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Micron-scale barcode particles enable labelling of small objects. Here, we demonstrate high-throughput barcode fabrication inside a microfluidic chip that can embed multiple, dye-doped high quality-factor whispering gallery mode cavities inside aqueous droplets at kilohertz rates. These droplets are then cured to form polyacrylamide hydrogel beads as small as 30 μm in diameter. Optical resonance spectra of the embedded cavities provide the hydrogels with unique barcodes with their diversity combinatorically scaled with the number of embedded cavities. Using 3 cavities per hydrogel, we obtain approximately one million uniquely identifiable, optically readable barcode microparticles.