We demonstrate a high pulse energy and low pulse rate 2 µm femtosecond fiber laser potentially applicable to laser surgery and vehicle laser detection and ranging. The laser is configured by a half kilometer long all-fiber ring cavity with two thulium-doped fiber gains deployed separately and is hybrid mode-locked by the coaction of a graphene saturable absorber and nonlinear polarization rotation. Numerical simulations were carried out to back up the mode-locking stability and estimate the maximally extractable pulse energy. We achieved a pulse energy of about 54.6 nJ, to the best of our knowledge the highest as a seed femtosecond pulse, and a pulse rate of 427 kHz, the lowest ever reported. The pulse duration was about 456 fs as measured by an intensity autocorrelator, and the output optical spectrum showed a wide FWHM of about 51.7 nm. We also examined the stability of the laser output over tens of hours.
This research was supported in part by the Korea Research Institute of Standards and Science project \u2018Establishment of National Physical Measurement Standards and Improvements of Calibration/Measurement Capability,\u2019 grants 20,011,046 and 20011270, and in part by the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education (NRF-2020R1A6A1A03047771).
