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.
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°.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Controller 50 also includes a carrier-signal generator 100 that generates an electrical RF carrier signal SC 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 SME and that is electrically connected to the second optical modulator 20.
In operation, controller 50 generates a laser control signal S14 that causes laser 14 to generate laser light 200 at a given wavelength λ. Laser light 200 travels along optical axis A1 to first modulator 20. Meanwhile, controller 50 causes carrier-signal generator 100 therein to generate carrier signal SC 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 SC. The associated carrier wave C(ω, φ) is shown in
Meanwhile, controller 50 causes envelope-modulation signal generator 110 to generate an envelope-modulation signal SME, 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 SME. 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
Because RF pulse generator system 10 is highly linear, phase and amplitude distortions are essentially eliminated. Thus, the phase φ of RF pulse SP 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.
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.
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.
Number | Date | Country | |
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61065996 | Feb 2008 | US |