Cairns, L; (2015) High Performance Optical Sampling of Microwave Signals. Doctoral thesis , UCL (University College London).

Abstract

High performance digital technologies are rapidly requiring increasing signal bandwidths and higher resolutions, and are being used in ever increasing areas. To be able to achieve the increasing digital requirements, higher speed analog-to-digital conversion (ADC), with greater resolution, is required. At the highest speeds and signal frequencies, applications include, optical communications, radar, high speed test and measurement, and electronic warfare. Current electronic ADC technology is stagnating. These limitations can be attributed to poor timing jitter of electrical systems. Optical sources have been demonstrated with timing jitter orders of magnitude lower than the best demonstrated for electrical systems. This work presents an ADC with a photoconductive switch sampling system using an In- GaAs photoconductive switch with a telecommunication wavelength cavity-less optical source. The photoconductive switch is able to operate over a wide frequency bandwidth (>20GHz), and the optical source used is able to produce a low timing jitter pulse train at gigahertz repetition rates. The optical source developed here is based on a cavity-less design, with a single electro optic modulator. A description of the design and implementation of the optical source is presented. The key parameters of the optical source, for optical sampling, are tested alongside two mode locked lasers for comparison. The lasers are a semiconductor laser which is fundamentally mode locked, and a bre ring laser which is harmonically mode locked. The cavity less source has been tested to have the lowest timing jitter of the three optical sources. Typically low temperature grown (LT) GaAs is used as the photoconductive material for optical sampling, however due to the large band gap energy, optical sources with a wavelength of 800nm or less is required. The choice of lasers at this wavelength are limited compared to optical sources at longer wavelengths. To be able to work with a laser operating at telecommunication wavelengths a photoconductive switch with a band gap energy of around 0.8eV is required. InGaAs has an appropriate band gap energy to work with telecommunication wavelength lasers, however InGaAs ultrafast materials are not as well established as LT-GaAs. Nitrogen implanted InGaAs has been used as the ultrafast material for photoconductive switches here. This material has been tested alongside LT-GaAs switches for comparison. It has been measured that InGaAs switches have a high dark current compared to LT-GaAs switches, which is expected with the lower band gap energy. The InGaAs device has been measured to have lower insertion losses (for the same optical power at 800nm) and better linearity than the LT-GaAs device measured here. Finally, a photonic sampling ADC system using an InGaAs sampling switch and a telecommunication wavelength laser has been demonstrated for the rst time. This system demonstrated a sampling rate of 7.2GHz and an ENOB of 3.9 for signals up to 8GHz, with a bandwidth limited by the electronic ADC to 900MHz. A similar system using a LT-GaAs and a Ti:sapphire laser has previously been published, which demonstrated an ENOB of 5.9 for a bandwidth of 750MHz and sampling rate of 1.5GHz. The di erence in ENOB is due to the lower sampling rate used in the LT-GaAs system. However, the optical source used for the InGaAs system is cheaper, smaller and more robust than the Ti:Sapphire laser used with the LT-GaAs system.

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