In the future digital video transmission, a multichannel subcarrier multiplexed system with M-ary quadrature- amplitude-modulation (M-QAM) signals over fiber-optic transmission is an excellent candidate. A system using 1.3 micrometers semiconductor optical amplifiers (SOAs) for increasing the span length or for power splitting is very economical at the situations of the penetration of 1.3-micrometers single-mode fiber with the having installed 1.3-micrometers optical transmitters. However, the significant intermodulation distortions (IMDs) resulted from the signal-induced carrier density modulation of SOA limits the system capability, including channel capacity and power budget. A method using the external light injection to the SOA used in a 77-channel 64-QAM signal transmission system as an in-line amplifier is capable of increasing system power budget. From the viewpoint of system design, two approaches are addressed, one is the optimization of the in-line SOA used the external light injection, the other is the consideration of the optical modulation index per channel (OMI/ch) applied to transmitted. From the experimental results, the pumping wavelength and the pump power in approach one are mainly decided by the requirement of BER and the dynamic range of SOA, in addition, choosing the pump resulting in a smaller accompanied signal gain reduction can enlarge the system power budget. In approach, the optimum position of the in- line SOA incorporated is determined by the value of applied OMI/ch.