Mode controlling study on narrow-linewidth and high power all-fiber amplifier

Suppressing mode instabilities by optimizing the fiber coiling methods

“…It reveals that the threshold of MI can be further increased by decreasing the increment. However, even the increment is set to be as small as 0.7 mm, the maximal MI threshold for the co-pumped fiber amplifiers is still less than 2 kW, which coincidences with the current power scaling progress [16,18]. One can find that, for fibers with these typical param eters, the optimization of the circle spiral shape alone is not enough to achieve MI-free 3 kW fiber laser.…”

“…Among these methods, suppressing MI by reducing the coil radius is a simpler way, which can be implemented without complicated fiber architectural design or accurate wavelength choosing. By coiling the gain fiber at smaller radius, the MI-free output power has nearly tripled [16,17]. However, for the commercial fiber employing current coiling technology, the narrow linewidth MI-free output power is no more than 2.3 kW [18][19][20] while the broadband spectral linewidth counterpart is less than 2.5 kW [21,22].…”

Study of dopant concentrations on thermally induced mode instability in high-power fiber amplifiers

“…Nevertheless, in some methods, the suppression of MI is implemented through mode-specific loss by tight-coiling the fiber [12,26,51], in which a longer length of fiber can provide higher high-order mode loss and stronger MI suppression. When adopting higher doped fiber to suppress the nonlinear effects in these cases, one should take the precaution of not reducing the fiber length too much so as to make the MI suppression invalid.…”

2.43 kW narrow linewidth linearly polarized all-fiber amplifier based on mode instability suppression

“…The onset of MI dramatically deteriorates the beam quality of the fiber laser, and limits the applications of fiber lasers in the aforementioned areas [2,5]. Since the first report of MI phenomena, various methods have been proposed to mitigate this undesired effect efficiently, such as tailoring the Yb-ion concentration profile [7,11], tuning the pump or signal wavelength [12,13], using gain fibers with low NA [14,15], and increasing the loss of high order mode (HOM) by coiling the fiber [16,17]. Among these methods, increasing the loss of HOM by tight coiling the fiber is a simple way to suppress MI, which can be implemented without the design of a complicated fiber or choosing an accurate wavelength.…”

“…Among these methods, increasing the loss of HOM by tight coiling the fiber is a simple way to suppress MI, which can be implemented without the design of a complicated fiber or choosing an accurate wavelength. Compared with the design without employing bend loss to suppress MI, the output power without MI is nearly tripled by properly choosing the coiling radius of the active fiber [17]. It is pointed out in [16] that, without the influence of the mode specific loss, the MI thresholds of the polarization maintaining (PM) fiber and the non-PM one are nearly the same.…”