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Optical Design & Engineering

Nanoscale all-optical analog-to-digital converter

Dispersively engineering photonic crystal structures provides tight control for new devices.
15 August 2007, SPIE Newsroom. DOI: 10.1117/2.1200708.0791

The emergence of high-speed parallel digital optical systems requires an all-optical analog-to-digital (A/D) converter. Optical or optoelectronic converters have received significant interest in areas such as telecommunication, sensors, and imaging. Optical A/D converters offer better performance than their electronic counterparts because optical signals are not subject to electronic noise and radiation, and are thus immune to electromagnetic interference. Moreover, all-optical converters eliminate the complexity and speed limitation of electrical-to-optical and optical-to-electrical conversions in photonic networks.

The all-optical A/D converter concept is illustrated in Figure 1. A cascaded array of beamsplitters and detectors splits the power of an incoming optical signal into multiple channels by specific splitting ratios that correspond to a digital representation of the signal. The splitting ratios are selected so that a strong signal will have enough energy for detection along the entire waveguide path. As the signal strength is reduced, the power in the output ports progressively becomes too weak for detection at the most distant detectors. Thus, the digital representation follows the characteristics shown on the right of Figure 1 from the maximum to minimum power level.

Photonic crystals are periodic dielectric structures that are designed to mainpulate the propagation of electromagnetic waves much like electrons in a semiconductor crystal. Similar to an electronic bandgap, these crystals have been shown to possess a photonic bandgap where light of certain wavelengths cannot propagate. They can also have unique dispersion characteristics that enable the structures to behave like a lens or super-prism.1,2 Our approach is to use our advanced hardware acceleration tools, developed in-house to engineer the dispersion characteristics of a photonic crystal structure for use in an all-optical analog to digital converter.

Figure 1. A schematic of the all-optical analog-to-digital converter concept.

Figure 2. Two-bit A/D converter consisting of three beam splitters in a self-guiding photonic crystal.

The A/D design relies on engineering the dispersive properties of photonic crystals instead of their confinement properties. It combines a self-collimation3 photonic crystal with a mirror structure to construct the elements of the beamsplitters. By engineering the transmission/reflection properties of the mirror, high-precision splitting ratios can be achieved and hence high resolution A/D designs can be implemented. Controlling the design parameters of the beamsplitting structure (mirror) controls the splitting ratio.

In order to construct a two-bit optical A/D converter, one needs four distinguishable states: this requires three splitters with splitting ratios of 50:50, 66:34, and 100:0, respectively. In our design, the splitters are realized in a self-guiding square lattice, as is shown in Figure 2. Each splitter is designed by changing the size of the air holes along a 45° line with respect to the propagation direction of the self-guided beam to break the lattice symmetry and thus perturb the beam. This results in an all-optical A/D converter that is compact and can be easily integrated with other photonic components.

Figure 3. Fabrication and characterization of a two-bit A/D converter: (a) the scanning-electron microscope photograph of the device, (b) the captured IR images of output ports at λ = 1560nm.

When the power of the incident wave is lower than twice the threshold value of the photodetectors, namely Pin < 2Pth, the output powers at the three output ports are smaller than the threshold level and the quantized states of three output ports yield ‘000’. When the input power is increased to 2Pth, but is less than 3Pth, the output power at port I reaches threshold, whereas the outputs at the remaining ports are still below threshold. Therefore, the state of the three-output device in this case can be recognized as 100. Similarly, as the output power at ports II and III reach the threshold value, their states change from 0 to 1. Consequently, we have four different states—000, 100, 110, and 111—at different levels of incident power. These can be coded into the binary representation as 00, 01, 10, and 11 for a two-bit optical A/D converter.

To experimentally validate the design concept of the two-bit optical A/D converter, a self-guiding photonic-crystal structure was fabricated in a silicon-on-insulator wafer, which has 260nm-thick silicon device layer on a 1μm-thick SiO2 insulating layer. E-beam lithography and inductively coupled plasma dry etching was used to pattern and transfer the structure to the silicon device layer. The SiO2 layer underneath was removed using buffered oxide etching. The self-guiding region of splitters consists of a square array of air holes. The radius of air holes is 220nm and the lattice constant a is 450nm. The splitters are diagonal rows of air holes with a different radius than the self-guiding region.4

To test these fabricated splitters, the sample was cleaved and light from a tunable laser was coupled into the input facet of a tapered dielectric waveguide by a tapered polarization-maintaining fiber. The waveguide is interfaced with the self-guiding lattice along the Γ-X1 (kx=0,ky=0)-X2 (kx=0.5,ky=0) direction. When the self-guiding beam propagates through the uniform lattice and arrives at the splitting structure, it splits into transmitted and reflected light. The splitting ratio varies with the size of air holes of the splitting structure. The diameter of the perturbed air holes in three splitting region are 370nm, 390nm, and 420nm, respectively. Figure 3 (b) shows the captured IR images of output ports at λ = 1560nm at different incident powers.

Figure 4 shows the dispersion properties of this lattice obtained using the iterative plane-wave method5, where (a) shows the dispersion surface and (b) shows the equi-frequency contours (EFCs)6 at frequencies (normalized to c/a, where c is the light speed and a is the lattice constant) of 0.28, 0.29, and 0.3. The plotted EFCs in (b) fall below the light cone and hence are confined to the slab region. Moreover, as can be seen from (b), the EFC at the frequency of 0.29 is approximately flat along the Γ-X direction except around corners. Since the relation between the group velocity vg and the dispersion function ω(k) can be expressed as: vg = ∇k ω (k), the group velocity, vg, or the direction of light propagation coincides with the direction of the steepest ascent of the dispersion surface, and is perpendicular to the EFC. To validate this, we launch a Gaussian beam into the lattice along the Γ-X direction and simulate the beam propagation within the lattice using the finite-difference time-domain (FDTD) method.7

Figure 4. Dispersion properties of the square lattice patterned in a silicon slab where air holes have radii of 0.25a and a slab thickness of 0.5333a: (a) the dispersion surface; (b) the equi-frequency contours of the second band.

To summarize, we have demonstrated an all-optical two-bit A/D converter by engineering the dispersive properties of photonic crystals. We then perturbed the uniform lattice to introduce a cascaded array of weighted taps. In this way, three splitters were constructed in cascade, which results in three unique outputs of the device. It should be noted that the design concept discussed could be extended to A/D converter with a higher number of bits: however, this requires finer adjustment to the splitting ratio of each splitter and thus demands more precise fabrication control.

This work was carried in collaboration between EM Photonics and the University of Delaware and was funded by Air Force Office of Scientific Research (Dr. G. Pomrenke, Program Manager).

Ahmed Sharkawy
Newark, DE

Ahmed Sharkawy is the director of photonic applications at EM Photonics. He holds five patents, and he has published more than 20 technical papers, four book chapters, and a book, all in the area of photonic crystals. During his PhD, he focused on the design and analysis of periodic photonic devices. His research intrrests include electromagnetic numerical modeling and simulation of nanophotonic devices, antennae, and metamaterials. He has also presented several oral/invited presentations at Photonics West.

Binglin Miao, Dennis Prather
University of Delaware
Newark, DE

Binglin Miao received his BS degree from the HuaZhong University of Science and Technology, China, in 1994. He is currently working toward his PhD in the Department of Electrical and Computer Engineering, University of Delaware. His research interests include micro- and nano-fabrication, micro-ring resonators and lasers, and photonic crystals.

Dennis Prather is a named professor at the University of Delaware. In 1999 he received both the National Science Foundation and Office of Naval Research Young Investigator Awards. He is currently pursuing research in the development of efficient electromagnetic models for both the analysis and synthesis of meso-scopic optical elements. He is also active in their fabrication, replication, and integration into hybrid optoelectronic systems.