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

[0002] This application is a non-provisional application of provisional application No. 60/303,877 filed on Jul. 9, 2001, the entire contents of which is hereby incorporated by reference as if fully set forth herein.

[0003] 1. Field of the Invention

[0004] This invention relates to the field of frequency standards, and in particular, to a miniature atomic frequency standard allowing length scales down to the order of ten microns and apparatus for accomplishing the same.

[0005] 2. Background of the Invention

[0006] Frequency stability is required for modern, high-speed communications, navigation, electronic instrumentation, and similar applications. Atomic frequency references or standards are based on the energy difference between two levels of a quantum system. In an atom, for example, quantum mechanics requires that the electrons can only exist in certain states with specific, discrete energies. Differences between the energies of these states define correspondingly specific frequencies that are, to a high degree, the same for every atom, and therefore atoms make good frequency references.

[0007] A dipole moment, oscillating at one of these frequencies, can be excited by an electromagnetic wave propagating in the same space as the atom. Frequency references are available that employ an excitation scheme in which microwave fields excite the atoms of a sample. When the microwave frequency is near the atomic oscillation frequency, a change in the atomic state occurs which can be detected by measuring the absorption or phase shift of an optical field propagating simultaneously through the atomic sample. The microwave excitation technique works well but poses problems for miniaturization since the microwaves are usually confined in a cavity necessitating a size scale constrained by the microwave wavelength.

[0008] All-optical excitation techniques have been developed, in which no microwaves are applied directly to the atoms. In one embodiment, an optical field produced by a laser is modulated with an external oscillator to produce two optical fields separated by the atomic oscillation frequency, and this laser field is passed through the atomic system. When the difference frequency of the two optical fields is near the atomic oscillation frequency, a change in the absorption or phase of the field by the atoms occurs. This change in absorption or phase, due to a phenomenon called coherent population trapping (CPT), can be used to lock the external oscillator frequency to the atomic transition. This locked frequency provides the output of the frequency reference and has the long-term stability and repeatability inherent to the atoms. Because no microwaves are applied directly to the atomic system, there is no constraint on the device size due to the microwave wavelength. As a result the system can be easily miniaturized to sub-cm dimensions. In addition, the device complexity associated with the microwave cavity is avoided in the all-optical frequency standard.

[0009] In another embodiment, the use of an external oscillator is avoided by using the atoms to generate the output microwave frequency directly. In this case, an optical field is sent through the atomic system and a second optical field is generated within the atomic system through the process of Raman scattering. These two fields produce a beatnote when incident on a high-speed photodetector; this beatnote becomes the output of the frequency reference. Since the output signal is generated directly from the atoms, no external oscillator (or microwave cavity) is necessary for the device to function.

[0010] Hence, all available atomic frequency standards generate a detection signal that quantifies a resonant interaction between and incident electromagnetic radiation and a quantum absorber.

[0011] U.S. Pat. No. 5,192,921 to Chantry et al., the entire contents of which is hereby incorporated by reference as if fully set forth herein, teaches a cell-type atomic frequency standard utilizing a miniaturized gas cell combined with a microwave cavity for excitation with an overall size that is on the order of 12 cm³when cesium is used as the vapor.

[0012] U.S. Pat. No. 5,327,105 to Liberman et al., the entire contents of which is hereby incorporated by reference as if fully set forth herein, teaches an atomic resonator that combines the functions of gas cell and resonator into a single structure which features, inter alia, allow reduction of the size of an atomic frequency standard to be reduced but not below 1 cm³overall.

[0013] U.S. Pat. No. 5,670,914 to Liberman et al., the entire contents of which is hereby incorporated by reference as if fully set forth herein, teaches a miniature atomic frequency standard that is on the order of 10 cm³.

[0014] No prior art devices have been disclosed for atomic frequency references with sizes below ˜1 cm³.

[0015] The present invention provides an apparatus and a method for fabricating an atomic frequency standard with very stable output frequency with very small physical dimensions, possibly as small as 1000 μm³. The atomic frequency standard, according to the present invention, is illustrated in schematic form in FIG. 1.

[0016] The compact micro-machined cell of the present invention can be used in an atomic frequency reference based on CPT, as illustrated in FIG. 2, or in a Raman-type oscillator, as illustrated in FIG. 3 or in a conventional microwave-excited design. In addition, the compact cell can be used in magnetometers based on Zeeman shifts in atomic systems, in which a known frequency from an external oscillator is compared to the frequency of an atomic transition known to be sensitive to magnetic fields. Finally the compact cell can be used in computational or information storage systems in which a series of vapor cells are networked together using optical fields and information is stored in the coherence properties of the quantum absorbers.

[0017] The frequency reference of the present invention is compact and very stable and reproducible over the short term. This kind of frequency reference can replace high-performance quartz-crystal oscillators in applications where long-term stability is required. The extremely small size and low power dissipation of the frequency reference of the present invention make it ideal for use in portable (hand-held) or remotely operated units, which operate on batteries or another limited power source. Other applications include LAN synchronization, instrumentation and calibration, secure communications, wireless communications, navigation (global positioning system) and radar systems. Additionally, the invention can be used in anti-jamming applications and in target identification systems.

[0018] FIG. 1 illustrates an atomic frequency reference incorporating a micro-machined cell, according to the present invention.

[0019] FIG. 2 illustrates the use of the micro-machined cell of the present invention in an atomic reference standard based on the phenomenon of coherent population trapping.

[0020] FIG. 3 illustrates the use of the micro-machined cell of the present invention in an atomic reference employing a Raman-type oscillator.

[0021] The present invention provides an apparatus and a method for fabricating an atomic frequency standard with very stable output frequency in a very small package that can be as small as 1000 μm³. The atomic frequency standard of the present invention is illustrated in outline in FIG. 1. Light from a semiconductor laser 10 is sent through a micro-machined cell 11 which encapsulates at least one quantum absorber 12. A photodetector 13, placed above the cell 11, collects the light transmitted through the vapor.

[0022] In a preferred embodiment, the encapsulation process is carried out using anodic bonding, in which a planar piece of silicon is bonded to a planar piece of glass using a combination of elevated temperature and an applied electric field. The bond is carried out in a controlled gaseous environment which enables an alkali atom vapor to be enclosed inside the cell.

[0023] In a preferred embodiment, the encapsulation process is carried out using compression bonding, in which a thin layer of metallic material is placed between two other materials, enclosing a small volume of space. The bond is activated by elevating the temperature of the interface.

[0024] In a preferred embodiment, the interior of the encapsulated cell is combined with a method that reduces the destruction of the quantum absorber coherence due to collisions with the walls of the cell. Two such methods are known from prior art: the use of a coating on the walls of a cell and the use of a buffer gas of inert gas atoms enclosed inside the cell with the quantum absorbers. In one preferred embodiment, the present invention is used in a frequency reference based on the CPT phenomenon, as illustrated in FIG. 2 wherein the quantum absorber is cesium or rubidium vapor, but not restricted to these atomic species. In this embodiment, an external oscillator 20 is used to modulate the injection current of the laser 10, producing optical sidebands used for CPT. The optical transmission through the container of quantum absorbers 11 is monitored and changes when the frequency of the external oscillator 20 is near that of the quantum absorbers. This change in transmission can therefore be used to stabilize the external oscillator frequency. The fractional frequency instability is estimated to be on the order of 10⁻⁸for an integration time of one second and a cell volume of 1000 μm³.

[0025] In another preferred embodiment the micro-machined cell of the present invention is used in a Raman-type oscillator, as illustrated in FIG. 3 wherein the quantum absorber is cesium vapor. Here there is no external frequency reference and a single-frequency optical field created by the laser 10 is sent through the cell 11. Through the process of stimulated Raman scattering, a second optical field is created, separated from the first by a frequency equal to the microwave transition in the quantum absorber 12. When these two optical fields are incident together on the photodetector, a beatnote is detected, with a frequency equal to the optical difference frequency. Since this beatnote frequency is derived from the quantum transition in the absorber, it can retain the intrinsic stability of the quantum system.

[0026] In another preferred embodiment, the micro-machined cell is used in a conventional microwave-excited design, in which an oscillating magnetic field is applied to the atoms using a microwave cavity or current loop. The atoms are optically pumped using an optical field from a laser or small lamp and the effect of the excitation is detected by measuring the change in transmission of the optical field through the cell.

[0027] One familiar with the art will realize that various fabrication methods are possible for the encapsulated micro-machined cell, depending on the particular application, and other all-optical excitation techniques can be employed, without departing from the essential scope or spirit of the invention as claimed. Accordingly, the examples of embodiments are presented, not in any limiting sense, but for purposes of illustration only.