*1.3.1. Modelocking methods*

Modelocked lasers can be produced by using either active or passive methods. In active modelocking, the optical modulator such as acousto-optic modulator is used with the help of an external electrical signal, as a modelocker. On the other hand, passive modelocking does not need an external signal to operate [15]. The modelocking is achieved by modulating an intracavity light using some intra-cavity elements, such as nonlinear polarization rotation and saturable absorber. Most of the passively modelocked lasers are achieved using a saturable absorber since it allows the generation of much shorter (femtosecond) pulses. This is attributed to the saturable absorber used, which can adjust the resonator losses much faster than an electronic modulator: the shorter the pulse becomes, the faster the loss modulation, if the absorber has a sufficiently short recovery time [14]. The pulse width can be even smaller than the recovery time of the absorber [6].

In addition, there are some passive modelocking schemes that do not require materials that directly display an intensity dependent absorption. These methods use nonlinear optical effects in intra-cavity components to provide a method of selectively amplify high intensity light in the cavity and attenuate low intensity light in the cavity. Among them, the most successful scheme is Kerr-lens modelocking (KLM), also sometimes referred to as "self-modelocking." This technique uses a nonlinear optical process, the optical Kerr effect, which results in high intensity light being focused differently from low intensity light. By careful arrangement of an aperture in the laser cavity, this effect can be made to produce the equivalent of an ultra-fast response time saturable absorber.
