Browsing by Author "Young, James F."
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Item All-optical CDMA with bipolar codes(1995-03-20) Nguyen, Lim; Aazhang, Behnaam; Young, James F.; Center for Multimedia Communications (http://cmc.rice.edu/)A method for the transmission and detection of bipolar sequences in a unipolar system is presented. It allows all-optical implementation, in noncoherent optical CDMA systems, of the bipolar codes that have been developed for the radio domain. A practical design is described that encodes the spectrum of a broadband optical source to support a large number of subscribers.Item BER of optical communication system using fiber source(1995-11-20) Nguyen, Lim; Aazhang, Behnaam; Young, James F.; Center for Multimedia Communications (http://cmc.rice.edu/)We analyze the bit-error rate (BER) of an optical communication system using the superfluorescent fiber source (SFS). The counting statistics of thermal light give improved performance relative to the Gaussian statistics that predict a BER floor. We consider a spectrum-sliced wavelength-division multiple access (WDMA) system that employs the SFS.Item 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.Item 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 demonstratedItem Development and characterization of XUV xenon laser(1994) Wu, Jun; Young, James F.Short wavelength lasers have many potential scientific and engineering applications. The goal of this research is to develop and improve a practical XUV Xe laser, and to use it for XUV holographic imaging. Measurements on gain lifetime values of this laser have been performed. The laser has been found to have gain lifetime around 250 psec to 350 psec under our experimental conditions, and thus requires a sub-nanosecond pump laser. The effects of using different target materials under identical experimental conditions have been studied. We found that a gold coated target provides an 80% increase in output energy over a plain stainless steel target, and thus becomes an attractive way of improving the Xe laser energy. In order to use Xe laser as a source for XUV holographic imaging, its coherence properties need to be studied. Some of these issues are addressed in this thesis. A possible way of improving Xe laser coherence and mode quality is also discussed at the end of this thesis.Item 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.Item 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.Item Experimental demonstration of wavelength shift keying in optical WDM networks(2002) Xiang, Chu; Young, James F.Fiber optic networking has developed incredibly over the past a few years. It offers the promise to support large numbers of users simultaneously, to provide users with integrated services at high speed, and to increase the length of transmission without regeneration of the signals. To achieve yet even bigger capacity and longer transmission spans, the physical properties of the link, such as nonlinear processes in the optical fiber have been extensively studied. This thesis reports a study of one of the principal fiber nonlinear processes---Four-Wave Mixing (FWM), which results in interchannel crosstalk in wavelength division multiplexing (WDM) networks. We introduce a coding and detection scheme---Wavelength Shift Keying (WSK)---to reduce this nonlinear noise. Wavelength shift keying using symmetrical wavelength assignment and balanced detection can easily be implemented on top of the standard WDM setup, with only moderate changes at the end equipment. Its capability to depress four-wave mixing crosstalk to first order will greatly improve performance in long-haul transport systems. An analytical study of the four-wave mixing noise characteristics is conducted in WDM networks using low-dispersion fibers. The results suggest a wavelength shift keying scheme that utilizes these properties to suppress four-wave mixing noise to first order. Simulations on system performance for various network scenarios are carried out in both standard WDM and WSK, and the results compared, using both Gaussian approximation and exhaustive calculations. A proof-of-principle experiment is then implemented on a testbed that emulates these systems to show the operability of the proposed scheme. Optical and electrical properties of the testbed such as power spectrum, transmission efficiency, and noise figure are also studied so that irrelevant factors are controlled. Bit-error rate (BER) measurements are conducted in the first order approximation, and the results compare well with the calculations. In the laboratory system, an erbium-doped superfluorescent fiber source is employed to generate the four-wave mixing noise spectrum, since normal laboratory-scale systems do not produce realistic FWM noise. The theory of importance sampling is considered for the second order behavior, and group measurements are conducted within the range of system limitations. Data analysis shows the validity of the first order comparison done previously, and provides a new approach to variance performance evaluation.Item Performance evaluation of an optical code-division multiplexing system with four-wave mixing effect(1996) Guo, Yile; Young, James F.The performance of an optical code-division multiplexing (CDM) system using spectral encoding and bipolar codes under the influence of four-wave mixing (FWM) is evaluated in terms of error probability. In the CDM system the first order crosstalk due to four-wave mixing can be cancelled using antisymmetric mask patterns in the encoders and decoders. The second order FWM crosstalk is proportional to the number of users supported by the system. For a CDM system using an ideal broadband source, shot noise is the major factor that causes error in the lower signal power range, while four-wave mixing crosstalk dominates in the higher signal power range. The CDM system has a lower error probability in the higher signal power range than the WDM system, i.e., the performance of the CDM system is better than that of the WDM system.Item Principle and practice of optical code-division multiple access communication(1996) Nguyen, Lim; Aazhang, Behnaam; Young, James F.This research examines the theoretical and experimental aspects of code-division multiple access (CDMA) communication in the incoherent, or direct detection, optical domain. We develop new modulation and detection principles that permit all-optical implementation of the bipolar, +1/$-$1, code and correlation detection available in the radio frequency (RF) CDMA systems. This is possible in spite of the non-negative, or unipolar, +1/0, nature of the incoherent optical system that only detects and processes the signal intensity. The unipolar optical system using the new principles is equivalent to the bipolar RF system in that the correlation properties of the codes are completely preserved. The optical CDMA system can be realized either in time or frequency domain encoding with all-optical components. All-optical implementation is extremely important in practice because the symbol rates of the individual users are far less than the bandwidth of the optical fiber. Frequency domain or spectral amplitude encoding significantly increases the number of available codewords that can be assigned to the subscribers, and is the focus of this work. The spectral amplitude encoding uses incoherent, broad bandwidth superfluorescent fiber source (SFS) that has the limiting signal-to-noise ratio characteristics associated with thermal light. Measurements of the photodetector illuminated by an erbium-doped SFS confirm that the noise distribution follows a negative binomial probability mass function as predicted for the photoelectron counting statistics of thermal light. The analysis based on this statistical model results in improved performance over the Gaussian assumption, which predicts a performance floor. The analysis also shows that optical CDMA system, even with the use of orthogonal codes, is interference limited. We implement an experimental prototype system that demonstrates the first use of bipolar codes for spectral amplitude CDMA. The experimental results validate the modulation and detection principles and demonstrate the feasibility of optical CDMA systems based on those principles.Item Short wavelength laser systems for applications(1993) Sharp, Tracy Elizabeth; Young, James F.Very short wavelength lasers have many potential scientific and technological applications. A practical extreme ultraviolet (XUV) laser system has been developed using the Xe Auger laser at 109 nm. This system is the first XUV laser system pumped by a standard, commercially available, Nd:YAG laser system at a high repetition rate. The Xe laser is pumped by the soft x-rays generated by a laser-produced plasma. A grazing incidence, traveling-wave pumping geometry is used to reduce the pump energy required to achieve saturated energy outputs. A total equivalent small signal gain of exp(25) has been achieved with a 20 cm long gain region. The maximum output energy of this system is about 1 $\mu$J. The extension of the laser-produced plasma pumping technique to extremely short wavelength lasers will require very high power, ultrafast laser pulses in order to produce sufficient upper state densities for gain within the short lifetime of the excited state. To this end, we have studied a new ultrashort laser pulse amplifier based on the broad bandwidth XeF(C $\to$ A) excimer transition that is capable of directly amplifying pulses as short as 10 fs duration and has high energy storage capability. Construction of a tunable, blue-green, subpicosecond source of laser pulses for injection into the XeF(C $\to$ A) excimer amplifier is described. Gain characteristics of the XeF(C $\to$ A) excimer amplifier were investigated for several pulse lengths. Saturation energy densities of 50 mJ/cm$\sp2$ and 80 mJ/cm$\sp2$ were measured for injected laser pulse durations of 250 fs and $\sim$100 ps, respectively. A gain bandwidth of 60 nm was observed. Using an optimized unstable resonator design, the laser amplifier produced 275 mJ pulses with a duration of 250 fs, and a 2.5 times diffraction limited beam quality, making the XeF(C $\to$ A) excimer amplifier the first compact laser system in the visible spectral region to reach peak powers of the terawatt level.Item System and method for performing optical code division multiple access communication using bipolar codes(1998-06-02) Young, James F.; Aazhang, Behnaam; Nguyen, Lim; Rice University; United States Patent and Trademark OfficeAn optical encoding and decoding system which performs code-division multiple access (CDMA) communication in the incoherent, or direct detection, optical domain using bipolar +1/-1 codes. The present invention uses code modulation and detection principles that permit all-optical implementation of the bipolar, +1/-1, code and correlation detection that have been developed for the radio frequency (RF) systems. This is possible in spite of the non-negative, or unipolar, +1/0, nature of the incoherent optical system that only detects and processes the signal intensity. The unipolar optical system of the present invention is equivalent to the bipolar RF system in that the correlation properties of the bipolar codes is completely preserved. The optical CDMA system can be realized both in time or frequency domain encoding with all-optical components.Item Wavelength encoding to reduce four-wave mixing crosstalk in multi-wavelength channels(1997) Guo, Yuanbin; Aazhang, Behnaam; Young, James F.; Center for Multimedia Communications (http://cmc.rice.edu/)The trend toward higher bit rates in lightwave communications has increased interest in dispersion-shifted fiber (DSF) to minimize dispersion penalties. In addition, wavelength-division multiplexing (WDM) is being used to gain even greater capacity, and broad bandwidth erbium-doped fiber amplifiers are extending link distances between signal regeneration. Taken together, these techniques can result in severe performance degradation due to four-wave mixing (FWM). Several different ways of reducing the FWM degradation in WDM systems have been proposed, but all of the approaches increase system complexity and make it difficult to add channels to the system. We show that wavelength-coded multiplexing can provide multiple channels on a simple fiber link while significantly reducing four-wave mixing interference compared to a WDM system. The FWM spectrum is symmetric and use of an antisymmetric code cancels the interference. Wavelength-coded multiplexing is also known as spectral code-division multiplexing (SCDM). We have calculated the error probability in the SCDM system considering only shot noise, thermal noise and FWM crosstalk, all of which are assumed to have a Gaussian distribution. For comparison, we also derived an expression for the error probability in a WDM system.Item Wavelength shift keying technique to reduce four-wave mixing crosstalk in WDM(2000) Xiang, Chu; Young, James F.In low dispersion fibers, the four-wave mixing (FWM) spectrum is symmetric with respect to the zero-dispersion wavelength. This thesis analyzes a wavelength shift keying (WSK) technique, a modification of the standard wavelength division multiplexing (WDM) system. WSK uses symmetric wavelength assignment and balanced detection to cancel FWM crosstalk to first order. Error probabilities for both dispersion shifted fiber and non-zero dispersion shifted fiber were calculated using Gaussian approximation. WSK out-performs standard on-off WDM at all signal power levels and allows a larger bit rate-distance product for a given BER requirement. Four-Wave mixing in a long-haul transmission scenario was simulated in an experimental testbed. BER measurements compared well with the calculations and proved that WSK is indeed noise tolerant.