Abstract:
Electroluminescent waveguide amplifier and methods for amplifying optical data signals in a fiber optical telecommunications system to achieve signal enhancement that compensates for losses incurred by attenuation, optical splitting, and routing through the optical communication system. The waveguide amplifier ( 30 ) includes an electroluminescent active layer ( 38 ) having a host medium doped with luminescent dopant atoms capable of amplifying a propagating optical data signal ( 45 ) by stimulated emission of photons ( 41 ). Confining and insulating cladding layers ( 36, 40 ) surround the active layer ( 38 ) and confine the propagating optical data signals ( 45 ) being amplified to the active layer ( 38 ) and cladding layers ( 36, 40 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT Application Serial No. PCT/US2004/021074 filed on Jun. 29, 2004 and claims the benefit of U.S. Provisional Application No. 60/483,710 filed on Jun. 30, 2003, the disclosures of which are hereby incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     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 Army Research Grant No. DAAD 19-02-2-0014. 
     
    
     FIELD OF THE INVENTION  
       [0003]     The present invention relates to optical fiber telecommunications systems and, in particular, to apparatus and methods for amplifying optical data signals in an optical fiber telecommunications system.  
       BACKGROUND OF THE INVENTION  
       [0004]     Modern optical fiber telecommunications systems transfer optical data signals over long distances with relatively low loss and minimal attenuation. A modulated light source or light source and modulator comprising a transmitter transmits information-modulated optical data signals at one or more distinct wavelengths over an optical fiber, which conveys the optical data signals to a light receiver. The intensity of the optical data signals is periodically amplified to compensate for signal attenuation from distribution and component-insertion losses. Conventional amplification devices boost the optical data signals without any conversion of the light into an electrical signal.  
         [0005]     Rare earth doped glasses in fiber form are a familiar amplification medium in optical fiber communication systems. The most interest has been directed towards erbium-doped fiber amplifiers (EDFA&#39;s). Although EDFA&#39;s present many advantages and can be used in a wide array of optical fiber telecommunication systems, a significant disadvantage is that EDFA&#39;s are not compact structures and typically require an amplifier length on the order of several meters. Erbium-doped waveguide amplifiers (EDWA&#39;s), which are related to EDFA&#39;s, combine the potential for large optical gains with a relatively small size and the ability to integrate the amplifier with other components such as optical taps (for signal and pump monitoring), splitters and other common integrated optical components on a single platform.  
         [0006]     EDFA&#39;s and EDWA&#39;s operate on the same physical principles. A waveguide glass structure, formed from a material such as silica, phosphate glasses, and soda lime glasses, is doped with atoms of the rare earth erbium (Er). An optical system injects 1.55 μm optical data signals in the C-band to be amplified in the waveguide along with pump light from an optical pumping source, usually a laser, emitting optical radiation typically in the 0.8 μm to 1 μm range. The erbium atoms mediate the transfer of energy from the optical pumping source to the optical data signals via absorption at the pump wavelength and stimulated emission at the signal wavelength, which yields amplification of the light forming the optical data signals.  
         [0007]     A principal difficulty with EDWA&#39;s, as compared with EDFA&#39;s, is that a high gain must be achieved over a short distance, which requires doping the waveguide glass structure with a relatively high optically-active Er concentration. High Er concentrations, however, introduce gain limiting effects, such as cooperative up-conversion interactions between Er ions, and concentration quenching. The pump power of the optical pumping source must be increased to compensate for these limiting effects, which can lead to excited state absorption that dramatically reduces pump efficiency.  
         [0008]     Successful integration of waveguide optical amplifiers on a silicon platform necessitates a material system having high amplification capability and increased functionality. Low-cost metro communication systems and high-speed microprocessor integration, among other applications, are contingent upon integrating optical amplifiers with optical components and microelectronics. Unfortunately, the use of EDWA&#39;s in such integrated systems is limited primarily by the need for a high Er dopant concentration and the necessity of an optical pumping source.  
         [0009]     Semiconductor optical amplifiers (SOA&#39;s) provide a compact alternative to EDFA&#39;s and EDWA&#39;s for light amplification. SOA&#39;s have a device structure similar to semiconductor Fabry-Perot laser diodes. However, optical feedback (e.g., the lasing effect caused by reflection between cavity mirrors defining a resonator cavity) is eliminated and low insertion loss is achieved by angle cleaving the input and output facets and applying anti-reflection coatings on the input and output facets. SOA&#39;s rely on electrically-stimulated intrinsic bandgap emission, which eliminates the need for an optical pumping source as in EDFA&#39;s and EDWA&#39;s. The emission wavelength is determined by bandgap engineering, such as by appropriately adjusting the composition of constituent compound semiconductors. Contemporary semiconductor processing has advanced to the point that SOA&#39;s can be produced at a significantly lower cost than EDWA&#39;s and EDFA&#39;s, present a smaller device footprint, and include a smaller parts count.  
         [0010]     With reference to  FIG. 1 , a typical conventional SOA  10  includes an active layer  12  sandwiched between lower and upper confining layers  14 ,  16  on a single crystal substrate  18 , a lower electrode  20  on the substrate  18 , a contacting layer  22  covering the upper confining layer  16 , and a stripe electrode  24  formed in an oxide layer  26  covering the contacting layer  22 . The active layer  12  of the SOA  10  provides electrically-stimulated intrinsic emission from the bandgap valence and conduction levels when sufficient DC voltage or potential is applied across the electrodes  20 ,  24 . The single crystalline semiconducting layers comprising the device heterostructure in the SOA (i.e., active layer  12 , confining layers  14 ,  16  and contacting layer  22 ) are fabricated by complex epitaxial crystal growth techniques, such as molecular beam epitaxy (MBE) or metallorganic chemical vapor deposition (MOCVD). These growth techniques are very expensive to implement and time consuming so that process throughput is limited. Moreover, the selection of an amplification wavelength is limited by the band-gap of constituent semiconductor material(s).  
         [0011]     SOA&#39;s have numerous disadvantages that limit their use for light amplification in fiber optic telecommunications systems. For example, low insertion losses are difficult to achieve in SOA&#39;s, which limits the coupling efficiency of the optical data signals into and out of the device. The gain of SOA devices is nonlinear and exhibits a polarization dependence due to the device geometry and dimensions. Moreover, it is also not practical to configure an SOA so that the entire amplifying region comprises an optical distribution device, such as integrating the SOA with splitters, multimode interference (MMI) couplers, arrayed waveguide gratings, and the like.  
         [0012]     What is needed, therefore, is an amplifier structure and method for amplifying optical data signals transmitted by optical fibers that does not require an optical pumping source for achieving amplification and that has an active layer that can be formed without resort to single crystal growth techniques.  
       SUMMARY OF THE INVENTION  
       [0013]     According to principles of the present invention, a waveguide amplifier includes an electroluminescent active layer consisting of a host medium doped with luminescent dopant atoms capable of amplifying a propagating optical data signal by stimulated emission of photons and a pair of electrodes supplying electrical excitation to the active layer when energized. The waveguide amplifier may further include a pair of electrically-insulating cladding layers disposed on opposite sides of the active layer. The cladding layers confine propagating light to the active layer. The waveguide amplifier may further include a low-reflection device facet receiving an optical data signal and directing the optical data signal into at least the active layer for amplification to create an amplified optical data signal and a low reflection output facet directing the amplified optical data signal out of the active layer to the surrounding environment.  
         [0014]     The electroluminescent waveguide amplifier (ELWA) of the invention is compact and relies upon electrical excitation, rather than pump light from an optical pumping source, to obtain high gains. This aspect of the invention represents a significant technological advance over conventional EDFA&#39;s and EDWA&#39;s. The gain medium or host material of ELWA&#39;s is easily fabricated as a simple amorphous thin film coating, similar to EDWA&#39;s, and does not require the use of sophisticated epitaxial growth techniques as required in the fabrication of SOA&#39;s. The gain medium of ELWA&#39;s may be electrically pumped (i.e., excited), as are SOA&#39;s, which eliminates the need for an optical pump source. The host material of ELWA&#39;s has a refractive index appropriate for a waveguide core, is compatible with a waveguide cladding material, and is capable of producing emission from embedded rare earth ions or other luminescent dopants.  
         [0015]     The ELWA&#39;s of the invention may be fabricated using inorganic host materials for enhanced compatibility with optical fibers formed from inorganic materials (e.g., silica) or organic host materials for enhanced compatibility with optical fibers formed from organic materials (e.g., plastics such as poly-methylmethacrylate (PMMA)). The amplification wavelength in ELWA&#39;s is determined by the selection of one or more luminescent dopant(s) and is not restricted by the band-gap of semiconductor material, as is true of SOA&#39;s. This represents a significant improvement over conventional SOA&#39;s. Furthermore, an ELWA can be designed to have a lower intrinsic optical attenuation than an SOA because an ELWA may rely on a highly-optically transparent material (e.g., oxide glasses, polymers) as a host material for the luminescent dopant. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Various advantages, objectives, and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings.  
         [0017]      FIG. 1  is a schematic cross-sectional view of a semiconductor optical amplifier in accordance with the prior art.  
         [0018]      FIG. 2A  is a schematic side cross-sectional view of an electroluminescent waveguide amplifier in accordance with the principles of the present invention.  
         [0019]      FIG. 2B  is a schematic end cross-sectional view of the electroluminescent waveguide amplifier of  FIG. 2A .  
         [0020]      FIG. 2C  is a diagrammatic view illustrating the electronic transition energy levels of the dopant in the active layer and photon emission from  
         [0021]      FIG. 3A  is a schematic end cross-sectional view of an electroluminescent waveguide amplifier in accordance with an alternative embodiment of the invention.  
         [0022]      FIG. 3B  is a schematic end cross-sectional view of an electroluminescent waveguide amplifier in accordance with an alternative embodiment of the invention.  
         [0023]      FIG. 4  is a schematic view of a platform integrating electroluminescent waveguide amplifiers of the invention with signal monitoring circuitry and waveguide devices. 
     
    
     DETAILED DESCRIPTION  
       [0024]     The present invention is directed to an electroluminescent waveguide amplifier that includes an electroluminescent active layer consisting of a host medium doped with luminescent atoms that amplify propagating signal light or optical data signals through stimulated emission and cladding layers disposed between the active layer and the electrodes, which confine propagating light having the form of optical data signals to the active layer and the cladding layers. The characteristics of the cladding layers also permit coupling of electrical excitation from the device electrodes to the active layer.  
         [0025]     With reference to  FIGS. 2A-2B , an electroluminescent waveguide amplifier  30  in accordance with the principles of the invention includes a substrate  32 , an electrode  34  applied to one surface of the substrate  32 , a lower cladding layer  36  applied to the opposite surface of the substrate  32 , an active layer  38  applied on the lower cladding layer  36 , an upper cladding layer  40  applied on the active layer  38 , and a stripe electrode  42  applied on an upstanding ridge  44  formed in the upper cladding layer  40 . The refractive index of the cladding layers  36 ,  40  is less than the refractive index of the active layer  38 .  
         [0026]     An input optical fiber  46  ( FIG. 2A ) supplies optical data signals  45  to the electroluminescent waveguide amplifier  30 , which propagate in a confined manner within a confined region  39  bounded by the cladding layers  36 ,  40  to an output optical fiber  48  ( FIG. 2A ). The intensity of the optical data signals  45  traveling from the input optical fiber  46  to the output optical fiber  48  is increased or amplified by stimulated emission of photons  41  ( FIG. 2C ) from the excited state of a dopant present in the host material of the active layer  38 .  
         [0027]     Although the electroluminescent waveguide amplifier  30  is depicted as having a linear device structure having uniform width features, a person of ordinary skill in the art will appreciate that different device geometries may be utilized. For example, the electroluminescent waveguide amplifier  30  may be implemented in a compact design, such as a coiled geometry, which effectively lengthens the optical path over which light amplification occurs while conserving space on the substrate  32 .  
         [0028]     With continued reference to  FIGS. 2A-2B , the substrate  32  may be any suitable substrate material having a smooth, relatively flat surface finish, such as silicon. Generally, the substrate  32  should be a material in which optical distribution devices, such as splitters, MMI couplers, and arrayed waveguide gratings, may be fabricated. The electrodes  34 ,  42  are formed from any electrically-conductive material, such as indium-tin-oxide (ITO), aluminum (Al), magnesium (Mg), calcium (Ca), indium (In), or gallium nitride (GaN).  
         [0029]     The host material of the active layer  38  may be any low crystallinity, non-crystalline or, preferably, amorphous material that is optically transparent at the amplified wavelength and that is capable of incorporating optically-active luminescent dopant atoms at a concentration effective to produce stimulated light emission of photons  41  at one or more wavelengths due to electronic transitions between energy levels  43   a  and  43   b , as diagrammatically shown in  FIG. 2C . In addition, the host material of the active layer  38  must be capable of either transporting electrons or holes as a semiconductor or undergoing electrical breakdown to produce hot electrons or holes, as is characteristic of an insulator, for exciting the luminescence centers supplied by the dopant. The host material of the active layer  38  must also exhibit compatibility with the material constituting the cladding layers  36 ,  40 .  
         [0030]     Among the suitable inorganic host materials for active layer  38  are oxides including, but not limited to, ZnSiGeO, SiGeO, BaMgAlO, InGaAlO, and YGeO, sulfides including, but not limited to, ZnMgSSe, SrInAlGaS, and BaInAlGaS, nitrides such as InAlGaN, arsenides such as AlGaAs, phosphides such as InAlGaP, and fluorides including, but not limited to, ZnF, CaF, and GdF. Suitable organic hosts include, but are not limited to, Alq3, poly-pheny-lene (PPP), poly-phenylene-vinylene (PPV), poly(N-vinylcarbazole) (PVK), poly(3-alkylthiophene) (PAT), oligo(p-phenyleneviny-lene) (OPV), and poly(methyl methacrylate) (PMMA). These and other potential organic hosts are described in “The Electroluminescence of Organic Materials” by Ulrich Mitschke and Peter Bäuerle and published in J. Mater. Chem., 2000, 10, 1471-1507, the disclosure of which is hereby incorporated by reference herein in its entirety.  
         [0031]     The dopant in the active layer  38  may be any element having electronic transition levels that can result in an inverted population of energy levels at a characteristic wavelength when incorporated into a wide band-gap semiconductor. Suitable dopants for inorganic host materials include elements selected from the Periodic Table, such as elements from the Transition metal series including chromium (Cr), titanium (Ti), manganese (Mn), copper (Cu), zinc (Zn), and silver (Ag), Rare Earth elements from, for example, the Lanthanide metal series including cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), and ytterbium (Yb), and other metals, such as lead (Pb). Typically, the elemental concentration of the dopant in inorganic host materials ranges from a minimum of about 0.1 at. % to a maximum of about 10 at. %. Suitable dopants for inorganic host materials have the form of organic complexes.  
         [0032]     A particularly preferred insulating material, that experiences suitable electrical breakdown, is zinc silicate-germanate (Zn 2 Si 0.5 Ge 0.5 O 4 ). Erbium as a dopant species in zinc silicate-germanate produces stimulated emission at about 1.55 μm, which is centered on the C-band used in optical fiber telecommunications systems. Similarly, praseodymium as a dopant species in zinc silicate-germanate produces stimulated emission at about 1.3 μm, which is centered on the L-band used in optical fiber telecommunications systems.  
         [0033]     With continued reference to  FIGS. 2A-2B , the active layer  38  is an amorphous thin film formed by, for example, physical deposition by sputtering or evaporation, laser ablation, or spin-on deposition. The dopant species can be incorporated into the semiconductor material during deposition by in situ methods or introduced into the semiconductor material post-deposition by a conventional technique, such as ion implantation or diffusion. The concentration of the dopant in the active layer  38  may be homogeneous or, in certain embodiments of the invention, may be inhomogenous (e.g., a Gaussian profile) in either the lateral direction parallel to the direction of light propagation or in the transverse direction perpendicular to the direction of light propagation. In addition, the refractive index of the active layer  38  may likewise be inhomogenous in either the lateral or the transverse direction, which may eliminate the necessity of ridge  44  for accomplishing transverse confinement.  
         [0034]     The active layer  38  may contain one or more sublayers that guide the propagating optical data signal  45  and/or one or more sublayers that serve the purpose of optical amplification. In particular, the active layer  38  may contain one or more sublayers that serve the purpose of coupling electrical excitation to one or more sublayers that provide optical amplification.  
         [0035]     The lower and upper cladding layers  36 ,  40  are formed from any suitable dielectric material, such as SiO 2 , Si 3 N 4 , BaTiO 3 , Y 2 O 3 , Al 2 O 3  or graded index combinations thereof to optimize transmission of the wavelength of optical data signals. The lower and upper cladding layers  36 ,  40  may also be formed from amorphous organic materials, such as perylenedicarboximide (PBD), Alq3, N,N′-Diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPD), poly-pheny-lene) (PPP), poly(N-vinylcarbazole) (PVK), poly(3-alkylthiophene) (PAT), oligo(p-phenyleneviny-lene) (OPV), poly(methyl methacrylate) (PMMA), poly-phenylene-vinylene (PPV), polyacteylene (PA), polyaniline (PAni), polypyrrole (PPy), polythiophene (PT), poly(3,4-ethylenedioxythiophene) (PEDOT), poly(pyridinevinylene) (PPyV), polyquinoxaline (PQx), and poly[2,2′-(p-phenylene)-6,6′-bis(3-phenylquinoxaline)] (PPQ). Amorphous organic materials are suitable for the lower and upper cladding layers  36 ,  40  if the host material of the active layer  38  is likewise an organic material. The cladding layers  36 ,  40  may be insulating, semi-insulating or conducting because the electroluminescent waveguide amplifier  30  with an organic host material in active layer  38  would be operated under DC bias.  
         [0036]     The refractive index of the lower and upper cladding layers  36 ,  40  is sufficiently less than the refractive index of the active layer  38  in order to maximize the transmission by preventing interaction with the electrodes  34 ,  42 , which would otherwise operate to attenuate the optical data signal  45  as it propagates through the electroluminescent waveguide amplifier  30 . Generally, to provide acceptable isolation, the refractive index of the cladding layers  36 ,  40  is a range of about 0.1 percent to about 20 percent smaller than the refractive index of the active layer  38 . The lower and upper cladding layers  36 ,  40  may be formed from the same or different dielectric materials or organic materials. The lower and upper cladding layers  36 ,  40  are characterized by a thickness t 3  and t 1 , respectively, and the active layer  38  has a thickness t 2 . The lower and upper cladding layers  36 ,  40  are sufficiently thick (typically about 300 nm to about 1000 nm) to isolate propagating light from the electrodes  34 ,  42 . Of course, the thickness and refractive index collectively determine the isolation effectiveness of the cladding layers  36 ,  40  for preventing interaction between the propagating light and the electrodes  34 ,  42 .  
         [0037]     The lower and upper cladding layers  36 ,  40  may also be formed by a gas or vacuum gap, which has a low relative permittivity of unity (1) and is less efficient at electrically coupling electric field to the active layer  38 . However, a vacuum gap or specialized gas possesses a very low refractive index of 1.0, which allows for strong optical confinement of the optical data signal  45  to the active layer  38 . This strong confinement allows the thickness (t 1  and t 3 ) of cladding layers  36  and  40  to be decreased, which increases the electrical coupling efficiency of the electric field established between electrodes  34  and  42  to the active layer  38 . Furthermore, the gas or vacuum gap may possess extremely high breakdown voltages allowing the waveguide amplifier  30  to be operated at voltages higher than that achievable with solid materials used for cladding layers  36 ,  40 . A gas or vacuum upper cladding layer  40  may also be electrically conducting through electron tunneling or breakdown. At sufficiently high voltages, a cathodoluminescence excitation of the active layer  38  may be achieved.  
         [0038]     The lower and upper cladding layers  36 ,  40  may be electrically conductive under alternating or direct current excitation or, alternatively, only under alternating current excitation. The refractive index and/or the free-carrier density of the lower and upper cladding layers  36 ,  40  may be inhomogenous in lateral or transverse directions. Preferably, the dielectric constant or relative permittivity of the lower and upper cladding layers  36 ,  40  is greater than about 20. In addition, the refractive index and/or free carrier density of the lower and upper cladding layers  36 ,  40  may be inhomogenous in either the lateral or the transverse direction, which may eliminate the necessity of ridge  44  for accomplishing transverse confinement.  
         [0039]     As the refractive index increases with increasing dielectric constant, it may be appropriate to form the lower and upper cladding layers  36 ,  40  from multiple sub-layers of different materials. For example, the lower and upper cladding layers  36 ,  40  may be formed from two different optically-transparent dielectric materials in which the effective index of refraction provides the desired light guiding effect and the effective dielectric constant is adequate to permit coupling of electrical energy with the active layer  38 . For example, a sublayer with a dielectric constant greater than about 20 may be separated from the active layer  38  by another sublayer of SiO 2 , which is particularly suitable for cladding ZSG and other oxides. SiO 2  itself has a dielectric constant of only about 3.9. The lower and upper cladding layers  36 ,  40  may contain one or more sub-layers that optically confine propagating light to the active layer  38  or one or more sub-layers that couple electrical excitation to the active layer  38 .  
         [0040]     With continued reference to  FIGS. 2A-2B , the ridge  44  extends along the length of the active layer  38 . Ridge  44  may be defined in the upper cladding layer  40  by standard lithographic techniques that apply a resist layer to the upper cladding layer  40 , expose the resist layer to impart a latent image pattern, and develop the resist layer to transform the latent image pattern into a final image pattern having a masked strip that defines the location of the ridge  44 . Material in the exposed areas flanking the masked strip is removed by etching, such as by plasma or reactive ion etching, to define the ridge  44 . The width, W, of the ridge  44 , in relation to its height, is selected in a known manner to ensure transverse confinement of the propagating optical data signals  45 . The ridge  44  is preferably equidistant from the lateral edges of the active layer  38 .  
         [0041]     One or more low-reflection device input facets, generally indicated by reference numeral  50 , are provided on a lateral input side of the electroluminescent waveguide amplifier  30 . The input optical fiber  46  is optically aligned with the device input facets  50 . Similarly, one or more low-reflection device output facets, generally indicated by reference numeral  52 , located on an opposite lateral side of the electroluminescent waveguide amplifier  30  to device input facets  50  are optically aligned with the output optical fiber  48 . For example, the input and output facets  50 ,  52  may be covered by corresponding anti-reflection coatings for reducing reflection. The number of output facets  52  may exceed the number of input facets  50 , which effectively splits the input optical data signal among multiple outputs. The optical amplification provided by the active layer  38  compensates for signal attenuation due to splitting the input optical data signal among the multiple output facets  52 .  
         [0042]     The upper cladding layer  40  is characterized by a slab height, H S , and a ridge height, H R , for ridge  44  defining the lateral and transverse boundaries of the optical waveguide. However, the ridge height may have a thickness of zero, depending on the specific embodiment of the amplifier  30 . The confinement of the optical signal power is indicated diagrammatically as confined region  39  in  FIG. 2B . Although not wanting to be limited by theory, it is believed that the optical waveguide amplifier  30  will support only a single propagating light mode.  
         [0043]     In use and with reference to  FIGS. 2A-2C , the input optical fiber  46  is aligned optically with the input facet  50  and the output optical fiber  48  is aligned optically with the output facet  52 . An AC bias source  54  is electrically coupled across the electrodes  34 ,  42  of the electroluminescent waveguide amplifier  30 . The invention contemplates that a DC bias source could be used as a substitute for AC bias source  54  to energize the electrodes  34 ,  42  and, thereby, to excite the dopant in the active layer  38 . The electroluminescent centers provided by the dopant species in the host material of the active layer  38  are excited, when energized by the AC bias source  54 , and an upper impurity level  43   a  provided by the presence of the electroluminescent impurity in the host material of active layer  38  is populated with electrons. The electrons exist in a metastable state after excitation and provide a population inversion, as indicated diagrammatically in  FIG. 2C .  
         [0044]     An optical data signal  45 , in the form of a string of pulses, is supplied from input optical fiber  46  to the input facet  50 . The optical data signal  45  propagates in a confined manner through the active layer  38  and cladding layers  36 ,  40  to the output optical fiber  48 . As best shown in  FIG. 2C , the optical data signal  45  stimulate electronic transitions from the populated upper impurity level(s)  43   a  to previously unpopulated lower impurity level(s)  43   b  in an abrupt cascade effect, accompanied by the emission of light or photons  41  at a wavelength substantially identical to the wavelength of optical data signal  45  and determined by the energy difference between the upper and lower impurity levels. The photons  41  of emitted light constructively add to the intensity of the input optical data signal  45 , so that the total light intensity supplied to the output optical fiber  48  is greater than the input light intensity (i.e., amplified). The electrical excitation provided by the AC bias source  54  creates another population inversion of electrons in the upper dopant energy level(s)  43   a  awaiting the arrival of another optical data signal  45 .  
         [0045]     With reference to  FIG. 3A  in which like reference numerals refer to like features in FIGS.  2 A-B and in accordance with an alternative embodiment of the invention, an electroluminescent optical amplifier  60  has an active layer  62  with a refractive index (n 2 ) surrounded on all sides by a single cladding layer  64  of a lower refractive index (n 1 ). An upper surface of the cladding layer  64  is etched to define a ridge  66  to which a stripe electrode  42  is applied or simultaneously defined by the etch. Depending on the specific embodiment of the device, the height of ridge  66  may be zero.  
         [0046]     With reference to  FIG. 3B  in which like reference numerals refer to like features in FIGS.  2 A-B and in accordance with an alternative embodiment of the invention, an electroluminescent optical amplifier  70  has an active layer  72  of refractive index n 2  deposited on a cladding layer  74  of refractive index n 3  and then patterned by lithographic techniques and etched to produce a structure (ridge  78 ) providing lateral optical confinement. After the active layer  72  is etched, an upper cladding layer  76  of refractive index n 1  is applied to the active layer  72  and a stripe electrode  42  is formed on the upper cladding layer  76 .  
         [0047]     With reference to  FIG. 4 , multiple electroluminescent optical amplifiers  30   a ,  30   b ,  30   c , each identical to either optical amplifier  30  (FIGS.  2 A,B), optical amplifier  60  ( FIG. 3A ), or optical amplifier  70  ( FIG. 3B ), are integrated on a single platform  80  with signal monitoring circuitry and waveguide devices, such as directional couplers  82  and  84  and optical splitters  86 , to create a chip-based amplifier  88 . The platform  80  may be a semiconductor wafer, such as silicon, or an electrical insulator, such as glass. Signal monitoring circuitry  90  and waveguide devices  92 ,  94 ,  96 ,  98  and  100  are formed in the platform  80  by appropriate fabrication methods. Additional circuitry (not shown) may be included on the platform  80 , such as signal filters that reduce undesired propagating wavelength(s) and propagating mode(s).  
         [0048]     While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept. The scope of the invention itself should only be defined by the appended claims, wherein