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We demonstrate a considerable enhancement of input power dynamic range (IPDR) for multiwavelength amplification and optical signal processing in a semiconductor optical amplifier (SOA) by using holding beam effect. The effect is used in SOA gain region at power level well below 10 dBm, which is much smaller than the required power as the setting at transparency wavelength. The IPDR is extended not only to high power range due to the reduction of nonlinear crosstalks, but also to low power range due to the compression of amplified spontaneous emission (ASE) noise. The technique is first applied to the amplification of wavelength-division multiplexed (WDM) channels using a continuous wave (CW) holding beam. The holding beam is particularly useful for the amplification of large number of channels thanks to the suppression effect of interactions among WDM channels. It is then carried out for one of extensively-used optical processing schemes, terahertz optical asymmetric demultiplexer (TOAD), for multiwavelength data format conversion without the need of an external CW light. The IPDR of TOAD reaches over 20 dB as control clock behaves simultaneously as the holding beam. We also present that the patterning effect and the impact of ASE noise on the converted signal are strongly dependent on the TOAD's switching window, which is optimized at 40% duty ratio for the largest IPDR.