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We introduce a class of highly compact refractometers integrated onto a planar microfluidic geometry that demonstrates high resolution refractive index measurements in 50μm fluid channels utilizing a Fabry–Perot cavity formed between resonant Bragg grating reflectors. This cavity forms a resonant peak in the transmission spectrum which is dependent upon the refractive index of the fluid in the microfluidic channel. We demonstrate this class of refractometer using optical fiber Bragg gratings; to provide high resolution, intracavity losses are minimized using integrated collimating optics. The refractometer can resolve refractive index changes of 0.2% and is simulated using coupled mode theory.