The key techniques for the realization of the femtosecond optical frequency comb are the detection of the carrier-envelop-offset frequency (f0) and the improvement of the signal to noise ratio (S/N) of f0. The influence of the length and the vibration of the fiber between the oscillator and the amplifier, and the pump power of the amplifier on the S/N of the f0 is experimentally studied. With optimized parameters, a f0 with 40 dB S/N is obtained in the Er-doped fiber femtosecond laser.