Solid-state lasers are lasers based on solid-state gain media such as crystals or glasses doped with rare earth or transition metal ions. Semiconductor lasers are also solid-state lasers, but they are not always meant with that term.
Solid-state lasers may generate output powers between a few milliwatts and (in high-power versions) many kilowatts.
The first solid-state laser – and in fact the first of all lasers – was a pulsed ruby laser, demonstrated by Maiman in 1960 . Later on, however, other solid-state gain media were preferred because of their superior performance. A major problem with ruby is its pronounced three-level nature.
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.
Laser diodes are now most 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.
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 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.
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 light 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). Some passively mode-locked solid-state lasers have a tendency for Q-switching instabilities, but these can usually be suppressed with suitable measures.
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).
Typical 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 with 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.
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See also: doped insulator lasers, all-solid-state lasers, lasers, laser gain media, laser crystals, composite laser crystals, laser glasses, rare-earth-doped laser gain media, transition-metal-doped laser gain media, YAG lasers, 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, laser specifications
and other articles in the category laser devices and laser physics