Review of mode-locked fiber lasers

The interest in mode-locked fiber lasers goes beyond the interesting physics that can be observed in their operation. There are a number of potential operating characteristics that make them particularly attractive and as a result a large number of designs have been examined, attempting to optimize the operation of the laser to suit the particular application. There have also been a number of difficulties and these have convinced a number of researchers that fiber lasers will never be practical sources. In fact there are now a number of practical mode-locked sources that can be purchased commercially, and more are being developed. As more is known about the various mechanisms at work, methods are being developed to control them. Mode-locking techniques generally divide into two categories: active and passive. The actively mode-locked lasers have cavities that most closely follow their solid-state counterparts. The differences are that the pulses can have a significant amount of nonlinear shaping as they propagate in the cavity and therefore violate the classical limitations on pulse-width. There are many more varieties of passive mode-locking than active. The major techniques (and a few of the more interesting minor techniques) will be covered. Some of the methods will be nonlinear loop mirrors, nonlinear polarization rotation, semiconductor mode-locking, and frequency-shift mode-locking. The physical basis for these techniques will be discussed and what determines the pulse width, repetition rate, and spectrum