Fig. 2: Characertization of losses in ultra low-loss waveguide.

a Photograph of an ULLW spiral with 1 m length under test. Scale bar: 5 mm. b Schematic of the Archimedean spirals used for loss measurement, composed of inward (green) and outward (blue) spirals, connected by an S-bend (red). In the measurement, laser light is injected into the spiral, and guided back-scattered photons originating along the spiral, collected from the spiral input, are detected in time-domain with a resolution of ≈ 200 ps. c Radius of curvature (RoC) as a function of length for the measured 1 m, 2 m, and 3 m spirals. \({R}_{\min }\) and R_S respectively mark the minimum spiral and S-bend radii. We note the large RoC discontinuity at the S-bend. d Back-scattered light intensity as a function of propagation length and RoC along the 1 m, 2 m, and 3 m spirals, relative to the intensity at the start of each spiral (dots: data; red lines: fits). In each panel, the top and bottom horizontal axes are, respectively, the RoC and length along the corresponding spiral. The spiral length uncertainty is < 1 mm, as described in the Supplementary Note 3.