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We studied the transverse mode instability (TMI) characteristics of few-mode fiber laser amplifier employing ytterbium-doped fiber (YDF) with core/cladding diameter of 30/400μm under different bending diameters. The gain fiber is coiled in ellipse shape with minimum bending diameter changed from 9cm, 10cm, 11cm to 12cm in the bending part, and the experimental results show an anomaly characteristic of TMI. The TMI threshold improved from 772W, 1149W and 1458W to 1641W when the bending diameter of YDF increase from 9cm, 10cm, 11cm to 12cm, accompanied by the deterioration of beam quality from 1.60, 1.67, 1.87 to 1.95. This is mainly because tightly bending in few-mode fiber will promote the overlap between modes and lead to mode coupling, which may trigger TMI. Besides, the bending losses of HOMs decrease while bending diameter increasing, so that there is higher output efficiency and deteriorated beam quality when bending diameter increase. Although the phenomenon is not agreed with common TMI with near single-mode fiber laser, the result will be helpful for the design and power scaling of few-mode fiber lasers.

Fiber lasers have been widely used in the industrial and scientific fields due to their advantages of high conversion efficiency, simple thermal management, and consistent stability. High brightness and high-power fiber lasers are affected by simulated Raman scattering and transverse mode instability, which limit the power scaling of fiber lasers. Therefore, there are only a few reports achieving a 10kW-level fiber laser system by laser diode direct pumping or tandem pumping. In this manuscript, we demonstrate an all-fiber laser amplifier based on home-made low numerical aperture fiber pumped by 976nm laser diodes. When the signal light is input to the gain fiber with a minimum bending diameter of 12cm, the beam quality factor M²~1.72. The onset of TMI is observed at 2467W output power, accompanied by beam quality degradation. In order to suppress the onset of TMI, the minimum bending diameter of the gain fiber was changed from 12cm to 20cm. And the signal light is input into the gain fiber with a bending diameter of 28cm. Benefiting from this operation, the fiber laser amplifier achieved a maximum output power of 10.53kW with an optical-to-optical efficiency of 74.04%, and there is no TMI onset observed. However, increasing bending diameter inevitably leads to the degradation of beam quality. At the maximum output power, the beam quality factor M² is 2.88. To the best of our knowledge, this is the highest optical-to-optical efficiency and the best beam quality among 10kW-level laser diodes pumping fiber lasers. Generally, it is believed that reducing bending diameter can suppress TMI by increasing high-order mode losses. However, this rule does not apply to few-mode fiber lasers. A larger bending diameter leads to more high-order modes being contained in the signal light instead of leaking into the cladding area. Thus, a higher output and poor beam quality are obtained. Also, it is believed that tightly coiled fiber can make mode coupling easier and trigger TMI, which results in a positive correlation between the TMI threshold and bending diameter. Low NA fibers are very sensitive to bending, and reducing the bend diameter to control the beam quality will result in lower efficiency and a lower TMI threshold. Therefore, although producing a 10kW-level fiber laser is simple, maintaining good beam quality during the power scaling process is still a challenge. The results of this article will be a valuable reference for high power fiber laser design.

Up till now, the spatial and temporal dynamics of transverse mode instability (TMI) in fiber laser oscillator have increasingly attracted a worldwide attention. Here, we develop a high-speed modal decomposition (MD) system to analyze the modal coupling for fiber laser oscillator above the TMI threshold. A set of angular-multiplexing transmission functions (TFs) are designed for simultaneous MD and monitoring the far-field beam profile. The TMI threshold of the deployed fiber laser oscillator is 181 W at a co-pumping power (CPP) of 279 W. As the CPP increases from 318 W to 397 W, the power fluctuations of the output laser become more drastic. The changes of the far-field beam profile and the centroid of far-field spot (COFFS) indicate an increased velocity of energy transfer between modes. The high-speed MD verifies above process and analyzes the modal components, indicating that the single cycle of modal coupling decreases from 11 ms to 4 ms. Otherwise, the strong mode coupling occurs between modes with relatively large weights. The high-speed MD provides a powerful tool to research the TMI effect.

Fiber laser performances including transverse mode instability (TMI), stimulated Raman scattering (SRS) and optical-to-optical efficiency are in connection with the pump wavelength. Here we studied the output characteristics of a 5-kW ytterbium-doped fiber laser oscillator pumped with two different pump sources, i.e., 915 nm and 981 nm laser diodes (LDs). The output characteristics of fiber laser oscillators pumped by 915 nm and 981 nm have been compared strictly and directly with the same structure in a high-power situation. Experimental results show that both pump wavelengths can scale the power up to more than 5 kW by suppressing the TMI effect. While in the case of pumping by the 981 nm LDs, the laser oscillator has an optical-to-optical efficiency of 87%, which is 13% higher than that of the 915 nm pumped scheme. In addition, due to the higher backward pumping ratio and lower total pump power, the laser oscillator has a better SRS suppression ratio when pumped at 981 nm. Thus, it reveals a great potential to balance the limitations of TMI and SRS for scaling up to an even higher output while pumping at 981 nm. All the devices of the oscillator are commercial, and it will be helpful for the commercialization of high-power fiber laser oscillators.