#12-1. Operation Principles and Methods of Different Lasers Ⅱ

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Types of Q-switched Laser

The most widely used Q-switched laser is the actively Q-switched solid state laser. Here, the word ‘active’ means that the lasing characteristics are controlled by external signals, and ‘Q-switching’ refers to the method of adjusting laser output by controlling the gain in the laser resonator. Adjusting the laser output involves modifying the pulse duration, energy and peak power. To understand these variables, we need to know what physical phenomena take place inside the resonator, and for that, we first need to understand the characteristics of the gain in the active medium(e.g. Nd:YAG rod) of the resonator and the process of energy transfer. Here, I would like to briefly discuss the factors affecting the process of energy storage, gain, as well as the main components of a laser resonator and their roles.

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Q-switching and Gain

The ‘Q’ in Q-switching refers to the quality of the resonator, that is, how effectively the resonator amplifies light and is closely related to laser characteristics. Light becomes stronger when it makes a feedback loop across the active medium(gain) but it also grows weaker due to diffraction or transmission by optical components(loss). If the gain is greater than loss while the light travels back and forth inside the resonator, and if this phenomenon occurs repeatedly, the light becomes increasingly stronger and generates laser, which is eventually emitted through the output coupler.

When the Q factor is prevented in the Q-switch, the reflection is reduced inside the resonator during optical pumping(flashlamp or diode laser). This prevents resonator oscillation and increases the population inversion at the active medium, resulting in the increased amplifier gain of the active medium. The amplifier gain, or single pass gain, refers to the gain obtained when light passes through the active medium once. When population inversion is maximized(generally before the end of optical pumping), the Q factor rapidly changes from low to high Q, allowing the normal formation of the resonator(the total mirror or total reflector starts to work); this results in optical amplification and generates a giant pulse for a short period of time.

Q-switching is carried out by inducing large losses in the resonator during optical pumping. This prevents the number of excited-state atoms and amplifier gain value from being reduced until they reach the maximum level. The Q-switch is basically a shutter between the active medium and total mirror.

When the shutter is closed, this blocks the total mirror, hindering the route of light amplification and thereby preventing light output. On the other hand, opening the shutter when the amplifier gain reaches a desired value creates a strong laser beam with a high peak power and short pulse duration.

The Effect of Q-switching on Laser Parameters

In order to further understand how energy is converted during Q-switching and requirements for Q-switching, it would be helpful to examine various parameters of lasing with regard to the presence or absence of a laser mirror and with Q-switching.

Lasing Parameters in the Absence of a Laser Mirror

There are no excited-state atoms immediately before the light output from flashlamp(pump beam) is absorbed into the active medium. However, the number of excited-state atoms and amplifier gain increase when the pump beam is absorbed in the active medium. The population inversion occurs when the amplifier gain value exceeds 1 by pump beam. But in the absence of a laser mirror to feed back the light, the loss increases and the loop gain(double pass gain) becomes 0. In other words, laser is not oscilated. Under these circumstances, optical pumping in the absence of a laser mirror allows; first, sufficiently increased number of excited-state atoms in the active medium and amplifier gain due to a great loss in the resonator, and second, the characteristics of light coming out from the active medium is spontaneous emission/fluorescence.

Lasing Parameters in the Presence of a Laser Mirror

As shown in <Figure 13-1>, in the presence of a laser mirror, the number of excited-state atoms and amplifier gain show similar patterns as those in the absence of a laser mirror. However, when the loop gain value exceeds 1 through the action of pump beam, in other words, when the gain is comparable to the loss, the lasing activity begins with a sharp drop in the number of excited-state atoms. This reduces the amplifier gain and in turn, lowers the loop gain value to below 1, terminating the lasing activity. After lasing stops, the amplifier gain keeps decreasing until it reaches a certain value, at which point the number of excited-state atoms starts to increase again by optical pumping. This triggers lasing to occur again through the same process. Lasing occurs repeatedly while optical pumping continues and stops when optical pumping is terminated. The amplifier gain and the number of excited-state atoms are very small compared to those in the absence of the laser mirror. This is because the loss is so great that the maximum gain value can be reached in the absence of the mirror, while the loss is moderate if the mirror is present, in which case lasing takes place with identical gain and loss values, precluding a high gain value. This state can be called gain saturation.