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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2010-03-04

Definition: a method for obtaining energetic pulses from lasers by modulating the intracavity losses

Q switching is a technique for obtaining energetic short (but not ultrashort) pulses from a laser by modulating the intracavity losses and thus the Q factor of the laser resonator. The technique is mainly applied for the generation of nanosecond pulses of high energy and peak power with solid-state bulk lasers.

The generation of a Q-switched pulse (sometimes called a giant pulse) can be described as follows:

• Initially, the resonator losses are kept at a high level. As lasing cannot occur at that time, the energy fed into the gain medium by the pumping mechanism accumulates there. The amount of stored energy is often limited only by spontaneous emission (particularly for continuous pumping), in other cases (with strong enough gain) by the onset of lasing or strong ASE, if not simply by the pump energy available. The stored energy can be a multiple of the saturation energy.
• Then, the losses are suddenly (with active or passive means, see below) reduced to a small value, so that the power of the laser radiation builds up very quickly in the laser resonator. This process typically starts with noise from spontaneous emission, which is amplified to macroscopic power levels within hundreds or thousands of resonator round trips.
• Once the temporally integrated intracavity power has reached the order of the saturation energy of the gain medium, the gain starts to be saturated. The peak of the pulse is reached when the gain equals the remaining (low) resonator losses. The large intracavity power present at that time leads to further depletion of the stored energy during the time where the power decays. In many cases, the energy extracted after the pulse maximum is similar to that before the pulse maximum.

The pulse duration achieved with Q switching is typically in the nanosecond range (corresponding to several cavity round trips), and usually well above the cavity round-trip time. The energy of the generated pulse is typically higher than the saturation energy of the gain medium and can be in the millijoule range even for small lasers. The peak power can be orders of magnitude higher than the power which is achievable in continuous-wave operation. Even for lasers with moderate size and with moderate focusing of the beam, the peak intensity can be sufficient for optical breakdown in air.

In most cases, Q-switched lasers generate regular pulse trains via repetitive Q switching. The pulse repetition rate is typically in the range from 1–100 kHz, sometimes higher. Passively Q-switched microchip lasers have reached pulse durations far below 1 ns and repetition rates up to several megahertz, whereas large (typically amplified) laser systems can deliver pulses with many kilojoules of energy and durations in the nanosecond range.

Lasers to which the Q-switching technique is applied are called Q-switched lasers. The first experimental demonstrations were performed in 1961 at Hughes Aircraft Company [2], shortly after the demonstration of the first laser in the same laboratory.

The resonator losses can basically be switched in different ways:

Active Q Switching

For active Q switching (Figure 1), the losses are modulated with an active control element (active Q switch), typically either an acousto-optic or electro-optic modulator. Here, the pulse is formed shortly after an electrical trigger signal arrives. There are also mechanical Q switches such as spinning mirrors, used as end mirrors of laser resonators. In any case, the achieved pulse energy and pulse duration depends on the energy stored in the gain medium, i.e. on the pump power and the pulse repetition rate.

Figure 1: Temporal evolution of gain and losses in an actively Q-switched laser. The Q switch is activated at t = 0. The power starts to rise exponentially at this point, but becomes high only after ∼ 0.2 μs.

Interestingly, the switching time of the modulator does not need to be comparable to the pulse duration – it can be much longer than that, since it takes many resonator round trips for an intense pulse to be formed. If it is too long, however, this may lead to double pulses or to certain instabilities.

Passive Q Switching

For passive Q switching (sometimes called self Q switching), the losses are automatically modulated with a saturable absorber (Figure 2). Here, the pulse is formed as soon as the energy stored in the gain medium (and thus the gain) has reached a high enough level. In many cases, the pulse energy and duration are then fixed, and the pump power only influences the pulse repetition rate.

Figure 2: Temporal evolution of gain and losses in a passively Q-switched laser. Shortly after the laser gain exceeds the resonator losses, a short pulse is emitted. Once the absorber starts to be saturated, the power rises rapidly.

A frequently used saturable absorber material for passive Q switching of 1-μm YAG lasers is Cr⁴⁺:YAG. For 1.5-μm erbium lasers, there are Co²⁺:MgAl₂O₄, Co²⁺:ZnSe and other cobalt-doped crystals, and glasses which are doped with PbS quantum dots. V³⁺:YAG crystals are suitable for the 1.3-μm region. Semiconductor saturable absorber mirrors can be applied at various wavelengths.

Compared with active Q switching, passive Q switching is simple and cost-effective (eliminating the modulator and its electronics), and is suitable for very high pulse repetition rates. However, the pulse energies are typically lower, and external triggering of the pulses is not possible (except with an optical pulse from another source).

Various Technical Issues

Doped insulator solid-state lasers are most suitable for Q switching, since their gain media have long upper-state lifetimes and high saturation energies, and hence the capability to store large amounts of energy. Bulk lasers are normally preferable over fiber lasers, since their larger mode areas allow more energy to be stored, and their shorter resonators allow for shorter pulses.

For both active and passive Q switching, higher pulse repetition rates usually imply longer pulses. This is because the reduced pulse energy leads to a weaker modulation of the net gain, and thus to a slower rise and decay of the optical power. When the pulse repetition rate of an actively Q-switched laser falls below the inverse upper-state lifetime, the maximum pulse energy is achieved, but the average power is reduced due to increased losses via fluorescence (spontaneous emission).

Pumping does not have to occur in a continuous-wave fashion; it is also possible to use flash lamps or quasi-cw laser diodes, fired shortly before the Q switch is opened. This reduces the energy losses via spontaneous emission and thus allows the use of gain media with shorter upper-state lifetimes.

In most cases, the pulses in a Q-switched laser are generated by amplifying noise from spontaneous emission in many resonator round trips. Therefore, there is usually no phase correlation between subsequent pulses, and the pattern of excited resonator modes can be random. Moreover, excitation of multiple modes results in the generation of beat notes, apparent as fast modulations on the Q-switched pulse envelope. In some cases, however, a Q-switched laser is seeded e.g. with the output of a small single-frequency seed laser in order to obtain a low-noise single-frequency output, avoiding beat notes and reducing the noise overall (→ injection seeding). It is also possible to generate such a seed in the laser itself (self-injection seeding) from prelasing at a low power level.

The nonlinear dynamics of Q switching sometimes lead to unexpected phenomena, such as the generation of double pulses and/or certain instabilities. Numerical simulations of pulse generation can be very helpful in understanding such effects and identifying the right cure.

In some laser applications, such as laser marking, the Q-switched pulse train must be switched off for certain time intervals. This often introduces the problem that the first pulse has a higher energy, if the pump source is continuously operated during the time without pulse emission. Various methods have been developed to solve or mitigate this problem.

Note that the high pulse energies and peak powers obtained with Q switching can raise serious laser safety issues even for lasers with fairly small average output power. Also, the optical intensities can become high enough to destroy intracavity optical elements such as laser mirrors. It can therefore be necessary to use a resonator design which avoids any strongly focused beams on optical components – which can be challenging particularly for short laser resonators (as are desirable for short pulses) with large mode areas. Further, a Q-switched laser has to be kept very clean in order to avoid the burning of dust particles.

See the article on Q-switched lasers for more details on the method of Q switching.

Bibliography

See also: pulse generation, modes of laser operation, pulses, double pulses, Q-switched lasers, Q switches, Q factor, injection seeding, gain switching, acousto-optic modulators, electro-optic modulators, Q-switching instabilities, Q-switched mode locking, mode locking, laser safety

Categories: methods, pulses

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