Several analog-to-digital converter (ADC) devices and part of a multispectral receiver have been designed, fabricated, and tested to demonstrate their feasibility as part of an effort to establish the organizational and technological foundation for development of photonic-crystal-based devices for commercial and military applications. Also known as photonic-band-gap devices, photonic crystals contain periodic structures having feature sizes in the submicron range — less than the wavelengths of light that the devices are intended to handle. Photonic crystals can be fabricated by techniques used in the integrated-circuit and microelectromechanical systems (MEMS) industries.

The periodic structures can be tailored to manipulate and control light in a wide variety of ways. In an important class of contemplated applications, photonic crystals of predominantly two-dimensional character would be used for in-plane distribution and routing of optical signals, while photonic crystals of three-dimensional character would be used for out-of-plane distribution and routing of optical signals. In both the two- and three-dimensional cases, the photonic crystals would make it possible to provide high-density optical interconnections among multiple photonic and/or optoelectronic circuits oriented at various angles to the photonic crystals.

Figure 1. A Photonic-Crystal-Based ADC would exploit the properties of beam splitters in conjunction with the nonlinear responses of photodetectors.
The basic principle of an ADC of the present type is illustrated in the upper part of Figure 1. A cascade of beam splitters at various positions along a waveguide apportions the power of an incoming optical signal (Pin) into multiple channels by specific fractions that correspond to a specific digital (typically, binary) representation of the signal power. In each channel, there is a photodetector. Relying on the nonlinear response of the photodetector, a signal is deemed to be present or not present in the channel depending on whether the signal power is above or below, respectively, a threshold power (Pth) for that photodetector. A relatively strong signal has enough power to be detected in all the channels along the cascade. As the signal strength is reduced, the power progressively falls below threshold at the most distant detectors. Thus, as the signal power decreases, the digital representation of the signal power changes as depicted on the right side of the upper part of Figure 1.

The lower part of Figure 1 depicts the layout of the waveguide-and-beam-splitting portion of a two-bit ADC of this type based on a two-dimensional photonic- crystal/waveguide structure. The splitting ratios shown in the figure are chosen in consideration of the nonlinear photodetector response. When Pin < 2Pth, the output powers at all three output ports are <Pth and, consequently, the quantized state of the photodetector outputs at the three output ports is hence defined as “000.” When the input power is slightly above 2Pth, the output power at port I only is slightly above threshold, making the quantized state “100.” Similarly, as Pin increases, causing the power levels at the other output ports to rise above Pth, their quantized states change from “0” to “1.” The net result is that as Pin increases past integer multiples of Pth, the quantized states exhibit the sequence “000,” “100,” “110,” and “111.” These four states can be coded into the states “00”, “01”, “10”, and “11” of a two-bit ADC.

the nonlinear responses of photodetectors.
Figure 2 illustrates the principle of operation of a photoniccrystal- based multispectral receiver, part of which has been built and tested. A multispectral infrared beam is launched into a two-dimensional main photonicband- gap waveguide that supports propagation at the wavelengths of interest. The waveguide is implemented as a periodic array of holes in a silicon-oninsulator chip. Resonators, implemented as localized defects in the photonic-crystal structure of the waveguide, are used for filtering light of individual wavelengths of interest into branch waveguides that, in a fully developed version, would lead to photodetectors. Proposed further developments include integration of a photonic- crystal-based multispectral receiver with ADCs and an optical switching network.

This work was done by Ahmed Sharkawy, Shouyuan Shi, and Caihua Chen of EM Photonics, Inc.; and Dennis Prather of the University of Delaware for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at under the Photonics category. AFRL-0007

This Brief includes a Technical Support Package (TSP).
Photonic-Crystal-Based Devices for Commercial Applications

(reference AFRL-0007) is currently available for download from the TSP library.

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This article first appeared in the June, 2007 issue of Defense Tech Briefs Magazine.

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