In the last few decades, ultrafast fiber lasers have gained increasing interest, thanks to their applications in biomedicine, fundamental research, and industry.
When compared to rare-earth-doped ultrafast fiber lasers that can operate only in restricted spectral range, ultrafast Raman fiber lasers have a peculiar advantage of wavelength agility. However, their characteristics like pulse width, pulse energy, and pulse train stability are worse than their rare-earth-doped equivalents.
Among various types of pulse shaping mechanisms, dissipative soliton (DS) is a sound method that has the ability to enhance pulse stability and energy.
With regards to saturable absorbers, nonlinear optical loop mirror (NOLM) is a method well-matched with all polarization-maintaining (PM) fiber configurations and is capable of realizing enduring stable pulse operation.
More recently, a team of researchers from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences used a NOLM and developed a mode-locked Raman DS fiber laser. The associated research has been published in Optics Express.
Spectral filtering is an essential technique for pulse and spectral shaping in the DS mode-locking. Thus, the researchers used an all-fiber Lyot filter based on the birefringence and dispersion of PM fiber in their experiment for the purpose of a spectral filter.
In addition, a 1064-nm amplified spontaneous emission source was used as a pump source in mode-locked Raman fiber laser for the additional enhancement of pulse stability.
The researchers described that stable 1120-nm Raman DS pulses with typical DS features could be achieved at a repetition rate of 1.23 MHz with their experiment. The highest pulse energy was 1.23 nJ, and the lowest pulse width was 63 ps. The signal-to-noise ratio of the radio frequency spectrum of the Raman DS was as high as 85 dB.
Their experimental finding revealed that NOLM-based Raman DS ought to be a potential solution to produce wavelength-flexible linearly polarized pulses with high pulse stability. Hence, this study could offer a new method to achieve linearly polarized ultrafast laser at versatile wavelengths.
This study was funded by the National Natural Science Foundation of China and China Postdoctoral Science Foundation.