Comparative study on transverse mode instability of fiber amplifiers based on long tapered fiber and conventional uniform fiber

Abstract

Compared with conventional uniform fibers, long tapered fibers provide an effective way to maintain beam quality and suppress the generation of nonlinear effects in fiber lasers. In this work, the transverse mode instability (TMI) of two amplifiers based on ytterbium-doped fiber (YDF) with uniform core diameter and tapered core diameter has been investigated experimentally. For a reasonable comparison of TMI in these two fiber amplifiers, the same effective core diameters and design parameters are applied to both. In the uniform YDF amplifier, the TMI threshold is around 1046 W, and the beam quality M 2 factor increases with the power and reaches 3.2 when the power is around 1500 W. In a tapered YDF (T-YDF) amplifier, the maximum output power is up to 2170 W with a beam quality of M 2 ~ 2.2 and no sign of TMI is observed. To the best of our knowledge, this is the first reported all-fiber tapered fiber laser with a 2 kW-level average output power. The experimental results and theoretical analysis indicate that the TMI threshold of the T-YDF amplifier is higher than the uniform YDF amplifier in spite of sharing the same effective core diameter.

Novel constant-cladding tapered-core ytterbium-doped fiber for high-power fiber laser oscillator

“…The constant cladding can make the uniformity and concentricity of the fiber coating much easier to control in the fabrication process. Compared with a traditional tapered fiber (in which the core and cladding vary simultaneously along the fiber) [29,30] , the constant cladding diameter of the CCTC fiber means a varying core-cladding ratio, which has a smoother thermal load profile along the fiber. In addition, the large core section provides a large mode area to suppress the SRS effect.…”

“…In this part, we designed new tapered fibers with different tapered structures, including CCTC fibers, and conducted a numerical simulation based on the coupled-mode theory compared with the conventional uniform fiber [30] . In our simulation, four types of fibers with different core diameter distributions are considered and plotted, as shown in Figure 1(a).…”

“…However, most of the reported tapered fiber lasers have been constructed based on a spatial optics configuration, which lacks stability and compactness [23][24][25][26][27][28] . In 2019, our research group conducted an experimental investigation into all-fiberized laser oscillators and amplifiers based on long tapered fibers [29,30] . However, because of the larger core size at the output end, the M 2 factor of the output laser was greater than 2.0, indicating a high number of HOMs in the output.…”

A 3.5-kW near-single-mode oscillating–amplifying integrated fiber laser

“…So it is difficult to balance TMI and SRS at the same time. In order to realize the simultaneous suppression of TMI and SRS, substantial research work has been carried out, including designing the new fiber structure [9][10][11][12][13][14], optimizing the laser system parameters and structure [15][16][17][18], and so on. From the perspective of laser structure, high-power fiber lasers are mainly divided into two types: the fiber amplifiers based on master oscillation power amplification (MOPA) structure and the fiber laser oscillators based on Fabry-Perot (FP) cavity structure.…”

Towards Ultimate High-power Scaling: Coherent Beam Combing of Fiber Lasers

“…Since the size of the core gradually increases along the fiber length, the mode content does not change through amplification, which resulted in excellent beam quality (𝑀 < 1.1). Indeed, T-DCFs have an inherent capacity to raise the NLEs thresholds [212]. However, for a stable high power/energy amplification, in addition to large MFD, polarization-maintaining is required.…”