We design an optical phase locked loop (OPLL) whose bandwidth is 1~7 GHz with ADF4107. Through a fast photo-detector, we can obtain a beat signal of two external cavity diode lasers. Then, two kinds of error signal are generated while passing OPLL, which are respectively loaded into the grating piezolectric ceramics and feedback current of semiconductor lasers to reduce the noise of the beat signal from MHz to Hz. The system is successfully used in electromagnetically induced transparency (EIT) spectral experiment. This method can be also applied to the Raman spectroscopy experiments of ultra-cold atoms.