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Optical Methods for Ranging and Analog-to-Digital Conversion


Optical methods are employed in various fields from communications to space research to improve the system performance. Each application uses different properties of light and light-matter interaction. For example, ultra-high frequency nature of light is desirable in applications like ranging and velocimetry to increase the resolution. On the other hand, in the telecommunication industry, ultra-wide bandwidth of the fibers is utilized with time and wavelength multiplexing techniques that make Gigabit internet possible today. Such multiplexing techniques are also useful in photonic assisted analog to digital converters (ADC) that multiply the sampling rates of electronic ADCs. Additionally, ultra-low (<10fs) jitter performance of lasers make an accurate sampling of fast events possible.

To be specific, this thesis covers multi-wavelength approaches to Lidar and photonic assisted ADCs. In ranging, multiple optical tones are generated through modulation with multiple RF frequencies and relative phase shift between individual tones are utilized to detect the range and velocity of a target. On the other hand, in photonic assisted ADC system, multiple continuous-wave lasers at different wavelengths are being utilized to achieve wavelength multiplexing, accurate delays, and to correct gain and offset mismatches between interleaved ADC channels.

In the first part of the dissertation, the multi-tone modulated continuous wave (MTCW) lidar system is analytically and numerically investigated. Also, a proof-of-concept experiment by employing 1550nm light source and multiple RF tone modulations ranging from 50MHz to 6GHz has been performed to demonstrate proof of principle for range finding with <1cm range resolution. Additionally, the velocity measurement capability of the system has been demonstrated by using a target attached to a speaker membrane or a stepper motor.

The second part of the dissertation presents a photonic assisted ADC system that employs wavelength-multiplexed pulse-modulated lasers as an optical sampler to capture an RF signal. The sampled signal is demultiplexed and quantized with electronic ADCs after optoelectronic conversion. The system is analytically and numerically investigated for its performance limitations and requirements to overcome the problems.

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