[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Pump-Power-Noise Influence on Mode Instabilities in High-Power Fiber Laser Systems

摘要： Transverse mode instability (TMI) is still the most limiting effect for the further average-power scaling of fiber lasers and amplifiers with excellent beam quality. For the modal energy transfer to happen, a phase shift between the thermally-induced refractive index grating and the modal interference pattern in the fiber is essential. In former experimental and theoretical studies it has been shown that such a phase shift can be induced by changes of the pump power. Thus, the investigation of the influence of pump-power noise on the TMI threshold is of high relevance since this noise could act as the main trigger for TMI. To experimentally examine this behavior, white noise with a bandwidth of 2 kHz was generated and transferred via an arbitrary waveform generator to the pump-diode driver. The connected laser diode pumped a 1.1 m long Large Pitch Fiber (LPF) with an active core of ~65 μm from the counter-propagating direction. The TMI threshold of the free-running system was measured according to [5] to be 256 W. The above mentioned frequency range of the white noise was chosen because it was found that only frequency components below 2 kHz have an significant impact on the TMI threshold of the particular system. To investigate the influence of the pump-power noise on the TMI threshold we varied the amplitude of the generated pump noise and measured the latter by sending a partial reflex of the pump beam onto a photodiode. The relative intensity noise (RIN) serves as a measure of the pump-noise amplitude and was calculated by integrating the power spectral density of the measured noise trace. Afterwards, the TMI threshold was determined according to [5] for different RINs, i.e., pump-noise amplitudes. The results are illustrated in Fig. 1. As can be seen, the fiber laser system started with a TMI threshold of 256 W (no artificial noise, RIN = 0.209 %, close to the measured dark noise) and could be decreased by almost a factor of three to 89 W (RIN = 2.927 %) by steadily increasing the pump-noise amplitude. Thus, the experiments have revealed that pump-power noise significantly influences the TMI threshold of fiber laser systems and, thus, likely acts as the main trigger for TMI. The nonlinear decrease of the TMI threshold with increasing pump-power noise (i.e., RIN), as shown in Fig. 1, can be explained with the nonlinearity with which the strength of the energy transfer grows with increasing average power. This correlation together with the current investigations on seed-power noise will also be discussed at the conference. From the experiments it can be concluded that developing low-noise pump sources and drivers could increase the TMI threshold of fiber lasers and amplifiers and, thus, it has the potential to enable a further average-power scaling of these systems with excellent beam quality.