Power scaling concepts in fiber lasers and amplifiers

Abstract

Fiber lasers and amplifiers have undergone rapid development in the past two decades, evidenced by the almost exponential rise in their output powers. This project investigates various concepts relating to high-power operation of fiber sources, focussing particularly on mode-area scaling, with the goal of developing a strategy for further power-scaling of fiber sources. The first concept examined is the phenomena of multimode interference (MMI), which occurs when fiber core areas are scaled to the extent that they support the propagation of more than one mode. MMI gives rise to self-imaging, whereby the initial phase relationship and spatial profile of an input beam is recovered at the output. This was investigated using a multimode ytterbium doped fiber amplifier, seeded with a wavelength tunable single mode ytterbium fiber laser. Tuning of the seed source showed that the amplifier underwent periodic self imaging of the fundamental mode beam incident from the seed, with a measured self-imaging wavelength period of 0.7 nm, consistent with predicted values. Despite significant higher order mode presence, self imaging yielded an excellent M2 value of 1.16. The laser beam M2 value in a non-self-imaging state reached a maximum value of 1.6. To illustrate the repercussions of these cyclical changes, the output of this amplifier was coupled into a single mode fiber. As the seed source was tuned the coupling efficiency underwent drastic changes from a maximum of 0.7 to a minimum of 0.04 due to changes in the amplifier’s beam pointing and quality as a result of MMI. A novel concept for mode selection was introduced which would preclude the negative effects of MMI by virtue of operating only on a single selected mode. The concept involves exploiting the mode-dependent spectral response of the reflectivity of fiber Bragg gratings. Experimental work with a multimode thulium fiber laser undertaken in collaboration with colleague Jae Daniel. Selected fundamental mode operation was successfully achieved, with an excellent M2 value of 1.1 compared to 3.3 for the same laser free-running without mode selection. Other higher-order modes could be selected by tuning the fiber laser. Analytical and numerical modelling showed that the reduced spatial overlap between the fundamental mode and the gain profile of the fiber would have minimal effect on the laser slope efficiency if operated at least 8 times above threshold. We speculate scalability of this novel technique to core diameters of up to 70 µm. To lay the groundwork for the transfer of the mode-selection technique to Q-switched multimode thulium fiber lasers, benchmarking experiments were conducted to probe the maximum achievable pulse energies and peak power from a thulium fiber. The highest pulse energy recorded was 618 µJ, with a corresponding peak power of 23 kW. However, the output pulse shape contained multiple peaks separated by one round-trip time (50ns). This multipeak phenomena was investigated numerically. It was found that the multipeak behaviour is initiated by the transient ASE wave injected into the cavity by the Q-switch as it is switched. A novel and elegant method for obtaining singly-pulsed, potentially high peak power output from a highly pumped Q-switch fiber source via regenerative amplification was proposed. This was proven experimentally in a Q-switched thulium fiber. We observed single-peaked pulses with a pulse width of < 14 ns. However, measured pulse energies and peak powers were low (20 µJ and 1.5 kW) due to high cavity losses and deleterious parasitic lasing. Inspection of the output spectrum confirmed that the Q-switched system was in fact a Q-switched ASE source. It is hoped that a more optimal experimental setup will yield better results in terms of the pulse energies and peak powers obtained

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