The limitations imposed on frequency stability of synthesized optical signal by intrinsic fluctuations of the femtosecond laser as well as the fluctuations in the optical readout system were studied. A conventional two-oscillator measurement technique was used to accomplish the noise measurements for studying the effects of noise sources on pulse repetition rate of a free-running femtosecond laser. The results showed an additional attenuation of 10.5 dB attenuation of the shot noise at 950 MHz and reduced the discrepancy level to 2.5 dB.