Pulsed Lasers
Definition: lasers emitting light in the form of pulses
More general terms: lasers
Opposite term: continuous-wave lasers
German: gepulste Laser
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Author: 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, 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 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.
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.
Suppliers
The RP Photonics Buyer's Guide contains 122 suppliers for pulsed lasers. Among them:


FYLA LASER
At FYLA we develop pulsed fiber lasers with pulse durations in the range of nanosecond, picosecond, and femtosecond. Our lasers are used in a lot of applications, from microscopy (as two-photon microscopy, SHG, SPIM, OCT) up to characterization of the semiconductors, providing a greater level of robustness, higher lifetimes, and a cost-effective solution.


Laser Quantum
Laser Quantum has a wide variety of ultrafast lasers with short pulses and high repetition rates, each offering unique capabilities and benefits to suit the needs of different applications.


Frankfurt Laser Company
Frankfurt Laser Company offers pulsed laser diodes with emission wavelengths from 850 nm to 1550 nm.


Lumibird
Lumibird manufactures a wide range of nanosecond pulsed lasers thanks to its expertise in three key technologies: solid-state lasers, fiber lasers and fiber amplifiers, and laser diodes. Various application areas are addressed in industry (manufacturing, lidar sensors), science (laboratories and universities), medical (ophthalmology) and defense.


Alpes Lasers
Alpes Lasers offers pulsed QCLs with wavelengths ranging from 4 to 14 μm and average powers up to hundreds of milliwatts. The unique nature of QCLs makes them perfectly suited for very short-pulse applications.


AdValue Photonics
AdValue Photonics offers different kinds of Q-switched nanosecond lasers, all emitting in the 2-μm spectral region (1950 nm):
- The Q-switched laser module AP-QS1-MOD generates pulses with 20 ns to 200 ns duration, up to 30 kHz repetition rate, up to 10 W average output power and high beam quality. The module also has an output modulation capability.
- The Q-switched laser AP-QS is a compact seed laser delivering 5-μJ pulses with up to 30 kHz repetition rate.
- The Q-switched laser AP-QS1 with a rack housing offers up to 5 W average power and 250 μJ pulse energy.
Besides, AdValue Photonics has picosecond and femtosecond fiber lasers.


Teem Photonics
Teem Photonics offers air-cooled diode-pumped passively Q-switched lasers – pulsed Microchip and Powerchip lasers series, as well the Picospark, PicoOne and PicoMega laser series, based on a MOFA architecture, i.e., using a fiber amplifier. All can generate intense sub-nanosecond pulses. Available emission wavelengths are 1064 nm, 532 nm, 355 nm, 266 nm and 213 nm.


RPMC Lasers
RPMC Lasers offers a wide range of pulsed laser sources with wavelengths from the UV through the IR. These offerings include DPSS lasers, flashlamp lasers, fiber lasers, and micro lasers/microchip lasers with both active and passive Q-switches.


TOPTICA Photonics
TOPTICA's FemtoFiber lasers provide reliable femto-/picosecond pulses based on polarization-maintaining fibers and SAM mode-locking. Different models (1560/780 nm, VIS/NIR tunable output, IR/NIR supercontinuum, short-pulse) cover a wide range of applications, e.g. time-domain terahertz, microscopy, attoscience and as seed lasers.


MPB Communications
MPBC’s line of pulsed fiber lasers are designed to address a range of market applications including medical and bio-medical research, semiconductor inspection, micro-machining, metrology, and multi-photon spectroscopy.
Products include:
- High-power mode-locked femtosecond fiber lasers which operate at 920 nm or 1190 nm. They generate linearly polarized nearly transformed-limited pulses with a pulse duration of 200 fs, at a repetition rate of 80 MHz, and an average power of 1 W.
- Mode-locked picosecond fiber lasers operate in the 1-µm or the 1.5-µm regime.
- Nanosecond pulsed fiber lasers, with user-selectable repetition rates, are also based on an all-fiber laser cavity, offering excellent beam quality and wall plug efficiency.


EKSPLA
EKSPLA offers a wide range of femtosecond, picosecond and nanosecond lasers as well as tunable wavelength systems for research and industrial applications.


AMPHOS
AMPHOS offers ultrafast lasers with femtosecond to nanosecond pulse durations. Due to the high repetition rate, that may also be used like cw lasers. We have a variety of versions, going up to the Amphos5000 series with 800 W or even 1000 W average output power and up to 50 mJ pulse energy. Common features are a high beam quality, narrow spectral bandwidth, burst mode features and flexibility concerning the pulse duration. Typical applications are OPCPA pumping, attosecond physics, high harmonic generation and laser material processing.
There are also frequency-converted versions emitting at 515 nm or 343 nm.


NKT Photonics
NKT Photonics offers a wide variety of pulsed fiber lasers, for industrial and scientific applications that covers a wide spectral range from UV to visible and infrared. The lasers are air-cooled and maintenance-free and packaged in a sealed, robust enclosure allowing for operation in the harshest environmental conditions. Our more than ten years’ experience and deep knowledge in laser science allow us to offer lasers having industrial-grade reliability and robustness. We offer ultrafast lasers for industrial and scientific applications as well as a wide range of picosecond pulsed diode lasers. Our gain-switched pulsed diode lasers offer triggable pulses down to 20 ps in a small footprint for both scientific and OEM applications.


Leukos
LEUKOS offers the passively Q-switched laser HLX-I, a compact microchip laser generated sub-nanosecond pulsed at 1064 nm. Different versions are available, with up to 500 MW average output power and 50 μJ pulse energy. It is suitable for micromachining, a seed laser, for LIDAR, 3 D scanning and imaging, biophotonics, supercontinuum generation and in other fields.
See our data sheet.
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See also: pulse generation, pulses, pulse trains, lasers, Q-switched lasers, mode-locked lasers, ultrafast lasers, gain switching
and other articles in the category light pulses
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