Properties Of All-normal-dispersion Modelocking Fiber Lasers

Fiber femtosecond lasers are appealing alternatives to bulk solid-state lasers owing to their compact size, excellent stability, and no-alignment requirement. However, the pulse energy increasing is usually limited by wave breaking. A promising scheme was recently proposed in the all-normal-dispersion (ANDi) cavities, where the gain-guided soliton, or dissipative soliton, exists. The normal dispersion "linearizes" the accumulated nonlinear phase shift, which increases the spectral bandwidth while does not destabilize the pulse. Pulse energies with tens of nJ have been reported.In this thesis, the ways of increasing the pulse energy were studied both in theory and experiment.1. The ANDi fiber laser based on nonlinear polarization evolution (NPE) effect was studied both in theory and experiment. A NPE based passive modelocking ANDi fiber laser has been turned out and dissipative solitons with a repetition rate of18.58MHz. The energy of the output pulse is1.88nJ and the spectral width is27.1nm. In the ANDi cavity a balance exists even if the nonlinearity is excessive. However, such a balance becomes so fragile that a tiny perturbation from the ASE noise breaks the balance and induces the wave breaking of dissipative soliton.2. Two ways of suppressing the ASE noise to make the pulse stable have been studied. First, we proposed to employ a linear spectral filter, with bandwidth much larger than similariton, to suppress the wave breaking. Such design has the advantage that the pulse energy could be further increased since the inserted filter shows no impact on the pulse shaping. Also we demonstrate here, for the first time, a long-relaxation-time saturable absorber (SA) will suppress the dissipative soliton wave breaking. Further more, too long relaxation time can also make the generated pulse have the unwanted characteristics such as longer pulse width, narrower spectral bandwidth and lower pulse energy.3. We firstly demonstrated that high energy dissipative solitons can be generated in an active mode-locking ANDi fiber laser theoretically. Our simulation showed that the shape and parameters of the ouput pulse depended on the modulation curve.4. The active mode-locking ANDi fiber laser has been turned out. Dissipative solitons with repetition rate of29.6MHz and2.43nJ energy have been got. And we found that the changing of modulation rate can make the central wavelength have an offset over10nm.