The figure schematically illustrates an optoelectronic security system in which binary information on the identity of a person or object is encoded in Bragg gratings in an optical fiber. This is not the first optoelectronic security system based on fiber-optic Bragg gratings, but it is the first in which such gratings constitute the basis of a key or an identification tag.
The system includes a source of broad-band light, which is fed into one of three terminals of a fiber-optic coupler. From the coupler, the broad-band light travels along an optical fiber to a central passageway in a receptacle, then travels along the passageway and emerges at the apex of a conical mouth in the receptacle. The system includes a key with a conical tip that mates with the conical mouth of the receptacle. The key comprises a carrier that holds the optical fiber that contains the Bragg gratings. When the key is inserted in the receptacle, this optical fiber is aligned with, and receives broadband light from, the passageway in the receptacle.
The Bragg gratings are in the form of periodic variations in the index of refraction along the core of the optical fiber in the key. The Bragg gratings are located at intervals along this fiber, interspersed with grating-free regions. Each grating has a unique spatial period, so that it reflects light maximally at a unique wavelength. As a result, when illuminated with broad-band light, the optical fiber in the key reflects light maximally at the wavelengths of the Bragg gratings; that is, the reflection spectrum is a series of spectral lines at these wavelengths. The presence or absence of a reflection-spectrum line at a given wavelength can be made to signify a one or a zero, respectively, in a binary identification code. Thus, the Bragg gratings can be fabricated to encode identity information.
The light reflected by the optical fiber in the key travels back along the route of the broad-band illumination to the fiber-optic coupler, then from the fiber-optic coupler to a decoder. The decoder contains a spectrometer, in which a diffraction grating disperses the light by wavelength onto a linear array of photodetectors. From the photodetector outputs as a function of position along the array, the array circuitry determines the presence or absence of each spectral line of interest, and thus determines the corresponding binary digit. A control circuit compares the binary number with a preselected binary number or set of numbers; if a match is found, then the control circuit generates an electronic command to open a lock. Alternatively, the key can serve as an identification tag for a person or object, in which case the control circuit generates a command that activates an authentication display device.
Among the advantages of this system are the ruggedness and compactness of the key or tag; typically, the carrier is made of metal and has a diameter of about 2 mm. The carrier portion of the key or tag need not be a cone-tipped cylinder as in the figure; alternatively, it could be a pen, a piece of jewelry, or any other convenient small object. In comparison with magnetic-card keys used in some electronic security systems, the key or tag is more easily carried and concealed, and less likely to be misplaced. Because of the smallness of the optical fiber, the key or tag cannot be duplicated easily; this feature enhances the degree of security. Finally, in common with other electronic security systems, this system makes it possible to tailor the system to accommodate multiple users, each of whom may have authority to open the same or a different set of locks in an installation.
This work was done by Charles K. Gary and Meric Ozcan of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
This invention has been patented by NASA (U.S. Patent No. 5,633,975). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Ames Research Center; (650) 604-5104.
Refer to ARC-12092.