Laser at 2.15 μm band has important applications in many fields. For example, it is located in the transmission window of the atmosphere and has remarkable transmittance in smoke, which makes it more reliable for data transmission in some special contexts. It has also realized the measurement of factors in the atmospheric environment, such as wind velocity, carbon dioxide, and water vapor. Lasers around 2.15 μm band are ideal pump sources for generating mid-infrared laser through nonlinear frequency conversion. Fiber lasers have great advantages in beam quality, stability, and portability. At present, most fiber laser at 2 μm band is generated by holmium-doped or thulium-doped fiber lasers. However, on one hand, due to the enormous decreases in the absorption and emission cross sections of Ho3+ and Tm3+ beyond 2.1 μm, the wavelength is difficult to expand to over 2.1 μm, and the power scaling is also restricted by insufficient gain. On the other hand, various nonlinear effects, such as self-phase modulation and stimulated Raman scattering (SRS), will lead to spectral broadening under high pump power, which restricts the applications of the lasers. Gas SRS in hollow-core fibers (HCFs) has been proved to be an effective method for expanding laser wavelengths. HCFs can provide an ideal environment for the interaction between the gas and the pump laser. Gas SRS in HCFs combines the advantages of traditional fiber lasers and gas lasers, such as high beam quality, high damage thresholds, narrow linewidths, and convenient heat management. In this paper, we report an all-fiber gas Raman laser at 2.15 μm band through fusion splicing between a solid-core fiber and an HCF. This paper provides a novel method to generate fiber lasers beyond 2.1 μm.
The 2.15 μm fiber gas Raman laser is realized based on the pure rotational SRS of deuterium filled in a 25.8 m long HCF. The 1971 nm pump laser is directly coupled into the HCF through the splicing between the solid-core fiber and the HCF. The output end of the HCF is sealed in a gas cell when it is filled with deuterium. After the internal gas pressure becomes balanced, the HCF is quickly withdrawn and spliced to the solid-core fiber, which forms an all-fiber gas cell. The Raman laser caused by the Fresnel reflection of the splice is well prevented from inducing damage to the pump source by introducing a self-written long-period fiber grating (LPG) in the solid-core fiber.
The self-written LPG functions well as an isolator of the Raman laser caused by Fresnel reflection, and the depth at 2147.1 nm is around -8 dBm [Fig. 1(c)]. This means most part of Fresnel reflection is attenuated, and thus the pump source can be protected from being damaged by the Raman laser. The splice loss at the input end and the output end is ~1.42 dB and ~1.00 dB, respectively. The gas pressure in the all-fiber gas cell is calculated to be around 1.4 GPa. Two first-order Raman lines are obtained in the experiment (Fig. 3). The 2147.1 nm Raman line is first generated because its threshold is lower than that of the 2043.6 nm Raman line. As the coupled pump power increases, part of the pump power is converted to 2043.6 nm Raman laser. With the increase in the pump repetition frequency, the pump laser has lower peak power (inversely proportional to the repetition frequency). Therefore, higher average pump power is necessary to exceed the Raman threshold, and more pump power is needed to generate Raman laser with the repetition frequency increasing [Fig. 4(a)]. Although the peak power is lower at the repetition frequency of 1.5 MHz and 2.0 MHz, the available pump power is much higher than that at 0.5 MHz and 1.0 MHz. Thus, the Raman power at these two repetition frequencies (1.5 MHz and 2.0 MHz) is much higher than that at the other two repetition frequencies. The power characteristics under different pumping pulse widths are also studied in detail (Fig. 5). Finally, a maximum Raman power of ~0.87 W at 2.15 μm is obtained [Fig. 4(a)] with a corresponding optical-to-optical conversion efficiency of 19% restricted by a high Raman threshold. The Raman power and conversion efficiency can be enhanced by optimizing the splice loss and fiber length of the HCF.
In this paper, we report a 2.15 μm all-fiber gas Raman laser. The all-fiber gas cell is obtained through the fusion between a single-mode solid-core fiber and a hollow-core photonic crystal fiber, and the Raman laser caused by the Fresnel reflection of the splice is well prevented from inducing damage to the pump source by introducing a self-written LPG in the solid-core fiber. Pumped by a 1971 nm pulse fiber amplifier, a maximum Raman power of ~0.87 W at 2147.1 nm is obtained with a corresponding optical-to-optical conversion efficiency of 19% restricted by a high Raman threshold. This paper provides a novel method to realize a fiber laser source beyond 2.1 μm.
