2 research outputs found

    Terahertz wireless communication through atmospheric atmospheric turbulence and rain

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    This dissertation focusses on terahertz (THz) wireless communication technology in different weather conditions. The performance of the communication links is mainly studied under propagation through atmospheric turbulence and rain. However, as real outdoor weather conditions are temporally and spatially varying, it is difficult to obtain reproducible atmospheric conditions to verify results of independent measurements making it a challenge to measure and analyze the impact of outdoor atmospheric weather on communication links. Consequently, dedicated indoor weather chambers are designed to produce controllable weather conditions to emulate the real outdoor weather as closely as possible. To emulate turbulent air conditions, an enclosed chamber is developed into which air with controllable airspeeds and temperatures are introduced to generate a variety of atmospheric turbulence for beam propagation. To emulate varying rain conditions, an enclosed chamber is built in which pressurized air forces drops of water through an array of 30 gauge needles. In order to study and compare propagation features of THz links with infrared (IR) links under identical weather conditions, a THz and IR communications lab setup with a maximum data rate of 2.5 Gb/s at 625 GHz carrier frequency and 1.5 μm wavelength, are developed. A usual non return-to-zero (NRZ) format is applied to modulate the IR channel but a duobinary coding technique is used for driving the multiplier chain-based 625 GHz source, which enables signaling at high data rate and higher output power. The power and bit-error rate (BER) on the receiver side are measured, which can be used to analyze the signal performance. To analyze the phase change in the turbulence chamber due to the refractive index change induced by turbulence, a Mach-Zehnder Interferometer with He-Ne laser at 632.8nm is developed. In the same weather conditions, the impact on THz in comparison with IR link is not equivalent due to the spectral dependence on atmospheric turbulence and rain. In the experiment, after THz (625 GHz) and IR (1.5 μm) beams propagate through the same condition, performance of both channels is analyzed and compared. Kolmogrov theory is employed to simulate the atmospheric turbulence which leads to attenuation of THz and IR signals. Mie scattering theory is employed to simulate the attenuation of THz and IR beams due to rain. Under identical turbulence conditions, THz links are superior to IR links. However, the performance of THz and IR links are comparable under identical rain conditions

    Low Reflectivity Facet Realization in GaAs-Based Optoelectronic Devices Using Self-Aligned Stripe Process

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    This thesis explores the realization of low facet reflectivity using self-aligned stripe buried waveguide configuration and its implementation in optoelectronic devices such as superluminescent diodes (SLDs) and semiconductor optical amplifiers (SOAs). I explored the development of the buried waveguide in AlGaAs/GaAs material system ,since its first presentation in 1974 by Tsukada, in order to identify the problems associated with this technology. A novel window-faceted structure is demonstrated. The experimental measurements demonstrated effective reflectivity <10-14 as a result of both divergence and absorption within these window-like regions (i.e. not transparent). Its implementation to suppress lasing in tilted and normal-to-facet waveguide SLDs was thoroughly investigated in chapters 3 and 4. In the tilted devices, ~40mW output power with spectral modulation depth < 2% is demonstrated. In the latter types of SLDs, up to 16mW output power with <5% spectral modulation depth was recorded, which is the highest power demonstrated for such configurations. The performance of the two types of devices was measured without the application of anti-reflective coatings on the rear facet, which makes them inherently broadband. By incorporating a windowed facet at each end of a waveguide I could realize an SOA with window structured facet. Promising results were demonstrated in this configuration including 33dB gain and <6dB noise figure, which are comparable to the state-of-the-art. A trial was held to extend the concept of absorptive rear window to visible wavelengths available in the GaInP/AlGaInP material system. Problems associated with such devices were explored briefly and two solutions are suggested. Simulations were performed to realize design of an optimized device. Unfortunately, the experimental implementation of the design was not successful but suggestions for strategies to overcome these problems are discusse
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