From spectral holeburning memory to spatial-spectral microwave signal processing
Wm. Randall Babbitt, Zeb W. Barber, Scott H. Bekker, Michael D. Chase, Calvin C. Harrington, K.D. Merkel, R Krishna Mohan, Tia Sharpe, Colton R. Stiffler, Aaron S. Traxinger, Alex J. Woidtke
Many storage and processing systems based on spectral holeburning have been proposed that access the broad bandwidth and high dynamic range of spatial-spectral materials, but only recently have practical systems been developed that exceed the performance and functional capabilities of electronic devices. This paper reviews the history of the proposed applications of spectral holeburning and spatial-spectral materials, from frequency domain optical memory to microwave photonic signal processing systems. The recent results of a 20 GHz bandwidth high performance spectrum monitoring system with the additional capability of broadband direction finding demonstrates the potential for spatial-spectral systems to be the practical choice for solving demanding signal processing problems in the near future.
Many storage and processing systems based on spectral holeburning have been proposed that access the broad bandwidth and high dynamic range of spatial-spectral materials, but only recently have practical systems been developed that exceed the performance and functional capabilities of electronic devices. This paper reviews the history of the proposed applications of spectral holeburning and spatial-spectral materials, from frequency domain optical memory to microwave photonic signal processing systems. The recent results of a 20 GHz bandwidth high performance spectrum monitoring system with the additional capability of broadband direction finding demonstrates the potential for spatial-spectral systems to be the practical choice for solving demanding signal processing problems in the near future.
It was envisaged that spectral holeburning based optical memory will provide orders of magnitude improvement in storage densities. The progress has basically been hampered by the lack of materials with proper performance characteristics. In the past two decades, novel atomic scale processes involved in holeburning have been investigated in detail in order to improve the performance of existing materials and to engineer new ones. On the other hand, significant advances have been made in optical storage techniques to use the materials available. And now, new materials that can approach the specifications of a memory device using different techniques for the storage are either available or are just over the horizon. This paper reviews the status of different classes of spectral holeburning materials suitable for memory devices.
Unique spectroscopic properties of isolated rare earth ions in solids offer optical linewidths rivaling those of trapped single atoms and enable a variety of recent applications. We design rare-earth-doped crystals, ceramics, and fibers with persistent or transient "spectral hole" recording properties for applications including high-bandwidth optical signal processing where light and our solids replace the high-bandwidth portion of the electronics; quantum cryptography and information science including the goal of storage and recall of single photons; and medical imaging technology for the 700-900 nm therapeutic window. Ease of optically manipulating rare-earth ions in solids enables capturing complex spectral information in 105 to 108 frequency bins. Combining spatial holography and spectral hole burning provides a capability for processing high-bandwidth RF and optical signals with sub-MHz spectral resolution and bandwidths of tens to hundreds of GHz for applications including range-Doppler radar and high bandwidth RF spectral analysis. Simply stated, one can think of these crystals as holographic recording media capable of distinguishing up to 108 different colors. Ultra-narrow spectral holes also serve as a vibration-insensitive sub-kHz frequency reference for laser frequency stabilization to a part in 1013 over tens of milliseconds. The unusual properties and applications of spectral hole burning of rare earth ions in optical materials are reviewed. Experimental results on the promising Tm3+:LiNbO3 material system are presented and discussed for medical imaging applications. Finally, a new application of these materials as dynamic optical filters for laser noise suppression is discussed along with experimental demonstrations and theoretical modeling of the process.
Optically active Spatial-Spectral (S2) materials are a unique resource for spectrally based optical memory and processing. At cryogenic temperatures, the rare-earth ions in these materials individually exhibit narrow optical resonances on the order of MHz to sub-KHz, but are inhomogeneously broadened over GHz to THz spectral bandwidths providing up to 107resolvable spectral channels. The material can be optically programmed to transform the spectral and spatial components of optical signals to perform signal processing operations such analog multiplications, time delay, filtering, convolutions, and correlations. We present work utilizing the S2 technology for high bandwidth (>32 GHz) and data rate selection and filtering, including processing of 1D data streams in real-time and 2D images. Despite cryogenic cooling, the power efficiency of the S2 technology compares favorably to CMOS in large scale systems. Finally, potential architectures for large (106× 106) vector-matrix multipliers using S2 materials are discussed.
Abstract : Optical materials for spectral hole burning and coherent transient applications were designed and evaluated, emphasizing relevance to devices including high performance memory, signal processing, high speed holographic recording and analysis, and optical switches, routers, and buffers. The ultimate limits on material performance set by atomic mechanisms were determined. The developed materials contained Eu3+, Pr3+, Tm3+, Tb3+, or Er3+ active ions in various oxide hosts. The first report was made of an Er3+ material for hole burning applications in 1.5 micron optical communications bands. Eight Er3+ materials for applications at 1.5 microns in optical routers and buffer memories were designed and characterized. A correlator' signal processing capability was demonstrated, and the first report was made of operation of a 1.5 micron packet header decoder for all-optimized optical packet switching in optical communications along optical fibers at 1531 nm. Eu3+:Y2SiO5 was optimized for use in 100 GByte ODRAM optical memory devices, considering Eu3+ concentration and temperature. Eight Tm3+ compounds for signal processing applications, such as true time delays for phased-array radar, at diode laser wavelengths near 795 nm were designed and characterized. Eight Tb3+ compounds for photon gated spectral hole burning were designed and characterized, and gated hole burning was observed in Tb3+:YAIO3.
Optical devices have been slower to mature than those based purely on electronics. Yet optical interactions offer a number of unique capabilities expected to support entirely new classes of devices-devices offering unprecedented performance in communications, computing, and data storage applications. In the present work, optical devices based on the interaction of light with frequency-selective recording materials are discussed. A frequency-selective recording material has the capability of supporting multiple independent recording channels. Each channel is addressed by light within a specific narrow band of optical frequencies. In some materials, operative at cryogenic temperatures, millions of distinct spectral channels are available. Through the mechanism of persistent spectral hole-burning, light can be employed to transiently or even permanently modify the absorptive or dispersive properties of the recording material within a selected channel. Such modifications provide a means of recording multiple bits of optical information within single spatial locations. Spectral multiplexing of information has clear potential in information storage systems. Less obviously, the ability to spectrally encode information provides an enabling basis for ultrahigh speed, content-controlled, all-optical switching devices operable at data speeds up to the Terabit/sec level.
We demonstrate application of spectral hole burning to the spectral analysis of broad-band rf signals. In quite the same way as an acousto-optic spectrometer, the device operates on an optically carried rf signal and achieves angular separation of the signal spectral components. An instantaneous bandwidth of 2.5 GHz has been achieved, with a power dynamic range of 35 dB, limited by the detector. Extension to greater than 10 GHz instantaneous bandwidth with greater than 1000 channels is consistent with the active material capabilities.