Since the 1990s, fiber lasers have been widely used in the medical, communications, national defense, and industrial processing fields because of their all-solid state, high efficiency, high reliability, and high beam quality. At present, in the 1 µm band, the output power of a single-link ytterbium-doped continuous fiber laser exceeds 20 kW, and there is a theoretical limit to further increases in this power. Most studies on high-power fiber lasers have focused on randomly polarized ytterbium-doped fiber lasers. However, the development of long-distance laser communication, coherent detection, high-power beam coherent synthesis, and other applications has higher requirements for laser power, linewidth, polarization state, and other properties. Obtaining a high-power, narrow-linewidth, and high polarization extinction ratio fiber laser output has become the focus and research direction in the field of high-power fiber lasers. The most important part of a high-power narrow-linewidth linearly polarized fiber laser is the ytterbium-doped polarization-maintaining fiber, which provides gain. In this study, based on a modified chemical vapor deposition (MCVD) process combined with solution doping technology, a ytterbium-doped polarization-maintaining fiber is successfully fabricated. The polarization extinction ratio (PER) is determined by using an extinction ratio tester. An all-fiber oscillator structure test platform is built, and the performance, polarization characteristics, and linewidth performance of the ytterbium-doped polarization-maintaining fiber lasers are studied.

In this study, a 11 µm/125 µm ytterbium-doped polarization-maintaining fiber (PM-YDF ) was fabricated using the MCVD process combined with solution doping technology. Using existing MCVD machine tool equipment in the laboratory, the preform was prepared after etching, deposition, liquid-phase doping, drying, vitrification, collapse, and burning. Two holes were drilled symmetrically along the fiber core in the cladding region of the preform, and a boron-doped low-refractive-index quartz rod was placed in the holes. Finally, at a temperature of 1980 ℃, the preform assembly was drawn into a fiber with the target size, coated, and UV-cured. The refractive index profile (RIP) of the fiber was analyzed and the absorption coefficient of the fiber was test. A birefringence coefficient measurement system was built, and hybrid fiber Sagnac interferometry was used to measure the birefringence coefficient of the polarization-maintaining fibers. An all-fiber oscillator structure test platform was built to evaluate the laser performance of the fiber.

The experimentally prepared 11 µm/125 µm PM-YDF is shown in Fig. 1, in which Fig. 1(a) shows the microscope cross-section of the fiber, and Fig. 1(b) shows the refractive index cross-section of the preform. It can be observed that the core ellipticity of the fiber is very low. The optical fiber cladding absorption coefficient is measured by the truncation method, and the cladding absorption coefficients of 11 µm /125 µm PM-YDF at 915 nm and 976 nm are 2.48 dB/m and 7.05 dB/m, respectively. The spectrum of the fiber beat length is shown in Fig. 3. After calculation, the birefringence coefficient value of the fiber to be tested, 11 µm/125 µm PM-YDF, is 3.0×10-4, with good performance. Simultaneously, the polarization-maintaining (PM) fiber is measured using a polarization extinction ratio tester. Linearly polarized light with a PER>25 dB and a wavelength of 1310 nm is passed into the 11 µm/125 µm PM-YDF sample with length of >2.25 m, and the PER measured by the PER tester is >18 dB, which confirms that 11 µm/125 µm PM-YDF can meet the needs of practical applications. An oscillator structure test platform is constructed. When the active fiber is 11 µm/125 µm PM-YDF , the output power of the laser varies with the pump power, as shown in Fig. 5(a). As the pump power increases, the output power also increases. The power tends to increase linearly, and there is no power jitter. When the pump power is 57 W, the output laser power is 48.9 W, which represents the maximum output power obtained. Further increase in the output power is limited by the pump power, and the linear fitting efficiency is 85.5%. Figure 5(c) shows the output spectrum of the fiber laser with a laser power of 12 W. The spectral shape is the Lorentzian type with a single peak. The full width at half maximum is 0.0466 nm. The frequency noise is well-suppressed, no spurious peaks appear, and the polarization extinction ratio at this power is >18 dB.

A domestically produced 11 µm/125 µm PM-YDF is successfully fabricated using an MCVD process combined with solution doping technology. It has a birefringence value of 3.0×10^-4 and maintains a degree of polarization greater than 18 dB over a fiber length of >2.25 m. An all-fiber oscillator structure test platform is built to achieve a laser output of 48.9 W at 1064 nm, and the slope efficiency is as high as 85.5%. The high laser performance of the PM fiber is verified, laying a solid foundation for the localization of high-performance PM fibers.