Abstract:
A flat or sheet-like electrooptical component for sending and receiving electrical and optical signals includes a central emission region with at least one light-emitting device for sending out optical signals. A sensor region is arranged around the emission region, and at least one device for sensing optical signals is configured in the sensor region. A reflector region is arranged around the sensor region for reflecting incident electromagnetic radiation. The electrooptical component also includes a device for driving the light-emitting device based on incoming electrical signals, and a device for outputting electrical signals based on received optical signals.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to the field of optical signal transmission. In particular, the invention relates to a flat or sheet-like electrooptical component for sending and receiving electrical and optical signals. The invention also relates to a light-guide configuration containing such components for serial, bidirectional signal transmission, and to an optical printed circuit board. 
   Electrical printed circuit boards for driving electronic devices are commonplace in modern electronics. For many years, the speed with which the devices are operated has steadily increased. Modern processors already run at clock rates of above 1 GHz. Clock rates of several 100 MHz are aimed for and in some cases are already realized, even for comparatively slow memory chips. 
   As the speed of signal transmission increases in purely electrical printed circuit boards, difficulties increasingly occur. While it is possible in low-frequency operation at several MHz to realize, for example, a parallel serial bus concept without any problems, in high-frequency operation a range of problems arise. 
   For example, when using high frequencies with signal lines routed in parallel, the problem of crosstalk in which signal changes on one line induce interfering signals on neighboring lines increasingly occur. To remedy this, the lines must either be routed far apart from one another, which reduces the achievable data parallelism, or elaborate measures to shield neighboring lines from one another must be taken. 
   In the transmission of signals, distortions of the signal waveform also occur, in particular, in the case of signals traveling over relatively great distances with relatively long transit times, and it is necessary for this to be elaborately corrected or taken into account during the design of a circuit. 
   In the DRAM (Dynamic Random Access Memory) area, for example, so far there has been a reliance on purely electrical connections and terminals, since they can be electrically wired to printed circuit boards and to other components with good soldered bonds. With switching times of 1 to 5 ns, corresponding to 200 to 1000 MHz, however, high-frequency phenomena become noticeable, and can only be countered by good shielding and signal line reduction. A higher signal transmission rate consequently restricts the usable parallelism, a nuisance which has to be overcome to obtain further increases in overall performance. 
   Consequently, altogether considerable design and production effort has to be undertaken with electrical printed circuit boards to ensure interference-free and transit-time-adapted signal or data transmission at high signal frequencies. 
   To obviate these problems, optical connections have also been used. However, optical connections generally only take a unidirectional form between an electrooptical signal generator and an electrooptical signal receiver and then either do not allow read/write operation, or require two separate signal lines between the two end stations. Genuine bidirectional signal transmission between two stations that can in each case operate as a transmitter and receiver has until now required complex electrooptical circuitry. 
   If the transmitted signals are picked off serially at several points along the link, with all of the known methods this leads to a significant deterioration in the signal, so that repeated optical coupling out is only possible to a very restricted extent. 
   At the same time, the effective optical coupling in and out of light into and from an optical line is contrary to the requirements for simplest possible, interference-free bidirectional signal transmission. This is attributable to the wave character of the light and the associated directed, transversal electromagnetic signal propagation. Electrical signal transmissions on purely electrical printed circuit boards are unaffected by this problem, since electric current can be coupled into or out of a current conductor without great effort. 
   On the level of the contact pads, a purely optical solution has the disadvantage that the optical interface for bidirectional communication has to have both an input and an output, which with massive parallelism and simultaneous miniaturization of the components, leads to problems of space (known as pad-out). 
   A solution to the problem provided by components for signal multiplexing and processing, for instance in fiber-optic technology, requires high-quality components, which are consequently correspondingly complex to produce and are expensive. Bidirectional communication is then not possible for many applications on account of the complex structural form, or is not cost-effective on account of the associated costs. 
   Similarly, it is often not possible to achieve optical signal transmission with continuity, since optical signals are refreshed. In other words, a residual optical signal is converted into an electrical signal, amplified, and is optically re-emitted. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a substantially flat electrooptical component for sending and receiving optical and electrical signals which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type. 
   The invention is based on the object of specifying a configuration for signal transmission which avoids the disadvantages mentioned, which in particular, makes it possible to achieve serial bidirectional signal transmission and which is also suitable for the parallel transmission of high-frequency signals in a way which can be easily realized. In this respect there is also the object of specifying a low-cost electrooptical contact pad that can form an interface between an optical signal transmission link and electrical devices. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a flat or sheet-like electrooptical component for sending and receiving electrical and optical signals. The electrooptical component includes:
         a central emission region, in which at least one light-emitting device for sending out optical signals is arranged,   a sensor region, arranged around the emission region, with at least one device for receiving optical signals,   a reflector region, arranged around the sensor region, for reflecting incident electromagnetic radiation, and   a device for driving the light-emitting device based on incoming electrical signals, and a device for outputting electrical signals based on received optical signals.       

   Realizing the electrooptical component is based on the idea of providing optical bidirectional drivability, in that a transmitter and receiver of optical signals are integrated in a space-saving manner in a small space, and on the idea of providing serial signal relaying at the same time. Consequently, serving further components without great losses is made possible by a high overall reflectivity of the configuration. 
   It is preferred for the central emission region to have a plurality of light-emitting semiconductor devices, in particular laser diodes, to increase the operational reliability. A failure of one or of some of the light-emitting devices then does not lead to a failure of the entire component. 
   It is particularly preferred for the central emission region to have a plurality of surface-emitting laser diodes, which act as punctiform light sources of great beam divergence. The wavelengths of such VCSELs (Vertical Cavity Surface Emitting Lasers) in the red or near infrared range are well-suited for optical signal transmission. 
   It goes without saying, however, that light-emitting diodes or other radiation sources with wavelengths from the ultraviolet to the infrared spectral range can also be used within the scope of the invention. With these, the principle applies that the achievable information density increases as the wavelength becomes shorter, and consequently the frequency becomes higher. 
   In one configuration, the sensor region is arranged in an annular form around the central emission region. The sensor region expediently includes a plurality of photodiode segments, so that the failure of one segment can be tolerated overall for the functionality of the component. 
   It has been found to be particularly advantageous if the reflector region is arranged in an annular form around the sensor region. The reflector region is expediently formed by a metal layer or a Bragg reflector layer. 
   Furthermore, the electrooptical component advantageously includes collimating optics for concentrating incoming radiation and for making outgoing radiation parallel. 
   A micro-lens arranged centrally on the component surface is preferred in this case, but the collimating optics may also be formed for example by a micro-parabolic mirror. 
   In an advantageous configuration, the light-emitting device is designed for emitting light of a first wavelength, and the device for sensing optical signals is designed for receiving light of a second wavelength, different from the first. This allows signals to be simultaneously sent and received separately without interference. 
   The components are advantageously produced based on a direct semiconductor material, for example GaAs/AlGaAs. This additionally allows fast receiving and sending electronics to be integrated on the regions adjacent to the electrooptical component. 
   With the foregoing and other objects in view there is also provided, in accordance with the invention, a light-guide configuration for serial, bidirectional signal transmission. The configuration includes:
         an optical signal line for carrying electromagnetic radiation along a principal direction of the line,   a plurality of optical access points, arranged along the principal direction of the optical signal line, for coupling electromagnetic radiation in or out along a direction that is substantially perpendicular to the principal direction of the line,   a plurality of diffusers arranged within the optical signal line and respectively assigned to an optical access point, and   a plurality of flat or sheet-like electrooptical components, described above, each arranged on a coupling-in/coupling-out surface of a respective optical access point.       

   In this case, each diffuser interacts with the assigned optical access point in such a way that part of the radiation carried in the optical signal line can be coupled out through the diffuser via the assigned optical access point from the optical signal line to the electrooptical component, and incident radiation from the electrooptical component of the optical access point can be coupled into the optical signal line via the assigned diffuser. 
   With the foregoing and other objects in view there is also provided, in accordance with the invention, an optical printed circuit board that contains a plurality of such light-guide configurations disposed in parallel. Since the optical signals in neighboring optical signal lines do not interfere with one at another, close parallel routing of the signal lines is possible. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a flat electrooptical component, a light-guide configuration for serial, bidirectional signal transmission and an optical printed circuit board, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows an exemplary embodiment of a light-guide configuration; 
       FIG. 2  schematically shows an exemplary embodiment of an electrooptical contact pad; 
       FIG. 3  is a cross-sectional view through the electrooptical contact pad taken through the line III—III shown in  FIG. 2 ; 
       FIG. 4  is a more detailed view of an optical access point of the light-guide configuration shown in  FIG. 1 ; 
       FIG. 5  is a cross-sectional view of an exemplary embodiment of an optical printed circuit board; and 
       FIG. 6  is a schematic representation of another exemplary embodiment of a light-guide configuration. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a light-guide configuration, denoted generally by  10 , into which an optical signal  20  is coupled via an optical connector  22 . The light signal  20  propagates in  FIG. 1  in the optical signal line  12  from the left side to the right side of the configuration  10  and thereby passes the optical access points  14   a  to  14   d  one after the other. The light propagation is in this case determined by multiple scattering at the reflecting side surfaces of the signal line  12 , is thus substantially diffuse and not restricted to one or a few modes. 
   Each of the optical access points  14   a  to  14   d  has an assigned prismatic diffuser  16   a  to  16   d , which is respectively provided opposite the inlet of the optical access point on a lateral surface of the optical signal line  12 . By light scattering at the prisms  16   a  to  16   d , part of the radiation intensity propagating in the optical signal line  12  is in each case coupled out into the optical access points  14   a  to  14   d  and is taken up by electrooptical contact pads  18 . 
   Radiation that has passed through the entire optical signal line  12  is absorbed in an absorber  24  at the end of the optical signal line  12  in order to obtain a defined termination of the signal line and to avoid interfering retroreflective effects. 
   The configuration of the contact pads  18  is explained in more detail below in conjunction with  FIGS. 2 and 3 . The contact pad  18  has a GaAs support  130 , on the surface of which an emission region  100  is centrally arranged. For reasons of redundancy, the emission region  100  includes a plurality, seven in the exemplary embodiment, of what are known as VCSELs (Vertical Cavity Surface Emitting Lasers)  102 . These VCSEL&#39;s  102  are ultra-quickly switching surface-emitting lasers that represent a punctiform light source of great beam divergence. Each of the VCSELs  102  has an extent of approximately 10 μm, so the entire emission region  100  has a diameter of approximately 50 μm. 
   A segmented ring  110  including a plurality of photodiodes  112  (six photodiodes in the exemplary embodiment) is arranged around the emission region  100 . Each of the photodiodes  112  respectively has a width of approximately 10 μm and together they cover the entire circumference of the emission region. 
   The segmented ring  110  with the photodiodes  112  is surrounded by a highly reflective ring region  120 , which has a width of approximately 200 μm. In the exemplary embodiment, the ring region  120  includes a vapor-deposited quarter-wave Bragg mirror, the reflectivity of which is designed for the light wavelength used, here 940 nm. The Bragg mirror of the reflector ring  120  can in this case be applied simultaneously with the Bragg reflector layer required for the laser resonators of the VCSELs  102 . 
   The total reflectivity of the contact pad  18  is 95-99%, so that a large part of the incident radiation is reflected back into the optical signal line  12  in order to relay the signal. The diameter of the entire contact pad  18  is approximately 500 μm, and is consequently slightly larger than the coupling-in/coupling-out opening of the optical access point  14 . In the exemplary embodiment, all of the components of the contact pad are produced on a GaAs/AlGaAs basis. 
   As can be seen in particular from  FIG. 4 , a micro-lens  26  is arranged centrally on the contact area. The micro-lens  26  concentrates incident radiation onto a central region of the contact pad  18  and consequently onto the segmented ring  110  with the photodiodes  112 . Radiation, which is emitted by the emission region  100 , is made parallel by the micro-lens  26  and is radiated into the optical access point  14 . Incident radiation that is not absorbed in the central region  100 ,  110  is reflected back highly effectively by the Bragg reflector layer  120 . 
   Returning to  FIG. 1 , the signal transmission in the light-guide configuration takes place bidirectionally and serially. An input signal can be picked off successively (serially) at a plurality of optical access points  14   a - 14   d  and the contact pads  18  provided there. For this purpose, each access point through the electrooptical pads  18  is set up both for receiving and sending optical signals (bidirectional transmission). 
   To ensure optimum transmission of the optical signals to the serially arranged access points  14   a  to  14   d , and a constant coupling-in performance for all of the access points  14   a  to  14   d , the shape and size of the prisms  16   a  to  16   d  along the direction of propagation of the optical signal line  12  are made to match one another. 
   As indicated in  FIG. 1 , the prisms  16   a  to  16   d  have a constant base area. However, the apex angle decreases along the direction of propagation, so the height of the prisms increases from prism  16   a  through prisms  16   b  and  16   c  to prism  16   d . As a result, an increasingly relative proportion of the radiation  20  still propagating in the light guide is coupled out via the prisms, which compensates for the decreasing radiation power after each coupling-out process. 
   On the other hand, the constant base area of all the prisms  16   a  to  16   d  ensures a constant coupling-in performance for each of the optical access points  14   a  to  14   d.    
     FIG. 4  shows the path of rays at an optical access point  14  during the operation of the light-guide configuration. In  FIG. 4 , the direction of propagation of the light in the signal line  12  runs from left to right. An optical signal pulse  140  falls from the left onto the diffuser prism  16 . A proportion  142  of the radiation, corresponding to the ratio of the height of the prism  16  to the overall height of the signal line  12 , is deflected toward the optical access point  14 . A small part of the radiation (1-5%) is absorbed by the optical contact pad  18 . The optical signal is thereby picked up by the photodiodes  112  and is converted into corresponding electrical signals. 
   The greatest proportion  144  of the radiation falling on the contact pad  18  is reflected back by the reflector ring  120  to the diffusing prism and is coupled again into the optical signal line  12 . The radiation intensity  148  transmitted from the optical access point consequently corresponds to the sum of the intensity propagating past the diffusing prism  16  and half of the intensity reflected back from the contact pad  18 . 
   If the contact pad  18  is operating as an emitter, the emitted intensity  144  is coupled half-and-half in both directions of propagation  146 ,  148  of the optical signal line  12 . The coupled-in optical signal is consequently available both at the input or output of the signal line  12  and at further connected optical access points  14 . 
   An exemplary embodiment of an optical printed circuit board  34  with four parallel optical signal lines  12  is represented in section in FIG.  5 . The optical signals coupled in via the optical connector  22  are led out serially on each signal line  12  to corresponding terminals of the devices  30 , which are memory chips in the exemplary embodiment. 
   The printed circuit board  34  in this case includes three layers, a lower printed circuit board  40 , which contains electrical connecting lines, a printed circuit board  42  containing the optical signal lines  12 , and an upper printed circuit board  44 . 
   The upper printed circuit board  44  terminates the optical signal lines between the optical access points  14  in the upward direction. For each optical access point  14 , the upper printed circuit board  44  has an aperture, on the upper side of which the described contact pads  18  are arranged. Connected to the electrical outputs of the contact pads  18  are electrical inputs of a device  30 . This takes place in a way that is known per se, for example, by using solder balls provided at the inputs of the device  30 . 
     FIG. 6  shows a further embodiment of the light-guide configuration, which differs from the configuration of  FIG. 1  in that devices  30 ,  34  are arranged on both sides of the optical signal line  62 . 
   In a corresponding way, the optical signal line  62  has optical access points in the upward direction (reference numeral  64 ) and downward direction (reference numeral  74 ). In each case, the optical access points  64 ,  74  are arranged perpendicularly to the direction of propagation of the radiation in the signal line  62 . Arranged respectively opposite the optical access points  64 ,  74 , in a way analogous to the configuration described in conjunction with  FIG. 1 , are diffusing prisms  66 ,  76 , which couple out a proportion of the propagating radiation in the upward direction (reference numeral  68 ) or in the downward direction (reference numeral  78 ) to the electrooptical contact pads  18 . 
   The upper and lower printed circuit boards  80 ,  82  in each case have optical apertures. At least one of the printed circuit boards is also designed for carrying electrical signals, in particular supply voltages for electrooptical components.