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 don't always refer to solid-state lasers. Ion-doped solid-state lasers (sometimes referred to as doped insulator lasers) can be made into bulk lasers, fiber lasers, or other types of waveguide lasers.
Solid-state lasers can produce output power between a few milliwatts and a few kilowatts in high-power versions. The first solid-state laser, and indeed the first of all lasers—was a pulsed ruby laser. Invented by Maiman in 1960, due to later on, other solid-state gain media were favored for their superior performance. A major problem with ruby is its pronounced level characteristic.
Light pumping
Many solid-state lasers are optically pumped with flash or arc lamps. This pump source is relatively inexpensive and can provide very high power. However, they result in fairly low power efficiency, moderate lifetime, and strong thermal effects, such as thermal lensing in gain media.
Laser diodes are now most commonly used to pump solid-state lasers. These diode-pumped solid-state lasers (DPSS lasers, also known as all-solid-state lasers) offer a number of advantages, notably their compact size, long service life, and generally very good beam quality.
Energy storage
Laser transitions of rare earth or transition metal-doped crystals or glasses are typically weakly permissible transitions, i.e., transitions with very low oscillation intensity, which achieve long high-energy state lifetimes, resulting in good energy storage, with microsecond to millisecond high-energy state lifetimes.
While energy storage is beneficial for the generation of nanosecond pulses (see below), it can also lead to unwanted spikes in CW lasers, such as when the pump source is turned on.
Generation of pulses
The long high-energy lifetime makes solid-state lasers ideal for Q-switching: laser crystals can easily store a certain amount of energy, which, when released in the form of nanosecond light pulses, results in peak power orders of magnitude higher than the achievable average power. As a result, bulk lasers can easily achieve millijoule pulse energies and peak power in the megawatt range.
In mode-locking operation, solid-state lasers can generate ultrashort pulses in picoseconds or femtoseconds (min.≈ 5 fs, achieved with a titanium:sapphire laser). Some passively mode-locked solid-state lasers have a tendency to Q-switch instability, but these can often be suppressed with appropriate measures.
Wavelength tuning
Different types of solid-state lasers vary greatly in terms of their potential for wavelength tuning. Most rare-earth-doped laser crystals, such as Nd:YAG and Nd:YVO4, have a fairly small gain bandwidth of about 1 nm or less, and are therefore only tuned in a fairly limited range. On the other hand, rare-earth-doped glasses, especially transition metal-doped crystals, such as titanium:sapphire, Cr:LiSAF, and Cr:ZnSe (oscillator lasers), can achieve a tuning range of tens of nanometers or more.
Typical solid-state lasers
- Small diode-pumped Nd:YAG (→YAG lasers) or Nd:YVO4 lasers (→ vanadate lasers) typically have output powers between a few milliwatts (for miniature devices) and a few watts. The Q-switch version produces pulses with a duration of a few nanoseconds, a pulse energy of microjoules, and a peak power of many kilowatts. Doubling the intracavity frequency can be used for green light output.
- Single-frequency operation, typically achieved with a unidirectional ring laser (e.g. NPRO = non-planar ring oscillator) or with a microchip laser, can operate in the low kilohertz region with very small linewidths.
- Large lasers in side-pumped or end-pumped configurations (see above) have the geometry of a rod, flat, or thin disk laser and are suitable for lasers with output powers of up to a few kilowatts. Thin-disk lasers, in particular, can still provide very high beam quality, as well as high power efficiency.
- Q-switched Nd:YAG lasers are still widely used in lamp-pumped versions. Pulse pumping allows for high pulse energies, while the average output power is usually modest (e.g. a few watts). The cost of such a lamp-pumped laser is lower than that of a diode-pumped laser with a similar output power.
- A fiber laser is a special solid-state laser with high average output power, high power efficiency, high beam quality, and wide wavelength tunability. See also articles on fiber lasers vs. in vitro lasers and high-power fiber lasers and amplifiers.
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