A proposed highly accurate clock or oscillator would be based on the concept of an opto-electronic oscillator (OEO) stabilized to an atomic transition. Opto-electronic oscillators, which have been described in a number of prior NASA Tech Briefs articles, generate signals at frequencies in the gigahertz range characterized by high spectral purity but not by longterm stability or accuracy. On the other hand, the signals generated by previously developed atomic clocks are characterized by long-term stability and accuracy but not by spectral purity. The proposed atomic clock would provide high spectral purity plus long-term stability and accuracy — a combination of characteristics needed to realize advanced developments in communications and navigation. In addition, it should be possible to miniaturize the proposed atomic clock.
When a laser beam is modulated by a microwave signal and applied to a photodetector, the electrical output of the photodetector includes a component at the microwave frequency. In atomic clocks of a type known as Raman clocks or coherent-population-trapping (CPT) clocks, microwave outputs are obtained from laser beams modulated, in each case, to create two sidebands that differ in frequency by the amount of a hyperfine transition in the ground state of atoms of an element in vapor form in a cell. The combination of these sidebands produces a transparency in the population of a higher electronic level that can be reached from either of the two ground-state hyperfine levels by absorption of a photon. The beam is transmitted through the vapor to a photodetector. The components of light scattered or transmitted by the atoms in the two hyperfine levels mix in the photodetector and thereby give rise to a signal at the hyperfine-transition frequency.
The proposed atomic clock would include an OEO and a rubidium- or cesium- vapor cell operating in the CPT/Raman regime (see figure). In the OEO portion of this atomic clock, as in a typical prior OEO, a laser beam would pass through an electro-optical modulator, the modulated beam would be fed into a fiber-optic delay line, and the delayed beam would be fed to a photodetector. The electrical output of the photodetector would be detected, amplified, filtered, and fed back to the microwave input port of the modulator.
The laser would be chosen to have the same wavelength as that of the pertinent ground-state/higher-state transition of the atoms in the vapor. The modulator/ filter combination would be designed to operate at the microwave frequency of the hyperfine transition. Part of the laser beam would be tapped from the fiber-optic loop of the OEO and introduced into the vapor cell. After passing through the cell, this portion of the beam would be detected differentially with a tapped portion of the fiber-optically-delayed beam. The electrical output of the photodetector would be amplified and filtered in a loop that would control a DC bias applied to the modulator. In this manner, the long-term stability and accuracy of the atomic transition would be transferred to the OEO.
This work was done by Lute Maleki and Nan Yu of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
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Refer to NPO-30557, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Atomic Clock Based on Opto-Electronic Oscillator
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Overview
The document outlines a novel technology report from the California Institute of Technology's Jet Propulsion Laboratory (JPL) regarding an atomic clock based on the Opto-Electronic Oscillator (OEO). The invention, conceived by Lute Maleki and Nan Yu, aims to enhance precision in communication and navigation, particularly for military applications and GPS devices.
The report details the historical development of the invention, including its conception on March 20, 2001, and subsequent disclosures and sketches leading to a written description by January 12, 2002. Although the device is currently in the conceptual stage, various elements of its design have been demonstrated, but laboratory prototyping has not yet been conducted.
The intended applications for this atomic clock are significant, particularly in anti-jam communication for military use and precision location in GPS technology. The report identifies potential competitors, such as Agilent and Datum, who are developing atomic clocks based on different technologies, which are described as more cumbersome. The document emphasizes that small atomic clocks based on this new technology do not currently exist, highlighting the invention's potential importance for the Department of Defense (DoD), NASA, and the National Reconnaissance Office (NRO).
The report also discusses the commercialization factors surrounding the invention. It states that the device is not yet ready for commercialization, as further development and prototyping are necessary. The technology is not fixed in its final form, indicating that additional work is required before it can be brought to market.
In terms of patent rights, the report notes that a written description has not yet been submitted for publication, but a proposal based on the invention is being made to DARPA. The document also mentions that no disclosures have been made without a non-disclosure agreement, ensuring that the invention's details remain protected during its development phase.
Overall, the report presents a promising advancement in atomic clock technology, with the potential to significantly impact military and civilian applications, while also outlining the current developmental status and future commercialization prospects of the invention.
