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We theoretically and experimentally demonstrate a novel scheme of bandwidth-variable ultra-wideband (UWB) monocycle generation, using a semiconductor-optical-amplifier (SOA)-based nonlinear optical loop mirror (NOLM). In our scheme, the probe light is split into two parts by the NOLM, and both of them are phase modulated by the pump light due to the cross-phase modulation in the SOA. An extra phase difference between these two parts is also introduced by tuning the polarization controller within the loop. When both parts interfere at the NOLM output port, the phase modulation is converted to the intensity modulation with a UWB monocycle waveform. In addition, the bandwidth of the UWB spectrum is variable by tuning the optical delay line within the loop.