Abstract:
The present invention is directed to systems and methods which provide a widely tunable, low-noise, synthesized microwave source with an OEO oscillator.

Description:
BACKGROUND OF THE INVENTION 
   Photonic systems use a combination of radio-frequency (RF) technology and optical technology to transport signals, and can be used for an opto-electronic oscillator (OEO). The oscillator uses the light energy to produce pure microwave signals. A typical oscillator comprises an optical modulator that is supplied with light from a low-noise laser source, a delay fiber, and a light detector. The detector forms an electrical signal that is amplified and filtered, and then is used to control the modulator. The modulator encodes or modulates the light from the light source with the electrical signal. The modulated light is then passed through the delay fiber, which introduces a delay in the light, and is then provided to the light detector. The light detector then converts the received light into the electrical signal that is used provided to the optical modulator. 
   The frequency at which the OEO oscillates can be controlled by placing a frequency selective element within the electronic portion of the loop. Also, the frequency at which the OEO oscillates can be controlled by placing a frequency selective element within the optical portion of the loop. For further information see “Optoelectronic Oscillator for Photonic Systems, Yao, X. S. et al, IEEE Journal of Quantum Electronics, Vol. 32, No. 7, July 1996, pgs 1141-1149, which is hereby incorporated herein by reference. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to systems and methods which provide a widely tunable, low-noise, synthesized microwave source with an OEO oscillator. 
   In one arrangement, the optical modulator is driven to produce at least a first pair of harmonic frequencies (with respect to the fundamental modulator frequency) about the fundamental optical frequency. One or more filters pass only the first pair of harmonic frequencies, and remove the other frequencies including the fundamental optical frequency. The photodetector then forms an electric signal that is equal to the difference between the pair of harmonic frequencies. The difference signal is a multiple of two of the fundamental modulator frequency. The difference signal is provided as an output from the OEO oscillator. One or more divide-by-two prescalers are used to reduce the difference signal back down to the fundamental modulator frequency. 
   In another arrangement, the optical modulator is driven to produce at least a third pair of harmonic frequencies (with respect to the fundamental modulator frequency) about the fundamental optical frequency. A coupler is used to split the light into two portions. One portion of light is used to provide the fundamental modulator frequency signal that is provided to the modulator. The other portion of light is used to provide an output from the OEO oscillator. The other portion of light is provided to one or more filters that pass only the third pair of harmonic frequencies, and remove the other frequencies including the fundamental optical frequency. A photodetector then forms an electric signal that is equal to the difference between the pair of harmonic frequencies. The difference signal may not be a multiple of two of the fundamental modulator frequency. The difference signal is provided as an output from the OEO oscillator. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  is an example of an OEO oscillator according to embodiments of the invention; 
       FIG. 2  is another example of an OEO oscillator according to embodiments of the invention; 
       FIGS. 3A-3C  depict the frequency spectrum of light at different points in the system of  FIG. 1 ; and 
       FIGS. 4A-4C  depict the frequency spectrum of light at different points in the system of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention use OEO concepts to provide different and/or higher frequencies by operating the OEO at harmonics of the modulator fundamental frequency. It is an advantage of at least one embodiment of the invention to use a lower bandwidth modulator (and hence less expensive) to generate a higher frequency output using an OEO oscillator. 
     FIG. 1  depicts an example of an optical electronic oscillator (OEO) according to embodiments of the invention. The OEO  100  is a second harmonic OEO with a prescaler. The OEO comprises an electronic or microwave portion  101  and an optical portion  102 . The electronic portion includes a phase lock  117  that receives a reference signal, e.g. a 10 MegaHertz (MHz), from signal source  118 . The phase lock  117  controls the phase shifter  104  to lock the OEO to a specific multiple of the reference frequency. 
   The phase shifter  104  provides an electronic signal to modulate the light of the laser input  103  via modulator  119 . Changing the phase shifter  104  (via the phase lock  117 ) causes the phase frequency of the OEO to shift. Thus, the phase shifter tunes the frequency of the OEO to force the oscillation to be locked to a harmonic of the reference signal  118 , e.g. a 10 MHz signal. For example, an OEO using a modulator with a 10 GigaHertz (GHz) modulation bandwidth may be set to oscillate at 10.1 GHz. Note that the OEO frequency can also be set to non-integer multiples of the reference frequency by using a fractional-n divider, as is typical for frequency synthesis. 
   The laser source provides light with a frequency that is several magnitudes greater than the fundamental frequency of the modulation. For example, the light may have a fundamental frequency on the order of 10 14  Hertz (Hz) (or a wavelength of about 1550 nanometers), while modulator may have a fundamental on the order of 10 GHz. 
   The light from the laser source  103  is encoded with the 10 GHz modulation signal by the modulator  119 . In other words, the light of the laser is amplitude and/or phase modulated with the modulation signal. In the frequency domain, the modulated light signal comprises a center frequency having the frequency of the laser light, and at least one pair of side bands located about the center frequency and spaced apart from the center frequency by the frequency of the modulation signal (i.e. plus and minus the modulation signal frequency). For example, the center frequency (or fundamental) may be at 10 14  Hz, and the side bands may be located 10 GHz from the center frequency, as shown in  FIG. 3A . The modulator may be driven to produce additional side bands, e.g. second and third harmonics. Thus, by selectively driving the modulator, other harmonic bands may be encoded into the light. 
   The modulated light is then coupled with a optical fiber  105 , which includes a length of fiber to provide additional optical delay, e.g. loop  106 . The longer the loop  106 , the more delay added to the light, such that a smaller frequency shift will cause the phase to change. Thus, longer loops (and hence longer delays) are desirable to place the frequency of the oscillation to be nearly exactly at the desired value and reduce the phase noise. The length of the loop  106  may be 10 meters. 
   Optical amplifier  107  may be optionally used to boost the modulated light prior to filtering. 
   The modulated light is then filtered by thin film filter  108 , e.g. a wavelength division multiplexing filter. In this arrangement, the thin film filter  108  passes the center frequency and the first set of side bands and blocks the remainder, as shown in  FIG. 3B . Note that in other arrangements other side bands may be passed and/or blocked. For example, second or third harmonic bands may be passed. 
   After filtering by filter  108 , the light passes into fiber coupler  109 , which splits the light into two paths. Each path passes into a respective narrow pass band filter, e.g. a Fabry-Perot filter. Each filter  110 ,  111  is tuned to pass a narrow range of frequency at side bands. For example, filter  110  passes the upper side band (e.g. the +10 GHz frequency) and the filter  111  passes the lower side band (e.g. the −10 GHz frequency). The filters  110 ,  111  are connected to the phase lock  117  to allow for the filters to be tuned to a desired frequency. The phase lock will set the center frequency of the phase shifter  104  as well as the filters  110 ,  111 . 
   Note that the arrangement of thin film filter  108  and filters  110 ,  111  is by way of example only, as another filter, other filters, and/or filter combinations could be used, so long as the desired frequencies are passed with noise as low as can be tolerated. Fabry-Perot filters tend to pass narrow bands of frequencies, but has a periodic response such that several periods of narrow bands would be passed. The use of the thin film filter removes the light that may be passed by the Fabry Perot in a period band. 
   After filtering by filters  110 ,  111 , the light passes into fiber coupler  112 , which recombines the light into a single path. In this arrangement, the center frequency has been filtered out, leaving the first set of side bands, as shown in  FIG. 3C . Thus, the two bands are separated by 20 GHz. 
   The light is then provided to the high-speed photodetector  113 , which produces an electric signal that has a frequency that represents the difference between the two received frequencies, e.g. 20 GHz. A tap  115 , allows the 20 GHz signal to be provided as an output for use by another device and/or with another application. 
   To allow the OEO oscillator to oscillate, the input signal that is provided to the modulator should approximate the output signal, thus, a divide-by-2 prescaler  114  is provided in the oscillating loop to divide the 20 GHz signal down to a 10 GHz signal. The value of the phase shifter  204  is chosen so there is an integral number of wavelengths around the combined electrical loop and the optical loop at the desired frequency of interest. An optional tap  116 , allows the 10 GHz signal to be provided as an output for use by another device and/or with another application. 
     FIG. 2  depicts another example of an optical electronic oscillator (OEO) according to embodiments of the invention. The OEO  200  is a sixth harmonic OEO with a separate detector. The OEO comprises an electronic or microwave portion  201  and an optical portion  202 . The electronic portion includes a phase lock  217  that receives a reference signal, e.g. a 10 MegaHertz (MHz), from signal source  218 . The phase lock  217  controls the phase shifter  204  to lock the OEO to a specific frequency. 
   The phase shifter  204  provides an electronic signal to modulate the light of the laser input  203  via modulator  219 . Changing the phase shifter  204  (via the phase lock  217 ) causes the phase frequency of the OEO to shift. Thus, the phase shifter tunes the frequency of the OEO to force the oscillation to be locked to a harmonic of the reference signal  218 , e.g. a 10 MHz signal. For example, a 10 GigaHertz (GHz) OEO may be set to oscillate at 10.1 GHz. Note that the OEO frequency can also be set to non-integer multiples of the reference frequency by using a fractional-n divider, as is typical for frequency synthesis. 
   The laser source provides light with a frequency that is several magnitudes greater than the frequency of the modulation. For example, the light may have a frequency on the order of 10 14  Hertz (Hz) (or a wavelength of about 1550 nanometers), while modulation of the light may be on the order of 10 GHz. 
   The light from the laser source  203  is encoded with the 10 GHz modulation signal by the modulator  219 . In other words, the light of the laser is amplitude and/or phase modulated with the modulation signal. In the frequency domain, the modulated light signal comprises a center frequency having the frequency of the laser light, and at least one pair of side bands located about the center frequency and spaced apart from the center frequency by the frequency of the modulation signal (i.e. plus and minus the modulation signal frequency). For example, the center frequency may be at 10 14  Hz, and the side bands may be located 10 GHz from the center frequency, as shown in  FIG. 4A . In this example, the modulator is driven to produce additional side bands, i.e. third harmonics. 
   The modulated light is then coupled with a optical fiber  205 , which includes a length of fiber to provide additional optical delay, e.g. loop  206 . The longer the loop  206 , the more delay added to the light, such that a smaller frequency shift that will cause the phase to change. Thus, longer loops (and hence longer delays) are desirable to place the frequency of the oscillation to be nearly exactly at the desired value and reduce the phase noise. The length of the loop  206  may be 10 meters. 
   An optical amplifier (not shown) may be optionally used to boost the modulated light prior to filtering. 
   The modulated light is then filtered by thin film filter  208 , e.g. a wavelength division multiplexing filter. In this arrangement, the thin film filter  208  passes the center frequency and up to the third set of side bands and blocks the remainder, as shown in  FIG. 4B . Note that in other arrangements other side bands may be passed and/or blocked. 
   After filtering by filter  208 , the light passes into fiber coupler  209 , which splits the light into three paths. Two of the paths are used to produce the fundamental frequency, namely the modulation signal. The third path is used to provide the 6 th  harmonic output. 
   Each path of the two of the paths are passed into a respective narrow pass band filter, e.g. a Fabry-Perot filter. Each filter  210 ,  211  is tuned to pass a narrow range of frequencies. For example, filter  210  passes one of the side bands (e.g. lower side band or the −10 GHz frequency) and the filter  211  passes the fundamental (i.e. the 10 14  Hz frequency). The filters  210 ,  211  are connected to the phase lock  217  to allow for the filters to be tuned to a desired frequency. The phase lock will set the center frequency of the phase shifter  204  as well as the filters  210 ,  211 . 
   Note that the arrangement of thin film filter  208  and filters  210 ,  211  is by way of example only, as another filter, other filters, and/or filter combinations could be used, so long as the desired frequencies are passed with noise as low as can be tolerated. Fabry-Perot filters tend to pass narrow bands of frequencies, but has a periodic response such that several periods of narrow bands would be passed. The use of the thin film filter removes the light that may be passed by the Fabry Perot in a period band. 
   After filtering by filters  210 ,  211 , the light passes into fiber coupler  212 , which recombines the light into a single path. In this arrangement, the center frequency has been passed, along with one of the side bands. Thus, the two bands are separated by 10 GHz. 
   The light is then provided to the high-speed photodetector  213 , which produces an electric signal that has a frequency that represents the difference between the two received frequencies, e.g. 10 GHz. A tap  216 , allows the 10 GHz signal to be provided as an output for use by another device and/or with another application. 
   The third path provides light to an optical amplifier  207  that may be optionally used to boost the modulated light prior to filtering. 
   The light is then optionally filtered by optional thin film filter  221 , which is similar to filter  208 . In this arrangement, the thin film filter  221  passes the center frequency and the third set of side bands and blocks the remainder, as shown in  FIG. 4B . Note that in other arrangements other side bands may be passed and/or blocked. 
   After filtering by filter  221 , the light passes into fiber coupler  222 , which splits the light into two paths. Each path passes into a respective narrow pass band filter, e.g. a Fabry-Perot filter. Each filter  216 ,  220  is tuned to pass a narrow range of frequencies. For example, filter  216  passes one of the side bands (e.g. upper side band or the −30 GHz frequency) and the filter  220  passes the other side band (e.g. lower side band or the +30 GHz frequency). The filters  216 ,  220  are connected to the band control circuitry  214  to allow for the filters to be tuned to a desired frequency. 
   Note that the arrangement of thin film filter  221  and filters  216 ,  220  is by way of example only, as another filter, other filters, and/or filter combinations could be used, so long as the desired frequencies are passed with noise as low as can be tolerated. Fabry-Perot filters tend to pass narrow bands of frequencies, but has a periodic response such that several periods of narrow bands would be passed. The use of the thin film filter removes the light that may be passed by the Fabry Perot in a period band. 
   After filtering by filters  216 ,  220 , the light passes into fiber coupler  223 , which recombines the light into a single path. In this arrangement, the center frequency, and the first and second sets of side bands have been filtered out, leaving the third set of side bands, as shown in  FIG. 4C . Thus, the two bands are separated by 60 GHz. 
   The light is then provided to the high-speed photodetector  224 , which produces an electric signal that has a frequency that represents the difference between the two received frequencies, e.g. 60 GHz. A tap  215 , allows the 60 GHz signal to be provided as an output for use by another device and/or with another application. 
   The arrangement of  FIG. 1  is preferable for harmonics that are a factor of two, as one or more divide by 2 prescalers may be used to provide the fundamental or modulating signal, while the arrangement of  FIG. 2  is more useful for non-factor of two harmonics. 
   Note that the arrangements of  FIGS. 1 and 2  use a 10 GHz electro absorption modulator, however other modulators may be used. For example, a 40 GHz Mach Zehnder modulator may be used in the arrangement of  FIG. 1  to produce a 80 GHz signal or used in the arrangement of  FIG. 2  to produce a 240 GHz signal. If the fundamental frequency of the OEO can be tuned, e.g. from 2-10 GHz, then the output can also be tuned from 2-20 GHz (for the arrangement of  FIG. 1 ) or from 2-60 GHz (for the arrangement of  FIG. 2 ). Most optical modulators have some nonlinearity that can be used to produce harmonics if driven hard. A Lithium Niobate Mach Zehnder modulator is particularly controllable in this regard. It has a sinusoidal transfer function (voltage to optical intensity). Biasing it at full-off point, even harmonics are produced. Biased in quadrature, odd harmonics are obtained. Intermediate bias points produce a controllable mixture of both. 
   Also note that the arrangements of  FIGS. 1 and 2  may use other harmonics, for example any of the harmonics from 1-6, or even higher. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.