Pulsed Lasers

Author: the photonics expert Dr. Rüdiger Paschotta

Pulsed lasers are lasers which emit light not in a continuous mode, but rather in the form of optical pulses (light flashes). The term is most commonly used for Q-switched lasers, which typically emit nanosecond pulses, but this article gives an overview of a wider range of pulse-generating lasers. Depending on the pulse duration, pulse energy, pulse repetition rate and wavelength required, very different methods for pulse generation and very different types of pulsed lasers are used. The article on pulse generation describes more in detail the technical methods, whereas this article discusses some types of pulsed lasers:

• Various types of actively or passively Q-switched lasers emit pulses in the nanosecond duration regime; particularly compact microchip lasers can also generate sub-nanosecond pulses. Most Q-switched lasers are solid-state bulk lasers, and some of them can achieve high pulse energies in the millijoule or even multi-joule region.
• Some solid-state bulk lasers are pumped with flash lamps but not Q-switched; in free-running mode (i.e., with pulsed pumping but no intracavity loss modulation), one obtains longer pulses and somewhat higher pulse energies.
• Excimer lasers are used for generating intense nanosecond pulses in the ultraviolet spectral region. They are pumped with rather short electric pulses.
• Some other gas lasers (e.g. nitrogen lasers) and metal vapor lasers (e.g. copper vapor lasers) are also driven with current pulses and could normally not work in continuous-wave operation.
• Ultrashort pulses with durations in the picosecond or femtosecond domain are usually generated with mode-locked lasers, which may be solid-state bulk lasers, fiber lasers or semiconductor lasers. Their pulse energies are generally quite small, and the pulse repetition rate is usually in the megahertz or gigahertz region. For higher pulse energies (roughly an order of magnitude more), one may use a cavity-dumped laser.
• Gain-switched semiconductor lasers are suitable for nanosecond or picosecond pulses with relatively low energy (→ picosecond diode lasers).
• Relatively long pulses can be generated e.g. with laser diodes in quasi-continuous-wave operation.

Difficulties with Continuous-wave Operation

Some types of lasers can hardly work in continuous-wave operation, but only in pulsed operation; that can have different reasons:

• It some cases, one can obtain a sufficiently high laser gain only with pump intensities which are practical only for pulsed pumping. For example, the required pump intensities would overheat the gain medium when applied for longer times.
• In some cases, one is dealing with self-terminating laser transitions.

For such reasons, one sometimes uses a pulsed laser with a high pulse repetition rate where continuous-wave operation would be fully suitable for the application, but hard to realize with the type of laser. An example is photolithography with excimer lasers.

Difficulties with Long Pulse Durations

While many laser applications benefit from very short pulse durations, there are a few cases where rather long pulses are desired. For example, long pulses allow for a very small optical linewidth and may avoid laser-induced damage due to their lower peak power.

However, it is often quite difficult to produce relatively long laser pulses – for example, with durations of several microseconds –, at least when a high pulse energy is required at the same time. The method of Q switching can be optimized for long pulse durations, but with limitations; for example, it is not practical to use a very long laser resonator in order to maximize the resonator round-trip time. Another approach is to work with a rather low gain, but that implies a high sensitivity to intracavity losses.

Single-pulse, Repetitive and Burst Mode

Single-pulse Mode

Some pulsed lasers are operated in a single-pulse mode where each pulse can be freely triggered when the application demands it. In that regime, one often achieves rather high pulse energies, but only quite limited pulse repetition rates. It is suitable, for example for lamp-pumped solid-state lasers.

Repetitive Pulsing

Some lasers emit pulses with a constant pulse repetition rate. In the case of Q-switched lasers, this is often between 10 Hz and 100 kHz, while mode-locked lasers emit with very high repetition rates, typically tens or hundreds of megahertz, sometimes even many gigahertz. The energy per pulse is correspondingly low.

The pulse repetition rate may be reduced by some possibly large factor by using a pulse picker.

Burst Mode

For some applications, it is advantageous to use bursts (bunches) of pulses. That means that some number of pulses is emitted with a close temporal spacing (e.g. a couple of nanoseconds), forming a burst, and the next burst may occur only after much longer time.

Many laser types are not suitable for that mode of operation, or only with substantial additional technical effort. A quite flexible approach is to produce pulses with a seed laser diode and amplify those in a fiber amplifier. One may then define a burst simply by appropriately driving the seed laser. In order to compensate for gain saturation during the burst (i.e., a drop of pulse energy), one may apply a seed pulse energy which is rising during the burst.

For more details, see the article on burst mode lasers.

Pulse Quality

There are various aspects of pulse quality, some of which may be quite relevant for applications:

• It is often desirable to have very reproducible (constant) pulse parameters like pulse energy, duration, center wavelength and bandwidth. Also, the temporal pulse shape (optical power vs. time) and the evolution of optical phase should be quite constant for some laser applications.
• In some cases, it is important to avoid any pre-pulses or post-pulses. For example, in laser-induced nuclear fusion or in other high intensity physics experiments, it is important that the target is “taken by surprise” by an intense laser pulse, and not already evaporated by some unwanted pre-pulse.
• The pulse timing can also be important. Some lasers – particularly well stabilized mode-locked lasers – exhibit an extremely small timing jitter, particularly over small measurement time intervals (e.g. pulse-to-pulse jitter).

Amplification of Laser Pulses

For boosting the average power (particularly of high repetition rate pulse trains with moderate pulse energies), high-power fiber amplifiers are often well suited. For cases with lower repetition rate but higher pulse energy, solid-state bulk amplifiers are better suited. These, however, usually do not provide as much gain, unless one uses sophisticated multipass arrangements.

For ultrashort pulses with much increased pulse energies, one uses regenerative amplifiers and multipass amplifiers.

A laser system combined with some kind of optical amplifier is often still called a laser as a whole.

More to Learn

Encyclopedia articles:

• pulse generation
• light pulses
• pulse trains
• lasers
• Q-switched lasers