Abstract:
Among others, RF receivers based on whispering gallery mode resonators are described. In one aspect, a photonic RF device includes a laser that is tunable in response to a control signal and produces a laser beam at a laser frequency. The RF device includes a first optical resonator structured to support a whispering gallery mode circulating in the first optical resonator, the optical resonator being optically coupled to the laser to receive a portion of the laser beam into the optical resonator in the whispering gallery mode and to feed laser light in the whispering gallery mode in the optical resonator back to the laser to stabilize the laser frequency at a frequency of the whispering gallery mode and to reduce a linewidth of the laser. The RF device includes a second optical resonator made of an electro-optic material to support a whispering gallery mode circulating in the optical resonator.

Description:
PRIORITY CLAIM AND RELATED APPLICATIONS 
     This application claims the benefits of the following two applications: 
     U.S. patent application Ser. No. 12/157,915, entitled “RF AND MICROWAVE RECEIVERS BASED ON WHISPERING GALLERY MODE RESONATORS” and filed on Jun. 13, 2008, which claims benefit of U.S. Provisional Application No. 60/934,800 entitled “Quadratic Photonic Receiver Based on Lithium Niobate Resonance Modulator with Optical Injection” and filed Jun. 13, 2007; and 
     U.S. Provisional Application No. 60/998,624 entitled “Superheterodyne receiver based on electro-optic high-Q resonator used as both modulator and optical delay for OEO” and filed Oct. 12, 2007. 
     The disclosure of the above referenced patent applications are incorporated by reference as part of the specification of this application. 
    
    
     BACKGROUND 
     This application relates to optical resonators and optical devices based on optical resonators. 
     Optical resonators may be used to spatially confine resonant optical energy in a limited cavity with a low optical loss. The resonance of an optical resonator may be used to provide various useful functions such as optical filtering, optical modulation, optical amplification, optical delay, and others. Light can be coupled into or out of optical resonators via various coupling mechanisms according to the configurations of the resonators. For example, Fabry-Perot optical resonators with two reflectors at two terminals may use partial optical transmission of at least one reflector to receive or export light. 
     Optical whispering gallery mode (WGM) resonators confine light in a whispering gallery mode that is totally reflected within a closed circular optical path. Unlike Fabry-Perot resonators, light in WGM resonators cannot exit the resonators by optical transmission. Light in a WGM resonator “leaks” out of the exterior surface of the closed circular optical path of a WGM resonator via the evanescence field of the WG mode. An optical coupler can be used to couple light into or out of the WGM resonator via this evanescent field. 
     SUMMARY 
     The specification of this application describes, among others, examples and implementations of RF receivers based on whispering gallery mode resonators. 
     In one example, a photonic RF device includes a laser that is tunable in response to a control signal and produces a laser beam at a laser frequency; and an optical resonator structured to support a whispering gallery mode circulating in the optical resonator. Thee optical resonator is optically coupled to the laser to receive a portion of the laser beam into the optical resonator in the whispering gallery mode and to feed laser light in the whispering gallery mode in the optical resonator back to the laser to stabilize the laser frequency at a frequency of the whispering gallery mode and to reduce a linewidth of the laser. The optical resonator exhibits an electro-optic effect in response to a control signal. This device includes electrodes formed on the optical resonator to apply the control signal to the optical resonator; an RF circuit that receives an RF signal carrying a baseband signal and applies the RF signal to the electrodes on the optical resonator at a frequency equal to a free spectral range of the optical resonator; a first optical detector coupled to detect modulated light coupled out of the optical resonator to produce a baseband signal of the input RF signal; a second optical detector coupled to detect modulated light coupled out of the optical resonator to produce a feedback signal; and an electrical feedback that applies the feedback signal to the electrodes to perform optical modulation in the optical resonator. 
     In another example, an RF photonic device includes a laser that is tunable in response to a control signal and produces a laser beam at a laser frequency and a first optical resonator structured to support a whispering gallery mode circulating in the optical resonator. The first optical resonator is optically coupled to the laser to receive a portion of the laser beam into the optical resonator in the whispering gallery mode and to feed laser light in the whispering gallery mode in the optical resonator back to the laser to stabilize the laser frequency at a frequency of the whispering gallery mode and to reduce a linewidth of the laser. The device includes a second optical resonator made of an electro-optic material to support a whispering gallery mode circulating in the optical resonator and the second optical resonator is optically coupled to the laser to receive a portion of the laser beam from the laser. An RF circuit is provided and receives an RF signal carrying a baseband signal and modulates the second optical resonator at a frequency equal to a free spectral range of the second optical resonator. A slow optical detector coupled to detect modulated light coupled out of the second optical resonator to produce a baseband signal of the input RF signal. 
     These and other examples and implementations are described in detail in the drawings, the detailed description, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of an RF receiver based on a laser stabilized by a WGM resonator and an electro-optic WGM resonator modulator driven by the stabilized laser. 
         FIGS. 1A and 1B  show an example of an electro-optic WGM resonator used for optical modulation in  FIG. 1 . 
         FIG. 2  shows an example of an RF receiver based on optical injection locking of a laser to an electro-optic WGM resonator that operates to both stabilize the laser via injection locking and to provide optical modulation via its electro-optic effect in response to a received RF signal. 
         FIGS. 3 and 4  show two exemplary implementations of an RF receiver based on the receiver design in  FIG. 2  where an optical detector is coupled to the WGM resonator and a feedback loop to the WGM resonator is provided to construct an opto-electronic oscillator. 
         FIGS. 5 ,  6  and  7  show operations of an RF receiver based on the design in  FIGS. 1-4 . 
         FIG. 8  shows an example of a multi-channel RF receiver formed by two or more RF receivers shown in  FIGS. 1-4  that share a common feedback loop for the opto-electronic oscillation in each WGM resonator. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of an RF receiver based on a laser  1100  stabilized by a WGM resonator  1400 . A diode laser  1100  is optically coupled to a resonator  1400  on the right hand-side based on optical injection locking. The laser output is directed via a GRIN lens coupler  1210  and an optical WGM evanescent coupler  1224  to direct laser light into the WGM resonator  1400 . The feedback light of the resonator  1400  is injected back to the laser  1100  to stabilize the laser  1100  so that the laser wavelength is locked at the wavelength of the WGM mode in the resonator  1400  and to reduce the linewidth of the laser  1100 . One way to achieve this injection locking is described in U.S. patent application Ser. No. 12/139,449 entitled “TUNABLE LASERS LOCKED TO WHISPERING GALLERY MODE RESONATORS” and filed on Jun. 13, 2008, which is incorporated by reference as part of the specification of this application. 
     In  FIG. 1 , the main components for the receiver are on the left-hand side of the laser  1100 . A high sensitivity lithium niobate resonance WGM light modulator is provided to receive the stabilized laser light from the laser  1100  and to modulate the received light based on the received RF signal  1500  via an RF port  1126  (e.g., at 35 GHz). The modulator includes an electro-optical WGM resonator  1300  made of an electro-optic material and has electrodes  1310  formed thereon to apply a control voltage to change the index of the resonator to cause optical modulation to light confined in one or more WG modes. The RF port  1126  is electrically coupled to the electrodes  1310  on the resonator  1300  to apply the received RF signal  1500  to the resonator  1300  to modulate light inside the resonator  1300 . An optical evanescent coupler  1124 , such as an optical prism, is provided to provide optical coupling to and from the WGM resonator  1300 . The laser light from the laser  1100  is injected via evanescent coupling into the resonator  1300  and to retrieve light inside the resonator  1300  from the resonator  1300  as output light. This output light can be coupled into a photodetector  1700 , which can be a detector of a sufficient response speed to detect the baseband RF signal modulated onto the light by the modulator  1300  in response to the received RF signal  1500  at the RF port  1126 . As an example, the detector  1700  can be a 5-MHz photodiode that detects video signals. 
     Therefore, the RF receiver in  FIG. 1  receives the RF signal  1500  carrying a baseband signal at the input RF port  1126  and outputs the baseband signal at the photodetector  1700 . The down-conversion operation is carried out in the optical domain by the optical modulator  1300 . As such, the RF receiver is a photonic-based receiver with an optical core or engine. 
       FIGS. 1A and 1B  show an example of a tunable electro-optic WGM resonator  1000  suitable for use for the modulator with the resonator  1300  in  FIG. 1 . The electro-optic material for the resonator  1000  may be any suitable material, including an electro-optic crystal such as Lithium Niobate and semiconductor multiple quantum well structures. One or more electrodes  1011  and  1012  (as the electrodes  1310  in  FIG. 1 ) may be formed on the resonator  1000  to apply the control electrical field in the region where the WG modes are present to control the index of the electro-optical material and to change the filter function of the resonator. Assuming the resonator  1000  has disk or ring geometry, the electrode  1011  may be formed on the top of the resonator and the electrode  1012  may be formed on the bottom of the resonator as illustrated in the side view of the device in  FIG. 1B . In one implementation, the electrodes  1011  and  1012  may constitute an RF or microwave resonator to apply the RF or microwave signal to co-propagate along with the desired optical WG mode. The electrodes  1011  and  1012  may be microstrip line electrodes. A varying DC voltage can be applied to tune the WGM frequency and an RF or microwave signal, which includes the RF signal  1500 , can be applied to modulate the WGM frequency. 
     The laser locking part of the RF receiver in  FIG. 1  can include an optical detector  1410  that receives output light from the coupler  1224  to monitor the laser locking condition. A second optical detector  1420  can be coupled to the resonator  1400  to detect light in the resonator  1400  to produce an output signal  1421  as an RF output for the RF receiver in  FIG. 1 . The laser  1100  has an electrical input  1101  to receive an RF signal  1102  for opto-electronic oscillation operation. 
       FIG. 2  shows another RF receiver which has only the electro-optic WDM resonator  1300  without the second WGM resonator  1400  for locking the laser  1100 . The resonator  1300  performs dual functions: an optical modulator for modulating the light in response to the received RF signal  1500  and an optical injection locking frequency reference to provide a narrow frequency reference to lock the laser  1100 . This design is to simplify the implementation of the receiver in which the standalone narrow-linewidth laser  1100  is electronically locked to a lithium niobate resonator mode of the resonator  1300 . The injection locking is achieved by optical feedback produced by the LN resonator  1300  itself. In presence of significant intracavity backscattering, the feedback can be achieved automatically by optical coupling methods between the laser  1100  and the resonator  1300 , such as prism coupling, during which light is inserted into a traveling WG mode inside the resonator  1300 , and is reflected in the cavity mode itself into the laser  1100 , forcing the laser to lase at the frequency of the WG mode for the injection locking In absence of significant intracavity backscattering, in a first embodiment, a diffractive coupler can be used to excite a standing-wave WG mode in the lithium niobate resonator  1300  directly. Because this coupling is reciprocal, the laser will receive optical feedback from the resonator automatically. 
     In the second embodiment, a partial mirror is placed after the traveling-wave coupler to WG mode, and partial standing wave is created between laser  1100  and this mirror. This standing wave will produce coupling to the corresponding standing-wave WG mode in the resonator  1300 , and will provide high Q optical feedback from the WG mode into the laser  1100  for injection locking and linewidth narrowing. As a result, a simple and inexpensive optical scheme of quadratic photonic receiver can be realized. 
     In operation, the RF frequency is equal to the free spectral range of the optical resonator  1300 . The optical detector  1700  is used at the output of the optical resonator  1300  to detect the baseband signal carried by the RF signal  1500 . Hence, the RF signal at the input of the device is now converted to a baseband signal. The electro-optic WGM resonator  1300  is used to provide both injection locking and the signal modulation. 
       FIG. 3  shows one implementation of an RF receiver with a single WGM resonator for modulation and laser injection locking. A near-field coupled high speed photodiode  3100  is evanescently coupled the resonator  1300  to detect light and to produce a detector signal to a feedback control circuit  3300  which conditions the signal, e.g., controlling the phase or delay of the signal and filtering the signal to select a particular frequency in the feedback loop. An amplifier  3310  is connected downstream from the circuit  330  to amplify the signal as a feedback signal to a signal combiner  3320 . The signal combiner  3320  is coupled to an antenna or receiver circuit  3400  that receives the RF signal  1500  and combines the signal from the amplifier  3310  and the RF signal  1500  into a control signal. This control signal is fed into the electrodes  1310  on the resonator  1300  to modulate the light inside the modulator  1300 . This design forms an opto-electronic loop with an optical portion that includes the optical resonator  1300  as an optical delay element and an optical modulator, and an electrical portion which includes the photodiode  3100 , the circuit  3300 , the amplifier  3310 , the signal combiner  3320  and the electrodes  1300 . This is a closed loop and can be operated to have a loop gain higher than the loop loss and the feedback to the resonator  1300  can be in phase. Under such conditions, the closed loop is a positive feedback loop and will oscillate as an opto-electronic oscillator (OEO) at a frequency at which the light in the resonator  1300  is modulated. In this OEO, the laser light from the laser  1100  is also modulated due to the feedback light from the resonator  1300 . The resonator  1300  provide the optical delay in the loop to reduce the phase noise of the loop that may be difficult to achieve with a conventional RF voltage-controlled oscillator. As indicated, an RF output can be generated in the electrical portion of the opto-electronic loop, e.g., at the signal combiner  3320 . A second detector  3200  is used to provide low frequency detection for monitoring the injection locking operation. 
       FIG. 4  shows a variation of the receiver in  FIG. 3  where an optical coupler  4100  is provided to receive output light from the coupler  1124  that provides optical coupling between the laser  1100  and the resonator  1300 . The detector  3100  for the OEO is used to receive a portion light from the coupler  4100  and the second detector  3200  is used for monitoring the injection locking. This design needs only one evanescent coupler  1124  in comparison with the design in  FIG. 3  which needs two: one for the detector  3100  and another one ( 1124 ) for injection locking with the laser  1100 . 
       FIGS. 5 ,  6  and  7  illustrate operations of the RF receiver in the frequency domain to show optical demodulation or frequency down-conversion in detecting the baseband signal carried by the RF signal  1500 . As illustrated, the oscillation frequency of the OEO, which is the frequency at which the light is modulated in the resonator  1300 , can be selected to achieve a desired frequency down-conversion in the optical domain. As illustrated in  FIG. 7 , such a photonic RF receiver can be used to directly detect the baseband signal at the detector  1700 , thus significantly simplifying the RF circuitry. The WGM resonator  1300  can be a resonator with a high Q value to produce significant advantages for the device performance and operations. 
       FIG. 8  shows a multi-channel RF receiver system with two or more RF receivers with interconnected  0 E 0  loops. In this example, two RF receivers are linked to receive two RF signals  1501  and  1502  carrying two different baseband signals. The electrical feedback signals  8010  and  8020  are combined at the circuit  3300  to produce a single feedback signal output by the amplifier. The feedback signal is split into two signals, one for each resonator. This design provides synchronous RF local oscillators that are in phase with each other. Three or more photonic receivers can be so linked to operate in synchronization. 
     While this specification contains many specifics, these should not be construed as limitations on the scope of an invention or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the invention. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or a variation of a subcombination. 
     Only a few implementations are disclosed. However, it is understood that variations, enhancements and other implementations can be made based on what is described and illustrated in this patent application.