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We investigated experimentally the effects of optical injection locking on the performance of a directly modulated semiconductor distributed feedback laser, aiming for radio-over-fiber (RoF) applications. The resonance frequency and noise level are measured at various frequency detuning and external injection ratios. Under optical injection locking with a -2.9dB injection ratio and -12.5GHz detuning, the resonance frequency improved 2.81 times and noise level at the peak reduced by 3.5dB compared to a free running laser. We also achieved 5dB dynamic range enhancement with the same injection conditions. A resonance frequency improvement of 4.2 times was achieved with increased detuning. Performances of a ROF system were evaluated at RF frequencies of 2, 2.5 and 3GHz using a 64-QAM signal. From the error vector magnitude measurements, the optical injection locking can both enhance the linearity and reduce the noise for direct modulation, and thus extending the dynamic range of input RF signals.