High-energy all-fiber all-positive-dispersion mode-locked ring Yb laser with 8 km optical cavity length

It has been demonstrated recently in that lengthening of the optical resonator in mode-locked fiber laser can be used as a design tool to increase energy of pulses generated in such systems. In this work we report stable mode synchronization in all-fiber pulsed laser using resonator with a record optical path of 8 km. The laser system scheme is shown in Fig 1. Laser mode synchronization was achieved by exloiting the nonlinear rotation of generated radiation polarisation. The cavity length has been controlled applying standard single mode fiber SMF-28. Average output power of radiation did not exceed 150 mW, a level that was limited by the maximal working power of fiber polarization beam splitter used for the out-coupling of the generated emission. Figure 2 demonstrates autocorrelation function of the generated laser pulses with duration about 10 ns and the energy of 4 mJ. Measured power spectrum of the generated output pulses is shown in Fig. 3.