**Abstract**

A 120° phase difference interferometer technology based on an unbalanced Michelson interferometer composed of a 3 × 3 optical fiber coupler is proposed, and based on this technology, the differential phase of the input laser can be obtained. This technology has many applications. This paper introduced its application in laser phase and frequency noise measurement in detail. The relations and differences of the power spectral density (PSD) of differential phase and frequency fluctuation, instantaneous phase and frequency fluctuation, phase noise, and linewidth are derived strictly and discussed carefully. The noise features of some narrow-linewidth lasers are also obtained conveniently without any specific assumptions or noise models. Finally, the application of this technology in the phase-sensitive optical time domain reflectometer (ϕ-OTDR) is also introduced briefly.

**Keywords:** interferometer, 120° phase difference, 3 × 3 optical fiber coupler, laser noise measurement, phase-sensitive optical time domain reflectometer
