Alan E. Johnson

Spectral Data Storage Using Rare-Earth-Doped Crystals

Conventional optical data-storage techniques, such as magneto-optic disks and CD-ROMs, record a single bit of information at each particular substrate location. In order to produce the gigabyte-class storage substrates demanded by todays computers using such conventional technologies, access to tens of billions of individual material locations is required. This brute-force approach to optical data storage has produced impressive results. However, there is increasing interest in methods for more efficiently accessing storage materials. One approach is to record multiple bits at a single storage-material location. This can be accomplished by multiplexing the bits spectrally, using differing optical frequencies to record data bits. It has been realized for over 20 years that when certain materials are cooled to appropriate temperatures, typically below 20 K, the possibility of spectrally multiplexing large numbers of bits in a single material location arises. Although this approach, known as spectral hole-burning, has been proposed as a data-storage mechanism, to date it has primarily been used as a tool to study material properties. Rare-earth-doped crystals have been demonstrated to have properties that lend themselves to a variety of different spectral hole-burning-based data-storage applications. In this article, we will review the principles of spectral hole-burning, discuss some specific material systems in which spectral hole-burning is of particular interest, and describe methods for producing high-capacity, high-data-rate spectral memories. Spectral hole-burning, and spectral memories based on spectral hole-burning, depend on a material property referred to as inhomogeneous absorption line broadening. Materials exhibiting this property contain active atoms or molecules that individually respond to (absorb) very specific frequencies of light, but the collective response of all of the materials active atoms or molecules covers a spectral region that is broad compared with the response of a particular active atom or molecule. Inhomogeneous absorption line broadening is caused by local variations in the structure of the host, which in turn lead to variations in the electronic levels of the active atoms or molecules. The absorption linewidth of an individual absorber is referred to as the homogeneous linewidth Γ h , and the absorption width of a collection of inhomogeneously broadened absorption centers is referred to as the inhomogeneous linewidth Γ i . Application of monochromatic light to such a material has the effect of exciting only a very small subset of active absorbing atoms—those residing in the illuminated spatial volume within a homogeneous width of the exciting lights specific frequency. If the frequency of the imposed light is shifted, a different subset of active absorbing atoms in the illuminated volume responds.

Spatially distributed spectral storage

A new technique for overcoming the effects of excitation-induced frequency shifts in swept-carrier optical memories is proposed and demonstrated. Spectral storage capacity increases of more than 2 orders of magnitude were achieved through implementation of the method.

Demonstration of code-division multiplexing with synchronous orthogonal coding using fiber Bragg gratings

We demonstrate two-channel synchronous code-division multiplexing using temporally orthogonal codes and fiber Bragg gratings. The use of synchronous orthogonal codes and properly gated detection permits simultaneous transmission of multiple data channels with minimal crosstalk.

Optical code-division multiple access (O-CDMA) interconnects and telecommunication networks based on temporally accessed spectral multiplexing (TASM)

A novel optical interconnect architecture based on temporally accessed spectral multiplexing (TASM) technology is presented. The use of TASM optical interconnects will be demonstrated in multiple access telecommunication applications. TASM technology builds on optical code- division multiple access concepts with promise of low cost implementation of Terabit/second optical networks. The use of TASM technology to enhance wavelength division multiplexing in optical communication systems will be discussed. The working principles and design of TASM system architectures will be presented with emphasis on the key functional component - the TASM encoder.

Applications of the arbitrary optical filter: Fiber Bragg grating filters for WDM and CDMA communication

The design and manufacture of advanced telecommunication devices based on Fiber Bragg gratings (FBG) is presented. With the maturing of optical communication comes the need for advanced optical filtering capabilities that permit the direct manipulation and processing of optical data streams in both the wavelength and time domains. By creating very complex grating amplitude and phase profiles inside fibers it is possible to approach an arbitrary transfer function capability. We demonstrate that with these filters it is possible, for example, to improve greatly the filtering characteristics vis-a-vis WDM or chromatic dispersion compensation applications. More importantly, more advanced filtering can be achieved in which temporal codes can be imprinted directly inside the optical bits and subsequently recognized by matched optical filtering. This optical code-division multiplexing (CDM) capability presents interesting new opportunities for advanced optical processing allowing for increased channel count, novel optical routing abilities, or more flexible bandwidth distribution. Moreover, the filtering can be made compatible with WDM and TDM systems, and hence takes a large step towards enabling the introduction of more complex modulation formats in optical communication systems. An important consequence of the arbitrary filtering capability is that multiple filtering functionalities can be combined inside a single FBG. In this context, we present a filter that combines wavelength selectivity with code discrimination and dispersion compensation.

Code-division-multiplexing-compatible coding and decoding of directly driven DFB laser bit streams

Optical implementation of code-multiplexing is heavily contingent upon compatibility of encoding devices with standard laser sources. We demonstrate encoding and decoding of RZ signals generated at 2.5 Gbps by direct-driving of a semiconductor DFB laser.

Implementation of optical dynamic RAM using spatially distributed spectral storage

Optical Dynamic RAM (ODRAM) is a high capacity, low latency optical memory architecture based on persistent spectral hole burning in frequency selective materials. This paper describes the basic ODRAM architecture and progress towards realization of a high capacity, low latency, tabletop demonstration unit. In particular, a new technique, Spatially Distributed Spectral Storage (SDSS) is introduced and demonstrated to provide over two orders of magnitude improvement in spectral capacity for materials that experience excitation induced frequency shifts. Finally, the relative strengths and weaknesses of ODRAM are emphasized in a competitive analysis that includes currently available memory technologies such as semiconductor DRAM and magnetic disks.