**1. Introduction**

Fiber laser is a mature technology that has become an essential tool facilitating a wide range of scientific, medical, and industrial applications. Fiber lasers are advancing rapidly due to their ability to generate stable, efficient, and diffraction-limited beams with significant peak and average powers.

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In fiber optics, optical signals travel in hair thin strands of glass or plastic fiber [10]. The light propagates in the core at the center of the fiber, surrounded by an optical material called cladding that confines the light in the core employing the phenomenon of total internal reflection. There are two basic types of fibers: single-mode and multi-mode fibers. Single-mode fiber has a core diameter of 8.3–10 microns and supports only one mode of transmission. Multi-mode fiber has a core diameter of either 50 or 62.5 microns. Multimode fibers support multiple modes as shown in **Figure 1**. Due to their smaller core size, these fibers cannot support high power pump sources and therefore cannot generate high output power.

To overcome the three limitations (small core radius, larger attenuation, and dispersion) of high speed transmission, a new type of optical fiber called double clad optical fiber is proposed [2]. It consists of three layers of optical material instead of the usual two as shown in **Figure 2**. The inner-most layer is called the core, surrounded by the inner cladding, while the inner cladding is surrounded by the outer cladding. All the three layers are made of materials with different refractive indices. The typical refractive index profiles are shown in **Figure 3**. Generally, the outer cladding is made of a polymer material rather than glass. Refractive index of inner cladding is higher than the outer cladding. This enables the inner cladding to guide light by total internal reflection, but for a different range of wavelengths than the core. Also, the inner cladding has larger area and higher numerical aperture so that it can support large number of modes. This allows multi-mode laser diodes to be used as the pump source, which possess high power but low brightness.
