Results and Discussions It is found that the threshold pump power is 2.1 W, which is approximately 40% lower than that previously reported for a thulium-doped random fiber laser with the same pump wavelength and nearly the same lasing wavelength. At the threshold power, the laser output spectrum contains many stochastic narrowband wavelength components or spikes (Fig. 2). When the pump power slightly exceeds the threshold, a quasi-continuous wave is generated. Based on previous studies, the generation of stochastic spikes is related to the combined effects of distributed Rayleigh scattering and cascade-stimulated Brillouin scattering, while nonlinear spectral broadening arising from nonlinear interactions, such as frequency mixing and cross-phase modulation, can broaden and superimpose the spikes and further suppress the stimulated Brillouin scattering process. Mode competition and hopping are observed between two laser modes with different peak wavelengths of 1951.2 nm and 1951.4 nm when the pump power is approximately 3.5 W and 4.5 W, respectively (Fig. 3). The output power increases almost linearly with the pump power, with a slope efficiency of 3.9% (Fig. 4). When the pump power reaches 6.0 W, the laser output power is 142.9 mW, the optical-signal-to-noise ratio is up to 43 dB, and the wavelength and power fluctuations of the output laser within 1 h are less than 0.1 nm (Fig. 5) and 3.7 mW (Fig. 6), respectively, demonstrating good stability. The relatively low slope efficiency is predominantly caused by the high loss of the pump laser power from the output to the net injection into the thulium-doped fiber, of approximately 7.5 dB. The mismatching fiber connection introduces the high loss, and therefore, this problem may be resolved by customizing a WDM made of fibers whose parameters match those of fibers to be connected, i.e., the output fiber of the pump laser and the thulium-doped fiber. If the insertion loss is minimized, the slope efficiency of the laser may reach 20.6%, which is close to the slope efficiency of ordinary thulium-doped fiber lasers.

The 2 μm wavelength band is an eye-safe optical band that contains the absorption lines of water molecules and many atmospheric molecules as well as the spectrum windows for atmospheric communications. Therefore, fiber lasers in the 2-μm-band have been widely studied for their potential applications in fields such as lidar, laser ranging, and surgery. Currently, random fiber lasers (RFLs) have attracted extensive research attention owing to their simple structure and low spatial/temporal coherence properties. These fibers usually utilize back Rayleigh scattering as the random feedback mechanism, combined with the distributed Raman or Brillouin gain in the fiber, to generate random lasers in a long single-mode fiber. However, the Rayleigh scattering intensity of the single-mode fiber is inversely proportional to the fourth power of the wavelength, and the transmission loss is as high as 30 dB/km at 2 μm. Compared with the common RFLs operated at 1.0-1.5 μm, RFLs operated within the 2-μm-band are rarely reported, as they are highly difficult to realize. In this work, an RFL operated in a 2 μm region is demonstrated using a thulium-doped fiber as the gain medium and a random fiber grating for random distributed feedback with enhanced Rayleigh scattering efficiency. A laser output of wavelength 1951 nm is achieved with a relatively low pump threshold of 2.1 W.

The proposed RFL is formed using a 793 nm pump laser, 793 nm/2000 nm wavelength-division multiplexer, fiber loop mirror, 1.5-m-long thulium-doped fiber, and random fiber grating. The random fiber grating containing over six thousand index modulation points along a 10-cm-long single-mode fiber is inscribed using a femtosecond Ti:sapphire regenerative amplifier, which is operated at a wavelength of 800 nm with a repetition rate of 100 Hz and a pulse duration of 80 fs. The neighboring refractive index modulation points are spaced at random distances between 7.5 μm and 12.5 μm. The random fiber grating provides an enhanced backward Rayleigh scattering, equivalent to that achieved from a several-km-long optical fiber. The thulium-doped fiber provides strong amplification, and the fiber loop mirror helps to form a half-open laser cavity structure. Therefore, a low pump threshold is expected for the proposed RFL.

In this work, a thulium-doped fiber random laser operated at 1951 nm is demonstrated using a random fiber grating as the distributed random feedback medium. Owing to the enhanced Rayleigh scattering provided by the random fiber grating, a random fiber laser with a relatively low threshold power of 2.1 W is achieved, which is approximately 40% lower than that of the previously reported thulium-doped random fiber laser with the same pump wavelength. When the pump power reaches 6.0 W, the output power of the random laser reaches 142.9 mW, and the optical-signal-to-noise ratio is up to 43 dB. The stability of the laser output is relatively good, as the wavelength shift and power fluctuation are less than 0.1 nm and 3.7 mW, respectively, over the testing period of one hour.