Femtosecond harmonic mode-locked fiber laser based on centimeter-level Er: YAG crystal-derived silica fiber

A ∼1.5 μm amplified spontaneous emissions spectral bandwidth of 60 nm and gain coefficient of 1.52 dB/cm were obtained from a 5-cm-long Er: YAG crystal-derived silica fiber (ECDSF), which was fabricated using a CO2 laser-heated drawing technique. An all-fiber-integrated passively femtosecond harmonic mode-locked fiber laser (fs-HMLFL) was constructed, in which a 5-cm-long ECDSF was used as a gain fiber and a long-period grating acted as a polarizer to meet nonlinear polarization rotation mechanism. To the best of our knowledge, this is the first demonstration of a fs-HMLFL in similar ECDSFs. Up to 751.7500 MHz repetition rate lasing pulse could be achieved with a pulse width of 629.75 fs, corresponding to the 21st-order harmonics of fundamental frequency. Moreover, its output spectral bandwidth, polarization degree, and super-mode suppression ratio are 8.37 nm, 98.7%, and 35.56 dB respectively. In addition, by adjusting the intracavity polarization state, a broad spectrum fundamental frequency mode-locked state was observed, exhibiting a spectral bandwidth of 11.11 nm and a pulse duration of 412.60 fs. These results suggest that this fs-HMLFL based on broadband and high-gain ECDSFs are promising for astronomical frequency combs and ultra-large capacity optical fiber communication systems. Especially, the utilization of centimeter-level ECDSF shows good performance in terms of spectral bandwidth, gain, and nonlinearity and provides an idea for the miniaturization and integration of ultrafast pulse lasers, which lays the foundation for applications in optical communications, precision spectroscopy, and micro-nano scale material processing.