At the core of this technology are Spatial Spectral Holographic (SSH) materials that
have unique intrinsic frequency selective properties, as well as high spatial resolution.
The materials typically contain rare earth ions like Tm (793 nm) or Er (1.5 microns)
that are doped into a crystal or glass. Typical hosts include oxide crystals like
YAG, a common laser material, or, more recently, LiNbO3, as well as other crystals,
glasses, and organic polymers.
The frequency selectivity of SSH materials comes from the sharp optical resonance of individual ions doped into the host material. The line-widths of individual dopants (the homogeneous line-width) can be less than a kilohertz at cryogenic temperatures (2-6 K). Due to microscopic defects in the crystal, the resonant frequency of each dopant is shifted from its nominal value. These shifts are randomly distributed and lead to a smooth inhomogeneous absorption profile that ranges from 20 GHz to over 200 GHz in crystals to several THz in glasses and organics. These shifts do not broaden the individual dopant resonances.
As a result, an inhomogeneously broadened SSH material acts like a broadband multi-channel spectral filter with an incredible number of channels. The ratio of the inhomogeneous bandwidth to the homogeneous bandwidth yields the time-bandwidth product of the processor. The material Tm:YAG has a measured Q of over 10e13 and a measured time-bandwidth product (TBP) exceeding 10e8. Other promising processing materials, that can provide over 300 GHz of bandwidth with sub-MHz resolution at 4K or TBPs approaching 10e6 are being developed.
The research at Spectrum Lab in collaboration with other groups has resulted in a number of publications related to SSH technology. Some of the recent publications are:
- Angle of arrival estimation using spectral interferometry , Z.W. Barber; C. Harrington; C.W. Thiel; W.R. Babbitt, and R. Krishna Mohan, Journal of Luminescence, vol. 130, issue 9, p. 1614-1618 (2010)
- Demonstrations of analog-to-digital conversion using a frequency domain stretched processor, Reibel, Randy R.; Harrington, Calvin; Dahl, Jason; Ostrander, Charles; Roos, Peter A.; Berg, Trenton; Mohan, R. K.; Neifeld, Mark A.; Babbitt, Wm. R., Optics Express, vol. 17, issue 14, p. 11281 (2009)
- Demonstration of geometric operations on the Bloch vectors in an ensemble of rare-earth metal atoms, Mingzhen Tian, Ijaz Zafarullah, Tiejun Chang, R. Krishna Mohan, and Wm. Randall Babbitt, Phys. Rev. A 79, 022312 (2009)
- Numerical modeling of optical coherent transient processes with complex configurations - III, Tiejun Chang, Mingzhen Tian, J. Lumin. 127, 76-82, (2007).
- Ultra-wideband spectral analysis using S2 technology, R. Krishna Mohan, T. Chang, M. Tian, S. Bekker, A. Olson, C. Ostrander, A.Khallaayoun, C. Dollinger, W.R. Babbitt, Z. Cole, R.R. Reibel, K.D. Merkel, Y. Sun, R. L. Cone, F. Schlottau, and K. H. Wagner, J. Lumin. 127, 116-128, (2007).
- Unambiguous Range - Doppler LADAR processing using 2 GSPS noise waveforms, Z. Cole, P. A. Roos, T. Berg, B. Kaylor, K. D. Merkel, W. R. Babbitt, and R.R. Reibel, J. Lumin. 127, 146-151, (2007).
- Broadband Analog to Digital Conversion with Spatial-Spectral Holography, W. R. Babbitt, M. A. Neifeld, and K. D. Merkel, J. Lumin. 127, 152-157, (2007).
- Preparation of inverted medium and processing in the inverted medium, I. Zafarullah, M. Tian, T. Chang, and W. R. Babbitt, J. Lumin. 127, 158-163, (2007).