Abstract:
A class of nanophotonic integrated circuit (nPIC) has been disclosed that is a platform technology for fiberoptic communication and computing, that is fabricated from waveguides that are based on natural index contrast (NIC) principle. A multifunctional nPIC and its fabrication details have been described. The nPIC is also known as an “optical processor”. A novel nanomaterial “dendrimer” is highlighted as the key ingredient that enables the fabrication of the nPICs and its multifunctionality from the same basic process. Other nanomaterials such as spin-on glass, nano-silica sol, and a combination of any of these materials can also be used via the natural index contrast method. Several preferred embodiments of the nPIC are described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/593,508, filed Jan. 20, 2005 Nanophotonic Integrated Circuit: A Platform for an Optical Processor and Fabrication Thereof, which application is hereby incorporated by reference in its entirety. This application is related to U.S. patent application Ser. No. 10/605,638, filed Oct. 15, 2003, entitled Reflective Arrayed Waveguide Grating, which is hereby incorporated by reference in its entirety and assigned to the assignee of the present invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is directed to a nanophotonic integrated circuit and more particularly to a multifunctional nanophotonic integrated circuit (nPIC) and devices based on the nPIC.  
       BACKGROUND OF THE INVENTION  
       [0003]     An integrated circuit (IC) is a microelectronic device that houses multiple electronic circuits on a chip. ICs are built by lithographic fabrication of numerous transistors on a silicon chip. Similarly a photonic IC (PIC) is a device that houses integrated photonic functions on a chip. A PIC fabricated from nanomaterials and/or via a nanolithography process is called a nanophotonic integrated circuit. Although the physics of photons (neutral particles) and electrons or holes (charge carrying particles) are different from each other, the evolution of the electronic IC provides an analogy to PICs. The development of PICs promises to increase functionality, density, and significantly reduced cost when compared to optical components that are assembled from discrete photonic elements. While there are several existing methods for designing and building fiberoptic networks and systems, the fundamental breakthrough required to meet dramatically reduced cost points is the introduction of viable PIC technology that must meet a few important criteria such as:  
         [0004]     (i) PIC technology must be capable of creating a broad range of optical functions out of a single fabrication process.  
         [0005]     (ii) The means must exist for PICs to be readily manufactured at low cost in high volume.  
         [0006]     (iii) The capability must be developed to aggregate individual optical functions into more complex arrangements within the PIC technology and with other optical technologies.  
         [0007]     While classical electronic computing has made significant advances to the modern generation of fast computers and the Internet, there are currently applications needs that require even faster speed, higher bandwidth, more robust deployment capability and cheaper methods of fabrication. In the field of photonics, efforts to achieve a true integrated platform for fiberoptic communication and computing that can parallel the electronic counterpart have been unsuccessful. Many of the optical components available today, whether passive, active, or hybrid combinations of the both, are analogous to the vacuum tubes that predate the modern transistors and ICs. This is primarily because silicon, as an optical material, poses several challenges that make it almost impossible to produce a true PIC that can replicate the IC revolution. These issues are fundamental limitations that arise from material properties, and result in difficulty of externally controlling.  
         [0008]     Silicon structures are difficult to control externally for optical switching. Poor light emission from silicon devices is attributed to silicon&#39;s indirect band gap. It is difficult to obtain optical amplification with silicon because of difficulty in doping silicon with amplifying ions. It is also difficult to fabricate modulators because of silicon&#39;s low electro-optic coefficient. Therefore, there is an expressed need to develop novel devices that can overcome the limitations of silicon-based photonics. Novel materials and novel methods are necessary for enhancing the interaction of light with matter. Devices with novel functionalities for guiding light, switching, amplifying, and modulating need to be developed. In addition, a long awaited need is the ability to combine electronics with photonics by fabricating PICs on a silicon wafer. Such devices will form the basis for an ultimate photonic integrated circuit. If a smart nanomaterial is used to build such a PIC, it is termed as an nPIC. In the field of photonics, a smart material is defined as a material that can provide multiple functionalities such as waveguiding, amplifying, and modulating of photonic signals  
         [0009]     For a truly integrated photonic technology, a smart material system is necessary that functions in a similar fashion for photonics, as silicon does for electronic IC. At the fundamental level, electronic IC requires only a periodic arrangement of numerous “transistors” (or p-n junctions) in a smaller dimension. Because IC functionality depends primarily on the movement of charge carriers, the features necessary to carry out these functionalities can be as small as lithographically possible, the latest being a 90 nm line width.  
         [0010]     Contrasting this with photonics, features suitable for photon guiding and coupling must first be able to accommodate the photons. As a result the waveguides must at least be of the order of the wavelength of the photon. This requirement is, in some aspects, an advantage for photonics because lithographic processes are readily available. On the other hand there exists a new set of challenges to overcome because of the difficulties with silicon material properties explained above, and because of the fact that silicon cannot do everything necessary for photon manipulations. Hence, the need for a new material-based technology.  
         [0011]     From a fundamental point of view the basic functionalities necessary to build a chip that can ultimately be termed as an “optical processor” include the following: light guiding, light amplifying, light modulating, signal processing, attenuating, sensing, on-chip light source and detector, on-chip interconnect, and inputting and outputting of the signal. Being able to build all of these functionalities on a single chip that would represent a true PIC is an ambitious goal. Nevertheless, this is what one must achieve to make a breakthrough, not only from a scientific and technology point of view, but also to be able to solve the problems that currently exist.  
         [0012]     The term “photonic integrated circuit” has been assigned to several different kinds of chip-based devices. For example, U.S. Pat. No. 5,863,809 (Jan. 26, 1999) titled “Manufacture Of Planar Photonic Integrated Circuits,” describes integration of a laser (active) waveguide with a passive waveguide in a coplanar configuration. As another example, U.S. Pat. No. 5,703,974 (Dec. 30, 1997) titled “Semiconductor Photonic Integrated Circuit And Fabrication Process Therefor,” describes InP based photonic integrated circuit wherein an active region and a passive region are coupled. Other examples of using the term photonic integrated circuit in one form or other can be found in US Pat. App. No. 2004/0067006 A1 (Apr. 08, 2004) titled “Transmitter photonic integrated circuit (TXPIC) chips,” in US Pat. App. No. 2003/0099425 A1 (May 29, 2003) titled “Optical Communication Module With One or More Photonic Integrated Circuit (PIC) Chips and an External Booster Optical Amplifier for Photonic Integrated Circuits,” in U.S. Pat. No. 6,224,667 (May 01, 2001) titled “Method for Fabricating Semiconductor Light Integrated Circuit,” and in US Pat. App. No. 2004/0105476 A1 (Jun. 03, 2004) titled “Planar Waveguide Surface Emitting Laser and Photonic Integrated Circuit.” 
         [0013]     In general, the term PIC has been used for any photonic device that accommodated optical functionality on a chip. It is apparent that, while the term PIC has been used for a number of years, devices based on those PICs are not in wide use in practical network equipment. The primary reason is the typical unfavorable cost and performance issues of the available PIC devices that need to be improved. Also the above referenced PICs lack features to be able to term them as a complete PIC or an optical processor.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention discloses a nanophotonic integrated circuit (nPIC) that has several distinctions. The waveguides, that are used to fabricate the nPIC, can be made from nanomaterials, e.g., dendrimer and dendrimer compositions, nano-silica sol, spin-on-glass and its compositions, etc. The waveguiding functionality is engineered by the natural index contrast (NIC) principle that is described in a previous invention (see U.S. patent application Ser. No. 10/710,303 (Jun. 03, 2004) titled “Novel Photonic Waveguide Structures for Chip-Scale Photonic Integrated Circuit”). The nPIC integrates multiple functionalities such as amplification, mux/demux, and modulation on a single chip via on-chip monolithic waveguide interconnects.  
         [0015]     The present invention is directed to a nanophotonic integrated circuit comprising an input section, an amplifier section and a multiplexing/demultiplexing unit arranged on a substrate. The input section has a plurality of input waveguides and a coupler portion for coupling two of the plurality of input waveguides to the amplifier section. The remaining input waveguides are connected to the multiplexing/demultiplexing unit, or alternately, to a modulator portion. The amplifier section is comprised of a continuous folded waveguide connected at a first end to the coupler portion of the input waveguide and connected at a second end to the multiplexing/demultiplexing unit.  
         [0016]     The multiplexing/demultiplexing unit has a plurality of input/output waveguides, a slab waveguide, an array waveguide, and a reflective mirror. The plurality of input/output waveguides are arranged for simultaneously inputting at least one signal to and outputting at least one signal for demultiplexing a multiplexed optical signal in to n different constituent wavelengths and for combining n input optical signals composed of n different constituent wavelengths in to a multiplexed signal. A slab waveguide has a first end and a second end, the first end coupled to the plurality of input/output waveguides to focus at least one input signal to the second end, and the second end coupled to an array waveguide, for focusing the at least one output signal to the input/output interface through the first end. The array waveguide has a plurality of waveguides for coupling the one or more input signals, separating the one or more input signals into the n different constituent wavelengths and focusing the n different constituent wavelengths back on to the slab waveguide first end coupling to the input/output interface. The plurality of waveguides of the array waveguide are optically coupled at one end with the second end of the slab waveguide, and terminated at an opposing end of the array waveguide by a reflective mirror. Each waveguide of the array waveguide has a predetermined path difference between successive waveguides. A reflective mirror is fabricated at the opposing end of the array waveguide for reflecting the one or more signals. The present invention also includes an external array interface portion attached to the input section, for connecting external fibers for input and output of signals to the nPIC.  
         [0017]     In a preferred embodiment, the present invention also includes a modulator array portion comprised of at least one modulator configured to modulate at least one input/output signal from the input/output waveguides. The modulators in the modulator array portion may be electro-optic phase modulators configured so that an electric excitation can be applied across the waveguide over a predetermined length; or amplitude modulators of the Mach Zehnder configuration. The modulator array portion may include both phase- and amplitude-modulator types. It may also include an electro-absorption type modulator.  
         [0018]     In another embodiment of the present invention, a nanophotonic integrated circuit comprises an input section, an amplifier section and a splitter section arranged on a substrate. The input section is configured to connect an input signal to the amplifier section, and the amplifier section is configured to amplify the input signal and transmit the amplified input signal into an input of the splitter section. The splitter section is coupled to a plurality of output channels configured to split the amplified input signal into a plurality of output signals of N branches. The circuit optionally includes a modulator section, wherein the modulator section is configured to modulate the plurality of output signals.  
         [0019]     The present invention is also directed to a method for tuning the wavelength ranges for amplification in an optical circuit, comprising doping dendrimer with rare-earth ions; and dissolving erbium (III) nitrate pentahydrate in a solution of the dendrimer and methanol and amplifying light waves in the 1500-1600 nm range.  
         [0020]     An advantage of the present invention is the ability to provide a monolithically integrated, nanophotonic chip incorporating functionality for light guiding, light amplifying, light modulating, signal processing, attenuating, sensing, on-chip light source and detector, on-chip interconnect, and inputting and outputting of the signal, and combinations thereof.  
         [0021]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The invention is described below by way of examples with reference to the accompanying drawings, throughout which like parts are referred to by like references, where in:  
         [0023]      FIG. 1  is a schematic diagram illustrating the overall structure of the nPIC of the present invention.  
         [0024]      FIG. 2  is a diagram of the amplifier section of the nPIC in  FIG. 1 .  
         [0025]      FIG. 3  is a diagram of the radius of curvature of the waveguide bend in  FIG. 2 ;  
         [0026]      FIG. 4  is a schematic diagram of the reflective arrayed waveguide grating (RAWG) section of the nPIC in  FIG. 1 ;  
         [0027]      FIG. 5  is a schematic diagram of the array of waveguides portion of the nPIC and the RAWG in  FIGS. 1 and 4 ;  
         [0028]      FIG. 6  is a schematic diagram of the waveguide termination with a reflective mirror of the array of waveguides of the RAWG;  
         [0029]      FIG. 7  is an illustration of the waveguide taper of the input end of the array of waveguides of the RAWG;  
         [0030]      FIG. 8  is a diagram of the modulation section of the nPIC in  FIG. 1 ;  
         [0031]      FIG. 9  is a fragmentary sectional view of the electro-optic phase modulator section;  
         [0032]      FIG. 10  shows the cross-section of a single waveguide with electrode arrangements along the AA′ line in  FIG. 9 ;  
         [0033]      FIG. 11  is a fragmentary sectional view of the so called Mach-Zehnder configuration for amplitude modulation;  
         [0034]      FIG. 12  shows the cross-section of a single Mach-Zehnder structure with electrode arrangements along the AA′ line in  FIG. 11 ;  
         [0035]      FIG. 13  shows a scheme of cascaded electrode structure of an array of electro-optic phase modulators.  
         [0036]      FIG. 14  shows a scheme of cascaded electrode structure of an array of Mach-Zehnder amplitude modulators.  
         [0037]      FIG. 15  shows a scheme of cascaded electrode structure of an array containing both phase modulators and Mach-Zehnder amplitude modulators.  
         [0038]      FIG. 16  shows a partial section of the nPIC chip at the fiber array coupling.  
         [0039]      FIG. 17  shows the measured refractive index of PAMAM dendrimer G2 film as a function of wavelength.  
         [0040]      FIG. 18  shows a sketch of molecular structure of ethylene diamine and a substituent.  
         [0041]      FIG. 19  shows a PAMAM G0 molecule.  
         [0042]      FIG. 20  shows an alternate embodiment of the nPIC of the present invention.  
         [0043]      FIG. 21  shows an enlarged sectional view of the modulation segment of the nPIC in  FIG. 20 .  
         [0044]      FIG. 22  shows measured refractive index of cured G0 film.  
         [0045]      FIG. 23  shows another embodiment of the nPIC with 2nd phase integration.  
         [0046]      FIG. 24  shows yet another embodiment of the nPIC with another 2nd phase integration.  
         [0047]      FIG. 25  shows yet another embodiment of the nPIC.  
         [0048]      FIG. 26  is an energy level diagram of Er 3+ . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0049]     In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention can be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments can be utilized and that changes can be made without departing from the scope of the present invention.  
         [0050]      FIG. 1  provides a diagrammatic representation of the overall structure of a preferred embodiment of the nPIC. A substrate  100  is preferably made from prime grade &lt;100&gt; silicon, on which a layer of oxide  101  is formed by either thermal oxidation or from a spin-on glass via spin coating and curing the subsequent film at a suitable temperature that may range from room temperature to a few hundreds of degrees of Celcius. Several photonic functionalities are provided via different segments: an amplifier section  110 ; a multiplexing/demultiplexing section  120 ; a reflective arrayed waveguide grating  145  (see, generally,  FIG. 4 ; the reflective arrayed waveguide grating is described in detail in commonly-assigned U.S. patent application Ser. No. 10/605,638 filed Oct. 15, 2003, entitled Reflective Arrayed Waveguide Grating, and the same is hereby incorporated by reference in its entirety); a modulation section  190 ; and an external fiber-array interface  210 .  
         [0051]     The amplifier section  110 , shown separately in  FIG. 2 , includes a folded waveguide that begins at the point  113 , which is composed of a plurality of straight segments  111  and a curved segment  112 , the curved segment having a minimum radius of curvature R as shown in  FIG. 3 . The core of the amplifier is made with a nanomaterial such as dendrimer that is doped with rare-earth ions such as Er 3+  and/or other amplifying ions.  
         [0052]     The value of the radius of curvature R, also called the bend radius, is a critical parameter. A minimum value of R is necessary for a transmission with minimal loss. The optimal value of R is material dependent and depends on material characteristics; for glassy materials it is usually ˜4-5 mm. For novel polymeric materials such as dendrimer, a lower value can be used to design an efficient amplifier. This value may range between 1-6 mm; however, a higher or lower value of R may also be used for some other materials and design.  
         [0053]     Referring next to  FIG. 4 , the multiplexing/demultiplexing (mux/demux) unit  120  is comprised of a reflective arrayed waveguide grating (RAWG)  145  that is fabricated on the same substrate. The RAWG  145  is composed of an input/output array  120 , a slab waveguide  140 , an array of waveguides  160  acting as a grating, and the said array of waveguides is terminated by a mirror  170  to reflect the input signal back into the array  160 . The array waveguide grating  160  splits the wavelengths of the input signal  230  into its constituent wavelengths λ 1 , λ 2 , . . . , λn that are focused on the individual output waveguides at the slab-waveguide interface  130 .  
         [0054]     Referring next to  FIG. 5 , the grating portion  160  of the nPIC is formed by an array of waveguides  160 . The total length of each successive waveguide in the array differ by a length of Δ1 from the nearest neighbor; the inner waveguide  161  being the shortest waveguide in the array  160  and the outer waveguide  162  being the longest waveguide in the array  160 . Both ends of the waveguides  160  have a taper to help couple the most light in and out of the grating  160 .  
         [0055]      FIG. 6  shows a magnified view of the array of waveguides  160  terminating at the mirror  170 . The mirror  170  is created by depositing a high reflectivity material such as gold or silver whose thickness, t m , usually has a value of ˜10 micron. The taper length L t1  and the shape of the taper are optimized such that light coupling is maximized and yet complies with practical lithographic dimensions. Usually a linear taper shape is chosen to simplify geometry and for process convenience, but other shapes such as parabolic, exponential, hyperbolic, or a combination of any of the above, can also be used.  
         [0056]     Similarly, the input ends  150  of the array of waveguides  160  interfacing with the slab  140  also have a taper that is designed for optimum performance (see  FIG. 7 ). Here the inter-waveguide separation S, distance between adjacent waveguides s, taper length L t2 , as well as the taper shape is optimized as design parameters where individual numeric values depend on the total length of the shortest waveguide in the array  160 , the refractive indices of the core and the cladding material, and the total number of channels of the device.  
         [0057]     Referring to  FIG. 8 , the modulation segment  190  of the nPIC includes a plurality of individual modulators  191  used for each output channel. All of the modulators in the array  190  may have identical or non-identical structure, allowing a design flexibility to accommodate the requirements of multiple services involving different modulation frequencies and/or signal encoding on a single chip.  
         [0058]      FIG. 9  shows the construction details of an electro-optic phase modulator. The waveguide  191  is the same as all other waveguides from which the nPIC is constructed with the exception that here a top and a bottom electrode  192  have been deposited such that an electric excitation can be applied across the waveguide over a length L pm .  
         [0059]      FIG. 10  shows the cross section of the phase modulator with electrodes through the line AA′ in  FIG. 9  wherein  102  is a cladding having the same index as  101 . The effective separation between the top and the bottom electrodes, g, is determined by the thickness of the waveguide which typically is a few microns. The exact value will depend on a given design, e.g., a range of 2-8 microns for the present invention, but other values can be used by other designs as determined by the material used. The linear electro-optic effect, also known as the Pockels effect, describes the change of the refractive index of the waveguide under applied electric field. The change in the refractive index, Δn, is proportional to the electric field strength, its direction and the polarization of light. For a linearly polarized light,  
                      Δ   ⁢           ⁢   n          =       1   2     ⁢     n   3     ⁢     r   33     ⁢     E   3         ,           [   1   ]               
         [0060]     where, r 33  is the electro-optic coefficient of the core material and E 3  is the applied electric field in the z-direction (across the thickness of the waveguide). For a homogeneous electric field applied over a length L pm , the change in phase of the guided light is given to the first approximation by,  
                 Δ   ⁢           ⁢   ϕ     =           π   ⁢           ⁢     L   pm       λ     ·     V   g     ·     n   3       ⁢     r   33     ⁢   Γ       ,           [   2   ]             
 
         [0061]     where, V is the applied voltage, λ is the wavelength, Γ is the efficiency of the field generated by the voltage V, whose value is less than 1, and π is a constant. The half-wave voltage, V π , which causes a phase shift of π is given by,  
               V   π     =         g   ⁢           ⁢   λ         n   3     ⁢     r   33     ⁢     L   pm     ⁢   Γ       .             [   3   ]             
 
         [0062]     As indicated by equation 3 above, the half-wave voltage is directly proportional to the electrode separation g and inversely proportional to the electrode length, L pm ; therefore, these geometry factors are optimized to obtain an optimum value of V π  for a given material system that also simultaneously optimizes the chip size.  
         [0063]     Some special polymeric nanomaterials such as dendrimers have a refractive index in the range 1.5 to 1.6. For instance, the refractive index of a cured PAMAM dendrimer generation 2 (G2) film formed via spin coating on a silicon wafer and cured between 150° C. and 3000° C. for a length of time between half-hour and one hour was measured using a Metricon 2010 prism coupler (Metricon Corporation, Pennington, N.J. 08534, U.S.A.). It was determined that the cured G2 film has a refractive index of 1.64 at 632.8 nm while it falls to 1.604 at 1553 nm (see  FIG. 17 ). As shown in  FIG. 17 , there is a wavelength dependent dispersion of the refractive index. While a similar dispersion is expected for other waveguide materials (e.g., glass), however, here the variation between 1300 nm and 1600 nm is very small which can be accommodated within the design. An important observation here is that, if a glassy material is used as cladding with a refractive index n 2 =1.46 and the G2 film (n 1 =1.604) is used for core, a natural index contrast of Δn=[(n 1 −n 2 )/n 2 ]100%=9.86% is obtained which is suitable for guiding light in the 1.0-1.6 μm bands. Similarly, for PAMAMOS dendrimer was spun-on silicon wafer and cured via similar process as outlined above. The said film&#39;s refractive index was 1.5.  
         [0064]     For some polymeric material that is optimized for electro-optic applications, r 33  can have a value on the order of 300 pm/V, while for other common materials such as lithium niobate r 33  is reported to be 33 pm/V. The value of Γ is also material dependent, and in part determined by the driving circuit  196  shown in  FIG. 13 ; however, it increases with an increasing value of r 33 . It is also evident from Equation (3) that higher the value of r 33 , the lower the value of V π . Since a lower value of V π  is beneficial for system design and operation, therefore, as high a value of r 33  as possible is desirable. Table-I shows typical values of parameters in Equation (3) and corresponding values of a polymer whose r 33  is assumed to be 100 pm/V. Since polymer is deposited on to a substrate, its thickness can be controlled from fraction of a micron up to tens of microns; in Table-I a thickness of 5 micron is assumed.  
                                           TABLE I                           Typical values for Equation [3].                Parameter (Unit)   Polymer                            λ (μm)   1.5           g (μm)   5           n   1.6           r 33  (μm/V)   0.100           L pm  (μm)   10           Γ   0.5           V π  (Volts)   2.93                      
 
         [0065]      FIG. 11  shows a so-called Mach-Zehnder structure (MZ) used for amplitude modulation whose cross-sectional view through the line BB′ is shown in  FIG. 8 . Here a pair of phase modulators is used in parallel as the individual arms of the MZ to produce amplitude modulation. As before, an external applied voltage creates a phase difference in both arms which causes a change in the power at the output end  195  due to the interference of signal carried in both arms. Here the transmittance of individual modulators can be controlled from a minimum power P min  to a maximum P max . The extinction ratio is given by P min /P max . The output power, P out , can be controlled by the applied voltage V according to equation [4]:  
               P   out     =       P   min     +       (       P   max     -     P   min       )     ·     [       1   2     +       1   2     ⁢   cos   ⁢     {       π   ⁡     (     V   -     V   0       )         V   π       }         ]                 [   4   ]               
 where V 0  is the voltage at peak power and other terms are as described before. The half-wave voltage can be thought of as a “switching” voltage because this is the minimum voltage required for switching the transmission state into the off state and vice versa. 
 
         [0066]      FIG. 13  shows an array of phase modulators where a single source circuit  196  is used to power all branches (modulators) in the array. If all modulators in the array have identical structure and excitation, they will produce the same modulation frequency.  
         [0067]      FIG. 14  shows an array of amplitude modulators in the MZ configuration. Owing to their identical construction and by virtue of a single driving circuit  197  exerting an identical excitation to all branches, all output branches will have identical modulation frequency.  
         [0068]      FIG. 15  shows an array of modulators where phase modulators and amplitude modulators have been embodied on the same unit. Other combinations of different modulators are also possible which will be best determined and practiced according to the requirements of a particular application.  
         [0069]     Referring to  FIG. 16 , at the input and output (I/O) segment of the nPIC an external fiber array  210  is used for signal in and out of the nPIC. Arc  220  encircles the individual fibers in the array where the fiber  221   a  is used to input the pump and usually the fiber  221   b  is used to input a multiplexed signal into the nPIC. A coupler or combiner  201  is monolithically built on the same chip that combines the pump and the signal and feeds them into the amplifier unit  110 . The fiber array is precisely aligned to the nPIC input  200  and fixed permanently so that it becomes a part of the packaged module.  
         [0070]      FIG. 17  shows the refractive index of G2 PAMAM dendrimer measured with a Metricon 2010 prism coupler (Metricon Inc., Pennington, N.J.). The thicknesses are also measured simultaneously as shown in  FIG. 17 . The G2 film was formed via spin-coating on prime grade &lt;100&gt; silicon wafer and cured by a procedure as described before. The curing temperature is important to obtain a fully cured film; however, a minimum duration of cure of 30 minutes is required, while a longer duration at this temperature does not make any significant difference in the final film properties. As seen from  FIG. 12 , cured G2 PAMAM film has an index of 1.6 at 1500 nm, thus used as a core with a suitable cladding, it is suitable for guiding light in the wavelength range of 1000-1600 nm. The PAMAMOS dendrimer has an index of 1.5, thus this can be used as the core material with glass being used as a cladding.  
         [0071]      FIG. 18  shows an ethylene diamine molecule  230  that is used as the central core of the PAMAM dendrimer where a substituent  240  is used to form a zero th  generation shell around the core, thus forming a zero t h generation PAMAM dendrimer, G0 ( FIG. 14 ); wherein  231  is a nitrogen atom bonded to a carbon atom  232 ; both also have required number of hydrogen atoms  233  attached to them. PAMAM dendrimers are synthesized by repetitive reaction of these two constituents, viz., the core  230  and the substituent  240 .  
         [0072]      FIG. 19  shows a G0 PAMAM dendrimer where 4 substituents have been attached to the core  230  whereby this molecule can chemically bind rare-earth ions and other molecules either to its branch ends or to its core, thus its properties can be tailored for optical amplification and modulation applications. For instance, Er 3 +ions can be attached either to the core  230  or to the substituent  240  to obtain amplification in the 1500-1600 nm range. Other rare earth ions can be used to dope the dendrimer to obtain amplification in other wavelength ranges. Additionally, other organic molecules such as chromophores can be attached to the dendrimer molecule to obtain a higher electro-optic response for modulation applications. Moreover, other dielectric and/or inorganic ions can be attached to the dendrimer molecules to obtain a higher dielectric constant of dendrimer films that will be suitable for photonic crystal fabrication (not described here).  
         [0073]      FIG. 20  shows another preferred embodiment of the nPIC where the modulation unit  250  is constructed of MZ amplitude modulators for each output channel that are connected to the input and output of the nPIC via a monolithic integration of waveguide interconnects fabricated on the same chip.  FIG. 21  shows a magnified view of the modulation segment  250  of the nPIC of  FIG. 20 .  
         [0074]      FIG. 22  shows the measured refractive index and thickness of PAMAM G0 film formed on a silicon wafer via spin coating which was cured at ˜250° C. for about an hour. Cured G0 film has a lower index than G2 film described before, therefore, can be used as a cladding material.  
         [0075]      FIG. 23  shows another preferred embodiment of the nPIC comprising of a “second-phase” integration, where only an amplifier section and a mux/demux section implemented via a RAWG is constructed on the same chip. This chip does not have the capability of modulating output signals; it will only amplify and perform multiplexing and demultiplexing of the I/O signals.  
         [0076]      FIG. 24  shows yet another preferred embodiment of the nPIC that is comprised of an amplifier section and a 1×N splitter section whereby the input signal is first amplified by the amplifying section that is fed into the input of the splitter. The splitter then just splits the signal into N branches that are coupled to the output channels.  
         [0077]      FIG. 25  shows yet another preferred embodiment of the nPIC that is comprised of an amplifier section, a 1×N splitter section, and a modulator section. The said nPIC is designed to modulate the output signal coming from the splitter section that was preamplified by the amplifier section.  
       METHODS OF FABRICATING THE ERBIUM DOPED DENDRIMER AMPLIFIER (EDDA)  
       [0078]     The first generation of Erbium-doped waveguide amplifiers (EDWAs)—glass-based waveguide amplifiers—used mainly laser diodes emitting at 1480 nm for optical pumping. Under this excitation scheme, the metastable  4 | 13/2  energy level of the erbium ions is excited directly via its high-lying Stark states. The ions act as a quasi-three-level laser system, which limits the population inversion with respect to the ground state to roughly 40% (corresponding to a population in the first excited state of 70%) because of stimulated emission by pump radiation. On the other hand, when pumping into the second excited state  4 | 11/2  using 980 nm radiation, a population inversion between metastable level and ground state of close to 100% can be obtained, therefore, 980 nm pumping is expected to be more efficient.  
         [0079]     This 980 nm excitation scheme of Er 3+  in EDWAs, however, requires considerably higher intensities, since the absorption cross-section of the  4 | 11/2  state is small. In the present invention, the excitation cross-section of Er 3+  around 980 nm is increased by codoping dendrimer with ytterbium (Yb 3+ ). The absorption cross-section of Yb 3+  at 980 nm is about an order of magnitude larger than that of Er 3+ , and the absorption band of Yb 3+  extends over a wider wavelength region, between 850 nm and 1000 nm. From Yb 3+  the energy is then transferred resonantly to the  4 | 11/12  state of Er 3+ . This energy transfer has been investigated in a wide range of materials, both glassy and crystalline [3-7], however, no data has heretofore been available for dendrimer.  
         [0080]     Under the present invention, enhanced efficiencies and higher output power are obtained by using high concentrations of Er 3+ , &gt;10,000 ppm (or even &gt;40,000 ppm). It is hypothesized that such high concentrations reduce the problem of ground state bleaching and simultaneously up convert an adjacent ion to a higher energy state which subsequently contributes to emission in the range of interest.  
         [0081]     The present invention uses the doping concentration as a parameter to control the amplification efficiency. Hence, the higher the doping level, the higher the amplification per unit length of the waveguide. The lower limit of doping concentration, below which no measurable amplification occurs, may be a few hundred ppm. The upper limit is unknown however, the inventor has demonstrated through simulations that concentrations of at least 40,000 ppm are effective.  
         [0082]     Every rare-earth metal has a unique characteristic absorption and emission ranges. This characteristic emission range is the one that will dominate when a higher concentration of this species is present. The characteristic emission of Er is around 1550 nm band. Neodymium (Nd 3+ ) will amplify in the 1060 nm range. Praseodymium (Pr 3+ ) will amplify in the 1250-1300 nm range, Thorium (Th 3+ )and Holmium (Ho 3+ ) will amplify in the 1300-1400 nm range, and Terbium (Tb 3+ ) will amplify in the 1400-1500 nm range. Yb is mainly used as a codopant , or sensitizer. In glass, up to 4% Yb has been used. In dendrimer, a percentage greater than 4% is possible.  
         [0083]     PAMAM dendrimer of generation 2 (G2), generation 3 (G3), and generation 4 (G4); and PAMAMOS dendrimer of generation 2 (G2′), generation 3 (G3′), and generation 4 (G4′), are doped with Erbium ions (Er 3+ ) via liquid phase coordination chemistry or chelating. The PAMAM and PAMAMOS generations are given by way of example and not limitations, and generation zero through generation 10 of each of the PAMAM and PAMAMOS dendrimers can be doped with Erbium ions via the same procedure. In addition to Erbium, this method can also be used to dope dendrimer with other rare-earth ions, leading to a mechanism to tune the wavelength range for amplification.  
         [0084]     The dendrimers are preferably standard Generation 2 poly(amido amines) with amine (—NH2) surfaces and a 1,2-ethylenediamine core (theoretical molecular weight: 3252 g/mol). They are synthesized by sequential Michael addition followed by amidation according to procedures known by those skilled in the art.  
         [0085]     Erbium-doped dendrimer solutions of the present invention can be prepared by dissolving erbium (III) nitrate pentahydrate in a solution of the dendrimer and methanol. A solution of 500 ppm (parts per million) erbium-doped dendrimer can be prepared by adding 5×10 −4  g erbium (III) nitrate to 0.5 g dendrimer and 0.5 g methanol. A solution of 1000 ppm erbium-doped dendrimer can be prepared by adding 0.001 g erbium (III) nitrate pentahydrate to 0.5 g dendrimer and 0.5 g methanol. Additionally, a 1% erbium (III) nitrate pentahydrate solution (no dendrimer) can be prepared by dissolving erbium (III) nitrate pentahydrate (0.005 g) in methanol (0.5 g). Although nitrate is the disclosed compound, other compounds such acetate, halides, isopropoxide, oxalate, phosphate hydrate, sulfate hydrate, and other similar Erbium compounds that are soluble in organic solvents, for example, methanol, isopropanol, toluene methoxypropanol, and similar compounds. Other concentrations can also be obtained following this method.  
         [0086]     While the invention has been described with reference to a few preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.