Patent Publication Number: US-7912327-B2

Title: Hybrid strip-loaded electro-optic polymer/sol-gel modulator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of priority under 35 U.S.C. 120 as a continuation-in-part of U.S. application Ser. No. 12/199,765 entitled “Hybrid Electro-Optic Polymer/Sol-Gel Modulator” filed Aug. 27, 2008 now U.S. Pat. No. 7,693,355, which claims benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 60/967,679 entitled “Hybrid Electro-Optic Polymer/Sol-Gel Modulators With Reduced Half-Wave Voltage, Lower Insertion Loss and Improved Contrast” and filed on Sep. 6, 2007, the entire contents of which are incorporated by reference. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was made with government support under NR0000-07-C-0030 awarded by the Department of Defense and under DMR0120967 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electro-optic (EO) modulators and more particularly to hybrid EO polymer/sol-gel modulators in which the sol-gel core waveguide does not lie below the active EO polymer waveguide in the active region. 
     2. Description of the Related Art 
     An EO modulator is fundamentally a device that is able to impress an electrical signal on the amplitude or phase of an optical input through the use of special materials that exhibit an EO effect. In such materials, when an electric field is applied to the material, an associated change in refractive index occurs, which can be used to create various kinds of EO modulators, including phase, Mach-Zehnder modulators and directional coupler modulators. The change in refractive index is directly proportional to the applied electric field, so that this is sometimes called the linear EO effect, which only occurs in materials lacking a center of symmetry, as opposed to the much smaller quadratic EO effect which occurs in all materials. In most applications, the EO modulator is part of a fiber-optic communications system, in which case input and output fibers are aligned and attached to an optical waveguide that has been created in the EO material by a variety of methods. EO modulators are generally used when direct modulation of the laser in the communications system is not a viable option, which occurs when very high bandwidth (&gt;10 GHz) or signal linearity are required. High bandwidth is needed in very high bit rate digital communications systems (OC-768 systems running at 40 Gbps), while both high bandwidth and high signal linearity are required for analog applications such as phased array antennas, optical digital to analog converters, and the like. The cw laser is typically a diode laser in the wavelength region from 800-1600 nm, predominantly near 1310 nm (O band) and near 1550 nm (C and L band). 
     The modulator is characterized by several critical parameters, the most important of which are as follows:
         Half-wave voltage, V π —the voltage change needed to take the modulator from its maximally transmitting state to its minimally transmitting state; it is generally desired that this voltage be as small as possible;   Insertion loss—the optical loss, in decibels, that is suffered by light as it travels from the input to the output of the modulator; it is always desired that the loss be as small as possible;   Extinction—the ratio of the output power in the “on” state to the output power in the “off” state; this ratio should be as large as possible and is generally measured in decibels; and   3 dB bandwidth—the electrical driving signal frequency at which the maximal optical output of the modulator, driven at the low frequency half-wave voltage, has dropped by 3 dB or 50%.       

     For a Mach-Zehnder waveguide EO modulator, the half-wave voltage is related to the various other parameters of interest via: 
                 V   π     =       λ   ⁢           ⁢   d         n   eff   3     ⁢     r   max     ⁢   Γ   ⁢           ⁢   L         ,         
where λ is the optical wavelength, d is the physical separation distance between the drive electrodes in the direction along the applied field, n eff  is the effective refractive index for light polarized along the direction in which the electric field is applied (generally a function of both waveguide materials properties and dimensions), r max  is the maximum value of the EO coefficient (in units of picometers per volt) that can be achieved in the material, which dictates the preferred directions for the applied field and the light polarization, Γ is the normalized dimensionless overlap integral that measures the degree to which the optical and electrical fields overlap, having a minimum value of 0 and a maximum value of 1 and L is the length of the active region of the modulator, defined as that region where an electric field is applied to an EO material waveguide.
 
     A hybrid EO polymer/sol-gel modulator was first described in Enami, et. al.  Appl. Phys. Lett.  82, 490 (2003) and most recently reviewed in Enami, et. al.,  Nature Photonics  1, 180 (2007) and involves using organically modified sol-gels as the cladding and EO polymers as the waveguide in the active region of the modulator and using just sol-gels in the passive region of the modulator where coupling to the optical fiber occurs. These sol-gels have an easily adjustable refractive index, which makes it possible to make waveguides with low coupling loss to optical fiber. They also have been shown to allow for efficient in-device poling of the EO polymer as discussed in C. T. DeRose, et. al.  Appl. Phys. Lett.  89, 131102 (2006) and U.S. Pat. No. 7,391,938. 
     As shown in  FIG. 1 , a hybrid EO polymer/sol-gel modulator  10  is formed on an insulating layer on a substrate (not shown). The modulator includes a bottom electrode  16 , a sol-gel under cladding  18  having a refractive index n 1 , a sol-gel core  20  having a refractive index n 2 &gt;n 1 , and a sol-gel over cladding  22  having a refractive index n 1 . The sol-gel core  20  is confined below and on either side by under cladding  18 . In order to provide a symmetric mode as the light propagates through the waveguide and to provide better coupling to the input and output fibers, the cladding indices n 1  are preferably the same. However, some slight variation (&lt;&lt;1%) may occur within a cladding or between cladding layers due to slight variations in fabrication or design. 
     A vertical taper  23  in layer  22  exposes the surface of core  20  above electrode  16 . An EO polymer waveguide  24  having a refractive index n 3 &gt;n 2  covers the exposed surface of the sol-gel core layer. In typical embodiments, the refractive index n 3  is designed to be uniform over the active region. Small variations from the uniform value on account of fabrication and poling can occur without degrading performance. A non-uniform index profile may be designed to provide the same accumulated phase change. A buffer layer  26  having a refractive index n 4 &lt;n 2 , typically &lt;n 1  and preferably &lt;n 1 −0.1 covers the EO polymer layer. A top electrode  28  defines an active region  30  between itself and bottom electrode  16  and passive regions  32  to either side. A voltage signal V sig    34  is applied between the top and bottom electrodes to apply an electric field along the poling direction of the EO polymer to change the refractive index of the EO polymer waveguide and modulate the amplitude or phase of light  36  passing through the modulator. Modulator  10  may be configured as a phase modulator  38  having one arm as shown in  FIG. 2   a  or as a Mach-Zehnder modulator  40  having a pair of arms as shown in  FIG. 2   b  and with the ability to produce on-chip amplitude modulation. The Mach-Zehnder can be driven using single-arm, dual-drive or push-pull techniques well known to those in the art. 
     Light  36  can be input to and output from the modulator using a standard single-mode optical fiber (SMF-28 from Corning); the dimensions of the input and output sol-gel waveguides (˜4 μm×4 μm) and the refractive index difference (˜1%) between the core and the cladding sol-gels can be optimized so that coupling loss to SMF-28 is minimized. Light  36  propagating in sol-gel core  20  proceeds and soon enters the region where the sol-gel over cladding  22  is physically tapered. As the light propagates further into this region it begins to “see” the EO polymer waveguide  24  that has been deposited in the recessed region over and between the physical sol-gel tapers on the input and output sides of the modulator. Since the EO polymer has a significantly higher refractive index (˜1.6-1.7) than the sol-gel core (˜1.5), the light from the sol-gel core is gradually or “adiabatically” pulled up into the EO polymer so that by the end of the taper as much light as possible has been transferred to the EO polymer. The same mechanism (in reverse) results in the transfer of light from the EO polymer back to the sol-gel core at the output. In the active region  30 , the refractive index of the buffer layer  26  is often chosen to be very low (˜1.3-1.4) which ensures that the optical waveguide mode in the EO polymer waveguide  24  drops off very rapidly as it enters the buffer layer thereby minimizing losses from the electrodes. In the case of a single-arm Mach-Zehnder modulator ( FIG. 2   b ), when a field is applied to the electrodes and light is propagating in the modulator, light propagating in the left-hand arm of the Mach-Zehnder receives a phase shift relative to the light propagating the right-hand arm, so that the intensity at the output of the Mach-Zehnder changes. 
     In a representative embodiment sol-gel claddings  18  and  22  and core  20  are comprised of 95/5 (n=1.487) and 85/15 (n=1.50) mixtures (mole %) of methacryloyloxy propyltrimethoxysilane (MAPTMS) and zirconium-IV-n-propoxide, where the MAPTMS provides both organic and inorganic character as well as photopatternability, while the zirconium-IV-n-propoxide is used as an index modifier. The under cladding  18  is deposited by spin coating and hard baked (˜8.5 μm), while the core 85/15  20  is also deposited by spin coating, but then is soft-baked and photopatterned, through a simple wet etching process, prior to hard baking (˜4 μm). Under cladding, also  18 , is deposited after the core is patterned to provide confinement on either side of the core. This photopatterned sol-gel waveguide provides excellent coupling to standard SMF-28 single-mode fiber, with coupling losses in the 0.5-1.0 dB per end face range routinely achieved. Propagation losses for this standard sol-gel are 3-4 dB/cm. An adiabatic vertical transition to an EO polymer waveguide (n˜1.65) is accomplished through the use of a grey scale mask that creates a vertical taper in the next layer of sol-gel (i.e. sol-gel over cladding  22 ). Note that this is not an evanescent field device, as the vertical adiabatic transitions result in more than 70% of the optical field being in the EO polymer waveguide  24 . The vertical tapers are also low loss, with less than 1 dB of radiation loss at each taper when the device is made to design. The EO polymer waveguide  24  is typically quite thin (˜1 μm) to achieve single-mode operation; the sol-gel over cladding  22  provides lateral confinement. The buffer layer  26  consists of CYTOP® (Asahi Glass), which has a very low refractive index (n=1.33) at 1550 nm as well as extremely low optical absorption, thereby providing good optical isolation from the top drive electrode  28  even for thin layers (˜1.5 μm). 
     SUMMARY OF THE INVENTION 
     The present invention provides hybrid EO polymer/sol-gel modulators with higher electric field/optical field overlap factor Γ and smaller inter-electrode separation d to reduce the modulator&#39;s half-wave drive voltage V π , reduce insertion loss and improve extinction. This is accomplished with modulator designs in which the sol-gel core waveguide does not lie below the EO polymer waveguide in the active region. 
     In an embodiment, a hybrid electro-optic (EO) polymer/sol-gel modulator comprises a sol-gel core waveguide in first and second passive regions for coupling light into and out of the modulator. An EO polymer waveguide in an active region between said first and second passive regions provides for transmission of light from said first passive region through said active region to said second passive region. The sol-gel waveguide is not established below the EO polymer waveguide in the active region but instead may provide a cladding for the EO polymer waveguide in the active region. First and second electrodes on opposite sides of the EO polymer waveguide in the active region are configured to receive a voltage signal to apply an electric field along a poling direction to change the refractive index of the EO polymer waveguide in the active region. 
     In a first index-tapered, core-tapered embodiment, the sol-gel over cladding has been eliminated and the vertical taper formed in the sol-gel core. Placing the EO polymer waveguide in the same plane as the sol-gel core eliminates the potential for deleterious coupling between the sol-gel core and the EO polymer waveguide. The EO polymer waveguide is physically extended to cover the vertical taper in the passive regions on either side of the active region. The index itself is tapered from a high uniform value in the active region to a lower value to approximately match the sol-gel core at the top of the taper. By tapering the EO polymer waveguide index the inter-electrode separation d is reduced and the electric field/optical field overlap factor Γ is increased, both having the beneficial effect of reducing the half-wave drive voltage. The formation of the vertical taper in the sol-gel core and the index taper of the EO polymer waveguide are separable features. 
     In a second phantom-core embodiment, the sol-gel core lies in a plane below the plane of the EO polymer waveguide. The sol-gel core is patterned so that the core waveguide stops at or near the end of the vertical taper in the sol-gel over cladding. The gap that is created is filled with a sol-gel or other material with a refractive index less than or equal to that of the sol-gel under cladding. Since this region is no longer a waveguide, no light is coupled from the EO polymer waveguide to the core thereby increasing Γ and reducing coupling to the bottom electrode. In fact, the bottom electrode may be placed on top of the sol-gel under cladding for reduced inter-electrode separation d. The lack of background light from the sol-gel core leads to significantly improved modulation efficiency as well. 
     In a third transverse-tapered embodiment, a sol-gel side-cladding lies in the same plane as the sol-gel core. The side-cladding has a transverse physical taper on one (phase modulator) or both (Mach-Zehnder modulator) sides of the sol-gel core that defines a gap on one or both sides of the core. The gap increases gradually from the core from both ends until reaching a fixed gap that is maintained over the active region of the modulator. The gap(s) between the sol-gel core and sol-gel side-cladding are filled with EO polymer to form an EO polymer waveguide. A buffer layer lies over the EO polymer. A top electrode and patterned bottom electrode define the active region there between. This transverse-taper design improves confinement in the EO polymer waveguide so that the inter-electrode spacing can be reduced and Γ increased. The transverse physical taper can be formed using standard lithographic techniques. The performance can be improved by tapering the refractive index of the EO polymer waveguide to gradually increase from a value slightly above that of the sol-gel core to its ordinary high value in the active region of the EO modulator. 
     A fourth strip-loaded embodiment comprises a sol-gel under cladding having a refractive index n 1 , a sol-gel core having a refractive index n 2 &gt;n 1  and a sol-gel side cladding having a refractive index n 3 ≈n 1 &lt;n 2  in the same plane as the sol-gel core. An EO polymer layer is provided with a graded refractive index. The EO polymer has an index n 4 &lt;n 2  in a first passive region that provides a top cladding for the sol-gel core to define a first sol-gel waveguide in the first passive region. The index is graded in a first transition region from n 4  up to n 5 &gt;n 2  in an active region so that the sol-gel core provides a bottom cladding for the EO polymer layer to define an EO polymer waveguide in the active region, The index is graded in a second transition region from n 5  down to n 6 &lt;n 2  in a second passive region that provides a top cladding for the sol-gel core to define a second sol-gel waveguide in the second passive region. A sol-gel buffer layer lies above the EO polymer layer and has a refractive index n 7 &lt;n 5  in the active region. First and second electrodes on opposite sides of the EO polymer waveguide in the active region are configured to receive a voltage signal to apply an electric field along the poling direction to change the refractive index of the EO polymer waveguide in the active region. The use of a uniform thickness EO polymer layer eliminates optical scattering caused by sidewall roughness due to etching, thereby leading to lower optical propagation loss in the active section. The sol-gel core also acts as a virtual side-cladding to confine light laterally in the overlying EO polymer layer thereby creating a well defined EO polymer waveguide in the active region for efficient coupling back into the sol-gel waveguide for coupling to optical fiber. 
     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , as described above, is a section view of they hybrid EO polymer/sol-gel modulator; 
         FIGS. 2   a  and  2   b , as described above, are perspective views of a hybrid EO polymer/sol-gel modulator configured as a phase modulator and a Mach-Zehnder modulator, respectively; 
         FIGS. 3   a  and  3   b  are section and end views of an embodiment of a hybrid tapered-core tapered-index Mach-Zehnder modulator in accordance with the present invention; 
         FIG. 4  is a plot of the tapered-index of the EO polymer waveguide; 
         FIGS. 5   a  and  5   b  are diagrams illustrating mode propagation through the known and hybrid tapered-index, tapered-core Mach-Zehnder modulators, respectively; 
         FIG. 6  is a section view of another embodiment of a hybrid tapered-index modulator; 
         FIG. 7  is a section view of another embodiment of a hybrid tapered-core modulator; 
         FIGS. 8   a  and  8   b  are section views of two embodiments of a hybrid phantom-core Mach-Zehnder modulator in accordance with the present invention; 
         FIGS. 9   a  and  9   b  are diagrams illustrating mode coupling through the known and phantom-core modulators, respectively; 
         FIGS. 10   a  through  10   d  are plan and section views in the passive, transition and active regions of another embodiment of a hybrid transversely-tapered modulator; 
         FIGS. 11   a  through  11   c  are plots of the optical power distribution for the mode propagation in the transversely-tapered modulator; 
         FIGS. 12   a  through  12   e  and  13   a  through  13   e  are a section in the action region and plan views of a sequence for fabricating the transversely-tapered modulator; 
         FIGS. 14   a  through  14   c  are a side view, a section view B-B through the passive region and a section view C-C through the active region, respectively, of a strip-loaded EO polymer/sol-gel modulator; 
         FIG. 15  is a plot of the EO polymer layer&#39;s graded refractive index including an inset showing the grading of the index in a transition region from a uniform low value in the passive region to a uniform high value in the active region; and 
         FIG. 16  is a side section view of an exemplary strip-loaded EO polymer/sol-gel modulator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention describes hybrid EO polymer/sol-gel modulators with higher electric field/optical field overlap factor Γ and smaller inter-electrode separation d to reduce the modulator&#39;s half-wave voltage V π , reduce insertion loss and improve extinction. Applications for these new designs include but are not limited to high speed analog modulators for cable TV, phased array radars, photonic analog-to-digital converts, RF photonics, fiber optic gyroscopes and true time delay lines as well as digital modulators for optical communications networks ranging from long haul dense wavelength division multiplexing (DWDM) networks, to metropolitan area DWDM, to passive optical networks (PON) for optical access. 
     The conventional design of the hybrid EO polymer/sol-gel modulator as illustrated in  FIG. 1  suffers from several limitations owing principally to the fact that the design employs a sol-gel core waveguide that lies directly below the active EO polymer waveguide—this has the effect of allowing light to couple from the EO polymer waveguide to the sol-gel core in the active region of the device resulting in the following performance impairments:
         Since the light in the sol-gel core is optically close to the bottom electrode, the bottom cladding must be made thick enough to assure that the light does not reach the electrode thereby causing high insertion loss; this extra thickness results in increased required electrode separation d, which increases the half-wave voltage; a typical separation required is 15 μm;   The residual light that is in the sol-gel core does not get modulated, leading to a lower extinction ratio—a typical extinction ratio achieved is 10 dB, whereas most commercial applications require 20 dB or higher; and   The coupling of the light into the sol-gel core and the limitation of the tapering technique used in the prior art lead to a moderate value of Γ, typically ˜0.7       

     The hybrid electro-optic (EO) polymer/sol-gel modulator comprises a sol-gel core waveguide in first and second passive regions for coupling light into and out of the modulator. An EO polymer waveguide in an active region between said first and second passive regions provides for transmission of light from said first passive region through said active region to said second passive region. The sol-gel waveguide is not established below the EO polymer waveguide in the active region but instead may provide a cladding for the EO polymer waveguide in the active region. First and second electrodes on opposite sides of the EO polymer waveguide in the active region are configured to receive a voltage signal to apply an electric field along a poling direction to change the refractive index of the EO polymer waveguide in the active region. 
     The present invention provides four different designs that do not employ a sol-gel core waveguide that lies below the EO polymer waveguide in the active region. All four of the new designs result in improvements in d, Γ, d/Γ and V π . At a minimum, d can be reduced to 10 μm from the baseline art value of 15 μm, resulting in at least a 33% reduction in V π . Γ can be improved to at least 0.8 from its baseline art value of 0.7, representing at least a 14% reduction in V π . Therefore, the ratio d/Γ is, at a minimum, reduced to 12.5 μm, compared to the prior art value of 21.5 μm. Thus, the V π  is at a minimum reduced by 42% from its prior art value. 
     More particularly, in one or more of the different designs the electrode separation d is at a minimum reduced by 33% from 15 μm to 10 μm, preferably reduced to 8 μm (47% reduction) and at a maximum reduced to 6 μm (60%). The overlap integral Γ is at a minimum increased to 0.8 from the prior art value of 0.7 (12.5%), typically increased to 0.85 (21%), and at a maximum increased to 0.95 (36%). The ratio d/Γ, which appears in the expression for the half-wave voltage, is, at a minimum, reduced to 12.5 μm, a 42% reduction from its prior art value of 21.5, typically reduced to 9.4 μm (56%), and at a maximum reduced to 6.3 μm (70.7% reduction). Although the half-wave voltage V π  is dependent on other parameters, the improvements in d and F should reduce V π  to less than 1 V for many modulator designs. 
     Index-Tapered EO Polymer, Vertically-Tapered Sol-Gel Core Modulator 
     An index-tapered, core-tapered modulator eliminates the sol-gel over cladding and forms the vertical taper in the sol-gel core. Placing the EO polymer waveguide in the same plane as the sol-gel core eliminates the potential for coupling between the sol-gel core and the EO polymer waveguide. The EO polymer waveguide is physically extended to cover the vertical taper in the passive regions on either side of the active region. The index is tapered from the high uniform value in the active region to a lower value to approximately match the sol-gel core at the top of the taper in the passive regions. By tapering the EO polymer waveguide index the inter-electrode separation d is reduced and the overlap Γ between the optical and electrical fields is increased, both having the beneficial effect of reducing the drive voltage V π . The formation of the vertical taper in the sol-gel core and the index taper of the EO polymer waveguide are separable features. 
     As shown in  FIGS. 3   a  and  3   b , an embodiment of an index-tapered, core-tapered modulator  50  is formed on an insulating layer on a substrate (not shown). The modulator includes a bottom electrode  56 , a sol-gel under cladding  58  having a refractive index n 1  and a sol-gel core  60  having a refractive index n 2 &gt;n 1 . Sol-gel under cladding  58  provides confinement below and on either side of sol-gel core  60 . A vertical taper  62  in core  60  exposes the surface of under cladding  58  above electrode  56 . An EO polymer waveguide  64  covers the vertical taper  62  in the passive regions and the exposed surface of under cladding  58  in the active region of the modulator. The EO polymer waveguide exhibits a refractive index  65  having a high value n 3 &gt;n 2  in the active region  70  that tapers in a transition region  71  to a lower value n 4 ≈n 2  (e.g. within +/−2%) at the top of the physical taper in the passive region  72  as shown in  FIG. 4 . Note, the ‘transition’ regions  71  for the index taper are part of the passive regions  72 . In typical embodiments, the refractive index n 3  is designed to be uniform over the active region. Small variations from the uniform value on account of fabrication and poling can occur without degrading performance. A non-uniform index profile may be designed to provide the same accumulated phase change. 
     A buffer layer  66  having a refractive index n 5 &lt;n 2 , typically &lt;n 1  and preferably &lt;n 1 −0.1 covers the EO polymer waveguide. The index of the buffer layer is preferably considerably less than core index n 2 . The EO polymer waveguide  64  is laterally confined by sol-gel under cladding  58  and vertically confined by buffer layer  66  and sol-gel under cladding  58  as shown in  FIG. 3   b . A top drive electrode  68  defines the active region  70  between itself and bottom electrode  56  and passive regions  72  to either side. An analog or digital voltage signal V sig    74  is applied between the top and bottom electrodes to change the refractive index of the EO polymer waveguide to modulate the amplitude or phase of light  76  passing through the modulator. Modulator  50  is configured as a Mach-Zehnder modulator but may be configured as a phase modulator. 
     In a representative embodiment of modulator  50  the sol-gel under cladding  58  and core  60  are comprised of 95/5 (n 1 =1.487@1550 nm) and 85/15 (n 2 =1.50@1550 nm) mixtures (mole %) of methacryloyloxy propyltrimethoxysilane (MAPTMS) and zirconium-IV-n-propoxide. The under cladding  58  is deposited by spin coating and hard baked (˜8.5 μm), while the core 85/15 layer is also deposited by spin coating, but then is soft-baked and photopatterned to form core  60 , through a simple wet etching process, prior to hard baking (˜4 μm). Cladding, also  58 , is deposited after the core and patterned to provide confinement on either side of the core. The preferred sol-gel material will have a high conductivity at poling temperatures to provide for efficient poling of the EO polymer, refractive indices n 1  and n 2  such that their difference (n 2 −n 1 ) is approximately equal to that of standard single-mode optical fiber (˜0.01), and low optical loss at 1550 nm and 1310 nm, preferably less than 1.0 dB/cm at both wavelengths. The EO polymer should exhibit a high EO coefficient r 33 &gt;30 pm/V and preferably &gt;60 pm/V. Unlike the known hybrid modulator design, the sol-gel core material lies outside the region of polymer poling, hence the sol-gel core does not have to be able to provide high poling efficiency. This removes a constraint, possibly allowing for a larger class of sol-gel materials to be used in each of the proposed designs. Furthermore, the proposed designs reduce inter-electrode spacing. To achieve the same performance, an EO polymer with a lower r 33  can be used with thinner devices. These materials may provide better stability and lower optical loss. A 6 μm thick thermally grown oxide SiO 2  layer beneath the sol-gel under cladding  58  is used to preserve optical transparency at 1550 nm wavelength and prevent coupling to the silicon substrate in the passive regions. 
     The EO polymer layer  64  is spin coated on the entire device and is typically quite thin (˜1 μm) to achieve single-mode operation. The sol-gel core layer with a thickness of 4 μm has a refractive index difference of 0.86% from the sol-gel under cladding (5 μm) and side-cladding to increase the efficiency of the optical coupling to standard single mode fiber (SMF-28™), and is vertically tapered  62  to produce an adiabatic transition between the sol-gel core  60  and the EO polymer waveguide  64 . To improve coupling, vertical taper  62  is preferably a long shallow taper e.g. 3.5 μm change in height over a 1 millimeter length or 0.0035 radians measured from the surface. An angle of taper less than 0.01 radians is suitable; less than 0.007 radians is typical and less than 0.005 radians being preferred. In an embodiment, the taper is at least 0.4 mm in length with an angle less than 0.01 radians. 
     An electrode is placed directly on the top of the EO polymer waveguide for poling and then removed. After poling, the modulator is irradiated with UV radiation (9 mW/cm 2 ) through a gray scale mask for 18 h to fabricate photobleached index tapers in the EO polymer waveguide  64  in the interface regions. The photobleached portion in the passive regions will become transparent, and the interface between the active and passive regions will evidence a gradual change from the original green to the colorless host polymer. The index of the photobleached EO polymer single film coated on SiO 2  was measured using the prism coupling method, and showed gradual index change from n 3 =1.65 to n 4 =1.50 after UV radiation. In a typical embodiment, n 3  is substantially uniform in the active region for a given device. The index value typically lies in a range from approximately 1.53 to approximately 1.80. 
     The buffer layer  66  is coated and the operating top electrode  68  is deposited. The electric field is applied between the electrodes along the poling direction to maximize the EO effect. The inter-electrode spacing is 15 μm or less. The buffer layer  66  consists of 2.0 μm-thick UV cured acrylate top buffer layer (n 5 =1.49). The buffer layer can be any material with reasonably low optical loss at telecommunications wavelengths (&lt;3 dB/cm) with an index that is less than n 2  and typically considerably less than the core index. The buffer layer principally serves to isolate light in the active EO polymer waveguide from the top electrode. In the event that the poling is performed directly on the EO polymer waveguide (and the electrode removed prior to the application of the buffer layer), the buffer layer must be a low temperature curing or drying material, so as to avoid depoling the EO polymer. If the poling is done after buffer layer deposition, the buffer layer should have conductivity higher than that of the EO polymer at the poling temperature in order to achieve efficient poling. 
     In the described embodiment, the EO polymer waveguide  64  is poled in the vertical direction. The electrodes  56  and  68  are placed to create an electric field along the poling direction for maximum effect. In an alternative embodiment, the EO polymer is poled in the horizontal direction and the electrodes are formed co-planar. This approach makes poling the EO polymer more difficult and may constrain the available addressing techniques but simplifies the microwave engineering of the modulator. Each of the index-tapered/core-tapered, phantom-core and transversely-tapered may be configured with either the vertical or co-planar electrodes, the general requirement being that the electrodes create an electric field along the poling direction of the EO polymer. 
     Compared to the known hybrid modulator design  10  shown in  FIG. 1 , modulator  50  exhibits improved mode confinement. The adiabatic transition is accomplished using a combination of a 0.5 mm-long sol-gel vertical taper  62  and a refractive index taper in the EO polymer waveguide  64 , which reduce the transition loss and prevents mode beating between the sol-gel core and the EO polymer waveguide. Improved mode confinement in the EO polymer waveguide in the active region also reduces waveguiding loss due to optical absorption from under and over electrodes. As shown in  FIG. 5   a  for the known modulator  10  and  FIG. 5   b  for index-tapered core-tapered modulator  50 , the confinement of light  36  and  76 , respectively, propagating through the modulators is markedly different. In the figures, the optical power density is highest where the dot density is lowest, and falls off rapidly with increasing dot density. Light  36  spreads out through the modulator, with much of the light in the cladding. Light  76  is much better confined to the EO polymer waveguide. Consequently the inter-electrode separation distance d can be reduced and the overlap factor Γ is increased. 
     In an alternate embodiment of the hybrid modulator  80  as shown in  FIG. 6 , a vertical taper  82  is formed in a sol-gel over cladding  83  having index n 1  above a sol-gel core  84 . The over and under cladding indices n 1  are preferably the same. However, some slight variation (&lt;&lt;1%) may occur within a cladding or between claddings due to slight variations in fabrication or possibly design. For simplicity of description both claddings are designated as having index n 1  but slight variations in their values are well understood by those skilled in the art. An EO polymer waveguide  86  having a refractive index taper is formed over the vertical taper  82  and an exposed portion of the sol-gel core. The EO polymer waveguide index tapers to a value approximately equal to that of the over cladding. The other aspects of the modulator are the same. 
     In an alternate embodiment of the hybrid modulator  90  as shown in  FIG. 7 , a vertical taper  92  is formed in a sol-gel core  94 . An EO polymer waveguide  96  having a uniform refractive index is formed inside the taper over an exposed portion of the sol-gel under cladding  98 . The other aspects of the modulator are the same. 
     Phantom-Core Modulator 
     A phantom-core modulator includes a patterned sol-gel core that stops at or near the end of the vertical taper in the sol-gel over cladding. The gap that is created in the core is filled with a sol-gel or other material with a refractive index less than or equal to that of the sol-gel under cladding. Since this region is no longer a waveguide, no light is coupled from the EO polymer waveguide to the core. If the index difference between the phantom core and the EO polymer waveguide is large enough, the bottom electrode may be placed on top of the sol-gel under cladding for reduced inter-electrode separation. The lack of background light from the sol-gel core layer leads to significantly improved modulation efficiency. 
     As shown in  FIG. 8   a , a hybrid EO polymer/sol-gel modulator  100  is formed on an insulating layer of a substrate (not shown). The modulator includes a bottom electrode  106 , a sol-gel under cladding  108  having a refractive index n 1 , a sol-gel core  110  having a refractive index n 2 &gt;n 1 , and a sol-gel over cladding  112  having a refractive index n 1 . The core  110  is confined below and to either side by under cladding  108 . The over and under cladding indices n 1  are preferably the same. However, some slight variation (&lt;&lt;1%) may occur within a cladding or between claddings due to slight variations in fabrication or possibly design. For purposes of description both claddings are designated as having index n 1  but slight variations in their values are well understood by those skilled in the art. 
     The sol-gel core is patterned to form a gap  114  that exposes the surface of the under cladding  108  so that the core stops at or near the end of a vertical taper  116  in over cladding  112 . Gap  114  is filled with a sol-gel or other material  118  with a refractive index n 3 &lt;=n 1 , preferably at least 0.10 less than and most preferably at least 0.15 less than. The material should exhibit low optical loss and reasonably high conductivity at typical poling temperatures. The taper exposes the surface of material  118  above electrode  106 . An EO polymer waveguide  120  having a refractive index n 4 &gt;n 2  covers the exposed surface of material  118 . In typical embodiments, the refractive index n 3  is designed to be uniform over the active region. Small variations from the uniform value on account of fabrication and poling can occur without degrading performance. A non-uniform index profile may be designed to provide the same accumulated phase change. 
     A buffer layer  122  having a refractive index n 5 &lt;n 2 , typically &lt;n 1  and preferably &lt;n 1 −0.1 covers the EO polymer waveguide. A top drive electrode  124  defines an active region between itself and bottom electrode  106  and passive regions to either side. An analog or digital voltage signal is applied between the top and bottom electrodes to change the refractive index of the EO polymer waveguide to modulate the amplitude or phase of light  126  passing through the modulator. Modulator  100  may be configured as a phase modulator or as a Mach-Zehnder modulator. If the index of material  118  is not at least 0.10 less than the index of the under cladding  108 , bottom electrode  106  should be placed beneath the under cladding  108  to avoid mode coupling to the bottom electrode as shown in  FIG. 8   b . Alternatively, the electrodes  106  and  124  can be co-planar and the EO polymer  120  poled along a horizontal direction. In another embodiment, the EO polymer can be index tapered in the physical taper region and over the passive sol-gel upper cladding so as to have the refractive index approximately match that of the sol-gel in the passive region, while gradually increasing in the physical taper region. 
     As shown in  FIG. 9   a , the known hybrid modulator design shown in  FIG. 1  permits directional coupling of light  36  between the EO polymer waveguide  24  and the sol-gel core  20 . Light remaining in the sol-gel core leads to increased insertion loss and low contrast. As shown in  FIG. 9   b , the phantom core design eliminates this possibility; no light is coupled between EO polymer waveguide  120  and material  118  and provides for reduced inter-electrode distance. 
     Transversely-Tapered Modulator 
     The vertical coupling approach exemplified by the known hybrid modulator of  FIG. 1  and the improved index-tapered/core-tapered and phantom-core designs demands excellent control over film thicknesses in all regions of the device, which can be both difficult and costly. A horizontally (transversely) coupled device in which the sol-gel core waveguide does not lie below the active EO polymer waveguide has fewer issues with respect to dimensional control, relying primarily upon lithographically defined features whose resolution is well beyond the requirements of the modulator. 
     A transversely-tapered modulator includes a sol-gel side-cladding that lies in the same plane as the sol-gel core. The side-cladding has a transverse physical taper on one (phase modulator embodiment) or both (Mach-Zehnder modulator embodiment) sides of the sol-gel core that defines a transverse gap on one or both sides of the core. The gap increases gradually from the core from both ends until reaching a fixed gap that is maintained over the active region of the modulator. The gap(s) between the sol-gel core and sol-gel side-cladding are filled with EO polymer to form an EO polymer waveguide(s). A buffer layer lies over the EO polymer. A top electrode and patterned bottom electrode define the active region there between. This transverse-taper design improves confinement in the EO polymer waveguide so that the inter-electrode spacing can be reduced. The transverse physical taper can be formed using standard lithographic techniques. The performance can be improved by tapering the refractive index of the EO polymer waveguide to gradually increase from a value slightly above that of the sol-gel core to its ordinary high value in the active region of the EO modulator. 
     As shown in  FIGS. 10   a  through  10   d  a transversely-tapered modulator  200  is formed on an insulating layer of a substrate (not shown). The modulator includes a patterned bottom electrode  206 , a sol-gel under cladding  208  having a refractive index n 1  and a sol-gel core  210  having a refractive index n 2 &gt;n 1 . A sol-gel side-cladding  212  having a refractive index n 1  lies in the same plane as sol-gel core  210 . The under and side cladding indices n 1  are preferably the same. However, some slight variation (&lt;&lt;1%) may occur within a cladding or between claddings due to slight variations in fabrication or possibly design. For purposes of description both claddings are designated as having index n 1  but slight variations in their values are well understood by those skilled in the art. 
     The side-cladding has a transverse physical taper  214  on one (phase modulator embodiment) or both (Mach-Zehnder modulator embodiment) sides of the sol-gel core that defines a transverse gap on one or both sides of the core. The gap increases gradually from the core from both ends over lengths L taper1  and L taper2 , respectively, until reaching a fixed gap that is maintained over the active region L active  of the modulator. The gap(s) between the sol-gel core and sol-gel side-cladding are filled with EO polymer having refractive index n 3 &gt;n 2  to form an EO polymer waveguide(s)  216 . In typical embodiments, the refractive index n 3  is designed to be uniform over the active region. Small variations from the uniform value on account of fabrication and poling can occur without degrading performance. A non-uniform index profile may be designed to provide the same accumulated phase change. Performance can be improved by tapering the refractive index from value n 3  over L active  over lengths L taper1  and L taper2  in the passive regions to a value n 5  to approximately match index n 2  of the sol-gel core similar to what is depicted in  FIG. 4 . 
     A buffer layer  218  having a refractive index n 4 &lt;n 2 , typically &lt;n 1  and preferably &lt;n 1 −0.1 lies over the EO polymer. A top electrode  220  and patterned bottom electrode  206  define the active region there between. As shown in this embodiment, a very thin layer of EO polymer  221  lies between core  210  and buffer layer  218 . This is a residual of the standard spin-on processing of the EO polymer and is optically insignificant. This layer is not poled or modulated by the electric fields created by the electrodes to either side. The residual layer could be removed using dry-etch techniques such as reactive ion etching (RIE), but is unnecessary. 
     The input and output regions of the transverse-tapered modulator are similar to those of the vertically-tapered modulator. Sol-gel core  210  and cladding  208  create a single-mode optical waveguide with excellent coupling to optical fiber. Light  222  coupled into the modulator from an input SMF is initially confined to travel in sol-gel core  210  and gradually couples transversely into EO polymer waveguide(s)  216  in which the light is modulated in accordance with changes in the refractive index of the EO polymer induced by the applied electric field. The modulated light  222  is coupled back into core  210  and coupled to the output SMF. The Mach-Zehnder waveguide modulator does not require Y-branches, light  222  traveling in the core splits in two with half traveling down each EO polymer waveguide  216 . The transverse design improves confinement of light in the EO polymer waveguide in the active region, which means the inter-electrode separation can be reduced without deleterious optical coupling to either electrode. This in turn means that the V π  can be reduced. 
     Optical power distribution for the mode propagation calculated using the 3D beam propagation method (BPM) is illustrated in  FIGS. 11   a - 11   c  with the X, Y, and Z axes correspond to transverse, vertical, and propagation directions. The top view shown in  FIG. 11   a  illustrates the integrated mode power for the total waveguide, the EO polymer core, and the sol-gel core layers. Light is confined to the sol-gel core  210  in the passive regions and the EO polymer waveguide  216  in the active regions. The side view shown in  FIG. 11   b  illustrates the center region of the sol-gel straight core  210 , demonstrating that no light is in the sol-gel core in the active region. The side view shown in  FIG. 11   c  illustrates the center of an EO polymer MZ waveguide arm  216  (both are identical). The negative regions on the Y axis in  FIG. 11   b  refer to positions in the under cladding  208 . 
     Strong mode confinement (&gt;90%) was calculated for the EO polymer core significantly exceeding that of previous hybrid modulators. The transverse transition loss was calculated to be &lt;0.2 dB. This reduces d and increases Γ. 
     As shown in  FIGS. 12   a - 12   e  and  13   a - 13   e , a transverse-taper modulator  250  can be fabricated using standard photolithographic masking and high precision lithography. This is simpler, less expensive and more precise than the grey-scale masking techniques required to form the vertical taper. A fabrication sequence for a particular embodiment of the modulator is now described. 
     A multilayer bottom electrode  252  of Cr(10 nm)/Au(100 nm)/Cr(10 nm) is electron beam evaporated onto an Si&lt;100&gt; substrate  254  with a 6-μm-thick thermally grown SiO 2  layer (not shown). A lift-off technique is used to pattern the bottom electrode for dual-drive modulation. An under cladding  258  3 to 3.7 μm thick of methacryloyloxy propyltrimethoxysilane (MAPTMS) doped with an index modifier, zirconium (IV)-n-propoxide (ZrPO) having a mole ratio of MAPTMS to ZrPO of 95/5% is spin-coated over the substrate. UV radiation (λ peak =365 nm, 9 mW/cm 2 ) accelerates the formation of the silica network in the sol-gel layers via the photoinitiator Irgacure 184 (CIBA). The cladding layers are UV-radiated through a photolithographic mask and wet-etched in isopropanol for photopatterning. 
     After coating and baking (150° C./1 h) of the sol-gel under cladding  258  a 3.5-μm-thick core layer was coated and wet-etched to give an 8-μm-wide straight channel core  260 . The core layer is formed from the same material as the cladding but with a mole ratio of 85/15%. 
     A 7-μm-thick side cladding layer is coated and wet-etched to provide a side-cladding  262  and trenches  264  for the tapers and trenches  266  the 4-μm-wide MZ EO polymer channel waveguide. To maximize the coupling efficiency of the hybrid waveguide with SMF-28™, the top cladding covering the core  260  in the passive regions was coated and wet-etched to deposit the top cladding only in the passive region, and finally baked at 150° C. for 1 h. 
     A 2.8-μm-thick-EO polymer layer  268  is deposited by spin coating from a cyclopentanone solution to fill the trenches  264  and  266  adjacent to the sol-gel core  260  on the sol-gel under cladding  258 . The guest-host EO polymer (25 wt % chromophore loaded AJLS102 in amorphous polycarbonate) was used as it readily photobleaches, has relatively low optical loss, and a good EO coefficient (˜50 μm/V). After baking at 85° C. in a vacuum oven overnight, the EO polymer was photobleached using UV radiation through a gray-scale mask for 18 h. The EO polymer was partially bleached with a gradual index change in 0.5-mm-long transition regions, and totally bleached (n=1.53) in the other passive regions. After the EO polymer was contact-poled (poling voltage of 300-400V at 150° C.) between the bottom electrode and a poling electrode directly deposited onto the EO polymer, the poling electrode was removed using a solution of iodine and potassium iodide for subsequent coating of a low index (˜1.33@1550 nm) top buffer layer. A 1.8-μm-thick CYTOP (Asahi Glass) top buffer layer  270  is formed over the EO polymer waveguide  268  and gold is sputtered to form a top electrode  272 . The refractive indices at 1550 nm for the sol-gel core, the cladding, the EO polymer, and buffer layer were 1.500, 1.487, 1.632, and 1.328, respectively. 
     Strip-Loaded Modulator 
     The vertical coupling approach exemplified by the known hybrid modulator of  FIG. 1  and the improved index-tapered/core-tapered, phantom-core designs and transverse tapered modulators all achieve excellent drive voltage performance (as low as 0.65V has been achieved) and reasonably good insertion loss (about 10 dB). For many applications, especially in RF photonics, very low insertion loss is required on the order of 5.5 dB or less. To achieve this level of insertion loss a further modification to the basic design of  FIG. 1  was developed, namely a strip loaded modulator. 
     A strip-loaded modulator includes a sol-gel side-cladding that lies in the same plane as the sol-gel core. The side-cladding index is chosen so as to achieve an optical mode shape at the input to the device that matches well to the mode field diameter of high numerical aperture optical fiber (NA˜0.2). A uniform EO polymer layer is deposited on top of the sol-gel core layer. In the passive section of the device, the EO polymer layer may be photobleached so that the refractive index of the bleached EO polymer is below that of the sol-gel core, in which case the bleached EO polymer acts as a top cladding for the sol-gel core A top electrode and patterned bottom electrode define the active region there between. The refractive index of the EO polymer layer is made to gradually increase from the passive region, where it is a top cladding for the sol-gel core, through a transition region to the active region, where the sol-gel now acts as an under cladding for the high index EO polymer, which acts to strip load the waveguide stack. Application of a voltage across the active region modulates the refractive index of the EO polymer, which in turn produces amplitude changes in the light beam. At the output side of the modulator, the EO polymer index is reduced again to its value at the input, so that light will be guided once again in the sol-gel core and coupled out to optical fiber. 
     The use of a uniform thickness EO polymer layer eliminates optical scattering caused by sidewall roughness due to etching, thereby leading to lower optical propagation loss in the active section. This is the primary origin of its significantly reduced insertion loss, in addition to excellent mode matching to optical fiber and low loss vertical adiabatic transitions. The sol-gel core and sol-gel buffer layer in the active region act as an under and top cladding that confine the light vertically in the EO polymer layer. The sol-gel core also acts as a virtual side-cladding to confine light laterally in the overlying EO polymer layer thereby creating a well defined EO polymer waveguide in the active region. 
     As shown in  FIGS. 14   a  (side),  14   b  (passive) and  14   c  (active) a strip-loaded modulator  300  is formed on an insulating layer of a substrate (not shown). The modulator includes a sol-gel under cladding  302  having a refractive index n 1  and a sol-gel core  304  having a refractive index n 2 &gt;n 1 . A sol-gel side-cladding  306  having a refractive index n 3  (with n 3 &lt;n 2  and n 3 ≈n 1 ) lies in the same plane as sol-gel core  310 . The under and side cladding indices n 1  and n 3  are suitably the same. However, some slight variation (&lt;&lt;1%) may occur within a cladding or between claddings due to slight variations in fabrication or possibly design. The index of the sol-gel core is uniform across the passive and active regions. 
     An EO polymer layer  312  lies over the sol-gel core  304  and side cladding  306 . The EO polymer layer is of uniform thickness and not etched within the footprint of the transition or active regions. As best shown in  FIG. 15 , the EO polymer layer  312  has a graded refractive index  313  along the length of the modulator  300 . The EO polymer layer has an index n 4 &lt;n 2  in a first passive region  314  that provides a top cladding for the sol-gel core to define a first sol-gel waveguide in said first passive region. The index is graded in a first transition region  316  from n 4  up to n 5 &gt;n 2  in an active region  318  so that the sol-gel core provides a bottom cladding for the EO polymer layer to define an EO polymer waveguide in the active region. The index is graded in a second transition region  320  from n 5  down to n 6 &lt;n 2  in a second passive region  322  that provides a top cladding for the sol-gel core to define a second sol-gel waveguide in the second passive region. n 4  may ≈n 6 . The difference in refractive index in the EO polymer layer between the passive and active regions may be at least 0.015. In the passive region, the refractive index of the sol-gel core may be at least 0.0075 greater than the index of the EO polymer layer. In the active region, the refractive index of the EO polymer layer may be at least 0.0075 greater than the index of the underlying sol-gel. The graded index may be formed by photobleaching the EO material in the passive and transition regions to reduce the refractive index. In the active region of the device the EO polymer may be unbleached. Alternately, direct write electron beam lithography may be used to destroy the chromophore in the EO polymer to form the graded index. The EO polymer layer is generally poled along a direction perpendicular to the substrate, but can also be poled transversely (in the plane of the substrate). 
     A top sol-gel buffer layer  324  lies above the EO polymer  312 . The buffer layer has an index n 7  where n 7 &lt;n 5  in the active region to provide an upper cladding for the EO waveguide. n 7  may ≈n 1 . 
     A top electrode  326  and patterned bottom electrode  328  define the active region  318  there between in which the EO polymer waveguide is defined. The electrodes are configured to receive a voltage signal to apply an electric field along the poling direction to change the refractive index of the EO polymer waveguide in the active region. 
     The input and output regions of the strip-loaded modulator are qualitatively similar to those of the other embodiments. Sol-gel core  304  and claddings  302 ,  306 ,  324  and the bleached EO polymer layer  312  create a single-mode optical waveguide with excellent coupling to optical fiber. Light  330  coupled into the modulator from an input SMF is initially confined to travel in the sol-gel waveguide defined in sol-gel core  304  and traverses transitions region  316  to couple into the EO polymer waveguide defined in EO polymer layer  312  in which the light  330  is modulated in accordance with changes in the refractive index of the EO polymer induced by the applied electric field. Single-mode as used herein means that the waveguide is single-mode both in the plane and perpendicular to the plane of the substrate.  FIG. 15  shows a typical refractive index profile  313  achieved in the EO polymer in order to efficiently transfer the light from the sol-gel core (passive region  314 ) to the EO polymer (active region  318 ). 
     As shown in  FIG. 14   c , the light  330  traveling in the EO polymer in the active region is vertically confined by the sol-gel core  304  and buffer layer  324 . Because the refractive index of the sol-gel core n 2  is greater than the refractive index n 3  of the side-cladding  306 , sol-gel core also acts as a virtual side-cladding  331  to confine light  330  laterally in the overlying EO polymer layer thereby creating a well-defined EO polymer waveguide in the active region; this is the so-called strip loading effect. The modulated light  330  is coupled back into the sol-gel waveguide defined in sol-gel core  304  through transition region  320  and coupled to the output SMF. The difference in refractive index between the sol-gel core  304  and the sol-gel side-cladding  306  may be at least 0.01 and may preferably be at least 0.04-0.06. The greater the contrast between the indices the better the lateral confinement of light in the EO polymer waveguide. However, if the difference is too large coupling to the optical fiber will suffer. Without this virtual side-cladding the light would expand laterally within the EO polymer layer in the active region and coupling back into the sol-gel waveguide would be poor; the waveguide would also become multimode in the plane. Conversely, because the light is confined within this virtual side-cladding without etching the EO polymer layer the light does not scatter, the waveguide remains single-mode and the insertion loss is much lower, typically 5 dB or lower. 
     The strip-loaded design provides excellent confinement of light in the EO polymer waveguide in the active region, which means the inter-electrode separation can be reduced without deleterious optical coupling to either electrode. This in turn means that the V π  can be reduced. Light  330  is well confined to the sol-gel core  304  in the passive regions and the EO polymer layer  312  in the active regions. Light  330  does not encounter rough edges as it transitions to and from the sol-gel and EO polymer waveguides. This reduces insertion loss. 
     A particular embodiment of the strip-loaded modulator  300  is depicted in  FIG. 16 . The structure is the same as that shown in  FIG. 14   a  but specific refractive indices are provided. Sol-gel under cladding  302  and sol-gel buffer layer  324  have refractive indices n 1 =n 7 =1.485. Sol-gel core  304  has refractive index n 2 =1.539. Sol-gel side-cladding  306  (not shown) also has a refractive index n 3 =1.485. The EO polymer layer has a refractive index n 4 =n 6 =1.529 in the passive regions and n 5 =1.609 in the active region. 
     Light  330  is confined to the sol-gel core  304  in the passive regions and the EO polymer layer  312  in the active regions. In the active region, where the sol-gel core  304  acts as a bottom cladding for the EO polymer waveguide, substantially no light penetrates to the sol-gel core. The side view shown in  FIG. 16  illustrates the center region of the sol-gel straight core  310 . Strong mode confinement (&gt;80%) was calculated for the EO polymer core, which is competitive with that of previous hybrid modulators. Both the optical fiber coupling loss and the taper transition losses were found to be very low, and with resulting insertion losses in the range of 5 dB or less. 
     A fabrication sequence for the particular embodiment of the strip-loaded modulator shown in  FIG. 16  is now described. Two sol-gel compositions were used for this device. A 95/5 molar ratio of methacrylpropyltrimethoxysilane (MAPTMS) to zirconium(IV)-n-propoxide (ZPO) was used as the cladding in both the active and passive sections with an index of 1.485. A 30/70 molar ratio of MAPTMS to diphenyldimethoxysilane (DPDMS) was used as the core in the passive section and as the strip-load in the active section with an index of 1.539. 1.5 weight % Irgacure 369 photoinitiator was added to make the sol-gel photo-patternable. Both sol-gels were hydrolyzed with a ratio of water:methoxy of 0.37 using 0.1 N HCl. The ZPO was chelated with a 1:1 molar ratio of methacrylic acid (MAA). The EO polymer used was a guest-host material, 25 weight % AJLS102 chromophore doped into amorphous polycarbonate, a high glass transition temperature (T g ) host polymer. 
     Mach-Zehnder modulators were fabricated as follows: A 6 μm layer of the 95/5 sol-gel was spin-cast onto a Ti coated Si wafer and cured for 1 hour at 135° C. A 1.3 μm layer of Shipley 1813 photo-resist was spin-cast onto the sol-gel layer and prebaked for 8 minutes at 110° C. Next, waveguide trenches were patterned in the resist by exposing for 23 s in a Carl Zuss MJB3 mask aligner and developing. 3 μm deep trenches were then wet-etched into the sol-gel at a rate of 0.1 μm per minute by immersion in a 1:6 buffered oxide etchant. The resist was then stripped and the 30/70 sol-gel core was spin-cast into the trenches, photo-cured for 30 s, and developed in a 50:50 mixture of acetone and ethanol. Due to oxygen inhibition in the free radical photo-polymerization of the sol-gel, the 30/70 MAPTMS/DPDMS sol-gel achieved planarization at the top of the trenches etched in the cladding. 
     Next, a 1.1 μm layer of EO polymer dissolved in cyclopentanone was spin-cast and dried under vacuum at 100° C. After drying the refractive index tapers were created in the EO polymer using a grayscale mask from Benchmark Technologies. The mask consists of two 1.5 mm long sections of linearly varying optical density connected by a wide chrome strip 1.5 cm in length which prevents the active section from becoming bleached. An exposure dose of 9 kJ/cm 2  of Hg i-line radiation was applied which created refractive index tapers that can be seen in  FIG. 15 . After index tapering, the top sol-gel buffer layer (MAPTMS/ZPO 95/5) was deposited. The top electrode was deposited and patterned, after which the polymer was poled, thereby completing the modulator. 
     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.