Patent Publication Number: US-2010121016-A1

Title: Low refractive index hybrid optical cladding and electro-optic devices made therefrom

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefit under 35 U.S.C. §119(e) from, and to the extent not inconsistent with this application, incorporates by reference herein U.S. Provisional Patent Application Ser. No. 61/097,166; filed Sep. 15, 2008; entitled “LOW REFRACTIVE INDEX HYBRID OPTICAL CLADDING AND ELECTRO-OPTIC DEVICES MADE THEREFROM”; invented by Danliang Jin, Guomin Yu, and Hui Chen. 
     This application is also related to U.S. Provisional Patent Application Ser. No. 61/097,172 (attorney docket number 2652-022-02); filed Sep. 15, 2008; entitled “ELECTRO-OPTIC DEVICE AND METHOD FOR MAKING LOW RESISTIVITY HYBRID POLYMER CLADS FOR AN ELECTRO-OPTIC DEVICE”; invented by Danliang Jin, Guomin Yu, Anna Barklund, Hui Chen and Raluca Dinu; and to the extent not inconsistent. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The inventions disclosed herein were made the U.S. Government support pursuant to NRO Contract No. NRO000-07-C-0123 and DARPA Contract No. W31P4Q-08-C-0198. Accordingly, the Government may have certain rights in the inventions disclosed herein. 
    
    
     BACKGROUND 
     Electro-optic devices, and especially poled hyperpolarizable organic chromophore-based electro-optic devices have typically been limited to using hybrid organic-inorganic cladding materials that have a relatively high index of refraction. For example, a crosslinked hybrid organic-inorganic silicon sol-gel may have an index of refraction of 1.45 to 1.47 at a wavelength of 1550 nanometers (nm). Other crosslinked hybrid organic-inorganic sol-gels made from titanate, aluminate, or zirconate precursors have also typically had respective indices of refraction that are substantially determined according to the particular type of sol-gel (i.e. titanium, zirconium, or aluminum-based). 
     SUMMARY 
     According to embodiments, a hybrid organic-inorganic cladding may be made including at least one precursor having a covalently bound fluorinated organic group. The fluorinated group may reduce the index of refraction of the cladding. 
     According to embodiments, a silicon sol-gel cladding may include covalently bound fluorinated groups that reduce the index of refraction of the cladding to below 1.45. According to embodiments, the index of refraction may be between about 1.35 and 1.44. 
     According to embodiments, an electro-optic device may include a hybrid organic-inorganic cladding may be made including at least one precursor having a covalently bound fluorinated organic group. The fluorinated group may reduce the index of refraction of the cladding. 
     According to embodiments, an electro-optic device may include silicon sol-gel cladding having covalently bound fluorinated groups that reduce the index of refraction of the cladding to below 1.45. According to embodiments, the index of refraction may be between about 1.35 and 1.44. The electro-optic device may include an electro-optic core having poled chromophores in a polymer matrix. The polymer matrix of the core may also include silicon sol-gel cladding having covalently bound fluorinated groups. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a cross-sectional diagram of an electro-optic device, according to an embodiment. 
         FIG. 2  is a simplified diagram of system including an electro-optic device of  FIG. 1 , according to an embodiment. 
         FIG. 3  a flow chart showing a method for making a hybrid organic-inorganic optical cladding according to an embodiment. 
         FIG. 4  is a cross-sectional diagram of an alternative device structure, according to an embodiment. 
         FIG. 5  is a cross-sectional diagram of another alternative device structure, according to an embodiment. 
         FIG. 6  is a diagram illustrating a device at several steps of fabrication, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or and other changes may be made without departing from the spirit or scope of the disclosure. 
       FIG. 1  is a cross-sectional diagram of an electro-optic device  101 , according to an embodiment. The electro-optic device  101  includes an electro-optic core  102  disposed between optical clads  104  and  106 . The electro-optic device  101  may be formed over a substrate  108  such as silicon, silicon-on-insulator, glass, or other semiconducting or insulating wafer. Two electrodes  110 ,  112  are arranged to apply a modulation voltage across the electro-optic core  102  through the clads  104 ,  106 . One or more light guiding structures  114 , such as a trench waveguide, etc. may be provided to guide light transmitted through the electro-optic core  102  for modulation. 
     The electro-optic core may include at least one type of hyperpolarizable organic chromophore and cross-linked polymer. The at least one hyperpolarizable organic chromophore and the polymer may form a guest-host material. Alternatively, the hyperpolarizable organic chromophore may be covalently bonded to the cross-linked polymer, or may be otherwise held in the cross-linked polymer. The cross-linked polymer may include an organic polymer, such as amorphous polycarbonate for example, or may include a hybrid material such as a sol-gel. 
     Typically, the electro-optic core material is poled, ideally to substantially align the chromophores. The core may be poled by applying a poling voltage from a poling electrode (not shown in  FIG. 1 ) across the electro-optic core  102  through some or all of the cladding  106 ,  104  thickness while the device  101  is heated to near a glass transition temperature, Tg, of the polymer in the core. After the chromophores are aligned, the device  101  is cooled to “lock” the chromophores into their poled orientations. The poling electrode  116  may include a temporary electrode that is removed after poling. Alternatively, a modulation electrode  112  may be used as a poling electrode  116 . 
     According to embodiments, the optical index of refraction or refractive index of the material in at least one of the optical clads  104 ,  106  is lower than the index of refraction of previous polymer cladding materials and especially of previous hybrid organic-inorganic polymer cladding materials, such as organic-inogranic sol-gels hybrids, which may typically have an index of refraction of 1.45 to 1.47 at 1550 nm. For example, the optical clads  104 ,  106  may have indices of refraction of about 1.35 or lower to just below the 1.45 to 1.47 index of previous materials. According to another embodiment, the optical clads  104 ,  106  may have indices of refraction between 1.39 or lower to just below 1.45. According to another embodiment, the optical clads  104 ,  106  may have indices of refraction of between 1.391 and 1.404. 
     The reduced index of refraction may be used to increase index contrast between the electro-optic core  102  and one or both of the clads  104 ,  106 . Alternatively or additionally, the reduced index of refraction at least one clad  104 ,  106  may allow modifications to the electro-optic core  102 , such as to decrease the index of refraction of the electro-optic core  102 , increase the size of the electro-optic core  102  while maintaining numerical aperture, etc. 
     According to other embodiments, fluorinated organically modified sol-gel precursors of silica, titania, zirconia, and/or alumina may combined with non-fluorinated organically modified sol-gel precursors of silica, titania, zirconia, and/or alumina along with hydrolysable precursors of silica, titania, zirconia, and/or alumina to produce a hybrid sol-gel optical cladding having a selected index of refraction. 
       FIG. 2  is a simplified diagram of system  201  including an electro-optic device  101 , according to an embodiment. In operation, light  202  such as laser light from a laser  204  at an infrared wavelength may be passed through the electro-optic core  102 . To provide light guidance and minimize optical losses, the optical clads  104 ,  106  typically have indices of refraction that are lower than the index of refraction of the electro-optic core  102 . For example, according to an embodiment, the nominal index of refraction of the electro-optic core  102  may be about 1.5 to 1.8 and the index of refraction of the clads  104 ,  106  may be less than 1.45 to 1.47. According to another embodiment, the nominal index of refraction of the electro-optic core  102  may be less than about 1.5-1.8 and the index of refraction of the clads  104 ,  106  may be less than about 1.45-1.47. According to another embodiment, the index of refraction of the clads 104, 106 may be about 1.35 to about 1.4. 
     During operation, one electrode  110  may be held at ground while the other electrode  112  is voltage modulated. In some applications, the electrode  112  may be a top electrode that is provided in the form of a high speed strip electrode configured to propagate modulation pulses along its length, parallel to and preferably at least somewhat velocity-matched to the propagation of light through the electro-optic core  102 . The poled hyperpolarizable chromophore in the electro-optic core  102  responds to the modulation voltage with a corresponding change in refractive index, which operates to modulate the phase of the propagated light  202 . A device may be used to provide a phase-modulated light signal  206  for transmission through a network  208 . Alternatively, a device, such as in a Mach-Zehnder modulator, may include plural optical channels, each modulating a portion of coherent light, which when the light is rejoined, may destructively or constructively interfere to provide an amplitude-modulated light signal  206  for transmission. 
     According to embodiments, the electro-optic device  101  may be combined with other components in an integrated device  210 . Such components may include a receiving circuit  212  configured to receive one or more signals along an input signal transmission path  213  from a network  214  or other signal source, and drive electronics  216  configured to provide the drive signal to the electrodes  110 ,  112 . 
     According to embodiments, the bottom clad  104  may be about 1-2 microns thick below the waveguide  114  and/or about 2-2.4 microns thick without the trench waveguide  114  or at locations not corresponding to a trench waveguide  114 . The electro-optic core  102  may be about 3 microns thick including a trench waveguide  114  and/or about 2 microns thick without the trench waveguide  114  or at locations not corresponding to the trench waveguide  114 . The top clad may be about 0.5 to 2.0 microns thick. 
     Referring again to  FIG. 1 , the low index of refraction material in the cladding layers  104 ,  106  includes a hybrid organic-inorganic material. The hybrid organic-inorganic material may be referred to as a sol-gel material. The chemical structure of the sol-gel may be expressed as: 
     
       
         
         
             
             
         
       
     
     where M is Si, Ti, Al, or Zr;
     R is a hydrolizable group;   R 1  is an organic crosslinker;   R 2  is a fluorinated organic group or fluorine; and   n1, n2, and n3 may be modified to provide selected mechanical, electrical, and/or optical properties.   

     The actual physical structure of a cured clad  104 ,  106  is typically a three-dimensional matrix of M&#39;s linked in an amorphous gel by a combination of M-O-M and M-R 1 -M linkages with pendent (unlinked) R (e.g., a trace amount), R 1 , and R 2  groups. This may be depicted as: 
     
       
         
         
             
             
         
       
     
     RO— may be a hydrolysable alkoxy group such as —OCH 3  or —OCH 2  CH 3 , or in hydrolyzed form, —OH. The M-O— backbone may link or gel to form silicate, titanate, aluminate, or zirconate (M-O-M) bonds through displacement of the hydrolysable groups (e.g. after being fully condensed). 
     R 1  is a reactive organic crosslinker such as an epoxy: 
     
       
         
         
             
             
         
       
     
     (e.g. glycidyl propyl ether), or an acrylate: 
     
       
         
         
             
             
         
       
     
     where R 3 , R 4 , and R 5  are alkyl or aromatic groups. 
     The organic crosslinker R 1  may link the M backbone through an organic (M-R 1 -M) linkage. Thus, the sol-gel includes both -M-O-M- and -M-R 1 -M- linkages. The M-O-M linkage is a very reactive linkage with large number density that may tend to make the material brittle. The inclusion of M-R 1 -M linkages may significantly improve the toughness of the material and makes it more processible and suitable for use as an optical cladding. 
     R 2  is a fluorinated organic group or fluorine that is pendent on the M backbone. For example, R 2  may be a partially or substantially fully fluorinated alkyl or aryl group. According to embodiments, R 2  may include fluorine, —F; a perfluorododecyl-1H,1H,2H,2H-triethyl group, —CH 2 —CH 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 3 ; a perfluorotetradecyl-H,1H,2H,2H-triethyl group, —CH 2 —CH 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 2 —CF 3 ; a pentafluorobenzyl group, 
     
       
         
         
             
             
         
       
     
     or other fluorinated and relatively non-reactive group. The fluorinated group R 2  tends to reduce the index of refraction of the organic-inorganic hybrid clads. Generally, a larger proportion of fluorinated groups within the hybrid materials composition may provide a greater reduction in index of refraction. The fluorinated groups R 2  listed above are selected based on their relatively wide commercial availability. Other fluorinated groups may be substituted as desired. 
     The stoichiometery of the groups 
     
       
         
         
             
             
         
       
     
     may be varied according to device design considerations, cost, etc. For example, larger values of n1 may tend to make the cladding  104 ,  106  relatively hard but also relatively brittle. Larger values of n2 may tend to make the cladding  104 ,  106  tougher. Larger values of n3 tend to reduce the index of refraction. According to one embodiment where M was 100% Si, R 2  was —(CH 2 ) 2 (CF 2 ) 5 CF 3 , and n1=2, n2=1, and n3=1; an optical cladding  104 ,  106  was produced having an index of refraction of 1.397 at 1550 nm. A comparison of two compositions, LIP1 and LIP2 with differing n1, n2, and n3 values is presented in the table below: 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                 tridecafluoro- 
                   
               
               
                   
                   
                 3- 
                 tetrahydro- 
               
               
                   
                   
                 glycidoxypropyl- 
                 octyltrieth- 
               
               
                   
                 Tetraethoxysilane 
                 trimethoxysilane 
                 oxysilane 
                 Index at 
               
               
                   
                 (mole) 
                 (mole) 
                 (mole) 
                 1550 nm 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 LIP1 
                 0.48 
                 0.12 
                 0.24 
                 1.3940 
               
               
                 LIP2 
                 0.20 
                 0.10 
                 0.20 
                 1.4040 
               
               
                   
               
            
           
         
       
     
     For low index of refraction optical clads  104 ,  106 , M=Si provides the lowest starting index of the group Si, Ti, Al, Zr, and may thus provide the lowest index of refraction. Fluorinated groups R 2  may alternatively be added to non-silicon sol-gels or partially non-silicon sol-gels to tune the index of refraction and/or to tune the dielectric constant. Adding fluorinated groups R 2  may generally decrease the dielectric constant of the optical cladding  104 ,  106 . Sol-gels produced from combinations of M&#39;s, for example Si and Ti, may be used to provide indices of refraction between the two materials when used alone. 
     For example a cladding  104 ,  106  with the formula: 
     
       
         
         
             
             
         
       
     
     (with no fluorinated groups R 2 ) may have an index of refraction of about 1.8, while a cladding  104 ,  106  with the formula: 
     
       
         
         
             
             
         
       
     
     (also with no fluorinated groups R 2 ) may have an index of refraction of about 1.45 to 1.47. Mixing titania and silica precursers to form a hybrid sol-gel having the formula: 
     
       
         
         
             
             
         
       
     
     (with no fluorinated groups R 2 ) may provide an approximate index of refraction equal to the average of the Ti- and Si-based sol-gels: ((1.45)+(1.8))/2=1.625. Similarly, combining titania and silica precursors in different ratios will generally produce a weighted average of the individual indices of refraction. 
     Adding fluorinated groups decreases the index of refraction further, and may allow the designer an extra degree of freedom with respect to the properties of the cladding  104 ,  106 . 
     To reduce the electrical resistivity, the hybrid organic-inorganic cladding material may be doped with an inorganic or organic salt. The concentration of the salt may be at a concentration equal to or less than about 5%, for example. According to an embodiment, the cladding is doped with an inorganic salt of lithium, sodium, or potassium at a concentration equal to or less than about 2%. According to an embodiment, the cladding is doped with lithium perchlorate at a concentration of between about 1% and 3%. According to an embodiment, the cladding may be doped with lithium perchlorate at a concentration of about 2%. 
       FIG. 3  is a flow chart showing a method  301  for making a hybrid organic-inorganic optical cladding according to an embodiment. In step  302 , a sol-gel solution including a sol-gel precursor for silica : 
     
       
         
         
             
             
         
       
     
     and organically modified silica precursors: 
     
       
         
         
             
             
         
       
     
     is mixed in solution. As described above,
     R is a hydrolizable group;   R 1  is an organic crosslinker; and   R 2  is a fluorinated organic group or fluorine.   

     According to embodiments, the silica precursor and organically-modified silica precursors may be mixed in a solution at a wide range of molar ratios. For example, embodiments of low index sol-gels whose indices of refraction are disclosed in this application include molar ratios of about 2:1:1, 2:1:2, and 4:1:2 silica precursor:cross-linker modified precursor:fluorination modified precursor. 
     Specific embodiments may be made by reference to the following examples: 
     Example 1 (LIP3):
         1. To a 1 liter round bottom flask, 49.92 gram (0.24 mol) of, 56.76 gram (0.24 mol) of 3-glycidoxypropyltrimethoxysilane, 91.87 gram (0.18 mol) of tridecafluoro-tetrahydrooctyltriethoxysilane, and 179 gram of ethanol were charged with magnetic stirring.   2. Mixed 50.88 gram of H 2 O and 9.8 gram of 2M HCl.   3. Slowly dropped the acid into the round bottom flask and stirred until the solution became clear.   4. The flask was equipped with condenser and purged with nitrogen and immersed in a 60° C. oil bath. The solution was maintained refluxing for four hours. The solution was then cooled to room temperature.   5. 1.60 gram of aluminum acetylacetonate was added into the solution while stirring. The solid dissolved slowly and a clear solution was obtained.   6. The solution was aged overnight. The solution was then ready for thin film deposition.       

     Example 2 (LPT1): 
     Preparation of a Fluorinated Sol-Gel:
         A fluorinated sol-gel was prepared by adding 99.96 g (0.48 mol) of tetraethoxysilane, 236.3 g (0.12 mol) of 3-glycidoxypropyltrimethoxysilane, 122.40 g (0.24 mol) of tridecafluorotetrahydrooctyltriethoxysilane, and 312 g of isopropyl alcohol to a 1 L round bottom flask. The resulting mixture was stirred and a solution of 4.32 g of 2M DCI in 60 g of D 2 O was added dropwise slowly until the mixture became clear. The resulting solution was refluxed for 3 h then allowed to cool to room temperature overnight. The isopropyl alcohol and other volatile reaction products were removed under reduced pressure. The resulting solution was diluted with 200 g of n-butanol, 40 g of cyclopentanone, and stored in 0° C. refrigerator.       

     Proceeding to step  304 , the solution is applied to a surface. For example, the solution may be spin-coated or sprayed onto a substrate such as a silicon, glass, or silicon-on-insulator wafer. The substrate may include one or a plurality of bottom electrodes ( FIG. 1 ,  110 ).
         For example, following steps 1-6 from Example 1, above:   7. The solution was spin-coated onto a silicon wafer having a plated or sputtered gold conductor at 1000 rpm and cured on a hot plate at 150 C for 1 hr. The film thickness was 2.0-2.2 mm. The refractive index was about 1.397 at a wavelength of 1550 nm.       

     Next, in step  306 , the applied layer is cured thermally or via an ultraviolet and thermal process. A backbone molecular structure for the cured material may be expressed as: 
     
       
         
         
             
             
         
       
     
     where:
     —OR is a hydrolizable group such as —OCH 2 CH 3 ;   R 1  is an organic crosslinker such as:   

     
       
         
         
             
             
         
       
         
         R 2  is a fluorinated organic group, such as: 
       
    
       —(CH 2 ) 2 (CF 2 ) 5 CF 3 ; 
     where R 3 , R 4 , and R 5  are alkyl or aromatic groups;
     n1=2;   n2=1; and   n3=1.   

     There are two types of gelling or crosslinking mechanisms. One is from the inorganic backbone ( . . . Si—O—Si . . . ) and the other is from the organic crosslinker ( . . . Si—R 1 —Si . . . ). The combination of crosslink types provides for the excellent mechanical and optical properties provided by the hybrid sol-gel clads. 
     Proceeding to step 308, the gelled material is further condensed and cured to form a solid film, which in turn forms the optical cladding. For example the film may be cured by placing the substrate on a hot plate at 150° C. for 1 hr. 
       FIG. 4  is a cross-sectional diagram of an alternative device structure  401 , according to an embodiment. In some embodiments, it may be advantageous to combine the low index of refraction hybrid organic-inorganic cladding layers with one or more other cladding layers formed from more conventional materials. For example, a bottom cladding layer may include a first cladding layer  402  made with a low index of refraction hybrid organic-inorganic material described herein. The bottom cladding may also include another cladding layer  404 . For example, the additional cladding layer  404  may include a higher index of refraction material such as a UV-cured polymer, a cross-linked polymer, a non-fluorinated sol-gel, or another conventional cladding material. The upper cladding layer  106  may be formed from a low index of refraction hybrid organic-inorganic material as described above, or may be made of an alternative material such as a UV-cured polymer, a cross-linked polymer, a non-fluorinated sol-gel, or another conventional cladding material. 
     One attribute of the device structure may be that the etching process used to form the waveguide structure  114  may be performed on an alternative material. Etching an alternative material may be advantageous in some embodiments for process considerations. 
       FIG. 5  is a cross-sectional diagram of another alternative device structure  501 , according to an embodiment. In the embodiment of  FIG. 5 , the bottom low index of refraction hybrid organic-inorganic cladding layer  104  is substituted with another type of cladding  502 . The device  501  uses a bottom clad  502  with dry-etched trench waveguide  114  formed from UV15LV, a conventional ultraviolet-cured cross-linked polymer. The top-cladding  106  is formed from a low index of refraction hybrid organic-inorganic material taught herein. 
       FIG. 6  is a diagram  601  illustrating a device  101  at several steps of fabrication  602  to  612 , according to an embodiment. First, as shown at step  602 , a bottom cladding layer  104  is deposited over a substrate  108  and bottom electrode  110 . The bottom cladding layer may be a low index of refraction hybrid organic-inorganic material as described elsewhere herein. Alternatively, a bottom cladding layer may be formed as a composite with a first cladding layer (see  402  in  FIG. 4 ) made with a low index of refraction hybrid organic-inorganic material described herein and another cladding layer (see  404  in  FIG. 4 ). For example, the additional cladding layer may include a relatively high index of refraction material such as a UV-cured polymer, a cross-linked polymer, a non-fluorinated hybrid sol-gel, or another conventional cladding material. 
     The bottom cladding layer  104  may be deposited as a low index of refraction sol-gel solution, as described above. For example, the bottom cladding layer may be deposited by spraying or spin-coating. Then, the bottom cladding may be dried and cured to form a solid film. For example, the wafer may be kept at about 100° C. to 200° C. for a period of time sufficient to provide the desired mechanical properties. For example, the temperature may be maintained for between 30 minutes and 10 hours. There has not been any detrimental effect found arising from 10 hour or longer dry and cure times. 
     In step  604 , a waveguide structure  114  may be formed in the bottom clad  104 . Generally, the waveguide structure  114  is formed parallel and below a top electrode. Etching may be performed by a number of methods. For example, plasma etching such as reactive ion etching or deep reactive ion etching may be used to form a trench waveguide  114 , and may be advantageous for forming smooth and vertical trench sides. 
     Proceeding to step  606 , a core material  102  including hyperpolarizable (aka non-linear) chromophores is deposited over the bottom cladding  104 , for example by spin-coating or spraying. If the core material includes a polymer material such as an amorphous polycarbonate, the core  102  may be applied from solution during spinning or spraying, and then baked at elevated temperature to remove the solvent. Optionally, the core material may be reheated to reflow the top surface of the core  102  flat. If the core material includes a hybrid organic-inorganic material such as those described herein, the core may be dried and cured similar to the method described in conjunction with step  604  above. The electro-optic core  102  may optionally also include a low index of refraction hybrid organic-inorganic polymer. 
     Proceeding to step  608 , a top cladding  106  is applied over the electro-optic material layer  102 . Preparation, application, drying, and curing of the low index of refraction hybrid organic-inorganic material may be done as described above. Alternatively, the top cladding  106  may include another material such as a UV-cured polymer, UV-cured fluorinated sol-gel materials, a cross-linked polymer, a non-fluorinated sol-gel, or another conventional cladding material. 
     Proceeding to step  610 , a poling electrode  116  may be formed over the upper cladding layer  106 , and the electro-optic core  102  poled to align the chromophores as described above. The top electrode  112 / 116  shown in  FIG. 1  may be configured as a modulation electrode and/or as a poling electrode. In some embodiments, such as that illustrated by  FIG. 6 , the poling electrode  116  may be removed after poling and a high speed electrode formed. 
     During step  610 , the poling electrode  116  may be formed, for example by sputtering or solution plating over the top cladding  106 . During poling, the core material  102  is brought up to near its glass transition temperature. Generally, it may be preferable for the temperature to be within ±10° C. of the glass transition temperature of the cross-linking core polymer. The elevated temperature makes it easier for the polar chromophore molecules to rotate to a parallel orientation responsive to the applied poling voltage. 
     Then, a poling circuit applies a poling voltage to the poling electrode and maintains the bottom electrode  110  at ground. The poling voltage may typically be up to about 900 to 1000 volts, depending on the device configuration. Typically, the poling voltage is maintained for about one to three minutes while the temperature is maintained, then the temperature is allowed to drop. The poling voltage is removed, typically shortly after the temperature reaches room temperature. The reduction in temperature causes the core polymer to drop below its glass transition temperature, which tends to immobilize the chromophores in the poled orientation. 
     According to alternative embodiments, the modulation electrode  112  may be used as a poling electrode  116 . However, the process  601  shows a more conventional approach where separate poling  116  and modulation  112  electrodes are used. 
     Proceeding to step  612 , the poling electrode  116  is stripped off the top of the top cladding  106 . Optionally, an additional thickness of top cladding material may be deposited over the stripped top cladding  106 . Then, a modulation electrode  112  is formed. The modulation electrode  112  is typically configured as a high speed (aka RF) strip electrode configured to conduct modulation signals at very high modulation bandwidths corresponding to optical signal transmission bandwidths. Typically, trace and electrode layouts take propagation delay and signal termination into account to maximize the transmission of in-phase, clean signals while minimizing reflections, impedence, and other deleterious effects. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.