Transverse mode instability of fiber oscillators in comparison with fiber amplifiers

Abstract

Transverse mode instability (TMI) is experimentally investigated in a fiber oscillator and a fiber amplifier. For a reasonable comparison of TMI in these two configurations, the same optical components and design parameters are applied to both. Our experimental results show that the TMI power threshold in a fiber oscillator is lower than in a corresponding fiber amplifier. By using simulation software, a fiber oscillator and an amplifier are designed with similar characteristics, to provide identical conditions for all effective parameters on TMI in both of them. Since the signal propagation in fiber oscillators is different from that of single-pass fiber amplifiers, and also since both forward and backward propagating signals in fiber oscillators can generate thermo-optic index gratings, the observed lower TMI threshold in the fiber oscillator is due to its different interaction of light with index gratings.

The effect of population inversion saturation on the transverse mode instability threshold in high power fiber laser oscillators

“…The reason for the sharp increase of backward light power beyond TMI threshold backs to the higher transmission of HR FBG for mode 23 , 24 . Coincidence of these two effects indicates the appearance of TMI 25 . Moreover, M2 factor before and after the TMI threshold is measured.…”

3.2 kW, 0.22 nm narrow-linewidth MOPA configuration fiber laser with a homemade polarization-maintaining Yb-doped fiber

“…High-power narrow-linewidth fiber lasers or amplifiers with linearly polarized have been a major area of interest within the field of gravitational wave detection (GWD), non-linear frequency conversion (NFC), spectral beam combining (SBC), coherent beam combining (CBC) [1][2][3][4][5][6], and ultrafast lasers [7][8][9][10][11][12], etc., Over the last decade, ytterbium (Yb)-doped master oscillator power amplifiers (MOPAs) with linearly polarized ones have made great progress, and the power scaling has reached the muti-kilowatt level [13][14][15]. However, a number of destructive non-linear phenomena, including stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and thermal mode instability (MI), can jeopardize the stability of the laser system because of the high intensity in the fiber core [6,[16][17][18]. Among them, MI and SBS are the main limiting factors in the power scaling of a high-power narrowlinewidth linearly polarized MOPA system [18,19].…”

“…However, a number of destructive non-linear phenomena, including stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and thermal mode instability (MI), can jeopardize the stability of the laser system because of the high intensity in the fiber core [6,[16][17][18]. Among them, MI and SBS are the main limiting factors in the power scaling of a high-power narrowlinewidth linearly polarized MOPA system [18,19].…”

Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

“…Transverse mode instability effect TMI is the most prominent power-scaling limitation responsible for beam quality degradation, which is the first representative result of thermally-induced non-linear effects in optical fibers [23,[65][66][67][68][69][70][71]. This threshold-like behavior refers to the energy transfer between the fundamental transverse mode and one or more high-order transverse modes, leading to temporal fluctuations of the output power on the kHz frequency scale.…”