**5. Conclusion**

A laser phase and frequency noise measurement technique based on a 120° phase difference unbalanced Michelson interferometer composed of a 3 × 3 optical fiber coupler and two Faraday rotator mirrors is proposed. In the method, the laser differential phase fluctuation accumulated by the interferometer delay time is demodulated directly at first and then the phase and frequency noise is calculated by the PSD estimation for the differential phase. Also the concepts and differences of differential phase and frequency fluctuation PSDs, instantaneous phase and frequency fluctuation PSDs, and phase noise are defined strictly and discussed carefully. The method can obtain the noise features of a narrow linewidth laser without any specific assumptions or noise models. Meantime, the technique is used to characterize a narrow linewidth external-cavity semiconductor laser, which confirmed the correction of the method and revealed the fact that the linewidth would increase with the increase of observation time, and the Lorentz fitted linewidth measured by the SDH method only includes the contribution of the white noise components and would be larger than the real value. Moreover, the technique can monitor the state change of commercial DFB semiconductor lasers in different processes of designing, installation, debugging, routine test, and final check test, and it offers suggestions to optimize design and improve its performance. The 120° phase difference technology is also applied to test the phase difference between Rayleigh scattering in phase sensitive optical time domain reflectometer (*ϕ*-OTDR).
