Patent Publication Number: US-11042051-B2

Title: Direct drive region-less polymer modulator methods of fabricating and materials therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of U.S. patent application Ser. No. 15/899,312, filed 19 Feb. 2018, and claims the benefit thereof. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to active region-less polymer modulators integrated into a common platform as direct drive modulators. 
     BACKGROUND OF THE INVENTION 
     Polymer modulators have become very popular in the current technology boom. Polymer photonics technology with customized core and cladding layers provides a number of significant advantages over the prior art. Among the many advantages, some of the most significant are that it allows efficient 3-layer modulators with high performance (multi GHz) and very-low voltage operation to allow direct-drive (DDPM) without the need for using a drive circuit. While this technique has been commercialized it is not optimized. That is it does not reach the low cost targets, or space/size requirements, and takes lots of time to align the components, place them, package them, and test them. Also, while some integration of lasers and modulators has occurred, optical alignment of the components can be a costly and tedious task. 
     Much of the recent work on polymer modulators has been focused on Si-organic-hybrids (SOH) often referred to as Si slot modulators. These prior art modulators exhibit very small V π -L products due to their short length (˜1 mm) and high single-layer r 33  coefficients. Due to the fact that only an electro-optic (EO) polymer is used in the structure, the poling is efficient and the single-layer r 33  (value achieved in a Teng-Man measurement) is also achieved in the device. This is contrasted with a typical 3-layer modulator (cladding/core/cladding) where the poling is inefficient due to the voltage division among the three layers (see  FIGS. 1 and 2  below). Typically, dielectric breakdown occurs in the cladding before complete poling can be accomplished in the core. 
     It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. 
     Accordingly, it is an object of the present invention to provide a new and improved direct drive region-less polymer modulator integrated on a common platform. 
     It is another object of the present invention to provide new and improved methods of fabricating direct drive region-less polymer modulators and materials therefor. 
     It is another object of the present invention to integrate the new and improved direct drive region-less polymer modulator on a common platform with a monolithic laser formed in/on the platform. 
     SUMMARY OF THE INVENTION 
     Briefly to achieve the desired objects and advantages of the instant invention in accordance with a preferred embodiment a direct-drive region-less polymer modulator is provided. The direct-drive region-less polymer modulator includes a platform and a polymer modulator monolithically built onto the platform. The polymer modulator includes a multilayer waveguide formed in/on the platform. The waveguide includes a first cladding layer, a passive core region with a surface abutting a surface of the first cladding layer, the passive core region extending to define an optical input and an optical output for the modulator. A shaped electro-optic polymer active component has a surface abutting a surface of a central portion of the passive core region. The shaped electro-optic polymer active component is polled to align dipoles and promote modulation of light and has a length that extends only within a modulation area defined by modulation electrodes. A second cladding layer encloses the shaped electro-optic polymer active component and is designed to produce adiabatic transition of light waves traveling in the passive core region into the shaped electro-optic polymer active component to travel the length of the shaped electro-optic polymer active component and return to the passive core region. At least a portion of the multilayer waveguide forms the polymer modulator into a direct-drive polymer modulator. 
     To further achieve the desired objects and advantages of the present invention a specific embodiment of a direct-drive polymer modulator is provided that includes a substrate and a lower electrode on the substrate. A multilayer waveguide is formed in/on the substrate and includes a bottom electro-optic polymer cladding layer, an electro-optic polymer core and a top electro-optic polymer cladding layer. The electro-optic polymer core has an electro-optic coefficient (r 33 ) greater than 250 pm/V, and a Tg 150° C. to &gt;200° C., and the top and bottom electro-optic polymer cladding layers having a Tg approximately the same as the Tg of the electro-optic polymer core. A polymer modulator is monolithically built into the multilayer waveguide and includes the electro-optic polymer core having a passive core region with a surface abutting a surface of the bottom cladding layer, the passive core region extending to define an optical input and an optical output for the polymer modulator. A shaped electro-optic polymer active component has a surface abutting a surface of a central portion of the passive core region, the shaped electro-optic polymer active component is polled to align dipoles and promote modulation of light and has a length that extends only within a modulation area defined by modulation electrodes. The top cladding layer encloses the shaped electro-optic polymer active component and is designed to produce adiabatic transition of light waves traveling in the passive core region into the shaped electro-optic polymer active component to travel the length of the shaped electro-optic polymer active component and return to the passive core region. An upper electrode overlies the shaped electro-optic polymer active component so as to define with the lower electrode the modulation area. At least a portion of the multilayer waveguide forms the polymer modulator into a direct-drive polymer modulator. 
     To further achieve the desired objects and advantages of the present invention a specific embodiment of a method of fabricating a direct-drive region-less polymer modulator is disclosed. The method includes steps of providing a platform and depositing on the platform a multilayer waveguide including bottom electro-optic polymer cladding layer, an electro-optic polymer core overlying the bottom electro-optic polymer cladding layer, and a top electro-optic polymer cladding layer overlying the electro-optic polymer core forming an electro-optic polymer waveguide. The electro-optic polymer core is formed from materials having an electro-optic coefficient (r 33 ) greater than 250 pm/V, a Tg 150° C. to &gt;200° C. and a resistivity approximately 10 8  Ohm-cm, and the top and bottom electro-optic polymer cladding layers have a Tg approximately the same as the Tg of the electro-optic polymer core, and a resistivity less than the resistivity of the electro-optic polymer core at poling temperature, and the top and bottom electro-optic polymer cladding layers have levels of conductivity equal to or higher than the electro-optic polymer core. A polymer modulator is monolithically built into the multilayer waveguide and includes the electro-optic polymer core having a passive core region with a surface abutting a surface of the bottom cladding layer and extending to define an optical input and an optical output for the polymer modulator. A shaped electro-optic polymer active component has a surface abutting a surface of a central portion of the passive core region, the shaped electro-optic polymer active component being polled to align dipoles and promote modulation of light, the shaped electro-optic polymer active component having a length that extends only within a modulation area defined by modulation electrodes. The top cladding layer encloses the shaped electro-optic polymer active component and is designed to produce adiabatic transition of light waves traveling in the passive core region into the shaped electro-optic polymer active component to travel the length of the shaped electro-optic polymer active component and return to the passive core region. An upper electrode overlies the shaped electro-optic polymer active component so as to define with the lower electrode the modulation area. At least a portion of the multilayer waveguide forming an electro-optic polymer modulator in the electro-optic polymer waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Specific objects and advantages of the invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof, taken in conjunction with the drawings in which: 
         FIGS. 1A and 1B  illustrate a concept of optimized polymer modulator poling with a single resistance layer in physical and schematic views, respectively; 
         FIGS. 2A and 2B  illustrate a concept of optimized polymer modulator poling in a three layer stack with matched resistance layers in physical and schematic views, respectively; 
         FIG. 3  is a table listing materials and their resistivity at the glass transition temperature (Tg); 
         FIG. 4  is a graph illustrating the relationship of V core /V pole  vs R clad /R core ; 
         FIG. 5  is a graph illustrating r 33  vs the poling field; 
         FIG. 6  illustrates Table 1 which includes various characteristics for maximizing the EO coefficient; 
         FIGS. 7A and 7B  illustrate two examples of polymer Mach-Zehnder modulators with Y-splitters/combiners and with MMI splitters/combiners, respectively; 
         FIGS. 8A and 8B  illustrate prior art polymer modulators using rf drivers with Mach-Zehnder modulators with Y-splitters/combiners and with MMI splitters/combiners, respectively; 
         FIGS. 9A and 9B  illustrate polymer modulators using direct-drive signals (no rf drivers) with Mach-Zehnder modulators with Y-splitters/combiners and with MMI splitters/combiners, respectively; 
         FIGS. 10A, 10B, and 10C  illustrate a method of fabricating a polymer modulator using a ridge waveguide with 3-layer stack, 4-layer stack, and 5-layer stack, respectively, on an InP platform, in accordance with the present invention; 
         FIGS. 11A, 11B, and 11C  illustrate method of fabricating a polymer modulator using an inverted-ridge waveguide with 3-layer stack, 4-layer stack, and 5-layer stack, respectively, on an InP platform, in accordance with the present invention; 
         FIG. 12  illustrates a method of fabricating a polymer modulator using a ridge waveguide with 3-layer stack and bottom and top microstrip conductors on an InP platform, in accordance with the present invention; 
         FIG. 13  illustrates a method of fabricating a polymer modulator using an inverted-ridge waveguide with 3-layer stack and bottom and top microstrip conductors on an InP platform, in accordance with the present invention; 
         FIG. 14  illustrates steps in a method of fabricating direct-drive polymer modulators using a ridge waveguide 3-layer stack; 
         FIG. 15  illustrates steps in a method of fabricating direct-drive polymer modulators using an inverted-ridge waveguide 3-layer stack; 
         FIG. 16  is a simplified perspective view illustrating and array of lasers/polymer modulators on a semiconductor platform; 
         FIG. 17  is a simplified perspective view illustrating the array of lasers/polymer modulators of  FIG. 16  assembled in a package module; 
         FIG. 18  is a top plan view of a Mach-Zehnder modulator with upper electrodes in accordance with the present invention; 
         FIG. 19  is a cross sectional view of the Mach-Zehnder modulator of  FIG. 24  as seen from the line  19 - 19 ; and 
         FIG. 20  (A)-(H) illustrate steps in the fabrication of the Mach-Zehnder modulator of  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     A primary object of the present invention is to provide a new and improved direct drive region-less polymer modulator which can be integrated with lasers and/or other light sources, preferably on a common platform, with novel new designs and processes. In this context, the term “region-less polymer modulator” is defined as a polymer modulator formed in a waveguide in which the active components do not reach the edge of the chip but are confined within the periphery of the chip (hence ‘region-less’). In the preferred embodiment, the common platform is InP, because lasers are naturally fabricated from InP and are already monolithic (part of the same material). It will be understood however, that the platform or substrate could be silicon, GaAs, metal, plastic, or any other suitable organic or inorganic material. 
     One specific method or concept for the formation of a direct drive region-less polymer modulator in accordance with the present invention, is to provide a platform/substrate. Electro-optic polymer waveguides are formed using a 3-layer structure, e.g. lower cladding layer, core, and upper cladding layer. The cladding layers are sufficiently thick so that negligible light leaks to the substrate. A laser or other light source can be coupled to an input of the waveguide and output light from the waveguide can be coupled to an output optical fiber. Alternatively, the modulator output can be coupled to a glass or nitride guide and directed to a waveguide mux (that can also be formed in the same material). 
     Referring to polymer modulators, it is understood in the art, before poling of a polymer the chromophores with a permanent dipole are randomly oriented. Poling consists of heating the polymer to a temperature near the glass transition temperature (Tg) of the material and applying an electric field to align the chromophores (or a substantial quantity of them). As is also understood, properly poling a polymer is a major determinate of some characteristics, such as the EO coefficient, of the material. Referring to  FIGS. 1A and 1B , it can be seen that poling a single layer of polymer material (e.g. a core) is relatively straightforward, since the poling voltage is equal to the voltage applied across the core or single layer. Referring additionally to  FIGS. 2A and 2B , it can be seen that once upper and lower matched cladding layers are formed with the core (e.g. a 3-layer modulator stack) the poling becomes more complex because of the voltage division between the core and the cladding layers. As described, when V core  is approximately equal to V pole  the eo coefficient (r 33 ) is maximized and the resulting drive voltage for the waveguide is minimized. Further, it can be determined from  FIG. 2B  and the included equation that V core  is approximately equal to V pole  when the conductivity of the cladding layers (σ clad ) is much greater than the conductivity of the core (σ core ). 
     Turning now to  FIG. 3 , the chart illustrates the resistivity of some typical polymer core and cladding materials. Usual polymer lower cladding materials may include commercially available polymers such as: UV/thermal cure types; OC-3021 from Dymax, CO-160 from ChemOptics, Level M10-44 from Brewer Science, AL-3355 from FOC, NOA61 from Norland; as well as Epoxies from DOW, Epoxylite; as well as Thermoplastics such as UDEL from Solvay, PI2555 from HD Microsystems, Matrimid 5218 from Huntsman, Polysulfone from Aldrich etc. The examples show that core materials have a much smaller resistivity than cladding materials. Further, from the graphs illustrated in  FIGS. 4 and 5  it can be determined that a smaller V core  results in a smaller r 33  and a smaller r 33  results in a larger required modulator drive voltage. 
     At the present time, there are commercially-available EO polymers that exhibit very large r 33  values, but the lack of a cladding material with the correct properties makes it difficult to efficiently pole these large r 33 -value EO polymers. Many commercially available EO polymers have a low T g , making them unsuitable for typical Polymer photonics applications. None have engineered the conductivity of the cladding materials to solve this problem. In summary, a large r 33  and high T g  are needed. 
     Referring additionally to Table 1 in  FIG. 6 , some characteristics for maximizing the EO coefficient are listed in the form of modeled modular approaches. In Table 1, the ‘length’ refers to the active electrode length and the ‘overlap’ is the overlap factor for the applied and optical fields. For a 3-layer microstrip configuration, the overlap is approximately 1. The ‘Strawman Si’ slot provides the shortest length modulator but currently has excessive optical loss. Table 1 provides the specification to achieve V π =0.5V and a high EO coefficient for the core (r 33 &gt;250 pm/V). For cladding with conductivity much larger than that of the core material at its poling temperature, the active electrode length would be approximately 7 mm for a V π =0.5V using the #1 3-layer material approach. 
     Some important criteria for cladding materials suitable for use in the novel direct-drive polymer modulator are as follows. A refractive index of the cladding material at 1550 nm equal to the core refractive index minus approximately 0.05. A coefficient of thermal expansion (CTE) of the cladding material as close to the CTE of the core materials as possible. A T g  of the cladding material near the T g  of the core (i.e. 150° C. to &gt;250° C.). A resistivity of the cladding material at room temperature greater than approximately 10 8  Ohm-cm. A resistivity of the cladding material at the poling temperature (˜T g ) much less than the core resistivity (less than approximately 10 8  Ohm-cm). An optical loss through the cladding layer less than 1 dB/cm of light at 1300 nm to 1600 nm. A dielectric loss tangent less than 0.002 at 10 GHz. A cladding material that is not chemically attacked by the core, core solvents, or photoresist solvents. A cladding material that forms an adhesive bond with Au, Ti/Au, or Cr/Au metallization and with the core. A cladding material that is dry-etchable with reasonable etch rates. A cladding material with endfaces that can be polished, cleaved, or direct cut. A cladding material that forms a hermetic seal or is at least O 2  blocking. 
     Some cladding materials that are suitable for use in the present novel direct-drive polymer modulator are as follows: ionically-doped polyimides and epoxies; colloidal-metal-doped polyimides; sulfonic acid-doped polyimides (dopants for various cladding polymers may include metal or organic trifles, such as triflic acid esters, or triflamides; and inorganic metal oxides, nitrides, etc, that can be vacuum deposited by evaporation or sputtering, or epitaxy (formation can include molecular beam epitaxy, metal organic chemical vapor deposition, or atomic layer epitaxy). Materials such as: Cladding P12555; Cladding AL3355; Cladding UDEL; Cladding UV15; and others can be used in the present invention, by doping the materials to lower their resistivity. 
     Turning to  FIGS. 7A and 7B , typical Mach-Zehnder modulator layouts are illustrated. In  FIG. 7A , Y-splitters/combiners are attached to a Mach-Zehnder type modulator and in  FIG. 7B  MMI (multimode interference) splitters/combiners are attached to the Mach-Zehnder type modulator in place of the Y-splitters/combiners. In  FIG. 7A , the input waveguide is 2000 um long, the input Y-splitter is 1432 um long, the Mach-Zehnder interferometer legs are 7000 um long, the output Y-splitter is 1432 um long, and the output waveguide is 2000 um long, for a total of 13,864 mm long. In  FIG. 7B , the input waveguide is 2000 um long, the input MMI splitter is 100 um long with connecting legs 1146 um long, the Mach-Zehnder interferometer legs are 7000 um long, the output mmi splitter is 100 um long with connecting legs 1146 um long, and the output waveguide is 2000 um long, for a total of 13,492 mm long. The standard Mach-Zehnder modulator also has ‘overhead’ length in addition to the active (electrode) length, which for the embodiment of  FIG. 7B  amounts to approximately 6.5 mm. Benefits of these structures are that the full length of the modulator is approximately 13.5 mm and as understood in the art, the length is important for packaging. For Mach-Zehnder modulator designs of this type, the higher the EO coefficient, r 33 , the shorter the device length. For this type of device to be useful, the waveguide loss is preferably reduced to &lt;2.2 dB/cm, assuming 1.5 dB coupling loss per end can be achieved, and a total insertion loss of 6 dB is acceptable. With an rf design, the performance at these levels is &gt;50 GHz. The very big advantage and key aspect of the present invention is that external rf drivers are not required. 
     Turning now to  FIGS. 8A and 8B , prior art Mach-Zehnder type modulators with Y-splitters/combiners attached and with MMI splitters/combiners attached, respectively, are illustrated. Input or modulating signals are applied to the modulators by means of U-shaped electrodes overlying each arm of the Mach-Zehnder modulator in a well-known manner. In these prior art examples rf modulating signals (set at approximately 0.5V in this example) are applied to the electrodes by way of rf drivers connected to one end of one of the U-shaped electrodes, the other end being connected through a 50-ohm termination to ground. It will of course be understood that the driver adds substantial cost, size, complexity, DC power, heat dissipation, etc. to any packaging of the modulator 
     Referring additionally to  FIGS. 9A and 9B , Mach-Zehnder-type modulators, fabricated in accordance with the present invention, with Y-splitters/combiners attached, designated  20 , and with MMI splitters/combiners attached, designated  20 ′, respectively, are illustrated. Input or modulating signals are applied to each of modulators  20  and  20 ′ by means of U-shaped electrodes,  22  and  23  for modulator  20  and  22 ′ and  23 ′ for modulator  20 ′, overlying each arm of the Mach-Zehnder modulator. In accordance with the present invention, an rf modulation signal  25  of approximately 0.5V, and generally in a range of 0.2V to 0.5V, is applied directly to one end of U-shaped electrode  22  and the other end is connected through a 50 ohm termination  26  to ground. Similarly, an rf modulation signal  25 ′ of approximately 0.5V is applied directly to one end of U-shaped electrode  22 ′ and the other end is connected through a 50-ohm termination  26 ′ to ground. Thus, two examples of direct-drive polymer modulators (DDPM) are illustrated in  FIGS. 9A and 9B . In this preferred embodiment, each of modulators  20  and  20 ′ use an EO polymer with an r 33 &gt;250 pm/V and generally in a range of 250 pm/V to 750 pm/V. Also, modulators  20  and  20 ′ include at least 3-layer waveguides with customized cladding layers having relatively high conductivity (see explanation above) which allows the DDPM waveguides to be optimally poled. Further, the waveguides have propagation losses less than 2.2 dB/cm and insertion losses less than 6 dB. Suitable EO polymers are commercially available and include, for example, the nonlinear optical chromophores described in U.S. Pat. Pub. Nos. US2009/0005561, US2007/0260063, US2008/0139812, US 2007/0260062 and US 2012/0267583, the entire contents of which are incorporated herein by reference. 
     Turning now to  FIG. 10A , a specific example of a 3-layer polymer modulator/waveguide  30  in accordance with the present invention is illustrated. Modulator/waveguide  30  includes an InP platform/substrate  32  with a ridge waveguide  34  monolithically formed thereon. It should be understood that substrate  32  could be silicon or any other semiconductor material and the InP is simply for purposes of example. Waveguide  34  may be fabricated using any convenient method for the specific application. In this specific example the first material deposited on the InP surface is a metallization  35 , such as Au, Ti/Au, or Cr/Au, in this example sequential layers of Ti/Au/Ti are deposited. A bottom cladding layer  36  is deposited on metallization  35 , a core  37  with upwardly directed ridge is deposited on bottom cladding layer  36 , and a top cladding layer  38  is deposited on core  37 . Modulator/waveguide  30  is referred to as a “ridge waveguide”. The modulator generally includes coplanar electrodes for poling and modulation unless a metal layer is deposited on the InP substrate below the polymer layers, hence metallization  35 . 
     In this and the following examples, the material for cladding layers  36  and  38  is selected to have the following characteristics. The refractive index of the cladding material at 1550 nm is equal to the refractive index of the material of core  37  minus approximately 0.05. The coefficient of thermal expansion (CTE) of the cladding material is as close to the CTE of the material of core  37  as possible. The Tg of the cladding material is near the Tg of the core material (generally 150° C. to &gt;200° C.). The resistivity of the cladding material, at room temperature, is greater than approximately 10 8  Ohm-cm. The resistivity of the cladding material, at poling temperature (˜Tg), is much less than the resistivity of the core material (&lt;˜10 8  Ohm-cm). The optical loss of the cladding material is less than 1 dB/cm from 1300 nm to 1600 nm. The dielectric loss tangent of the cladding material is less than 0.002 at 10 GHz. The cladding material is not chemically attacked by the core material, core material solvents, or photoresist solvents. The cladding material forms an adhesive bond with Au, Ti/Au, or Cr/Au metallization and with the core material. In addition, the cladding material must be dry etchable with reasonable etch rates, the endfaces must be able to be polished, cleaved, or direct cut, and it must provide a hermetic or at least an O 2  block. 
     Referring additionally to  FIGS. 10B and 10C , 4 and 5 layer ridge waveguides are illustrated, respectively. The 4-layer ridge waveguide illustrated in  FIG. 10B  is basically the same as the 3-layer ridge waveguide illustrated in  FIG. 10A , except that an additional cladding layer  39  is deposited on the upper surface of core layer  37 . The 5-layer ridge waveguide illustrated in  FIG. 10C  is basically the same as the 4-layer ridge waveguide illustrated in  FIG. 10B , except that an additional cladding layer  33  is deposited on the bottom cladding layer  36 , i.e. between core layer  37  and the previous bottom cladding layer  36 . Additional cladding layers  39  and/or  33  may be added to enhance the characteristics of the cladding layers, allow the selection of a wider range of materials, further enhance the operation, etc. 
     A modulator/waveguide  30 ′ is illustrated in  FIG. 11A , which is the same as modulator/waveguide  30  of  FIG. 10A  except that an inverted ridge and core are included. Modulator/waveguide  30 ′ is referred to as an “inverted ridge waveguide”. Referring additionally to  FIGS. 11B and 11C , 4 and 5 layer inverted-ridge waveguides are illustrated, respectively. The 4-layer inverted-ridge waveguide illustrated in  FIG. 11B  is basically the same as the 3-layer inverted-ridge waveguide illustrated in  FIG. 11A , except that an additional cladding layer  39 ′ is deposited on the upper surface of core layer  37 ′. The 5-layer inverted-ridge waveguide illustrated in  FIG. 11C  is basically the same as the 4-layer inverted-ridge waveguide illustrated in  FIG. 11B , except that an additional cladding layer  33 ′ is deposited on the bottom cladding layer  36 ′, i.e. between core layer  37 ′ and the previous bottom cladding layer  36 ′. Additional cladding layers  39 ′ and/or  33 ′ may be added to enhance the characteristics of the cladding layers, allow the selection of a wider range of materials, further enhance the operation, etc. 
     Turning now to  FIG. 12 , a direct-drive polymer modulator (DDPM)  40  is illustrated in accordance with the present invention. DDPM  40  includes a 3-layer stack forming a ridge waveguide similar to the example illustrated in  FIG. 10A  with an InP platform/substrate  42  having a ridge waveguide  44  monolithically formed thereon. Ridge waveguide  44  may be fabricated using any convenient method for the specific application. In this specific example, the first material deposited on the InP surface is a metallization  45 , such as Au, Ti/Au, or Cr/Au, in this example sequential layers of Ti/Au/Ti are deposited. A bottom cladding layer  46  is deposited on metallization  45 , a core  47  with upwardly directed ridge is deposited on bottom cladding layer  46 , and a top cladding layer  48  is deposited on core  47 . An electron blocking layer  49  is deposited on the upper surface of top cladding layer  48  above the ridge formed in the core  47  and a top microstrip conductor  50  is deposited on the upper surface of electron blocking layer  49 . Electron blocking layers or charge barriers can be composed of thinly deposited metal oxides such as titanium oxide, spin on glasses such as PDMS, organic materials such as BCB, Organo-silane compounds, as well as polyvinylcarbazole. These materials provide an electrical charge barrier to reduce leakage in the device. In this specific example, metallization  45  acts as a microstrip bottom conductor and top microstrip conductor  50  completes the microstrip input. Thus, a simple and inexpensive microstrip input is conveniently integrated into the present inventive direct-drive polymer modulator. Similarly, a direct-drive polymer modulator (DDPM)  40 ′ with inverted-ridge waveguide and microstrip input is illustrated in  FIG. 13 . 
     Referring additionally to  FIG. 14 , a flow chart is illustrated showing a preferred process for the fabrication of ridge waveguides, such as ridge waveguide  30  illustrated in  FIG. 10A . The process starts with a platform/substrate having, in this preferred embodiment a trench formed for the deposition of the ridge waveguide. The trench may also have a metallization on the bottom surface, or not. Starting with these features, in a first step  60  a bottom cladding layer is deposited. In a second step  61  a core layer is deposited. A third step  62  includes forming waveguide ridges, which in this preferred example includes reactive ion etching. A fourth step  63  consist of depositing a top cladding layer. A seed layer is deposited on the upper surface of the structure, step five  64 , and Au electrodes are plated on the seed layer, step six  65 . The seed layer is etched to separate the Au electrodes, step seven  66 , and the core is poled, step eight  67 , using the Au electrodes to apply the proper field. In a step nine  68 , the Au poling tab electrodes are removed by etching and in a final step  69  the waveguide is completed by preparing the endfaces as required by the specific type of device being fabricated. 
     Referring additionally to  FIG. 15 , a flow chart is illustrated showing a preferred process for the fabrication of an inverted-ridge waveguides, such as ridge waveguide  30 ′ illustrated in  FIG. 11A . All of the steps in the process of fabricating an inverted-ridge waveguide are the same as those described above for a ridge waveguide, except that RIE waveguide inverted ridges are formed in the bottom cladding in a step preceding the deposition of the core layer. 
     Referring now to  FIG. 16 , an example of multiple DDPM structure, designated  80 , of the type described above are arrayed on a generic InP chip  82 . The chip can be silicon based or an InP substrate may be deposited on the silicon and becomes the platform as used in the above descriptions. The various components may be fabricated in silicon or, as in this specific example the InP chip becomes the platform as used in the above descriptions. In the example illustrated, four monolithic lasers, designated  83 , are formed in the InP platform/substrate. Four direct-drive polymer modulators  84  are formed in the InP platform/substrate in optical alignment with the four lasers. While four laser/polymer modulator sets are illustrated in this example, it will be understood that to form a coarse wavelength division multiplexing (CWDM) transmitter chip, several lasers each of a different wavelength are formed on the InP chip and an array of, for example, polymer photonic (PP) modulators are fabricated to align one to each laser. The output guides could be directed to a multiplexer, for example an echelle grating or ring resonator array (not shown), and the output of the multiplexer could be coupled to an optical fiber. In this example, 4×25 G modulators result in a 100 G transmitter chip or 8×50 G could be provided for a 400 G transmitter chip. 
     Referring additionally to  FIG. 17 , an example of a package for multiple DDPM structure  80  is illustrated. In this example, multiple DDPM structure  80  is mounted in an optical subassembly  90 . Optical subassembly  90  is then mounted in a package module  95  designed around optical subassembly  90 . 
     In the various above described structures, it is most desirable to include a polymer modulator that can be efficiently coupled into a photonic integrated circuit (PIC), for example. A polymer modulator  230  having the desired characteristics is illustrated in  FIGS. 18, 19 and 20 , described in detail below. Additional details as to the various characteristics, materials included, operation and methods of construction, etc. can be obtained from copending United States patent application entitled “Active Region-Less Modulator and Method”, application Ser. No. 16/680,900, filed 12 Nov. 2019, and incorporated herein by reference. While a Mach-Zehnder modulator is used in the examples below, it will be understood that other modulators can used, including intensity modulators, for example, a Mach-Zehnder modulator, a directional-coupler modulator, a polarization modulator, etc. 
     Turning now to  FIGS. 18 and 19 , polymer modulator  230  fabricated as a Mach-Zehnder modulator is illustrated in which a passive core region  232  is formed that extends from an optical input  234 , through a Y-branch  235 , into legs  237  and  238 , into another Y-branch  231 , and to an optical output  236 . Legs  237  and  238  include parallel central portions  237   a  and  238   a  which include the modulation area of modulator  230 . Shaped active components  239  and  240  are formed on passive core region  232  and overlying central portions  237   a  and  238   a  of legs  237  and  238 . Shaped active components  239  and  240  are preferably fabricated with a host-guest system made of a host polymer and a guest chromophore. The active component is the electro-optic guest-host that is polled to align dipoles that will promote the modulation of light with electrical energy (i.e. active component). Additional poling information can be found in a copending U.S. patent application entitled “Polymer Modulator and Laser Integrated on a Common Platform and Method”, Ser. No. 15/692,080, filed 31 Aug. 2017 and included herein by reference. Active components  239  and  240  are confined within the periphery of the chip and do not reach the outside edges. Passive core region  232  can be any material that defines a core for a waveguide, other than an active polymer, and in this invention, is designed for more efficient optical coupling to optical fibers and free space light at optical input  234  and optical output  236 . Because passive core region  232  is not active and not included in the modulation process, the dimensions are less critical and passive core region  232  can be fabricated with, for example, a larger cross-section. In this example passive core region  232  is sized larger for higher more tolerance alignment with single mode optical fibers. 
     Referring additionally to  FIGS. 18 and 19 , the various components of Mach-Zehnder modulator  230  are illustrated in more detail. A substrate  242  is provided of a semiconductor material such as silicon, InP, or the like which allows Mach-Zehnder modulator  230  to be integrated directly into other electronic circuitry and more specifically into a PIC. An insulating layer  244  of an oxide, such as silicon oxide or the like, is deposited or formed on substrate  242 . Lower or ground electrode  246  of some material, such as gold, is formed on insulating layer  244  at least beneath central portions  237   a  and  238   a  of legs  239  and  240  of modulator  230 . A lower cladding layer  248  is deposited over lower electrode  246  and planarizes the structure. Passive core region  232  is deposited on lower cladding layer  248  with a lower surface abutting an upper surface of lower cladding layer  248 , as described above. A side cladding layer  249  (see  FIG. 20  (D)) is deposited beside passive core region  232  to planarize the structure. A shaped portion  250  of an upper cladding layer  252  is deposited over passive core region  232  and side cladding layer  249  and shaped active components  239  and  240  are deposited over passive core region  232  between shaped portions  250  of upper cladding layer  252 . Shaped active components  239  and  240  have lower surfaces that abut upper surfaces of passive core region  232 . A remaining portion of upper cladding layer  252  is deposited to enclose shaped active components  239  and  240  and has a lower surface that abuts an upper surface of shaped active components  239  and  240 . It should be understood that shaped portion  250  and shaped active components  239  and  240  can be deposited in any order commensurate with the particular shapes selected. Referring specifically to  FIG. 24 , upper electrodes  254  and  256  are formed on upper cladding layer  252  overlying portions of shaped active components  239  and  240  and lower electrode  246 . Upper electrodes  254  and  256  are preferably formed of some easily deposited metallic material such as gold or the like. Upper cladding regions  250  and  252  are designed to allow adiabatic transition of the input optical wave up from passive region core  232  to active components  239  and  240  and back down to passive region core  232  for exiting the modulator. In this disclosure the term “adiabatic transition’ is defined as meaning a transition of light from one region to another with substantially no loss or gain of energy. 
     In the operation of modulator  230  in  FIG. 19  (for example), the main mode of single mode light enters passive core region  232  at the left and travels from left to right. The main optical mode travels until it sees a higher refractive index than the refractive index of passive core region  232 . In modulator  230 , as an example, the refractive index of passive core region  232  is 1.5 and the refractive index of shaped active components  239  and  240  is 1.7. In modulator  230 , as the main optical mode travels to the right, it slowly sees the higher refractive index of the shaped active components  239  and  240 , because of the slanted or angled portion  251  of shaped portion  250  of upper cladding layer  252 , and slowly leaks into the higher refractive index material as active components  239  and  240  are brought closer to passive core region  232 . The slanted portion  251  of shaped portion  250  is defined herein as an adiabatic transition and the angle determines the amount or speed with which the main optical mode leaks into active components  239  and  240 . It should be noted that both ends of active components  239  and  240  are tapered, along with shaped portion  250 , so that the main optical mode leaks into active components  239  and  240  at one end and back out into passive core region  232  at the other end. In this example the optical properties and the angle are intentionally designed to leak the main optical mode into active components  239  and  240 . Preferably the angle of slanted portion  251  is about 0.4 degrees to 1 degree to the horizontal although the adiabatic transition and the optical properties and angle can be designed so that the main optical mode will leak at higher angles (e.g. up to 30 degrees) but increased light will be lost. To maintain a good leakage to higher order material, a slow adiabatic transition is preferred from the passive region to the active component and then back down to the passive region. 
     Turning now to  FIG. 20  steps (A) through (H) in the fabrication of polymer modulator  230  as a Mach-Zehnder modulator are generally illustrated. In step (A), lower electrode  246  is deposited on insulating layer  244  on substrate  242 . Lower cladding layer  248  is deposited on lower electrode  246  in step (B). In step (C), passive core region  232  is formed on lower cladding layer  248  in the shape of a Mach-Zehnder modulator with an optical input  234 , an optical output  236  and legs  237  and  238 . Side lower cladding layer  249  is deposited in step (D) to planarize the structure. Shaped portion(s)  250  of upper cladding layer  252  are formed on portions of passive core region  232  and side lower cladding layer  249 . Shaped portion(s)  250  preferably include evanescent optical coupling planes  256  and  258 . In step (F), shaped active components  239  and  240  are formed on the exposed portions of passive core region  232  (central portions of legs  237  and  238 ) between evanescent optical coupling planes  256  and  258  of shaped portion(s)  250 . Here it should be understood that light is coupled from passive core region  232  up into active components  239  and  240  and back down into passive core region  232  by an adiabatic transition enhanced by the evanescent optical coupling. Upper cladding layer  252  is completed in step (G) to enclose active components  239  and  240  and planarize the structure. Upper electrodes  260  and  262  are formed on upper cladding layer  252  with central portions  254  and  256  overlying active components  239  and  240 , respectively. 
     It should be noted that in active region-less polymer modulator  230  the active components do not reach the edge of the chip but are confined within the periphery of the chip (hence ‘region-less’). The use of a passive core with EO polymer active components adiabatically coupled thereto results in more efficient optical coupling between the modulator chip and the outside world, and with higher reliability, since the active material is never exposed to the outside world-atmosphere and with higher reliability, since the active material is not subject to optical reflections, rough surfaces, and other effects that could affect optical light transmission. Also, waveguiding in a Mach-Zehnder modulator results in equal separation between the two branches. 
     Thus, a new and improved direct-drive region-less polymer modulator is disclosed. The direct-drive region-less polymer modulator is smaller than prior-art modulators that required external rf drivers and can easily be integrated in an operating structure and/or on a common platform with semiconductor lasers and the like. The laser and polymer modulator can be integrated on a common platform with more efficient coupling between source laser and optical modulator. Efficient 3-layer direct-drive polymer modulators are disclosed that have higher performance (multi GHz), and very low voltage to allow direct drive without the use of a driver circuit. 
     Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.