A semiconductor laser amplifier (SLA) may be used as a nonlinear element in ultrafast optical asymmetric demultiplexers. For demultiplexing to take place in the optical domain it is necessary to create a switching window by placing the SLA asymmetrically in an optical loop and saturating it with a short duration control pulse. The exact size of this window, for selecting the required pulse at the output port, depends mainly on the precise location of the SLA within the loop. In the paper a finite length model of an SLA, used as a switching gate within the loop is presented and results for the carrier density, gain and phase time responses together with transmission (switching) windows are also given. The result obtained for the latter is compared with practical data.