Silicon Nanowires Make Ultratiny Directional Couplers
As several laboratories around the world pursue silicon lasers, other researchers are exploring a variety of silicon-based optical components. Directional couplers are the basic building blocks for a host of optical devices, from multiplexers to switches. Recently, scientists with NEC Corp. and the Optoelectronic Industry and Technology Development Association, both in Tsukuba, Japan, and with the University of Tokyo performed what they believe is the first investigation of silicon nanowire directional couplers.
Figure 1. The tiny, silicon-nanowire directional coupler (top) coupled virtually all the light input in the upper left port to the lower right port over a coupling distance (L) of about 8 µm. The cores of the silicon nanowires (bottom) were 0.3 µm square and were separated by 0.3 µm. Images ©2005 IEEE.
Silicon nanowire directional couplers can be fabricated on a much smaller scale than their conventional counterparts, which are made with glass optical fibers or with waveguides based on semiconductors, silica or lithium niobate. Conventional directional couplers typically have coupling lengths of hundreds of microns or even millimeters, but the coupling length of a silicon nanowire coupler may be 10 µm or less.
Figure 2. A calculation of the electric fields in the two arms of the coupler (top) shows optical power being transferred from the top arm to the bottom arm over a 10-µm distance. A plot of the intensity in each arm (bottom) quantifies the calculated transfer taking place. "Parallel" indicates the intensity in the upper arm, and "cross" indicates the intensity in the lower arm.
Moreover, because the difference in refractive index between the silicon core and the silica cladding is so great (3.5 and 1.5, respectively), the S-curves of a silicon nanowire waveguide can have a much smaller radius of curvature without introducing significant bending losses. Thus, the total length of the silicon nanowire coupler can be 50 µm or less versus several millimeters for a conventional coupler.
Figure 3. The experimental data differs from the calculation because the prediction did not take into account the coupling that occurs along the near-parallel portions of the S-curves.
The scientists fabricated the couplers from square-core waveguides usually less than 100 µm in length (Figure 1). They calculated the electric fields that would propagate in the coupler and concluded that, for the dimensions shown, the coupling length was 10 µm (Figure 2). To confirm this, they fabricated directional couplers of various lengths and measured the power emerging from the ports of each (Figure 3). The experimental data varied from the calculation somewhat because, in the experimental case when L = 0, the actual coupling length is not zero. Rather, the ends of the S-curves add an estimated ~2 µm to the effective length.
Figure 4. The wavelength-dependent coupling from one arm to the other in a 100-µm-long directional coupler shows a 20-dB extinction ratio. The ripple in the data may be due to the Fabry-Perot resonator formed by the coupler's uncoated facets.
To show that the couplers are good candidates for building multiplexers or interleavers, the investigators fabricated several longer devices and measured their coupling efficiency as a function of wavelength (Figure 4). They observed an extinction ratio of 20 dB. The device's insertion loss at 1550 nm was 15 dB, most of which may have been due to the coupling loss that is associated with the tapered fibers connected to the silicon nanowire waveguides. Even in a much longer (800 µm) coupler, the insertion loss was essentially the same as in shorter devices, indicating that the loss of the coupler itself is quite small.
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