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.

 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 lowpower 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 normallyweakly 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. For example, a laser

crystal pumped with 10 W of power and having an upper-state lifetime of 1 ms

can store an energy of the order of 10 mJ. Although energy storage 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.

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.

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 withtransition-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.