**1.1. Cladding pump**

The pump light can be easily coupled into the large inner cladding and propagates through the inner cladding, while the signal propagates in the smaller core as shown in **Figure 4**. The

doped core gradually absorbs light from the cladding as it propagates through the core and amplifies it [4]. This type of pumping scheme is referred as cladding pumping, an alternate scheme to the conventional core pumping, where the pump light is coupled into the small core. Cladding pumping has revolutionized the design of fiber amplifiers and fiber lasers [3]. Using this technique, contemporary fiber lasers can generate continuous power up to several kilowatts, besides allowing the signal light to maintain near diffraction limited beam

Cladding Pumped Thulium-Ytterbium Short Pulse Fiber Lasers

http://dx.doi.org/10.5772/intechopen.81060

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quality [4].

**Figure 4.** Cladding pump scheme.

**Figure 2.** Structure of single and double clad fibers.

**Figure 3.** Typical refractive index profile of double clad optical fiber.

**Figure 1.** Mode propagation in single- and multi-mode fibers [1].

**Figure 2.** Structure of single and double clad fibers.

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

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,

The pump light can be easily coupled into the large inner cladding and propagates through the inner cladding, while the signal propagates in the smaller core as shown in **Figure 4**. The

power pump sources and therefore cannot generate high output power.

which possess high power but low brightness.

**Figure 1.** Mode propagation in single- and multi-mode fibers [1].

**1.1. Cladding pump**

24 Laser Technology and its Applications

**Figure 3.** Typical refractive index profile of double clad optical fiber.

**Figure 4.** Cladding pump scheme.

doped core gradually absorbs light from the cladding as it propagates through the core and amplifies it [4]. This type of pumping scheme is referred as cladding pumping, an alternate scheme to the conventional core pumping, where the pump light is coupled into the small core. Cladding pumping has revolutionized the design of fiber amplifiers and fiber lasers [3]. Using this technique, contemporary fiber lasers can generate continuous power up to several kilowatts, besides allowing the signal light to maintain near diffraction limited beam quality [4].

In a double clad fiber, cladding shape is of extreme importance, particularly in case of small core diameter compared to the cladding size. Circular symmetry is considered the worst in a double clad fiber, because many modes of light in the cladding missed the core, therefore cannot pump the core [5]. Normally, claddings are noncircular, which enhance the absorption of the pump light in the doped core [7]. Different shapes of inner cladding are shown in **Figure 5** for rare earth doped fibers.

Hence, double clad cladding pumped fiber lasers are regarded as devices that can generate diffraction limited single-mode laser light using multi-mode laser diodes as the pump. Due to large size of inner cladding, high pump powers can be injected in a double clad fiber. However, the core size puts limits on the output power due to the danger of optical damage and thermal effects [7].

Different lasers operating in continuous wave or quasi-continuous wave mode have limited optical output power, linked with the maximum available pump power [10]. The peak output power of a laser can be enhanced by concentrating the available energy in a single, short optical pulse, or in a periodic sequence of optical pulses as in Q-switched and modelocked fiber lasers.
