1：Experimental Design and Method Selection:

The linewidth measurement system consists of an unbalanced Michelson interferometry (MI) with a 3 × 3 coupler, a 50 m long single mode fiber used as the fiber delay line, two Faraday rotator mirrors (FRMs), two photodetectors (PDs), and a data acquisition card (DAQ).

2：Sample Selection and Data Sources:

A high-performance SF thulium-doped fiber laser (TDFL) was proposed and employed for the demonstration.

3：List of Experimental Equipment and Materials:

3 × 3 coupler, 50 m long single mode fiber, FRMs, PDs, DAQ, TDFL.

4：Experimental Procedures and Operational Workflow:

The laser to be measured is connected to port 1 of the 3 × 3 coupler. The laser signal goes through the coupler and is transmitted to ports 4, 5 and

5：The lights of ports 4 and 6 reach FRMs after a certain light path. The two beams reflected by FRMs interfere again at the coupler and output from ports 2 and The lights emitted from ports 2 and 3 are received by two identical PDs, which then conducted photoelectric conversion. The converted electrical signals are collected by the DAQ and then put into the computer for processing and calculation. Data Analysis Methods:

The phase signal φ(t) can be obtained by integrating the differential phase fluctuation of the laser output in delay time τ. The power spectral density (PSD) of Δφ(t) can be estimated and the laser linewidth can be calculated according to the β-separation line theory.