Abstract:
An optical transmission system having a single mode laser that generates an optical signal that is carried by a multimode optical fiber to a receiver is disclosed. The single mode laser has an emitting aperture from which the optical signal is routed to the input end of the multimode optical fiber. The receiver receives light from the output end of the optical fiber. The receiver includes an equalizer that corrects the received light for modal dispersion introduced by the multimode optical fiber. Light leaving the emitting aperture of the laser is introduced into the multimode optical fiber in a pattern that excites a subset of the plurality of optical transmission modes thereby reducing the modal dispersion introduced into the light signal and stabilizing the dispersion in time. The improved dispersion enables further correction of the dispersion through the utilization of equalization techniques.

Description:
RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/394,840 filed Mar. 21, 2003, which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The transmission of electronic signals by converting the signals to optical signals that are transmitted via optical fibers has significant advantages over using metallic conductors to transmit the electronic signals. Optical fibers have higher bandwidth, and hence, can carry more data per unit time. In addition, optical fibers have reduced noise and are less expensive than copper conductors.  
         [0003]     Signals are transmitted on optical fibers by first converting the electrical signals to an optical signal using a light converter such as a laser or an LED. The optical signal is then coupled into the optical fiber transmission line that may include a number of amplification stations. At the receiving end of the optical fiber, the optical signal is converted back into an electrical signal.  
         [0004]     Light traveling down the optical fiber is dispersed in time. The time dispersion is the result of the range in wavelengths generated by the light conversion device and/or the different optical paths through the optical fiber. The dispersion effects can be corrected to some degree by the use of equalization techniques provided the properties of the transmission system are stable over a sufficient period of time. In principle, if the distortion properties of the optical fiber are known, a filter can be provided at the receiver that corrects for the distortion. Some examples of such equalization are described in the text book, E. A. Lee et al., Digital Communication, Kulwer Academic Publishers (1988). Adaptive equalization utilizes equalization that is adjusted while signals are being transmitted in order to adapt to changing line characteristics. However, even with adaptive equalization, the properties of the communication link must remain constant over a time period that is long compared to the time needed to transmit one bit of information.  
         [0005]     To maximize the amount of information that can be transmitted on a fiber, both the total dispersion and the variations in the dispersion over time need to be minimized. This dispersion limits the distance over which the signals can be transmitted. Dispersion is introduced both by the light source and the transmission fibers. Single mode lasers generate a very small wavelength range, which results in the light traveling down the fiber with a very small spread in transmission times provided all of the light traverses the same path through the fiber. In principle, a transmission system that utilizes single-mode fibers that are driven by single mode lasers has the least dispersion; however, single mode fibers present additional problems that discourage such uses, especially for low-cost applications  
         [0006]     There are two types of optical fibers, single-mode optical fibers (SMFs) and multi-mode optical fibers (MMFs). Single-mode fibers provide an inherently less dispersive transmission path; however, such fibers present other problems. Compared with a multimode fiber, a single-mode fiber has higher bandwidth and can carry signals for longer distances due to the reduced signal dispersion. Also, since a single-mode fiber only has one mode there is no modal dispersion, i.e., all of the light traverses the same optical path. However, it is more expensive to manufacture fiber optic modules for single-mode fibers due, at least in part, to the tighter alignment requirements between the light source and the optical fiber.  
         [0007]     Multimode fibers are large enough in diameter to allow low cost light sources such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs) to be coupled into the fiber utilizing low cost assembly methods. More expensive single-mode lasers may also be coupled into a multimode fiber. However, when light rays from multimode or single-mode lasers are directly coupled into a MMF, the light rays travel through multiple path lengths (zigzag with varying numbers of bounces from the walls of the fiber) through the MMF, causing signal or modal dispersion. The specific path or mode taken by any given light ray depends on the position and angle of incidence on the end of the fiber at which the light ray enters. This modal dispersion has limited the transmission distance of a multimode fiber compared to a single-mode fiber. For this reason, MMFs have generally been employed to transmit light signals from sources such as VCSELs only for short distances, typically less than 300 meters.  
         [0008]     Modal dispersion may, in part, be compensated for with the use of equalization techniques on the fiber optic link at the receiver. Adaptive equalization utilizes equalization that is adjusted while signals are being transmitted in order to adapt to changing line characteristics. Adaptive equalization introduces components to an analog or digital circuit to compensate for signal attenuation and delay distortion in the transmission system as a function of frequency. In such systems, the transmission link is periodically examined to determine the distortions introduced in to the link by using a signal of known composition. Once the distortions are determined, a “filter” can be introduced into the receiver. The filter introduces the inverse distortions into the received signal, and hence, corrects for the known distortions of the communication link. For this type of strategy to succeed, the distortions introduced by the communication link must change slowly compared to the update rate of the filter.  
         [0009]     The distortion introduced by modal dispersion changes rapidly with time, and hence, MMFs have not been good candidates for equalization techniques. The precise modes that are excited when a single mode laser is coupled to a MMF depend on the coupling mechanism that is employed and the stability of the mode and output spot on the laser face. Small changes in the coupling conditions can result in very large changes in the specific modes that are excited, and these changes can take place at rates that approach the bit rate of the transmission link.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention includes an optical transmission system having a single mode laser that generates an optical signal that is carried by a multimode optical fiber to a receiver. The single mode laser has an emitting aperture from which the optical signal is routed to the input end of the multimode optical fiber. The receiver receives light from the output end of the optical fiber. The receiver includes an equalizer that corrects the received light for modal dispersion introduced by the multimode optical fiber. Light leaving the emitting aperture of the laser is introduced into the multimode optical fiber in a pattern that excites a subset of the plurality of optical transmission modes thereby reducing the modal dispersion introduced into the light signal and stabilizing the dispersion in time. The improved dispersion enables further correction of the dispersion through the utilization of equalization techniques. The light from the emitting aperture can be routed to the input end of the optical fiber by an optical element that provides a non-uniform pattern of illumination over the input end of the optical fiber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]      FIG. 1A  is a block diagram of an optical communication system according to one embodiment of the present invention.  
         [0012]      FIG. 1B  illustrates one embodiment of an input apparatus according to the present invention for coupling a signal from a laser to a MMF.  
         [0013]      FIG. 2A  illustrates another embodiment of an input apparatus according to the present invention for coupling a signal from a laser to a MMF.  
         [0014]      FIG. 2B  is a front view of an offset patch cord.  
         [0015]      FIG. 3  is a side view showing laser offset with respect to the centerline of a MMF. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]     The present invention is based on the observation that even with a MMF, a light signal can be launched into the MMF in a manner that limits the number of modes that are excited in the MMF. This restricted set of modes is more stable in time, and hence, the dispersion characteristics of the MMF over time are also substantially more stable than the dispersion characteristics obtained with a conventional launch of a single-mode light signal into a MMF. This increase in stability makes adaptive dispersion correction possible, and hence, provides a means for implementing a long distance MMF connection that makes use of the large installed base of MMF channels.  
         [0017]     The manner in which the present invention provides its advantages can be more easily understood with reference to  FIG. 1A , which is a block diagram of an optical communication system  10  according to one embodiment of the present invention. Communication system  10  converts an input signal to a light signal via a laser  11 . The output of laser  11  is launched into a MMF  13  with the aid of a conditioning optical element  12 . The light signal is converted back to an electrical signal by a receiver  14  that includes an equalization circuit that corrects for dispersion of the signal in MMF  13 . The equalization circuit utilizes gain parameters that are computed by comparing the signal output from the receiver in the absence of equalization with the input to the laser when a known training signal is sent over the communication system.  
         [0018]     A controller  15  can be utilized to compute the gain parameters that bring the output signal as close to the input signal as possible for the equalization algorithm employed by the receiver/equalizer  14 . These parameters can be determined periodically if the communication link changes over time. As noted above, for this strategy to operate successfully, the parameters must remain constant over a time period that is long compared to the time period required to send a single bit of information over the MMF. In particular, the parameters must remain constant between calibrations. The present invention provides a transmission environment that is sufficiently stable to allow such equalization.  
         [0019]     The typical laser is a laser having a single mode such as a single-mode VCSEL or a Fabry-Perot laser; although other single mode lasers can be utilized. The output of the laser can be modulated by an external light modulator that responds to the input signal or by modulating the electrical signal across the gain medium within the laser. The advantages of the present invention can be realized with any such lasers.  
         [0020]     The present invention utilizes a conditioning optical element  12  to limit the modes of the MMF that are excited. The conditioning optical element is a connection block between laser  11  and multimode fiber  13 . The conditioning optical element transmits a conditioned light source data signal into the MMF thereby launching the signal into a restricted set of modes of the MMF. The use of a single-mode laser in combination with the conditioning optical element improves the stability of the launch as compared to the use of multimode lasers, or even a single-mode laser without conditioning. MMFs are commonly found in present day installations, and the present invention takes advantage of the installed base.  
         [0021]     The present invention provides an improved bandwidth-distance product as a result of employment of a single mode laser and MMF combination with a conditioned launch, to enable effective use of adaptive equalization techniques, which is possible due to the more stable signal impulse response resulting from use of the invention. The usual combination of a laser with multiple modes when launched into a MMF with loose alignment tolerances will result in an impulse response that may vary rapidly with time. This resulting impulse response will be due to variations in the spatial modal profile of the laser itself, variations in coupling to the various modes of the MMF, and transmission variations through the MMF. In a severe case, the resulting impulse response may vary on a time scale similar to the bit rate, making it very difficult to implement adaptive equalization techniques as the equalization would need to adapt on a bit by bit time basis. The modal dispersion will result in severe receiver errors for some link lengths.  
         [0022]     In one exemplary embodiment of the present invention a diffractive optical element (DOE) is placed between a single-mode laser and the MMF. This DOE functions to transform the incoming light from the laser into a specific shaped (for example, a doughnut shape or some other uniform shape) light signal that launches into the MMF such that it excites a restricted set of modes in the MMF.  
         [0023]     In another exemplary embodiment an offset patch cord is utilized. The patch cord is placed between the single-mode laser and the MMF. The patch cord consists of a SMF and a MMF piece. The laser is aligned into the SMF input portion of the offset patch cord, the SMF is, in turn, coupled with a fixed offset from the center of the core of the MMF portion of the offset patch cord, and then the MMF output of the offset patch cord is connected to the desired MMF link.  
         [0024]     In yet another exemplary embodiment the laser is directly offset relative to the MMF via alignment, thereby serving the same function as an offset patch cord. That is, the end of the MMF is illuminated with a spot of light that is smaller than the diameter of the MMF and which is offset such that only a restricted set of modes is excited.  
         [0025]     In one embodiment, a single-mode VCSEL is used to convert an initial output light source data signal into a conditioned launch, using one of the above techniques for achieving the desired conditioning. The conditioned launch functions to condition the output light source data signal from the VCSEL into a light core (MMF) so that a more stable and restricted set of modes are excited in the fiber. The conditioned launch restricts the set of modes that are excited in the fiber and results in reduced modal dispersion. When used in conjunction with a single-mode VCSEL, the conditioned launch results in a very time-stable optical pulse. The result of this method of using the VCSEL and conditioned launch MMF combination results in a greatly improved stabilization of the impulse response of the fiber over time. The resulting optical output light source data signal from the MMF has reduced dispersion when compared to the optical output of a standard multimode VCSEL launched into a MMF with no attempt to condition the launch. This results in an output that is more stable with time and has less dispersion due to the launch into a restricted set of MMF modes. As a result, the impulse response of the MMF will change on a slow time scale relative to the bit rate. Thus the impulse response will be time invariant on the time scale of interest.  
         [0026]     As noted above, this slow time scale change enables adaptive equalization techniques to now be applied to the fiber optic link. Thus, the MMF output signal can be converted into an electronic output signal and adaptive equalization techniques can be applied to this signal to result in an output electronic signal that is compensated for modal dispersion. The ability to apply adaptive equalization techniques is a direct result of the reduction of time variation achieved by using a single mode laser, conditioned launch, and MMF combination.  
         [0027]     One benefit of the present invention is an improved bandwidth-distance product as a result of employment of the single-mode laser-MMF combination, conditioned launch, and adaptive equalization techniques. A higher bandwidth-distance product allows sending a faster signal over the same distance or increasing a distance for a set signal bandwidth. An improvement of about ten-fold or more, when compared to prior art, is expected with the use of the apparatus and method of the present invention.  
         [0028]     Yet another benefit of the present invention is that it takes advantage of the existing fiber optic installation base, which commonly has MMF installed. Hence, the capacity of this installed base is effectively increased without replacing the existing MMFs.  
         [0029]     Referring now to the drawing,  FIG. 1B , which illustrates one embodiment of an input apparatus according to the present invention for coupling a signal from a laser  400  to a MMF  404 , along with input and output light intensities. Single mode laser  400  launches optical input  401 , which is conditioned by diffractive optical element (DOE)  402  placed between the laser and MMF  404 . DOE  402  conditions optical input  401  by transforming it into a specific shape (for example, a doughnut shape or other uniform shape) that launches the signal into the center core or channel  405  of MMF  404  such that it would excite a restricted set of modes in the MMF. The controlled launch condition provided by the DOE, and the restricted set of modes in MMF  404 , results in a very time-stable optical output  406 . The resulting optical output  406 , as it exits core  405 , has little dispersion, as well as being time stable.  
         [0030]     The use of a single mode laser with DOE  402  thus provides a much more stable optical output as it exits from MMF  404  as compared to the use of a multi mode laser launched into a MMF. As stated above, this allows use of adaptive equalization techniques as shown in  FIG. 1A  to further improve the output signal.  
         [0031]     The input apparatus shown in  FIG. 2A  is similar to that shown in  FIG. 1B  with the exception that offset patch cord  505  replaces DOE  402 . The offset patch cord includes a single-mode fiber (SMF) element  503  and MMF element  502 . Offset patch cord  505  conditions the launch of optical input  401  from single mode laser  400  into channel  405  of MMF link  404 , enabling further improvement in the stability of the optical output as it exits the MMF link. As with the  FIG. 1B  embodiment, the controlled launch thereby excites a restricted set of optical modes of MMF fiber core  405 . The offset patch cord creates a launch that restricts the set of modes that are excited in MMF  404  and results in a very time-stable optical output  406 .  
         [0032]      FIG. 2B  is a front view of offset patch cord  505  showing SMF element  503  and MMF element  502 . Referring to  FIG. 2A , laser  400  is aligned with SMF  503  whereas MMF element  502  is coupled to and aligned with MMF  404  with connecting collar  504 .  
         [0033]     Refer now to  FIG. 3 , which is a side view showing laser  400  offset with respect to the centerline of MMF  404 . This use of directly offsetting laser  400  into MMF  404  does not use an optical element or an offset patch cord. The result, however, is to excite a restricted set of modes of MMF  404  with resulting optical output  406  having time stabilization and low dispersion, to enable the application of adaptive equalization techniques.  
         [0034]     Although the present invention has been described with reference to preferred embodiments, numerous modifications and variations can be made and still the result will come within the scope of the invention. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred.