Abstract:
An optical fiber amplifier utilizing a phosphate glass optical fiber highly doped with rare-earth ions such as erbium to exhibit high gain per unit length, enabling the use of short fiber strands to achieve the needed gain in practical fiber optical communication networks. The high-gain phosphate optical glass fiber amplifiers are integrated onto substrates to form an integrated optics amplifier module. An optical pump such as a semiconductor laser of suitable wavelength is used to promote gain inversion of erbium ions and ultimately provide power amplification of a given input signal. Gain inversion is enhanced in the erbium doped phosphate glass fiber by co-doping with ytterbium. A phosphate fiber amplifier or an integrated optics amplifier module utilizing this power amplification can be combined with other components such as splitters, combiners, modulators, or arrayed waveguide gratings to form lossless or amplified components that do not suffer from insertion loss when added to an optical network. The fiber amplifier can be a single fiber or an array of fibers. Further, the phosphate glass fibers can be designed with a temperature coefficient of refractive index close to zero enabling proper mode performance as ambient temperatures or induced heating changes the temperature of the phosphate glass fiber. Large core 50-100 μm fibers can be used for fiber amplifiers. The phosphate glass composition includes erbium concentrations of at least 1.5 weight percentage, preferably further including ytterbium at 1.5 weight percentage, or greater.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contracts DASG60-98-M-0090 and DASG60-99-C-0064 awarded by the Ballistic Missile Defense Organization. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention is related to optical amplifiers, more particularly, to rare-earth doped phosphate optical fibers for amplifiers pumped by diode or solid state laser sources. The rare-earth ions, erbium and ytterbium, are co-doped at high levels to provide for extremely high gain. Erbium ion concentration is increased to provide very high active ion content. Ytterbium ion concentration is increased to provide efficient absorption of pump power. The phosphate glass fiber provides for a low-loss and stable fiber host. Phosphate glass fibers can be produced with a temperature coefficient of refractive index close to zero. 
     BACKGROUND OF THE INVENTION 
     Deregulation, long distance price declines, bandwidth stockpiling, and internet usage are driving bandwidth demand in telecom and datacom networks. Data traffic is now growing at 100 percent or more per annum, straining global fiber capacity. Dense Wavelength Division Multiplexing (DWDM), where multiple wavelength channels propagate within a single fiber multiplies fiber capacity by 2-128 times or more, is an approach for extending carrier capacity without the need of deploying new fiber. Systems being deployed today generally can transmit from 8 up to 128 channels in the 1550 nm low-dispersion window. Channel spacing ranges from 1.6 nm (200 GHz) to 0.4 nm (50 GHz). 
     Optical amplifiers are considered enabling components for bandwidth expansion in DWDM fiber optic communications systems. In particular, silica glass Erbium Doped Fiber Amplifiers (EDFA) exhibit many desirable attributes including high gain, low noise, negligible crosstalk and intermodulation distortion, bit-rate transparency, and polarization insensitive gain. These properties make optical fiber amplifiers superior to semiconductor devices as amplifiers in fiber optic systems. Moreover, fiber-based amplifiers do not require conversion from electrical energy to photon energy as do semiconductor devices. In a communications system of any significant size, there is typically a distribution network that includes long communication paths and nodes where the network branches. In such a network, amplifiers are required in order to maintain the amplitude of the signal and the integrity of any data in route between a source and destination. For these amplifiers to function properly, the amplifiers must exhibit high small signal gains and/or high output saturation powers. 
     Application of erbium-doped optical fibers as amplifiers has received considerable attention recently because the characteristic gain bandwidth of these fibers is within the telecommunications window of 1.5 μm commonly used in fiber optic communications systems. Since the announcement of a single mode Er 3+ doped fiber amplifier (EDFA) in 1987 at the University of Southampton, enormous research has been performed, and more than 400 U.S. patents have been issued in fiber amplifiers. To date, all erbium fiber amplifiers use erbium doped silica fibers more than one meter long to achieve greater than 20 dB gain near the 1.54 μm range. More commonly, the length of the erbium doped silica fiber is approximately 10 to 20 meters. Such lengths are not practical for assembly into integrated optical components. There is a compelling need for amplifiers that can introduce high gain into an integrated, compact package. 
     To shorten length in fiber amplifiers, high gain must be achieved. In order to enable fiber amplifiers of only a few centimeters in length, magnitudes of doping two orders higher than what is commercially achievable (˜10 18  cm −3 ) in silica fiber amplifiers is required. However, in silica fiber, cooperative upconversion and ion clustering effects develop from the interactions between nearby ions in silica glass, and electrons depopulate from the erbium metastable level ( 4 I 13/2 ). Thus, increased doping in silica glass does not improve gain. 
     Other glasses such as for example phosphate glasses exhibit high solubility and large emission cross sections for many rare-earth ions. Phosphate glasses for optical components have been investigated. Y. L. Lu, Y. et al., in “Fluorescence and attenuation properties of Er +3 -doped phosphate glass fibers and efficient infrared-to-visible up-conversion,”  Applied Physics B , Vol. 62, pp.287-291 (1996) and Ya Lin Lu et al., in “Properties of Er +3  doped phosphate glasses and glass fibers and efficient infrared to visible upconversion,”  Journal of Materials Science , Vol. 30, No. 22, Nov. 15, 1995, pp.5705-10, (1995) discuss phosphate glass fiber for use in up-conversion schemes. S. Jiang et al., in “Er +3  doped phosphate glasses and lasers,”  Journal of Non Crystalline Solids , Vol.239, No. 1-3, October 1998, pp. 143-8, show phosphate glasses for application as bulk lasers. T. Nishi et al., in “The amplification properties of a highly Er +3  doped phosphate fiber,”  Jpn. J Appl. Phys ., Vol. 31 (1992), Pt. 2, 2B, pp. L177-L179, show phosphate fiber with moderate erbium oxide doping. The maximum gain per unit length reported by Nishi et al. was only 1 dB/cm. S. Jiang, T. Luo et al. in “New Er 3+ doped phosphate glass for ion-exchanged waveguide amplifiers,”  Optical Engineering , Vol. 37, No. 12, December 1998, pp. 3282-6, disclose phosphate glasses for application in ion-exchanged waveguide amplifiers. 
     In addition, a number of patents have addressed doped glasses in various optical applications. For example, Hsu et al. (U.S. Pat. No. 5,425,039), Myers (U.S. Pat. No. 4,962,067), Myers et al. (U.S. Pat. No. 4,333,848), Myers et al. (U.S. Pat. No. 4,248,732), Myers et al. (U.S. Pat. No. 4,075,120), each disclose doped fibers for application as fiber lasers. In addition, Myers et al. (U.S. Pat. No. 5,322,820) and Myers (U.S. Pat. No. 5,164,343) disclose various glass compositions for laser applications. Grubb et al. (U.S. Pat. No. 5,225,925) disclose silica fibers or phosphorous doped silica fiber. Andrews et al. (U.S. Pat. No. 4,962,995) disclose glasses that are optimized for pumping by 800 nm laser light. 
     Recently, Y. C. Yan et al., in “Net optical gain at 1.53 μm in an Er-doped phosphate glass waveguide on silicon,”  Optical Amplifiers and Their Applications, Topical Meeting. OSA Trends in Optics and Photonics Series , Vol. 16. Opt. Soc. America, Washington, D.C., USA; 1997; xlv+526, pp.93-5, investigated doped phosphate glasses as a high gain medium for planar waveguide amplifiers at wavelength of 1.5 μm. Y. C. Yan et al., in “Erbium-doped phosphate glass waveguide on silicon with 4.1 dB/cm gain at 1.535 μm,”  Applied Physics Letters , Vol.71, No. 20, Nov. 17, 1997, pp. 2922-4 reported a gain of 4.1 dB in a 1 cm long phosphate glass waveguide prepared by an R-F sputtering technique. D. Barbier et al., in “Net gain of 27 dB with a 8.6-cm-long Er/Yb-doped glass-planar-amplifier,”  OFC  &#39;98  Optical Fiber Communication Conference and Exhibit, Technical Digest , Conference Edition 1998 OSA Technical Digest Series Vol.2 (IEEE Cat. No.98CH36177), Opt. Soc. America, Washington, D.C., USA; 1998; vii+421, pp.45-6, demonstrated a net gain of 27 dB in a 8.6 cm long ion-exchanged Er/Yb-doped phosphate glass waveguide. 
     Despite the high gains achieved for example in phosphate glass waveguides, planar waveguide amplifiers have significant disadvantages when compared with fiber amplifiers including polarization sensitivity, optical mode mismatch between waveguides and fiber networks, large propagation losses, and complicated fabrication processes. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a phosphate glass optical fiber amplifier with a gain per unit length, greater than 1.5 dB/cm and preferable over 3 dB/cm. 
     Another object of the invention is to provide a high gain per unit length doped phosphate glass fiber which can be utilized as a fiber amplifier in an optical communications system. 
     Another object of the invention is to provide an erbium and ytterbium codoped phosphate glass fiber with high (concentrations well above concentrations deemed practical by current wisdom) erbium and ytterbium co-doping concentrations for high gain amplification within a short length of the optical fiber. A short-length optical fiber amplifier utilizing the high gain, short-length fiber is compatible with V-groove and micro-machining fabrication processes, making the short-length fibers compatible and integratable into optical component modules. 
     Still a further object of the invention is to provide a phosphate glass fiber with a core containing erbium (as Er 2 O 3 ) and ytterbium (as Yb 2 O 3 ) and a phosphate glass clad without erbium or ytterbium, wherein the phosphate glass fiber is manufactured using a rod-in-tube technique. 
     Another object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless splitter module, wherein a system signal is amplified (with the assistance of a pump laser diode) and fed to a splitter. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns. 
     Yet, another object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless combiner module, wherein a system signal is amplified (with the assistance of a pump laser diode) and fed to a combiner. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns. 
     A further object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless arrayed waveguide grating module, wherein the system signal channels are amplified (with the assistance of a pump laser diode) and fed to an arrayed waveguide grating. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns. 
     Still a further object of the invention is the application of the phosphate fiber in an optical amplifier, preferably in an integrated amplified or lossless modulator module, wherein the system signals are amplified (with the assistance of a pump laser diode) and fed to a Lithium Niobate optical modulator. The fiber may be only a few centimeters in length but exhibits a gain coefficient greater than 3 dB/cm at 1.54 microns. 
     Still another object of the invention to provide an efficient, long-lived erbium and ytterbium glass optical amplifier that is generally suitable for a variety of components in metro and local network applications, specifically in the area of fiber optic communication networks. 
     As such, one object of the invention is to provide doped phosphate glasses with a temperature coefficient of refractive index close to zero. 
     Another object is to provide an array of doped phosphate glass fibers mounted in a groove on a substrate such as for example a V-groove, where the array is pumped by a multi-mode laser diode bar orthogonal to the array. 
     Another object of the invention is to provide an erbium doped fiber with a large light-guiding region (diameter of the core ranging from 50 to 300 μm, which is much larger than in erbium doped fibers deemed practical by current wisdom). The fiber may be only a few centimeters in length and is pumped with one or more high-power multi-mode 980 nm light-emitting laser diodes, each having an emitting cross-sectional area on the order of 1 μm×100 μm. 
     These and other objects are achieved according to a fiber amplifier of the present invention utilizing a phosphate glass optical fiber highly doped with rare-earth ions such as erbium, and preferably co-doped with ytterbium to enhance gain. The phosphate glass optical fibers exhibit high gain per unit length, enabling the use of short fiber strands to achieve the needed gain in practical fiber optical communication networks. 
     According to one aspect of the present invention, the high-gain phosphate optical glass fiber amplifiers are integrated onto substrates, such as in grooved substrates, to form an integrated optics amplifier module. An optical pump such as a semiconductor laser of suitable wavelength is used to promote gain inversion of erbium ionic metastable states and ultimately provide power amplification of a given input signal. 
     According to another aspect of the present invention, a phosphate fiber amplifier is integrated with other components such as splitters, combiners, modulators, or arrayed waveguide gratings to form lossless or amplified components that do not suffer from insertion loss when added to an optical network. 
     According to a further aspect of the present invention, the fiber amplifier includes a single fiber or an array of fibers. Further, the phosphate glass fibers are designed with a temperature coefficient of refractive index close to zero enabling proper mode performance as ambient temperatures or induced heating changes the temperature of the phosphate glass fiber. Fiber core diameters from standard sizes such as for example 5 μm to large core sizes such as for example 50-100 μm fibers are used for fiber amplifiers in the present invention. 
     According to one aspect of the present invention, it is recognized that erbium doped glass fibers, containing erbium concentrations far beyond the generally accepted optimum concentration for erbium ions in silica fiber, show fiber amplification in a short length and thus enable production of integrated high gain optical components. 
     According to a further aspect of the present invention, the erbium doped glass fibers are co-doped with ytterbium to enhance pumping of the erbium metastable levels and the resultant gain in the phosphate fibers. 
     Further, according to the present invention, there is provided a novel phosphate glass composition including the following ingredients by weight percentages: P 2 O 5  from 30 to 80 percent, Yb 2 O 3  from 0 to 12 percent, Er 2 O 3  from 2.5 to 12 percent, R 2 O from 0 to 5 percent L 2 O 3  from 5 to 30 percent, MO from 5 to 30 percent, where the sum of the weight percentages of Yb 2 O 3  and Er 2 O 3  is 2.5% or greater, R 2 O is selected from the alkali metal oxide group consisting of Li 2 O, K 2 O, Na 2 O, Rb 2 O, and mixtures thereof, MO is selected from the alkaline earth oxide group consisting of BaO, BeO, MgO, SrO, CaO, ZnO, PbO and mixtures thereof, and L 2 O 3  is selected from the transition metal oxide group consisting of Al 2 O 3 , B 2 O 3 .Y 2 O 3 , La 2 O 3 , and mixtures thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a schematic energy level diagram of the Yb +3 /Eb +3  co-doped system; 
     FIG. 2 is a graph depicting energy transfer efficiency from Yb 3+  ionic energy levels to Er 3+  ionic energy levels under different pumping powers for different doping concentrations of Yb 2 O 3  and Er 2 O 3 ; 
     FIG. 3 is a graph depicting the influence of glass composition with different alkali earth ions on refractive index and on the effective emission linewidth; 
     FIG. 4 is a graph depicting the influence of glass composition with different BaO and Al 2 O 3  concentrations on refractive and effective emission linewidth; 
     FIG. 5 is a graph showing the absorption and emission cross sections for the  4 I 15/2  &lt;-&gt; 4 I 13/2  transition band; 
     FIG. 6 is a flowchart showing the method steps for producing doped single mode phosphate glass fibers; 
     FIG. 7 is a diagram showing schematically the stages of the fabrication process for producing the single mode doped phosphate glass fibers; 
     FIG. 8 is a schematic diagram showing the experimental setup used to determine gain characterization of a co-doped phosphate glass fiber; 
     FIG. 9 is a graph showing the net gain and noise figure for an erbium doped phosphate glass fiber as a function of pump power; 
     FIG. 10 is a graph showing the net gain and noise figure for an erbium doped phosphate glass fiber as a function of signal wavelength; 
     FIG. 11 is a graph showing the gain saturation at 1535 nm and 1550 nm for an erbium doped phosphate glass fiber; 
     FIG. 12 is a schematic of an amplifying splitter according to the present invention; 
     FIG. 13 is a schematic of a fiber amplifier pumped with a multi-mode laser; 
     FIG. 14 is a schematic of a fiber array amplifier according to the present invention; 
     FIG. 15 a  is a schematic of one embodiment of the fiber array amplifier; 
     FIG. 15 b  is a schematic of another embodiment of the fiber array amplifier; 
     FIG. 15 c  is a schematic of another embodiment of the fiber array amplifier; 
     FIG. 16 is a schematic of an amplifying combiner according to the present invention; 
     FIG. 17 is a schematic of an amplifying arrayed waveguide grating according to the present invention; 
     FIG. 18 is a schematic of an amplifying modulator according to the present invention; 
     FIG. 19 is a schematic illustration of a large core fiber amplifier. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, FIG. 1 is a schematic energy level diagram of a Yb 3+  and Er 3+  ionic energy levels in a phosphate glass host. 
     According to one embodiment of the present invention, pumping of an erbium-doped phosphate glass populates the erbium metastable state creating a population inversion which under an input signal produces stimulated emission and amplification of the input signal. More specifically, the amplification process is explained in reference to energy levels shown in FIG.  1 . The Er 3+  ionic energy states are shown on the right hand side of FIG.  1 . Typically, a pump light excites electrons from the ground state  4 I 15/2  to an upper energy state such as  4 I 11/2 . Higher erbium doping levels allows more absorption of the pump light and ultimately higher gain. Once electrons are excited to the  4 I 11/2  state, relaxation occurs through phonon processes in which the electrons relax to the  4 I 13/2  state, giving up energy as phonons to the glass host material. The state  4 I 13/2  is a metastable state which normally does not readily emit a photon and decay to the ground state (i.e., the  4 I 15/2  state). In the fiber amplifiers of the present invention, stimulated emission occurs when an input signal at 1.5 μm is introduced into the optical fiber. The stimulated emission amplifies the input 1.5 μm signal. 
     According to another embodiment of the present invention, co-doping with ytterbium enhances population inversion and stimulated emission from the erbium  4 I 13/2  metastable state. More specifically, the enhancement process is explained in reference to energy levels shown in FIG.  1 . As shown in FIG. 1, the Yb 3+  excited state  2 F 5/2  are pumped from the Yb 3+ 2 F 7/2  ground state with the same pump wavelength that is used to excite upward transitions from the erbium ground state  4 I 15/2 . Energy levels of the excited ytterbium  2 F 5/2  state coincide with energy levels of the erbium  4 I 15/2  state permitting energy transfer (i.e. electron transfer) from the pumped ytterbium  2 F 5/2  state to the erbium  4 I 11/2  state. Thus, pumping ytterbium ionic energy states provides a mechanism for populating the metastable erbium  4 I 13/2  state, permitting even higher levels of population inversion and more stimulated emission than with erbium doping alone. 
     Ytterbium ions exhibit not only a large absorption cross section but also a broad absorption band between 900 and 1100 nm. Furthermore, the large spectral overlap between Yb 3+  emission ( 2 F 7/2 - 2 F 5/2 ) and Er 3+  absorption ( 4 I 15/2 - 4 I 13/2 ) results in an efficient resonant energy transfer from the Yb 3+ 2 F 5/2  state to the Er 3+   4 I 13/2  state. The energy transfer mechanism in a Yb 3+ /Er 3+  co-doped system is similar to that for cooperative upconversion processes in an Er 3+  doped system. However, interactions are between Yb 3+  (donor) and Er 3+  (acceptor) ions instead of between two excited Er 3+  ions. 
     Thus, the present invention utilizes either erbium doped or erbium/ytterbium co-doped phosphate glass fibers doped at concentrations orders of magnitude higher than what is available in silica fibers. The increased doping levels in the phosphate glass fibers do not suffer from a high cooperative upconversion rate. Thus, cooperative upconversion effects in phosphate glass at high levels of erbium doping concentration, while significantly smaller than in silica glass, can be further reduced in phosphate glasses by the introduction of ytterbium doping. Further, it can be seen from FIG. 2, a graph depicting energy transfer efficiency from Yb 3+  ions to Er 3+  ions, that the energy transfer efficiency is greater than 91% for the co-doping concentrations shown. FIG. 2 shows that 6 weight % Yb 2 O 3  and 2 weight % Er 2 O 3  produce measured lifetimes for the ytterbium  2 F 5/2  state of about 180 μs with a transfer efficiency to the erbium  4 I 13/2  state of 92%. Compared to silicate glasses, the large phonon energy in the phosphate host increases the transition probability for  4 I 11/2 - 4 I 13/2  relaxation which prevents the back energy transfer from Er 3+  to Yb 3+ . 
     In one embodiment of the present invention, optical fibers utilize a doped phosphate glass fiber containing the following ingredients by weight percentages: P 2 O 5  from 30 to 80 percent, Yb 2 O 3  from 0 to 12 percent, Er 2 O 3  from 2.5 to 12 percent, R 2 O from 0 to 5 percent L 2 O 3  from 5 to 30 percent, MO from 5 to 30 percent, where R 2 O is selected from the alkali metal oxide group consisting of Li 2 O, K 2 O, Na 2 O, Rb 2 O, and mixtures thereof, where the sum of the weight percentages of Yb 2 O 3  and Er 2 O 3  is 2.5% or greater, MO is selected from the alkaline earth oxide group consisting of BaO, BeO, MgO, SrO, CaO, ZnO, PbO and mixtures thereof, and L 2 O 3  is selected from the transition metal oxide group consisting of Al 2 O 3 , B 2 O 3 .Y 2 O 3 , La 2 O 3 , and mixtures thereof As a result, an active fiber length for 20 dB gain is expected to be less than 7 cm. 
     Thus, the high doping concentrations possible, the low cooperative upconversion rate, the high transfer efficiencies, and the high gains make doped phosphate glass fibers ideal for high gain optical fiber amplifiers. 
     Besides these properties, doped phosphate glasses in one embodiment of the present invention can be comprised of components which yield a temperature coefficient of refractive index close to zero or negative. Athermal performance can be achieved when the temperature coefficient of refractive index is negative. An optical path length for a glass is a product of a length of the glass and the refractive index. Achieving athermal performance results in a temperature coefficient for the optical path length close to zero, which can be described by the following equation: 
     
       
           G =α( n −1)+ dn/dt   (2) 
       
     
     where, G is the temperature coefficient of optical path length, cc is the linear coefficient of thermal expansion, dn/dt is the temperature coefficient of refractive index. Since o&amp; is always positive, dn/dt has to be negative in order to make G equal to zero. 
     In active laser glass elements, heating occurs from the pump source. In order to dissipate the heat, laser glass elements are typically gas or liquid cooled to maintain the laser glass elements within a specified operating temperature range. Cooling of the active laser glass elements creates a thermal gradient. The temperature near the center is higher than the temperature close to the cooling. When dn/dt is positive, the optical path length near the center becomes longer, causing a distortion of laser beam. For a typical silicate glass, the refractive index increases with temperature because polarizability of electron clouds in the silicate glasses increases with increased spacing (i.e. expansion of the material upon heating). 
     In optical fiber amplifiers, the mode profile of the optical fiber changes with the index of the core glass and the cladding glass. However, maintaining a temperature coefficient of the optical path length is not as critical as in a laser glass since the beam in the fiber amplifier is confined by the cladding layer. Nonetheless, a fiber amplifier for telecommunication application has to pass a standard Bellcore test, where the temperature changes from −40° C. to 80° C. The multi-component phosphate glasses must achieve a temperature coefficient of refractive index close to zero by adjusting components in the glass composition. Achieving a temperature coefficient of refractive index close to zero eliminates the influence of changing temperatures from heat generated by unused pumping power and changing environmental temperatures on the mode profile of the fiber. By adjusting BaO and ZnO content in the phosphate glass structure, an approximately zero temperature coefficient of refractive index can be achieved since BaO and ZnO exhibit negative and positive temperature coefficient of refractive index, respectively. Glass network intermediates such as B 2 O 3  and Al 2 O 3  are used to fortify the phosphate glass structure. The combination of an aluminum-oxygen tetrahedron or a boron-oxygen tetrahedron with a neighboring phosphorus-oxygen tetrahedron increases the number of bridging oxygen, resulting in a stronger glass structure. In one embodiment of the present invention, Al 2 O 3  in concentrations from 6 to 11 mole % is used to fortify the glass structure. 
     Tables 1, 2, and 3 (see below) list glass compositions in mole % of phosphate glasses designed and fabricated, the temperature coefficients of refractive index for aluminum phosphate, barium phosphate and zinc phosphate, and the predicted temperature coefficient of refractive index for the fabricated glasses. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Glass compositions designed and fabricated 
               
             
          
           
               
                 Glass 
                 P2O5 
                 Al2O3 
                 Er 2 O 3  + Yb 2 O 3  + La 2 O 3   
                 BaO 
                 ZnO 
               
               
                   
               
             
          
           
               
                 PZI 
                 63 
                 8.5 
                 3.0 
                 25.5 
                 0 
               
               
                 PZ2 
                 63 
                 8.5 
                 3.0 
                 25.5CaO* 
                 0 
               
               
                 PZ3 
                 63 
                 8.5 
                 3.0 
                 25.5MgO* 
                 0 
               
               
                 PZ4 
                 63 
                 8.5 
                 3.0 
                 0 
                 25.5 
               
               
                 PZ5 
                 63 
                 8.5 
                 3.0 
                 9.0 
                 16.5 
               
               
                 PZ6 
                 63 
                 8.5 
                 3.0 
                 19 
                 6.5 
               
               
                 PZ7 
                 63 
                 9.5 
                 3.0 
                 21 
                 4.5 
               
               
                 PZ8 
                 63 
                 8.5 
                 3.0 
                 23 
                 2.5 
               
               
                   
               
               
                 *where 25.5 CaO indicates CaO has been substituted for the BaO additive and 25.5 MgO indicates MgO has been substituted for the BaO additive.  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 The temperature coefficient of refractive index 
               
               
                 of common phosphate glasses 
               
             
          
           
               
                   
                 Glass 
                 dn/dt (10 −6 ) 
               
               
                   
                   
               
             
          
           
               
                   
                 Al(PO) 3   
                 +5.0 
               
               
                   
                 Ba(PO) 2   
                 −10.6 
               
               
                   
                 Zn(PO) 2   
                 +5.1 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Predicted temperature coefficient of refractive index 
               
               
                 of the fabricated glasses 
               
             
          
           
               
                   
                 dn/dt (10 −6 ) 
                   
               
             
          
           
               
                   
                 Glass 
                 Using Reference [1] 
                 Using Reference [2] 
               
               
                   
                   
               
             
          
           
               
                   
                 PZI 
                 −1.8 
                 −1.4 
               
               
                   
                 PZ2 
                 +0.3 
                 +3.4 
               
               
                   
                 PZ3 
                 +1.6 
                 +5.5 
               
               
                   
                 PZ4 
                 +2.2 
                 +6.5 
               
               
                   
                 PZ5 
                 +0.8 
                 +3.7 
               
               
                   
                 PZ6 
                 −0.8 
                 +0.6 
               
               
                   
                 PZ7 
                 −1.1 
                 0 
               
               
                   
                 PZ8 
                 −1.4 
                 −0.6 
               
               
                   
                   
               
             
          
         
       
     
     Although there are some numerical differences in the predicted temperature coefficient of refractive index using reference [1]: Optical Properties of Glass, Edited by Donald R. Uhlmann and Norbert J. Kreidl, The American Ceramic Society, Inc., 1991 and reference [2]: Optical Glasses (Chinese), Edited by Fuxi Gan, Academic Publication, Beijing, 1985, Table 3 indicates that the temperature coefficient of refractive index of the glass decreases when the ZnO content increases and the BaO content decreases. The temperature coefficient of refractive index can be adjusted by changing the content of ZnO and BaO. Glasses with a temperature coefficient close to zero can be obtained near the composition of PZ6. 
     Thus, according to another embodiment of the present invention, the phosphate glass fibers of the present invention can be composed with compositions such that a temperature coefficient close to zero or negative is realized and these compositions can be fortified with B 2 O 3 or Al 2 O 3 . 
     Phosphate Glasses 
     The following disclosure describes in more detail the manufacture and characterization of phosphate glasses suitable for utilization as core or cladding glasses in the phosphate glass fibers of the present invention. 
     First, two series of glass specimens, 64P 2 O 5 ·12Al 2 O 3 ·3.5(Er 2 O 3 +La 2 O 3 )·20.5MO (M=Mg, Ca, Ba) (mole %) and 64P 2 O 5 ·3.5(Er 2 O 3 +La 2 O 3 )·(21.5−x)Al 2 O 3 ·(11−x)BaO, (x=0, 3.5, 6.5, and 9.5) (mole %) were prepared to investigate the influence of glass composition on spectral properties, refractive index, and thermal properties. Compounds with less than 10 ppm of iron or copper were used as the starting chemicals for glass preparation. The mixed chemicals were melted in an electric furnace using a quartz crucible at 1250° C. The liquid was then held at temperature for thirty minutes. After fusing, the glass liquid was transferred to a platinum crucible, and the temperature was increased to 1350° C. Nitrogen gas was purged through the liquid to remove hydroxyl ions (OH −1 ). The liquid was cast into an aluminum mold. A variety of samples from these specimens were prepared for the measurements. The refractive index of glass was measured with a prism coupler (e.g., a Metricon Model 2010) at 632.8 nm and 1550 nm. The optically polished glass samples with a size of 2 cm×2 cm×0.5 cm were used in the measurements of the absorption spectrum. The thermal expansion coefficient, glass transition temperature, and softening temperature of each sample were measured on a dilatometer. The absorption spectra were recorded on a spectrophotometer (e.g., a Cary 5G). 
     The effect of glass compositions on the absorption linewidth of the  4 I 15/2 - 4 I 13/2  transition of the emission spectrum of erbium ions was investigated. The linewidth becomes increasingly important due to the emerging wavelength division multiplexing (WDM) technology. FIGS. 3 and 4 show the influence of glass composition on the refractive index and the effective linewidth. FIG. 3 shows that the refractive index of phosphate glass increased when the ionic size of alkaline earth metal increased from Mg, to Ca, to Ba. FIG. 4 illustrates the effect of the concentration of BaO and Al 2 O 3  on the refractive index of the samples. The refractive index increased when the concentration of BaO increased and the concentration of Al 2 O 3  decreased. The effective linewidth is defined as the integration of a band over wavelength and dividing by the maximum. FIG. 3 indicates that the effective emission linewidth for the  4 I 13/2 - 4 I 15/2  transition increases with increasing ionic size of alkaline earth ions. When the concentration of BaO increases and the concentration of Al 2 O 3  decreases, the effective emission linewidth of the  4 I 13/2 - 4 I 15/2  transition increases, as illustrated in FIG.  4 . 
     Emission spectra were measured on samples with a thickness of 0.4 mm to reduce reabsorption. The emission spectrum measurement at 1.5 μm was carried out using a 980 nm diode as a pumping source. Absolute emission cross section were calculated using McCumber theory. According to McCumber theory, the absorption and emission cross sections are related by equation (1) if the time required to establish a thermal distribution within each manifold is short compared with the lifetime of that manifold. 
     
       
         σ e ( v )=σ a ( v )exp[(ε− hv )/ kT]   (1) 
       
     
     where σ a  and σ e  are the absorption and stimulated emission cross section, respectively, v is the photon frequency, ε is the net free energy required to excite one Er 3+  ion from the  4 I 15/2 - 4 I 13/2  state at temperature T, h is the Planck constant, and k is the Boltzman constant. The absorption cross section was determined from the absorption coefficient. The peak absorption cross section for a 63P 2 O 5 ·8.5Al 2 O 3 ·3(Er 2 O 3 +La 2 O 3 )·9.5BaO·16LiO (mole %) glass is 0.75×10 20  cm 2 /ion at 1.5335 μm. The peak stimulated emission cross section is calculated to be 0.82×10 20  cm 2 /ion at 1.5335 μm. The absorption and fluorescence spectra in the phosphate glass bulk exhibit wide linewidths for  4 I 13/2 - 4 I 15/2  transition of erbium ions (48.63 nm). The absorption and fluorescence spectra of erbium doped phosphate glass bulk are illustrated in FIG.  5 . 
     Next, undoped glasses (no erbium or ytterbium ions) were fabricated as cladding glasses. In order to insure a low stress and birefringence in the fiber, similar thermal characteristics between the core glass and the cladding glass are required. To form waveguide in the core glass with good confinement, a slight lower refractive index of the cladding glass is also required. Theoretical calculation was performed based upon experimental results listed previously (FIGS.  3  and  4 ). Several melts were made to achieve a thermal behavior match and a required difference of refractive index by modifying the Al 2 O 3 , alkali ions and alkaline earth ions content. The thermal expansion coefficient, glass transition temperature, and softening temperature of each melt were measured on a dilatometer. The refractive index at various wavelengths was determined by prism coupling technique. Cladding glasses were developed with differences in thermal expansion coefficients between the cladding glasses and the core glass less than 3% in the temperature range from 80° C. to 400° C. In addition, glass transition temperatures and softening temperatures of these cladding glasses are also very close to that of the core glass (see Table 4 below). The 10% difference in thermal expansion coefficient between the core glass and the cladding glass is acceptable for fiber fabrication without serious stress in the fiber. Small differences in thermal expansion coefficient, glass transition temperature and softening temperature between the core and cladding glasses ensures high quality of the fiber. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Glass transition and softening temperatures of core and cladding glasses 
               
             
          
           
               
                   
                   
                 Glass transition 
                 Softening temperature 
               
               
                   
                 Glass type 
                 temperature (T g ) 
                 (T f ) 
               
               
                   
                   
               
               
                   
                 Core glass (P25) 
                 439° C. 
                 480° C. 
               
               
                   
                 Cladding 1-1 
                 435° C. 
                 479° C. 
               
               
                   
                 Cladding 2-1 
                 436° C. 
                 470° C. 
               
               
                   
                 Cladding 3-1 
                 443° C. 
                 486° C. 
               
               
                   
                   
               
             
          
         
       
     
     The respective compositions of a core glass co-doped with erbium and ytterbium and various cladding glasses are given as follows: 
     Core Glass P25: 63P 2 O 5 ·8.5Al 2 O 3 ·3 (Er 2 O 3 +La 2 O 3 +Yb 2 O 3 )·9.5BaO·16Li 2 O (mole %) 
     Cladding 1-1: 63P 2 O 5 ·10Al 2 O 3 ·1.5La 2 O 3 ·7BaO·17Li 2 O·1.5Na 2 O (mole %) 
     Cladding 2-1: 63P 2 O 5 ·11Al 2 O 3 ·4BaO·17Li 2 O·2.5Na 2 O·2.5CaO (mole %) 
     Cladding 3-1: 62P 2 O 5 ·12Al 2 O 3 ·4BaO·17Li 2 O·2.5Na 2 O·2.5CaO (mole %) 
     A good refractive index match with the core glass is another key parameter for the cladding glass besides the thermal behavior match. The refractive index of the cladding and core glasses is listed in the Table 5 (see below). The refractive index at 1.55 μm of cladding glass is 0.48% to 1.14% lower than the core glass P25, which results in numerical apertures from 0.149 to 0.230. With such values of numerical aperture, erbium doped optical fibers with a mode profile perfectly matched with standard telecommunication fiber can be fabricated. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Refractive index of core and cladding glasses 
               
             
          
           
               
                   
                 Refractive index 
               
             
          
           
               
                 Glass type 
                 632.8 nm 
                 830 nm 
                 1300 nm 
                 1550 nm 
               
               
                   
               
               
                 Core glass (P25) 
                 1.5431 
                 1.5389 
                 1.5318 
                 1.5290 
               
               
                 Cladding 1-1 
                 1.5365 
                 1.5309 
                 1.5249 
                 1.5217 
               
               
                 Cladding 2-1 
                 1.5298 
                 1.5250 
                 1.5187 
                 1.5158 
               
               
                 Cladding 3-1 
                 1.5257 
                 1.5206 
                 1.5150 
                 1.5116 
               
               
                   
               
             
          
         
       
     
     Finally, an ytterbium-doped glass composition of 63P 2 O 5 ·8.5Al 2 O 3 ·3 (Er 2 O 3 +La 2 O 3 +Yb 2 O 3 )·9.5BaO·16Li 2 O (mole %), chosen based upon modeling and the spectral characterization results of the non-ytterbium doped glasses, was melted at 1350° C. in a platinum crucible using high purity starting chemicals. This glass composition contains 3.5 weight % erbium. The refractive index of this glass composition was measured with a prism coupler at 632.8 nm, 1300 nm and 1550 nm. The absorption and emission cross sections were determined to be 0.75×10 −20  cm 2  and 0.82×10 −20  cm 2  at 1534 nm, respectively. The thermal expansion coefficient, glass transition temperature, and softening temperature of the ytterbium-doped core glass were measured to be 8.7×10 −6 ° C. −1 , 439° C., and 480° C., respectively. The cladding glass was designed to match the thermal performance of the ytterbium-doped core glass to ensure low stress in the fiber. The difference in thermal properties between the ytterbium-doped core glass and the designed cladding glass is less than 3%. 
     Fabrication and Characterization of Erbium-doped Phosphate Glass Fiber 
     In another embodiment of the present invention, a rod in tube technique was utilized to prepare single mode phosphate glass fibers. FIG. 6 illustrates the steps of producing the single mode phosphate glass fibers. Referring to FIG. 6, a phosphate glass ingot containing dopants is formed in step  200 . Once the ingot is formed, the ingot is cored to produce in step  210  a first core glass rod. The first core glass rod has a barrel of the glass rod polished in step  220 . In step  230 , a cladding glass tube (i.e. a tube without erbium or ytterbium dopants) is drilled to have an inside diameter within 0.1 mm of the outside diameter of the first core glass rod. In step  240 , the first core glass rod is placed inside the first cladding glass tube, and the assembly is drawn to form a second core glass rod. The second core glass rod is placed, in step  250 , inside a second cladding glass tube, and the assembly is drawn into a single mode fiber. Fiber drawings were performed in an argon gas atmosphere to reduce absorption of water from air exposure which causes fluorescence quenching of Er 3+  ions. 
     FIG. 7 shows schematically the representative steps of the present invention. A bulk glass rod (not shown) was doped for example with 3.5 weight % erbium. A first core glass rod  2  was drilled from a bulk glass, and the barrel of the first core glass rod  2  was polished. Both inside and outside surfaces of a glass cladding tube  4  made from the cladding glass were polished. The inside diameter of the cladding tube  4  was matched to within 0.1 mm of the diameter of the first core glass rod  2 . Next, the 3 mm core glass rod  2  and the 12 mm cladding tube  4  were drawn into a second core glass rod  6  with a 3 mm outside diameter. The second core glass rod  6  together with a second cladding tube  8  were drawn into a single mode fiber  10 . The fiber drawing was performed at 765° C. No plastic coating was applied to the fiber. 
     After fabrication of erbium doped glass fibers, gain characterizations of the fibers were made. FIG. 8 shows the experimental setup for gain characterization. An erbium-doped fiber  20  whose core contained 3.5 weight % erbium was pumped using a 980 nm laser diode  22  stabilized by fiber Bragg gratings (not shown). An external cavity tunable laser diode  24 , tunable from 1530 nm to 1570 nm was used as a signal source. An attenuator  26  was used to vary the input signal strength. Pump and signal beams were combined by a 980/1550 fiber multiplexer (not shown). The erbium-doped fibers were placed on V-grooves. Two fiber in-line isolators  28  for the signal were used. The output fiber is connected to the optical spectrum analyzer (OSA)  30 . A pump monitor  32  was used to ensure that the pump signal did not vary during measurement of optical gain. 
     A single mode fiber consisting of a 3.5 weight percent concentration erbium doped phosphate glass core with an undoped glass cladding was fabricated using the rod in tube technique previously discussed. The phosphate glass fibers with the 3.5 weight percent concentration of erbium were fabricated by the rod in tube technique. One phosphate glass rod uniformly doped with 3.5 weight % erbium and two phosphate glass tubes without erbium doping were used in the drawing processes. A single mode fiber was obtained. The fiber core was 4 μm in diameter with an erbium concentration of 3.7×10 −20  cm −3 . 
     A 980 nm semiconductor laser diode was used as pumping source. An external cavity tunable laser diode, tunable from 1530 nm to 1570 nm was used as a signal source. A net gain of 15.5 dB was achieved at 1535 nm from a 5.1 cm long fiber. The internal gain is 17.5 dB. The net gain per unit length is 3 dB/cm, which is the highest erbium doped fiber net gain ever obtained. 
     The refractive indices were measured by the prism coupling method. The numerical aperture of erbium-doped fiber was calculated to be 0.25 using the measured refractive indices of the core and cladding glasses at the wavelength of 1535 nm. Absorption cross section was calculated from the measured absorption spectrum. The effective linewidth is 46 nm for the  4 I 15/2 - 4 I 13/2  transition. Emission cross section was calculated from the absorption cross section and emission spectrum by McCumber theory. The emission spectrum and fluorescence lifetime for the  4 I 13/2 - 4 I 15/2  transition of Er +3  were measured in a 2 mm long phosphate fiber at low pump power. The measured emission spectrum for 1.5 μm band in the fiber was similar to that in the bulk material. The propagation loss of 0.1 dB/cm was measured at a wavelength of 1.3 μm. 
     The rod-in-tube technique was once again utilized to fabricate erbium doped phosphate glass fibers. A core glass rod was drilled from the bulk glass material formed in the melt at 1350° C., and the barrel of the rod was polished. The inside diameter of a cladding tube was matched to within 0.1 mm with the diameter of the core glass. The fiber drawing temperature was 765° C. As before, two drawings were used. The erbium ion concentration of the fiber core is 3.5 weight %. No plastic coating was applied to the fiber. The numerical aperture of the erbium-doped fiber was calculated to be 0.216 at 1550 nm. Fibers with core diameters of 4 μm, 5 μm, and 6 μm were fabricated. The propagation loss of 0.3 dB/cm was estimated at 1.3 μm using the cut-back method. 
     A fiber with a core diameter of 5 μm was tested. A 5.1 cm long erbium doped fiber was pumped using a 980 nm laser diode stabilized by fiber Bragg gratings. An external cavity tunable laser diode, tunable from 1530 nm to 1570 nm was used as a signal source. Pump and signal beams were combined by a 980/1550 fiber multiplexer. 245 mW of pump power were available at the output of the multiplexer. The 5 μm fiber was placed on a V-groove. Two in-line isolators for the signal were used. The output fiber was connected to the optical spectrum analyzer. The coupling loss was estimated to be 1 dB at 1.3 μm. 
     FIG. 9 illustrates the net gain and noise figure versus the pump power at 1535 nm and 1550 nm for the 3.5 weight % erbium, 5 μm core diameter erbium-doped fiber of the present invention. The input signal power was −31 dBm. A net gain of 15.5 dB was achieved at 1535 nm, and the internal gain is 17.5 dB. The net gain per unit length of 3 dB/cm, was once again obtained. The internal noise figures are approximately 1 dB below noise figures shown in FIG. 9, which indicates that a noise figure close to 4 dB could be readily achieved by reducing the coupling losses. Gain saturation was not observed in this experiment, suggesting a higher gain is achievable at higher pumping power. 
     According to the present invention, doping with erbium improves the gain of the phosphate glass fibers Based on internal modeling results, the pump power needed to reach a 15 dB net gain is around 100 mW, given the high energy transfer efficiency. 
     FIG. 10 illustrates net gain as a function of signal wavelength with different signal powers for the 5 μm core diameter erbium-doped optical fiber of the present invention. The pump power is 245 mW. The signal wavelength was tuned from 1530 to 1570 nm. As shown in FIG. 10, the gain profile extends towards longer wavelengths and decreases dramatically at shorter wavelengths. This profile agrees well with the above-noted absorption and emission cross section results and numerical modeling. FIG. 10 shows that the net gain decreases significantly near the peak wavelength when the signal power was increased from −6 dBm to 0 dBm. This decrease is caused by gain saturation. 
     FIG. 11 shows gain saturation at 1535 nm and 1550 nm for the 5 μm core diameter erbium-doped fiber of the present invention. The saturation output power (defined as the output power where the gain is 3 dB below the maximum gain) at 1535 nm and 1550 nm are 8 dBm and 10 dBm, respectively. The input saturation power at 1550 is 5.5 dBm. This result agrees with the results in FIG. 10 where the net gain at the longer wavelength with different signal power does not change too much. 
     Thus, the phosphate fibers of the present invention, manufactured according to one embodiment of the present invention using a rod in tube technique, show minimal inefficiency from cooperative upconversion. Consequently, with respect to manufacture, cost, and performance, the phosphate fibers of the present invention are integrated into optical components such as splitters, couplers, waveguide gratings, and modulators. An integrated approach dramatically reduces component cost and meets the demand for high amplification in metro and local network applications. 
     Amplifying Splitter 
     An amplifying splitter  40  of the present invention shown in FIG. 12 includes a splitter  42  and a phosphate glass fiber amplifier  44  of the present invention. The amplifying splitter  40  is a 1×n device with one input port  46  and multiple output ports  48  for dividing optical signals about 1.54 μm, the optical fiber communication window. The amplifying splitter  40  divides optical signals without any intensity loss by employing the phosphate glass fiber amplifier  44  with high gain per unit length. The amplifying splitter  40  integrates a passive optical component, such as for example the splitter  42 , with a high performance, a compact fiber amplifier, such as for example the fiber amplifier  44 . The fiber amplifier  44  is a doped phosphate glass fiber  50 , inserted and epoxied to a groove, such as for example a V-groove  52 , and coupled to an optical pump  54 , such as for example a solid state laser emitting at 980 nm. The optical pump  54  propagates laser light in a direction opposite to a propagation direction of the split signals. The laser light is coupled into the phosphate glass fiber  50  by a 980/1550 fiber multiplexer  56 . The fiber amplifier  44  and the splitter  42  are bonded to a substrate  58  to form a quasi-monolithic device. The amplifying splitter  40  is pigtailed with single mode fibers and packaged in a compact, rugged environmentally stable case (not shown). The amplifying splitter  40  exhibits high gain, high port-to-port uniformity, low insertion loss, low back reflection, and low polarization dependent loss. 
     Existing splitters are only 50% efficient for a single source, two output port (1×2) device. That is, the device divides the light intensity. In a more typical source, a sixteen output port (1×16) splitter, the loss would be 94%. To counter this loss, current networks must employ expensive optical power boosting amplifiers. 
     The amplifying splitter  40  of the present invention with high gain compensates for losses in splitters. The amplifying splitter uses low cost erbium and ytterbium co-doped phosphate glass fiber amplifier technology. The amplifying splitter  40  is a device that can be used in a fiber optic network to split an optical fiber input into several fiber optic outputs, without any loss of output light intensity from each port. Optical splitter use is expected to grow strongly as fiber networks reach closer and closer to end users, such as in short haul terrestrial systems and metro optical networks. The amplifying splitter  40  finds application in fiber-to-the-curb and fiber-to-the-home applications. 
     Besides using a fiber multiplexer to couple laser light to a fiber amplifier, the fiber amplifier  44  is pumped, according to one embodiment of the present invention, with a low cost multi-mode diode laser. The multi-mode laser emits light from 900 to 1000 nm. FIG. 13 is a schematic illustrating pumping of a fiber amplifier with a multi-mode diode laser lasing near 980 nm. A silica fiber  60  introduces an optical signal to a single fiber amplifier  62 . The single fiber amplifier  62  includes a doped phosphate single-mode glass fiber  64  attached to a substrate  66 . A multi-mode diode laser  68  pumps the single-mode fiber  64 . The beam diameter of the diode laser is focused to approximately 80 μm. The amplified signal is transmitted forward to an output silica fiber  70 . 
     Further still, as shown in FIG. 14, more than a single fiber can be pumped with a multi-mode laser. A fiber array amplifier  72  is utilized to amplify input optical signals. The fiber array amplifier  72  includes a multi-mode diode laser bar  74  which pumps a side of an array  76  of doped phosphate glass fibers. The array  76  of doped fibers exhibit high gain per unit length and are placed orthogonal to the multi-mode diode laser bar  74 . The length of the rare-earth doped fibers is a few centimeters, such as for example 5 cm. The multi-mode diode laser bar  74  excite the array  76  from the side and/or the top of the substrate. As shown in FIG. 14, the phosphate glass fibers are placed within two V-grooved substrates  78 . Inside surfaces of two V-grooved substrates  78  are coated with a metallic coating  80  to reduce scattering of the pump laser, ensuring a high pumping efficiency. Less than 100 mW absorbed pump is needed to excite each fiber. With this approach, significant cost reduction is realized since the cost of multi-mode laser diode bar is significantly lower than the cost of a single mode diode laser. 
     The fiber array amplifier  72  could be used in an amplifying splitter by adding the fiber array amplifier  72  to the passive splitter  42  after the input signal has been split. The fiber amplifier  44  may optionally compensate for optical loss in the passive splitter  42 . 
     FIG. 15 is a schematic diagram illustrating the array of glass fibers placed on a surface of a substrate  78  (as shown in FIG. 15 a ), between two substrates (as shown in FIG. 15 b ), or in a fiber bundle  80  (as shown in FIG. 15 c ). The multi-mode diode laser  74  is shown at several alternative positions by which the array  76  of phosphate glass fibers can be pumped. The optical transparency of the substrates  78  and the high absorption strength of the high gain doped phosphate fiber enables pumping of the fiber amplifier. 
     Amplifying Combiner 
     FIG. 16 illustrates an amplifying combiner  82  of the present invention. Combiner  82  integrates a passive wavelength combiner  84  with a phosphate glass fiber amplifier  86 . The combiner  84  and the amplifier  86  are separately designed and optimized before being assembled together into one package. The amplifying combiner  82  is a n×1 device with n multiple input ports  88  and one output port  90  for combining a variety of optical signal wavelengths around 1.54 μm, the optical fiber communication window. The amplifying combiner  82  combines the n multiple ports  88  without any intensity loss by employing the phosphate glass fiber amplifier  86  with extremely high gain per unit length. The fiber amplifier  86  is a co-doped phosphate glass fiber  92 , inserted and fixed to a V-groove  94 , coupled to an optical pump  96  such as for example a solid state laser emitting at 980 nm. The optical pump  96  propagates laser light in a direction opposite to a propagation direction of the combined signals. The laser light is coupled into the phosphate glass fiber  92  by a 980/1550 fiber multiplexer  98 . The wavelength combiner  84  and fiber amplifier  86  are bonded to a substrate  100  to form a quasi-monolithic device. The amplifying combiner  82  is pigtailed with single mode fibers and packaged in a compact, rugged environmentally stable case (not shown). The amplifying combiner  82  exhibits high gain, low insertion loss, low back reflection, and low polarization dependent loss. 
     The amplifying combiner  82  is a device which could be used in a fiber optic network to combine and amplify optical signals. Use of the amplifying combiner  82  is expected to grow strongly as DWDM devices evolve and become more prevalent in optical networks. 
     As with the amplifying splitter  40 , the fiber array amplifier  72  can be used in an amplifying combiner by adding the fiber array amplifier  72  to the combiner  84  before the input signals have been combined. The fiber amplifier  86  may or may not be utilized to compensate for optical loss in the combiner  84 . 
     Amplifying Arrayed Waveguide Gratings 
     As shown in FIG. 17, the present invention includes amplifying arrayed waveguide gratings  102  which integrate arrayed waveguide gratings (AWG)  104  with a compact phosphate glass fiber amplifier  106 . The typical attenuation for AWG DWDM devices is 5 to 7 dB. The compact co-doped phosphate glass fiber amplifiers of the present invention can compensate this attenuation in order to simplify the system design. The amplifying arrayed waveguide gratings  102  is a device which resolves optical signal wavelengths around 1.54 μm, the optical fiber communication window, into specific frequency signals (i.e signals at 1.50 μm, 1.52 μm, 1.54 μm, 1,56 μm, etc.). The fiber amplifier  106  is a doped phosphate glass fiber  108 , inserted and fixed to a V-groove  110 , coupled to an optical pump  112  such as for example a solid state laser emitting at 980 nm. Light from the solid state laser is coupled into the phosphate glass fiber  108  by a 980/1550 fiber multiplexer  114 . The arrayed waveguide gratings  104  and the fiber amplifier  106  are bonded to a substrate  116  to form a quasi-monolithic device. The amplifying arrayed waveguide gratings  102  is packaged in a compact, rugged environmentally stable case (not shown). The amplifying arrayed waveguide gratings  102  exhibits high gain, low insertion loss, low back reflection, and low polarization dependent loss. 
     Utilization of the amplifying arrayed waveguide is expected to grow strongly as DWDM devices evolve and become more prevalent in optical networks. 
     As with the amplifying splitter  40 , the fiber array amplifier  72  could be used in an amplifying arrayed waveguide gratings  102  by adding the fiber array amplifier  72  to the passive waveguide grating  104  after the input signals have been resolved. The fiber amplifier  106  may or may not be utilized to compensate for optical loss in the grating  104 . 
     Amplifying Modulator 
     Lithium niobate modulators are known in the art and have the following advantages: compatibility with optical fiber, high frequency bandwidth (up to 40 GHz), low driving voltage, and rugged and all solid state construction. A further advantage of a lithium niobate modulator is that multiple optical components can be integrated into a single chip. In a similar approach to the above-mentioned splitter, combiner, and arrayed waveguide gratings, an amplifying modulator  118  of the present invention as shown in FIG. 18 integrates a lithium niobate modulator  120  with a compact phosphate glass fiber amplifier  122 . This integration provides a mechanism to compensate for insertion loss from the lithium niobate modulator  120 . The amplifying modulator  118  compensates for intensity loss from the lithium niobate modulator  120  by employing the fiber amplifier  122  with extremely high gain per unit length. The fiber amplifier  122  is a doped phosphate glass fiber  124 , inserted and fixed to a V-groove  126 , coupled to an optical pump  128  such as for example a solid state laser emitting at 980 nm. The laser light is coupled into the phosphate glass fiber  124  by a 980/1550 fiber multiplexer  130 . The amplifying modulator  118  and the fiber amplifier  122  are bonded to a substrate  132  to form a quasi-monolithic device. The amplifying modulator  118  is packaged in a compact, rugged environmentally stable case (not shown). The amplifying modulator  118  exhibits high gain, low insertion loss, low back reflection, and low polarization dependent loss. 
     Typical attenuation for a lithium niobate modulator can be between 0.5 to 3 dB. The low cost erbium and ytterbium co-doped phosphate glass fiber amplifier of the present invention allows for mated structures to the lithium niobate integrated optical component chip package and provides amplification before sending or receiving an optical signal. The amplifying modulator will help substantially in management of power losses in metro and local loop applications. 
     As with the amplifying splitter  40 , the fiber array amplifier  72  could be used with an amplifying modulator  118 . Adding the fiber array amplifier  72  to the modulator  118  enables amplification of multiple input signals before modulation. 
     Large Core Fiber Amplifier 
     FIG. 19 is a schematic illustration of a large core fiber amplifier  160  of the present invention. The core diameter ranges from 50 μm to 300 μm. Typically in fiber amplifiers, the light-guiding region (i.e., the core of the fiber) is only about 5 μm in diameter. Such a small core diameter allows only a single-mode propagation of the light at a signal wavelength around 1550 nm. According to the present invention, the high gain coefficient enables the use of short amplifying fibers that can be assembled in V-grooves without bending the fibers. This compactness enables the use of large core fibers that are not single-mode. Due to the short length and stable and rugged packaging, light that is coupled to the fundamental mode of the fiber, remains mostly in the fundamental mode with very little coupling to the other modes of the fiber. As shown in FIG. 19, light is coupled, from input single-mode fiber  162 , predominantly into the fundamental mode of a large core fiber  164  using conventional micro-optic techniques, such as for example a collimating lense  166 . A key benefit of this embodiment is that high power 980 nm multi-mode light-emitting laser diodes  168  pump the core fiber  164  by introducing the laser light with a lense  170  through a split mirror  172  onto the core fiber  164 . Light is coupled from the multi-mode laser diodes  168  into the fiber core with high efficiency, which is not possible with the small core diameter fiber amplifiers. The multi-mode laser diodes have an emitting cross-sectional area on the order of 1 μm×100 μm. A second collimating lense  174  focuses the light onto an output fiber  176 . 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.