The Physics of WGM Resonators

Whispering Gallery Mode Resonators (WGMRs) are a marvel of optical physics, enabling light to circulate within tiny, ultra-smooth structures for extended periods. Much like a whisper traveling around the dome of St. Paul’s Cathedral, light in a WGMR is confined to glide around the curved boundary by total internal reflection, creating circulating optical modes with extraordinary finesse.

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How Light is Trapped

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WGMRs are typically made from transparent, low-loss materials such as silica, calcium fluoride, or magnesium fluoride. When light enters the resonator at just the right angle, it becomes confined near the outer surface, guided by the curved interface between the resonator and the surrounding medium. This is possible thanks to the total internal reflection (TIR) phenomenon, which occurs when light within the resonator hits the boundary at a shallow angle, reflecting back into the resonator instead of escaping.

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The circulating light forms what are known as whispering gallery modes—intense electromagnetic fields that can orbit the resonator’s circumference hundreds of thousands of times before attenuating.

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Ultra-High Q Factors

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The quality factor (Q) of a WGMR quantifies how long light can circulate within it—essentially, how “clean” the resonator is in retaining light energy. High-Q WGMRs, like those made at OEwaves, can achieve Q-factors exceeding 10⁹, meaning the light can circle the resonator millions of times before dissipating.

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Three main factors enable these ultra-high Qs:

1. Material Purity: Crystalline materials like calcium fluoride exhibit low absorption losses.
2. Geometry: A smooth, curved shape ensures that bending and scattering losses are minimized.
3. Surface Perfection: Advanced fabrication techniques polish the resonator’s surface to nanometer-level smoothness, reducing scattering at the interface.

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OEwaves’ Unique WGMR Advantage

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OEwaves’ WGMRs stand out due to their advanced fabrication methods and choice of ultra-pure crystalline materials, resulting in devices with some of the world’s highest Q factors across an exceptionally broad wavelength range—from ultraviolet to mid-infrared. Unlike conventional resonators, OEwaves’ designs exploit custom crystalline geometries to tailor performance for demanding applications like microwave photonic signal generation and ultra-stable laser sources. This versatility, combined with the company’s proprietary self-injection locking techniques, allows OEwaves’ WGMR-based solutions to push the boundaries of spectral purity and stability, unlocking new possibilities in cutting-edge photonics.

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A Playground for Nonlinear Optics

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Because of their exceptional Q factors, WGMRs are hotspots for nonlinear optical phenomena. The intense circulating light can trigger:

• Four-wave mixing and Kerr frequency comb generation: Turning a single input frequency into a cascade of new frequencies.
• Brillouin and Raman lasing: Where interactions between photons and phonons (vibrations in the resonator material) generate new laser lines.
• Second-harmonic generation: Converting light to double its frequency for new wavelengths.

These nonlinear effects underpin cutting-edge applications in precision metrology, quantum optics, and telecommunications.

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Real-World Impact

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WGMRs are far more than lab curiosities—they’re foundational to advanced photonics technologies:

• Laser Stabilization: The ultra-narrow bandwidth of WGMR modes acts as a “mirror” for laser self-injection locking, producing lasers with linewidths below 1 Hz.
• Microwave Photonics: WGMRs enable RF signal processing in the optical domain, offering ultra-low noise performance for high-frequency communications and radar.
• Quantum Sensing: Their precise resonance properties are exploited in atomic clocks and other quantum devices for unparalleled accuracy.

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Pushing Boundaries in Research and Industry

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At OEwaves, WGMRs are at the heart of many of our photonic solutions, from low-noise RF photonic oscillators to ultra-narrow linewidth lasers. Their extreme stability and versatility are driving breakthroughs in quantum computing, LiDAR, and next-generation networks.

As the demands for precision, coherence, and miniaturization in photonics grow, the humble whispering gallery mode—born from an ancient architectural curiosity—remains a beacon of innovation at the frontier of science and technology.