- Parabolic Reflector Couples Between Fiber and Silicon-on-Insulator Waveguide
Simulation shows that coupling efficiency as high as 81 percent is possible.
Optical fibers and silicon-on-insulator waveguides both are crucial to today’s photonic systems, but coupling between the two is problematic. The small, tightly confined modal area in a silicon-on-insulator waveguide with high-index contrast typically is three orders of magnitude smaller than the modal area in the low-contrast fiber (i.e., 0.1 μm2 vs. 100 μm2), so simply jamming them together in a butt joint would introduce 30 dB of coupling loss. Recently, Thomas Dillon and his colleagues at the University of Delaware in Newark designed a unique coupling mechanism with the potential for much lower loss.
Figure 1. In the vertical-J coupler, the fiber fits into a snug hole fabricated in the silicon-on-insulator dielectric, and a parabolic reflector couples light from the fiber into the waveguide, or vice versa. Images reprinted with permission of Optics Letters.
They call the configuration a “vertical J” coupler because the vertical fiber and the parabolic mirror resemble the letter J (Figures 1 and 2). A parabola has the unique ability to focus a plane wave to a point, and in this case, the tiny parabolic reflector matches the large mode of the fiber to the much smaller mode of the waveguide. Because the coupler’s operating principle is total internal reflection rather than diffraction or resonance, it is relatively independent of wavelength and polarization.
Figure 2. This diagram’s color shows a computer simulation of light being coupled between the horizontal waveguide and the vertical fiber. If the light is coming from the fiber, the parabolic reflector focuses it to the optimal location for coupling into the waveguide, which is not necessarily in the center of the waveguide. The inset shows the mode in the waveguide.
The researchers created a finite-difference time-domain simulation of the coupler using the commercial program FastFDTD from EM Photonics, also of Newark. The result shows that coupling efficiencies between 81 and 61 percent can be achieved, depending on wavelength and polarization (Figure 3). The lower efficiency for the TE polarization (defined as having its electric field in the plane on Figure 2) probably results because that polarization propagates in the waveguide as a multiple peak optical mode (Figure 2, inset) and because that mode is more difficult to couple into.
Figure 3. A computer simulation shows that the vertical-J coupler is relatively insensitive to both wavelength and polarization (TM and TE).
The scientists also believe that the diminishing efficiency at longer wavelengths results because the longer wavelengths are focused to a larger spot size at the waveguide.
Conceptually, the vertical-J coupler offers several attractive features in addition to its coupling efficiency. It is compact and can be located anywhere on the dielectric chip, so it is well suited for massive parallelism. Moreover, because the fiber would fit snugly into a hole on the chip with no need for additional alignment, chip packaging is straightforward.
Figure 4. The scientists at the University of Delaware used gray-scale lithography to fabricate a prototype device.
Realistically, issues still remain with the laboratory demonstration of an operational device. Fabricating the three-dimensional structure pictured in Figure 1 is challenging, and the researchers have used gray-scale lithography to create a prototype (Figure 4). Although they believe they have made “significant progress” toward positioning a fiber into a hole in the chip as diagrammed in Figure 1, to date they have tested their prototype device simply by focusing laser light with a microscope objective up onto the parabolic reflector. By measuring the light in the waveguide, they calculated a coupling loss of 16 dB, but they expect that improvements in their gray-scale lithographic technique, together with mechanical advances in joining a fiber to the chip, will bring significant improvements in device performance.
Optics Letters, May 1, 2008, pp. 896-898.
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