An standard commercialized fiber-pigtailed distribution feedback (DFB) semiconductor laser without internal isolator is chosen as the slave laser to study the amplification properties of a semiconductor laser under optical injection. A signal consisting of a carrier and two second order sidebands is used as a probe signal, which is generated by a Mach-Zehneder modulator (MZM) biased at the maximum transmission point. The amplification properties of the slave DFB laser under different injection powers are measured by measuring the power of the resonantly amplified probe signal after photodetector. Relationship between the optimum detuning frequency at largest gain of -2nd order sideband and the gain of the carrier and the -2nd order sideband in different injection ratios is also investigated. Optical single sideband (SSB) signal with high sideband suppression (SSR) is achieved, and a microwave signal with doubled frequency of the local oscillator (LO) signal with a good signal to noise ratio (SNR) is generated after photodetector. It provides a new way to generate a high frequency microwave signal.