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The basis of Q-switching is the use of a device which can alter the Q factor or quality factor of the optical resonator of the laser. The Q is a measure of how much light from the gain medium of the laser is fed back into itself by the resonator. A high Q factor corresponds to low resonator losses per roundtrip, and vice versa.
In the technique, initially the laser medium is pumped while the Q-switch device prevents feedback of light into the gain medium (producing an optical resonator with low Q). This produces a population inversion, but laser operation cannot yet occur since there is no feedback from the resonator. Since the rate of stimulated emission is dependent on the amount of light entering the medium, the amount of energy stored in the gain medium will increase as the medium is pumped. Due to losses from spontaneous emission and other processes, after a certain time the stored energy will reach some maximum level; the medium is said to be gain saturated. At this point, the Q-switch device is quickly changed from low to high Q, allowing feedback and the process of optical amplification by stimulated emission to begin. Because of the large amount of energy 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. The net result is a short pulse of light output from the laser, known as a giant pulse, which may have a very high peak intensity.
There are basically two types of Q-switching:
Here, the Q-switch may be a mechanical device (e.g. a shutter, chopper wheel or spinning mirror placed inside the cavity), or (more commonly) some form of modulator such as an acousto-optic or electro-optic device. The reduction of losses (increase of Q) is triggered by an external event, typically an electrical signal. The pulse repetition rate can therefore be externally controlled.
In this case, the Q-switch is a saturable absorber , e.g. an ion-doped crystal material (e.g. Cr:YAG for Q-switching of Nd:YAG lasers) or a passive semiconductorA semiconductor is a material that is an insulator at very low temperature, but which has a sizable electrical conductivity at room temperature. The distinction between a semiconductor and an insulator is not very well-defined, but roughly, a semiconducto device. Initially, the loss of the absorber is high, but still low enough to permit some lasing once a large amount of energy is stored in the gain medium. As the laser power increases, it saturates the absorber, i.e., rapidly reduces the resonator loss, so that the power can increase even faster. Ideally, this brings the absorber into a state with low losses to allow efficient extraction of the stored energy by the laser pulse. After the pulse, the absorber recovers to its high-loss state before the gain recovers, so that the next pulse is delayed until the energy in the gain medium is fully replenished. The pulse repetition rate can only indirectly be controlled, e.g. by varying the laser's pump power.
A typical Q-switched laser (e.g. a Nd:YAG laser) with a resonator length of e.g. 10 cm can produce light pulses of several or some tens of nanosecondsTo help compare orders of magnitude of different times this page lists times between 10-9 seconds and 10-8 seconds (1 nanosecond and 10 nanoseconds) See also times of other orders of magnitude. shorter times 1 nanosecond 1. 0 ns cycle time for frequency 1 duration. Even when the average power is well below 1 W, the peak power can be many kilowatts. Large-scale laser systems can produce Q-switched pulses with energies of many joules and peak powers in the gigawatt region. On the other hand, passively Q-switched microchip lasers (with very short resonators) have generated pulses with durations far below one nanosecond and pulse repetition rates from hundreds of Hertz to several MHz.
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