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Microstructured optical fiber inline cavity designs are presented with lengths less than 60 μm, mode volumes less than 3 (λ0/n)3, and Q factors exceeding 3000. The device geometries are consistent with the fiber postprocessing capabilities of focused ion beam or femtosecond micromachining. The devices are based on introducing a longitudinally periodic hole array into a microstructured optical fiber. The micromachined fiber dispersion is calculated using the 3-D finite-different time-domain method. Bandgap frequencies, confined cavity mode frequencies, and quality factors are presented. Application of the device as a fast-response-time refractometer is explored, and sensitivities of 150 nm per refractive index unit are predicted.