Fiber optic technology has revolutionized communications as it has enabled the flow of large data streams over long distances. All long-haul, metropolitan and an increasing fraction of local ground-based communications links are based on optical fibers. Their tremendous success has been largely driven by the extremely low-loss (< 0.2 dB/km at 1.55 micron) over very broad bandwidth of silica glass. The availability of low-cost, optical fiber has also created applications outside of communications such as fiber-optic gyros [1,2], high-performance electrical oscillators , continuum generation , and optical buffers . In each of these applications, the fiber is coiled-up into a small footprint. Such applications have fueled interest in chip-based waveguides with "fiber-like optical loss" as an alternative. Such a waveguide could be lithographically fabricated and even integrated with electronics into a robust solid-state device. However, high-performance chip-based waveguides exhibit optical loss in the range of 0.1 dB/cm or roughly 5 orders worse than for optical fiber. It is important to realize that this 5 order-of-magnitude factor appears in the exponent when computing the end-to-end power transmission through a waveguide!
In effect, one can view a whispering gallery resonator as a closed-loop waveguide. Therefore the ability to fabricate ultra-high-Q chip-based resonators enables the fabrication of ultra-low-loss waveguides. Our group has demonstrated a monolithic silica waveguide as long as 27 m with loss-rate of 0.1 dB/m and a 7m-long waveguide featuring record-loss rate values of 0.05 dB/m near 1.55 micron . The photograph in figure 1 shows a slicon wafer containing four, cascaded spiral waveguides having a combined length of 27 meters. The input and output to the waveguide are at the upper left and upper right of the image. The function of such a waveguide as an optical buffer was also confirmed. These waveguides are formed in a pair of interleaved spirals, providing input and output ports, connected at the center by an adiabatic coupler. The details of the design algorithm can be found in [7,8]. With design optimization, we envision that silica waveguides can be packed with higher density on a silicon chip.
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