We present comprehensive theoretical study of mode instability with a universal model for the first time, which includes quantitative study on the various influence factors and mitigation methods. With large numerical results, the influence of various fiber and system parameters has been studied to summary mitigation methods. The mode instability threshold behavior of multi-wavelength pumping configurations has been studied for the first time, and the influence of pump wavelength shifting in tandem pumping technique and amplitude modulation frequency have also been given for the first time. The mitigation methods are discussed with review of the latest developments in experimental results. The drawbacks and possible overcome measures are also discussed. By properly combining the mitigation methods, an approach to achieve beyond 10kW diode pumped fiber laser with near diffraction limited beam quality has been proposed, which provides a creative way to overcome the mode instability limitation on diode pumped fiber lasers, and demonstrates the effectiveness of the mitigation methods.

We report on the high-power amplification of a 1064 nm linearly polarized laser in an all-fiber polarization-maintained master oscillator power amplifier, which can operate at an output power level of 1.3 kW. The beam quality ($M^2$) was measured to be $<1.2$ at full power operation. The polarization extinction rate of the fiber amplifier was measured to be above 94% before mode instabilities (MIs) set in, which reduced to about 90% after the onset of MI. The power scaling capability of strategies for suppressing MI is analyzed based on a semianalytical model, the theoretical results of which agree with the experimental results. It shows that mitigating MI by coiling the gain fiber is an effective and practical method in standard double-cladding large mode area fiber, and, by tight coiling of the gain fiber to the radius of 5.5 cm, the MI threshold can be increased to three times higher than that without coiling or loose coiling. Experimental studies have been carried out to verify the idea, which has proved that MI was suppressed successfully in the amplifier by proper coiling.

Suppressing mode instabilities (MI) by optimizing the fiber coiling methods has been studied numerically. By employing our semi-analytical model, the MI threshold of two typical high power fibers in various coiling methods has been calculated. It reveals that fiber laser systems with gain fibers being coiled in cylinder shape has higher MI threshold than those with gain fibers being coiled in spiral shape, and MI-free output power of fiber lasers can be scaled up to above 3 kW even with the typical commercial fibers in the co-pumped configurations.

We demonstrate an experimental study on scaling mode instability (MI) threshold in fiber amplifiers based on fiber coiling. The experimental results show that coiling the active fiber in the cylindrical spiral shape is superior to the coiling in the plane spiral shape. When the polarization maintained Yb-doped fiber (PM YDF: with a core/inner-cladding diameter of 20/400 µ m) is coiled on an aluminous plate with a bend diameter of 9–16 cm, the MI threshold is ~1.55 kW. When such a PM YDF is coiled on an aluminous cylinder with diameter of 9 cm, no MI is observed at the output power of 2.43 kW, which is limited by the available pump power. The spectral width and polarization extinction ratio is 0.255 nm and 18.3 dB, respectively, at 2.43 kW. To the best of our knowledge, this is the highest output power from a linear polarized narrow linewidth all-fiberized amplifier. By using a theoretical model, the potential MI-free scaling capability in such an amplifier is estimated to be 3.5 kW.

Influence of pump power distribution on modal instabilities (MI) is studied numerically, which reveals that the MI threshold is dependent on the pump power distribution in fiber amplifiers and can be increased by optimizing the pump power distribution without any adjustment of other amplifier parameters. It shows that amplifiers with backward or bi-direction pump schemes have a higher threshold than those employing forward pump schemes. For backward pumped amplifiers employing fiber with core/clad diameter being 20/400 µ m, the MI threshold yields a 42% increase compared to the forward pumped ones. For bi-direction pumped amplifiers, there exists an optimal power ratio between forward and backward pump power, which results in the highest threshold power. When amplifiers with core/clad diameter being 20/400 µ m employ a bi-direction pump scheme at the optimal backward pump power fraction, the threshold can be increased by a factor of approximately 60% with respect to the forward pump configuration. The threshold increment factor reduces as the gain saturation effect weakens. It also shows that the MI threshold can be increased by employing multi-point side pump schemes.

Transverse mode instability (TMI) is one of the main limiting factors in kW-level fiber lasers. Unlike fiber amplifiers, TMI in fiber laser oscillators attracts less attention from researchers. In this work, we construct an all-fiber ytterbium-doped laser oscillator and investigate the performance in co-pumping and bidirectional-pumping configurations, respectively. In the co-pumping scheme, TMI occurs at ~1.6kW and restricts further output power scaling. Different from the characteristic of dynamic TMI in fiber amplifiers, quasi-static TMI is observed in the laser oscillator. Details of the temporal characteristic around the TMI threshold are provided. In the bidirectional-pumping scheme, experimental results validate that the TMI is mitigated notably by employing bidirectional-pumping instead of co-pumping. The output laser power is further scaled to 2.5kW with a slope efficiency of 74.5% and good beam quality (M²~1.3). At the maximum power, the FWHM bandwidth of optical spectra is 5.2nm, and the Raman stokes light is ~20dB below the signal.