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Photonic Analog-to-Digital Converters

Speed and resolution limits of all-electronic ADCs would be surpassed.

Early steps have been taken toward the development of analog-to-digital converters (ADCs) that would incorporate photonic quantizers based on the technology of InP semiconductors. These photonic ADCs are intended to overcome the sampling speed and temporal resolution limitations of state-of-the-art all-electronic ADCs, so that outputs of radar and other sensor systems at frequencies as high as tens of gigahertz could be sampled directly, without need for analog signal processing to effect down-conversion in frequency prior to sampling.

The Photonic Analog-to-Digital Converter incorporates a mode-locked laser, electro-optical modulator and a photonic quantizer.

The figure depicts the architecture of a basic photonic ADC as envisioned. The primary photonic subsystems would be the following:

  • A mode-locked laser, which would generate an optical pulse train to be used as a sampling source and clock signal;
  • An electro-optical modulator, which would amplitude-modulate a portion the laser pulse train power with the analog radio-frequency (RF) signal to be digitized; and
  • A quantizer.

Because the specifications of these components differ substantially from those of photonic components now commercially available, further development effort will be necessary to enable realization of a practical photonic ADC.

There are many potential alternative approaches to development of the quantizer. In the primary approach considered thus far, the quantizer would include a module containing passive semiconductor (e.g., InGaAs) saturable absorbers having saturation levels corresponding to the various quantization levels. The saturable-absorber module would be followed by an optoelectronic module, which would include photodiodes, photonic integrate-and-reset circuits, and electronic comparators. The outputs of the electronic comparators would be the desired electronic digital samples of the incoming analog RF signal, in a format suitable for further digital signal processing.

There are also potential alternative approaches to development of the electro-optical modulator: (1) one based on LiNbO3 Mach-Zehnder modulators (which are commercially available but may not satisfy performance requirements for original intended military applications), (2) one based on electro-absorption in such semiconductor material systems as InAlAs/InGaAlAs, and (3) one based on polymer modulators (the state of development of which is not mature as are the states of development of LiNbO3 and semiconductor devices). Part of the early development effort was focused on electro-absorption modulators (EAMs) for two main reasons: (1) they can be made smaller and lighter in weight (relative to LiNbO3 Mach-Zehnder modulators); and (2) because the EAM semiconductor materials are compatible with the semiconductor materials to be used for other components, it may become possible to integrate the EAMs the saturable absorbers, photodiodes, and associated electronic circuitry.

Another part of the early development effort was focused on a laser that is required to be environmentally stable and compact and to emit pulses characterized by timing jitter of less than 10 fs over a repetition- frequency range from 10 Hz to 5 MHz. One laser considered was based on a semiconductor laser diode placed within an external optical cavity. Another, taking advantage of experience with fiber lasers, was a coupled optoelectronic oscillator, the basic nature of which eliminated the need for an expensive, large, heavy RF source to provide mode-locking. Additional investigations addressed the use of high-concentration-Er-doped fibers and an Er-doped waveguide as gain media.

This work was done by Rebecca Bussjager, Michael Hayduk, Steven Johns, Michael Fanto, Reinhard Erdmann, Joseph Osman, John Malowicki, and David Winter 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-0026

Topics:
Radar/LiDAR Systems Architecture Electronic equipment Electronics Fibers Lasers Lasers & Laser Systems Optics Optics Photonics Polymers Polymers Radar Research and development Research and development Semiconductor devices Semiconductors Semiconductors & ICs Sensors Sensors and actuators
This Brief includes a Technical Support Package (TSP).
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Photonic Analog-to-Digital Converters

(reference AFRL-0026) is currently available for download from the TSP library.

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    Magazine cover
    Defense Tech Briefs Magazine

    This article first appeared in the June, 2007 issue of Defense Tech Briefs Magazine (Vol. 1 No. 3).

    Read more articles from the archives here.

    Overview

    The document is a technical report on the development of photonic analog-to-digital converters (ADCs) based on Indium Phosphide (InP) technology, produced by the Air Force Research Laboratory (AFRL). It outlines the historical context of electronic ADCs and presents a detailed exploration of various photonic ADC architectures pursued by the AFRL's in-house Photonic ADC team.

    The report begins with an introduction to the significance of ADCs in modern technology, particularly in applications requiring high-speed data conversion. It emphasizes the potential advantages of photonic ADCs, such as higher bandwidth and lower power consumption compared to traditional electronic counterparts.

    Key components essential for the development of photonic ADCs are discussed, including lasers, modulators, and quantizers. The report highlights the innovative approaches taken by the team, particularly focusing on the use of saturable absorbers in quantization processes. Several concepts and modifications of saturable absorber architectures are presented, along with their characterization and initial modeling efforts.

    The document also details the challenges faced during the development process, including issues related to phase noise and timing jitter, which are critical for the performance of photonic systems. Despite the ambitious goals set for the project, including a demonstration of a photonic ADC for the AFRL Monobit cueing receiver, the program was ultimately concluded before achieving this milestone.

    In addition to the technical details, the report summarizes the results obtained up to the program's termination, providing insights into the progress made and the lessons learned throughout the research period from February 1999 to September 2005. The conclusions drawn from the findings emphasize the importance of continued research in photonic technologies and their potential applications in various fields.

    Overall, this report serves as a comprehensive overview of the efforts to advance photonic ADC technology, showcasing the innovative work of the AFRL team and the potential future directions for research in this area. It is a valuable resource for understanding the state of photonic ADC development and its implications for future technological advancements.

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