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For the alignment of ultradense wavelength-division-multiplexing (UD-WDM) channels, we propose to use the beat-frequency-locking (BFL) method that can fix the channel frequencies very accurately using the beat-frequency signals between UD-WDM channels. Every UD-WDM channel is classified as an ordinary channel or as a reference channel. The ordinary channels are stabilized with respect to the reference channels through the BFL method. As a demonstration, we align 12.5-GHz-spaced nine UD-WDM channels within ±200 MHz channel spacing errors.