in the following detailed description , for purposes of explanation and not limitation , exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention . however , it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure , that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein . moreover , descriptions of well - known devices , methods and materials may be omitted so as to not obscure the description of the present invention . briefly , in accordance with exemplary embodiments of the present invention described in detail herein , an optical apparatus and method reduces mpi in dispersion compensating ( dc ) optical waveguides used in a variety of optical links by pumping the dc waveguides with light . usefully , this enables dc waveguides to be designed to provide a greater degree of dispersion compensation , or dispersion slope compensation , or both to be achieved , while suppressing higher order modes relative to the fundamental optical mode . this results in a reduction in the mpi in the optical link in which the apparatus and method are used . fig1 shows a schematic diagram of an optical apparatus 100 in accordance with an exemplary embodiment of the present invention . the optical apparatus 100 receives an input optical signal that traverses an input waveguide 101 , which is illustratively an optical fiber . the input optical signal is input to a dispersion compensating module ( dc module ) 102 , which has a dc optical waveguide 110 disposed therein . as described in further detail below , the dc optical waveguide is illustratively a dcf that has a germanium - doped core , which fosters an improved bend sensitivity , but which may support higher - order modes ( i . e ., the dc waveguide may be few - moded ). as described previously , these higher order modes may result in mpi in an optical link . the dc waveguide of the dc module 102 is pumped by light 104 with an optical pump 103 . illustratively , the optical pump 103 is a raman pump . as is well known , raman pumping results in optical gain from optical phonons within the guiding material of the waveguide ( in this example , the material of the dc waveguide ). to this end , if the pump light 104 sufficiently excites the glass molecules to high - energy vibrational states , their relaxation results in photon emission . these emitted quanta of light can be used to increase the gain of the fundamental mode of the dc waveguide . for example , an emitted photon is approximately − 12 thz less than the frequency of the pump signal . this translates to approximately 100 nm in a common 1550 nm optical communications link . as a result , without providing additional dopants to the core of the dc waveguide , an optical pump near 1450 nm can provide gain to an optical signal having wavelength of 1550 nm . in the exemplary embodiments described herein , and as will become more clear as the present invention proceeds , when the dc waveguide is raman pumped by light 104 from the raman pump 103 , most of gain is realized by the fundamental mode because the higher order modes do not overlap significantly spatially with the single - mode transverse profile of the raman pump 103 . the use of raman optical pump 103 in combination with the dc module 102 not only can improve the overall gain of the fundamental mode , but can also decrease mpi in the system while enabling improved dc and dispersion slope compensation . advantageously , therefore , an exemplary embodiment of the present invention provides dc and dispersion slope compensation , while reducing mpi . for purposes of illustration , and not limitation , the following quantitative description of the pumping of a few - moded dcf is given . in a 3 km length of dcf , raman pumping may be effected to realize a gain of approximately 12 db . the maximum mpi in such a system can be shown to be : where i 1 is the intensity of the fundamental mode and i 2 is the intensity of a higher - order mode . in the present exemplary embodiment of the present invention , the fundamental mode would experience 12 db of gain , while the higher - order mode ( s ) would experience substantially no gain from the pumping . this follows , of course , from the fact that the mode field of the raman pump is most similar to the fundamental mode of the dc fiber , while higher order modes of the dc fiber , which can cause mpi , have much less spatial overlap with the raman pump field . this approximation does not take into account the different mode field of the pump and signal , which could actually provide some gain to the higher - order mode ( s ). from the above , it can be shown that 12 db of raman gain can result in a reduction of approximately 12 db of mpi . it is noted that in the less common case of coherent mpi , the reduction in mpi may be only 6 db . in either case the reduction of mpi enables greater cd and dispersion slope compensation with mpi that is below specified levels . clearly , this is beneficial in optical links , particularly as bandwidths are increased . it is noted that in the above exemplary embodiments the use of a raman pump is merely illustrative . however , it is noted that in accordance with other exemplary embodiments of the present invention other amplification methods may be used to realize gain in the fundamental mode of the dc waveguide , while substantially not amplifying the higher - order modes . an alternative amplification technique includes , but is not limited to , erbium - doped dc fiber where er + 3 ions or other rare earth ions in the fiber core produce gain selectively for the fundamental mode , and the fiber profile is designed in order to accomplish proper dispersion compensation , or dispersion slope compensation , or both , while reducing mpi . it is further noted that in the exemplary embodiment shown in fig1 , the raman pump 103 is counter - propagating . this too is merely illustrative of the present embodiment , and co - propagating pump light or a combination of counter - and co - propagating light may be used . it is further noted that more than one pump may be used in carrying out the exemplary embodiments of the present invention . a better understanding of the problems that the exemplary embodiments of the present invention overcome , and of the benefits of the exemplary embodiments may be obtained through a discussion of fig2 , read in light of the optical apparatus and it method of use in fig1 . fig2 is a graph of the relative index of refraction ( n r ) between the core of a dispersion compensating optical fiber and its cladding layer ( or between the guiding layer of a dc waveguide and the dielectric layer thereabout ) versus wavelength λ . a lower curve 201 is for a standard dcf , while an upper curve is of a dcf having a greater concentration of dopants ( e . g ., ge ) to increase the index differential across the operational spectrum . as is commonplace , the wavelengths at which many optical systems operate are generally at the ‘ knee ’ of the curves ; or in the first region 202 in the lower curve 201 , and at the second region 204 of the upper curve 203 . as mentioned previously , the lower curve 201 may be useful in avoiding the capability of supporting higher - order optical modes in the dcf . moreover , any higher order optical modes in a fiber having an index characteristic such as that of lower curve 201 would likely be weakly supported ; and may be stripped out to a great extent via bends in the deployed dcf . this would leave the fundamental mode supported in the dcf , and therefore the optical link . however , providing dcf &# 39 ; s having an index characteristic over wavelength such as that of lower curve 204 also places constraints on the fiber designer &# 39 ; s ability to provide increased chromatic dispersion and dispersion slope compensation capability . to wit , it may be useful to have an increased index differential between the core and cladding layers . alternatively , or additionally , the core or guiding layer size may be increased ( compared to known dcf ) to improve the dispersion compensation capabilities , or dispersion slope capabilities , or both , of the dcf ( or other dc waveguide ). these changes , alone or in combination , will improve the cd and slope compensation capabilities of the dispersion compensating fiber ( or waveguide ). as previously described , operation along an index differential such as upper curve 203 enables the higher order modes to be supported . in this case , bends and other known techniques are not as effective at stripping these higher - order modes , and the incidence of mpi and other problems associated with higher - order modes in the optical system can be deleterious to optical performance . as such , in dc fibers and waveguides that turn out to be ‘ few - moded ’ in the quest to improve the dispersion and dispersion slope compensating capabilities of mpi can be unacceptable . however , by virtue of the exemplary embodiments of the present invention , dc fibers and waveguides have the requisite characteristics ( e . g ., core or guiding layer doping concentrations ) to provide desired levels of chromatic dispersion ( cd ) and dispersion slope compensation , without increasing ( if not decreasing ) mpi . for purposes of illustration , and not limitation , the following description depicts a typical dcf and resultant mpi , dc and dispersion slope compensation readily attained using an exemplary embodiment of the present invention . in the presently described exemplary embodiment , the dcf is illustratively a leaf ® compensation fiber that is commercially available . this dcf would be part of a dc module such as dc module 102 of the exemplary embodiment of the present invention . the dcf is capable of providing dispersion compensation in the range of approximately − 350 ps / nm - km to approximately − 70 ps / nm - km ; and can provide dispersion slope compensation in the range of approximately − 7 ps / nm 2 - km to approximately − 1 ps / nm 2 - km . the invention having been described in detail in connection through a discussion of exemplary embodiments , it is clear that modifications of the invention will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure . such modifications and variations are included in the scope of the appended claims .