Photonic Analog-to-Digital Converter

Analog-to-digital conversion (ADC) is an essential operation in many applications including radar, sensing, communications, and instrumentation. Two key parameters of an ADC are its sampling rate and resolution, measured in samples per second and number of bits, respectively. Unfortunately, as the sampling rate increases the maximum resolution is reduced. High-speed electronic ADCs (>10 GS/s) are currently limited to about 4-6 effective bits of resolution, while moderate speed ADCs (<500 MS/s) can have 10 or more effective bits of resolution. It is a goal of this STTR project to simultaneously achieve both high sampling speeds and high resolution. ADCs which use photonic technology to improve their performance have achieved drastic advances in sampling rate. This is especially true for time-limited signals as sample rates of many hundreds of GS/s have been recorded, exceeding purely electronic ADCs by 1-2 orders of magnitude. However, this does not translate into high resolution samples, especially for continuous-time signals. In fact, high speed continuous-time photonic-assisted ADCs typically have resolutions of just 3-5 bits even at the more modest 10 GS/s sampling rates. The resolution limitation comes from several sources, including mismatches between components and signal-to-noise ratio issues. We have proposed a new method of photonic-assisted ADC which has increased tolerance for component mismatches and a higher signal-to-noise ratio. As such it is uniquely poised to address the problems that previous optical ADCs have had with achieving high resolutions. BENEFIT: High resolution ADCs are routinely used at lower sampling rates. For instance, even the standard 802.11g Wi-Fi receivers on typical computers can have > 8 bits of resolution. Obtaining high resolution at high speeds could allow for many advances in communication systems. For example, software-defined radios could have a fully digital receiver which digitizes the carrier frequency directly rather than forcing a down-conversion operation. A fully digital radio is a key goal in the community, but is not practical at commonly used high carrier frequencies (>1 GHz). Maintaining high resolutions at high sampling rates will enable efficient bandwidth utilization and the compensation of various distortions such as those arising from multiple reflections at much higher carrier frequencies that are used in the super high frequency (SHF) band employed in various satellite communication systems. Additionally, great interest has recently been given to using fast ADCs in high speed optical communication systems. These ADCs are used to compensate for various distortions such as dispersion and usually only have a few bits of resolution. However, as data rates increase and more complex optical signaling become available (coherent systems), higher sampling rates and higher resolutions will be required. More bandwidth-efficient optical M-ary systems (as opposed to binary systems) can also be developed. In addition to communications, there are also applications in advanced instrumentation and radar systems. As a diagnostic and signal analysis tool, high speed and high resolution ADCs which bridge the gap between the analog and digital worlds are indispensable.