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Crosstalk in a balanced bridge interferometric-type optical switch is highly suppressed by using Mach-Zehnder (MZ) structures as intensity trimmers. An imbalance in optical power is caused by deviations from the ideal dividing ratio of directional couplers in the switch, which causes an increase in the crosstalk. This imbalance was compensated for by adjusting the transmittances of the nested MZ structures. A crosstalk of less than 55 dB has been achieved even for alternating voltage signals. It is also shown that the crosstalk reduction is possible for a control voltage signal of more than 10 GHz, which is based on double-sideband suppressed-carrier modulation spectra measurement results.