fig1 shows in a block diagram a single stage optical amplifier in accordance with one embodiment of this disclosure . it is to be understood that this amplifier is fabricated and assembled using conventional optical technology and components . the input light beam incident from the left side of the figure is in this case a wavelength division multiplexed optical signal including wavelengths in the s band , c band , and l band . typically this signal is carried in a conventional optical fiber which is conventionally connected ( e . g ., pigtailed ) to the input port of a conventional optical demultiplexer 14 . the depicted optical amplifier 10 includes , downstream of demultiplexer 14 , three optical paths . the lowest optical path ( as drawn ) carries the demultiplexed l band signal from demultiplexer 14 which is input via an optical fiber into a conventional l band edfa 18 . instead of an edfa 18 , a raman optical amplifier may be used , if suitable , for the l band . raman optical amplifiers are somewhat similar to edfas , however they use conventional optical fiber , rather than doped optical fiber . raman optical amplifiers require high pump power and suffer from other drawbacks , so currently they are used less often than edfa &# 39 ; s the amplified l band output signal from edfa 18 is then applied to one input port of a demultiplexer 22 which is also a conventional optical element commercially available from , for instance , jds - uniphase . the center path carries the demultiplexed c band optical signal which is input to an optical input port of a c band edfa 26 . it is to be understood that each edfa conventionally includes one or more lengths of conventional doped optical fiber pumped by suitable pump lasers , not shown . the amplified c band optical signal is then input to a second optical input port of multiplexer 22 . the uppermost path in amplifier 10 is of most interest . the demultiplexed s band signal is applied to the input terminal of a wavelength converter 32 which converts the s band signal into a c band signal . ( the c ′ designation indicates the result of this conversion .) the c band signal is then applied to the input terminal of a c band edfa 38 , which is essentially identical to c band edfa 26 . the amplified c band signal output from edfa 38 is then input to the input port of a second wavelength converter 42 which down - converts this signal to the s band and the resulting signal is then applied to the third optical input port of multiplexer 22 . such wavelength converters are mixers , analogous to the type known in rf ( radio frequency ) technology . the output wavelength is a known function of the pump wavelength and the input signal wavelength ; the pump wavelength may be in or out of the band of the input and output wavelengths . conventionally , the multiplexer 22 outputs at its output port the multiplexed s band , c band , and l band signals . it is to be understood that the horizontal lines shown in fig1 typically are lengths of optical fiber ( or other suitable waveguides ) and in each case the connections to the optical elements are , e . g ., conventional pigtail connections or conventional free space connections using lenses . the various mechanical supports , amplifier housing , optical isolators and electric power supplies for the pumping lasers associated with the various edfas and wavelength converters are not shown . the fig1 optical amplifier is typically housed in an enclosure of standard size , such as used with a conventional edfa “ line card ” and included in the enclosure are a power supply , monitor and control circuitry , and diagnostic circuitry , all of the type known in the optical fiber communications field . also , typically temperature control ( e . g ., electric heating ) is provided to the wavelength converters for the fig1 amplifier . for a typical ppln wavelength converter , the temperature is typically maintained at 60 - 120 ° c . this heating is enhanced by thermally insulating the enclosure . while fig1 shows an optical amplifier 10 amplifying s band , c band and l band signals , which are multiplexed together , it is to be understood that in other embodiments , the lower two paths in fig1 are not present and also the demultiplexer and multiplexer are not present . instead , this embodiment includes only elements 32 , 38 and 42 and the intervening optical fibers ( or other types of waveguides ), thereby allowing amplification using a edfa or other type of optical amplifier of an s band signal , even though the edfa itself is not capable of amplifying s band signals . hence , this embodiment is a sub - combination of the fig1 amplifier which may have utility on its own . for instance , there may be applications where an s band optical signal is required for optical communication and it is desired to amplify it in the optical domain , even though the optical signal itself does not include c band or l band signals . while there is reference here to wavelength bands , it is to be understood that each band may include numerous channels , each of which is a sub - band within that band . however , there may be only one frequency in use in each band in other embodiments . hence , in fig1 it is assumed in one embodiment that the input optical signal extends over all the available bandwidth of s , c and the l bands . the s , c , and l band signals are demultiplexed by demultiplexer 14 into the three separate bands . in one embodiment , all the components shown in fig1 are packaged as a single module within a single housing . this packaged amplifier thus functions as a universal band amplifier covering all of the s , c , and l bands together . in one example , the three bands s , c and l include in total 40 to 120 or more channels , depending on the bandwidth of each channel . a typical performance goal is a bit error rate of less than 10 − 9 . the typical optical power input to the c band edfa 38 is approximately 6 dbm . of course , this is merely exemplary . fig2 shows a two stage optical amplifier arrangement with components similar to those of fig1 identically labeled , for another embodiment . in fig2 there is a first stage amplifier 58 and a second stage amplifier 60 . the first stage 58 converts , in its upper path , the s band signal to c band and then amplifies in the c band using edfa 38 . also , it converts the c band into the s band using an additional wavelength converter 62 . the lower ( l band path ) in stage 58 is identical to that in fig1 . the second amplifier stage 60 is typically coupled to first stage 58 by a substantial span 80 ( many km ) of optical fiber 70 . the second stage 60 , shown in the lower portion of fig2 includes a second demultiplexer 66 which demultiplexes the received c , l , and s band signals again into the c band , the s band , and l band . as shown , the c band signal is then converted back to the s band by converter 42 while the s band signal is converted back to the c band by converter 64 and then amplified by edfa 26 . the two stage amplifier arrangement of fig2 ( which in other respects is similar to that of fig1 ) is intended to overcome any power limitations inherent in a single stage amplifier as in fig1 . as shown in fig2 where there are a succession of such amplifiers 58 , 60 , each separated by a span of fiber optic cable 70 and serving as , for instance , repeaters in a telecommunications system , the c band and s band signals “ switch places ” ( with regard to the original information - bearing signals ) in each consecutive fiber optic span 70 . by this is meant that at each stage the incoming c band signals are translated to the s band , and the incoming s band signals are translated to the c band . this place switching helps ensure that the wavelength conversion elements 32 , 62 , 42 , 64 are not exposed to the high input signal power present at the output of an edfa . fig2 shows that the l band signals are amplified in every other span 70 along with the c band signals . alternatively the l band signals are amplified with the s band signals or are amplified in both consecutive stages . note that as described above the amplifiers of fig1 or 2 may be housed in the same size enclosure as is a conventional c band edfa . as explained above , the individual elements of fig1 and fig2 are conventional . the wavelength converters , unlike the other elements , are typically not now commercially available . however , such wavelength converters are well known in the field . various known devices achieve this wavelength conversion . one such device is a three - wave mixer using a non - linear material such as periodically poled lithium niobate ( ppln ). other types of material suitable for three - wave mixing are magnesium oxide - lithium niobate or orientation patterned gallium arsenide . it is also possible to use four - wave mixing for wavelength conversion in a material such as a non - linear optical fiber or semiconductor optical amplifier . another conversion approach is cross - gain or cross - phase modulation in semiconductor optical amplifiers . all of these devices are known in the field . for example , yoo u . s . pat . no . 5 , 434 , 700 , incorporated herein by reference in its entirety , discloses an all optical wavelength converter . this uses , as do the above - described devices , an optical pump signal applied to a passive waveguide device . the combination performs wavelength conversion on an input optical signal . yoo discloses , for instance , receiving wavelengths around the 1530 nanometer wavelength and shifting them by , for instance , 20 nanometers in wavelength . yoo uses a gallium arsenide material for the waveguide device . yoo also refers to another type of optical converter as disclosed in applied physics letters , vol . 63 , pp . 1170 - 1172 “ wavelength conversions - 1 . 5 micrometer by difference frequency generation periodically domain inverted linbo three channel wave guides ”, august 1993 by xu et al ., incorporated herein by reference in its entirety . further work on such wavelength conversion devices is disclosed in a dissertation entitled “ optical frequency mixers using three wave mixing for optical fiber communications ” by ming - hsien chou , stanford university , 1999 , g . l . no 5671 , also incorporated herein by reference in its entirety . this dissertation shows , at page 50 , a tapered waveguide providing nonlinear frequency mixing , suitable for use in the present converter , and explains how to fabricate same . in this waveguide the information bearing optical signal and the pump optical signal are both coupled to one edge of the waveguide . the above publications and others in the field explain in detail how to manufacture and use such wavelength converters . the particular waveguide material selected and parameters of the devices are chosen so as to provide the desired wavelength conversion . note that a particular device which performs an up - conversion in terms of wavelength will also perform a down - conversion depending on the pump wavelength . since the c band is narrower than the s band , conversion of s band signals to c band necessarily reduces bandwidth , resulting in fewer channels . thus typically in the ( original ) s band signal not all channels will be used . to make the wavelength converter used in one embodiment , a process similar to those described in the above - referenced patent and publications is used to fabricate a ppln wave guide for wavelength conversion . each process step is conventional . first , starting with a lithium niobate wafer , a layer of silicon dioxide is sputtered on the principal surface thereof . then the electrode configuration is formed on a mask and by lithography the electrode configuration is patterned on the silicon dioxide layer , by conventional etching . the ppln structure is then formed by applying a suitable high voltage to the wafer which inverts the ferro - electric domain underneath the electrodes . the high voltage is applied using a salt solution as a liquid electrode . see , e . g ., the chou thesis at pages 33 - 38 . next , a proton exchanging step takes place using benzoic acid involving the acid in its molten state . then the wafer is annealed and then diced and polished and then subject to conventional wave guide characterization . in use , each such converter waveguide is coupled by a length of optical fiber to a pump source such as a laser diode outputting a light beam of suitable wavelength and power . the optical fiber carrying the pump light , as are the optical fibers carrying the information signal to / from the waveguide , is optically coupled to the end surfaces of the waveguide by pigtailing or by a lens , conventionally . the waveguide is installed in the system to receive the input optical signal and output the converted ( wavelength shifted ) optical signal . the pump wavelength conventionally determines the center wavelength of the signal output from the converter . if the input wavelength is greater than the pump wavelength , there is a wavelength down conversion . if the input wavelength is less than the pump wavelength , there is a wavelength up conversion . the above described ppln waveguides support only a single polarization . the chou thesis shows ( page 86 ) two different schemes to overcome this using either a polarizing beam splitter with feedback , or using two polarizing beam splitters using , e . g ., two waveguides on one chip . these polarization diversity schemes are well known in the field . if a polarization insensitive waveguide is used , no such diversity scheme is needed . while the above disclosure is of a ppln waveguide for the converter , this is not limiting . other suitable waveguides use variants of lithium niobate . ( doping advantageously lowers the operating temperature .) further , other nlo ( non - linear optical ) waveguide materials , such as lithium tantalite , are suitable . various pumping schemes are available to pump the waveguides , including co - and counter - propagation . the pumping wavelength is typically “ in band ” relative to the information bearing signal &# 39 ; s wavelength ( s ) so as to allow use of a smaller and lower power pump source , but this is not required . the pumping wavelength can also be a subharmonic of the average of the wavelengths of the input and output pumped signals . it is to be understand that various types of wavelength converters can be used , depending on the operating parameters of the amplifier in which it is to be installed . similarly , the type of pump source , and pump power are determined by the particular optical band to be converted and the input power of the optical signal to the wavelength converter . this disclosure is illustrative and not limiting ; further modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims .