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  Communications system employing single-mode lasers and multimode optical fibersRelated Patent Categories: Optical Waveguides, With Disengagable Mechanical Connector, Optical Fiber To A Nonfiber Optical Device ConnectorThe Patent Description & Claims data below is from USPTO Patent Application 20060039656. Brief Patent Description - Full Patent Description - Patent Application Claims
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. Continue reading... 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