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Photonic Recirculating Delay Line for Analog-to-Digital Conversion

This approach modifies an analog fiber-optic link with a recirculating optical loop.

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 www.defensetechbriefs.com/tsp  under the Photonics category. AFRL-0125

Topics:
Amplifiers Architecture Education and training Electronic equipment Lasers Optics Oscilloscopes People and personalities Photonics Research and development
This Brief includes a Technical Support Package (TSP).
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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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    Defense Tech Briefs Magazine

    This article first appeared in the December, 2009 issue of Defense Tech Briefs Magazine (Vol. 3 No. 6).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document presents a conference paper detailing a novel approach to high-speed, high-resolution Analog-to-Digital Conversion (ADC) using a photonic recirculating delay line. The authors, Henry Zmuda, Michael Fanto, Thomas McEwen, Jared Pawloski, and Kristina Norelli, explore how this technology modifies an analog fiber optic link to enable the digitization of multi-gigahertz microwave signals with a resolution exceeding 10 effective bits.

    The core concept involves utilizing a recirculating optical loop to store time-limited microwave signals, which allows for their subsequent digitization by slower, conventional electronic ADCs. This method addresses the challenges associated with high-speed signal processing, particularly the limitations imposed by the effective number of bits (ENOB) and the dynamic range of the system. The paper emphasizes that the performance of ADCs is not solely determined by conversion speed but also significantly influenced by factors such as signal-to-noise ratio (SNR) and intermodulation products introduced by modulator nonlinearities.

    Key findings include the establishment of a relationship between SNR and ENOB, where the SNR is derived from the ratio of the full-scale voltage to the noise voltage. The authors provide analytical insights into the dynamic range and noise figure, demonstrating that under optimal conditions, the degradation of microwave signals is minimal, thus facilitating effective digitization.

    The paper also discusses the technical requirements for achieving high ENOB, including the need for precise timing accuracy and low aperture uncertainty. For instance, a 10-bit, 10 Giga-sample per second ADC necessitates an aperture uncertainty of approximately 65 femtoseconds, a challenging requirement that can be met using advanced clocking systems like mode-locked lasers.

    Experimental results are presented to support the theoretical analysis, showcasing the practical viability of the proposed system. The findings indicate that the photonic recirculating delay line can effectively enhance the performance of ADCs in high-speed applications, making it a promising solution for future developments in microwave photonics and optical signal processing.

    In summary, this document outlines a significant advancement in ADC technology, highlighting the potential of photonic systems to overcome traditional limitations and improve the resolution and speed of signal digitization.

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