Wide-band electrooptic guided-wave analog-to-digital converters

Progress in the development of high-speed electrooptic A/D converters is reviewed. A/D converters of this type have been operated at 1 gigasample/second (GS/s) in 2- and 4-bit structures for 500-MHz analog bandwidth. The converter consists of an array of LiNbO3guided-wave interferometric modulators that function as an analog amplitude analyzer, pulsed lasers for optical sampling, and high-speed monolithic comparators/demultiplexers to generate the digital levels and slow the data to ECL-compatible rates. The operational principles of the converter are summarized and a performance analysis presented. The analysis indicates that with currently attainable components, conversion in the 4- to 6-bit range at rates from 1 to 3 GS/s is feasible. Experimental results for several converters are summarized, including a description of beatfrequency tests for analog signals with frequency content up to 500 MHz that indicate the analog bandwidth capabilities of this device. The electrooptical technology is compared to competing high-speed A/D technologies in Si, GaAs, and superconducting materials and the relative merits analyzed. It is found that the electrooptic approach eliminates some of the fundamental and severe problems of conventional converters (e.g., sampling pickup and large numbers of comparators). Finally, application of this converter to wide-band signal-processing problems is described. It is noted that there are numerous applications where a moderate number of bits at a high (gigahertz) sampling rate is attractive.