In this work we present investigation of TMI in Yb-doped FA-LPF using spatially and temporally resolved imaging. We present ST-measurements that are clearly describing the system noises and that can be used to optimize amplifier systems in order to mitigate TMI. Comparative TMI measurements between the same FA-LPF in amplifier and laser configuration are also presented. These measurements show similar results with respect to optical efficiencies but large differences with respect to TMI.
In this work we investigate transverse mode instability (TMI) in the presence of pump intensity noise and a controlled perturbation of the input coupling for a rod-type fiber amplifier using spatially and temporally resolved imaging (ST). We show that inherent pump intensity noise from the power supply can define significant peaks in the resulting TMI spectrum. ST measurements in the transition region of TMI also indicates that the simple picture of TMI being seeded by the combination of a static initial fraction of LP11 and pump or signal intensity noise is not valid for our measurements. Furthermore, we present seeding of TMI by perturbing the input coupling dynamically which allows measurements of the TMI gain as a function of frequency and signal power.
In this work a novel accurate method of measuring beam fluctuations is presented and applied to analyze transverse mode instabilities (TMI). The new measurement, ST-measurement, uses Fourier analysis on data from a high-speed camera to achieve raw spatial information about beam fluctuations. TMI in a 65 μm mode- field-diameter aeroGain-ROD-PM85 fiber is investigated using both the ST- and standard photo detector measurement. A comparison of the two measurements shows the quantitative and qualitative superiority of the new ST-measurement due to the spatial information. Numerical simulations are carried out to support the interpretation of the data.
In this work, an all-solid Ytterbium doped double-clad single-mode (SM) large-mode-area (LMA) photonic crystal fiber (PCF) is characterized in a monolithic amplifier setup. The characterized all-solid PCF combines the SM operation of LMA PCFs while maintaining simple splicing of all-solid fibers. The fiber has been tested in a pulsed monolithic forward pumped amplifier system achieving excellent modal properties. The fiber is polarization maintaining and has a mode field diameter of approximately 24 μm. At optimal fiber length the output signal power of the amplifier was measured to 43 W limited by pump power with an optical to optical efficiency of 61 %. Polarization extinction ratios (PER) above 18 dB was observed for all fiber lengths. Simple splicing with splice losses < 0.2 dB and maintained polarization are reported using a standard Fujikura fusion splicer.
Large-mode-area (LMA) photonic crystal fiber (PCF) amplifiers are attractive for high-power amplification of single-mode (SM) narrow-linewidth light. Traditionally, LMA PCF designs include air holes but splicing and interfacing of fibers with air holes is complicated. Recently, this was addressed with the development of a LMA PCF amplifier fiber without air holes but with SM operation maintained. This all-solid fiber can be spliced with standard splicers and thus enables integration of PCFs into all-fiber monolithic laser systems. The new fiber, named DC-250/30-PM-Yb-FUD, is demonstrated in a free-space configuration for high-power amplification of 1064 nm light with a narrow linewidth of less than 20 kHz. A seed power of 380 mW is amplified to 40 W, corresponding to a single-stage gain of 20 dB, with an optical to optical efficiency of 69%. No indication of stimulated Brillouin scattering is observed, and the output is stable during frequency modulation of the seed laser.
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