**1.4. Optical solitons**

A fascinating manifestation of the fiber nonlinearity is the development of optical solitons, created due to the balance between dispersive and nonlinear effects [8]. Solitons are a unique type of wave packets, capable of propagating unaltered over long distances inside a fiber. Solitons have been discovered in many branches of physics. In the field of photonics, especially in fiber optics, solitons not only are of fundamental interest, but they have also found practical applications in the field of fiber-optic communications [16].

The pulse maintains the shape and width, along the entire length of the fiber, if the effects of SPM and GVD cancel out with each other. Such a pulse is called solitary wave pulse or soliton. The pulse that has the above property is the sech profile pulse, which is a solution of the nonlinear Schrodinger equation NLSE [13].

**Figure 8** shows the spectrum of different types of modelocked fiber lasers. The conventional soliton and stretched pulse are obtained in anomalous dispersion fiber laser setup, where the pulse shaping is mainly due to the natural balance between the anomalous dispersion and the fiber nonlinearity. On the other hand, dissipative solitons (DSs) are obtained in the normal dispersion region as a result of the combined effects among the fiber nonlinearity, cavity dispersion, gain and loss, and spectral filtering [13]. Moreover, the soliton shaping is strongly dependent on the dissipative effects. Consequently, dissipative solitons have a wider pulse duration and lower peak power, compared to conventional solitons and stretched solitons [17].

**Figure 8.** Different types of solitons [17].

the minimum pulse duration can be calculated consistent with the measured laser spectral

Modelocked fiber lasers are capable of producing pulses with widths from close to 30 fs to 1 ns at repetition rates, ranging from less than 1 MHz to 100 GHz. This broad range along with a compact size of optical fiber lasers is quite unique in laser technology, making them feasible for a large range of applications. As modelocked fiber laser technology was developed and these lasers became commercially available, they have been used in various fields, such as laser radar, all-optical scanning delay lines, THz generation, injection-seeding, twophoton microscopes, optical telecommunications, and nonlinear frequency conversion, just to mention the most widely publicized areas [12]. Surely, modelocked fiber lasers are a premier source of short optical pulses sharing an equal position with semiconductor and

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

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

A fascinating manifestation of the fiber nonlinearity is the development of optical solitons, created due to the balance between dispersive and nonlinear effects [8]. Solitons are a unique type of wave packets, capable of propagating unaltered over long distances inside a fiber.

width [13].

30 Laser Technology and its Applications

solid-state lasers [14].

*1.3.1. Modelocking methods*

the recovery time of the absorber [6].

ultra-fast response time saturable absorber.

**1.4. Optical solitons**
