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We present a novel post-fabrication laser trimming technique to adjust the power splitting ratio of rib waveguide Y-junctions made in thermally crosslinked electrooptic polymers. The trimming is based on the index change due to photobleaching, and it takes only a few seconds. An adjustable range of the splitting ratio as wide as /spl plusmn/20% is achieved with less than 0.2 dB of excess loss. This in situ trimming technique is effective for both the TE and TM modes of the waveguide and is very suitable for automated device processing.