The radical suppression of the photodarkening effect and laser performance deterioration via H2 loading were demonstrated in high-power Yb-doped fiber (YDF) amplifiers. The photodarkening loss at equilibrium was 114.4 dB/m at 702 nm in the pristine fiber, while it vanished in the H2-loaded fiber. To obtain a deeper understanding of the impact of photodarkening on laser properties, the evolution of the mode instability threshold and output power in fiber amplifiers was investigated. After pumping for 300 min, the mode instability threshold of the pristine fiber dropped from 770 to 612 W, and the periodic fluctuation of the output power became intense, finally reaching 100 W. To address the detrimental effects originating from photodarkening, H2 loading was applied in contrast experiments. The output power remained stable, and no sign of mode instability was observed in the H2-loaded fiber. Moreover, the transmittance at 638 nm confirmed the absence of the photodarkening effect. The results pave the way for the further development of high-power fiber lasers.

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Effective suppression of the photodarkening effect in high-power Yb-doped fiber amplifiers by H 2 loading

A gain-tailored Ge-free Yb/Ce codoped aluminosilicate fiber is fabricated by MCVD combined with solution doping technique. Through regulating the temperature in the tube and designing the solution doping process, the refractive index profile of this fiber is close to a step-index without any center dip. The laser performance of this fiber is proved through contrast experiments with conventional fiber in a kW-level MOPA setup. The gain-tailored fiber amplifier presents a beam quality of M2 ~1.43 at 1.2 kW. Its MI threshold is 1.25 kW, about 1.74 times as much as that of the conventional fiber amplifier. The laser slope efficiency of the gain-tailored fiber amplifier is 86.75%. Stabilized at an output power of 1.1 kW for 15 hours, the MI threshold does not decrease after this long-term operation, demonstrating a strong resistance to photodarkening effect. These results have confirmed that MCVD-fabricated gain-tailored Yb/Ce codoped aluminosilicate fibers have great potential in power scaling and output stability of high-power fiber lasers and amplifiers.

Gain-tailored Yb/Ce codoped aluminosilicate fiber for laser stability improvement at high output power

The high absorption confined-doped ytterbium fiber with 40/250 μm core/inner-cladding diameter is proposed and fabricated, where the relative doping ratio of 0.75 is selected according to the simulation analysis. By employing this fiber in a tandem-pumped fiber amplifier, an output power of 6.2 kW with an optical-to-optical efficiency of ∼82.22% is realized. Benefiting from the large-mode-area confined-doped fiber design, the beam quality of the output laser is well maintained during the power scaling process with the beam quality factor of ∼1.7 of the seed laser to ∼ 1.89 at the output power of 5.07 kW, and the signal-to-noise ratio of the output spectrum reaches ∼40 dB under the maximum output power. In the fiber amplifier based on the 40/250 μm fully-doped ytterbium fiber, the beam quality factor constantly degrades with the increasing output power, reaching 2.56 at 2.45 kW. Moreover, the transverse mode instability threshold of the confined-doped fiber amplifier is ∼4.74 kW, which is improved by ∼170% compared with its fully-doped fiber amplifier counterpart.

High-power tandem-pumped fiber amplifier with beam quality maintenance enabled by the confined-doped fiber

Abstract In this work, an all-fiberized and narrow-linewidth fiber amplifier with record output power and near-diffraction-limited beam quality is presented. Up to 6.12 kW fiber laser with the conversion efficiency of approximately 78.8% is achieved through the fiber amplifier based on a conventional step-index active fiber. At the maximum output power, the 3 dB spectral linewidth is approximately 0.86 nm and the beam quality factor is Mx2 = 1.43, My2 = 1.36. We have also measured and compared the output properties of the fiber amplifier employing different pumping schemes. Notably, the practical power limit of the fiber amplifier could be estimated through the maximum output powers of the fiber amplifier employing unidirectional pumping schemes. Overall, this work could provide a good reference for the optimal design and potential exploration of high-power narrow-linewidth fiber laser systems.

Six kilowatt record all-fiberized and narrow-linewidth fiber amplifier with near-diffraction-limited beam quality

Mode instability with different mode excitation has been investigated by off-splicing the fusion point in a 4 kW-level monolithic fiber laser system, which reveals that the fiber systems exciting more high order mode content exhibits lower beam quality but higher mode instability threshold. The static-to-dynamic mode degradation and dynamic-only mode degradation have also been observed in the same high power fiber amplifier by varying the mode excitation, which implicates that the mode excitation plays an important role in mode characteristics in high power fiber lasers. By employing a seed with near fundamental mode beam quality, only dynamic mode degradation-mode instability sets in with negligible static beam quality degradation. Then the fusion point in the seed laser is offset spliced to excite high order mode. As the output power of the main amplifier scales, the beam quality degrades with the beam profile being static, and then the dynamic mode instability sets in, the power threshold of which is higher than that with good beam quality seed. We consider that the static mode degradation is caused by the presence of incoherent supposition of fundamental and high order mode, which leads to that the measured dynamic mode instability threshold is higher.

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Experimental study of the influence of mode excitation on mode instability in high power fiber amplifier.

In this work, we conduct a detailed experimental study on the impact of signal bandwidth on the TMI threshold of fiber amplifiers. Both the filtered superfluorescent fiber sources and the phase-modulated single-frequency lasers are employed to construct seed lasers with different 3 dB spectral linewidths ranging from 0.19 nm to 7.97 nm. The TMI threshold of the fiber amplifier employing those seed lasers are estimated through the intensity evolution of the signal laser, and different criteria have been utilized to characterize the spectral linewidth of the seed lasers. Notably, the experimental results reveal that the TMI threshold of fiber amplifiers grows, keeps constant, and further grows as a function of spectral linewidth of seed lasers. Our experimental results could provide a well reference to understand the mechanism of the TMI effect and optimize the TMI effect in high-power fiber amplifiers.

Experimental study on the impact of signal bandwidth on the transverse mode instability threshold of fiber amplifiers.

A confined-doped fiber was fabricated by a modified chemical vapor deposition (MCVD) process based on refractive index matching technology. With theory and experiments, we compared the confined-doped fiber and normal-doped fiber. We found that the confined-doped fiber with a core of 35 μm and 0.07 numerical aperture could achieve single-mode output and improve the beam quality from 2.8 to 1.5 in the fiber laser. Meanwhile, it still possesses high laser efficiency and has good stability of beam quality with the increase in pump power. It suggests that the confined-doped fiber with a MCVD process may be the key material for a high-power fiber laser with excellent beam quality.

Confined-doped fiber for effective mode control fabricated by MCVD process.

We systematically investigated the dependence of mode instability (MI) thresholds on bending diameters in ytterbium-doped fiber amplifiers. The MI thresholds of a fiber amplifier, which was based on a multimode ytterbium-doped fiber, were measured at various bending diameters. It is found that the correlation between MI thresholds and fiber bending diameters depends on the operation state of the active fiber. When the multimode active fiber is in the few-mode operation, the MI threshold is successfully raised from 717 W to 1084 W by increasing the fiber bending diameter from 11 cm to 60 cm. When the multimode active fiber operates in quasi-single mode, the MI threshold is raised from 717 W to 953 W by reducing the bending diameter from 11 cm to 8 cm. According to our experiments and previous reports, it is suggested that the influence of changing fiber bending diameter on MI might be closely related to the mode coupling effect.

https://iopscience.iop.org/article/10.1088/1612-202X/aaff4b/pdf

Yisha Chen

Papers

Effective suppression of the photodarkening effect in high-power Yb-doped fiber amplifiers by H 2 loading

Experimental study on the impact of signal bandwidth on the transverse mode instability threshold of fiber amplifiers.

Confined-doped fiber for effective mode control fabricated by MCVD process.

Bending diameter dependence of mode instabilities in multimode fiber amplifier

Six kilowatt record all-fiberized and narrow-linewidth fiber amplifier with near-diffraction-limited beam quality