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.

This block diagram shows the modification of a fiber-optic link with a Recirculating Delay Line to realizean optically assisted ADC.
Since it lacks a frequency selective element, the ring laser acts as a noise source and swamps any signal present in the loop unless lasing is prevented. This is accomplished by using gates in both the input and ring circuits. The RF-modulated laser signal is gated into the loop for a specified time period and then disabled. Disabling the input gate prevents the laser signal from continuing to enter the ring once the RF signal has terminated. The ASE noise from the loop amplifier would again tend to stimulate lasing unless this effect is accounted for. This is accomplished by ensuring that the total loop delay is greater than the duration of the RF signal circulating in the loop. A second gate then essentially introduces significant loss in the loop, thereby preventing lasing from occurring except for the time period required for the RFmodulated optical signal to pass through the loop gate. In this way, the loop gain is unity for a length of time, which is less than the total loop delay, so constructive interference cannot occur.

A concern involves the gain dynamics of the loop’s optical amplifier. With little or no light entering the amplifier when the loop gate is open, the turn-on time of the SOA when the light does enter will prevent the total required loop gain from being present when needed. This effect is mitigated by using distributed amplification in the loop. Two optical amplifiers were used in the loop, which allowed for a lower gain in each amplifier and thus a faster turn-on time. Future implementation of the system would perhaps use either more amplifiers or else a continuously distributed amplification scheme.

Upon exiting the loop via the 3-dB coupler, the signal is directed to a 20- GHz PIN photodetector, and the resulting periodic signal is viewed on a highspeed oscilloscope. Since the system was constructed with (connectorized) bulk optical components, this resulted in reduced SNR performance.

This work was done by Henry Zmuda of the University of Florida; Jared Pawloski of the State University of New York, Binghamton; Kristina Norelli of Syracuse University; and Michael Fanto and Thomas McEwen of the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at under the Photonics category. AFRL-0125

This Brief includes a Technical Support Package (TSP).
Photonic Recirculating Delay Line for Analog-to-Digital Conversion

(reference AFRL-0125) is currently available for download from the TSP library.

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This article first appeared in the December, 2009 issue of Defense Tech Briefs Magazine.

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