fig1 depicts an illustration of a doped fiber amplifier 100 in accordance with the present invention . in the preferred embodiment of the instant invention , the fiber is doped with erbium . however , one of ordinary skill in the art will recognize that the fiber may be doped with other elements without departing from the spirit of the invention . an input signal is presented at 105 . pump radiation from a first pump laser module 110 and a second pump laser module 120 are combined using a fiber wavelength division multiplexer 115 . the combined output of first waveguide division multiplexer 115 is further combined with the input signal at 105 in a second wavelength division multiplexer 125 producing a combined output into a doped fiber 135 . first pump laser module 110 and the second pump laser module 120 are pigtailed laser diodes which generate light radiation at wavelengths λ 1 and λ 2 respectively within a tolerance of plus or minus 15 to 20 nm of a specified wavelength , λ c . for an erbium amplifier , wavelengths in the 960 - 1000 nm wavelength window are presented to wavelength division multiplexer 115 , and combined . while two pump lasers are depicted in fig1 this is for exemplary purposes only , and a multiplicity of pump lasers may be combined at wavelength division multiplexer 115 without departing from the scope of the invention so long as the lasers radiate light within the required wavelength tolerance , approximately plus or minus 20nm for an erbium doped fiber amplifier pumped at 980 nm , for generating stimulated emission in the doped fiber 135 . while this invention provides the greatest benefit for pump lasers that emit a single longitudinal mode because this permits more efficient and higher density wavelength multiplexing , it also applies to multi - longitudinal mode pump lasers . the doped fiber 135 has a high numerical aperture ( n . a .) with a typical mode field diameter of less than 4 μm . the mode field diameter of the fiber used in a typical wavelength division multiplexer is typically larger . with a mode field diameter of 8 μm for the latter the splice loss is ≃ 2 db between wavelength division multiplexer 125 and fiber 135 . this translates to a loss of 40 % of the pump power . this loss can be significantly reduced by fabricating the output fibers of laser modules 110 and 120 with the same mode field diameter ( mfd ) as the input fibers of wavelength division multiplexer 115 same as the output fiber of wavelength division multiplexer 115 , the input fiber of wavelength division multiplexer 125 , and the output fiber of wavelength division multiplexer 125 with same mfd as the doped fiber 135 . the splice losses between the pump laser modules , the wavelength division multiplexers , and the doped fiber are thereby greatly reduced . use of a fiber with a small mode field diameter in laser modules 110 and 120 enhances the coupling efficiency between the laser and fiber since diode lasers typically have small mode field diameters of a few microns depending on the laser geometry . using the approach described above , the only splice loss of any significance is at the input 105 and output 150 where the small mode field diameter fiber is typically spliced to a 1550 nm transmission fiber with a larger mode field diameter which is typically 9 μm . this loss can be minimized using beam expansion of the smaller beam before splicing . one approach to achieve this is the use of thermal diffusion of the dopant for example ge to expand the beam as described by k . shiraishi et al . in the journal of lightwave technologyy , vol . 8 at page 1151 . in a further embodiment , the wavelength division multiplexer is fabricated with an mfd to match the transmission fiber and an output fiber with mfd matched to the doped fiber 135 . the insensitivity of erbium doped fiber amplifiers to pump wavelength is illustrated in fig2 which displays the signal output power as a function of pump wavelength . a ge / al / p / er - doped silica fiber with an n . a . of 0 . 18 and a cut - off wavelength of 940 nm was used . the signal launched into the erbium - doped fiber was - 1 . 4 dbm . the experimental results are indicated by marks for three different fiber lengths each pumped with 40 and 80 mw . the solid curves are computer simulations of the experiment obtained using a numerical model . all basic parameters used by the model were determined experimentally and the calculated values are seen to be in good agreement with experiment . also indicated in fig2 is the ground - state absorption ( gsa ) cross section spectrum which peaks at 979 nm . as expected , signal output increases with pump power , reaching 16 . 3 dbm , for the 16 m fiber pumped with 80 mw at 979 nm . this compares to 12 . 7 dbm and 0 . 71 for a 12 m fiber pumped with 40 mw . fig2 reveals that the magnitude of the gain penalty for pump wavelengths away from 979 nm is significantly reduced at high pump powers . at 80 mw the penalty is & lt ; 0 . 5 db for a pump wavelength range of ± 9 nm about the peak if one does not alter the fiber length from the optimum for 979 nm ( 16 m ). the corresponding range for 40 mw is only ± 5 nm . detuning the pump wavelength decreases the absorption coefficient . fig2 indicates that increasing the fiber length partially compensates for this and reduces the gain penalty . note also that the output when pumped at 979 nm is less sensitive to fiber length for higher pump powers ; for 80 mw with this fiber design the gain penalty is & lt ; 0 . 1 db with an uncertainty of ± 25 % around the optimum fiber length . based on these results , it is seen that using pump lasers at wavelength intervals convenient for wavelength multiplexing ( 10 - 20 nm ) does not introduce a significant gain penalty . fig3 shows a schematic of the apparatus for the experiment . two pump lasers with operating wavelengths of 963 nm and 992 nm , respectively , are combined using a wavelength division multiplexer , wdm 1 . the wdm used is commercially available and makes use of dichroic filters to combine closely spaced wavelengths . wdm 1 combines the output of the two pump lasers , one with its wavelength in the 960 - 975 nm range and the other having its wavelength between 985 and 1000 nm , into an output fiber with an insertion loss of 0 . 5 db . the output of wdm 1 is combined with the input 1532 nm optical signal using a broadband wavelength multiplexer , wdm 2 . a similar arrangement of wdm &# 39 ; s is placed at the other end of the erbium doped fiber amplifier . the total insertion loss for the pump lasers through both wdm &# 39 ; s is 0 . 8 db . fig4 shows the spectrum of the pump lasers at the input of the 20 m erbium doped fiber which has a numerical aperture of 0 . 18 and cutoff wavelength of 1300 nm . erbium doped fiber amplifier output is shown in fig5 as a function of the pump input . the circles and triangles represent the experimental results and the corresponding calculations are shown by the solid line . the dashed line indicates the calculated signal power for pumping exactly at 979 nm . as expected there is a 0 . 9 db penalty due to the ± 5 nm deviation of the pump wavelengths from 979 nm . the wdm couplers used can combine any pump between 960 and 975 nm with another in the 985 and 1000 nm range . thus by selecting pump lasers at 10 ( 20 ) nm apart , a lower gain penalty of ≃ 0 . 2 ( 0 . 5 ) db can be achieved . while there has been shown and described what is at present considered the preferred embodiment of the invention it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims .