Browsing by Author "Dr. John F. Muth, Committee Chair"

Now showing 1 - 4 of 4

A 1 Mbps Underwater Communications System using LEDs and Photodiodes with Signal Processing Capability
(2008-12-07) Simpson, Jim Anto; Dr. Brian Hughes, Committee Member; Dr. Leda Lunardi, Committee Member; Dr. John F. Muth, Committee Chair
The inability of radio frequency electromagnetic waves to propagate without attenuation in seawater has traditionally limited underwater communications to acoustics or tethered systems. High bandwidth optical communication systems have been demonstrated for terrestrial and space applications. There is growing interest to see if short range high bandwidth optical wireless systems can be made for the underwater environment. In this thesis we demonstrate a 1 Mbps optical wireless system using LEDs and PIN photodiodes that also incorporates capabilities for signal processing of the received data to be performed. Lasers and Photomultiplier tubes offer high performance, and are generally used in most underwater optical communication systems. However, these components are relatively expensive and can have large form factors. As an alternative solution the much cheaper and more compact LEDs and photodiodes are used as transmitters and receiver components. However, compared to a laser and PMT based system, such a system would be strongly disadvantaged in photon limited environments. If one assumes that photons actually reach the receiver, using signal processing techniques, optimized modulation formats, and error-correction coding, one expects that the range of the system can be extended. The development of a prototype system for the experimentation and verification of this proposition is the main motivation of this thesis. Small, compact transmitters using High Power LEDs and receivers using Si Photodiodes where the data can be digitally sampled such that signal processing techniques can be applied were constructed and demonstrated using a 12 foot, 1200 gallon tank that was also constructed for the project. It was shown that the LED and photodiode based system works well for short ranges, and that advantages can be obtained using digital signal processing. The applicability of this strategy to use digital signal processing techniques can be easily extended to higher performance Laser/PMT based systems.
Gallium Nitride Ultraviolet Optical Modulators
(2005-02-28) Oberhofer, Andrew Edward; Dr. John F. Muth, Committee Chair; Dr. Richard T. Kuehn, Committee Member; Dr. Salah M. Bedair, Committee Member; Dr. Dennis M. Maher, Committee Member; Dr. Mark Johnson, Committee Member
In narrower band gap semiconductors researchers have exploited the ability to manipulate the exciton resonance via the Quantum Confined Stark Effect to make a variety of different types of optical modulators at infrared wavelengths. In this thesis, the large exciton binding energy of Gallium Nitride is used as the basis for ultraviolet optical modulators without the need for quantum confinement. A 5x5 array of UV optical modulators at 360 nm was fabricated. The modulators operated in a transverse geometry and consisted of a GaN active layer surrounded by transparent AlGaN insulating and electrical contact layers. The typical thickness of the GaN layer was 0.4 um so the effects of the electric field on the exciton resonance could be directly observed. A hydrogenic model for the bulk exciton was assumed. The applied electric field opposed the attractive coulomb potential between the electron and hole and broadens the exciton resonance. This results in more or less light through the device depending on the spectral position. To understand the magnitude of the applied field within the device structure a 1D Poisson Solver was used. Spontaneous polarization and piezoelectric effects due to lattice strain between the AlGaN and GaN layers were included in the model and were found to have influence on the device at lower operating voltages. In the electric field modulated devices a contrast ratio of about 20 percent was obtained. In thermally modulated devices, at low frequencies less than 200 Hz large shifts in the band edge led to large contrast ratios as expected. The temperature dependence of the device followed the Varshni relationship and allowed the magnitude of the temperature shift to be quantified. At higher frequencies from 1kHz to 120 kHz an optical modulation of ~ 5 percent was readily observed and was attributed to electronic effects. The limitation of 100 kHz was equipment related and it is conjectured that the modulation bandwidth would extend into the MHz.
Short Range Underwater Optical Communication Links
(2005-10-25) Chancey, Mark Alan; Dr. Leda Lunardi, Committee Member; Dr. Gianluca Lazzi, Committee Member; Dr. John F. Muth, Committee Chair
The future tactical ocean environment will be increasingly complicated. In addition to traditional communication links there will be a proliferation of unmanned vehicles in space, in the air, on the surface, and underwater. To effectively utilize these systems improvements in underwater communication systems are needed. Since radio waves do not propagate in sea water, and acoustic communication systems are relatively low bandwidth the possibility of high speed underwater optical communication systems are considered. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this thesis is to understand how to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. The secondary focus of the thesis was to construct LED based optical communication systems. This required understanding the behavior of Gallium Nitride LEDs operated under intense electrical pulsing conditions. An optical FM wireless system was constructed for transmitting speech. An LED based Ethernet compatible digital communications system that was capable of operating at 10 Mbps was also constructed and packaged for underwater operation.
Underwater Free Space Optics
(2006-12-08) Gawdi, Yash Jagdishlal; Dr. John F. Muth, Committee Chair; Dr. Leda M. Lunardi, Committee Co-Chair; Dr. Kevin G. Gard, Committee Member
Radio waves propagate poorly in water and acoustics have been the dominant method for undersea communications, but its data rates are bandwidth limited. Free Space Optics potentially provides an alternative solution to acoustical communications with wider bandwidth and wireless flexibility that would benefit many undersea applications over short ranges. Compared to atmospheric propagation, ocean waters are a more complex medium for light propagation presenting a very high attenuation, depending on a variety of different parameters and conditions. In this work, a model is investigated to that estimates the total attenuation of the light propagation in natural waters in the context of optical communications. Using a one parameter model of absorption and scattering, a general framework has been structured in MathCAD, where the absorption and scattering coefficients are iteratively calculated for a variable vertical chlorophyll profile. The beam spread function is implemented to estimate the total beam attenuation. Link budget simulations are also computed to address the feasibility of the underwater free space optical links. Laboratory experiments of light attenuation in sea water with red (633nm) and green (532nm) wavelengths corroborate simulated results obtained with the beam spread function within 25% error margin. Particulate scattering experiments with polystyrene latex spheres of sizes 500nm and 6μm were carried out to confirm its dependence on wavelength and particle size. These phase function of the measured experimental results are in agreement with the previously published data.