Abstract:
An optical waveguide supporting a plurality of modes including an annulus which attenuates a desired mode to a lesser degree than any other mode in the plurality of modes. In one embodiment, the annulus is disposed in the core of the waveguide. In another embodiment, the annulus is concentric about the core. In another embodiment, the annulus has a predetermined width and radius. In yet another embodiment the optical waveguide includes an annulus that is disposed at a radial position corresponding to a region in the core in which the desired mode has substantially no energy. In yet another embodiment the desired mode includes the LP 02  mode. In another embodiment, the annulus includes a scattering material. In still another embodiment, the annulus includes a conductive dopant material. Another embodiment includes a sharp change of refractive index within the core. Still other embodiments include disposing a region of increased refractive index in the core of the waveguide to attenuate undesired modes. Another embodiment includes disposing a region of decreased refractive index in the core of the waveguide to attenuate undesired modes. Other embodiments include combinations of annuli. These combinations of annuli can include, for example, absorbing annuli, scattering annuli, annuli comprising conductive dopant material, regions of increased refractive index, and regions of decreased refractive index or any combination thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 09/464,629 filed Dec. 17, 1999, entitled “REDUCING MODE INTERFERENCE IN TRANSMISSION OF LP02 MODE IN OPTICAL FIBERS” and claims priority to provisional U.S. patent application Ser. No. 60/138,369 which was filed on Jun. 10, 1999. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to optical fibers for use in communication systems and, more specifically, to devices which reduce mode interference in the fiber. 
     BACKGROUND OF THE INVENTION 
     Propagating a high order mode in optical fiber transmission systems can have advantages over propagating a fundamental or basic mode. For example, propagating a high order mode can improve the overall performance of transport systems. One way performance is improved is due to the choice of fiber that will support the propagation of the high order mode. A significant advantage of this high order mode fiber is that it can be designed to have strong negative dispersion and high effective areas. Therefore, it can be used to compensate for chromatic dispersion. There is a drawback to these high order mode fibers, however. Propagating a high order mode can generate interferometric noise due to mode coupling in the fiber. To substantially reduce this noise, it would be advantageous to propagate a single high order mode. 
     Various methods have been suggested for transforming or coupling light energy in the fiber from one mode to another different mode. For example, a long period fiber grating may be used to transfer energy from one mode to another mode. Unfortunately, this method can also transform some of the light energy from one mode to other undesirable modes. Other methods can have the same undesired results. 
     Energy transfer or coupling from the desired high order modes to undesirable modes can also occur due to inhomogeniety of the high order mode fiber. These inhomogenieties can occur in the manufacturing process. Inhomogenieties can also appear due to imperfect splicing of the fiber, periodic bending (micro bending), and scattering mechanisms (i.e., Riley scattering), for example. 
     The result of imperfect mode transformation and mode coupling in the fiber is that undesirable modes will propagate in the fiber. These modes can interfere with the desired mode through a process called multipath interference (MPI). MPI causes significant reduction in signal quality by distorting its phase and amplitude. Therefore, in order to realize reasonable signal quality, the ratio between the energy transmitted through the undesired modes to the energy transmitted through the desired mode should be below 1:10000 or approximately 40 dB. This ratio should be maintained for any length of high order mode fiber being used in order to maintain reasonable signal quality. 
     It is therefore desirable to suppress the undesired modes in a high order mode fiber in order to improve the signal quality in the fiber. 
     SUMMARY OF THE INVENTION 
     The invention relates to an optical waveguide for attenuating undesired modes. In one embodiment, the waveguide includes either an absorbing or scattering annulus disposed substantially within a core of the waveguide. In another embodiment, the absorbing or scattering annulus is concentric about the core of the waveguide. In another embodiment, an optical fiber is used as a waveguide and the optical fiber supports the LP 02  mode. In still other embodiments, the width and the radius of the absorbing or scattering annulus are predetermined so as to attenuate undesired modes. In another embodiment, the annulus is disposed at a radial position corresponding to a region in the core in which the desired mode has substantially no energy. Other embodiments include a plurality of absorbing and/or scattering annuli. In another embodiment, the absorbing or scattering annulus includes titanium. In still another embodiment, the absorbing or scattering annulus includes boron. In still another embodiment, the absorbing or scattering annulus includes Erbium. In still other embodiments, the annulus includes any suitably absorbing material. In other embodiments, the annulus includes any suitably scattering material. In still another embodiment, the annulus includes any suitably conductive material dopant. 
     In another embodiment, the optical waveguide includes a multimode waveguide having a core and supporting a plurality of modes, the core having a core radius and a core refractive index. The optical waveguide further includes a sharp change of refractive index within the core. The refractive index discontinuity in another embodiment occurs at a radial position which is predetermined so as to attenuate a desired mode to a lesser degree than any other mode in the plurality of modes. The sharp change in index of refraction in the core, in one embodiment, comprises a discontinuity in the refractive index profile of the core. 
     In another embodiment, the optical waveguide includes a multimode waveguide having a core and supporting a plurality of modes, the core having a core radius and a core refractive index. The optical waveguide further includes a region of increased refractive index disposed within the core, and the region of increased refractive index step has a refractive index which is greater than the core refractive index and the radial position of the region of increased refractive index is predetermined so as to attenuate a desired mode to a lesser degree than any other mode in the plurality of modes. Another embodiment includes a plurality of regions of increased refractive index within the waveguide. 
     In another embodiment, the optical waveguide includes a multimode waveguide having a core and supporting a plurality of modes, the core having a core radius and a core refractive index. The waveguide further includes a region of decreased refractive index disposed within the core, and the region of decreased refractive index has a refractive index which is less than the core refractive index and the radial position of the region of decreased refractive index is predetermined so as to attenuate a desired mode to a lesser degree than any other mode in the plurality of modes. Another embodiment includes a plurality of regions of decreased refractive index within the waveguide. 
     In one embodiment, the desired mode is a high order mode. In another embodiment, the desired mode is an even high order mode. In yet another embodiment, the desired mode is the LP 02  mode. Other embodiments include combinations of annuli. These combinations of annuli can include, for example, absorbing annuli, scattering annuli, annuli comprising conductive dopant material, regions of increased refractive index, and regions of decreased refractive index or any combination thereof. 
     In one embodiment of the invention, a method for attenuating undesired modes includes providing a multimode waveguide having a core, transmitting an optical signal having a plurality of modes through the waveguide, and absorbing the desired mode to a lesser degree than any other mode in the plurality of modes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taking conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates the intensity distribution of the LP 01 , LP 11 , LP 02 , and LP 21  modes in a conventional step index fiber known to the prior art. 
     FIG. 2 shows an approximation of the variation in index of refraction with radial distance for a multiclad approximated as a step profile known to the prior art. 
     FIG. 3 a  shows an approximation of the variation in index of refraction with radial distance for one embodiment of an optical fiber including an absorbing annulus according to the present invention. 
     FIG. 3 b  shows an approximation of the variation in the absorption or scattering coefficient with radial distance for one embodiment of an optical fiber including an annulus according to the present invention. 
     FIG. 3 c  shows an approximation of the variation in index of refraction with radial distance for one embodiment of a multiclad fiber approximated as a sharp decrease in refractive index according to the present invention. 
     FIG. 3 d  shows an approximation of the variation in index of refraction with radial distance for one embodiment of a multiclad fiber approximated as a sharp increase in refractive index according to the present invention. 
     FIG. 3 e  shows an approximation of the variation in index of refraction with radial distance for one embodiment of a multiclad fiber approximated as a step profile with an additional index step according to the present invention. 
     FIG. 3 f  shows an approximation of the variation in index of refraction with radial distance for one embodiment of a multiclad fiber approximated as a step profile with an additional index depression according to the present invention. 
     FIG. 4 is a cross-sectional view of one embodiment of an optical fiber constructed in accordance with the principles of the invention. 
     FIG. 5 shows the actual variation in index of refraction with radial distance according to one embodiment of the present invention. 
     FIG. 6 shows the LP 02  mode distribution in an embodiment of a fiber according to the present invention. 
     FIG. 7 illustrates actual measurements of various materials deposited in an embodiment of a fiber manufactured according to the present invention. 
     FIG. 8 shows an actual measurement of the attenuation of the LP 02  mode compared with the measured attenuation of the LP 01  mode for one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, therein is illustrated the intensity distribution of the LP 01  mode ( 10 ), LP 11  mode ( 11 ), LP 02  mode ( 12 ), and LP 21  mode ( 13 ) in a conventional step index fiber known to the prior art. The intensity of each mode varies with the radius of the fiber. For example, the LP 01  mode ( 10 ) has its maximum intensity close to the center of the fiber core and its intensity trails off to a minimum value as it gets deeper into the cladding of the fiber. 
     One realization of the present invention is that each of the modes LP 01 , LP 11 , LP 02 , and LP 21 , vary in intensity at different radii in the fiber core. Another realization of the present invention is that the most favorable modes for the application of compensation for chromatic dispersion are the even modes (LP 01 , LP 02 , LP 03  . . . ) because their properties are independent of polarization when being transmitted in a circularly symmetric fiber. Unwanted mode interference in the transmission of the LP 02  mode in an optical fiber can be reduced by reducing mode coupling or by selectively attenuating undesired modes. Please note that the terms “mode” and “spatial mode” are interchangeable throughout the application. 
     Selective attenuation can be achieved by changing the radial transmission profile of the optical fiber. Referring back to FIG. 1, the radial distribution of the intensity of the LP 02  mode ( 12 ) is shown. The LP 02  mode ( 12 ) has maximal intensity close to the center of the fiber core and no intensity at the radial distance a 0    14 . All other guided modes ( 10 ), ( 11 ), and ( 13 ) have significant energy at this radial distance a 0    14 . 
     FIG. 2 illustrates a rectangular approximation of the variation in index of refraction with radial distance for a fiber known to the prior art. This fiber has a profile  17  that can support both high and low order modes in the fiber. 
     Referring now to FIG. 3 a,  therein is shown an embodiment of the index profile of the present invention including an absorbing annulus  20 . The absorbing annulus  20  in one embodiment is an absorbing ring introduced at the radial distance a 0    14 . In this embodiment, the index profile shows several index changes. The core  22  has an index n core . The first cladding  24  has an index n cld1 . The second cladding  26  steps up in index to n cld2  and the next cladding section  28  of the fiber has an index n cld3 . The annulus  20  can be used to significantly attenuate all modes except the LP 02  mode ( 12 ). As noted above with reference to FIG. 1, at radial distance a 0    14  the LP 02  mode ( 12 ) has minimum intensity. Therefore, it will not be significantly attenuated by the absorbing annulus. Precise positioning of the absorbing annulus can be achieved through the use of conventional and specialized manufacturing methods known to those skilled in the art such as MCVD and OVD. 
     Referring now to FIG. 3 b,  therein is shown an embodiment of the absorption or scattering coefficient in the core area of the present invention including an annulus. The horizontal axis represents radial distance and the left portion of the diagram represents the center of the fiber and the right portion represents the first cladding. The vertical axis represents the absorption or scattering coefficient in arbitrary units. The absorption or scattering coefficient shows a large peak at the radial distance a 0    14 , indicative of the significant attenuation of the annulus. 
     It should be understood that a similar result may be obtained in another embodiment by inserting a scattering annulus in place of the above described absorbing annulus. The scattering annulus can be implemented in one embodiment by utilizing a scattering or conductive material dopant placed substantially at the radial distance a 0    14 , or at whatever point unwanted energy exists. The scattering annulus in one embodiment has a sharp discontinuity in refractive index. 
     Referring now to FIG. 3 c,  therein is shown an embodiment of the invention which includes a sharp change in index of refraction at a 0    14 . This discontinuity  16  is shown as a sharp decrease in refractive index in the core. The refractive index n core1  is greater than the refractive index n core2  in the embodiment shown in the figure. Discontinuity  16 , in one embodiment, has the effect of scattering energy in the core of the fiber. Since mode LP 02  has substantially no energy at a 0    14 , it is minimally affected by discontinuity  16 . 
     Referring now to FIG. 3 d,  therein is shown another embodiment of the invention which includes a sharp change in index of refraction at a 0    14 . This discontinuity  19  is shown as a sharp increase in refractive index in the core. The refractive index n core1  is less than the refractive index n core3  in the embodiment shown in the figure. Discontinuity  19 , in this embodiment, has the effect of scattering energy in the core of the fiber. Since mode LP 02  has substantially no energy at a 0    14 , it is minimally affected by discontinuity  19 . 
     Another implementation of this concept is illustrated in FIG. 3 e,  which shows a discontinuity in the refractive index in core  23  at the desired area. Step  25  is placed at the desired radial distance, and functions to scatter any energy located in the area of the step  25 , which is herein shown to coincide with location a 0    14 . Width  27  of step  25  can be varied so as to optimize the effect of step  25 . In one embodiment, width  27  is made as narrow as possible. As indicated in FIG. 3 e,  step  25  is shown with refractive index n step , whereas core  23  is shown with refractive index n core . In this embodiment, step  25  has an index of refraction which is greater than the refractive index of the core  23 . Step  25 , in one embodiment can be manufactured using an appropriate dopant. The effect of step  25 , in one embodiment, is to scatter any energy found in the region of width  27  at radius a 0    14 . As discussed earlier, mode LP 02  has practically no energy at radius a 0    14 . Other modes have significant energy at a 0    14  and those modes have energy which will be scattered by step  25 . 
     Although dip  21  at the core center is not required by the present invention, it does have purpose which will become clear to one skill in the art. Dip  25 , in one embodiment, functions to better discriminate the modes from each other in the waveguide. In another embodiment, dip  21  serves to force mode LP 01 , further away from the center of the core towards the region of a 0    14 . This has the effect of improving the attenuation of mode LP 01 , since more of its energy is now in the attenuating region of a 0    14 . Therefore, in this embodiment, dip  21  is used to increase the effect of the absorption or scattering of mode LP 01  by increasing its intensity at a 0    14 . 
     Referring now to FIG. 3 f,  therein is illustrated a discontinuity in the refractive index n core  of core  23  consisting of a dip or depression  29  in the region of core  23 . The refractive index n dep  of the dip or depression  29  is less than the core refractive index n core . The depression  29  of refractive index n dep  is placed at the desired radial distance a 0    14 , and functions to direct any energy in this region to the adjacent regions. As described above, the region is chosen as consisting primarily of unwanted energy, and thus the unwanted energy is directed into the desired regions. Width  27  of depression  29  as discussed above, can be varied so as to optimize the effect of depression  29 . In one embodiment, width  27  is made as narrow as possible. This will have the effect of attenuating any modes that have energy in unwanted areas. 
     Referring to FIG. 4, therein is shown an embodiment of a waveguide according to the present invention. FIG. 4 shows a cross-sectional view of one embodiment of an optical fiber of the invention. Absorbing or scattering annulus  20  is positioned at a radial distance a 0    14  from the center of the fiber. This corresponds to the location in which the LP 02  mode ( 12 ) has substantially zero intensity. Various index regions of this embodiment are shown including the region of core  22 , the region of first cladding  24 , the region of second cladding  26 , and the region of the third cladding  28 . This embodiment is only an example of one possible cross-section of a fiber according to the invention. It should be appreciated that other fiber profiles could be used within the scope of the invention. Furthermore, absorbing or scattering annulus  20  may vary in both width and radius, and may be located anywhere within the region defined by the core index  22 . In addition, the absorbing or scattering annulus  20  may also surround the region of core index  22  ( 24 ,  26 ). In another embodiment, absorbing or scattering annulus  20  is designed to be thin so as not to substantially diverge from location a 0    14  and, hence, unintentionally attenuate the desired mode. In yet another embodiment, multiple absorbing or scattering annuli, or a combination of both may be used in the core and/or the cladding of the fiber. The absorbing annulus  20 , in another embodiment, may be of any absorbing material, and may comprise titanium, boron, Erbium or any other absorbing material. Scattering materials can be used as long as special care is made to reduce scattering from the desired mode to undesired modes. Below the desired threshold, this can be accomplished by reducing the scattering concentration or by using scattering at large angles. 
     FIG. 5 is a measurement of the actual variation in index of refraction with radial distance according to one embodiment of the present invention including an absorbing annulus  20 . The actual index profile  30  is shown. The profile  30 , is actually the index difference from the nominal index  34  of the cladding. The center vertical axis (CVA) in FIG. 5 corresponds to the center of the fiber. The dip  32  shown in the region of the core index  22  is a by-product of the manufacturing process. The region of the annulus  20  exhibits a negligible dip in the index of refraction. The region of the first cladding index  24 , the region of the second cladding index  26 , and the region of the next cladding index  28  are also shown. Similar results can be achieved for another embodiment which includes a scattering annulus, or a conductive material dopant annulus. 
     FIG. 6 illustrates the LP 02  mode ( 12 ) distribution in an embodiment of the fiber presented in FIG.  5 . In FIG. 6 the horizontal axis is the radial distance in microns and the vertical axis is the intensity in arbitrary units. In this embodiment, the LP 02  mode ( 12 ) has maximum intensity  40  at the center of the fiber and a null  42  in energy at a distance of approximately 2.25 microns from the center of the fiber. 
     FIG. 7 illustrates the various materials deposited in the fiber as they were measured in the pre-form used to make an embodiment of the fiber utilizing an absorbing annulus. The horizontal axis represents radial distance where the left side of the diagram represents the center of the fiber and the right side is the cladding. The vertical axis represents material concentration in arbitrary units. The core  52  is doped with a germanium concentration D germ  which increases the refractive index of the core of the fiber. The first cladding region  55  is doped with fluoride materials D fluor  used to reduce refractive index. The absorbing annulus  60  comprises titanium D titan  which was deposited in a ring in the core at radius a 0  in order to attenuate undesired modes, by increasing the local absorption coefficient. It can be seen in this diagram that, at the ring location a 0  where the titanium concentration D titan  was high, the germanium concentration D germ  was reduced in order to maintain a near constant refractive index in the core region. 
     The absorbing material that comprises the absorbing annulus, in one embodiment, can be introduced into the optical fiber pre-form using conventional known technology. Preferably, the absorbing material has a low scattering coefficient. For example, titanium or boron can be used. Low scattering by the absorber material reduces energy coupling from the LP 02  mode into other modes. The width of the absorbing ring in one embodiment is preferably as narrow as possible in order to reduce losses to the LP 02  mode. 
     FIG. 8 is the measured attenuation of the LP 02  mode for one embodiment of the present invention utilizing an absorbing annulus. In FIG. 8, the horizontal axis represents the wavelength in nanometers, and the vertical axis represents the loss in dB per kilometer. The loss of the LP 01  mode with respect to wavelength is shown by line  70 . The loss of the LP 02  mode with respect to wavelength is shown by line  72 . There is significant increase of loss for these modes around 1400 nanometers ( 75 ) due to OH absorption in the fiber. There is another increase in loss for the LP 02  mode around 1600 nanometers due to cut off of this mode ( 77 ). However, in the typical operating wavelengths, between 1525 and 1565, the loss in the fiber of the LP02 mode is lower than in any other mode. 
     From these results, it is observed that the relative intensity of the LP 02  mode will remain constant or even improve as it propagates through the fiber. Therefore, the fiber is suitable for dispersion compensation applications. For example, the dispersion of an embodiment of this fiber was measured to be in the order of 250 psa/nm.km at 1550 nm. 
     The absorption coefficient for each mode with respect to the absorbing annulus can be calculated by overlapping the intensity distribution of the mode with the absorbing annulus. As an example of one embodiment, an absorbing annulus comprising boron having a width of 0.5 microns and located at a radial distance of 2.3 microns from the fiber center and having a local absorption of 30 dB/km increases the loss of the LP 02  mode by 0.11 dB/km. The undesired modes have a much higher loss: 
     LP 01  5.26 dB/km more loss 
     LP 11  5 dB/km more loss 
     LP 21  5 dB/km more loss 
     The calculation of the above values assumed a wavelength of 1550 nm. Changing the wavelength by ±50 nm caused low changes in losses of less than 0.001 dB/km. 
     It should be appreciated to those skilled in the art that the invention could be used to isolate any desired high even mode including the LP 02 , LP 03  and LP 04  modes. It should also be appreciated that multiple absorbing annuli may be used without departing from the spirit or scope of the invention. 
     This fiber with the absorbing annulus can be applied in dispersion compensating fibers, transmission fibers or Erbium doped fiber amplifiers (EDFAs) using the LP 02  mode for example. The absorbing annulus can be applied to an optical fiber regardless of its refractive index profile. Similar results may be achieved utilizing a scattering annulus consisting of scattering material or a conductive material dopant. Placing an additional step index in the desired area, or an additional index depression or well, will also achieve similar results. A combination of an annulus and a step or depression may also be utilized. 
     Having described and shown the preferred embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts may be used and that many variations are possible which will still be within the scope and spirit of the claimed invention. It is felt, therefore, that these embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the following claims.