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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2010-01-06

Acronym: SSL

Definition: lasers based on solid-state gain media (usually ion-doped crystals or glasses)

Solid-state lasers are lasers based on solid-state gain media such as crystals or glasses doped with rare earth or transition metal ions, or semiconductor lasers. (Although semiconductor lasers are of course also solid-state devices, they are often not included in the term solid-state lasers.) Ion-doped solid-state lasers (also sometimes called doped insulator lasers) can be made in the form of bulk lasers, fiber lasers, or other types of waveguide lasers. Solid-state lasers may generate output powers between a few milliwatts and (in high-power versions) many kilowatts.

Figure 1: Typical setups of solid-state bulk lasers, converting pump light (blue) into laser light (red): end-pumped (top) and side-pumped (bottom) versions.

Optical Pumping and Energy Storage

Many solid-state lasers are optically pumped with flash lamps or arc lamps. Such pump sources are relatively cheap and can provide very high powers. However, they lead to a fairly low power efficiency, moderate lifetime, and strong thermal effects such as thermal lensing in the gain medium. For such reasons, laser diodes are very often used for pumping solid-state lasers. Such diode-pumped solid-state lasers (DPSS lasers, also called all-solid-state lasers) have many advantages, in particular a compact setup, long lifetime, and often very good beam quality. Therefore, their share of the market is rapidly rising.

The laser transitions of rare-earth or transition-metal-doped crystals or glasses are normally weakly allowed transitions, i.e., transitions with very low oscillator strength, which leads to long radiative upper-state lifetimes and consequently to good energy storage, with upper-state lifetimes of microseconds to milliseconds. Although this is beneficial for nanosecond pulse generation (see below), it can also lead to unwanted spiking phenomena in continuous-wave lasers, e.g. when the pump source is switched on.

Pulse Generation

The long upper-state lifetimes makes solid-state lasers very suitable for Q switching: the laser crystal can easily store an amount of energy which, when released in the form of a nanosecond pulse, leads to a peak power which is orders of magnitude above the achievable average power. Bulk lasers can thus easily achieve millijoule pulse energies and megawatt peak powers.

In mode-locked operation, solid-state lasers can generate ultrashort pulses with durations measured in picoseconds or femtoseconds (minimum: ∼ 5 fs, achieved with Ti:sapphire lasers). With passive mode locking, they have a tendency for Q-switching instabilities, if these are not suppressed with suitable measures.

Wavelength Tuning

In terms of their potential for wavelength tuning, different types of solid-state lasers differ considerably. Most rare-earth-doped laser crystals, such as Nd:YAG and Nd:YVO₄, have a fairly small gain bandwidth of the order of 1 nm or less, so that tuning is possible only within a rather limited range. On the other hand, tuning ranges of tens of nanometers and more are possible with rare-earth-doped glasses, and particularly with transition-metal-doped crystals such as Ti:sapphire, Cr:LiSAF and Cr:ZnSe (→ vibronic lasers).

Types of Solid-state Lasers

Examples of different types of solid-state lasers are:

• Small diode-pumped Nd:YAG (→ YAG lasers) or Nd:YVO₄ lasers (→ vanadate lasers) often operate with output powers between a few milliwatts (for miniature setups) and a few watts. Q-switched versions generate pulses with durations of a few nanoseconds, microjoule pulse energies and peak powers of many kilowatts. Intracavity frequency doubling can be used for green output.
• Single-frequency operation, typically achieved with unidirectional ring lasers (e.g. NPROs = nonplanar ring oscillators) or microchip lasers, allows for operation with very small linewidth in the lower kilohertz region.
• Larger lasers in side-pumped or end-pumped configurations (see above), having the geometry of rod lasers, slab lasers or thin-disk lasers, are suitable for output powers up to several kilowatts. Particularly thin-disk lasers can still offer very high beam quality, and also a high power efficiency.
• Q-switched Nd:YAG lasers are still widely used in lamp-pumped versions. Pulsed pumping allows for high pulse energies, whereas the average output powers are often moderate (e.g. a few watts). The cost of such lamp-pumped lasers is lower than for diode-pumped versions with similar output powers.
• Fiber lasers are a special kind of solid-state lasers, with a high potential for high average output power, high power efficiency, high beam quality, and broad wavelength tunability. See also the articles on fiber lasers versus bulk lasers and on high-power fiber lasers and amplifiers.

Bibliography

See also: doped insulator lasers, all-solid-state lasers, lasers, gain media, laser crystals, composite laser crystals, rare-earth-doped gain media, transition-metal-doped gain media, YAG lasers, laser crystals versus glasses, fiber lasers versus bulk lasers, diode-pumped lasers, lamp-pumped lasers, end pumping, side pumping, rod lasers, slab lasers, thin-disk lasers, ring lasers, nonplanar ring oscillators, semiconductor lasers

Category: lasers