چكيده به لاتين
In recent years, the average output power of fiber lasers and fiber amplifiers, doped with rare earth ions, reached few kilowatts .The mode instability as a nonlinear effect in high power fiber lasers and fiber amplifiers limits the average output power and at the same time decreases the beam quality. The output beam profile exhibits temporal instability, ranging from quasi periodic fluctuations in the 1-5 kHz range to quite chaotic behavior.
The purpose of this thesis is to present a theoretical model for understanding of the modal instability in high-power ytterbium doped fiber amplifier. Thermo-optical effects are the main factor in the presented model. In the presented model, wave equation, coupling mode equations, together with heat equation are solved. The Green function technique is used to solve the heat equation. In this thesis, nonlinear coupling coefficient, power spectral density and higher order mode contents have been calculated using the model and the results are interpreted physically.
