Abstract:
Systems and methods for generating RF pulses that have a reduced phase error are disclosed. The systems are optical based and thus are highly linear, so that phase errors, including jitter, are significantly reduced as compared to electrical RF pulse generation systems and methods. The optical-based RF pulse generation methods includes generating laser light, imparting an envelope modulation to the laser light, imparting a carrier modulation to the laser light, and detecting the envelope-modulated and carrier-modulated light to form the electrical RF pulse. The electrical RF pulse can then be carried by a cable to an external device.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 61/065,996 filed on Feb. 15, 2008, which application is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to radio-frequency (RF) signals, and in particular to the generation of RF signals in the form of pulses that have small phase error. 
       BACKGROUND ART 
       [0003]    Pulsed RF signals are used in a variety of applications from distance measurement systems to magnetic resonant imaging (MRI) systems to quantum computers. For many applications, the RF pulses can have relatively large phase errors (e.g., up to 10°) and still achieve their purpose. However, for certain applications such as quantum computing and MRI systems, the phase errors need to be as small as possible, in some cases as low as 1° or even 0.1°. 
         [0004]    Pulsed RF signals are typically generated by using two orthogonal baseband signals. One signal is an in-phase signal (the “I signal”), and the other signal is a quadrature-phase signal (the “Q signal”). The I and Q baseband signals are converted to a bandpass signal by a device called an “IQ mixer.” The IQ mixer modulates the I signal with an in-phase carrier, and modules the Q signal with a quadrature-phase carrier. The modulated signals are summed together to form a bandpass signal in the form of an RF pulse. 
         [0005]    One of the problems with IQ mixers is that they tend to generate phase errors in the modulation process. These phase errors arise due to a number of factors, including nonlinearities in the electronic components, and path length mismatches that arise when the IQ mixer is fabricated. In the case where the phase errors are random or quasi-random, they represent a source of jitter in the RF pulse. 
         [0006]    While for many applications phase errors can be considered negligible, for other applications the phase errors are unacceptable. And while the phase errors can be reduced by electronic means, some applications such as quantum computing require a degree of phase precision that is not readily obtainable through electronic compensation schemes. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention relates generally to radio-frequency (RF) signals, and in particular to the generation of RF signals in the form of pulses that have very low phase errors, including random and quasi-random phase errors that cause jitter. 
         [0008]    One aspect of the invention is a RF pulse generator system. The system includes a laser that emits laser light, and a first optical modulator arranged to receive the laser light. The first modulator is configured to impart to the received laser light one of an envelope modulation and a carrier modulation. The system also includes a second optical modulator arranged downstream of the first optical modulator so as to receive the laser light from the first modulator. The second modulator is configured to impart to the laser light therefrom the other of the carrier modulation and the envelope modulation imparted by the first modulator so as to form an optical RF pulse. The system also includes a photodetector arranged to receive and detect the optical RF pulse from the second modulator and form a corresponding RF electrical pulse. 
         [0009]    Another aspect of the invention is a RF pulse generator system that includes a laser that emits laser light. The system also includes an optical modulator arranged to receive the laser light and configured to impart to the received laser light one of an envelope modulation and a carrier modulation. The system also includes an envelope-modulation generator operably connected to one of the laser and the optical modulator and configured to generate an envelope-modulation signal that drives one of the laser and the optical modulator with the envelope modulation. The system further includes a carrier-modulation generator operably connected to the other of the laser and the optical modulator and configured to generate a carrier modulation signal that drives with the carrier modulation the other of the laser and the optical modulator driven by the envelope-modulation generator so that the laser light exiting the optical modulator includes an envelope modulation and a carrier modulation that forms an optical RF pulse. The system also includes a photodetector arranged to receive the optical RF pulse and form therefrom an electrical RF pulse. 
         [0010]    Another aspect of the invention is a method of generating an electrical RF pulse. The method includes generating laser light with a laser, and imparting an envelope modulation to the laser light. The method also includes imparting a carrier modulation to the laser light. The method further includes detecting the envelope-modulated and carrier-modulated light to form the electrical RF pulse. The method can also include modulating the laser directly with an envelope-modulation signal or a carrier-modulation signal so that the light exiting the laser is already modulated. The remaining modulation is then applied to the laser light to form the optical RF pulse. The extinction of the optical RF pulse and thus the subsequent electrical RF pulse can be enhanced by applying the envelope modulation to the laser and then again after an optical modulator imparts the carrier modulation. 
         [0011]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
         [0012]    It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  is a schematic diagram of a first example embodiment of the RF pulse generator system that uses first and second optical modulators for imparting carrier modulation and envelope modulation; 
           [0014]      FIG. 2  is a schematic diagram of a second example embodiment of the RF pulse generator system that uses a single optical modulator for imparting the carrier modulation while directly envelope-modulating the laser; 
           [0015]      FIG. 3  is a schematic diagram of a third example embodiment of the RF pulse generator system that uses a single optical modulator for imparting the envelope modulation while the laser is directly carrier-modulated; and 
           [0016]      FIG. 4  is a schematic diagram of a third example embodiment of the RF pulse generator system that uses respective optical modulators for imparting the carrier modulation and the envelope modulation, while the laser is also directly envelope-modulated. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIG. 1  is a schematic diagram of a first example embodiment of an RF pulse generator system  10 . System  10  includes a laser  14 , such as a semiconductor laser or a diode-pumped Nd:YAG laser. Laser  14  is arranged along an optical axis Al and is optically coupled to a first optical modulator  20 , which in turn is optical coupled to a second modulator  20  arranged in order along optical axis Al. In an example embodiment, modulators  20  are or otherwise include lithium-niobate modulators or like optical modulators. 
         [0018]    A photodetector  30  is arranged along optical axis Al downstream of the second optical modulator and is optically coupled thereto. An electrical cable  40 , such a coaxial cable, is electrically connected to photodetector  30 . 
         [0019]    System  10  further includes a controller  50  operably connected to laser  14  and the first and second modulators  20 , and optionally to photodetector  30 . In an example embodiment controller  50  is or includes a programmable computer with a processor  54  and includes an operating system such as Microsoft WINDOWS or LINUX. In an example embodiment, controller  50  may also be or otherwise include an analog circuit with feedback, or analog and digital circuits that provide a combination of analog and digital control. 
         [0020]    In an example embodiment, processor  54  is or includes any processor or device capable of executing a series of software instructions and includes, without limitation, a general- or special-purpose microprocessor, finite state machine, controller, computer, central-processing unit (CPU), field-programmable gate array (FPGA), or digital signal processor. In an example embodiment, the processor is an Intel XEON or PENTIUM processor, or an AMD TURION or other in the line of such processors made by AMD Corp., Intel Corp. or other semiconductor processor manufacturer. 
         [0021]    Controller  50  also preferably includes a memory unit (“memory”)  56  operably coupled to processor  54 . As used herein, the term “memory” refers to any processor-readable (or “computer-readable”) medium, including but not limited to RAM, ROM, EPROM, PROM, EEPROM, disk, floppy disk, hard disk, CD-ROM, DVD, or the like, on which may be stored a series of instructions (e.g., the aforementioned software) executable by processor  54 . In an example embodiment, controller  50  includes a drive or port  58  (e.g., a disk drive or USB port) adapted to accommodate a removable processor-readable medium  60 , such as CD-ROM, DVE, memory stick or like storage medium. 
         [0022]    The methods of the present invention may be implemented in various embodiments in a machine-readable medium (e.g., memory  56 ) comprising machine readable instructions (e.g., computer programs and/or software modules) for causing controller  50  to perform the methods and the controlling operations for operating system  10 . The computer programs and/or software modules may comprise multiple modules or objects to perform the various methods of the present invention, and control the operation and function of the various components in system  10 . The type of computer programming languages used for the code may vary between procedural code type languages to object oriented languages. The files or objects need not have a one to one correspondence to the modules or method steps described depending on the desires of the programmer. Further, the method and apparatus may comprise combinations of software, hardware and firmware. Firmware can be downloaded into processor  54  for controlling the operation of system  10  and for generally implementing the various example embodiments of the invention. 
         [0023]    Controller  50  also optionally includes a display  70  that can be used to display information using, for example, a wide variety of alphanumeric and graphical representations. 
         [0024]    Controller  50  also includes a carrier-signal generator  100  that generates an electrical RF carrier signal S C  and that is electrically connected to the first optical modulator  20 . Controller  50  also includes an envelope-modulation signal generator  110  that generates an envelope-modulation signal S ME  and that is electrically connected to the second optical modulator  20 . 
         [0025]    In operation, controller  50  generates a laser control signal S 14  that causes laser  14  to generate laser light  200  at a given wavelength λ. Laser light  200  travels along optical axis A 1  to first modulator  20 . Meanwhile, controller  50  causes carrier-signal generator  100  therein to generate carrier signal S C  with an associated carrier frequency ω and phase φ. This signal drives the first optical modulator  20  so as to modulate laser light  200  with the RF carrier signal, thereby forming once-modulated RF-modulated laser light  201  having the same carrier frequency ω and phase φ as the electrical carrier signal S C . The associated carrier wave C(ω, φ) is shown in  FIG. 1 . 
         [0026]    Meanwhile, controller  50  causes envelope-modulation signal generator  110  to generate an envelope-modulation signal S ME , which drives the second optical modulator  20 . The RF-modulated light  201  then passes through second modulator  20  and is modulated thereby based on envelop-modulation signal S ME . This signal causes the second optical modulator  20  to impart a modulation envelope A(t) to light  201 , thereby forming a twice-modulated optical RF pulse  202  having the wavepacket form as shown in  FIG. 1 . Optical RF pulse  202  is then detected by photodetector  30 , which converts the optical pulse into an electrical RF pulse S P . In an example embodiment, controller  50  gates photodetector  30  via a gating signal S 30 . Pulse S P  is then transmitted over cable  40  to an external device D, as shown. In an example embodiment, device D is configured for holding elements (e.g., one or more atoms, one or more ions, etc.) that can be place in a quantum state to form a quantum bit or “qubit.” In an example embodiment, device D is or includes an ion trap or an atom trap. 
         [0027]    Because RF pulse generator system  10  is highly linear, phase and amplitude distortions are essentially eliminated. Thus, the phase φ of RF pulse S P  is precisely imparted with very little or no substantial phase error Δφ. In an example embodiment, the phase error Δφ can be kept to below 0.1°, or even 0.01°. This includes reducing or eliminating random or quasi-random phase errors that give rise to jitter in the electrical RF pulse. Thus, RF pulse generator system  10  can be said to generate low-jitter electrical RF pulses. 
         [0028]      FIG. 2  is a schematic diagram of a second example embodiment of the RF pulse generator system  10  similar to that shown in  FIG. 1 , except that the system only uses one optical modulator  20 , which is used to impart the modulation envelope. With reference to  FIG. 2 , carrier-signal generator  100  sends carrier signals S C  to laser  14  so as to cause the laser to directly generate RF-modulated laser light  201  having the same carrier frequency ω and phase φ as the electrical carrier signal S C . In this case, the electrical carrier signal S C  also serves the same role as laser control signal S 14 , e.g., the two signals can be considered as being combined. Otherwise, system  10  of  FIG. 2  operates in the same manner as system  10  of  FIG. 1  as described above. 
         [0029]      FIG. 3  is a schematic diagram of a third example embodiment of the RF pulse generator system  10  similar to that shown in  FIG. 2 , except that the system only uses one optical modulator  20 , which is used to impart the carrier modulation. With reference to  FIG. 3 , envelop-modulation generator  110  sends envelop-modulation signals SME to laser  14  so as to cause the laser to directly generate once-modulated envelope-modulated laser light  203 . This light is then carrier-modulated by modulator  20 , as described above, to form the twice-modulated optical RF pulse  202 . In this case, electrical envelope-modulation signal S ME  also serves the same role as laser control signal S 14 , e.g., the two signals can be considered as being combined. 
         [0030]      FIG. 4  is a schematic diagram similar to that of  FIG. 1  and  FIG. 3  and illustrates a fourth example embodiment of the RF pulse generator system  10  wherein the system uses two optical modulators  20  as described in connection with  FIG. 1 , but wherein the controller also envelope-modulates laser  14  directly as described above in connection with  FIG. 3 . This embodiment applies the envelope modulation twice to the optical signal—once when the signal is generated by the laser, and then again downstream of the carrier modulation. This provides a triple-modulated optical RF pulse  202 ′, which forms an enhanced electrical RF pulse S P  having an enhanced extinction ratio (i.e., Power on/Power off). 
         [0031]    It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.