Search

Linewidth measurement based on phase noise analysis using a 3 × 3 coupler has been applied in the 1550 nm band. One of the obvious advantages of this method compared with the common delayed self-heterodyne method is the low loss caused by the short delay line used. Taking full advantage of this particular characteristic, the linewidth of the single frequency (SF) fiber laser in the 2 μm band can also be measured. However, a specifically experimental demonstration of this method in the 2 μm band is absent to date. In this paper, a high-performance SF thulium-doped fiber laser (TDFL) was proposed and employed for the demonstration. The TDFL operates at the center wavelength of 1942.03 nm with high stability. By means of the phase noise analysis-based linewidth measurement system with a fiber delay line of only 50 m, the linewidth of the proposed TDFL is successfully measured. When the measuring time is 0.001 s, the linewidth is ~47 kHz. Furthermore, the noise characteristics of the SF laser were simultaneously studied by this method.

A passively mode-locked thulium-doped fiber laser (TDFL) based on a nonlinear amplifying loop mirror (NALM) is presented. By adjusting the polarization controllers, stable noise-like (NL) mode-locked femtosecond pulse operation is obtained at the 2 μm band. In the experimental period of 200 min, the output power fluctuation is less than 0.06 dB and the 3 dB spectral bandwidth variation is less than 0.02 nm, indicating that the pulsed TDFL possesses good long-term stability. To the best of our knowledge, this is the first 2 μm band NALM-based TDFL with small net anomalous dispersion for a NL femtosecond pulse. At the maximum pump power of 3.52 W, the emitting laser has a NL pulse width of 460 fs, the repetition rate of 9.1 MHz, and the NL pulse energy of 32.72 nJ.