Single-fibre power-scaling & pulsed phase characterization and optimization for coherent beam combination

Abstract

Pulsed fibre-based lasers are already proven and widely used in low-energy regimes. In addition, they are increasingly considered for new applications requiring high peak power, high average power, as well as high energy. However, the pulse energy is limited by e.g., optical damage, nonlinearities in a single-fibre laser. A promising approach is coherent beam combination which has been proposed for scaling the average and peak power of ultrafast pulsed lasers using an array of matched fibre amplifiers or a sequence of pulses in an amplifier.
This thesis describes a narrow-linewidth linearly-polarised nanosecond pulse-burst Yb-doped fibre amplifier system built to explore the phase distortions within the pulses in the highly nonlinear regime. The phase distortions are mainly induced by self-phase modulation (SPM) and are further complicated with the emergence of gain-saturation-induced sub-pulse reshaping and variations, when the extracted energy is comparable to, or larger than the saturation energy. Large phase distortion (up to a B-integral of 21 rad) was successfully characterized through a single-shot homodyne phase measurement scheme based on a 90° optical hybrid technique, and the phase fluctuations were also analysed. On the basis of this phase diagnosis technique, I developed a method to equalise the inter- and intra-phase differences between pulses and realized a theoretical 90% coherent combination with a Bintegral of >10 rad. This method is based on fast amplitude and phase electro-optic modulators (EOMs) which are controlled by proportional integral derivative (PID) algorithms.
Furthermore, taking advantage of the merits of multi-core fibre and parabolic-pulse amplification for ultrafast pulse combination, I experimentally demonstrated 90% passive coherent combination of an ultrafast twin-core fibre amplifier at a B-integral of 29 rad. In addition to the beam combination, I also advanced the power scaling and high beam quality operating in terms of a single fibre. The power and efficiency of erbium fibre lasers are limited by a small absorption cross-section and concentration quenching. Firstly, using an in-house fabricated large-core Er-doped Yb-free fibre which is cladding-pumped by 0.98-μm diode lasers, I demonstrated a 656-W record output power from any Er-doped fibre source. Then, via a high-concentration nanoparticle doping technique, a high Er-concentration (4×1025 ions/m3 ) Yb-free fibre was fabricated. An experimentally achieved laser slope efficiency as high as ~41% confirms that the concentration quenching effect is well suppressed.
Last but not least, a large effective area of the fundamental mode and quasi-single-mode operation is desirable for overcoming nonlinear limits in high power fibre amplifiers and lasers. Here, a bendable non-circular core large mode area Yb-doped fibre has been designed and fabricated to avoid bending distortions and mode degeneracy. The ability of the fibre to mitigate bend induced mode-distortions was also experimentally demonstrated.

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Identifiers

ORCID for Huaiqin Lin: orcid.org/0000-0002-9805-4570

ORCID for Johan Nilsson: orcid.org/0000-0003-1691-7959

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