Abstract:
In a first aspect, the invention includes a radio frequency photonic transceiver, comprising: a radio frequency receiver; a radio frequency photonic transmitter; and a switch between the input of the radio frequency photonic receiver and the output of the radio frequency photonic transmitter. In a second aspect, the invention includes an apparatus, comprising: a radio frequency photonic receiver; a radio frequency photonic transmitter; and a switch between the input of the radio frequency photonic receiver and the output of the radio frequency photonic transmitter. In a third aspect, the invention includes a radio frequency, photonic transceiver, comprising: means for generating a radio frequency modulated optical signal; a radio frequency photonic transmitter; and means for switching between the input of the radio frequency photonic receiver and the output of the radio frequency photonic transmitter. In a fourth aspect, the invention includes a method, comprising: generating a first optical signal; electro-optically generating a first radio frequency signal from the first optical signal; transmitting the first radio frequency signal; receiving a second radio frequency signal; and electro-optically modulating a second optical signal with the frequency of the second radio frequency signal. In a fifth aspect, the invention includes a zero-intermediate frequency radio frequency photonic transmitter-receiver unit. In a sixth aspect, the invention includes a radio frequency photonic transmitter-receiver unit including a direct conversion receiver with receiver output.

Description:
[0001]    The priority of co-pending U.S. Provisional Application Ser. No. 61/052,810, entitled “Radio Frequency Photonic Transceiver”, filed May 13, 2008, in the name of the inventors Brett A. Williams and Kurt S. Schuder is hereby claimed. This application is also hereby incorporated by reference for all purposes as if set forth verbatim herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention pertains to radio frequency transceivers and, more particularly, a radio frequency photonic transceiver. 
       SUMMARY OF THE INVENTION 
       [0003]    In a first aspect, the invention includes a radio frequency photonic transceiver, comprising: a radio frequency receiver; a radio frequency photonic transmitter; and a switch between the input of the radio frequency photonic receiver and the output of the radio frequency photonic transmitter. 
         [0004]    In a second aspect, the invention includes an apparatus, comprising: a radio frequency photonic receiver; a radio frequency photonic transmitter; and a switch between the input of the radio frequency photonic receiver and the output of the radio frequency photonic transmitter. 
         [0005]    In a third aspect, the invention includes a radio frequency, photonic transceiver, comprising: means for generating a radio frequency modulated optical signal; a radio frequency photonic transmitter; and means for switching between the input of the radio frequency photonic receiver and the output of the radio frequency photonic transmitter. The generating means includes: first means for generating a laser signal; first means for focusing the laser signal; means for electro-optically, actively modulating the focused laser signal with a received radio frequency signal; and means for detecting the modulated laser signal. The radio frequency photonic transmitter includes: second means for generating a laser signal; second means for focusing the laser signal; means for generating a radio frequency signal from an optical signal output by the laser generating means and received through the focusing means; and means for detecting the radio frequency signal. 
         [0006]    In a fourth aspect, the invention includes a method, comprising: generating a first optical signal; electro-optically generating a first radio frequency signal from the first optical signal; transmitting the first radio frequency signal; receiving a second radio frequency signal; and electro-optically modulating a second optical signal with the frequency of the second radio frequency signal. 
         [0007]    In a fifth aspect, the invention includes a zero-intermediate frequency radio frequency photonic transmitter-receiver unit. In some embodiments, this zero-intermediate frequency radio frequency photonic transmitter-receiver unit may be a variable zero-intermediate frequency radio frequency photonic transmitter-receiver unit. 
         [0008]    In a sixth aspect, the invention includes a radio frequency photonic transmitter-receiver unit including a direct conversion receiver with receiver output. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
           [0010]      FIG. 1  is a block diagram of one particular embodiment of a radio frequency, photonic transceiver constructed and operated in accordance with one aspect of the present invention; 
           [0011]      FIG. 2  is a block diagram of one particular embodiment of the transceiver in  FIG. 1 ; 
           [0012]      FIG. 3A-FIG .  3 B illustrate the receiver and transmitter of the embodiment in  FIG. 2  in greater detail; 
           [0013]      FIG. 4  illustrates the relationship of components common to both the receiver and the transmitter to open a discussion of the operation of those components; 
           [0014]      FIG. 5A  and  FIG. 5B  show a disk resonator fabricated from a sphere with two different orientations of the optic axis of the electro-optical crystal; 
           [0015]      FIG. 6 ,  FIG. 7 , and  FIG. 8  show examples of RF electrodes for a disk-like whispering-gallery-mode resonator; 
           [0016]      FIG. 9  is a mechanical drawing of one particular embodiment of the receiver of  FIG. 3A ; 
           [0017]      FIG. 10  illustrates an embodiment in which the receiver section and the transmitter section are combined by sharing the same electro-optically active modulator; 
           [0018]      FIG. 11  shows one concept application in which a single opto-electronic oscillator (“OEO”) local oscillator (“LO”) is used to feed four receive channels as in monopulse radar architecture using the transceiver of  FIG. 2 ; 
           [0019]      FIG. 12  depicts an alternative embodiment in a block diagram in which that photonic receiver of the radio frequency photonic transceiver in  FIG. 1  has been replaced by a conventional radio frequency mixer; and 
           [0020]      FIG. 13  shows a concept application in which a single opto-electronic oscillator (“OEO”) local oscillator (“LO”) is used to feed four receive channels as in monopulse radar architecture using the transceiver of  FIG. 12 . 
       
    
    
       [0021]    While the invention is susceptible to various modifications and alternative forms, the drawings illustrate specific embodiments herein described in detail by way of example. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but 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 OF THE INVENTION 
       [0022]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort, even if complex and time-consuming, would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0023]      FIG. 1  is a block diagram of an apparatus  100  constructed and operated in accordance with one aspect of the present invention. The apparatus  100 , in the embodiment of  FIG. 1 , is a radio frequency photonic transceiver. The apparatus  100  comprises a radio frequency photonic receiver (“RFPR”)  105 , a radio frequency photonic transmitter (“RFPT”)  110 , and a switch  115 . The switch  115  connects the line  128  to the input  120  of the radio frequency photonic receiver  105  or the output  125  of the radio frequency photonic transmitter  110 . In operation, the apparatus  100  is typically associated with an antenna (not shown), such that the switch  115  switches the input  120  and output  125  to the antenna in respective receive and transmit modes. 
         [0024]      FIG. 2  is a block diagram of one particular embodiment  200  of the apparatus  100  in  FIG. 1 . More particularly,  FIG. 2  depicts a radio frequency photonic transceiver  100 ′ operationally connected to an antenna  203  over an RF connection  206 . The RF connection  206  may be made using any suitable means known to the art. RF waveguides, for instance, of various types are frequently used. In the illustrated embodiment, the RF connection  206  is implemented as an RF microstrip line, as will be discussed further below. 
         [0025]    The switch  115 ′ in this particular embodiment  200  is implemented as a 3-port circulator  209 . Such circulators are well known in the telecommunications arts and any suitable circulator known to that art may be used. The circulator  209  routes an RF signal received via the RF connection  206  or the output  125 ′ to the appropriate output  212 ,  215 , respectively, depending on whether the apparatus  200  is operating in transmit or receive mode. 
         [0026]    The radio frequency photonic transceiver  100 ′ comprises a radio frequency photonic receiver  105 ′ and a radio frequency photonic transmitter  110 ′. The photonic receiver  105 ′, which may also be referred to as a “modulator” or “modulator receiver”, is better shown in  FIG. 3A . The photonic transmitter  110 ′, which may also be referred to as an “optoelectronic oscillator” (“OEO”), is better shown in  FIG. 3B . As will be discussed further below, the photonic transmitter  110 ′ provides a master frequency generator (“MFG”) and a local oscillator (“LO”) for the radio frequency photonic transceiver  100 ′. 
         [0027]    Referring now to both  FIG. 2  and  FIG. 3A , in receive mode, the antenna output  300  enters the circulator  209  directing received signals to the receiver  105 ′, then to the low noise amplifier  216 , and finally to the photonic receiver  105 ′. The photonic receiver  105 ′ includes an electro-optically active modulator  303 . The electro-optically active modulator  303 , as will be discussed in greater detail below, generally comprises a horseshoe-shaped electrode  325  atop a disk resonator  326 . The electro-optically active modulator  303  receives the RF input  300  into the disk resonator  326  from the circulator  209  through the LNA  216  via the electrode  325 . 
         [0028]    The electro-optically active modulator  303  is also evanescent-wave coupled to a lens and prism package (“LP”)  306 . The lens (not individually shown) collimates the laser diode and the prism (also not individually shown) properly k-matches it. The electro-optically active modulator  303  converts the radio frequency input  300  into an optical signal  309  that is then transmitted to the LP  306 . A diode laser  312  outputs a laser signal  315 , which passes through the LP  306 , the resulting evanescent wave  316  entering the disk resonator  326  of the electro-optically active modulator  303 . More particularly, the diode laser  312 , LP  306  and disk modulator  303  form an injection locked external cavity diode laser (“ECDL”), creating what is known as a “wavelocker”. In the illustrated embodiment, the wavelocker generates a ˜200 THz optical laser frequency in the kHz range or less for line width. 
         [0029]    The electro-optically active modulator  303  also receives a local oscillator (“LO”) signal  221  from the transmitter  110 ′ generated as discussed further below. 
         [0030]    The voltage of the RF input  300  modulates the disk index of refraction of the disk resonator  326  at the RF frequency, thereby modulating laser light internal to the disk resonator  326 . The internal light originates from the laser signal  312  and the evanescent wave  316 . The laser light trapped in the whispering gallery mode of the disk cycles about the periphery (˜26 times in this particular embodiment) before exit through evanescent coupling. It is during these multiple cycles that RF modulates laser light, imposing the desired information (i.e., Doppler frequency) on the IR carrier. 
         [0031]    The output light  318  is delivered to a baseband photodiode  321 . In the illustrated embodiment, “baseband” is, e.g., low pass DC to &lt;150 kHz. The resultant modulated optical signal  318  is then detected by an optical detector  321 . The optical detector  321  then converts the modulated optical signal  318  into an output electrical signal  324  representative of the modulated optical signal  318 . The output electrical signal  324  is then sampled by ADCs (not shown) in a manner suitable for its intended use. 
         [0032]    Turning now to the photonic transmitter  110 ′, shown in  FIG. 3B , a second diode laser  327  generates a laser signal  330  that enters a second electro-optically active modulator  333  through a LP  336 . The electro-optically active modulator  333  converts the collimated laser signal  339  into an optical frequency signal  342 , that it outputs to a photodiode satisfying the desired radio frequency bandwidth as detector  345 . The photodiode detector  345  converts the optical frequency signal  342  into a radio frequency signal  348  representative thereof. The radio frequency signal  348  is then passed through a RF filter  351  and the filtered signal  354  is passed through an RF amplifier  357 . The filtered and amplified RF signal  360  is then fed back to the electro-optically active modulator  333  and is output to the switch  115 ′ through an amplifier  218 , both shown in  FIG. 2 . The signal  360  is used for a number of functions discussed further below, such as a master frequency generator and a local oscillator. 
         [0033]    The local oscillator (“LO”) function oscillates at the frequency to which it is tuned, energized by the input laser  327 . That is, the LO frequency is that of the maximum gain point of the microdvisk bandwidth centered on its free spectral range (“FSR”) through means of an RF feedback loop  363 . The RF photodiode (“RF PD”)  345  is coupled to the electro-optically active modulator  333  as a lowpass filter (though high enough to pass desired RF, e.g., S-band, X-band, Ka-band, etc.). This signal  348  is then high-passed through the RF filter  351 , amplified and returned to the electro-optically active modulator  333 , a sample  366  of which is forwarded to the receiver  105 ′. 
         [0034]    Referring again to  FIG. 3A , the OEO accesses the electro-optically active modulator  303  of the receiver  105 ′ (though injecting into the receive line makes for less manufacturing effort). The OEO LO signal  221  modulates laser light internal to the electro-optically active modulator  303  along with target signal modulation (i.e., the RF input  300 ) already present, again via the electro-optic effect. Self-mixing internal to the electro-optically active modulator  303  results in the usual heterodyning action between LO and input signals  221 ,  300 , respectively, with their resulting sum and difference frequencies passed and filtered out respectively by the baseband lowpass photodiode  321 . 
         [0035]    The modulated optical signal  324  is a sinusoidal electrical waveform at the Doppler frequency of all signals received which are low enough to clear the lowpass photodiode  321  (e.g., target(s), clutter). After ADCs (not shown) sample the composite signal  324  (target and clutter) numerical results are passed to a discrete Fourier transform (not shown) for spectral separation of each component in order to pull target from rain or ground clutter and/or other targets moving at different velocities and hence different Doppler frequencies. 
         [0036]    Thus, the embodiment of  FIG. 2  combines microwave-photonic receiver and coherent, microwave-photonic transmitter technologies. Both the receiver  105 ′ and the transmitter  110 ′ are based on lithium niobate (LiNbO 3 ) microdisk technology or comparable electro-optically active material. The receiver  105 ′ accepts RF at the desired RF frequency and converts this from radio wavelengths to optical laser wavelengths via electro-optic effect of the receive microdisk  303 . Baseband signals  318  resulting from laser modulation are filtered by a baseband photodiode  321  and sampled by an analog-to-digital converter (“ADC”) (not shown). The baseband signals  318  are derived by internal self-mixing within the receive microdisk  303  of both target return combined with an local oscillator (“LO”) signal—i.e., the LO signal  221 —as generated by the transmitter  110 ′. 
         [0037]    Still referring to  FIG. 2 , notice that, in this embodiment, no offset intermediate frequency (“IF”) is provided for, though another mixer stage or stages, analog or digital, may be available at the output or input of RF signal per application requirements. Hence this particular layout acts like a Zero-IF or Direct Conversion receiver with receiver output at the Doppler frequency of a reflecting target. Note that, in some embodiments, this unit may be a variable zero-intermediate frequency radio frequency photonic transmitter-receiver unit. This radio frequency photonic transmitter-receiver unit can be the basic building block of a microwave-photonic radar as delineated below. 
         [0038]    The design and operation of the receiver  105 ′, shown in  FIG. 3A , and the transmitter section  110 ′, shown in  FIG. 3B , are centered on a similar combination of components shown in  FIG. 4 . These components are a laser  400 , a collimating LP  403 , and an electro-optically active modulator  406 . Although these components are utilized somewhat differently depending on whether they are included in the receive section  105 ′ or the transmit section  110 ′, they are basically the same set of components. 
         [0039]    The laser  400 , the collimating LP  403 , and the disk modulator  406  together, as noted above, form an injection locked external cavity diode laser (“ECDL”). The laser  400  and collimating LP  403  may be implemented in any of variety of ways known to the art to be suitable for this purpose. In the illustrated embodiment, the laser  400  is a diode laser, of which a variety are known. One particular implementation employs a semiconductor laser. The collimating LP  403  focuses the beam onto a particular point on the disk perimeter. Together, they form what is known to the art as a “wavelocker”  409 . 
         [0040]    The electro-optically active modulator  406  of the illustrated embodiments is a microstructure, and, more particularly, a microdisk. Note that the invention is not limited to disks and disk-like geometries. Other geometries have been developed and may be satisfactory for some embodiments. For example, microspheres, micro-rings, and micro-octagons have been developed. In general, however, microdisks have, to date, demonstrated superior performance in a wider range of conditions than these other geometries. The invention therefore is not limited to microdisks as the invention admits variation in this aspect of the invention. 
         [0041]    Suitable microdisks are commercially available on specification from, for example, OEwaves, Inc., at 1010 East Union Street Pasadena, Calif. 91106; telephone: (626) 449-5000; facsimile: (626) 449-1215; or electronic mail: info@oewaves.com. Additional information is also available over the World Wide Web of the Internet at http://www.oewaves.com/index.html. 
         [0042]    Design of the microdisk includes material selection, diameter, thickness and polishing of the outside perimeter. Disk diameter is made to accept the RF frequency of interest by the equation 
         [0000]    
       
         
           
             FSR 
             ~ 
             
               c 
               
                 
                   n 
                   disk 
                 
                  
                 2 
                  
                 π 
                  
                 
                     
                 
                  
                 R 
               
             
           
         
       
     
         [0043]    where:
       FSR is free spectral range, or the RF frequency to which one wants to design (e.g., S-band, X-band, Ka-band, etc.);   c is the speed of light;   n disk  the disk resonator refractive index; and   R its radius.       
 
         [0048]    The thinner a disk the more sensitive it becomes to a fixed applied voltage because modulation index, or disk sensitivity, depends on thickness by: 
         [0000]    
       
         
           
             
               sensitivity 
               ~ 
               
                 V 
                 d 
               
             
             . 
           
         
       
     
         [0049]    where:
       V is applied RF voltage; and   d is disk thickness.       
 
         [0052]    Optical polishing of the outer perimeter of the disk improves Q, i.e., narrow bandwidth. A number of materials may be selected, some better than others, as long as they are electro-optically active. A list materials from http://www.kavelaby.npl.co.uk/general_physics/2 — 5/2 — 5 — 11.html includes C 6 H 5 O 2 N (nitrobenzene), Pb 0.814 La 0.124 —(Ti 0.6 Zr 0.4 )O 3 (PLZT), β-Zns, ZnSe, ZnTe, Bi 12 SiO 20 , KH 2 PO 4  (KDP), KD 2 PO 4  (KD*P), CsH 2 AsO 4  (CDA), BaTiO 3 , SrTiO 3 , KTa 0.35 Nb 0.65 O 3  (KTN), Ba 0.25 Sr 0.75  Nb 2 O 6 , LiNbO 3 , LiTaO 3 , Ag 3 AsS 3 , and KNbO 3 . 
         [0053]    The basic process makes use of the electro-optic effect in which an RF voltage applied to an electro-optic material causes it to vary index of refraction at the rate of the RF oscillation. When laser light is coupled into the material and it is properly shaped such that this laser light proceeds on a continuous path allowing interaction with applied RF voltage—such as a disk allows when laser light cycles about its internal perimeter—then this laser light is modulated by index variation. In the case presented here, this modulation is simply a continuous or pulsed RF frequency tone. That tone includes a Doppler shift from the illuminated target. 
         [0054]    Microdisks of the type shown and other, alternative structures, are known to the art. Principles of design, construction, and operation appear in the patent literature, e.g.:
       U.S. Letters Patent 5,929,430, entitled “Coupled Opto-electronic Oscillator”, issued Jul. 27, 1999, to California Institute of Technology as assignee of the inventors X. Steve Yao et al.;   U.S. Letters Patent 6,389,197, entitled “Coupling System to a Microsphere Cavity”, issued May 14, 2002, to California Institute of Technology as assignee of the inventors Vladimir Iltchenko et al.;   U.S. Letters Patent 6,473,218, entitled “Light Modulation in Whispering-Gallery-Mode Resonators”, issued Oct. 29, 2002, to California Institute of Technology as assignee of the inventors Lute Maleki et al.; and   U.S. Letters Patent 7,133,180, entitled “Resonant Impedance Matching in Microwave and RF Device”, issued Nov. 7, 2006, to OEwaves, Inc., as assignee of the inventors Vladimir Iltchenko et al.       
 
         [0059]    Selected portions of U.S. Letters Patent 6,473,218 shall now be excerpted with some modification to further an understanding of the electro-optically active modulator  406 . The electro-optically active modulators are formed from what are known as “gallery-whispering-mode resonators.” Referring now to  FIG. 4 , the electro-optically active modulator  406  comprises a resonator  410  driven through an electrode  411 . The optical energy from the input laser beam  418  is coupled to a resonator  410  in one of the whispering gallery modes through a microprism  413 . The applied electrical signal—i.e., the RF input signal  415 —modulates the dielectric constant of the resonator  410  and hence the phase of the whispering gallery modes. This modulates the intensity of the output from the second optical coupler. 
         [0060]    In the illustrated embodiment, the whispering-gallery-mode resonator  410  defines a disk cavity. Optical energy can be coupled into a resonator  410  by evanescent coupling, e.g., using an optical coupler near the microdisk by less than one wavelength of the optical radiation. Such resonators have a special set of resonator modes known as “whispering gallery modes”. The resonators may be designed to have a high quality factor, Q, that are only limited with attenuation of radiation in the dielectric material and the surface inhomogeneities. 
         [0061]    The resonator  410  may be formed from any electro-optic material such as lithium niobate “or a similar electro-optic crystal”. The whispering gallery modes essentially exist near the equator of the resonator  410 , the resonator  410  may be not a whole sphere but a portion of the sphere near the equator that is sufficiently large to support the whispering gallery modes. Hence, rings, disks and other geometries formed from spheres may be used. 
         [0062]    Under proper configurations, either the resonator  410  alone or the resonator  410  in connection with a proper electrical strip line—i.e., the electrode  411 —can form an electrical resonator  410  to support electrical energy in the same whispering gallery modes with proper phase matching conditions. Hence, electrical and optical wave  426   s  can coexist and co-propagate in certain whispering gallery modes. When the resonator  410  is formed of an electro-optic material, the electrical wave can be used to alter or modulate the dielectric constant of the resonator  410  and hence modulate the light based on the electro-optic effects. 
         [0063]      FIG. 5A  shows a disk-like whispering gallery mode resonator  410  such as that used in the illustrated embodiment. It is formed from a sphere (not shown) by removing top and bottom portions of the sphere to leave a portion containing the sphere equator  500 . This embodiment of the resonator  410  includes a top circular surface  510  and a bottom circular surface  520 , both with diameters less than the diameter of the original sphere. The side surface  530  may be a spherical surface. The spacing, d, between the top and bottom surfaces  510  and  520  is sufficiently large that the optical and electrical modes centered at the equator  500  remain essentially undisturbed by the geometry. A small spacing d can be used to achieve a sufficient electrical field strength for the electro-optic modulation at a low voltage, e.g., on the order of millivolts. 
         [0064]    In general, the optic axis of the electro-optic material for the resonator  410  may be in any direction.  FIG. 5B  shows a disk configuration where the optical c-axis  503  is different from the z-axis  502  perpendicular to the equatorial circular plane. In certain applications, the optical c-axis  503  may be aligned with the z-axis  502  as in  FIG. 5A . 
         [0065]    The optical modulator  500  in  FIG. 5  may support RF (i.e., mm and microwave) signals, and light, simultaneously in a sphere of material with the electro-optic effect. Lithium niobate, for example, changes its real part of the index of refraction in response to the applied is electric field. Other materials may respond to the electric field differently. Multiple quantum well structures of III-V compounds, for example, change their imaginary part of the index of refraction when the electric field is applied. 
         [0066]      FIG. 6 ,  FIG. 7 , and  FIG. 8  show examples of the microstrip line electrode  600  when the resonator  410  is similar to the disk configuration shown in  FIG. 5A  and  FIG. 5B . In  FIG. 6 , the electrode  600  is formed on the top surface of the resonator  410  and another electrode  610  is formed in contact with the bottom surface of the resonator  410 .  FIG. 7  shows a half-circuit microstrip line as the top electrode  600  on the rim of the top surface.  FIG. 8  shows two pieces of circular microstrip lines  600 A and  600 B (solid lines) as the top electrode  600  and two pieces of circular microstrip lines  610 A and  610 B as bottom electrodes (broken lines with shades). 
         [0067]    The invention admits some variation in implementation specific detail.  FIG. 9  is a mechanical drawing of one particular embodiment  900  of the receiver  105 ′ shown in  FIG. 3A  and shows a LP  903  not found in  FIG. 3A . Similarly, in some embodiments, the lenses of the LPs  306 ,  336  may be replaced by lens assemblies (not shown) comprising more than one lens. Still other variations not shown might also be desirable in still other embodiments. 
         [0068]    Another variation may be found in the implementation of the switch  115 . The switch  115  is shown, conceptually, in  FIG. 1 , as a double pole, single throw switch. The invention is not so limited, as is evidenced by the disclosure herein of embodiments employing circulators. However, such a switch could be employed in some embodiments. The switch  125  let in the receive position until time to transmit, wherein it is thrown until transmission is completed, whereupon it is returned to the receive position. Other embodiments may employ alternative switching strategies and implementations. These and other such variations will be encountered in alternative embodiments and will arise from implementation specific considerations and design constraints. 
         [0069]    Thus, the particular structure and acts set forth herein are, by way of example and illustration, but one set of means for performing the disclosed functionality. For example, referring to  FIG. 3A-FIG .  3 B:
       the diode lasers  312 ,  327  are but one means for generating a laser signal;   the LPs  306 ,  336  are but one means for collimating the laser signal, and, in turn, for focusing the laser signal onto the microdisks  303 ,  333 ;   the microdisks  303 ,  333  are but one means for electro-optically, actively modulating the collimated laser signal with a received radio frequency signal; and   the microdisks  303 ,  333  are but one means for generating a radio frequency signal from an optical signal output by the laser generating means and received through the collimating means; and   the photodiodes  321 ,  345  are but one means for detecting the modulated laser signals.       
 
         [0075]    Some of these alternative means are set forth in the specification. For instance, the microdisks may be replaced using any of a variety of microstructures as previously described. Others will become apparent to those skilled in the art having the benefit of this disclosure. 
         [0076]    Turning now to  FIG. 9 , the drive mechanism for the electro-optically active modulator  303  will now be described in the illustrated embodiment. The electro-optically active modulator  303  is shown comprising a disk resonator  905  and a drive electrode  910  as described above relative to  FIG. 5-FIG .  8  and a microprism  915 . The drive mechanism  920  comprises a RF microstrip  925  formed in a substrate  930  that is wire bonded by a lead  933  to the electrode  910  of the electro-optically active modulator  303 .  FIG. 9  also illustrates the local oscillator feed  935 , which comprises a RF microstrip  939  formed in a substrate  942  that is wire bonded by a lead  945  to a second electrode  948 . 
         [0077]    Techniques for the manufacture of RF microstrips and use are well known in the art. Those skilled in the art will be able to readily adapt conventional techniques to the present invention. However, alternative embodiments may employ alternative technologies, such as printed circuit board (“PCB”) or printed wiring board (“PWB”) technologies that will also be readily adaptable. 
         [0078]    Note that, in the embodiment of  FIG. 2 , the receiver section  105 ′ and the transmitter section  110 ′ are separated even though they use similar designs. The receiver section  105 ′ and the transmitter section  110 ′ may be combined in some embodiments, both sharing the same microdisk. Such an embodiment is shown in  FIG. 10 . However, there is risk of pulling the resonant frequency of the disk  406  by injection locking to received signals close to but different from resonance when such inputs are at high levels (i.e., high power Doppler returns at close approach). The illustrated embodiment therefore separates the receiver section  105 ′ from the transmitter section  110 ′ to prevent this occurrence. 
         [0079]      FIG. 11  shows one concept application  1200  in which a single OEO LO  110 ′ is used to feed four receive channels  1203  as in monopulse radar architecture. A mixer conversion gain is assumed due to high power OEO LO as compared to received RF. This value zo decreases once target signal level nears LO power. For this example, analysis has shown that for a particular target RCS, at least 50 mW of peak transmit power is required in 4 mm/hr of rain at a range of 500 m. Power loss and division of a typical 10 mW OEO output requires 17 dB transmit amplification before additional losses in the circulator, feed network and antenna result in transmit powers of ˜16 mW per channel, spatially combined to exceed 50 mW on the mainbeam center axis. 
         [0080]    This same OEO LO signal is shared with each receive channel as a phase reference against which target return is mixed in the usual process of coherent detection. Using this OEO LO as both receive reference and transmit signal ensures phase coherence required for Doppler processing. Notice that Doppler processing may be pulse Doppler, requiring pulse amplification output, or linear frequency modulation (“LFM”), requiring a ramp output from the OEO, as determined by the particular application. 
         [0081]    Note that not all embodiments of the present invention  FIG. 12  depicts an alternative embodiment employing a photonic transceiver  1200  in a block diagram in which the photonic receiver  105  of the radio frequency photonic transceiver  100  in  FIG. 1  has been replaced by a conventional radio frequency mixer  1220 . Similarly,  FIG. 13  shows a concept application in which a single opto-electronic oscillator (“OEO”) local oscillator (“LO”) is used to feed four receive channels as in monopulse radar architecture using the transceiver of  FIG. 12  using the photonic transceiver  1200  of  FIG. 12 . 
         [0082]    Thus, the present invention generally provides coherent operation in a lightweight, miniature device. In some embodiments, neither the receiver nor the oscillator of the transmitter have powered mixers. Radio wavelengths are converted in the photonics to light wavelengths yielding small size with ideal device and channel isolation difficult to achieve in RF. The master frequency source does not multiply phase noise to the RF transmit frequency as any RF frequency may be chosen by a simple change in disk dimensions deriving RF from 200 THz light. 
         [0083]    Antenna output enters a circulator directing received signals to the receive section, to an LNA and finally to the modulator via RF-resonator horseshoe atop the disk. This RF voltage modulates disk index of refraction at the RF frequency thereby modulating laser light internal to the disk converting radio wavelengths to optical laser wavelengths via electro-optic effect. The modulator is evanescent wave coupled to an injection locked ECDL (external cavity diode laser) creating a wavelocker of ˜200 THz optical laser frequency in the kHz range or less for linewidth. Laser light trapped in the whispering gallery mode of the disk cycles about the periphery before exit via evanescent coupling. It is during these multiple cycles that RF modulates laser light, imposing the information we desire (Doppler frequency) on an IR carrier. 
         [0084]    Thus, in some embodiments two photonic devices, both based on LiNbO3 (lithium niobate) microdisk technology, one a modulator receive unit, the other acting as an LO/MFG (master frequency generator), may be combined in the form of a transmit-receive module. A four channel photonic receiver occupies a single card 1.08″ in diameter (size of a quarter). The LO is approximately ¼th of this. Dense packaging of such units can be combined as an option to traditional TR-modules and the dense microstrip phased array previously investigated in another area. 
         [0085]    A photonics approach provides miniature volumes (fractions of a cubic inch, 0.02 in cubed for a single modulator receiver channel including all support elements). No powered mixers are required, only the electro-optic effect of LiNbO 3 . Complete isolation is provided between channels due to all optical connections vs. RF cross talk problematic in microwave TR modules. 
         [0086]    The OEO LO oscillates at the frequency to which the OEO is tuned, energized by an input laser. That is, the OEO frequency is that of the maximum gain point of the microdisk bandwidth centered on its FSR (free spectral range) through means of an RF feedback loop. An RF photodiode (RF PD) is coupled to the OEO disk as a lowpass filter (though high enough to pass desired RF, e.g. S-band, X-band, Ka-band, etc.). This signal is then high-passed through an RF filter, amplified and returned to the modulator, a sample of which is forwarded to the receive section. 
         [0087]    The OEO makes access to the receiver microdisk as shown by a second horseshoe RF-resonator (though injecting into the receive line makes for less manufacturing effort). The OEO LO modulates laser light internal to the disk with target signal modulation already present, again via the electro-optic effect. Self-mixing internal to the disk results in the usual heterodyning action between LO and input signals with their resulting sum and difference frequencies passed and filtered out respectively by the baseband lowpass photodiode of the receive section. Output is a sinusoidal electrical waveform at the Doppler frequency of all signals received which are low enough to clear the lowpass photodiode (target(s), clutter). Using this OEO LO as both receive reference and transmit signal ensures phase coherence required for Doppler processing. Each OEO in an array can be easily phase locked to the others. 
         [0088]    Note that not every embodiment will necessarily possess all the characteristics nor exhibit all the advantages discussed above to the same degree or in the same way. 
         [0089]    The following references are hereby incorporated by reference for the noted purposes as if set forth verbatim herein:
       U.S. Letters Patent 5,929,430, entitled “Coupled Opto-electronic Oscillator”, issued Jul. 27, 1999, to California Institute of Technology as assignee of the inventors X. Steve Yao et al., for disclosure regarding design, construction, and operation of electro-optically active modulators;   U.S. Letters Patent 6,389,197, entitled “Coupling System to a Microsphere Cavity”, issued May 14, 2002, to California Institute of Technology as assignee of the inventors Vladimir Iltchenko et al., for disclosure regarding design, construction, and operation of electro-optically active modulators;   U.S. Letters Patent 6,473,218, entitled “Light Modulation in Whispering-Gallery-Mode Resonators”, issued Oct. 29, 2002, to California Institute of Technology as assignee of the inventors Lute Maleki et al., for disclosure regarding design, construction, and operation of electro-optically active modulators;   U.S. Letters Patent 7,133,180, entitled “Resonant Impedance Matching in Microwave and RF Device”, issued Nov. 7, 2006, to OEwaves, Inc., as assignee of the inventors Vladimir Iltchenko et al., for disclosure regarding design, construction, and operation of electro-optically active modulators; and   U.S. application Ser. No. 11/421,504, entitled “Millimeter Wave Electonically Scanned Antenna”, filed Jun. 1, 2006, in the name of Cole A. Chandler, and commonly assigned herewith, for disclosure regarding design, construction, and operation of RF microstrips.       
 
         [0095]    This concludes the detailed description. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.