Fig. 1. (a) Cross-section of the dual-layer Si3N4 waveguide. (b) Electric field intensity distribution of the even and odd supermodes of the dual-layer Si3N4 waveguide. (c) Schematic of the spiral-shaped dual-layer Si3N4 waveguide interferometer. (d) Schematic structure of the interlayer Y junction. (e) Simulated insertion loss and power imbalance of the interlayer Y junction under various lengths.

Fig. 2. (a) Group index difference and temperature sensitivity of the dual-layer Si3N4 waveguide when the separation gap changes from 0.2 to 0.3 μm. (b) Group index and temperature sensitivity of the single-layer MZI. (c) Extinction ratio of the two interferometer structures as a function of the FSR.

Fig. 3. (a) Microscope image of the fabricated chip. (b) SEM image of the dual-layer waveguide cross-section. (c) Picture of the packaged chip.

Fig. 4. (a)–(c) Measured transmission spectra and (d)–(f) extracted central wavelength shift under various temperatures. (a) and (d) Dual-layer Si3N4 spiral waveguide; (b) and (e) MRR in the upper Si3N4 layer; and (c) and (f) MRR in the bottom Si3N4 layer.

Fig. 5. (a) and (b) Measured transmission spectra of (a) the shortest and (b) the longest dual-layer spiral waveguide interferometers of the FTS chip. The pink-shaded areas illustrate the wavelength range for spectral reconstruction. (c) Normalized calibration matrix obtained by a tunable laser.

Fig. 6. (a) Reconstructed spectra of the single narrowband laser source. (b)–(f) Measured and reconstructed spectra of the two narrowband laser sources with a wavelength spacing of (b) 0.2 nm, (c) 0.4 nm, (d) 0.6 nm, (e) 0.8 nm, and (f) 1 nm. PSD, power spectral density.

Fig. 7. Measured and reconstructed spectra of a broadband optical signal with the bandwidth increasing from 1 to 4 nm. PSD, power spectral density.

Fig. 8. Reconstructed spectra of (a) the single-line laser source and (b) the broadband optical signal under various temperatures.