Since its first report in 2010, transverse mode instability (TMI) remains one of the primary limiting factors in the power scaling of high-power fiber lasers. Fiber bending presents a straightforward and effective TMI suppression technique based on mode control. Typically, a negative correlation exists between the TMI threshold and fiber bending diameter. This implies that decreasing the bending diameter increases the TMI threshold. In near-single-mode fiber laser systems, the laser's output power usually increases by reducing the bending diameter to suppress TMI. However, to counteract nonlinear effects exemplified by the SRS and enhance the pumping capacity, fibers with larger mode fields and cladding diameters gain widespread use in high-power fiber lasers. Throughout this transition, an unusual TMI phenomenon is observed where the threshold power increases with the fiber bending diameter's increase. In this scenario, the beam quality of the output laser can be sacrificed to a certain extent in favor of power enhancement, thereby boosting brightness. In this study, an experimental investigation is conducted on the abnormal TMI effect in high-power fiber lasers with respect to varying fiber bending diameters. It is expected that these findings will pave new paths for the evolution of high-brightness fiber lasers.
In this study, we designed a high-power fiber laser amplifier based on a double cladding ytterbium doped fiber with a core/cladding diameter of 30/600 μm, combined with water-cooled columns with cylindrical fiber grooves of different diameters. The nominal absorption coefficient of this fiber was 0.40 dB/m@915 nm. In the experiment, we used water-cooled columns with diameters of 13, 14, 15, and 16 cm to compare the output characteristics of the lasers with different fiber bending diameters. First, under the condition of a fiber length of 20 m, the TMI threshold of the laser with different bending diameters was examined using a wavelength-stabilized 976 nm LDs as the pump source, and the abnormal TMI phenomenon was examined. Subsequently, the fiber length was increased to 45 m by combining the pump wavelength optimization and abnormal TMI suppression to realize a high-brightness fiber laser amplifier.
When the fiber length is 20 m and wavelength stabilized 976 nm LDs serve as the pump source, the TMI thresholds of the laser are 1650, 2839, 3182, and 3740 W as the fiber bending diameter gradually increases from 13 to 16 cm (Fig. 4). The corresponding calculated relative brightness values of the output laser are 519, 631, 751, and 970, respectively (Fig. 4). As the fiber bending diameter increases, even though this might lead to a slight degradation in the beam quality, the TMI threshold of the laser significantly rises, and the progressively increasing maximum output power results in an increase in the relative brightness of the output laser. After optimizing the fiber length and pump wavelength, as the bending diameter gradually increases from 13 to 15 cm, the TMI thresholds of the laser are 2825, 4020, and 6789 W (Fig. 6). When the bending diameter increases to 16 cm, the maximum output power reaches 7100 W without TMI and SRS. The beam quality (M2) at this power is about 2.17, and the relative brightness is 1293 (Fig. 6). As the fiber bending diameter increases, the relative brightness of the laser also gradually increases. At a bending diameter of 16 cm, the maximum output power obtains limited from the pump power, leading to no further advancements in the output power or brightness.
In this study, a design for a high-power fiber laser amplifier is presented based on a double cladding ytterbium doped fiber with a core/cladding diameter of 30/600 μm and water-cooled columns of different diameters. Based on this, experiments are conducted on abnormal TMI effects. The results indicate that when the fiber supports many modes, increasing the bending diameter can raise the TMI threshold of the laser and also enhance the brightness of the output laser. In the experiment, using a wavelength-stabilized 976 nm LDs, as the bending diameter increases from 13 cm to 16 cm, the TMI threshold of the laser rises from 1650 W to 3740 W, showing an increase of approximately 1.27 times. The relative brightness of the output laser rises from 519 to 970, marking an increment of 0.87 times. Although increasing the fiber bending diameter might slightly degrade the beam quality of the output laser, the notable rise in the TMI threshold significantly augments the laser's maximum output power, thereby elevating the brightness of the output laser. Ultimately, by expanding the fiber bending diameter and optimizing the pump wavelength, TMI is successfully suppressed, achieving a 7100 W high-brightness laser output with a relative brightness of 1293. This is 1.53 times the relative brightness of the output laser under identical conditions when the bending diameter is 13 cm. The findings from this study offer guidance for TMI suppression and enhancements in the output power and brightness of high-power fiber lasers.
