To achieve high modulation efficiency and low RF threshold gain, direct modulated semiconductor lasers are very attractive. Sung et al. have proposed an OEO scheme based on directly modulated lasers under strong optical injection, in which a 7 dBm RF threshold gain is attained [13]. However, an EBPF with fixed central frequency is still used, which limits its tunability. Xiong et al. have demonstrated a simple OEO based on direct modulated DFB laser [14]. However, the tuning range is very limited due to the falling of the resonance peak when the bias current or the temperature increase. For an improved scheme presented in [15], the frequency tuning range is enhanced through additional optical injection, but another tunable laser source is needed to form the optical injection system, which results in a complicated system.