Photonic Recirculating Delay Line for Analog-to-Digital Conversion

Aconventional analog fiber-optic link can be augmented with a recirculating optical delay loop so as to realize an optically assisted analogto- digital converter (ADC) that provides improved performance in terms of both speed and resolution using one (slower) electronic ADC (see figure). The overall architecture readily integrates with any electronic ADC system. Moreover, the highspeed ADC performance is fundamentally limited by the performance of the fiber-optic link. The system was constructed on an optical bench. A 1,550-nm, 50-mw diode laser was used as the optical source. The link was modulated using an 18 GHz - LiNbO3 Mach- Zehnder modulator electrically driven with a 1-GHz tone burst. The RFmodulated optical signal was injected into the recirculating delay loop via 3-dB coupler. A loop time delay of roughly 100 nanoseconds was achieved using approximately 22 meters of single mode fiber with fine time delay adjustment (+3 nanoseconds) obtained from a variable delay line. The fundamental ring architecture with unity gain is essentially a laser.