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The variations of the thermal load boundaries between three stages S1, S2, and S3 of LP01 (a) and LP11o/e (b & c) modes via Δn0
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The adiabaticity criterion of the thermally-guided very-large-mode-area (TG VLMA) fiber is presented based on the mode-coupling theory firstly, to the best of our knowledge. The requirement for the adiabatic propagation of fundamental mode is discussed systematically. It is revealed that the pump absorption plays the most important role and the adi...
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Citations
... As a common suppression strategy, highly multimode fiber can expand the effective mode area and mitigate intensity-depended nonlinear effects effectively [24]- [28]. Thus, the 20/400 LMA GDF was replaced with an equal length 50/400 multimode LMA GDF [29], which had the core NA of 0.11 and was coiled tightly in spiral configuration to avoid the HOMs excited by the scattering from the bent fiber [30], [31]. The bending diameter of 50/400 multimode LMA GDF was 0.2 m, which has limited impacts on the output performances and would be explained by the stronger HOMs constraint capacity of 50/400 highly multimode GDF [29], [32]. ...
The spatial-spectral evolution properties of laser beam in the presence of stimulated Raman scattering (SRS) effect have been studied in large mode area (LMA) fiber. With the employment of long/short pass filters, spatial evolution of the signal and Raman light has been studied individually. After the onset of SRS, the simultaneous optimizing of signal light and degrading of Raman light was observed, which has been attributed to the SRS effect of multimode laser beams in LMA fiber. The spectral evolution characteristics indicated that the SRS effect in LMA fiber will trigger other nonlinear effects, mainly inter-modal four-wave-mixing effect and cross-phase-modulation induced modulation instability. Furthermore, the impacts of bending strategy and multimode LMA fiber on SRS-induced spatial-spectral distortion have also been studied, which shows that the SRS-induced mode distortion can be suppressed by increasing the fiber size while the bending strategy plays a limited role.
... The possible reason is that the core size of GRIN fibers are far larger than that in previous amplifier with MI, which all have rather large mode numbers and similar sophisticated mode intensity distribution, hence the fluctuation will not be sensitive to this change. From the experimental results it can be summarized that in several-kilowatt high power CW RFL based on GRIN fiber, the distortion of beam spot occurs with characteristic frequency at kHz-level, which is related to the possible nonlinear effects in the GRIN fiber, i.e. thermal lens effect, MI or the thermal-induced gain competition and coupling of modes [51][52][53][54][55]. It is a sort of instability of modes in RFA, however, there are different features compared with the common MI in RE-doped fiber laser systems, which still deserves more theoretical and experimental study. ...
In this paper, we study the power scaling in high power continuous-wave Raman fiber amplifier employing graded-index passive fiber. The maximum output power reaches 2.087 kW at 1130 nm with an optical conversion efficiency of 90.1% (the output signal power versus the depleted pump power). To the best of our knowledge, this is the highest power in the fields of Raman fiber lasers based merely on Stokes radiation. The beam quality parameter M² improves from 15 to 8.9 during the power boosting process, then beam spot distortion appears at high power level. This is the first observation and analysis on erratic dynamic properties of the transverse modes in high power Raman fiber amplifier.
... In these studies, the shrinking of transverse mode induced by the thermal-lens effect was taken into account, and its effects on the gain abstract (induced by the variation of mode overlap with the doped core because of the mode shrinking) and single-mode amplification were investigated. Besides, the adiabatic criterion of fundamental mode propagation in the TG-IAG fiber was studied in [18]. In [19], a normalized thermal lensing parameter was also proposed to characterize the effect of thermal-lens and revealed that HOMs can be supported by the single-mode fiber when the thermal load is large enough. ...
... Thus, in this section, the mode coupling between the LP 01 and LP 02 mode will be discussed. Considering a co-pumping or counter-pumping scheme and assumed that pump power is exponentially varying with the pump absorption coefficient α p , the mode coupling coefficient can be given by [18] 2 0 ...
... Although the local modes are orthogonal to each other at each propagation position z, the mode coupling coefficient can be non-zero because of the variation of thermally-induced refractive index (i.e. Δn) caused by the thermal load variation induced by the pump absorption [18,28]. Generally, α p is a constant for a given active fiber. ...
In this paper, the thermal-lens induced mode coupling in step-index large mode area fiber laser is systematically investigated. The pertinent mode coupling coefficient is studied firstly, to the best of our knowledge. It is demonstrated that the mode coupling can be induced by the thermal-lens induced waveguide changing along the active fiber. It is found that the mode coupling can be enhanced mainly by the thermally-induced mode distortion and refractive index variation, both of which will become severe with the large thermal load. The impacts of fiber configuration parameters on the mode coupling are discussed. It is found that in the straight fiber, the mode coupling in a larger-core fiber can be weakened when the thermal load is low, but it will become stronger when thermal-lens effect is severe enough. However, in the bent fiber, enlarging core size, reducing core numerical aperture (NA), or decreasing bend radius will all aggravate the mode coupling. Especially when NA is excessively reduced, the mode coupling will be dramatically raised even with a small thermal load. The pertinent study is significant for understanding the mode coupling phenomenon in high-power fiber lasers.
... The adiabaticity criterion of the thermally-guided very-large-mode-area (TG VLMA) fiber is presented by W. Liu et al. [6] based on the mode-coupling theory. The requirement for the adiabatic propagation of fundamental mode is discussed systematically. ...
The Advanced Solid State Lasers 2017 Conference (ASSL) was held from October 1 to 5, 2017. It was an extraordinary conference at the Nagoya Congress Center in Nagoya, Japan. ASSL 2017 again suggested an impressive platform where miscellaneous perceptions with a variety of approaches to optics, photonics, sensing, laser technology, laser systems, and solid state lasers were presented. This international meeting was highly selective, leading to high level contributions through one plenary conference, 17 invited presentations, 70 regular talks, and 121 posters. The present joint issue of Optics Express and Optical Materials Express features 27 articles written by ASSL 2017 authors and covering the spectrum of solid-state lasers from materials research to sources, and from design innovation to applications.
Thermal-lens induced coupling between the fundamental and higher-order modes in the high-power fiber amplifier is revealed firstly, to the best of our knowledge, and the variation of pertinent coefficient is discussed.
The Advanced Solid State Lasers 2017 Conference (ASSL) was held from October 1 to 5, 2017. It was an extraordinary conference at the Nagoya Congress Center in Nagoya, Japan. ASSL 2017 again suggested an impressive platform where miscellaneous perceptions with a variety of approaches to optics, photonics, sensing, laser technology, laser systems, and solid state lasers were presented. This international meeting was highly selective, leading to high level contributions through one plenary conference, 17 invited presentations, 70 regular talks, and 121 posters. The present joint issue of Optics Express and Optical Materials Express features 27 articles written by ASSL 2017 authors and covering the spectrum of solid-state lasers from materials research to sources, and from design innovation to applications.
