Involvement of a saturable absorber in ultrafast fiber lasers design presents one of the critical bottlenecks restraining their flexibility, energy endurance and performance scaling. There is a serious urge for improved saturable absorbers or finding new ultrafast modulation techniques. Self-mode-locking may remedy this as no additional components are required other than the rare-earthdoped fiber that is anyway required as an active laser medium. In the work, the researchers have presented an advanced concept of self-modelocked Thulium-doped fiber laser without any saturable absorber or modulator, generating 350-fs solitons tuneable within 90 nm wavelength range around 1900 nm with 80 mW of average power at excellent longterm stability. This generation was entirely enabled by the unique design of Thulium-doped optical fibers with reinforced ion-pair concentration to ensure ultrashort pulse generation and to break the picosecond barrier. Furthermore, the unique level structure of the Thulium-doped fiber can be exploited in such a way that its absorption and emission wavelength spectrum is shifted by changing the degree of excited Thulium ions. By changing the intra-cavity power through a variable ratio of output coupling, the laser generation wavelength could be adjusted without a spectral filter. The ultrashort pulse generation was ensured within 90-nm wavelength range.

Overall, the researchers demonstrated that the single rare-earth-doped fiber can play three roles in ultrafast laser cavity: gain medium, saturable absorber and enable wavelength tuneability. Such laser design represents a very promising platform for reliable, compact easy-to-operate ultrafast instruments for a variety of applications. Furthermore, translation of the proposed concept to other rare-earth ions can also facilitate streamlined mid-IR laser research.