Abstract:
An opto-electronic oscillator including a modulator for outputting modulated light and a tunable filter for receiving modulated light output from the modulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional application Ser. No. 60/325,351, filed on Sep. 26, 2001, which is expressly incorporated by reference in its entirety as though fully set forth herein. 
    
    
     ORIGIN OF THE INVENTION 
     The systems and techniques described herein were made in the performance of work under a contract issued by DARPA, and are subject to the provisions of Public Law 96-517 (35 U.S.C. § 202) in which the Contractor has elected to retain title. 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 number DAAD17-02-C-0085 awarded by DARPA. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to opto-electronic systems, and in particular, to an opto-electronic oscillator including a tunable electro-optic filter. 
     BACKGROUND OF THE INVENTION 
     Opto-electronic oscillators (“OEOs”) are electro-optic systems that may be used to generate microwave frequency signals having high stability and low phase noise. Typically, OEOs include a single-mode laser and an electro-optic modulator coupled to at least one active feedback loop having an open-loop gain greater than one to sustain microwave frequency oscillation. The active feedback loop includes a photodetector that converts the optical signals output from the electro-optic modulator into microwave frequency electrical signals that are, in turn, used to control the modulation of light passing through the electro-optic modulator. Thus, the active feedback loop converts the laser&#39;s output photon energy into microwave signals. 
     Many oscillation modes can oscillate simultaneously in the OEO as long as the gain of the active feedback loop exceeds the loop&#39;s losses. The active feedback loop typically includes a radio frequency (“RF”) amplifier that amplifies the electrical signal output from the photodetector. The OEO&#39;s active feedback loop also includes an RF bandpass filter that is used to select a single mode of oscillation for the OEO&#39;s. The RF bandpass filter is also used for coarse frequency tuning of the OEO&#39;s single oscillation mode. The electrical signal output from the RF bandpass filter is the OEO&#39;s output microwave signal. The electrical signal output from the RF bandpass filter is also used to control modulation of the light that propagates through the electro-optic modulator. 
     One challenge associated with OEOs is that the OEO&#39;s RF bandpass filter can only provide for coarse frequency tuning. Also, the RF bandpass filter does little to reduce phase noise in the output microwave signal. Furthermore, the RF bandpass filter does not provide for fast tunability of the OEO&#39;s oscillation frequency. Therefore, there is a need for an OEO that provides for fine frequency tuning with reduced phase noise and fast tunability. 
     SUMMARY OF THE INVENTION 
     An opto-electronic oscillator that embodies the invention includes a modulator for outputting modulated light and a tunable filter for receiving modulated light from the modulator. 
     A tunable filter includes an optical resonator, a first optical coupler, and a second optical coupler. The optical resonator has a refractive index that depends on an electric field applied to the optical resonator. The first optical coupler is adjacent the optical resonator. The second optical coupler is adjacent the optical resonator. 
     A method for generating an oscillatory signal includes generating light, modulating the light into sideband optical signals, selecting a single sideband optical signal from the modulated light using a tunable filter, mixing the single sideband optical signal with the light, generating an electrical signal based on the mixed single sideband optical signal and light, amplifying the electrical signal, and controlling the modulation of the light using the amplified electrical signal. 
     Other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, which shows and describes exemplary embodiments of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention. The invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features, aspects, and advantages of the invention will be better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a diagram depicting an embodiment of the invention: 
         FIG. 2  is a cross-sectional diagram of an embodiment of the tunable electro-optic filter shown in  FIG. 1 ; 
         FIG. 3  is a perspective diagram of the optical resonator shown in  FIG. 2 ; 
         FIG. 4  is a partial cross-sectional diagram of the optical resonator shown in  FIGS. 2 and 3 ; and 
         FIG. 5  is a diagram depicting another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Optical resonators are electro-optical devices that are included in optical feedback loops to store energy having only specific resonant mode frequencies. Optical resonators are often small in size, having diameters on the order of millimeters, and may be used in many electro-optical system applications including tunable electro-optic filters. The optical resonators are curved optical waveguides, for example, a cylinder, a sphere, or a toroid within which light is internally reflected at the inner surface of the optical resonator. 
     Optical resonators can support resonator modes of light called whispering-gallery modes (“WGMs”), and thus, are often referred to as whispering-gallery mode resonators. WGMs occur when light having an evanescent wave component travels via internal reflection around the periphery of the optical resonator. The evanescent waves extend beyond the optical resonator&#39;s outer surface and may be coupled into an adjacent optical coupler as long as an optical coupler is located within the extent of the evanescent wave, typically on the order of the light&#39;s wavelength. Coupling losses between the optical coupler and the optical resonator are exponentially dependent upon the distance d between the surface of the optical coupler and the optical resonator ˜ exp (−d/r*), where r* is the effective scale length of evanescent field of the resonator for the excited WGM as expressed in the following equation: 
         r   *     =     λ   /       (       4   ⁢       Π   ⁡     (       n   res     /     n   out       )       2       -   1     )             
         where:
           λ is the wavelength of light evanescently coupled between the   optical coupler and the optical resonator;   n res  is the index of refraction of the optical resonator; and   n out  is the index of refraction outside the surface of the optical resonator.   
               

     Many optical resonators which propagate WGMs of light have extremely low transmission losses, and as a result, have a very high quality factor (“Q”). High-Q optical resonators are desirable because the higher the Q, the longer the amount of time the internally reflected light remains within the optical resonator and the greater the reduction of the spectral line width and phase noise. The ultimate intrinsic Q of the optical resonator (Q 0 ) is limited by the optical losses of the resonator material. Optical resonators having a radius of  10  to a few hundred micrometers have been produced with Q&#39;s in excess of 1×10 9  (see V. B. Braginsky, M. L. Gorodetsky, V. S. Ilchenko, Phys.Lett. A37, 393 (1989); L. Collot, V. Lefevre-Seguin, M. Brune, J. M. Raimond, S. Haroche, Europhys.Lett. 23, 327 (1993)). In particular, a Q in excess of 1×10 10  was demonstrated for optical resonators, and a Q of 10 11  is expected for glass microsphere optical resonators with a resonant wavelength of light at 1550 nanometers, where the intrinsic loss of glass is a minimum. 
     The present invention involves OEOs that include optical resonators.  FIG. 1  is a diagram, not to scale, of one exemplary embodiment of an OEO  10  that includes a continuous-wave laser  12 , electro-optic modulator  14 , delay optical fiber  16 , tunable electro-optic filter  18 , photodetector  20 , RF amplifier  22 , first optical fiber  24 , second optical fiber  26 , third optical fiber  28 , first coaxial cable  30 , second coaxial cable  32 , third coaxial cable  34 , and output terminal  36 . 
     In preferred embodiments, the laser  12  is FLD5F10NP made by Fujitsu located in San Jose, Calif. The electro-optic modulator  14  is integrated with the laser. In preferred embodiments the electro-optic modulator is an electro-absorption type electro-optic modulator in which phase modulation modulates light intensity. Other types of modulators known to those skilled in the art may be used, for example, an electro-absorption modulator, an acoustic-optic modulator, a polarization modulator, and a directional coupler modulator. The first, second, third, and delay optical fibers  24 ,  26 ,  28 , and  16 , respectively, are made from SMF- 28  manufactured by Corning Incorporated of Corning, N.Y. The photodetector  20  is Lasertron QDMH2 made by Lasertron located in Burlington, Mass. The RF amplifier  22  is MSH-6312202-MOD made by Microwave Solutions located in National City, Calif. The first, second, and third coaxial cables  30 ,  32 , and  34 , respectively, are PE-34182-8 made by Pasternack Enterprises located in Irvine, Calif. 
     The laser  12  has an output  38  that is coupled to an optical input  40  of the electro-optic modulator  14  by the first optical fiber  24 . Also, the output  38  of the laser is coupled to an input  42  of the photodetector  20  by the second optical fiber  26  that is coupled at one end  44  to the first optical fiber  24  adjacent the output  38  of the laser  12 . The length of each of the first and second optical fibers  20  and  24  is approximately 50 centimeters. The electro-optic modulator  14  includes an output  46  that is coupled to one end  48  of the delay optical fiber  16 . The other end  50  of the delay optical fiber  16  is coupled to an input  52  of the tunable electro-optic filter  18 . The length of the delay optical fiber  16  is approximately one kilometer or greater. An output  54  of the tunable electro-optic filter  18  is coupled to the input  42  of the photodetector  20  by the third optical fiber  28  that is coupled at one end  56  to the second optical fiber  26  adjacent the input  42  of the photodetector  20 . The length of the third optical fiber  28  is approximately 50 centimeters. An output  58  of the photodetector  20  is coupled to an input  60  of the RF amplifier  22  by the first coaxial cable  30  that is approximately 20 centimeters in length. An output  62  of the RF amplifier  22  is coupled to an electrical input  64  of the electro-optic modulator  14  by a second coaxial cable  32  that is approximately 20 centimeters in length, and to the output terminal  36  by a third coaxial cable  34  that is approximately 20 centimeters in length. The third coaxial cable  34  couples at one end  66  to the second coaxial cable  32  adjacent the output  62  of the RF amplifier  22 . 
       FIG. 2  is a diagram, not to scale, depicting the tunable electro-optic filter  18  which includes a first lens  68 , a second lens  70 , a first optical coupler  72 , a second optical coupler  74 , and an optical resonator  76  which may be co-located on a single chip  78 . Even though  FIG. 2  depicts the first and second lenses as converging thin lenses, the first and second lenses can take on other forms, for example, gradient index lenses. In preferred embodiments, the first and second lenses are lens model 024-0690 made by Optosigma located in Santa Ana, Calif. The first and second optical couplers are both prism model  2 A made by Drukker International located in Cuijk, Netherlands. The optical resonator is made of lithium niobate which is a material that allows for the propagation of light. However, the refractive index of the optical resonator is dependent upon electric fields established in the lithium niobate. The optical resonator is typically 3 to 6 millimeters in diameter, which corresponds from 16 to 8 GHz in frequency. The Q for the lithium niobate optical resonator is approximately 1×10 7 . 
     One end  50  of the delay optical fiber  16  is coupled to the input  52  of the tunable electro-optic filter  18 . The first lens  68  is positioned such that an optical axis of the delay optical fiber  16 , indicated by the straight arrow  80 , is aligned with a midpoint  82  of the first lens and a point  84  at an edge  86  of the first optical coupler  72  at which light is evanescently coupled into the optical resonator  76 . Similarly, an optical axis of the third optical fiber  28 , indicated by the straight arrow  88 , that couples the output  54  of the tunable electro-optic filter to the photodetector  20  is aligned with both a midpoint  90  of the second lens  70  and a point  92  at the edge  94  of the second optical coupler  74  at which light is evanescently coupled out of the optical resonator. 
     The optical resonator  76  is positioned adjacent to both the point  84  at the edge  86  of the first optical coupler  72  where light is evanescently coupled into the optical resonator and the point  92  at the edge  94  of the second optical coupler  74  where light is evanescently coupled out from the optical resonator. The optical resonator is spaced away from the first optical coupler by a distance “d 1 ” and the second optical coupler by a distance “d 2 ” both of which typically are approximately 0.1 to 3 times the wavelength of the light. 
     In some embodiments, the midpoint  82  of the first lens  68  is positioned approximately two focal lengths of the first lens away from the one end  50  of the delay optical fiber  16 , and the midpoint  82  of the first lens is positioned approximately two focal lengths of the first lens away from the point  84  at the edge  86  of the first optical coupler  72  that is closest to the optical resonator  76  where light is evanescently coupled into the optical resonator. Similarly, the midpoint  90  of the second lens  70  is positioned approximately two focal lengths of the second lens away from the point  92  at the edge  94  of the second optical coupler  74  where light is evanescently coupled out from the optical resonator. Also, an end  96  of the third optical fiber  28  that couples the tunable electro-optic filter  18  to the photodetector  20  is positioned approximately two focal lengths of the second lens away from the midpoint  90  of the second lens. 
     Referring additionally to  FIGS. 3 and 4 , the optical resonator  76  is cylindrical and configured between a substrate  98  that functions as a ground electrode and a top electrode  100 . The top electrode  100  may cover a top surface  102  of the optical resonator  76 , as depicted in  FIGS. 3 and 4 , or may cover only a portion of the top surface (not shown). An interface wire  104  is coupled to the top electrode  100 . 
     In operation, referring to  FIG. 1 , light having a single optical carrier frequency is generated by the laser  12  and travels through the output  38  of the laser  12  and into the first optical fiber  24 . Light from the laser travels along the optical axis of the first optical fiber  24  and is input to the electro-optic modulator  14 . Light output from the laser  12  is also propagated through the first optical fiber  24  and into the second optical fiber  26 . The light travels along the optical axis of the second optical fiber  26  and is coupled into the input  42  of the photodetector  20 . 
     The electro-optic modulator  14  modulates the light that travels through the electro-optic modulator  14  as a function of an electrical signal input to the electro-optic modulator  14  through the electrical input  64 , as discussed in greater detail below. The modulated light, which includes the optical carrier frequency generated by the laser  12  and sideband frequencies, leaves the electro-optic modulator  14  through the output  46  of the electro-optic modulator  14  and is coupled into one end  48  of the delay optical fiber  16 . The modulated light travels the long length of the delay optical fiber  16  along its optical axis indicated by the straight arrow  80 , which provides for a long energy storage time and in turn decreases the phase noise associated with the modulated light. After traveling the length of the delay optical fiber  16 , the modulated light is coupled into the tunable electro-optic filter  18  through the input  52  of the tunable electro-optic filter  18 . Referring additionally to  FIG. 2 , light entering the tunable electro-optic filter  18  passes through the first lens  68  as indicated by straight arrow  106  and is focused by the first lens  68  on the point  84  at the edge  86  of the first optical coupler  72 . The light is totally internally reflected relative to a perpendicular  108  to the edge  86  of the first optical coupler  72  at the point  84  at the edge  86  of the first optical coupler  72  where the light is evanescently coupled into the optical resonator  76  and propagates away from the edge  86  of the first optical coupler  72  as indicated by the straight arrow  110 . An evanescent component of the totally reflected light extends away from the edge  86  of the first optical coupler  72  toward the optical resonator  76  where the light is coupled into the optical resonator  76 . The light that is coupled into the optical resonator  76  travels around the outside edge of the optical resonator  76 , as indicated by curved arrows  112  and  114  and establishes WGMs that extend beyond the surface of the optical resonator  76 . 
     Referring additionally to  FIGS. 3 and 4 , an external electrical potential, typically between −100 volts and +100 volts depending upon the height of the optical resonator  76  is applied from an external voltage source (not shown) to the top electrode  100  via the interface wire  104  and produces an electrical field between the top electrode  100  and the substrate  98  which passes through the region  116  where light propagates in the optical resonator  76  near the outer edge of the optical resonator  76 . The external electrical potential is used to change the refractive index of the lithium niobate optical resonator, and thus shift the frequency of the light propagating through the optical resonator  76 . Thus, the tunable electro-optic filter  18  can be adjusted to select a single sideband from the modulated light output from the electro-optic modulator  14 . 
     Evanescent components of the light traveling around the periphery of the optical resonator  76  are then coupled from the optical resonator  76  into the second optical coupler  74  at the point  92  at the edge  94  of the second optical coupler  74 . The light coupled into the second optical coupler  74  from the optical resonator  76  travels toward the second lens  70  as indicated by the straight arrow  118 . Next, the light travels through the second lens  70  that focuses the light on the output  54  of the tunable electro-optic filter  18  on one end  96  of the third optical fiber  28 . The light then travels through the third optical fiber  28  along the optical axis of the third optical fiber  28 , indicated by straight arrow  88 , and into the second optical fiber  26  that couples the light into the input  42  of the photodetector  20  along with the light from the laser  12  that traveled through the second optical fiber  26 . Thus, the sideband frequency selected using the tunable electro-optic filter  18  is then recombined and mixed with the optical carrier at the input  42  of the photodetector  20 . 
     The photodetector  20  generates an electrical signal that quantifies the intensity of the light mixed at the input  42  of the photodetector  20 . The electrical signal generated by the photodetector  20  is output  58  through the output  58  of the photodetector  20  through the first coaxial cable  30  and into the input  60  of the RF amplifier  22 . The amplification provided by the RF amplifier  22  is selected such that the total open-loop gain for the electro-optic feedback loop  120 , that includes the delay optical fiber  16 , tunable electro-optic filter  18 , third optical fiber  28 , photodetector  20 , first coaxial cable  30 , RF amplifier  22 , and second coaxial cable  32 , is greater than one. Thus, the gain provided by the RF amplifier  22  is sufficient to drive the OEO  10  into self-sustained oscillation. The RF amplifier  22  amplifies the input electrical signal by approximately +30 dB to +50 dB and then outputs an amplified electrical signal that travels through the second coaxial cable  32  to the electrical input  64  of the electro-optic modulator  14  and through the third coaxial cable  34  to the output terminal  36 . The amplified electrical signal input to the electro-optic modulator  14 , as discussed above, modulates the intensity of the light from the laser  12 . The amplified electrical signal provided at the output terminal  36  is a microwave signal that oscillates at the sideband frequency selected by the tunable electro-optic filter  18 . 
       FIG. 5  is a diagram, not to scale, depicting another OEO embodiment  122  that includes a laser  12 , electro-optic modulator  14 , delay optical fiber  16 , tunable electro-optic filter  18 , first photodetector  124 , second photodetector  126 , RF amplifier  22 , first optical fiber  24 , second optical fiber  26 , third optical fiber  128  (optional), fourth optical fiber  130 , fifth optical fiber  132 , first coaxial cable  134 , second coaxial cable  136 , third coaxial cable  138 , fourth coaxial cable  140 , and output terminal  36 . 
     In preferred embodiments, the laser  12 , electro-optic modulator  14 , tunable electro-optic filter  18 , and RF amplifier  22  are the same devices used in the embodiment of FIG.  1 . FIG.  5 &#39;s first and second photodetectors  124  and  126 , respectively, are the same photodetectors as the photodetector  20  in FIG. 1&#39;s embodiment. Also, the first, second, third, fourth, fifth and delay optical fibers  24 ,  26 ,  128 ,  130 , and  132 , respectively, in the FIG.  5 &#39;s embodiment are made from the same fiber optic material as that used for the first, second, third, and delay optical fibers  24 ,  26 , and  28 , respectively, of the  FIG. 1  embodiment. In addition, the first, second, third, and fourth coaxial cables  134 ,  136 ,  138 , and  140 , respectively, in  FIG. 5  are made from the same coaxial cable used for the first, second, and third coaxial cables  30 ,  32 , and  34 , respectively, in the embodiment of FIG.  1 . 
     Referring to  FIG. 5 , the output  38  of the laser  12  is coupled to an optical input  40  of the electro-optic modulator  14  by the first optical fiber  24 . Also, the output  28  of the laser  12  is coupled to an input  142  of the first photodetector  124  by the second optical fiber  26 . The second optical fiber  26  is coupled to the first optical fiber  24  adjacent the output port  38  of the laser  12 . The optional third optical fiber  128  couples between the input  144  of the second photodetector  126  and the second optical fiber  26 . The length of each of the first, second, and third optical fibers  24  and  26  is approximately 50 centimeters. The electro-optic modulator  14  includes an optical output  46  that is coupled to the input  52  of the tunable electro-optic filter  18  by the fourth optical fiber  130 . The length of the fourth optical fiber  130  is approximately 50 centimeters. The output  54  of the tunable electro-optic filter  18  is coupled to the input  142  of the first photodetector  124  by the fifth optical fiber  132  that is approximately 50 centimeters in length. The fifth optical fiber  133  is coupled to the second optical fiber  26  adjacent the input  142  of the first photodetector  124 . One end  146  of the delay optical fiber  16  is coupled to the fourth optical fiber  130  between the electro-optic modulator  14  and the tunable electro-optic filter  18 . The other end  148  of the delay optical fiber  16  is coupled to the third optical fiber  128  adjacent to the input  144  of the second photodetector  126 . The length of the delay optical fiber  16  is approximately 2000 meters to 4000 meters. 
     The output  150  of the first photodetector  124  is coupled by the first coaxial cable  134  to the input  60  of the RF amplifier  22 . The output  152  of the second photodetector  126  is also coupled to input  60  of the RF amplifier  22  by the second coaxial cable  136  that couples to the first coaxial cable  134  adjacent to the input  60  of the RF amplifier  22 . The length of each of the first and second coaxial cables  134  and  136  is approximately 20 centimeters. The output  62  of the RF amplifier  22  is coupled to the electrical input  64  of the electro-optic modulator  18  by the optional third coaxial cable  138 . The output  62  of the RF amplifier  22  is also coupled to the output terminal  36  by a fourth coaxial cable  140  that couples to the optional third coaxial cable  138  adjacent the output  62  of the RF amplifier  22 . The length of each of the third and fourth coaxial cables  138  and  140  is approximately 20 centimeters. 
     Therefore, the embodiment depicted in  FIG. 5  is a dual-loop OEO  122  where one loop  154  includes the electro-optic modulator  14 , fourth optical fiber  130 , tunable electro-optic filter  18 , fifth optical fiber  132 , a portion of the second optical fiber  26 , first photodetector  124 , first coaxial cable  134 , RF amplifier  22 , and third coaxial cable  138 . The other loop  156  includes the electro-optic modulator  14 , a portion of the fourth optical fiber  130 , delay optical fiber  16 , a portion of the optional third optical fiber  128 , second photodetector  126 , second coaxial cable  136 , a portion of the first coaxial cable  134 , RF amplifier  22 , and third coaxial cable  130 . 
     Referring additionally to  FIGS. 2 ,  3 , and  4 , the tunable electro-optic filter  18  in the embodiment of  FIG. 5  is similar to that of the embodiment of  FIG. 1 , however, the input  52  of the tunable electro-optic filter  18  is coupled to one end  158  of the third optical fiber  130  in  FIG. 5  instead of the one end  50  of the delay optical fiber  16  as shown in FIG.  1 . Also, the output  54  of the tunable electro-optic filter  18  is coupled to one end  160  of the fourth optical fiber  132  in  FIG. 5  instead of one end  96  of the third optical fiber  28  in FIG.  1 . 
     The operation of the embodiment of  FIG. 5  is similar to that of the embodiment depicted in  FIG. 1 , however, there are the following differences. The modulated light output from the electro-optic modulator  14  propagates through the fourth optical fiber  130  to the input  52  of the tunable electro-optic filter  18 . The light output from the tunable electro-optic filter  18  propagates through the fifth optical fiber  132  and is coupled into the second optical fiber  26 , which in turn couples both the light output from the tunable electro-optic filter  18  and light from the laser  12  into the first photodetector  124 . The modulated light from the electro-optic modulator  14  is also coupled from the fourth optical fiber  130  into one end  146  of the delay optical fiber  16 , through the delay optical fiber  16 , and into a portion of the optional third optical fiber  128 , which in turn couples both the modulated light that travels through the delay optical fiber  16  and light from the laser  12  into the input  144  of the second photodetector  126 . 
     The first photodetector  124  generates a first electrical signal by mixing the light from the laser  12  and light from the tunable electro-optic filter  18 . The second photodetector  126  generates a second electrical signal by mixing the light from the laser  12 , when the third optical fiber  128  is included, and modulated light from the delay optical fiber  16 . The first electrical signal is output from the first photodetector  124  and input to the input  60  of the RF amplifier  22  via the first coaxial cable  134 . The second electrical signal is output from the second photodetector  126  and input to the input  60  of the RF amplifier  22 &#39;s input via the second and first coaxial cables  136  and  134 , respectively. The RF amplifier  22  amplifies the first and second electrical signals by approximately +30 dB to +50 dB, which is sufficient to drive the OEO  122  into self-sustained oscillation, and then outputs an amplified electrical signal that travels through the third coaxial cable  138  to the electrical input  64  of the electro-optic modulator  14 . The amplified electrical signal also travels through the fourth coaxial cable  140  to the output terminal  36 . The amplified electrical signal input to the electro-optic modulator  14 , as discussed above, modulates the intensity of the light from the laser  12 . The amplified electrical signal provided at the output terminal  36  is a microwave signal that oscillates at the sideband frequency selected by the tunable electro-optic filter  18 . 
     Advantageously, the optical resonator  76  in the tunable electro-optic filter  18  of both the  FIG. 1  embodiment and the  FIG. 5  embodiment has a high Q that allows for narrow filtering of a selected sideband frequency sustained by the OEO  10  and  122 . Because the optical resonator  76  provides for high-Q frequency selection, the optical resonator  76  also provides for frequency stabilization and broadband suppression of noise generated by the other components of the OEO. In addition, the embodiments depicted in  FIGS. 1 ,  2 ,  3 ,  4 , and  5  are advantageous in that they provide for a high-Q tunable electro-optic filter  18  that can be tuned by an external electrical potential, and as such, may be tuned in a short amount of time. Also, the tunable electro-optic filter  18  need only be tuned over a very limited fraction of its full optical bandwidth in comparison with typical RF bandpass filters which may be tuned across their entire bandwidth. Therefore, only a small variation in the external electrical potential applied to the top electrode  100  of the optical resonator  76  is needed to provide a wide range of filtering in the microwave domain. Also, the tunable electro-optic filter  18  depicted in  FIGS. 2 ,  3 , and  4  and discussed above can be configured within one chip  78 , thus reducing costs by reducing the total number of components to be configured during manufacturing of the OEO. 
     Although exemplary embodiments of the present invention have been described, they should not be construed to limit the scope of the appended claims. Those skilled in the art will understand that various modifications may be made to the described embodiments. For example, the optical resonator  76  may be spherical-shaped, cylindrical-shaped, torodial-shaped, or may have other physical configurations. In addition, even though  FIG. 2  only depicts an embodiment of the tunable electro-optic filter  18  where the optical resonator  76  is physically separated from the first and second optical couplers  72  and  74 , respectively, by free space, the distance between the optical resonator  76  and first and second optical couplers  72  and  74  may be maintained by a spacer (not shown) comprised of a thin film of material, for example, FIBERCOAT QLI manufactured a Navitar Coating Labs located in Newport Beach, Calif. One side of the spacer would contact the optical resonator  76  while the opposing side of the spacer would couple to either the first or second optical coupler. 
     Moreover, to those skilled in the various arts, the invention itself herein will suggest solutions to other tasks and adaptations for other applications. The present embodiments must therefore be considered in all respects as illustrative and not restrictive. The scope of the invention is not limited to those embodiments, but must be determined instead by the appended claims, along with the full scope of equivalents to which those claims are legally entitled.