Abstract:
A dispersion compensation device uses at least two chromatic dispersion compensation fibers to compensate for chromatic dispersion present in an optical communication system. Two dispersion orders can be corrected using appropriate lengths of two serially coupled compensation fibers having different dispersion characteristics. The device can compensate for N additional orders of dispersion by using N additional compensation fibers with unique dispersion characteristics. The device can be coupled directly to a transmission fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to provisional U.S. patent application No. 60/079,423 which was filed Mar. 26, 1998, provisional U.S. patent application No. 60/089,350 which was filed Jun. 16, 1998 and provisional U.S. patent application No. 60/091,026 which was filed Jun. 29, 1998 and incorporates by reference U.S. patent applications “Transverse Spatial Mode Transformer for Optical Communication” (attorney docket no. LCM-001) and “Optical Communication System with Chromatic Dispersion Compensation” (attorney docket no. LCM-002) filed concurrently herewith. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to fiber optic telecommunication systems and more specifically to chromatic dispersion compensation in such systems.  
         BACKGROUND OF THE INVENTION  
         [0003]    The tendency of a pulse of light propagating through an optical fiber to broaden is a result of the fact that different wavelengths of light pass through the fiber at different speeds. This speed differential which causes the pulse to broaden is termed chromatic dispersion. Chromatic dispersion presents a problem in modern optical communication systems because the tendency of light pulses to broaden as they propagate down the fiber causes the closely spaced light pulses to overlap in time. This overlap can have an undesirable effect since it restricts how closely spaced the pulses can be. This in turn limits the data bandwidth of the optical fiber.  
           [0004]    There are many characteristics of dispersion. First order dispersion is the rate of change of index of refraction with respect to wavelength in the fiber. First order dispersion is also referred to as group velocity. Second order dispersion is the rate of change of the first order dispersion with respect to wavelength. Second order dispersion produces the pulse broadening. Third order dispersion is the rate of change of broadening with respect to a change in wavelength. This is often referred to as the dispersion slope.  
           [0005]    Several solutions have been proposed to mitigate the effects of dispersion in transmission fibers. One technique involves the use of a compensating optical fiber having an appropriate length and which has a dispersion that is opposite to the dispersion characteristic of the transmission fiber. The result is dispersion in the transmission fiber is substantially matched and canceled by the total dispersion in the compensating fiber. While this technique offers a solution to the dispersion problem, it may be impractical in actual use because of the attenuation due to the required length of the compensating fiber. In such a case, the total transmission length of the fiber is significantly increased thereby increasing the signal attenuation in the fiber. Furthermore, it may be difficult to find a fiber of the desired length with the required dispersion properties.  
           [0006]    It is also difficult to design a fiber having a changing index of refraction across the diameter of the fiber (the fiber index profile) that will compensate simultaneously for the second and third dispersion orders. It is even more difficult to control the material properties of such fibers even in the most accurate fabrication process necessary to produce such fibers. In addition, the process of fabricating the single compensating chromatic dispersion fiber is expensive and generally not practical.  
           [0007]    When a pulse of light is transmitted through an optical fiber, the energy follows a number of paths which cross the fiber axis at different angles. A group of paths which cross the axis at the same angle is known as a mode. Sometimes it is necessary to limit or control the number of modes used in a transmission system. The fundamental mode LP 01  in which light passes substantially along the fiber axis is often used in high bandwidth transmission systems using optical fibers commonly referred to as single mode fibers.  
           [0008]    The dispersion properties of high order modes have been investigated at length. There is a dependence of high order mode dispersion on wavelength and on the properties of the fiber. By properly designing the fiber index profile it is possible to make the dispersion slope be positive, negative or zero. It is also possible to make the magnitude of the dispersion be negative, zero or slightly positive. Using these two properties one can either control or compensate for the dispersion in any transmission fiber.  
           [0009]    Systems have been developed to take advantage of higher order modes to compensate for dispersion in a typical optical communication system. In such systems it has been necessary to first convert the lower order fundamental mode of the light to a higher order spatial mode. This is accomplished using longitudinal mode conversion.  
           [0010]    Conventional methods for longitudinal mode conversion are based on introducing a periodic perturbation along the fiber axis. The length of each period and the number of periods in these longitudinal converters must be determined accurately according to the wavelength, the strength of the perturbation, and the modes involved. By constructing a longitudinal mode converter it is possible to achieve good efficiency in transferring the energy from one mode to the other in a limited spectral bandwidth. This spectral property has been used in Dense Wavelength Division Multiplexing (DWDM) applications in telecommunications for other applications. Unfortunately, this technique is accompanied by significant energy attenuation and it cannot be used over broad spectral bandwidths.  
           [0011]    Another deficiency associated with longitudinal mode converters is related to the fact that after the conversion, only a single mode should be present in the fiber. It can be difficult to discriminate between desired modes and undesired modes having almost the same group velocities because unwanted modes can appear at the output of the converter. As the modes propagate, modal dispersion occurs and the pulse broadens. Generally, longitudinal mode converters introduce significant energy attenuation and noise. Therefore, a trade-off must be made between having broad-spectrum capability and the demand for converting the original mode to a pure, single, high-order mode.  
           [0012]    One such longitudinal mode converter is discussed in U.S. Pat. No. 5,802,234. Here, a single mode transmission fiber carries the LP 01  to a longitudinal mode converter. Before conversion in this system, however, it is necessary to couple the single mode transmission fiber to a multimode fiber while maintaining the signal in the basic LP 01  mode. This coupling is typically difficult to achieve without signal degradation and any misalignment or manufacturing inaccuracies can result in the presence of higher order modes. It is desirable that only the LP 01  mode propagate initially in the multimode fiber in order to avoid significant noise that degrades the system performance and typically such coupling results in the propagation of additional modes.  
           [0013]    The present invention overcomes the disadvantages of longitudinal mode converters and previous attempts to control dispersion in a fiber optic system.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention relates to an apparatus and method for chromatic dispersion compensation of optical systems. The apparatus and method make use of multiple chromatic dispersion compensation optical fibers. The number of orders of dispersion that can be corrected increases with the number of compensation fibers in the apparatus. Specifically, N orders of dispersion can be corrected by serially coupling N chromatic dispersion compensation optical fibers.  
           [0015]    In one embodiment, the present invention features a chromatic dispersion compensation module which includes a first and second dispersion compensation fiber. The optical signal is dispersion compensated by each compensation fiber. In one embodiment, one compensation fiber compensates for first order chromatic dispersion and the other compensation fiber compensates for second order chromatic dispersion. In another embodiment, at least one of the compensation fibers is optical coupled to a transmission fiber.  
           [0016]    In another aspect, the invention features a method of compensating for chromatic dispersion in an optical system which includes the steps of receiving an optical signal from a transmission fiber, injecting the optical signal into a first compensation fiber and dispersion compensating the optical signal. The method includes the additional steps of injecting the optical signal exiting the first compensation fiber into a second compensation fiber, additionally compensating the optical signal, and injecting the optical signal into a second transmission fiber. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    These and other advantages of the invention may be more clearly understood with reference to the specification and the drawings, in which:  
         [0018]    [0018]FIG. 1 is a block diagram of an embodiment of a typical fiber optic transmission system known to the prior art;  
         [0019]    [0019]FIG. 2 is a block diagram of an embodiment of the fiber optic transmission system of the present invention including a chromatic dispersion compensation fiber module;  
         [0020]    [0020]FIG. 3 is a block diagram of an embodiment of the chromatic dispersion compensation fiber module shown in FIG. 2 showing transverse mode transformers and a chromatic dispersion compensation fiber;  
         [0021]    [0021]FIG. 4 is a block diagram of another embodiment of the chromatic dispersion compensation fiber module of the present invention showing transverse mode transformers and two chromatic dispersion compensation fibers;  
         [0022]    [0022]FIG. 5 is a highly schematic diagram of an embodiment of a transverse mode transformer shown in FIG. 3;  
         [0023]    [0023]FIG. 6 a  is a block diagram of an alternative embodiment of a fiber optic transmission system of the current invention with the leading transmission fiber replaced by a transmission source;  
         [0024]    [0024]FIG. 6 b  is a block diagram of an alternative embodiment of a fiber optic transmission system of the current invention with the receiving transmission fiber replaced by a detector;  
         [0025]    [0025]FIG. 7 a  is a graph of the intensity as a function of position along the diameter of a fiber in an ideal case;  
         [0026]    [0026]FIG. 7 b  is a graph of the intensity as a function of position along the diameter of the fiber after transformation to the LP 02  mode;  
         [0027]    [0027]FIG. 8 is a graph of the relative energy in the higher order mode relative to the LP 01  mode for an element optimized for operation at a wavelength of 1550 nm in an ideal case;  
         [0028]    [0028]FIG. 9 is a block diagram of an alternative embodiment of a transverse mode transformer using two phase elements;  
         [0029]    [0029]FIG. 10 a  is a highly schematic diagram of an alternative embodiment of the present invention showing two chromatic dispersion compensation fibers used for multiple order dispersion compensation;  
         [0030]    [0030]FIG. 10 b  is a highly schematic diagram of an alternative embodiment of the present invention showing two chromatic dispersion compensation fibers sandwiching a single mode transmission fiber used for multiple order dispersion compensation;  
         [0031]    [0031]FIGS. 11 a - 11   e are graphs of different solution spaces showing relative design characteristics resulting from the use of first and second order dispersion;  
         [0032]    [0032]FIGS. 12 a - 12   c  are illustrations of alternative embodiments of the transverse mode transformer shown embedded in a fiber optic transmission system;  
         [0033]    [0033]FIGS. 13 a - 13   c  are graphs of the amplitude versus position plot of the pulse across the diameter of the fiber before, during and after mode transformation;  
         [0034]    [0034]FIG. 14 is an illustration of an alternative embodiment of the current invention using a polarization beam splitter and a polarization combiner;  
         [0035]    [0035]FIG. 15 is a schematic diagram of a single bulk component that can be used to replace the discrete bulk optical components in the embodiment shown in FIG. 14;  
         [0036]    [0036]FIG. 16 shows a representation of the polarization of propagating modes through the element described in FIG. 15;  
         [0037]    [0037]FIG. 17 shows a representation of the polarization of propagating modes using a birefringent element;  
         [0038]    [0038]FIG. 18 is a block diagram of an alternative embodiment of the current invention designed to eliminate the sensitivity of the system to polarization mode dispersion by using a circulator and a Faraday mirror; and  
         [0039]    [0039]FIG. 19 is a block diagram of an alternative embodiment of the current invention designed to eliminate the sensitivity of the system to polarization mode dispersion without using a circulator.  
         [0040]    [0040]FIGS. 20 a - 20   c  are diagrams of alternative embodiments of a transverse mode transformer using internal reflection. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]    A typical optical fiber transmission system known in the prior art is shown in FIG. 1. Such a system includes a signal transmitter  2  in optical communication with a single mode fiber (SMF)  3  which is in turn in optical communication with a signal receiver  4 . (Other components common to optical fiber systems, such as amplifiers, circulators, isolators, etc. are not shown.) A signal is transmitted from the transmitter  2  into the fiber  3  where it propagates some distance. Depending on the length and other properties of the fiber, significant signal attenuation and dispersion can occur in the fiber. The receiver  4  acquires the attenuated signal as it exits the fiber  3 .  
         [0042]    A basic configuration of the system of the present invention is presented in FIG. 2. A transmitter  2  transmits an optical signal into a communication fiber  3 . The communication fiber  3  introduces dispersion that requires compensation. The chromatic dispersion compensation module  10  compensates for signal dispersion introduced by the communication fiber  3  before propagating the signal into a receiver  4 .  
         [0043]    An embodiment of the chromatic dispersion module  10  is shown in FIG. 3. A signal propagating in a single mode fiber (SMF)  3  enters a mode transformer  28  which converts the basic lower order spatial mode, generally LP 01 , to a higher order spatial mode, generally LP 02 , that propagates in a special chromatic dispersion compensating fiber  30 . The chromatic dispersion compensation fiber (DCF)  30  is designed to compensate for the first order dispersion of the signal. A second chromatic dispersion compensation fiber  31  with different compensation properties may be coupled to the first chromatic dispersion compensation fiber  30  in order to compensate for dispersion slope as shown in FIG. 4. If required, more than two chromatic dispersion compensation fibers may be used to compensate even higher order dispersion or alternatively for mode filtering applications. Once compensation is complete, the signal is then converted back to the lower order mode by a second mode transformer  28 ′ and emerges from the chromatic dispersion compensation module  10  in the single mode fiber  3 ′.  
         [0044]    The mode transformer  28  of the present invention is a bidirectional transverse mode transformer. It can be used to convert a lower order spatial mode to a higher order spatial mode. Conversely, the same transverse mode transformer  28  can be used to convert a higher order spatial mode to a lower order spatial mode. Unlike prior mode transformers which used the longitudinal axis of the fiber to accomplish longitudinal mode conversion, the present transverse mode transformer uses transverse properties of the wavefront of the light to mode convert by selectively altering the phase of at least one portion of the wavefront. One embodiment of a transverse mode transformer is shown in FIG. 5. A transverse phase element  58  arranged perpendicular to the longitudinal axis of the fiber is used to accomplish mode transformation. A pulse of light propagates in a single mode fiber  50  with a small diameter core  54 . The pulse broadens into an expanded region  56  as it emerges from the fiber. As the pulse passes through the transverse phase element  58  the phase distribution of the pulse is changed. The phase element  58  can consist of a spatially selective phase element which alters the phase of points on the wavefront as a function of their transverse position. A focusing lens  62  focuses the pulse back into the special chromatic dispersion compensation fiber  64 , shown as having a broader core  66  simply for explanatory purposes. In many conventional systems the lens  62  is a compound lens. In one embodiment, gradient index (GRIN) lenses are used. The phase element  58  can be any spatially selective phase element, including but not limited to, lenses, mirrors, gratings, electro-optic devices, beamsplitters, reflective elements, graded indexed materials and photolithographic elements.  
         [0045]    Phase transformation can be achieved using the properties of spherical aberration inherent in optical lenses. After a wavefront passes through a lens, it will experience spherical aberration. The resulting distorted wavefront can be used with or without a phase element  58  in the transverse mode transformer  28  of the present invention to transform the spatial mode of the original wavefront to a higher order spatial mode.  
         [0046]    [0046]FIG. 6 a  depicts a system in which a transmission source  24  replaces the optical fiber  3  shown in the embodiment in FIG. 4. Here the system does not require an input transmission fiber and retains all the functionality and advantages of the present invention. The transmission source  24  injects an optical signal directly into the chromatic dispersion compensation module  10  where it is pre-compensated before being received by the transmission fiber  3 ′. Precompensation can be desirable when the transmission fiber  3 ′ has a known dispersion that requires compensation.  
         [0047]    [0047]FIG. 6 b  describes a system in which a detector  36  replaces the transmission fiber  3 ′ shown in the embodiment in FIG. 4. Here the system does not require an exit transmission fiber  3 ′ and the functionality of the system is not affected. In this case the optical signal propagates in the optical fiber  3  before being compensated by the chromatic dispersion compensation module  10 . Once the signal is down converted by mode transformer  28 ′, it is detected directly by detector  36 . This method can conserve energy since there will not be fiber coupling losses exhibited before the detector.  
         [0048]    The physical mechanism of the transverse mode transformation presented in this invention is explained with reference to FIGS. 13 a  to  13   c . (FIGS. 13 a  to  13   c  share the same horizontal scale.) FIG. 13 a  illustrates the gaussian-like amplitude distribution of mode LP 01  in a single mode fiber, wherein the horizontal axis represents the transverse position across the diameter of the fiber in arbitrary units and the vertical axis represents the amplitude in arbitrary units. In one embodiment, the transverse phase element  58  (FIG. 5) introduces a step function to the wavefront  20  of the pulse such that the center region  20   a  of the wavefront  20  is retarded with respect to the outer region  20   b  of the wavefront  20 . Therefore, the inner region  20   a  and the outer region  20   b  of the wavefront  20  will differ in phase by 180°. After propagation and transformation, the resulting distribution  22  shown in FIG. 13 c  enters the chromatic dispersion compensation fiber  64  (see FIG. 5). More than ninety percent of the transverse intensity distribution in the LP 01  mode (see FIG. 7 a ) is present in the LP 02  mode (see FIG. 7 b ) after transformation. The remaining energy is distributed among higher order modes which are not supported by the special chromatic dispersion compensation fiber  66 . Therefore, the fiber will contain substantially a single high order mode (LP 02 ). The same process, but in the reverse order, occurs in the second mode transformer  28 ′ at the opposite end of the compensation fiber  66 . This technique can also be applied to convert between other spatial modes.  
         [0049]    One of the advantages of this transverse transformation mechanism is its high efficiency over a broad spectrum. FIG. 8 shows the residual energy in the LP 01  mode for an element optimized for operation at 1550 nm. The horizontal axis represents the wavelength of the pulse in nanometers, and the vertical axis represents the ratio between the energy remaining in the low order mode to the total energy of the pulse. Less than one half of a percent of the pulse energy is left in the lowest order mode over greater than 100 nm of spectral range.  
         [0050]    In order to further improve the transformation efficiency it is possible to use multiple phase elements  74  and  74 ′ as shown in FIG. 9. The pulse emerging from fiber  54  is collimated by lens  72 , then it passes through the two phase elements  74  and  74 ′ and is finally focused by lens  72 ′ into a special chromatic dispersion compensation fiber  64 . This technique reduces longitudinal sensitivity in the placement of the phase elements. The design of phase elements  74  and  74 ′ can be based on a coordinate transformation technique for converting between spatial modes. The first phase element  74  is designed to have local phase changes across the pulse. Each local phase change redirects (i.e., steers) a small section of the wavefront  20  to a predetermined coordinate on the second phase element  74 ′. As a result, a predetermined intensity pattern is generated at the second phase element  74 ′. The second phase element also induces local phase changes across the wavefront so that the resulting wavefront  20  with predetermined intensity and phase distributions at the second element  74 ′ yields the desired spatial mode.  
         [0051]    Another embodiment of the chromatic dispersion compensation module  10  of the present invention is shown in FIG. 10 a . This embodiment may be used with transverse mode transformers  28 , but is not limited to their use. Any means that propagates a pulse with a higher order mode into an optical coupler  6  can use the invention. After the higher order pulse passes through optical coupler  6 , the pulse then enters the first chromatic dispersion compensation fiber (DCF 1 )  8  which is designed to compensate for the dispersion of the communication fiber  3 . DCF 1    8  is spliced to a second dispersion compensation fiber (DCF 2 )  10  through a splice  12 . DCF 2    10  is designed to have minimal second order dispersion at the point where the dispersion slope is maximum. By properly choosing the design parameters, a minimal length of DCF  8  and is required to compensate for dispersion. DCF 1    8  and DCF 2    10  can be designed to operate with the basic LP 01  mode as long as they have different dispersion characteristics. The order in which DCF 1    8  and DCF 2    10  are arranged can be changed. Generally, more chromatic dispersion compensation fibers are required as the number of dispersion orders to be compensated increases. The chromatic dispersion compensated pulse passes into the outgoing optical transmission fiber  3 ′ at splice  14 . FIG. 10 b  illustrates another embodiment of the invention. A single mode fiber is sandwiched between two dispersion compensation fibers. Any number of combinations can be realized without detracting from the essence of the invention.  
         [0052]    Graphs of possible solutions using the chromatic dispersion compensation fibers of the present invention are shown in FIGS. 11 a - 11   e . The horizontal axes represent the second order dispersion, and the vertical axes represent the second order dispersion slope (i.e., third order dispersion). The dispersion compensation introduced by the chromatic dispersion compensation fibers is presented as arrow  24 . FIG. 11 a  represents an ideal system, where the desired dispersion solution is presented as the point  20 . By choosing the proper length of chromatic dispersion compensation fiber, the desired results are achieved. Unfortunately, in conventional communication systems it is difficult to change the relationship between the dispersion orders. Moreover, it is difficult to even predict this relationship before fabrication of the compensation fiber is completed. In addition, this relationship varies strongly according to fabrication processes. Therefore, if the desired amount of dispersion compensation presented at point  20  is displaced as illustrated in FIG. 11 b , it is impossible to achieve the desired compensation. It is possible, however, to increase the length of the DCF in order to add length  26  to the arrow  24 , so that the actual magnitude of dispersion is increased and the resulting dispersion  27  will approximate the desired dispersion  20 .  
         [0053]    By combining two or more different fibers it is possible to achieve a variety of dispersion properties. The dispersion properties of DCF 1    8  and DCF 2    10  in FIG. 10 are represented as  32  and  34  in FIG. 11 c . The area  36  represents the solution space of dispersion compensation which can be achieved by proper combination of the two fibers  8  and  10 .  
         [0054]    [0054]FIG. 11 d  represents an example of such a combination. Using a combination of two or more DCFs, one can compensate for higher orders of dispersion. In order to achieve better coverage of the dispersion possibilities it is desirable to increase the angle between the arrows  32  and  34  in FIG. 11 c . It is difficult to achieve this result by using conventional single mode DCFs, however, high order mode-dispersion compensation fibers (HOM-DCF) can achieve more than 90 degrees difference between two different DCFs as presented in FIG. 11 e . This system is insensitive to the exact properties of the DCFs, because changing the length of the fibers can compensate for any deviation in the result.  
         [0055]    [0055]FIG. 12 a  depicts an alternative embodiment of the transverse mode transformer of the present invention and shows a connection, between two fibers, designed to modify the wavefront. Both fibers include a core  10  and cladding  12 . The face of the transmission fiber  14  can be perpendicular to the face of the dispersion compensation fiber  6  or at a small angle to the DCF  6  in order to eliminate reflection noise. The end face of at least one of the fibers has a predetermined binary pattern  16 . The pattern  16  can be etched onto the fiber or be in optical communication with the fiber. The pattern is designed to redistribute a gaussian wavefront such as that corresponding to the LP 02  mode as described in FIG. 7 b . In order to achieve an instantaneous change of the wavefront, the height of the binary pattern is set in one embodiment to 1.5 microns. This height is much smaller than the ‘Rayleigh range’, which is approximately 50 microns in a conventional fiber. The Rayleigh range is defined as πr 2 /λ where r is the radius of the wavefront and λ is the wavelength of the light.  
         [0056]    [0056]FIG. 12 b  depicts an embodiment in which the fibers  4 ,  6  are in contact with each other in order to reduce the relative motion and losses. FIG. 12 c  depicts the same architecture as in FIG.  12   b  except that a transparent material (for example the cladding itself) fills the gap  17 . In this architecture the height of the pattern  16 ′ can be larger. If the relative refractive index difference between the filled gap  17  and the pattern  16 ′ is set to 4%, then the pattern height is set to 13 microns. This height is still smaller than the ‘Rayleigh range’.  
         [0057]    The width of the wavefront in a fiber is of the order of microns. Since modem photolithographic methods can achieved sub-micron resolution, photolithography can be used to create the desired pattern on the face of the fiber.  
         [0058]    Just as photolithography makes it is possible to accurately etch or coat the desired pattern on the edge of the fiber, multiple lithographic processes make it possible to approximate any continuous pattern. Accurate alignment of the fiber core to the desired pattern can be achieved by illuminating the fiber through the core.  
         [0059]    Another method for creating a pattern  16  on the end face of a fiber is to attach a short (i.e., a few tenths of microns in length) fiber having the desired pattern  16 . It can also be done by attaching a long fiber to the fiber end face and cutting it to the desired length. This method is more convenient and less expensive in mass production.  
         [0060]    An internally reflective spatial mode transformer  190  of the present invention is illustrated in FIG. 20 a . The gaussian beam emerging from the end of a single mode fiber  186  includes a center portion  192  and an outer portion  194 . The gaussian beam  192  and  194  enters the spatial mode transformer  190  where only the outer portion  194  is reflected from an internal surface  196  back into the center portion  192  so that the interference between the portions  192  and  194  results in a wavefront similar to that of the LP 02  mode. The resulting wavefront passes through one or more lenses  198  which couple the wavefront into a high order mode fiber  188 . The internal surface  196  can be made from a variety of reflectors including, but not limited to, metallic reflective materials and refractive index interfaces (e.g., a segment of optical fiber having a core-cladding interface). FIG. 20 b  illustrates an internally reflective spatial mode transformer  190  attached to the single mode fiber  186 . In another embodiment shown in FIG. 13C, a fiber-based spatial mode transformer  190 ′ is disposed between the ends of the two fibers  186  and  188 . The mode transformer  190 ′ includes a short segment of optical fiber with an expanded core  200  of high refractive index. The cores of the two fibers  186  and  188  can be expanded in order to improve the coupling efficiency between spatial modes.  
         [0061]    The transverse transformation process is insensitive to the polarization of the propagating pulse. However, in many applications it is necessary to introduce different phase shifts to the different polarizations of the pulse. This can be desirable because the polarization of the LP 01  mode in the single mode fiber can be different from that of the higher order modes such as the TE 01  mode. FIG. 14 depicts an embodiment for such an application. In this embodiment a collimating lenses  88 , a polarization beam splitter  92 , and a combiner  96  are conventional bulk elements. Special mirrors  100  and  102  perform the transverse mode transformation. These mirrors  100  and  102  are designed to introduce phase changes to the reflected wavefronts. One way of achieving this is by etching patterns on the mirrors themselves. In another embodiment, the transverse mode transformer  28  is constructed as a single bulk component  109  as shown in FIG. 15. The incident optical beam  110  is split into two orthogonally polarized beams  111  and  1113  by a polarization beam splitter  115 . Each beam is then reflected by total internal reflection from sides  114 , and recombined at polarization beam splitter  115  into a single output beam  112 .  
         [0062]    The effect of this element  109  on the polarization of the light passing through it is illustrated in FIG. 16. An arbitrarily polarized pulse  120  is split to its two orthogonal polarization components  124   a  and  124   b  by the polarization splitter  115 . The phase of each component  124   a  and  124   b  is changed by the phase elements on the mirrors  114  resulting in altered components  128   a  and  128   b . A polarization beamsplitter  115  combines the components  128   a  and  128   b  into a single annular distribution  132 . The orientation of the phase elements on the mirrors  114  which are used to generate the altered components  128   a  and  128   b  can be rotated so that all LP 11  modes can be generated separately. As a result, only a single mode propagates in the fiber  84 . One advantage is that a polarization-maintaining fiber is not required.  
         [0063]    If the polarization of the incident pulse is known (after a polarizer or a polarizing splitter) then it is possible to transform its polarization to match that of the high order modes in the fiber. This polarization transformation can be done with a fine transverse grating. For example, the polarization of the LP 01  mode (the lowest order mode), which is basically linear and uniform across the mode, can be transformed to an azimuthal one (as that of the TE 01 ) by using a transverse grating with a varying local period.  
         [0064]    Alternatively, a birefringent element can be used. FIG. 17 represents a physical description of the process of transforming a linear polarization towards angular polarization by using a retardation plate. The linear polarization  140  passes through a waveplate having primary axes oriented at an angle to the orientation of the linear polarization  142 . The height of the plate is designed to have an angular dependence according to the equation H 1 (r,θ)=D/(2π)θ, where D is defined as the depth for which the birefringence waveplate is not changing the orientation of linear polarization. The resulting polarization  144  is shown in FIG. 17. However, this wavefront may have a residual angular phase. Therefore, another non-birefringent element  146  is used to compensate for any residual angular phase. This element introduces the negative angular phase. This phase can be presented as H 2 (r,θ)=−F/(27π)θ, where F is calculated according to the residual angular phase. The same effect can be achieved also by using two retardation waveplates having opposite angular phases and their primary axis oriented at opposite angles to the linear polarization.  
         [0065]    The transverse phase elements can be implemented in a few configurations according to the requirements of the complete system. FIG. 18 represents a conventional system designed to eliminate the sensitivity of the system to polarization mode dispersion. The light propagating in a single mode fiber  3  enters a circulator  160  or a coupler (not shown). Then the light passes through the transverse mode transformer  162 . The light is propagated as a higher order mode in the dispersion compensation fiber  164 . A Faraday mirror  166  then reflects the light. After the light has passed again through the dispersion compensation fiber  164  and transverse mode transformer  162 , the circulator  160  separates the outgoing light for propagation through fiber  3 ′ from the incoming light propagating through fiber  3 .  
         [0066]    However, in many applications circulators  160  are not desired because of their expense and complexity. Couplers (i.e., beamsplitters) are also undesirable because they introduce an inherent 50% loss. FIG. 19 represents a configuration in which a circulator or coupler is not needed. The light is separated into its orthogonal polarizations by the polarization splitter  172 . Then, each polarization passes through a Faraday rotator  174  imparting a 45° polarization rotation to the polarization and then through a phase element  178 . A polarization conserving special fiber  180  or an elliptical special fiber  180  is oriented at 45° so it is parallel to the transmitted polarization. The influence of the two Faraday rotators  174  cancels the rotation introduced by the special fiber  180 . As a result, the two polarizations return to their original state and are combined at the polarizer  172  in the same orientation. As the two polarizations are counter-propagating in the special fiber  180 , they have the same orientation. Therefore, they will be combined without time difference.  
         [0067]    Thus, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein.