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In this paper, we show the influence of intrachannel crosstalk in an optical switch network. The intrachannel crosstalk is mainly due to a signal that is not sufficiently blocked by the optical switch. The signal has a broadband spectrum and causes coherent crosstalk at the optical switching node. We investigate analytically the relationship between the channel isolation of the optical switching node that the optical switch network comprises and the crosstalk penalty due to the broadband signal. We confirm experimentally that the relationship is suited to the optical switch network that emulates an optical cross-connect system. We also show experimentally that it is important to use the optical switch with a high channel isolation level in the entire range of the signal bandwidth in order to suppress the impairment due to the crosstalk. We confirm that in a 50-GHz spaced system, a 44-Gb/s return-to-zero differential quadrature phase-shift-keying signal is sufficiently transmitted without a crosstalk penalty through 24 optical switching nodes, comprising wavelength selective filters with a high channel isolation level in the entire range of the signal bandwidth.