Abstract:
A photodiode and a method of fabricating a photodiode for reducing modal dispersion and increasing travel distance. The central region of the photodiode is made less responsive to incident light than a peripheral region of the photodiode. The less responsive central region discriminates the lower order modes such that only the higher order modes are incident on the more responsive peripheral region. Because the lower order modes are subtracted, the range of propagation constants is reduced and modal dispersion is also reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/653,747, titled Photodiode with Fiber Mode Dispersion Compensation filed Feb. 16, 2005, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates in general to the fabrication of a photodiode to achieve fiber modal compensation in optical fiber systems. 
     2. Description of the Related Art 
     In an optical communication system that includes a multi-mode fiber (MMF), an optical signal launched into a fiber propagates along the fiber in multiple mode groups, each of which exhibits a different group velocity. Portions of the source optical signal reside in the different mode groups. Each of the mode groups can have different arrival times at the end of the fiber due to the different group velocities. The different group velocities of the mode groups cause a pulse, representing a binary symbol, formed from more than one mode group to spread out as it propagates. This is because portions of the symbol travel faster than other portions of the symbol. This is referred to as modal dispersion. Modal dispersion causes the optical signal initially launched through a fiber at a predetermined frequency and an initial phase to vary as a function of the length of the fiber. 
     Modal dispersion reduces the maximum data transmission rate of the optical communication system and thus diminishes the total transmission capacity of the fiber. This results at least in part from the fact that modal dispersion spreads the optical pulse as it propagates. Adjacent pulses may spread into each other causing intersymbol interference. Short pulses typically also include short times between pulses and thus are limited to very short transmission distances to prevent or minimize intersymbol interference. Longer pulses can be transmitted further because the relative distortion of the pulse is smaller and distance between pulses is greater. Shorter pulses typically require more bandwidth for propagation. A common specification criteria for multimode fiber is a bandwidth-distance product. Importantly, the bandwidth-distance products of typical multimode fiber are severely limiting. Typically the fiber bandwidth is specified using a light launching condition that fills all of the modes of the optical fiber. 
     Modem multimode fiber incorporates a graded optical index profile within the core of the fiber to reduce modal dispersion. This is done by engineering the index profile of the graded region, typically in the shape of a parabola along the transverse dimension of the optical fiber. However, manufacturing variances and defects such as uncontrolled changes in grading profiles, a so called “alpha change”, or index peaking or dipping in the center, lead to variances in the fiber modal bandwidth. Modal dispersion remains the dominant bandwidth limiting mechanism in multimode fibers. These deviations can lead to significant variations in the optical bandwidth of the fiber depending on which mode groups are excited in the launching condition. Furthermore, as compared to single mode fibers, these limits reduce the capacity of multimode fiber by orders of magnitude. 
     The different propagation velocities of the distinct optical mode groups can lead to large differential mode delay (DMD) in multimode fibers. The severity of this modal dispersion can limit multimode fiber links to bandwidth-distance products of a few tens of MHz-kilometers depending on the original mode excitations. 
     The mode groups inside the fiber are characterized with different divergences at the distal end of the optical fiber. Specifically, higher order mode groups have a wider divergence angle than lower order mode groups. Stated differently, when higher order mode groups exit a multimode fiber onto a detector, such as a photodiode, the higher order modes will impinge the detector within a larger radius than the lower order modes. 
     Therefore, it is possible to spatially separate the various mode groups in the optical fiber, and by appropriate reconstruction and delays, correct the effects of modal dispersion in the fiber. 
     In practice it is often sufficient to only spatially resolve two areas, the inner and outer portion. Electronic compensation of the fiber modal dispersion can then be used to restore the signal integrity. This has been demonstrated to improve the bandwidth of the optical system. In previously described systems, a photodiode has been fabricated with two concentric active regions. The response of the two active regions can be subtracted electronically to reduce the effects of the modal dispersion. The approach therein is to use a Metal-Semiconductor-Metal (MSM) photodiode with two active areas. However, this has the disadvantage of requiring complex fabrication technology, and external subtraction circuits to operate effectively. 
     Thus, a need exists in the industry to address the aforementioned and/or other deficiencies and/or inadequacies. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly described, one embodiment of the invention can be implemented as a system that includes a photodiode fabricated with two regions: a central region that is rendered optically dead by deep proton implantation, and a peripheral region. The photodiode is coupled to an end of an optical fiber for detecting optical signals exiting the optical fiber and for converting the optical signals to an electrical output. The central region of the photodiode intercepts a mode in a manner distinct from the peripheral regions. The system also includes reception of a resultant signal output from the photodiode regions and forwarding the signal to circuitry. 
     The present invention can also be embodied as a method that, in one embodiment, includes the following steps: forming an n-type layer on the semiconductor substrate, forming an intrinsic layer on the n-type layer, forming a p-type layer on the intrinsic layer, and rendering the photodiode less responsive to incident light on the central region than to incident light on the peripheral region. The photodiode detects different portions of a plurality of optical signals radiating from an end of a multi-mode fiber. The effect of the central region is to reduce the effects of modal dispersion among the plurality of optical signals by selectively reducing the response to particular mode groups of the fiber. 
     Embodiments of the invention may also be described in terms of method for receiving and processing optical signals. For example, one embodiment includes a method for receiving information through a multimode fiber. The method includes receiving an optical signal having higher order mode groups and lower order mode groups from the multimode fiber onto a surface having a central region and a peripheral region. The lower order mode groups are incident on the central region. The higher order mode groups have portions that are incident on the peripheral region. The method further includes generating an output electrical signal in response to incident light on the peripheral region. The method includes being less responsive to incident light on the central region than to incident light on the peripheral region. Advantageously, this embodiment allows for modal dispersion to be reduced by implementing a system that effectively subtracts the lower order mode groups from the received optical signal. 
     These and other advantages and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIGS. 1A and 1B  is a schematic diagram of an optical fiber system including a photodiode system having a lower order mode group incident on the central region in accordance with an embodiment of the invention. 
         FIGS. 2A and 2B  is a schematic diagram of an optical fiber system including a photodiode system having a higher order mode group incident on the peripheral region of the photodiode in accordance with an embodiment of the invention. 
         FIG. 3  is a flow chart of a method of fabricating an optoelectrical system in accordance with an embodiment of the invention. 
         FIG. 4  is a schematic diagram of an optoelectrical system including the photodiode shown in  FIGS. 1A ,  1 B,  2 A and  2 B, and a circuit for processing the output electrical signal of the photodiode. 
         FIG. 5  is a flow chart of a method of fabricating a photodiode, including a mask above, on, or in the central region, being irradiated from the end of a multimode optical fiber by light composed of higher order mode groups and lower order mode groups. 
         FIG. 6  depicts the photodiode device being irradiated from the end of a multimode optical fiber by light composed of higher order mode groups and lower order mode groups in accordance with an embodiment of the invention. 
         FIG. 7  is a flow chart of a method of fabricating a photodiode including an opaque mask in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In an optical communication systems, modal dispersion limits both the bandwidth and the travel distance of the communications system. An optical signal from a laser is launched into a fiber and propagates along the fiber as multiple modes, each of which travels a different path through the fiber. The different mode velocities cause a pulse formed from more than one mode to spread out as it propagates. Each of the modes can have different arrival times at the end of the fiber, resulting in modal dispersion. 
     Modal dispersion causes each digital pulse of the optical signal launched through a fiber to spread as a function of the length of the fiber. Uncompensated modal dispersion limits the transmission bit rate of a digital communications system because the resultant distortion of the received signal causes high bit rate pulses to merge causing intersymbol interference. Pulses that overlap cannot be reliably distinguished. 
     Embodiments illustrated herein include photodiode fabrications that functionally perform subtraction of one or more modes so as to reduce dispersion in a received signal. 
     A photodiode generally includes a p-type semiconductor material connected to an n-type semiconductor material. P-type semiconductor materials differ from intrinsic semiconductor materials in that the p-type semiconductor materials include a dopant that causes the p-type material to naturally include free positive charge carriers, also known as holes. In n-type semiconductor materials, dopants are added to intrinsic semiconductor materials to create semiconductor materials with free negative charge carriers, also known as electrons. Between the n-type semiconductor material and the p-type semiconductor material is an intrinsic region with no net charge carriers. A photon, if it has enough energy, can cause free carriers including a free electron and a free hole to be created in the intrinsic region. This allows a current to flow when the photodiode is reverse biased at an appropriate voltage. Current flowing in the photodiode is proportional to the number of photons and the energy of the photons impinging the intrinsic region. 
     Deep proton implantation is used in one embodiment to define an optically dead region in the center of the photodiode. Proton implants serve to damage the dead region such that the dead region is not able to generate free carriers when photons impinge the dead region. The optically dead region does the same subtraction of the two fiber modal responses but requires significantly less complex processing and external circuit connections to achieve the desired fiber modal compensation. When light impinges the optically dead region, essentially no free carriers are created. Thus, as shown below, only power from the higher order mode groups are measured while power from low order mode groups are not. This has the net effect of subtracting low order mode group power from the total optical power including the low order mode groups and the high order mode groups. By eliminating one of the mode groups, modal dispersion is decreased by only measuring modes that travel at similar speeds on an optical fiber. 
       FIGS. 1A and 2A  depict one embodiment of an optical fiber system of the invention.  FIG. 1A  illustrates the propagation of a lower order mode group  140  of optical signals through the optical fiber system and  FIG. 2A  illustrates the propagation of a higher order mode group  240  of optical signals through the optical fiber system. Although the mode groups are depicted in separate figures for the purposes of illustration, both mode groups propagate through the same optical system simultaneously with the lower order mode group  140  arriving before the higher order mode group  240 . 
       FIGS. 1A and 2A  are schematic diagrams of an optical fiber system, including a receiver  130  that compensates for modal dispersion. The receiver&#39;s surface  160  includes a central region  180  and a peripheral region  170 , illustrated in cross-sections of  FIGS. 1B and 2B . 
     An optical fiber system includes a transmitter  110  adapted to transmit information. A digital signal is converted to an optical signal by an electrical-to-optical (E-O) converter which includes the use of a digital-to-analog converter and light source such as a laser. Suitable light sources include a vertical cavity surface emitting laser (VCSEL), light emitting diodes (LED), distributed feedback (DFB) laser, and Fabry-Perot (F-P) lasers. The light source launches an optical signal into a multimode optical fiber  100 . 
     The optical signal propagates through the multimode optical fiber  100  in both lower  140  and higher order  240  mode groups. The higher order mode groups  240  travel a less direct path through the multimode optical fiber  100 . The result is that in one example the higher order mode groups  240  travel through the multimode optical fiber  100  at a lower group velocity than the lower order mode groups  140 . Therefore, the width of each digital pulse of the optical signal is stretched as it propagates through the multimode optical fiber  100  due to the higher order mode groups  240  falling behind the lower order mode groups  140 . The stretching distorts the pulses received at the end of the multimode optical fiber  100  and limits the frequency of the digital pulses. Notably, because of index perturbations, lower order modes may arrive later than higher order modes. In any case, there exists a difference in group velocity that results in modal dispersion. 
     The receiver  130  is positioned at the end of the multimode optical fiber  100 . Referring now to  FIGS. 1B and 2B , multiple modes of light exiting from the multimode optical fiber  100  are incident to the surface  160 , the surface  160  having a central region  180  and a peripheral region  170 . The lower order mode groups  140  are for the most part incident on the central region  180 , while the higher order mode groups  240  are incident on the peripheral region  170 . The receiver  130  has a surface that is less responsive, or preferably unresponsive, to incident light on a central region  180  than to incident light on a peripheral region  170 . 
     As a result of this process of subtracting the lower order mode groups from the optical signal, the dispersion of the optical signal is compensated for. The receiver  130  outputs an electrical signal that contains the information encoded in the optical signal. In one embodiment, the information is transmitted to a circuit or a computer for further processing. 
       FIG. 3  shows a flow chart of a method of transmitting information in an optical system of the present invention. 
     At  300 , the transmitter launches an optical signal, with high and low order mode groups, into an optical fiber. The propagation of the higher order mode groups along the length of the fiber lags behind the propagation of the lower order mode groups as described herein. 
     At  310 , the optical signal is received onto a surface or a receiver having a central region and a peripheral region. The lower order mode groups are incident substantially on the central region. The higher order mode groups are incident on the peripheral region. 
     At  320 , a generator generates an output signal that is less responsive or unresponsive to light on the central region than to light on the peripheral region. As a result, the output signal is primarily responsive to the higher order mode groups incident on the peripheral region. 
     At  330 , the output electrical signal is processed by a circuit. Processing of the output electrical signal by means of a computer, a signal conditioning circuit, or a decoding device is within the inventive method described herein. 
       FIG. 4  shows an optoelectronic device that includes a photodiode  400  and a circuit  470 . 
     The photodiode  400  comprises a substrate  445 , an n-type layer  450  on the substrate  445 , an intrinsic layer  455  on the n-type layer  450  and a p-type layer  460  on the intrinsic layer  455 . A cathode contact  440  and an anode contact  435  are coupled to the n-type layer  450  and the p-type layer  460 , respectively. The photodiode  400  has a surface comprising a central region  425  and a peripheral region  430 . 
     A fiber cable having cladding  405  and a core  410  is configured such that the high order mode groups  420  and low order mode groups  415  are emitted to be incident on the peripheral region  430  and the central region  425 , respectively. 
     The photodiode  400  is rendered less responsive, or completely dead, to incident light on the central region as compared to the incident light on the peripheral region by implanting a proton implant  465  into the central region  425 . The proton implant  465  penetrates the central region  425  through the p-type layer  460 , the intrinsic layer  455 , and at least a portion of the n-type layer  450 . Therefore, the receiver  130  responds preferentially to the higher order mode groups  420 , and is less responsive to the lower order mode groups  415  because the lower order mode groups  415  are incident on the less responsive central region  425  while the higher order mode groups  420  are incident on the peripheral region  430 . Other types of implants may also be used. For example, in one alternative embodiment, an oxygen implant is used to render the photodiode  400  less responsive, or completely dead, to incident light on the central region  425  as compared to the incident light on the peripheral region  430 . 
     The photodiode  400  comprises a circuit  470  coupled to the cathode contact  440  and the anode contact  435 . The circuit  470  is configured to process the electrical signal generated by the photodiode  400 . 
       FIG. 5  shows a flow chart of a process of fabricating one embodiment of a photodiode of the present invention. At  500 , an n-type layer is formed on a semiconductor substrate. At  510 , an intrinsic layer is formed on the n-type layer. At  520 , a p-type layer is formed on the intrinsic layer. At  530 , a proton implant is performed to render the photodiode less responsive, or unresponsive, to light on the central region as compared to light on the peripheral region. 
     While  FIGS. 4 and 5  illustrate an n-type layer  455  formed directly on the substrate  445 , the intrinsic layer  455  directly on the n-type layer  450 , and the p-type layer  460  directly on the intrinsic layer  455 , it should be understood that other epitaxies may also be used. For example, the layer order could be reversed with the p-type layer  460  directly on the substrate  445 , the intrinsic layer  455  directly on the p-type layer  460  and the n-type layer  450  directly on the intrinsic layer  455 . Additionally, when forming or formed on is used herein, they do not require a direct connection. Other intervening layers may exist between a layer and a structure on which it is formed. For example, as used herein, a recitation of “forming an n-type layer on a semiconductor substrate” allows for the n-type layer to be formed directly on the substrate, or directly on intervening layers that are formed on the substrate. 
       FIG. 6 , illustrates an alternative embodiment of a photodiode of the present invention. 
     The photodiode  600  comprises a substrate  645 , an n-type layer  650  on the substrate  645 , an intrinsic layer  655  on the n-type layer  650  and a p-type layer  660  on the intrinsic layer  655 . A cathode contact  640  and an anode contact  635  are coupled to the n-type layer  650  and the p-type layer  660 , respectively. The photodiode  600  has a surface comprising a central region  625  and a peripheral region  630 . 
     A fiber cable having cladding  605  and a core  610  is configured such that the high order mode groups  620  and low order mode groups  615  are emitted to be incident on the peripheral region  630  and the central region  625 , respectively. 
     The photodiode  600  is rendered less responsive, or unresponsive, to incident light on the central region than the incident light on the peripheral region by forming a mask  665  over the central region  625 . The mask  665  substantially prevents the photodiode  600  from responding to light incident on the central region  625 . Therefore, the photodiode  600  responds preferentially to the higher order mode groups  620 , and is less responsive to the lower order mode groups  615  because the lower order mode groups are incident on the less responsive central region  625  while the higher order mode groups are incident on the peripheral region  630 . 
     In one embodiment, the mask  665  comprises a metal that reflects incident light. It will be apparent to one skilled in the art that other opaque materials can be used. It will be apparent that the mask  665  may be formed in alternative configurations to substantially reduce responsiveness to light incident on the central region  625 . 
       FIG. 7  shows a flow chart of a process of fabricating a photodiode with a mask at the central region. At  700 , an n-type layer is formed on a semiconductor substrate. At  710 , an intrinsic layer is formed on the n-type layer. At  720 , a p-type layer is formed on the intrinsic layer. At  730 , an opaque mask is formed on the central region to render the photodiode less responsive to incident light on the central region than to incident light on the peripheral region. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.