Q-switching

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Q-switching, sometimes known as giant pulse formation, is a technique discovered circa 1962 by R.W. Hellwarth and F.J. McClung using electrically switched Kerr cell shutters and is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high (gigawatts) peak intensity, much higher than would be produced by the same laser if it were operating in a continuous wave (constant output) mode. 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.

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Principle of Q-switching

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:

Active 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.

Passive Q-switching

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 semiconductor 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.

Typical performance

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 nanoseconds 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.

Applications

Q-switched lasers are often used in applications which demand high laser intensities in nanosecond pulses, such as dentistry and metal cutting. However, Q-switched lasers can also be used for measurement purposes, e.g. for distance measurements (range finding) by measuring the time it takes for the pulse to get to some target and the reflected light to get back to the sender.

See also modelocking and gain-switching.

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