Dual optical frequency comb analog to digital conversion

Abstract

Analog to digital converters (ADCs) are the fundamental technology that allows the capture and analysis of signals across all scientific and engineering disciplines and underpin the digital links that connect our analog world. Modern communications systems demand high bandwidth, high resolution ADC in order to detect higher order modulation formats at high baud rates and maximise the spectral efficiency of the channel. However, the resolution of high speed (i.e. 1 GHz) electronic ADCs is typically limited by clock jitter and, at especially high frequencies, the speed of the component transistors that results in comparator ambiguity. This presents a trade-off between the frequency of the detected signal and accuracy, defined by the SINAD or ENOB. In a jitter limited ADC, the SINAD decreases quadratically with increasing frequency, giving a 6 dB SINAD penalty for every doubling of the input frequency. This thesis proposes a frequency interleaving photonic front end for analog to digital converters, based on dual optical frequency combs, in order to meet this challenge of high speed, high resolution signal digitisation. Firstly, the dual frequency comb technique is described and modelled, both analytically and through simulations, to establish the potential performance of the dual comb approach in analog to digital conversion and other radio frequency signal processing applications. Secondly, a dual frequency comb prototype is experimentally demonstrated based on phase coherent electro-optic combs. The phase noise characteristics of the architecture are established and the prototype is evaluated using the IEEE ADC testing standard, outperforming any reported electronic ADC. Finally, arbitrary signal detection using the dual comb technique is demonstrated using a novel phase locking approach that efficiently utilises the comb bandwidth, and the impact of possible implementation errors is investigated