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Study Of Thermal-Induced Modal Instabilities In High Power Narrow-Linewidth Fiber Amplifiers With Near Diffraction-Limited Beam Quality

Posted on:2016-11-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:R M TaoFull Text:PDF
GTID:1108330509461084Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Fiber laser, which has a linewidth on the order of or less than 0.1nm and M2≤1.5, is called narrow-linewidth fiber laser(NLFL) with near diffraction limited beam quality. Due the intrinsic advantages in excellent temporal coherence and beam quality, high power NLFL with near diffraction limited beam quality can be widely employed in the fields like earth science, atomics physics, frequency conversion, beam combination and so on. By employing large mode area fibers and spectral broaden technology, the output power of NLFLs with near diffraction limited beam quality has already reached kilowatt level. With the power scaling of NLFLs, thermal induced modal instabilities(MI) have set in, which results in the degradation of beam quality. At present, the onset of MI has become one of the main limitations, which limits the further power scaling of NLFL systems with diffraction-limited beam quality and is being studied intensively. Therefore, in this thesis, theoretical and experimental study on MI in Yb-doped high power NLFL system is presented in an attempt to mitigate MI and construct all fiberized NLFL system with higher power while maintaining near diffraction limited beam quality. The main work included as follows:1. A novel theoretical model has been built up to investigate MI in NLFL. Based on classical mode coupling theory, a semi-analytical model is built up by taking into account mode profile in large mode area fiber and temperature profile. Analytical expression of nonlinear mode coupling coefficient has been achived, which indictes the relationship between physical parameters and MI in fiber laser.2. Quantitative analysis of MI in NLFL has been carried out by studying the influence of configuration parameters and system parameters. It revealed that the threshold power can be increased by reducing the core diameter and numerical aperture(NA), increasing the pump cladding diameter and seed power, and shifting the pump wavelength from 976 nm absorption peak. Threshold can also be increased by counter-propagation pump, bi-direction pump, and multi-point side pumping. It also shows that dopant concentration, the way of cooling and polarization effects has little impact on MI threshold.3. Physical phenomenan of MI in all-fiberized NLFL and influence factors are studied experimentally. Analysis on beam quality saturation has been carried out. A novel method to detecting MI by detecting scattering light has been proposed and compared with the traditional method, which proves to be reliable and trustworthy. The experimental results agree with the theoretical prediction.4. The performance of various mitigating stratigies of MI in all-fiberized NLFL are inveistigated in the area from high order mode supressing, increasing gain saturation, reducing quantum defect. Mitigating MI by pumping at 915 nm is proposed for the first time. In theory, it is found that mode filter by bending, detuning the pump wavelength, tandem pump are promising approach to mitigate MI in all-fiberized fiber laser. In experiment, 1.5k W linearly-polarized fiber laser with near diffraction-limited beam quality has been achieved with mode filter by bending, and the power is limited by power handle capability of the signal/pump combiner. 1.26 k W linearly-polarized fiber laser with near diffraction-limited beam quality has been achieved by pumping at 915 nm.5. Beam propagation model of NLFL from low NA and large mode area fibers has been setup while modification has been made to the classical M2 criterion. The modified M2 criterion has eliminated the disadvantage that the traditional M2 criterion is affected by the relative phase between LP01 and LP11 mode. The modified M2 criterion can be used to evaluate the mode content of the high power NLFLs from low NA and large mode area fibers. Propagation properties of NLFL with LP11 mode content has been studied by employing the beam propagation model. It revealed that the relative phase between LP01 mode and LP11 mode has no impact on power in the bucket(PIB). If adaptive optics system is used to correct the peak intensity deviation due to relative phase, the results indicate that there exists an optimization of PIB as relative phase increases with optimal phase being π/2.
Keywords/Search Tags:Modal instability, narrow linewidth, fiber amplifier, thermal effects, mode filter by bending, gain saturation, beam propagation
PDF Full Text Request
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