The optical-phase-locked-loop (OPLL) at 9.2 GHz between two independent narrow linewidth diode lasers is realized. Ultrabroad servo bandwidth at 4 MHz is first achieved and it is guaranteed that the full spectral characteristics of the master laser can be transferred to the slave laser. The experimental results prove that the coherence between two lasers is about 99%. This offers a new method to study the interaction between lasers and atoms based on the ground hyperfine structure of caesium atoms.