Generation of ultra-flat broad spectrum with stable single-pulse mode-locking in double-clad Yb-doped fiber laser

We achieved an ultra-flat broad spectrum output with a 20-dB bandwidth of 77.85 nm in a double-clad Yb-doped fiber laser. The intensity difference between the highest and lowest points of the spectrum indicates a flatness better than 4 dB. More notably, this ultra-flat broad spectrum maintains a stable single-pulse mode-locking state. With the increase of pump power, an ultra-wide spectrum with a 20-dB bandwidth approaching 100 nm was formed at a pump power of 2.25 W. Additionally, we obtained a 9-pulse mode-locked state at another PC station with the same pump, which is the highest number of stable mode-locked pulse bursts observed so far with a first-order Raman frequency shift. This fiber laser shows its benefits of ultra-flat broad spectrum, high stability, and ease of fabrication, which provides a new method of obtaining the broadband light source for multiple practical applications.