ULL Spirals

An equivalent way to think about UHQ Resonators is as closed-loop ultra-low-loss (ULL) waveguides. The techniques that we have developed to fabricate record, high-Q resonators on a chip have also been used to make the longest and lowest loss waveguides ever developed [1]. The basic structure and properties of these waveguides is reviewed in this section and then in the sections on Continuum Generation and Reference Resonators two applications of the waveguides are described.

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 [3], continuum generation [4], and optical buffers [5]. 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!

Figure 1: 27 m long ULL waveguide on a silicon chip.

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 [6]. 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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T. Chen, H. Lee, and K. J. Vahala, "Design and characterization of whispering-gallery spiral waveguides," Opt. Express 22, 5196 (2014)

Ultra-low-loss(ULL) Spirals