Postdoc in Mitigation of Modal Instabilities in Double-pass Rod Amplifiers

Technical University of Denmark - DTU Fotonik

A 30-months postdoctoral research position is available at the Technical University of Denmark (DTU), Department of Photonics Engineering (DTU Fotonik). The position is funded by the Danish Council for Independent Research, and the project will be carried out in close collaboration with the Danish company NKT Photonics A/S. 

DTU Fotonik focuses its research and education efforts on a range of photonics technologies, including optical fibers, signal processing components and systems, metamaterials, and terahertz technology. In addition, the department has strong research groups within network technology and coding. The department comprises around 200 researchers in total. 

NKT Photonics A/S is located within cycling distance of DTU Fotonik, and has around 150 employees. It is the world-leading manufacturer of photonic crystal fibers, and a major player within pulsed high-power fiber laser sources. 

Double-clad photonic crystal fibers and rods constitute the state of the art in large mode-area high-power optical amplifiers based on Yb-doped silica glass. Such amplifiers are essential ingredients in next-generation nanosecond, picosecond and femtosecond laser systems for industrial processing. However, in recent years it has been discovered that average-power scaling of these laser systems is limited by a spatio-temporal modal instability phenomenon, commonly denoted transverse modal instabilities (TMI) which is rooted in thermo-optical nonlinearities. While a basic understanding of this phenomenon has emerged, in part through research efforts at DTU Fotonik and NKT Photonics, work on mitigation is still ongoing. 

The use of double-pass fiber amplifiers in high-power laser systems is highly desirable from considerations of simplicity and compactness. It allows in principle simple polarization control, without the need for polarization-maintaining fibers, and use of an advanced large mode-area fiber/rod in two amplification stages. Therefore, understanding of the TMI phenomenon in double-pass amplifiers is of great interest, but is complicated by the fact that thermo-optic interactions between forward- and backward-propagating waves must be taken into account, leading to a complex nonlinear self-consistency problem. Recent theoretical work done at DTU Fotonik has predicted the existence of a low-power static modal instability, which contrasts the dynamic instabilities observed in single-pass amplifiers. The theoretical model also predicts regions of multistability, and in general the nonlinear problem appears much more rich and complex than in the single-pass case. 

The aim of the present project is to perform experimental investigations of static and dynamical instabilities in double-pass amplifiers, and identify mitigation strategies that will allow power scaling to several hundred Watts of average power. The experimental work will be closely tied to ongoing theoretical efforts at DTU Fotonik. The large-core amplifiers used in the investigations will be state-of-the-art photonic crystal fiber rods manufactured by NKT Photonics, and the experiments will be taking place in the well-equipped labs of this company, with technical support from company staff. 

Application procedure
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Application deadline: 25 November 2016 

DTU is a technical university providing internationally leading research, education, innovation and scientific advice. Our staff of 5,800 advance science and technology to create innovative solutions that meet the demands of society; and our 10,300 students are being educated to address the technological challenges of the future. DTU is an independent academic university collaborating globally with business, industry, government, and public agencies.

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