Miniature frequency standard based on all-optical excitation and a micro-machined containment vessel

• US 6,806,784 B2
• Filed: 06/20/2002
• Issued: 10/19/2004
• Est. Priority Date: 07/09/2001
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Commerce, The Board of Regents of the University of Colorado (University of Colorado System)
• Original Assignee: National Institute of Standards and Technology (Government of the United States of America), THE GOVERNMENT OF THE UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY OF COMMERCE
• Inventors: Kitching, John, Hollberg, Leo
• Primary Examiner(s): Wells, Kenneth B.
• Application Number:
  US10/175,498
• Publication Number:
  US 20020163394A1
• Time in Patent Office: 852 Days
• Field of Search: 331/94.1, 331/3, 331/176
• US Class Current: 331/94.1
• CPC Class Codes: G04F 5/14 using atomic clocks
• CPC Class Codes: G04F 5/145 using Coherent Population Trapping
• CPC Class Codes: H01S 3/1304 by using an active reference, e.g. second laser, klystron or other standard frequency source H01S3/139 takes precedence; automatic control of electronic generators H03L7/00
• CPC Class Codes: H01S 3/305 in a gas
• CPC Class Codes: H01S 5/0687 Stabilising the frequency of the laser
• CPC Class Codes: H01S 5/183 having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
• CPC Class Codes: H03L 7/26 using energy levels of molecules, atoms, or subatomic particles as a frequency reference

Abstract

A microwave frequency standard is provided which allows for miniaturization down to length scales of order one micron, comprising a modulated light field originating from a laser that illuminates a collection of quantum absorbers contained in a micro-machined cell. The frequency standard of the present invention can be based on all-optical excitation techniques such as coherent population trapping (CPT) and stimulated Raman scattering or on conventional microwave-excited designs. In a CPT-based embodiment, a photodetector detects a change in transmitted power through the cell and that is used to stabilize an external oscillator to correspond to the absorber'"'"'s transition frequency by locking the laser modulation frequency to the transition frequency. In a stimulated Raman scattering (SRS) embodiment, a high-speed photodetector detects a laser field transmitted through the cell beating with a second field originating in the cell. Both the locked laser modulation frequency and the beat frequency are very stable as they are referenced directly to the atomic transition.

Claims

1. A micro-machined cell for an atomic frequency standard comprising:

a container having an interior, walls and a window to allow light to pass therethrough, said container operable to hold a vapor;

a first vapor contained in said container, said first vapor comprising a plurality of quantum absorbers, said absorbers having a transition frequency; and

means for reducing the destruction of the coherence of the quantum absorbers due to collisions of the quantum absorbers with the cell walls.

2. A micro-machined cell according to claim 1, wherein the means for reducing the destruction of the coherence of the quantum absorbers is a coating applied to the walls of the container.

3. A micro-machined cell according to claim 1, wherein the means for reducing the destruction of the coherence of the quantum absorbers is a second vapor of atoms enclosed inside the container.

4. A micro-machined cell according to claim 1, wherein the size of the cell is 1000 μ

m³.

5. A micro-machined cell according to claim 1, wherein said cell is fabricated by bonding using a combination of elevated temperature and applied electric field of a plurality of cell materials in order to for said cell to enclose a small volume of space in an air-tight manner;

wherein said plurality of quantum absorbers is enclosed in said small volume.

6. A micro-machined cell according to claim 1 wherein said cell is fabricated by compression bonding at an elevated temperature of a plurality of cell materials each separated by a thin layer of a metallic material.

7. An atomic frequency standard comprising:

a micro-machined cell according to claim 1;

a laser operable to generate light and positioned such that said light passes through said vapor; and

a photodetector for detecting said light having passed through said vapor to provide a signal that is referenced to a characteristic of said detected light.

8. An atomic frequency standard according to claim 7 further comprising a system to apply an oscillating magnetic field to the quantum absorbers, wherein the photodetector monitors said detected light to provide adjustments to the frequency of said oscillating magnetic field, detects a frequency of modulation that coincides with one of said quantum absorbers, a harmonic of said quantum absorber frequency or a subharmonic of said quantum absorber frequency by detecting a change in the detected light and outputs the detected frequency as a reference.

9. An atomic frequency standard according to claim 7, further comprising an external oscillator to modulate an injection current of the laser to produce optical sidebands for coherent population trapping, wherein the photodetector monitors said detected light to provide adjustments to the frequency of modulation of the external oscillator, detects a frequency of modulation that coincides exactly with the quantum absorbers, a harmonic of the quantum absorber frequency, or a subharmonic of the quantum absorber frequency, by detecting a change in the detected light and outputs the detected frequency as a reference.

10. An atomic frequency standard according to claim 7, further comprising:

a single-frequency first optical field created by said laser;

a second optical field, created through the process of Raman scattering, separated from the first field by a frequency equal to the transition in the quantum absorbers,

wherein said photodetector detects a beatnote having a frequency equal to the optical difference frequency between the first and second fields and outputs said beatnote frequency as a reference.

13. The atomic frequency reference of claim 7, wherein said vapor comprises an alkali atom.

14. The atomic frequency reference of claim 13 wherein said alkali atom is one of cesium, rubidium, and potassium.

15. A LAN comprising the atomic frequency reference of claim 7.

16. An instrument calibration system comprising the atomic frequency reference of claim 7.

17. An identification system comprising the atomic frequency reference of claim 7.

18. An anti-jamming apparatus comprising the atomic frequency reference of claim 7.

19. A magnetometer comprising:

a micro-machined cell according to claim 1;

a laser operable to generate light and positioned such that said light passes through said vapor; and

a photodetector for detecting said light having passed through said vapor to provide a signal that is referenced to a characteristic of said light.

20. A computational device based on a network of micro-machined cells according to claim 1 wherein said cells are used to store information by encoding at least one coherence property of the quantum absorber.

11. A method for a micro-machined cell of an atomic frequency standard, comprising the steps of:

micro-machining a container as a quantum resonator to have walls and a window to allow light to pass therethrough, said container operable to hold a vapor,

enclosing in said cell a vapor comprising a plurality of quantum absorbers having a transition frequency; and

reducing the destruction of the coherence of the quantum absorbers due to collisions of the quantum absorbers with the container walls.

12. A method of fabricating a micro-machined cell according to claim 11 further comprising the steps of bonding silicon to glass using a combination of elevated temperature and applied electric field in order for said container to enclose a small volume in an air-tight manner.