expansion of the parts to be connected must be matched to one another as far as possible. In addition, in the case of particularly high demands placed on the wavelength stability, temperature regulation of individual filters, or else of the entire structure, is provided by means of Peltier cooling or a heater. The entire platform is then placed with the housing onto a Peltier cooler or a heater. A temperature sensor, for example an NTC thermistor, is fitted in this case on the platform for the purpose of temperature regulation. Regulation is performed, for example, by means of a P/I controller. 
     Illustrated schematically in FIG. 7 is an exemplary embodiment for holding, adjusting and fastening an optical beam path on a housing  2  which surrounds a platform (not illustrated) with filters and, possibly, prismatic and mirror elements. The housing  2  has light entry/exit ports  7  in this case, via which light is coupled into or out of the housing  2 . 
     A spherical lens  1  and a glass fiber F are fastened on the outer wall of the housing above the light entry/exit port  7 . The spherical lens  1  is arranged in a lens flange  5  which is fastened on the housing  2 . The flange  5  has a cylindrical bore for holding the spherical lens  1 . The spherical lens  1  is inserted into the flange  5 , for example, against a depth stop, in such a way that its focus is set with respect to the housing  2 . Instead of a spherical lens, it is also possible to insert graded-index lenses or other optics into the lens flange  5 . 
     The glass fiber F is arranged in a capillary  3 , in particular being bonded into the same. In this case, the fiber end face is slightly ground obliquely in order to avoid back-reflections. In order to fasten the glass fiber F on the lens flange  5 , a further flange  6  with the fastening edges  61  is provided, which flange  6  holds the capillary  3  with the glass fiber F. As an alternative to a lens flange, it is possible to provide a precisely designed rigid guiding sleeve (ferrule) into which the fiber is plugged with the capillary. The guiding sleeve (ferrule) is worked very accurately on its outer side and is plugged into the lens flange  5  (fiber plug). 
     After the performance of active adjustment (including: setting the associated wavelength-selective filter and the beam offset arising therefrom) on a widened edge  51 , the lens flange  5  is fastened on the housing  2  by means of laser welding in the direction of the arrow A. The flange  5  is actively adjusted in this case parallel to the housing surface. 
     After the performance of active adjustment with respect to the spherical lens  1 , the fiber flange  6  is subsequently likewise fastened on the lens flange  5  via laser welding. 
     The housing  2  and parts of the flanges  5 ,  6  preferably consist of a weldable material such as, for example, structural steel, iron or Kovar in the vicinity of the spot welds or seams. The flanges are preferably designed in a cylindrical shape, and the contact surfaces between the platform and/or housing and the holding flanges and/or holding sleeves are ground flat in order to permit optimum adjustment and a low welding warpage. 
     The bevelling of the fiber end faces of the fiber F is taken into account in the design of the beam path and/or by a small lateral offset of the fiber axis with reference to the lens axis when adjusting these parts. 
     If an airtight and hermetically sealable mounting of the filter platforms is required, the light entry/exit ports  7  of the housing  2  are provided with optical windows which are preferably obliquely positioned and antireflection-coated, and which seal the ports  7  in an airtight fashion. Alternatively, the lens flange  5  is of airtight design and is welded tightly to the housing  2 . 
     FIG. 8 a  shows a plan view of an exemplary embodiment for the arrangement of the light entry and light exit ports  7  on a housing  2 , on which the lens and fiber flanges  5 ,  6  are fastened. The light entry and light exit ports  7  are arranged in this case offset relative to one another in level, something which permits the lens and fiber flanges  5 ,  6  to be fitted on the housing  2  in a simple and compact way, and is particularly easy to execute. In this exemplary embodiment, the mirrors or prisms in the housing interior have surfaces orientated in accordance with the offset of the light entry/exit ports  7 . 
     FIG. 8 b  shows a further exemplary embodiment for the arrangement of the light entry and light exit ports  7  on a housing  2 . The lens and fiber flanges  5 ,  6  are also illustrated here. In this case, the light entry and/or light exit ports  7  and/or the individual surfaces of the housing  2  in which these are located are fitted respectively at the same level, but tilted with respect to one another. 
     All the optical surfaces in the beam path for the module are antireflection-coated as far as possible, in order to keep insertion losses low. To suppress undesired reflection by the rear side of the filter, an alternative provides that in addition to an antireflection coating of this surface the filter substrate is designed approximately in the form of a wedge. 
     The invention is not limited to the exemplary embodiments explained above. It is essential for the invention only that at least one wavelength-selective filter can be set with reference to the angle of incidence of the light beams, such that the center wavelength of the filter can be set exactly and, moreover, the same filters can be used for multiplexing and/or demultiplexing light beams of different wavelengths.