External-Cavity Semiconductor Diode Ring Laser For Application In Hybrid Optoelectronic Analog-To-Digital Converter

Current state-of-the-art electronic analog-to-digital converters (ADCs) operating at multigigahertz sampling frequencies are known to exhibit fairly limited resolution (∼4 bits at room temperature and ∼6 bits when superconducting). These high-frequency restrictions stem primarily from the response time of the constituent transistors that make up the ADC\u27s comparators (also known as comparator ambiguity). In an effort to improve the resolution of ADCs operating at ultrahigh sampling frequencies, several areas of investigation are currently underway regarding the capabilities of hybrid optoelectronic systems. High-power optical pulses can be used as sampling windows and high-bandwidth electro-optic modulators as voltage-to-intensity transducers to provide a means for digitizing ultrafast voltage waveforms with much greater accuracy than conventional (purely electronic) ADCs. When optical sampling is employed, the primary limiting factors determining ADC conversion accuracy (now that comparator ambiguity is no longer an issue) become the noise in the sampling pulsetrain and the extent of the sampling time (optical pulsewidth). Detrimental pulsetrain noise is associated with either phase modulation (PM noise) or amplitude modulation (AM noise), and recent measurements of AM and residual PM noise on our 10 Ghz ring laser show the best results to date for an actively-modelocked semiconductor diode system. Carrier offset integration bands extending from 10 Hz to 10 MHz exhibit RMS levels of AM and PM noise as low as 0.12% and 43 fs, respectively. In addition, linear dispersion compensation has successfully reduced the optical pulsewidth from 13 ps to 1.2 ps. Based on these experimental numbers, this laser could form the front end for an optoelectronic ADC capable of a theoretical resolution as high as 8.6 bits (10 Ghz sample rate)