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The characteristics of an optical signal passing though optical add/drop multiplexing (ADM) devices, which mainly consist of optical (de) multiplexers, are experimentally evaluated through 10 Gb/s/spl times/8-channel dispersion-managed wavelength-division multiplexing (WDM) transmission over 4000 km of dispersion-shifted fiber (DSF). A novel recirculating loop configuration is used to change the insertion positions and intervals of the ADM devices. Inserting a single ADM device near the receiver causes more electrical signal-to-noise ratio (SNR) degradation than if inserted near the transmitter. Periodically inserting ADM devices at small intervals into the transmission line also causes significant SNR degradation, The SNR degradation is sensitive to optical signal wavelength detuning. The resulting SNR degradation basically arises from the bandwidth limitations of the wavelength (de)multiplexers, and is exacerbated in long-distance transmission by the optical signal spectral broadening due to fiber nonlinearity.