Sub-wavelength Grating Coupling of Laser Diode Stack for Side-pumped High Power Fiber Laser
Date Issued
2012
Date
2012
Author(s)
Huang, Chieh-Wei
Abstract
In recent years, diode-pumped solid-state fiber laser technology is gradually becoming main-stream of high-power laser field, owing to its superior beam quality, narrow linewidth, and high electrical-optical efficiency. Besides, as the pump source, the laser diode array/stack (LDA/S) is a reliable high-output power source and has been widely used in different applications. When the pump source shifts to an even higher power LDS with many more elements and emitters, the pump power is limited by the complexity of the source itself. Presently, LDS can only be used as a bulk gain medium pumping source combined with spatial multiplexing technique due to the distributed multi-mode emission on the slow-axis. This shows that improved pump sources and methods are still needed for better performance.
Therefore, in this work, a more compact and highly efficient grating side-pumped configuration has been implemented. Through the use of a specially designed grating coupler, the emission from LDA/LDS was directly side-launched into the clad of a double-clad fiber through a set of brightness-preserved focusing/collimating optics. In the experiments, an electron-beam was used to fabricate the sub-wavelength grating. In addition, with the consideration of the backward diffraction loss and the groove wall non-verticality due to fabrication distortion, the grating pitch and groove width were optimized for the highest coupling efficiency. The experimental results show that the gold-embedded silica grating coupler is superior to the surface relief gold grating coupler, because of its higher resistance to thermal expansion and better heat removal capability.
For the LDA side-coupled scheme, the experimental results show that, the gold-embedded silica grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave LDA into the inner cladding of a 400-μm-diameter double-clad fiber with an overall coupling efficiency of 50%. Furthermore, a side-pumped ytterbium-doped double-clad fiber laser by using the grating coupler was demonstrated. The output power of 10 W with a slope efficiency of 61% was demonstrated for the bi-directional side-pumped fiber laser. Besides, for the LDS side-coupled scheme, a single LDS distributed side-coupled scheme using grating couplers is presented. To the best of our knowledge, this is the first demonstration of this compact distributed side-pumped configuration in the world. The launched power was 124.6 W at 304.4 W LDS emission with a coupling efficiency of 41%. Furthermore, the output power of 58 W with a slope efficiency of 50% was demonstrated for the distributed side-pumped fiber laser. The central wavelength is at 1090 nm, and the narrow linewidth shows the advantage of the distributed side-pumped scheme, which is nearly 0.11 nm.
Finally, this technique would not only be an efficient way to achieve a high-power and high-quality light source but also has the advantage for mass production of the grating couplers by using nano-imprint technique, which has the potential in business competition.
Subjects
high-power fiber laser
laser diode array/stack
grating
grating coupler
diffraction
Type
thesis
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