We report our proof-of-principle experiment and modeling of hexagonal micro-pillar (µ-pillar) cavities. A commercial hexagonal silica fiber (125µm plane-to-plane) was side-coupled perpendicularly with a Gaussian beam, thus the fiber acted as a µ-pillar cavity. We observed multimode resonances with typical Q ≈ 2,500 in the tangential directions that are ≈ 120° to the input-coupling cavity sidewall. The observed free spectral range (FSR) ≈ 4.5 nm is consistent with a six-bounce cavity round-trip

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... ength. By using wavefront-matching concept, the observed multimode resonances can be attributed to open-loop trajectories. The multiple wavefront-matched open-loop trajectories of the same ray incident angle can result in resonance linewidth broadening. We employed a k-space representation to calculate the hexagonal cavity normal mode locations.