*1.2.1. Active Q-switching*

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**

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

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

Q-switching, also known as giant pulse formation or Q-spoiling, [8] is a powerful technique by which a laser can be made to produce a pulsed output beam. This technique is capable of producing light pulses with extremely high (gigawatt) peak power, much higher than the continuous wave mode (constant output) operation of the laser. As compared to modelocking, another technique for pulse generation with lasers, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations than it. It was in 1958 when Q-switching was proposed by Gordon Gould [9]. Practically, it was achieved in 1961 or 1962 by Hellwarth and McClung using Kerr cell shutters in a ruby laser,

A variable attenuator is required inside the laser's optical resonator to produce Q-switched laser light. When the attenuator is functioning, light which leaves the gain medium does not

**Figure 5.** Cross-sectional images of a few inner cladding shapes used in double clad fiber lasers (a) octagon (b) double-D

for rare earth doped fibers.

26 Laser Technology and its Applications

and thermal effects [7].

**1.2. Q-switched lasers**

and these require electricity for switching [10].

fiber lasers.

(c) decagon [6].

In active Q-switching, the losses are modulated with an active control element so-called active Q-switcher, either by using an acousto-optic or electro-optic modulator, which requires an external electrical signal to operate. The pulse is formed shortly after an electrical trigger signal arrives. There is also mechanical type Q-switchers such as spinning mirrors, used as end mirrors of laser resonators. The pulse repetition rate can be controlled by the active modulator in an actively Q-switched laser. Higher repetition rates lead to lower pulse energies.
