A picosecond laser is a laser which emits optical pulses with a duration between 1 ps and (usually) some tens of picoseconds. It thus also belongs to the category of ultrafast lasers or ultrashort pulse lasers.
A variety of laser types can generate picosecond pulses, with other performance parameters varying in wide ranges:
- The most common sources are actively or passively mode-locked solid-state bulk lasers. These can provide very clean (transform-limited and low-noise) ultrashort pulses with pulse repetition rates varying from a few megahertz to more than 100 GHz. For example, a passively mode-locked Nd:YAG or vanadate laser can easily generate e.g. 10-ps pulses with several watts of output power, and thin-disk lasers can generate many tens of watts in shorter pulses.
- Mode-locked fiber lasers can also cover a wide range of repetition rates from a few megahertz up to more than 100 GHz (with harmonic mode locking). Particularly with MOPA or MOFA systems, very high average output powers are possible. The pulse quality from such sources varies; for example, the pulses may or may not be close to bandwidth-limited.
- Lower repetition rates are possible with an additional pulse picker and also allow for amplification to higher pulse energies e.g. with a regenerative amplifier, possibly using chirped-pulse amplification. Cavity dumping of a mode-locked laser is another option.
- Laser diodes can be mode-locked for picosecond pulse generation (→ mode-locked diode lasers). This leads to compact sources with typical pulse repetition rates between 1 GHz and hundreds of gigahertz. However, the pulse energy is severely limited, and the pulse quality is not always high.
- Laser diodes can also be gain-switched with carefully designed electronics to achieve pulse durations of well below 1 ns, sometimes even below 100 ps. This leads to very compact and potentially cheap sources, and another advantage is that the pulse repetition rate can easily be varied in a very wide range simply via the driver electronics. See the article on picosecond diode lasers.
- Although Q-switched lasers typically generate nanosecond pulses, Q-switched microchip lasers can reach pulse durations far below 100 ps.
- More exotic sources of picosecond pulses are free electron lasers, which can provide high pulse energies even in extreme wavelength regions.
Applications of Picosecond Lasers
Picosecond lasers are used in a wide range of laser applications. Some of these lasers are industrial lasers, while others are scientific lasers. Some typical applications are discussed in the following.
Laser Material Processing
In laser material processing, e.g. laser drilling or cutting, it is often advantageous to use very short light pulses having correspondingly high peak powers for a given pulse energy. Nanosecond pulse durations (from nanosecond lasers) are often too long, because a substantial spread of deposited energy can occur by thermal conduction during the pulse duration. This is quite different for pulse durations of e.g. 10 ps or less, where there is minimum heat diffusion during the pulse duration. As a result, substantially finer structures can be processed with high quality (laser micromachining). Note, however, that high quality results usually require a careful optimization of many process details.
Compared with femtosecond lasers, picosecond laser sources are often more economical: a higher average output power is available at a lower price. In applications such as laser micromachining, one sometimes achieves better quality results with femtosecond pulses, but picosecond pulses are often sufficient when the process is sufficiently optimized overall. In such cases, picosecond lasers are often preferred.
There are some medical applications where picosecond pulses have advantages. A common application is the removal of tattoos, and similarly one may reduce pigments of natural origins. There are also surgical procedures where precise material ablation can be achieved with picosecond pulses.
Some laser microscopes are operated with picosecond pulses, although femtosecond pulses have substantial advantages in some cases.
Many synchronously pumped optical parametric oscillators are pumped with picosecond lasers. Sometimes, the whole setup is still called a picosecond laser, even though it also contains an OPO.
Picosecond laser pulses are useful for a very wide range of measurements. For example, distance measurements with LIDAR, e.g. based on time-of-flight measurements, can be performed. Picosecond pulses are also often used in pump–probe measurements on timescales of multiple picoseconds to nanoseconds.
In the area of optical fiber communications, picosecond lasers can be used in different ways. For example, picosecond lasers may be used for generating soliton pulses in optical fibers, which propagate without dispersive broadening. For such purposes, compact and cheap lasers with gigahertz repetition rates, often with emission in the 1.5-μm telecom bands, are required.
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See also: mode-locked lasers, mode locking, ultrafast lasers, femtosecond lasers, ultrashort pulses, picosecond diode lasers, nanosecond lasers, solid-state lasers, mode-locked fiber lasers
and other articles in the categories laser devices and laser physics, light pulses