A looped waveguide that resonates only at wavelengths for which one round trip's optical length is an integer number of wavelengths. It is used as a wavelength filter or compact modulator. In silicon the resonance shifts by about 0.08 nm per °C, so heaters are often used to control its temperature.
Japanese name: リング共振器
Ring resonator appears in 3 practice questions across 1 topic, including Silicon photonics.
Field: Silicon photonics
Answer: C. At wavelengths where the optical path length of one round trip is an integer multiple of the wavelength, the circulating light interferes constructively
In a ring resonator, part of the light in an adjacent bus waveguide transfers into the ring and circulates many times. At wavelengths where the optical length of one round trip (effective index × circumference) is an integer multiple of the wavelength, the phase lines up on every pass and the light builds up (resonance), causing a sharp dip in the light passing through the bus waveguide at that wavelength only. This property is used for filters that extract specific wavelengths and for compact modulators that shift the resonant wavelength electrically. There are multiple discrete resonant wavelengths, and rings with radii from a few µm to several tens of µm are used.
Answer: A. The spacing between adjacent resonant wavelengths (FSR) becomes wider
The FSR (free spectral range) is the spacing between adjacent resonant wavelengths, approximately FSR ≈ λ² / (ng × L) (ng is the group index, L the circumference), so it is inversely proportional to the circumference. For example, with λ = 1.55 µm, ng ≈ 4.2 and L ≈ 63 µm (radius about 10 µm), the FSR is about 9 nm. Halving the circumference roughly doubles the FSR, making it easier in WDM, which handles many wavelengths, to keep one ring from also responding to another wavelength. However, sharper bends increase bending loss, so there is a limit to how small the ring can be.
In a ring resonator, part of the light in an adjacent bus waveguide transfers into the ring and circulates many times.
Which correctly explains why a silicon ring resonator respo…Silicon's refractive index changes strongly with temperature (thermo-optic coefficient about 1.8×10⁻⁴ /K), so the resonant wavelength of a C-band ring resonator shifts toward longer wavelengths by about 0.07–0.08 nm per 1 °C.
Why do optical circuits using ring resonators often place a…As an Amazon Associate I earn from qualifying purchases.
Waveguide WDM (wavelength division multiplexing) Refractive index C-band Chromatic dispersion CPO (co-packaged optics) Edge coupler Extinction ratio Grating coupler Mach-Zehnder modulator