Static information storage and retrieval – Radiant energy – Color centers
Reexamination Certificate
2001-09-24
2004-01-20
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Radiant energy
Color centers
C365S106000, C365S215000, C365S216000, C365S234000
Reexamination Certificate
active
06680860
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to an electromagnetic coherent transient device and, more particularly, to a continuous processor that can simultaneously, asynchronously, and continuously process an input signal while it is continuously being programmed. The invention has application in, but is not limited to, the areas of memory, correlations/convolutions, true-time delay, pulse shaping, distortion compensation and spatial routing of optical signals.
2. Description of the Related Art
The present invention utilizes several unique properties of condensed phase spectral hole burning materials as versatile coherent transient processors. Absorption features of ions or molecules doped into condensed phase materials are spectrally broadened by two main classes of mechanisms. Homogeneous broadening is the fundamental broadening experienced by all ions or molecules independently, and arises from the quantum-mechanical relationship between the lineshape and the dephasing time of the excited ion. At cryogenic temperatures, such homogeneous linewidths have been measured, using the photon-echo technique, to be as narrow as 100 Hz or less, orders of magnitude sharper than most gas phase transitions (R. W. Equall, Y. Sun, R. L. Cone, R. M. Macfarlane, Ultraslow Optical Dephasing in Eu3+:Y2SiO5
, Phys. Rev. Lett
. 72, 2179 (1994)). Inhomogeneous broadening, such as depicted in
FIG. 1A
, arises from the overlap of the quasi-continuum of individual spectra of all of the ions or molecules in the condensed phase material, which have microscopically different environments and therefore slightly different transition frequencies. The extent of this envelope can be anywhere from hundreds of megaHertz (MHz) to the teraHertz (THz) range.
Spectral hole burning (W. E. Moemer, ed., Persistent Spectral Hole Burning: Science and Applications, (Springer-Verlag, Berlin 1988); R. M. Macfarlane and R. M. Shelby, “Coherent Transient and Spectral Holebuming Spectroscopy of Rare Earth Ions in Solids,” in
Spectroscopy of Solids Containing Rare Earth Ions
, A. A. Kaplyanskii and R. M. Macfarlane, eds. (North Holland, Amsterdam 1987)) uses a narrow-band laser to selectively excite only the small fraction of ions or molecules whose frequencies coincide with that of the laser. If some mechanism exists to remove those ions from the absorbing population, or to change their resonant frequencies, then the inhomogeneous absorption profile can be temporarily or permanently altered, leaving a “spectral hole” or ion population shift at the frequency of the laser. In the cases of interest here, the homogeneous linewidth is many orders of magnitude smaller than the inhomogeneous linewidth. A very flexible advantage of hole burning, in contrast to isolated atomic transitions, is that the center frequency for the hole may be chosen anywhere within the wide inhomogeneous band of absorbing frequencies. Furthermore, multiple holes or ion population variations may be placed within one inhomogeneously broadened absorption band to generate an absorption spectrum. In addition, by varying the intensity of the burning spatially, as with an interference pattern among two or more laser beams, a spatial-spectral grating can be produced.
Mechanisms exist to provide permanent change to preserve the hole; the most common being (a) excitation-induced changes in the lattice near the optically active ion or molecule, (b) photoionization of that ion or molecule itself, and (c) photochemical reactions. Possibilities exist to use photon-gated processes where a second, possibly broadband, light source is required to make the hole persistent.
An electromagnetic wave coherent transient device is one with a broadband spectral grating that extends over several homogeneous profiles, and part or all of the available inhomogeneous broadening absorption profile. An optical coherent transient (OCT) device is one with a broadband spectral grating in the optical range that extends over several homogeneous profiles, and part or all of the available inhomogeneous broadening absorption profile. All the components of an optical spectral grating are typically programmed simultaneously by recording the spectral-spatial interference of two or more optical pulses separated in time and/or space. This spectral grating has the ability to generate an optical output signal that depends on an optical input waveform (referred to as a processing waveform) impinging on that grating, now referred to as a device. Optical coherent transient (OCT) devices have been disclosed such as an optical memory (for example, T. W. Mossberg, “Time-Domain Frequency-Selective Optical Data Storage,”
Opt. Lett
. 7, 77 (1982)), a swept carrier optical memory (for example, T. W. Mossberg, “Swept-Carrier Time-Domain Optical Memory,”
Opt. Lett
. 17, 535 (1992)), an optical signal cross-correlator (for example, W. R. Babbitt and J. A. Bell, “Coherent Transient Continuous Optical Processor,”
Appl. Opt
. 33, 1538 (1994)), an optical true-time delay regenerator (see for example, K. D. Merkel and W. R. Babbitt, “Optical Coherent Transient True-time Delay Regenerator,”
Opt. Lett
. 21, 1102 (1996)) and optical spatial router (for example, W. R. Babbit and T. W. Mossberg, “Spatial Routing of Optical Beams Through Time-domain Spatial-spectral Filtering,”
Opt. Lett
. 20, 910 (1995)), among others. While each device has different aspects in its programming and processing stages, all are implementations of a generalized OCT processor. The term processor here indicates the most generalized conceptualization of such a device.
OCT devices can only process data as long as the programmed spatial-spectral grating survives. When the programming stage is a single shot event, writing a strong spectral grating in a non-persistent hole-burning material, the processing stage can only occur while the excited absorbers have not decayed back to their ground states. After the grating decays away fully, the programming stage can be repeated, but this leads to dead time for the processor, which is several times the excited state lifetime T
1
. An alternative implementation is to utilize persistent spectral holes. But for single photon persistent holes (e.g., hyperfine storage), the processing stage is partially destructive to the stored gratings, (for example, M. Zhu, W. R. Babbitt and C. M. Jefferson, “Continuous Coherent Transient Optical Processing in a Solid,”
Opt. Lett
. 20, 2514 (1995)). For two-photon persistent holes, i.e., gated storage, (for example, W. E. Moemer, Editor.
Persistent Spectral Holeburning: Science and Applications
, Topics in Current Physics, Vol. 44, Springer-Verlag, 1988), the processing stage is non-destructive and can be continuous. However, low writing and gating efficiencies of available materials make this currently impractical. Each of the above techniques requires strong programming pulses and weak processing pulses, thereby putting stringent specifications on the laser source and optical modulators. These constraints could be lessened if the grating were accumulated by repeating the programming process. However, the temporal distinction between the programming and processing stages still remains for all the above techniques, along with the constraints on efficiency, materials and devices.
Previously, a continuous OCT processor has been proposed and demonstrated whereby the programming stage and the processing stage are two temporally distinct steps. Furthermore, previous continuous OCT processors required two spatially distinct programming beams to write a spatial-spectral population grating and a third beam to read. However, in the previous OCT processor, the first and third beams are typically specified to be the same, which makes the emitted output signal spatially distinct from the subsequently applied continuous processing data stream. Consequently, the programming and processing stages cannot be overlapped in time.
Previously, storage of the grating required a persistence of the spectral holes at least as long as the processing stage step. Herein sp
Babbitt William
Merkel Kristian
Research and Development Institute Inc.
Tran Andrew Q.
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