Raman Soliton Generation in Holmium All-Fiber MOPA

Mode-locked fiber lasers emitting in the wavelength range beyond 2 μm [1] are promising for a number of applications including environmental sensing, material processing, medicine etc. Mode-locked holmium fiber lasers are long-wavelength-limited at 2138 nm [2]. For some applications it is important to shift the laser wavelength further to the infrared. Wavelength of 2.8 μm is achievable with Er:fluoride fiber laser [3], but the spectral region between 2.14 μm and 2.8 μm forms a gap, where mode-locked fiber lasers does not exist. This gap can be bridged using the effect of a soliton self-frequency shift. All-fiber Raman soliton laser systems have been reported with solitons initially generated around 1.5 μm in erbium-doped fiber, or around 1.9 μm in thulium-doped fiber [4]. Here we report for the first time to our knowledge the Raman soliton generation starting at 2.09 μm in a holmium all-fiber MOPA system. The additional advantage of the reported system is the all-fiber format and only single-mode diode pumping.