**1.2. Q-switched lasers**

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, and these require electricity for switching [10].

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

return or cannot travel back and forth, this restricts the lasing, putting the optical cavity in low Q factor or high loss condition. A high Q factor represents low resonator losses per round trip.

Cladding Pumped Thulium-Ytterbium Short Pulse Fiber Lasers

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First, the laser medium is pumped, while the Q-switch is set to prevent feedback of light into the gain medium (producing an optical resonator with low Q factor). This produces a population inversion by losing pump energy through absorption by electrons, thus pumping electrons to higher energy level; this accumulates energy in the gain medium. However, the laser operation cannot yet begin, because there is no feedback from the resonator [3]. As the rate of stimulated emission depends on the amount of light entering the medium, therefore, the amount of energy stored in the gain medium increases as the medium is pumped. Due to losses from spontaneous emission along with other processes, the stored energy takes some time to reach some maximum level; the medium is said to be gain saturated. At this point, Q-switch rapidly changes from low to high Q, allowing feedback and the process of optical amplification by stimulated emission to begin [3]. Since a large amount of energy is already stored in the gain medium, the intensity of light in the laser resonator builds up very quickly; this also causes the energy stored in the medium to be depleted almost as quickly. This develops a short pulse of light output from the laser, known as a giant pulse, which may have very high peak intensity. Generally, several round trips are needed to completely depopulate the upper energy level and several more round trips to empty the optical cavity, so the duration of the pulse is greater than one round trip. The peak power (the pulse energy divided by its duration) of these lasers can be in the megawatt range or even higher. There are two main

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 passive Q-switching, the losses are modulated or controlled by optical cavity light, rather than some external electrical source. A saturable absorber device is normally used as a Q-switcher in this technique. The transmission of this device increases when the intensity of light exceeds some threshold. The material may be an ion-doped crystal like Cr:YAG (chromium-doped Yttrium-Aluminum garnet), which is used for Q-switching of Nd:YAG (neodymium-doped Yttrium-Aluminum garnet) lasers, a bleachable dye, graphene mechanical exfoliation and PVA thin film, semiconductor saturable absorber mirrors (SESAM), and carbon nanotubes embedded in PVA thin films. Initially, the loss of the absorber is high, once a large amount of energy is stored in the gain medium, the laser power increases, and it saturates the absorber, and light can pass through as there are no electrons in the ground state

in an actively Q-switched laser. Higher repetition rates lead to lower pulse energies.

The variable attenuator is commonly known as "Q-switch."

techniques for Q-switching: active and passive.

*1.2.1. Active Q-switching*

*1.2.2. Passive Q-switching*

**Figure 5.** Cross-sectional images of a few inner cladding shapes used in double clad fiber lasers (a) octagon (b) double-D (c) decagon [6].

return or cannot travel back and forth, this restricts the lasing, putting the optical cavity in low Q factor or high loss condition. A high Q factor represents low resonator losses per round trip. The variable attenuator is commonly known as "Q-switch."

First, the laser medium is pumped, while the Q-switch is set to prevent feedback of light into the gain medium (producing an optical resonator with low Q factor). This produces a population inversion by losing pump energy through absorption by electrons, thus pumping electrons to higher energy level; this accumulates energy in the gain medium. However, the laser operation cannot yet begin, because there is no feedback from the resonator [3]. As the rate of stimulated emission depends on the amount of light entering the medium, therefore, the amount of energy stored in the gain medium increases as the medium is pumped. Due to losses from spontaneous emission along with other processes, the stored energy takes some time to reach some maximum level; the medium is said to be gain saturated. At this point, Q-switch rapidly changes from low to high Q, allowing feedback and the process of optical amplification by stimulated emission to begin [3]. Since a large amount of energy is already stored in the gain medium, the intensity of light in the laser resonator builds up very quickly; this also causes the energy stored in the medium to be depleted almost as quickly. This develops a short pulse of light output from the laser, known as a giant pulse, which may have very high peak intensity. Generally, several round trips are needed to completely depopulate the upper energy level and several more round trips to empty the optical cavity, so the duration of the pulse is greater than one round trip. The peak power (the pulse energy divided by its duration) of these lasers can be in the megawatt range or even higher. There are two main techniques for Q-switching: active and passive.
