Abstract:
A method of attaching electrodes to optical substrates with embedded waveguides includes applying the electrode pattern to a separate superstrate from the electro-optic material containing the waveguide. This allows for the change of the index of refraction and/or the dipole moment of an electro-optic material using low voltages. In0 addition, the electrode superstrate can be detached from the waveguide substrate and repositioned and aligned to different waveguides. Removable electrodes add flexibility and increase yield by allowing the electrodes to be re-aligned to the waveguides when improper alignment occurs.

Description:
GOVERNMENT LICENSE RIGHTS 
   The U.S. government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. F29601-01-C-0016, awarded by the U.S. Air Force. 

   FIELD OF THE INVENTION 
   The present invention is directed generally to electro-optics and is believed to be particularly useful in interfacing micro-electrodes mounted on a non-conductive superstrate to electro-optic substrate materials with embedded waveguides. 
   BACKGROUND OF THE INVENTION 
   Optical waveguides embedded in materials such as silicon, lithium niobate, and potassium titanyl phosphate (KTP) have found many important applications such as wavelength division multiplexing and demultiplexing, phase modulation and laser stabilization. Other applications are related to phase control of channel waveguides, wavelength tuning with a Bragg waveguide, and low voltage poling of an electro-optic material. Many of these applications require an electric field to pass through the waveguide. The electric field may be generated by pairs of electrodes attached to one or more surfaces of the waveguide substrate. The electric field generated by the electrodes may be used for one of two purposes, either to change the index of refraction of the waveguide material or to change the dipole moment of the waveguide material. 
   A change in the index of refraction of the waveguide material may be used to control the characteristics of the radiation as it propagates through the waveguide. Some of these characteristics are its phase, center wavelength reflected by a Bragg grating, beam steering, and spatial mode profile. A change in the dipole moment of the waveguide material may be used to create quasi-phase-matched waveguides, which in turn may be used for second harmonic generation and various other forms of nonlinear frequency conversion. 
   There are two common approaches to apply this electric field through the waveguide, both approaches having the electrodes mounted on the same substrate which houses the waveguide. The first approach has the electrodes on opposite surfaces of the substrate with the waveguide sandwiched between the electrodes. The second approach has the electrodes on the same surface of the substrate, the electrodes straddling the waveguide. Both approaches typically require photo-lithographic procedures applied directly to the substrate material which houses the waveguide. Also, in this configuration it is typical to deposit the electrodes on the substrate after the waveguide region has been defined by diffusion, ion exchange, or other acceptable processing steps. In the event that problems arise during the electrode deposition steps, the entire substrate may be rendered useless. For example, the electrode mask pattern may be improperly aligned to the substrate geometry yielding unacceptable results. Also, in some cases there may be a high density of multiple components on the same substrate material. Given this the loss of an entire substrate may represent significant financial loss. 
   SUMMARY OF INVENTION 
   In view of the situation described above there is a need for an improved method to mount or attach electrodes to optical substrates with embedded waveguides. The present invention allows for the change of the index of refraction and/or the dipole moment of an electro-optic material using low voltages by applying the electrode pattern to a separate superstrate. This separate superstrate can be aligned and fixed over a particular waveguide housed by the substrate. In addition, the electrode superstrate can be detached from the waveguide substrate and repositioned and aligned to different waveguides. Removable electrodes add flexibility and reduce loss by allowing the electrodes to be re-aligned to the waveguides when improper alignment occurs. 
   One particular embodiment of the invention is directed to an electro-optic device that comprises a first optical waveguide in an electro-optic crystal, the electro-optic crystal having a first surface. An electrode plate has an electrode pattern on a second surface, the electrode pattern forming an electric field profile when a voltage is applied to the electrode pattern. The second surface of the electrode plate is removably disposed proximate the first surface of the electro-optic crystal so that the electric field profile passes through the first optical waveguide when the voltage is applied to the electrode pattern. 
   Another embodiment of the invention is directed to an electro-optic device that comprises crystal waveguiding means for guiding light within an electro-optic crystal. Electric field forming means forms an electric field in the crystal waveguiding means so as to control an electro-optic action of the crystal waveguiding means on light propagating along the crystal waveguiding means. The electric field forming means is disposed on a plate separate from the crystal waveguiding means. The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
       FIG. 1  shows an electro-optic device with a positive electrode and a ground electrode on opposite sides of a KTP substrate with an embedded waveguide. 
       FIG. 2  shows an electro-optic device with a positive electrode and a ground electrode on the same side of a KTP substrate with an embedded waveguide. 
       FIG. 3  shows an embodiment of electro-optic device which depicts an oblique view of a glass superstrate with electrodes, the superstrate shown positioned under a KTP chip containing waveguides, according to principles of the present invention. 
       FIG. 4  shows an embodiment of an alignment device to mount the KTP chip to the superstrate, according to principles of the present invention. 
       FIG. 5  shows a top view of a method for aligning the KTP chip to the superstrate, according to principles of the present invention. 
       FIG. 6  shows a low-power injection seed laser unit with an adjustable linewidth to reduce Stimulated Brillouin Scattering (SBS) in fiber amplifiers and lasers, according to principles of the present invention. 
       FIG. 7  shows an embodiment of an electro-optic device which may both stabilize a semiconductor laser and frequency modulate its output using removable micro-electrodes, according to principles of the present invention. 
       FIG. 8  shows a schematic representation of an embodiment of an electro-optic device with a superstrate structure designed with mechanical features and landmarks to aid in aligning optical fibers to embedded waveguides nested in separate substrates. 
   

   While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION 
   The present invention is applicable to electro-optic materials and is believed to be particularly useful in interfacing micro-electrodes mounted on a non-conductive superstrate to electro-optic substrate materials with embedded waveguides. 
   Generally, the present invention relates to changing the index of refraction and/or the dipole moment of an electro-optic substrate material with an embedded waveguide by introducing an electric field across or through the waveguide region. Historically, there have been at least two common approaches to apply an electric field across or through the embedded waveguide. 
   The first approach is depicted schematically in  FIG. 1 , which shows an electro-optic device  100  with a positive electrode  101  and a ground electrode  102  on opposing sides of a KTP substrate  103 . The electric field created by the voltage applied across the electrodes  101  and  102  passes through a waveguide  104  embedded in the KTP substrate  103 . The strength of the induced electric field is inversely related to the spacing between the electrodes. A typical thickness of the KTP substrate  103  may be approximately 1 millimeter (mm), which may be large enough to require a high voltage across electrodes  101  and  102  to induce a discernable change in index of refraction in the waveguide region  104 . 
   A second historical approach is depicted schematically in  FIG. 2 , which shows an electro-optic device  200  with a positive electrode  201  and ground electrode  202  on the same side of a KTP substrate  203 . Typically, photolithographic techniques are required to deposit the coplanar electrodes  201  and  202  directly onto the surface of the substrate  203  in the optimum position relative to the location of the waveguide  204 . Tight alignment tolerances and small substrate size may make this a difficult and complex operation. 
   The most efficient way to direct the electric field across an individual waveguide may be to place both the positive and ground electrode on the same surface rather than on the opposite sides of the substrate. In this configuration the electrodes may be positioned arbitrarily close to each other, which may allow for the achievement of strong electric fields with relatively low voltages. The direction of the electric field inside the waveguide  204  depends on the position of the waveguide relative to the electrodes  201 ,  202 . For instance, if the waveguide  204  is positioned at the edge of either electrode as shown in  FIG. 2  for a z-cut material, a large component of the electric field lies along the vertical direction. Other substrate orientations, for example x- or y-cut materials, may require the waveguide to be positioned directly between the electrodes to take advantage of a horizontal electric field. 
   According to one embodiment of the present invention, an electrode pattern may be designed on computer-aided design (CAD) software and duplicated onto a laser cut, one-to-one optical mask following standard procedures. The mask may then be used to transfer the pattern onto separate substrate or superstrate materials using standard contact lithography. In one embodiment of the present invention the mask may be used to pattern subsequent electrode geometry onto an 0.8 mm thick glass superstrate using contact lithography. From the pattern, electrodes may be made, typically, by applying a 0.05 μm thick chrome layer followed by a 0.5 μm gold layer. The gold may aid in wire bonding, a welding technique for attaching an electrical lead to the superstrate. 
   One particular embodiment of an electro-optic device  300  according to the present invention is depicted schematically in  FIG. 3 , which depicts an oblique view of a glass superstrate  301  with electrodes  302 . The superstrate  301  shown positioned under a KTP chip  303  containing waveguides  304 . Typical dimensions for superstrate  301  may be 0.8 mm (thick)×10 mm (wide)×11 mm (long). The superstrate  301  may be attached to a thermoelectric cooler  305 , which in turn may be attached to an aluminum plate  306 , both by means of a thermally conductive epoxy  312 . A thermistor  311  may be attached to the superstrate  301  to monitor the temperature of the waveguide region  304 . The electrodes  302  may be attached to bond pads  307  using gold wire bonds  308 . The bond pads  307  may be soldered to standard wire  309  and connected to a voltage supply  310 . 
   One particular embodiment of an alignment device  400  is depicted schematically in FIG.  4 . The alignment device  400  may be utilized to assemble the optical components in substantially the same vertical configuration as shown in  FIG. 3 , although other configurations may also perform in substantially the same manner. The superstrate  301  oriented with the electrodes  302  facing up and with attached thermoelectric cooler  305  and aluminum base  306  may be held in a clamp  402 , all supported by a 5 degree-of-freedom (x, y, z, θ and φ) positioning stage  403 . The KTP chip  303  may be suspended directly above the electrodes  302  by a vacuum chuck  404  mounted to an aluminum pivot arm  405 . The KTP chip  303  may be inverted so that the surface containing the waveguides  304  is down, facing the electrodes  302  on the glass superstrate  401 . The vacuum chuck  404  holding the KTP chip  303  may be machined of transparent material such as polycarbonate so that the alignment of the waveguides  304  to the electrodes  302  may be viewed through the vacuum chuck  404  from above using a far field microscope. The positioning stage  403  may be used to translate, rotate and side-to-side tilt the electrodes  302  on the glass superstrate  401  underneath the KTP waveguides  304  until the waveguides  304  are aligned with the electrodes  302 . Front and rear tilt may be adjusted by vertically raising the superstrate  401  using the z-axis knob  406  of the positioning stage  403 . One may also change the angle of the aluminum arm  405  supporting the vacuum chuck  404  relative to the KTP chip  303  by pivoting about the bearing  407  located at the rear of the pivot arm  405 . A stop at the back of the pivot arm  405  may hold the arm horizontal until during initial alignment and may be later released during the final stage of alignment. Therefore, the KTP chip  303  may be rocked front-to-back until it is level and flat with respect to the glass superstrate  401 . In addition, the pivot arm  405  may include weights  408  to compress a thin layer of UV cure epoxy  313  that may be placed between the electrodes  302  and the KTP chip surface  303 . 
   The KTP waveguides  304  may be aligned and fixed over the electrodes  302  on the glass superstrate  401  using the following process. Approximately 1 milligram of Norland NOA61 UV cure epoxy or equivalent  313  may be placed over the electrodes  302  on the glass superstrate  401 . The superstrate  401  may be heated to and maintained at approximately 60° C. using the thermo-electric cooler  305 , thermistor  311  and temperature control circuit to reduce the viscosity of the epoxy  313 , and in conjunction with applied pressure may reduce the spacing between the waveguides  304  and the electrodes  302 . The glass superstrate  401  may then be raised using the z-axis knob  406  of the positioning stage  403  until the KTP chip  303  comes into contact with the epoxy  313  leaving a small distance between the waveguides  304  and the glass superstrate  401 . The superstrate  401  may be intentionally made slightly shorter than the length of the KTP chip  303  to keep epoxy  313  from blocking the front and back facets of the waveguides  304 . 
     FIG. 5  schematically depicts a magnified top view  500  of the KTP chip  502  to the superstrate  504  in the same vertical orientation as depicted in  FIG. 3  while aligning the waveguides  501  in the KTP chip  502  to the electrodes  503  in the superstrate  504 . The bond pads  505  on the surface of the superstrate  504  may be used for making external connections to the electrodes  503 . The magnified view may be obtained with a medium power microscope, typically at 60× magnification, viewing the waveguides  501  through the KTP chip  502  and transparent vacuum chuck  404 . Dynamic alignment of the superstrate  504  to the KTP chip  502  may be accomplished by raising the superstrate  504  via the Z axis knob  406  of the 5 degree-of-freedom stage  403  until the weights  408  compress the epoxy  313  between the KTP chip  502  and the superstrate  504 . 
   Once compressed, it may be possible to view interference fringesformed between the KTP chip  502  and the transparent vacuum chuck  404  when illuminated with white light. Typical dimensions for the KTP chip  502  are 3 mm (width) by 10 mm (in length). There are typically four to five white light fringes are measured over the length of the KTP chip  502 . If the superstrate  504  is parallel to the KTP chip  502 , then the superstrate  504  applies even pressure to the KTP chip  502  when the two are brought into contact, and so the white light fringes remain largely unaffected. If, on the other hand, the superstrate  504  is not parallel to the KTP chip  502 , then the superstrate  504  applies a greater pressure to one end of the KTP chip  502  when the two are brought into contact, which may be detected by a change in the separation of the white light fringes. Typically, the fringes concentrate at that end of the chip  502  where the pressure being applied by the superstrate  504  is greatest. 
   Also, by utilizing the alignment capabilities of the 5 degree-of-freedom stage  403 , it may be possible under 60× magnification to align the electrodes  503  to less than a 2 micron offset Δ from the waveguides  501 . If the maximum displacement between the waveguides  501  and electrodes  503 , at at one end of the chip  502 , is less than 2 microns, this translates into an angular misalignment θ of less than 1 milliradian in those cases where the length of the KTP chip  502  is approximately 10 millimeters. 
   One particular embodiment of an electro-optic device according to the present invention is depicted schematically in  FIG. 6 , which shows a low-power injection seed laser unit  600  with an adjustable linewidth that may be useful for reducing Stimulated Brillouin Scattering (SBS) in fiber amplifiers and lasers. SBS is a nonlinear effect that may limit the available output power of single or multi-mode fiber amplifiers and lasers by setting up a reflection in the fiber core thereby clamping the forward output power. A seed laser  600  with adjustable linewidth may be useful for broadening the spectral output sufficiently to reduce or eliminate this reflection, yet maintain sufficient coherence length to allow for coherent addition of multiple fiber amplifiers. The fiber lasers and amplifiers may be based on fibers doped with rare earth species, for example ytterbium (Yb) or erbium (Er). 
   The particular embodiment of seed laser  600  may employ optical waveguides  604  and  612  embedded in the KTP chip  603  for optical feedback to a short section of optically pumped, double clad, Yb-doped fiber  606  as the gain element. This may ensure the wavelength of the seed laser  600  will lie within the gain bandwidth of Yb-doped fiber amplifiers, typically from 1040 to 1120 nm. The optical fiber can be side-pumped by high power 976 nanometer (nm) diode lasers  607 , the output of which may be coupled by lens  608  into the inner cladding and crossing the core of the fiber by reflecting off V-grooves  609  etched into the fiber or end-pumped directly into the inner cladding and core using emission from the 976 nm diode lasers  607 . 
   One end of the doped fiber  606  may receive a highly reflective coating acting as an end mirror  610 . The opposite end of the fiber  606  may be directly coupled to a channel waveguide  604  embedded in KTP chip  603 . The channel waveguide  604  couples to a Bragg section  612  of the waveguide that may provide a wavelength selective reflection back to the doped fiber  606  and acts as a partially reflective end mirror for the laser output  613 . 
   The KTP waveguide  604  may be aligned over the superstrate  601  containing the micro-electrodes  602  and  611  using techniques described earlier. The temperature of the KTP  603 /superstrate  601  device may be controlled using a thermoelectric cooler  605  for added stability. The variation of a voltage V λ  applied to the electrodes  611  across the Bragg section  612  of the waveguide may be used to tune the wavelength of the laser output  613 . A voltage V φ  applied to the electrodes  602  across the channel waveguide section  604  may tune the phase of light  613 . Continuous tuning may be achieved by simultaneously changing the voltage to the phase electrodes  602  and the Bragg electrodes  611 . This dual adjustment may allow for a single longitudinal mode to be resonant within the external cavity defined by the reflective end of the fiber  610  and the Bragg section of the waveguide  612 . Also, continuous tuning of the voltage V λ  may permit the frequency of the laser  600  to be matched, or closely matched to the frequency of the laser or amplifier device being injection seeded by this laser  600 . 
   Average linewidth control may be achieved by applying a rapid, oscillating voltage to the channel electrodes  602  and the Bragg electrodes  611  together in tandem, or independently. The electric field applied across the KTP Bragg waveguide  612  section typically affects the laser output  613  by the following process. The period of the Bragg section  612  determines the wavelength λ B  of the light reflected back to the Yb-doped amplified fiber  606  or a laser diode according to equation 1 below: 
               λ   B     =       2   ⁢           ⁢   n   ⁢           ⁢   Λ     m             (   1   )             
 
where n is the index of refraction in the Bragg waveguide region  612 , Λ is the period of the Bragg grating elements, and m is the order of the Bragg reflection. The effective optical path length of the Bragg period nΛ may be changed either by a combination of thermal induced change in index of refraction n and thermal expansion or contraction of the KTP chip  603 , or by applying an electric field to change the index of refraction n. Both effects operating alone or in tandem may be used to tune the reflected wavelength λ B . The index of refraction of KTP may be changed very rapidly by applying an external electric field. An optical field with extraordinary polarization (parallel to the z axis of the crystal) propagating through the Bragg section  612  will experience a change in the index of refraction, δ n (E), given by equation 2 below: 
               δ   ⁢           ⁢     n   ⁡     (   E   )         =       1   2     ⁢     n   3     ⁢     r   33     ⁢   E             (   2   )             
 
where E is the magnitude of the external field applied parallel to the z axis, and r 33  is the electro-optic coefficient appropriate for the KTP substrate. Replacing λ B  with λ B +δλ B  and n with n+δn in equation 1, and using equation 2, an expression for the change in the Bragg wavelength, δλ B  can be derived 
               δλ   B     =       Λ   m     ⁢     n   3     ⁢     r   33     ⁢   E             (   3   )             
 
Equation 3 suggests that electro-optic control of a Bragg grating  612  embedded in a KTP chip  603  may be possible. In addition, the channel section of the waveguide  604  may be electro-optically tuned to adjust the optical path length within the external cavity formed between the Bragg grating  612  and the end mirror  610 . This may be used to keep the same longitudinal mode of the cavity resonant as the Bragg section  612  selects the wavelength. Simultaneous control of both the phase and wavelength of the light in the resonator cavity may provide continuous wavelength tuning of the laser output  613 .
 
   Other advantages this seed laser unit  600  may have over current technologies include: only one laser may be needed to achieve a narrow or broad linewidth emission, the seed laser unit  600  may be factory set to a specific wavelength between 1040 nm and 1120 nm using a single KTP chip  603  which may host multiple waveguides with channel  604  and Bragg  612  sections, each designed to reflect a certain wavelength. The center wavelength may be temperature tuned to ±1 nm to hit target wavelengths. The electro-optic wavelength control may be scanned at gigahertz-type rates, and a simple compact design may be realized with a minimum of custom components, thereby reducing manufacturing costs. 
     FIG. 7  depicts a schematic representation of an electro-optic device  700  which may be used to both stabilize a laser and frequency modulate its output using removable micro-electrodes. The electro-optic device  700  may comprise back-to-back dual-element waveguides  704  and  710  embedded in an electro-optic material  703  such as potassium titanyl phosphate (KTP). In one embodiment, the KTP substrate  703  containing the optical waveguides  704  and  710  may be aligned and fixed to an electrically insulating superstrate  701  containing microelectrodes  702  using the techniques described earlier. The KTP substrate  703  may rest on a thermo-electric cooler  705  in order to temperature control the Bragg waveguide region  710  to stabilize the reflected wavelength to the Fabry-Perot type diode laser  707 , or other suitable laser source. 
   Emission from a standard Fabry-Perot type diode laser  707  may be coupled into the waveguide  710  by varying techniques including, but not limited to, a condensing lens, a series of positive focal length lenses, or direct end-face coupling the laser  707  to the waveguide  710 . In one embodiment of the present invention, the emission from the Fabry-Perot type diode laser  707  may be collimated using a high numerical aperture (NA) lens  708  such as a Geltech Corporation part number C330TM (f=3.1 mm, 0.68 NA) then focused into the waveguide  710  using a condensing lens  709  such as a Geltech Corporation part number C110TM (f=6.24 mm, 0.25NA). A typical coupling efficiency of 30% to 40% from the lens  708  to the KTP chip  703  may be achieved using this configuration, which is sufficient to stabilize the output of the Fabry-Perot type diode laser  707  to a single frequency via an approximately 50% reflective Bragg grating  710 . The light transmitted through the Bragg grating region  710  exits the channel waveguide  704  as the laser output  711 . 
   The diode laser  707  may rest on a thermal electric cooler  706  in order to temperature control the laser  707  for added wavelength stability and extended lifetime. In this configuration, the electro-optically controlled section of the waveguide  704  lies outside the external cavity defined by the diode laser  707  and the Bragg section  710  of the waveguide. Applying an oscillating voltage V φ  across the microelectrodes  702  at a specific frequency f causes an oscillating variation in the index of refraction. This results in a change in phase of the wavelength-stabilized light that can generate side bands at plus and minus the modulation frequency about the main optical carrier frequency. This device may be useful for, but not limited to, ultra-sensitive detection of trace gases and atmospheric gases using frequency modulated (FM) spectroscopy. 
     FIG. 8  depicts a schematic representation of an embodiment of an electro-optic device  800  with a superstrate structure  801  designed with mechanical features and landmarks to aid in aligning optical fibers to embedded waveguides nested in separate substrates  803 . Microelectrodes  802 , V-groove  807 , and adhesion pads  809  may be simultaneously patterned onto the superstrate  801  using standard photolithography techniques. The electrode pattern  802  may include an impedance matching electronics unit  805  for fast modulation of an applied voltage potential. 
   Once the superstrate  801  is patterned, the electrodes  802  and adhesion pads  809  may be protected with a chemical resistant mask, so only an alignment feature, such as the V-groove  807 , is chemically etched into the superstrate  801 . The chemically resistant mask may have alignment features referenced to the superstrate  801  which may ensure the V-groove  807  is etched parallel to and terminates adjacent to electrodes  802 . Once the masking material is removed, the KTP waveguide  804  may be aligned and affixed to the electrodes  802  utilizing the procedures and hardware outlined in FIG.  4 . Due to the nature of photolithography, the patterned features may be precisely aligned relative to each other to sub-micron tolerances. This may enable a high coupling efficiency from the fiber  806  into the KTP waveguide  804 . 
   The optical fiber  806  may then be inserted into the V-groove  807  and butted up to the KTP waveguide  804 , thus completing the fiber  806  to waveguide  804  alignment. The fiber  806  may then be sandwiched between a lid  808  and the V-groove  807  for mechanical integrity. Adhesion pads  809  may be provided on both the top of the superstrate  801  and the bottom of the lid  808  in corresponding locations. The adhesion pads  809  may be coated with a thin layer of standard epoxy, or other suitable materials such as an ultraviolet (UV) curable or low temperature solder  810 . Good coupling efficiency between the fiber  806  and the KTP waveguide  804  may be expected because the core dimensions and numerical aperture of both the fiber  806  and KTP waveguide  804  may be similar. 
   The superstrate  801  may be extended to support a second, passively aligned fiber at the opposite end of the KTP waveguide  804  resulting in a fiber pigtailed design. One advantage of having the electrodes  802  on the superstrate  801  separate from the KTP substrate  803  containing the optical waveguide  804 , is that the substrate material  803  may not need to be exposed to the chemical etchant which may damage the waveguide  804 . Additionally, alternative materials may be chosen for the superstrate  801  that preferentially etches well, such as silicon. This configuration may enable light to be coupled from the optical fiber  806  into the KTP waveguide  804  (or vice versa) and electro-optical control of the KTP waveguide  804  structure on a single, mechanically stable, integrated superstrate  801 . 
   The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. For example, the electro-optic crystal need not be KTP, but may be another material. One example of another material is an isomorph of KTP. The claims are intended to cover such modifications and devices.