Browsing by Author "Dennis, Tasshi"

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    Comparison of laser-produced plasma target materials for pumping the 109 nm xenon(2+) Auger laser
    (1995) Dennis, Tasshi; Young, James F.
    Short wavelength lasers have many potential applications in scientific and engineering fields. The goal of this research was to improve the output energy of the Xe$\sp{2+}$ 109 nm laser. We compared cadmium, copper, gold, and zinc targets to stainless steel for laser-produced plasma pumping the Xe laser. A unique target geometry allowed us to measure the laser output using two materials under identical conditions. Small signal gain coefficients and maximum output powers are presented for each material relative to stainless steel; we observed a $\sim$20% improvement in the gain coefficient and a $\sim$112% improvement in total energy using a cadmium target. This increase probably results from an improved overlap of the laser-produced plasma emission spectrum with the Xe inner shell 4d photoionization cross section, although published data to support this hypothesis are scarce and inconsistent.
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    Demonstration of all-optical CDMA with bipolar codes
    (1997) Dennis, Tasshi; Aazhang, Behnaam; Young, James F.; Center for Multimedia Communications (http://cmc.rice.edu/)
    Using a fiber-based testbed, we experimentally verify that the advantageous correlation properties of bipolar spread-spectrum codes can be preserved in an optical channel using direct detection and all-optical encoder/decoders. The power spectrum of an erbium-doped superfluorescent fiber source is encoded, the codeword correlations are verified and rejection of multiple-access interference is demonstrated
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    Experimental demonstration of all-optical CDMA using bipolar codes
    (1999) Dennis, Tasshi; Young, James F.
    Fiber optic networks capable of supporting a large pool of subscribers, many simultaneous users, and high data rates are receiving heightened interest as solutions to a growing communications need. The experiments reported in this study constitute the first experimental demonstration of a novel bipolar equivalent code-division multiple-access (CDMA) scheme. The sophisticated encoding increases noise tolerance, provides user security, and enables network flexibility. The scheme is based on an established bipolar radio frequency (RF) technique adapted to the unipolar optical domain. Whereas the phase of an RF signal can be readily detected, the high carrier frequency ( ∼ 200 THz at 1.5 mum) of an optical wave necessitates that optical signals be detected and processed solely by intensity. Asynchronous operation makes the CDMA scheme data rate independent, while all-optical implementation avoids the bandwidth limitations imposed by electrical processing. A proof-of-principle experiment was conducted by spectrally encoding an erbium-doped superfluorescent fiber source (SFS) using a diffraction grating and an amplitude mask. The optical properties of the system were measured and the bipolar correlation of codes was verified. The practical implementation of the scheme was investigated by the design, construction, and operation of a fiber-based testbed. Correlation measurements performed with modulated signals confirmed that the scheme can recover a binary information symbol while rejecting multiple access interference. A theoretical analysis of the optical correlation process was conducted, which identified key optical parameters important to future implementations. The theory of excess noise associated with the photodetection of a thermal source was considered, followed by noise measurements of a light bulb and the erbium-doped SFS used for spectral encoding. Finally, the ability of the proposed scheme to effectively transmit data was investigated. Signal-to-noise-ratio and eye diagram measurements of the testbed were recorded. Bit-error rate (BER) measurements were conducted for various network scenarios, including operation with one and two active users, as well as with varying amounts of broadband optical interference. The analysis of the BER measurements addresses the limitations due to excess noise, the impact of imperfect correlation performance) and the anticipated capacity for multiple users.
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    Experimental demonstration of bipolar codes for direct detection multi-user optical communication
    (1996-10-20) Dennis, Tasshi; Nguyen, Lim; Aazhang, Behnaam; Young, James F.; Center for Multimedia Communications (http://cmc.rice.edu/)
    We report the first experimental demonstration of a broadband spectral encoding and decoding technique using bipolar codes for optical code-division multiple access. The codeword correlations are verified and rejection of multiple-access interference is demonstrated.