Experimental investigation on linewidth characteristics of a single-frequency phosphate fiber laser at 1.0 mu m

We demonstrate an experimental investigation on the linewidth of a single-frequency Yb-doped phosphate fiber laser through a loss-compensated recirculating delayed self-heterodyne (LC-RDSH) method at 1.0 mu m. A resolution of 3 kHz is achieved by only using a 6 km fiber delay line in the linewidth measurement system. With optimal parameters of the LC-RDSH system, more than 16 orders of beat note are detected. By analyzing the behaviors of different orders of beat note, a broadening effect on the linewidth is observed with an increasing pump strength of the under-tested fiber laser. Associated with theoretical analysis, the frequency noise incorporated with the linewidth broadening is shown to be the attribution of the Lorentzian noise. Frequency noise of the laser is also measured, to verify the observed linewidth characteristics. To the best of our knowledge, this is the first time that the LC-RDSH has been applied to the 1.0 mu m window.