Abstract:
A bidirectional optical transmitting and receiving module for connection to an optical waveguide, having an optical receiving device and an optical transmitting device, with the receiving device being arranged in one region of the optical axis of the optical waveguide, in front of its end surface. In order to make it possible with a transmitting and receiving module such as this for this module to be produced very easily and at the same time in order to very largely avoid electrical crosstalk between the transmitting device and the receiving device, the transmitting device is arranged away from the region of the optical axis of the optical waveguide, and at least one deflection device is arranged in the beam path between the optical waveguide and the receiving device and injects the light which is emitted from the transmitting device into the optical waveguide.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to opto-electronic devices, and in particular to a bidirectional optical transmitting and receiving module.  
         BACKGROUND OF THE INVENTION  
         [0002]    A first known transmitting and receiving module is disclosed, for example, in German Laid-Open Specification DE 198 23 213. In this already known transmitting and receiving module, a laser (optical transmitting device) and the optically active zone of a photodiode (optical receiving device) are arranged immediately in front of the end surface of an optical waveguide. The photodiode and hence the active zone of the photodiode are integrated in a substrate and have a structure which is in the form of a frame and frames a central region of the substrate. The laser is mounted on the substrate in this central region, as a separate component. From the physical point of view—along the optical axis of the optical waveguide—the laser is thus located between the plane that is covered by the photodiode and the end surface of the optical waveguide. The laser and the photodiode are thus both located in the region of the optical axis of the optical waveguide.  
           [0003]    Furthermore, a second known transmitting and receiving module is disclosed in “Optical Transceiver for OP i.LINK S200/S400”, Hideaki Fukita, Yorishige Ishii, Toshihisa Matsuo, Yoshifumi Iwai, Tetsuo Iwaki, Kazuhito Nagura, Takatoshi Mizoguchi and Yukio Kurata; Proceedings of the 11th International POF Conference 2002, Tokyo, September 18-20, in which a deflection mirror is arranged between the optical receiving device and the end face of the optical waveguide. The transmission light from the optical transmitting device is injected without any physical deflection into an edge area of the end surface of the optical waveguide. The transmitting device and the receiving device in this already known transmitting and receiving module do not physically lie on a plane, so that the module has to have a relatively complicated physical design.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention is directed to a bidirectional optical transmitting and receiving module which can be produced easily, and in which electrical crosstalk between the transmitting device and the receiving device is at the same time very largely avoided.  
           [0005]    On this basis, the present invention provides that the transmitting device is arranged away from the region of the optical axis of the optical waveguide near to the receiving device, and at least one deflection device is arranged in the beam path between the optical waveguide and the receiving device and injects the light which is emitted from the transmitting device into the optical waveguide.  
           [0006]    A first major advantage of the bidirectional transmitting and receiving module according to the invention is that the arrangement of the transmitting device away from the region of the optical axis of the optical waveguide—in particular in contrast to the first-mentioned already known transmitting and receiving module (discussed above)—allows physical separation between the receiving device and the transmitting device. The distance between the two devices may in this case be chosen to be sufficiently large to very largely avoid electrical crosstalk between the receiving device and the transmitting device.  
           [0007]    A second major advantage of the bidirectional transmitting and receiving module according to the invention is that it can be produced at very low cost. The transmitting device and the receiving device can thus be arranged on a plane such that—in particular in contrast to the second-mentioned already known transmitting and receiving module (discussed above)—the two devices can be arranged, for example, on a printed circuit board, in particular a lead frame.  
           [0008]    A third major advantage of the transmitting and receiving module according to the invention is that—in particular in contrast to the second-mentioned already known transmitting and receiving module—this results in very high coupling efficiency between the receiving device and the optical waveguide, specifically because the receiving device is arranged immediately in front of the end surface of the optical waveguide. Owing to the arrangement of the receiving device immediately in front of the end surface, it is furthermore possible to choose the distance between the optical waveguide and the receiving device to be very small as well, so that high coupling efficiency is achieved very largely independently of the numerical aperture of the optical waveguide.  
           [0009]    A fourth major advantage of the optical transmitting and receiving module according to the invention is that, owing to the beam guidance of the light from the transmitting device, that is to say specifically owing to the deflection device between the transmitting device and the end surface of the optical waveguide, any light from the transmitting device which is reflected by the end surface of the optical waveguide effectively cannot fall or can fall only insignificantly, on the receiving device; optical crosstalk between the transmitting device and the receiving device is thus largely avoided.  
           [0010]    A fifth major advantage of the optical transmitting and receiving module according to the invention is that the module is highly independent of any radial offset between the optical waveguide and the “nominal position”, because—in particular in contrast to the second-mentioned already known transmitting and receiving module—the light from the transmitting device can be injected directly into the center of the end surface of the optical waveguide (i.e., relatively independently of tolerances), that is to say not into an edge region of the end surface (i.e., relatively dependent on tolerance).  
           [0011]    With respect to simple and thus low-cost mounting of the transmitting and receiving module, it is regarded as being advantageous for the receiving device and the transmitting device to be arranged alongside one another on a printed circuit board, in particular a lead frame. That is because, if these devices are mounted on a printed circuit board, then, for example, conventional automatic component placement machines, such as those which are normally used for the production of electrical printed circuit boards, can be used. The transmitting device may in this case be mounted directly or indirectly on the printed circuit board. If it is mounted directly, the transmitting device is mounted directly on the printed circuit board; if it is mounted indirectly, the transmitting device is first of all mounted on a submount, and is then fitted to the printed circuit board, together with this submount.  
           [0012]    In order to avoid the possibility of the transmitting and receiving module becoming dirty or being damaged by moisture, it is regarded as being advantageous for at least the optically active zone of the receiving device and/or the optical transmitting device to be provided or sealed with a translucent material, in particular an encapsulation material.  
           [0013]    As already explained above according to the invention a deflection device is provided between the transmitting device and the optical waveguide. With respect to the deflection device, it is regarded as being advantageous for the outer face of the translucent material which faces the optical waveguide to have an optical disturbance point which acts as a deflection element for the deflection device. An optical disturbance point in the translucent material or in the translucent encapsulation material can be produced easily and at a low cost, for example by means of a “stamp”, during application of the material, so that in this way it possible to form at least one deflection element for the deflection device particularly easily and at low cost.  
           [0014]    In order to ensure that the light which is produced by the transmitting device is coupled into the optical waveguide with an efficiency which is particularly independent of tolerance, it is regarded as being advantageous for the disturbance point to be located in the central region of the optical axis which is formed by the longitudinal axis of the optical waveguide. This is because, when the light is injected in the central region the coupling efficiency is less dependent on any radial offset of the optical waveguide than is the case when light is injected in the edge area of the optical waveguide.  
           [0015]    The optical disturbance point may preferably be formed by a groove; a groove such as this may be formed, for example, by a stamp, which is used during the application of the translucent material or of the encapsulation material.  
           [0016]    In order to inject the light that is produced by the transmitting device into the optical waveguide, the groove should preferably have a side surface which refracts the light generated by the transmitting device in the direction of the optical axis of the optical waveguide. The deflection device may preferably and additionally have a further deflection element which interacts with the first deflection element and, together with it, forms the “deflection device”. The further deflection element may advantageously be formed by a mirror.  
           [0017]    The mounting of the further deflection element is in this case particularly simple and is thus advantageously possible, if the further deflection element is arranged on the printed circuit board, physically between the transmitting device and the receiving device.  
           [0018]    An edge-emitting laser may advantageously be used, for example, as the transmitting device in the transmitting and receiving module. A particularly compact modular configuration can be achieved using an edge-emitting laser by the light outlet opening of the transmitting device (laser) being arranged vertically (i.e., perpendicular to the horizontally-aligned optical axis), at least essentially vertically, with respect to the end surface of the optical waveguide. The deflection device must then produce a deflection angle of 90 degrees.  
           [0019]    A deflection angle of 90 degrees can be achieved if the further deflection element, that is to say for example a mirror, produces a deflection angle relative to the optical axis of the optical waveguide which forms a right angle together—that is to say as the sum of the angles—with the deflection angle which is formed by the first deflection element. A deflection angle of 90 degrees means that any light from the transmitting device which is reflected on the end face of the optical waveguide is likewise thrown back at right angles and therefore cannot strike the optical receiving device.  
           [0020]    In general terms, it is regarded as being advantageous for the outer face of the translucent material (for example the encapsulation material) to form a lens which focuses the light that is admitted from the optical waveguide in the direction of the receiving device onto the active surface of the receiving device. A focusing lens such as this allows particularly high coupling efficiency to be achieved between the receiving device and the optical waveguide.  
           [0021]    According to another advantageous refinement of the transmitting and receiving module according to the invention, it is regarded as advantageous for the entire deflection device to be arranged exclusively directly in the central region of the optical axis of the optical waveguide. An arrangement such as this results in a particularly compact modular configuration which is independent of tolerances. Independence of tolerances is in this case based in particular on the fact that the light which is produced by the transmitting device is injected directly into the central region of the optical waveguide.  
           [0022]    The deflection device may preferably be formed by a single mirror which is arranged “on” the receiving device. The mirror should preferably be positioned at an angle of approximately 45° with respect to the optical axis of the optical waveguide. The active zone of the receiving device may have a structure in the form of a frame and, “as a frame” may frame the mirror which is arranged in the central region of the optical axis.  
           [0023]    In general terms, it is regarded as advantageous for a lens which focuses the light from the transmitting device onto the deflection device to be arranged between the transmitting device and the deflection device. A lens such as this allows the coupling efficiency between the transmitting device and the optical waveguide to be increased further. The lens may, for example, be formed by a spherical lens; instead of this, the lens may be formed by an appropriate shape of a housing for the transmitting device.  
           [0024]    Since the encapsulation materials for encapsulation of the transmitting and receiving module normally have a refractive index of approximately n=1.5, the use of GaP material as the lens material is regarded as advantageous; this GaP has a refractive index of approximately n=3, which is less than the refractive index of the semiconductor material of the transmitting device (approximately n=3.5) and is greater than the refractive index of the encapsulation material (for example casting resin). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 shows a first exemplary embodiment of a bidirectional transmitting and receiving module according to the invention, in which a deflection device which is formed from two deflection elements is provided, and the transmitting and receiving devices are accommodated in separate housings,  
         [0026]    [0026]FIG. 2 shows a second exemplary embodiment of a transmitting and receiving module according to the invention in which a mirror which is arranged in the region of the optical axis of the optical waveguide is provided as the deflection device,  
         [0027]    [0027]FIG. 3 shows a third exemplary embodiment of a bidirectional transmitting and receiving module according to the invention, in which the transmitting and receiving devices are encapsulated with one another, and the shape of the encapsulation compound forms a lens,  
         [0028]    [0028]FIG. 4 shows a fourth exemplary embodiment of a bidirectional transmitting and receiving module according to the invention, in which the transmitting and receiving devices are encapsulated with one another, but with the shape of the encapsulation compound not forming a lens,  
         [0029]    [0029]FIG. 5 shows a fifth exemplary embodiment of a bidirectional transmitting and receiving module according to the invention, in which the light from the transmitting device is injected into the optical waveguide away from the central region of the optical waveguide, and  
         [0030]    [0030]FIG. 6 shows a sixth exemplary embodiment of a bidirectional transmitting and receiving module according to the invention—in the form of a plan view along the optical axis of the optical waveguide—in which a beam-forming element is located between the transmitting device and a mirror. 
     
    
     DETAILED DESCRIPTION  
       [0031]    [0031]FIG. 1 shows an optical waveguide  10  which is optically connected to a photodiode  20 , which is provided with polarization filter  25  as the receiving device, and to a laser  30  as the transmitting device. When seen along the (first) optical axis  40  defined by the optical waveguide  10 , the photodiode  20  is in this case arranged in front of the surface  50  of the optical waveguide.  
         [0032]    A translucent isolation material  60 , for example an encapsulation compound, is provided between the photodiode  20  and the surface  50  of the optical waveguide  10 , and protects the photodiode  20  against dirt or moisture. The shape of the outer surface of the isolation material  60  forms a lens  70 —for example, a spherical lens or an aspherical lens, in this case referred to by way of example in the following text as a spherical lens—which focuses the light from the optical waveguide  10  so as to achieve particularly high coupling efficiency between the optical waveguide  10  and the photodiode  20 .  
         [0033]    The photodiode  20  and the laser  30  are arranged on a printed circuit board (base)  80 —possibly indirectly on a submount  35 —so that the laser light which is produced by the (for example edge-emitting) laser  30  at the light outlet opening of a laser housing  90  of the laser  30  emerges (i.e., directed along a second optical axis) essentially at right angles to the optical axis  40  of the optical waveguide  10 . In order to ensure that the light which is produced by the laser  30  is injected into the optical waveguide  10 , a deflection device is provided which comprises a (first) deflection element  100  and a further (second) deflection element  110 .  
         [0034]    The first deflection element  100  is in this case formed by a recess—that is to say a disturbance point—in the spherical lens  70 . The recess is in the form of a groove and is located in the central region of the optical axis  40  of the optical waveguide  10 . The groove  100 , or a side surface  105  of the groove  100 , is in this case at an angle to the optical axis  40  such that the light which is generated by the laser  30  is refracted in the direction of the optical axis  40  of the optical waveguide  10 .  
         [0035]    The second deflection element  110  is formed by a mirror, which is mounted on the printed circuit board  80  between the laser  30  and the photodiode  20 . The mirror  110  is in this case aligned such that the “light deflection angle” of the groove  100  and the “light deflection angle” of the mirror  110  together form an “overall deflection angle” 90 degrees.  
         [0036]    The arrangement of the two deflection elements  100  and  110  therefore ensures that the light which is produced by the laser  30  is deflected through an angle of approximately 90° and can be injected into the optical waveguide  10  at the end surface  50 .  
         [0037]    The size of the first deflection element  100  is in this case chosen such that the deflection element produces only a minor disturbance in the beam path between the optical waveguide  10  and the photodiode  20 . Although the first deflection element  100  is arranged in the central region of the optical axis  40 , the physical size of the first deflection element  100  is, however, so small that, overall, it represents only a minor disturbance, so that the coupling efficiency between the photodiode  20  and the optical waveguide  10  is reduced only slightly and is thus still sufficiently high overall.  
         [0038]    As can be seen from FIG. 1, the deflection effect of the first deflection element  100  is based on the sudden change in the refractive index between, on the one hand, the material of the spherical lens  70  and, on the other hand, the air between the spherical lens  70  and the optical waveguide  10 . The side surface  105  of the groove  100  therefore may not be aligned at an angle of 45° to the optical axis  40  of the optical waveguide  10 —as is accidentally the impression given by FIG. 1—but such that the light which is reflected by the second deflection element  110  can be injected into the optical waveguide  10  with the maximum possible coupling efficiency. The angle of the groove  100  therefore depends on the sudden change in the refractive indices of the materials, and must be chosen appropriately.  
         [0039]    Furthermore, FIG. 1 shows a further lens  120  which is arranged immediately adjacent to the housing outer surface of the laser housing  90  of the laser  30 . The function of this further lens  120  is to focus the light which emerges from the laser  30  before it strikes the second deflection element  110 , in order to achieve particularly high coupling efficiency between the laser  30  and the optical waveguide  10 .  
         [0040]    [0040]FIG. 2 shows a second exemplary embodiment of a bidirectional optical transmitting and receiving module according to the invention. In this exemplary embodiment as well, the laser  30  and the photodiode  20  are arranged on a printed circuit board  80 . A mirror  200  is arranged in the region of the optical axis  40 , immediately adjacent to the photodiode  20 .  
         [0041]    The mirror is positioned at an angle of 45° to the optical axis  40  of the optical waveguide  10 , so that the laser light which is produced by the laser  30  is injected directly into the optical waveguide  10  from the mirror  200 . The size of the mirror  200  is in this case chosen to be as small as possible, in order to avoid the coupling efficiency between the photodiode  20  and the optical waveguide  10  being excessively reduced.  
         [0042]    In summary, the mirror surface of the mirror  200  should thus be chosen to be sufficiently small that, on the one hand, sufficient coupling is achieved for the light which is produced by the laser  30  into the optical waveguide  10  and, on the other hand, such that any disturbance to the light transmission between the optical waveguide  10  and the photodiode  20  is reduced to the minimum extent.  
         [0043]    In the exemplary embodiment shown in FIG. 2, the mirror  200  deflects the beam of the laser light through 90°, in order to ensure that it is coupled into the optical waveguide  10 . In the exemplary embodiment shown in FIG. 2, there is therefore no need for any first deflection element  100 , such as that which is formed by a groove in the exemplary embodiment shown in FIG. 1.  
         [0044]    Overall, the mirror  200  means that the laser  30  and the photodiode  20  can be arranged physically separately from one another, thus effectively avoiding, or at least greatly reducing any electrical crosstalk between the photodiode  20  and the laser  30 .  
         [0045]    Further variants and exemplary embodiments of transmitting and receiving modules according to the invention are illustrated in FIGS.  3  to  6 .  
         [0046]    In the exemplary embodiments shown in FIGS.  3  to  6 , the laser  30  does not have its own housing  90 ; instead of this, the laser  30  is encapsulated together with the photodiode  20 , and is thus embedded in the isolation material  60 . In the exemplary embodiment shown in FIG. 3, the isolation material  60  forms the spherical lens  70 , which has already been explained in conjunction with FIGS. 1 and 2; the exemplary embodiments shown in FIGS. 4 and 5 have no such spherical lens  70 .  
         [0047]    In the exemplary embodiments shown in FIGS. 3, 4 and  5 , a spherical lens  300  is arranged between the laser  30  and the mirror  200  for beam focusing. The spherical lens  300  is mounted on the printed circuit board  80  and is encapsulated together with the laser  30  and the photodiode  20  by means of the isolation material  60 . In the exemplary embodiment shown in FIG. 6, a beam-forming element  310  is provided instead of a spherical lens, and is formed by the shaping of the isolation material  60  (encapsulation compound) or by a separate mirror element. The beam-forming element  310  is preferably mirrored, for example by means of a metallization layer.  
         [0048]    In general terms, as can be seen from FIG. 5, the light from the laser  30  can also be injected eccentrically into the optical waveguide  10 . In the case of multimode optical waveguides, for example, an adequate coupling efficiency can still be achieved even when the light is injected eccentrically.  
         [0049]    In the two exemplary embodiments, shown in FIGS.  3  to  5 , the spherical lens  30  may, for example, be formed by a gallium phosphide GaP) lens; gallium phosphide has a refractive index of approximately n=3, which is higher than the refractive index of n=1.54 of the isolation material  60  (for example casting resin), and is less than the refractive index of the laser  30  (n=3.5).  
         [0050]    In order to avoid the light which is generated by the laser  30  from being reflected on the end surface  50  of the optical waveguide  10  and partially being coupled back into the laser  10  again, polarization films with quarter-lambda delay elements may be used; quarter-lambda delay elements such as these rotate the polarization direction of the light which is generated by the laser  30  so that the light which is reflected back from the optical waveguide has a polarization which differs from that of the light which was originally produced by the laser  30 . The light which is reflected back can then be filtered out by means of an appropriate polarization filter, so that this avoids feedback into the laser  30 .