Q Switching

Is there any method or technique to produce a high output pulse? Yes, the method is known as Q-switching. Q-switching is a technique used to produce a high output pulse. It is accomplished by using a device to prevent the reflection of photons back and forth  in the active medium. This produces a  higher population inversion in the  metastable state. Then suddenly the optical cavity is opened to permit a large fraction of stored energy to be emitted in the form of very intense pulse of laser radiation. Q-switched lasers produce pulses of  10 to 250 nanoseconds.

Q-switching is also known as Q-spoiling.

As the quality factor Q of a laser cavity shows the ability of the cavity to store energy, thus, high Q means that high energy can be stored  in the cavity and a low Q means that the cavity will rapidly dissipate its energy. As the technique of Q-switching involves switching the optical cavity quality factor Q from a low to a high value, therefore, it is known as Q-switching.

Techniques of the Q-Switching :

There are four different techniques of Q-switching known as:

a) Mechanical Shutters

b) The rotating reflector method

c) Electro-optical shutters

d) Passive shutter


Let us discuss them one by one:

a) Mechanical Shutters.

In this  case, a shutter is introduced in front of  one of the mirrors in laser cavity. If the active medium is continuously pumped keeping the shutter closed, then the population inversion in the cavity goes on increasing and reaches a high value. Now, if the shutter is opened suddenly, the population inversion would correspond to a value much above the  threshold and the energy stored in the cavity will be released in the form of a short pulse of light with a high value of intensity. If the shutter is opened in a  time much shorter than the time required for the building of laser oscillation, the output would consist of a giant pulse of light. If the shutter opening is slow, the output would be a series  of  pulses having smaller peak power.

Next time, I will discuss the more techniques of Q-switching.

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