Abstract:
A method and apparatus for generating microwave signal frequencies. An incident reference signal is provided. A first stimulus signal is also provided, the first stimulus signal having a first polarization and having a first predetermined relationship with the incident reference signal. A second stimulus signal is also provided, the second stimulus signal having a second polarization and having a second predetermined relationship with the incident reference signal. The incident reference signal is split into a first polarization reference signal and into a second polarization reference signal. The first stimulus signal is coupled with the first polarization reference signal to provide first polarization mixed signals. The second stimulus signal is coupled with the second polarization reference signal to provide second polarization mixed signals. The first polarization mixed signals are combined with the second polarization mixed signals to provide output signals having only a first component signal at the first predetermined relationship with the incident reference signal and a second component signal at the second predetermined relationship with the incident reference signal. The first predetermined relationship with the incident reference signal is provided by a first phase lock loop and the second predetermined relationship with the incident reference signal is provided by a second phase lock loop.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of microwave signal generation and, more particularly, to a method and apparatus for generating microwave frequency signals using polarization selective photonic mixing, such method and apparatus being useful for photodiode testing, for local oscillator distribution, or for phased array antenna systems. 
     BACKGROUND OF THE INVENTION 
     Microwave frequency signal processing is a common aspect of many modern electronic and/or optical systems. One such system is Doppler radar. If light of a single frequency is incident on a reflecting object that has some component of motion along the light&#39;s direction of travel, the frequency of the light will be changed by an amount related to the speed of the object. This is called the Doppler effect. The velocity of the object may be determined if the frequency of the reflected light is compared with that of the incident light. This comparison will be made automatically if the return light and the reference light fall on the receiver. The photocurrent produced will contain components related to the frequency difference that describes the velocity of the reflector, provided that the polarizations are parallel. If two reflectors with different velocities are present, two RF tones will be present in the stimulus. If the receiver is linear each reflector will be represented by a unique RF tone. Otherwise, other tones will be present. This is an important problem if these tones (distortion) are very close to those actually generated by the reflectors. Such tones will be created by 3rd order harmonic distortion. They are difficult to distinguish from those originating from actual reflectors and cannot be filtered out of the RF response since they are mixed in with genuine signal. This is illustrative of one motive to characterize the linearity of an optical receiver. A receiver&#39;s 3rd order distortion is often characterized in the laboratory using tones produced synthetically. Therefore, a need exists for an effective method and apparatus for signal generation that can be useful for the testing of photodiodes for intermodulation product distortion. 
     Two RF tones similar to those originating from moving reflectors may be produced from three optical frequencies if one of the three pairings can be eliminated. This is important since the third tone is not independent of the other two and will create problems in the Two Tone test of a receiver. 
     Further, many systems applications, such as in microwave signal generation and local oscillator distribution systems for space based radars and antenna systems, require the generation and delivery of very “clean” microwave signals, i.e., signals without unwanted harmonic mixing products and of narrow line width that is achieved via the phase locked loop. 
     The present invention provides a method and apparatus to meet such needs. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a method and apparatus for generating microwave frequencies is provided. 
     In accordance with a first embodiment of the invention, an incident reference signal is provided. A first stimulus signal is also provided, the first stimulus signal having a first polarization and having a first predetermined relationship with the incident reference signal. A second stimulus signal is also provided, the second stimulus signal having a second polarization and having a second predetermined relationship with the incident reference signal. In accordance with the first embodiment of the present invention, the first polarization is horizontal and the second polarization is vertical. The incident reference signal is split into a first polarization reference signal and into a second polarization reference signal. The first stimulus signal is coupled with the first polarization reference signal to provide first polarization mixed signals. The second stimulus signal is coupled with the second polarization reference signal to provide second polarization mixed signals. The first polarization mixed signals are combined with the second polarization mixed signals to provide output signals only having a first component signal at the first predetermined relationship with the incident reference signal and a second component signal at the second predetermined relationship with the incident reference signal. 
     Further, in the first embodiment the first predetermined relationship with the incident reference signal is provided by a first phase lock loop and the second predetermined relationship with the incident reference signal is provided by a second phase lock loop. The first phase lock loop first couples a portion of the first stimulus signal and a portion of the first polarization reference signal to provide a first phase lock loop difference signal. The first phase lock loop difference signal is compared with a first predetermined difference reference signal to provide a first difference correction signal. The first stimulus signal is then tuned by the first difference correction signal to maintain the first predetermined relationship with the incident reference signal. Similarly, the second phase lock loop first couples a portion of the second stimulus signal and a portion of the second polarization reference signal to provide a second phase lock loop difference signal. The second phase lock loop difference signal is compared with a second predetermined difference reference signal to provide a second difference correction signal. The second stimulus signal is then tuned by the second difference correction signal to maintain the second predetermined relationship with the incident reference signal. 
     In accordance with a second embodiment of the present invention, an incident reference signal is provided. A first stimulus signal is also provided, the first stimulus signal having a first polarization and having a first predetermined relationship with the incident reference signal. A second stimulus signal is also provided, the second stimulus signal having a second polarization and having a second predetermined relationship with the incident reference signal. The first polarization is horizontal. The second polarization is vertical. The incident reference signal is split into a first reference signal and a second reference signal. However, the incident reference signal, the first reference signal, and the second reference signal are each at 45° polarization. The first stimulus signal is coupled with the first reference signal to provide first polarization mixed signals. The first polarization mixed signals are combined with the second stimulus signal to provide output signals only having a first component signal at the first predetermined relationship with the incident reference signal and a second component signal at the second predetermined relationship with the incident reference signal. 
     Further, in the second embodiment the first predetermined relationship with the incident reference signal is provided by a first phase lock loop and the second predetermined relationship with the incident reference signal is provided by a second phase lock loop. The first phase lock loop first couples a portion of the first stimulus signal and a portion of the first reference signal to provide a first phase lock loop difference signal. The first phase lock loop difference signal is compared with a first predetermined difference reference signal to provide a first difference correction signal. The first stimulus signal is then tuned by the first difference correction signal to maintain the first predetermined relationship with the incident reference signal. Similarly, the second phase lock loop first couples a portion of the second stimulus signal and a portion of the second reference signal to provide a second phase lock loop difference signal. The second phase lock loop difference signal is compared with a second predetermined difference reference signal to provide a second difference correction signal. The second stimulus signal is then tuned by the second difference correction signal to maintain the second predetermined relationship with the incident reference signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a topological block diagram of one embodiment of the present invention. 
     FIG. 2 shows a schematic representation of a vertical polarization of an microwave signal. 
     FIG. 3 shows a schematic representation of a horizontal polarization of an microwave signal. 
     FIG. 4 shows a schematic representation of a 45° polarization of an microwave signal. 
     FIG.  5 . shows a topological block diagram of a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown a topological block diagram of one embodiment of the present invention, wherein frequency generator  10  provides microwave frequencies for the testing of photodiode  12 , typically using an RF Spectrum Analyzer  14 , such as a Hewlett-Packard model 8565E. RF Spectrum Analyzer  14  measures (and displays, as required for testing purposes) the photocurrent produced by photodiode  12  when light (i.e., at microwave signal frequencies) is incident upon it. The photocurrent is related to the total power in the incident optical field. The goal of the measurement of photodiode  12  is to identify those diodes which have performance characteristics such that third order distortion signals  16  are below predetermined product characteristic specifications levels for such distortion with respect to electrical signal level representations  18  of the signals incident upon photodiode  12 . Frequency generator  10  provides a system which processes three different microwave frequencies of light such that when mixed together in a certain relationship amongst themselves, they will interfere so that a desired input light (stimulus) is provided to the photodiode under test. 
     Frequency generator  10  includes center laser  20 , left laser subsystem  22  and right laser subsystem  24 . Nodes  26 ,  28 ,  30 ,  32  interconnect a polarization maintaining (PM) fiber optic network having fiber links  34   a - 34   k  for efficient light channeling. Center laser  20  is a tunable non-planar ring oscillator (NPRO) pigtail laser, for example, Lightwave Technology NPRO Laser product model No. 125-1319- xxx-w, having a wavelength of 1319 nm, and is coupled to PM fiber link  34   b.    
     Referring briefly to FIGS. 2,  3  and  4 , there is depicted, respectively, vertical signal polarization V, horizontal signal polarization H, and a 45° signal polarization. When considering the endface of PM fiber links  34   a - 34   k  as seen in FIG. 1, a coordinate system for each endface can be established such that any polarization state that is launched into the fiber will be maintained with respect to the coordinate system. For example, referring back to FIG. 1, signal v 0  is launchable into PM fiber link  34   b  and has a vertical polarization. 
     Left laser subsystem  22  includes laser  40 , whose characteristics are similar to that of center laser  20  but provides a different signal v  1 , for example, one that is+10 GHz greater in frequency than v 0 , which is launchable into PM fiber link  34   a  and also has a vertical polarization. However, to control the stability of the microwave frequency difference v 0 -v 1 , left laser subsystem  22  includes equipment which is used to make sure that frequency v 1  maintains a constant relationship to frequency v 0 . In practicing the present invention, there is no particular concern if the frequency v 0  of center laser  20  varies slightly, but that the difference between v 1  and v 0  is maintained. 
     Referring back to center laser  20 , laser light v 0  feeds into node  26 , which is a coupler/splitter, for example, a Wave Optics PM 50:50 splitter product model no. 650/967-0700. At the splitter 50% of the v 0  power is diverted to PM fiber link  34   d  which outputs from coupler/splitter node  26  with a vertical polarization remaining. All of the v 1  NPRO laser  40  light and the 50% feed into line  34   d  from coupler splitter node  26  combine in a fiber optic coupler/splitter at node  28 , a similar Wave Optics PM 50:50 splitter product to that of couple splitter node  26 . Since the polarization states are the same, namely, both being vertically polarized, they mix. 50% of the output from coupler/splitter node  28 , as indicated by mixing arrows  60 , falls upon photodiode  44  in left laser subsystem  22  from PM fiber link  34   f , where the difference frequency generated by photodiode  44  is fed into RF mixer  46 . The RF mixer is such that it is compatible with the frequency range being measured, for example, Anaren RF mixer product model no. 73230. The difference frequency is compared with a reference frequency, i.e., the desired difference frequency, from very stable synthesized oscillator  48  set at the desired frequency and the desired frequency is compared with the frequency generated in photodiode  44 . The output of RF mixer  46  is coupled to laser offset locking accessory (LOLA)  42 , for example, Lightwave Technology product model No. LOLA 2000 or 2005. LOLA  42 , which, in turn, is coupled to laser  40 , allows for agile temperature tuning and for piezoelectric tuning of its coupled laser&#39;s frequency. As such, left subsystem  22  provides a phase lock loop, wherein the RF mixer produces the difference frequency between the reference and the actual difference frequency between the two light sources, lasers  20  and  40 . If the difference frequency is non-zero, then a periodic function is output from the RF mixer and will cause a periodic variation in the frequency of v 1 , which is being controlled by the LOLA  42 . In essence, when the frequency difference between the reference and the actual frequency being generated is zero, but of the appropriate phase, then there is derived a steady-state voltage of the proper value so that the frequency of laser  40  is locked. It then follows that if small changes in the v 0  frequency from laser  20  are made, it will remain in lock. Therefore, v 1  is now controlled with respect to v 0  to obtain a desired difference frequency. 
     Similarly, right laser subsystem  24  includes laser  50 , whose characteristics are similar to that of center laser  20  but provides a different signal v 2 , for example, one that is +9 GHz greater in frequency than v 0 , which is launchable into PM fiber link  34   c  and has a horizontal polarization. However, to control the stability of the frequencies v 0  and v 2 , right laser subsystem  24  similarly includes equipment which is used to make sure that frequency v 2  maintains a constant relationship to the frequency v 0 . As discussed similarly above with regard to v 1  and v 0 , in practicing the present invention, there is no particular concern if the frequency v 0  of center laser  20  varies slightly, but that the difference between v 2  and v 0  is maintained. 
     Referring again back to center laser  20 , laser light v 0  feeds into coupler/splitter node  26 . At the splitter the other 50% of the v 0  power is diverted to PM fiber link  34   e  which outputs from coupler/splitter node  26  with a horizontal polarization. At coupler/splitter node  26  the left side output onto PM fiber link  34   d  is one polarization, namely vertical, while the right side output polarization onto PM fiber link  34   e  is orthogonal. This is achieved by having a linear polarization in the incident PM fiber link  34   b  to node  26 . Because the polarization maintains a fixed relationship to the established coordinate system, a simple 90° rotation can provide the orthogonal polarization. Alternatively, light is not rotated 90° but launched at a 45° polarization, as is depicted in FIG.  4 . All of the v 2  NPRO laser  50  light and the 50% feed into line  34   e  from coupler splitter node  26  combine in a fiber optic coupler/splitter at node  30 , a similar Wave Optics PM 50:50 splitter product. to that of coupler/splitter node  28 . Since the polarization states travelling along fiber links  34   e  and  34   c  are the same, namely, both being horizontally polarized, they likewise mix. 50% of the output from coupler/splitter node  30 , as indicated by mixing arrows  62 , falls upon photodiode  54  in right laser subsystem  24  from PM fiber link  34   i , where the difference frequency generated by photodiode  54  is fed into an RF mixer  56 . The RF mixer is such that it is compatible with the frequency range being measured, for example, as in left subsystem  22 , Anaren RF mixer product model no. 73230. The difference frequency is compared with a reference frequency, i.e., the desired difference frequency, from very stable synthesized oscillator  58  set at the desired frequency and the desired frequency is compared with the frequency generated in photodiode  54  . The output of RF mixer  56  is coupled to laser offset locking accessory (LOLA)  52 , for example, as in left subsystem  22 , Lightwave Technology product model No. LOLA 2000 or 2005. LOLA  52 , which, in turn, is coupled to laser  50 , allows for agile temperature tuning and for piezoelectric tuning of its coupled laser&#39;s frequency. As such, right subsystem  24  likewise provides a phase lock loop, wherein the RF mixer produces the difference frequency between the reference and the actual difference frequency between the two light sources, lasers  20  and  50 . If the difference frequency is non-zero, then a time varying function is output from the RF mixer and will cause a time varying variation in the frequency of v 2 , which is being controlled by the LOLA  52 . In essence, when the frequency difference between the reference and the actual frequency being generated is zero, but of the appropriate phase, then there is derived a steady-state voltage of the proper value so that the frequency of laser  50  is locked. It then follows that if small changes in the v 0  frequency from laser  20  are made, it will remain in lock. Therefore, v 2  is now controlled with respect to v 0  to obtain a desired difference frequency. 
     Referring back to coupler/splitter node  28 , the remaining 50% of the optical power mix of v 0  and v 1 , vertically polarized, flows through PM fiber link  34   g , as indicated by mixing arrows  64 , and is input into coupler/splitter node  32 , also a Wave Optics PM 50:50 splitter product. Similarly, referring back to coupler/splitter node  30 , the remaining 50% of the optical power mix of v 0  and v 2 , horizontally polarized, flows through PM fiber line  34   h , as indicated by mixing arrows  66 , and is likewise input into coupler/splitter node  32 . At coupler/splitter node  32  there is another coupler/splitter, much like that of coupler/splitter nodes  28  and  30 , but not necessarily equal at this point. The splitting ratio at coupler/splitter node  32  is kept equal if it is desired to combine equal amounts of the light coming from node  28  and node  30 . But such is not always needed. 
     At the right hand port of coupler/splitter node  32  there is output some v 2  (horizontal polarization), some v 0  (both horizontal and vertical polarization), and some v  1  (vertical polarization). Some v 0  polarization will mix with v 1  and some v 0  polarization will mix with v 2 . On the other hand, v 1  and v 2  will not mix. The v 0  mixing with v 1  and the v 0  mixing with v 2  are represented by mixing direction arrows  68  and represent the stimulus light projected onto photodiode  12  under test. An optional portion of output from coupler/splitter node  32  along fiber link  34   j  can be monitored as desired by a reference photodiode  70 , which, in turn, can be switched to for test comparison references measurements by RF Spectrum Analyzer  14 . 
     Referring to FIG. 5, there is shown another embodiment of the present invention in topological block diagram form. Frequency generator  110  provides microwave frequencies for the testing of photodiode  12 , typically using an RF Spectrum Analyzer  14 . As in the embodiment shown in FIG. 1, RF Spectrum Analyzer  14  measures (and displays, as required for testing purposes) the photocurrent produced by photodiode  12  when light (i.e., microwave signal frequencies) is incident upon it. The photocurrent is related to the total power in the incident optical field. Frequency generator  110  similarly provides a system which processes three different microwave frequencies of light such that when mixed together in a certain relationship amongst themselves, they will interfere so that a desired input light (stimulus) is provided to the photodiode under test. 
     Frequency generator  110  includes center laser  120 , left laser subsystem  122  and right laser subsystem  124 . Nodes  126 ,  128 ,  130 ,  132  interconnect a polarization maintaining (PM) fiber optic network having fiber links  134   a - 134   k  for efficient light channeling. Center laser  120  is a tunable non-planar ring oscillator (NPRO) pigtail laser, for example, Lightwave Technology NPRO Laser product model No. 125-1319-xxx-w, having a wavelength of 1319 nm, and is coupled to PM fiber link  134   b.    
     Left laser subsystem  122  includes laser  140 , whose characteristics are similar to that of center laser  120  but provides a different signal v 1 , for example, one that is +10 GHz greater in frequency than v 0 , which is launchable into PM fiber link  134   a  and also has a vertical polarization. However, to control the stability of the frequencies vo and v 1 , left laser subsystem  122  also includes equipment which is used to make sure that frequency v 1  maintains a constant relationship to frequency v 0 . Similar to that of the embodiment shown in FIG. 1, in practicing the present invention, there is no particular concern if the frequency vo of center laser  120  varies slightly, but that the difference between v 1  and v 0  is maintained. 
     Referring back to center laser  120 , laser light v 0  at a 45° polarization feeds into node  126 , which is a coupler/splitter, for example, a Wave Optics PM 90:10 splitter product model no. WX-902-352-73C4. At the splitter 90% of the v 0  power is diverted to PM fiber link  134   d  which outputs from coupler/splitter node  126  with a 45° polarization remaining. All of the v 1  NPRO laser  140  light and the 90% feed into line  134   d  from coupler splitter node  126  combine in a fiber optic coupler/splitter at node  128 , a Wave Optics PM 50:50 splitter product. Since the polarization states are both vertical and 45°, they partially mix. 50% of the output from coupler/splitter node  128 , as indicated by mixing arrows  160 , falls upon photodiode  144  in left laser subsystem  122  from PM fiber link  134   f , where the difference frequency generated by photodiode  144  is fed into RF mixer  146 . The RF mixer is such that it is compatible with the frequency range being measured, for example, Anaren RF mixer product model no. 73230. The difference frequency is compared with a reference frequency, i.e., the desired difference frequency, from very stable synthesized oscillator  148  set at the desired frequency and the desired frequency is compared with the frequency generated in photodiode  144 . The output of RF mixer  146  is coupled to laser offset locking accessory (LOLA)  142 , for example, Lightwave Technology product model No. LOLA 2000 or 2005. LOLA  142 , which, in turn, is coupled to laser  140 , allows for agile temperature tuning and for piezoelectric tuning of its coupled laser&#39;s frequency. As such, left subsystem  122  provides a phase lock loop, wherein the RF mixer produces the difference frequency between the reference and the actual difference frequency between the two light sources, lasers  120  and  140 . If the difference frequency is non-zero, then a time varying function is output from the RF mixer and will cause a time varying variation in the frequency of v 1  which is being controlled by the LOLA  142 . In essence, as with the embodiment shown in FIG. 1, when the frequency difference between the reference and the actual frequency being generated is zero, but of the appropriate phase, then there is derived a steady-state voltage of the proper value so that the frequency of laser  140  is locked. It then follows that if small changes in the v 0  frequency from laser  120  are made, it will remain in lock. Therefore, v 1  is now controlled with respect to v 0  to obtain a desired difference frequency. 
     Similarly, right laser subsystem  124  includes laser  150 , whose characteristics are similar to that of center laser  120  but provides a different signal v 2 , for example, one that is +9 GHz greater in frequency than v 0 , which is launchable into PM fiber link  134   c  and has a horizontal polarization. However, to control the stability of the microwave frequency difference, v 0 -v 2 , right laser subsystem  124  similarly includes equipment which is used to make sure that frequency v 2  maintains a constant relationship to the frequency v 0 . As discussed similarly above with regard to v 1  and v 0 , in practicing the present invention, there is no particular concern if the frequency v 0  of center laser  120  varies slightly, but that the difference between v 2  and v 0  is maintained. 
     Referring again back to center laser  120 , laser light v 0  feeds into coupler/splitter node  126 . At the splitter the other 10% of the v 0  power is diverted to PM fiber link  134   e  which outputs from coupler/splitter node  126  with a 45° polarization. At coupler/splitter node  126  the left side output onto PM fiber link  134   d  and the right side output polarization onto PM fiber link  134   e  are both 45°. At 50% coupler/splitter node  127  the left side output onto PM fiber link  134   h  and the right side output polarization onto PM fiber link  134   m  are both horizontal. The 10% light on fiber link  134   e  and the 50% feed into line  134   m  from coupler splitter node  127  partially combine in a fiber optic 50:50 coupler/splitter at node  130 . Since the polarization states travelling along fiber links  134   e  and  134   m  are not the same, namely, one being 45° polarized and the other horizontally polarized, respectively, they likewise partially mix. The output from coupler/splitter node  130 , as indicated by mixing arrows  162 , falls upon photodiode  154  in right laser subsystem  124  from PM fiber link  134   i , where the difference frequency generated by photodiode  154  is fed into RF mixer  156 . The RF mixer is such that it is compatible with the frequency range being measured, for example, as in left subsystem  122 , Anaren RF mixer product model no. 73230. The difference frequency is compared with a reference frequency, i.e., the desired difference frequency, from very stable synthesized oscillator  158  set at the desired frequency and the desired frequency is compared with the frequency generated in photodiode  154  . The output of RF mixer  156  is coupled to laser offset locking accessory (LOLA)  152 , for example, as in left subsystem  122 , Lightwave Technology product model No. LOLA 2000 or 2005. LOLA  152 , which, in turn, is coupled to laser  150 , allows for agile temperature tuning and for piezoelectric tuning of its coupled laser&#39;s frequency. As such, right subsystem  124  likewise provides a phase lock loop, wherein the RF mixer produces the difference frequency between the reference and the actual difference frequency between the two light sources, lasers  120  and  150 . If the difference frequency is non-zero, then a time varying function is output from the RF mixer and will cause a time varying variation in the frequency of v 2 , which is being controlled by the LOLA  152 . In essence, when the frequency difference between the reference and the actual frequency being generated is zero, but of the appropriate phase, then there is derived a steady-state voltage of the proper value so that the frequency of laser  150  is locked. It then follows that if small changes in the v 0  frequency from laser  120  are made, it will remain in lock. Therefore, v 2  is now controlled with respect to v 0  to obtain a desired difference frequency. Referring back to coupler/splitter node  128 , the remaining 50% of the optical power mix of v 0  and v1, flows through PM fiber link  134   g , as indicated by mixing arrows  164 , and is input into 50:50 coupler/splitter node  132 . Similarly, referring back to coupler/splitter node  127 , the remaining 50% of the optical power, horizontally polarized, flows through PM fiber line  134   h  and is input into coupler/splitter node  132 . At coupler/splitter node  132  there is another 50:50 coupler/splitter. 
     At the right hand port of coupler/splitter node  132  there is output some v 2  (horizontal polarization), some v 0  (45° polarization), and some v 1  (vertical polarization). Some v 0  polarization will mix with v 1  and some v 0  polarization will mix with v 2 . On the other hand, v 1  and v 2  again will not mix. The v 0  mixing with v 1  and the v 0  mixing with v 2  are represented by mixing direction arrows  168 . 
     At coupler/splitter node  180  with ballast laser  182 , the ballast laser/splitter arrangement allows the amount of microwave frequency producing light to be varied while maintaining a constant total optical power. This will occur if the total power in fiber  180  equals the total power in  168  since the splitter simply controls the  180 / 168  split, while the total amount of light is constant. Further, it should be noted that node  32  in FIG. 1 could be configured as node  180  in FIG. 5, if needed. 
     An optional portion of output from coupler/splitter node  180  along fiber link  134   j  can be monitored as desired by a reference photodiode  170 , which, in turn, can be switched to for test comparison references measurements by RF Spectrum Analyzer  14 . 
     Therefore, in accordance with the present invention there is provided a system which processes three different frequencies of light (i.e., three colors of light) such that when mixed together in such away they will interfere so that only two RF frequencies will be generated when this resultant light falls on a photodiode. The present invention provides a control of the polarization of the three frequencies in such a way that only the desired two RF frequencies will result, and furthermore, the total optical power may be constant while the power in the microwave frequencies is varied. 
     Those skilled in the art can appreciate that variations in various aspects of the present invention can be implemented in the embodiments described herein. For example, instead of using PM fiber links, freely propagating optical beams can be used. 
     While the embodiments of the present invention have been described in the context of a frequency generator for use in testing performance characteristics of photodiodes, those skilled in the art can appreciate that the frequency generator of the present invention can have other applications. 
     For example, the frequency generator of the present invention can be used in systems applications requiring discrete frequency signal generation for local oscillator (LO) distribution or for phased array antenna transmissions. These systems require the generation and delivery of very “clean” microwave signals, i.e., signals without unwanted harmonic mixing products. As such, the frequency generators described above can be readily used as a microwave signal source for such LO distribution and antenna transmissions.