Methods of modelocking fiber lasers

It has long been imagined that the unique characteristics of the fiber laser cavity might allow for the development of versatile, compact, efficient sources of coherent light. Particularly in the area of ultrashort pulse production, the fiber geometry provides access to the nonlinear index of the cavity providing additional degrees of freedom for pulse formation mechanisms. The first modelocked fiber laser, reported in 1983, actually used a material saturable absorber to provide the passive loss mechanism necessary for modelocking. The UV radiation from a flashlamp produced transient color centers in the glass matrix which in turn modelocked the laser, producing Q-switched trains of ultrashort pulses (Dzhibladze et al., 1983).

Since that time nearly every method of modelocking that has been used in a bulk laser has been translated into the fiber geometry. In this chapter we will attempt to provide an overview of the more successful attempts and provide the insight necessary to choose the best technique for a given application.

The chapter is organized according to modelocking technique, starting with active modelocking, both amplitude and phase modulation, and then proceeding with the newer techniques of passive modelocking. Of the methods that deal with artificial fast saturable absorbers, this chapter will concentrate on the use of the nonlinear loop mirror (Sagnac interferometer) in the figure eight laser (F8L) configuration (4.2), rather than polarization rotation which will be treated in Chapter 5. Included in this section will be an extensive discussion of the generation and consequences of the sidebands present in the spectra.