Solid-state laser

A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers (see Semiconductor laser).

Solid-state media

Generally, the active medium of a solid-state laser consists of a glass or crystalline host material to which is added a dopant such as neodymium, chromium, erbium, or other ions. Many of the common dopants are rare earth elements, because the excited states of such ions are not strongly coupled with thermal vibrations of the crystalline lattice (phonons), and the lasing threshold can be reached at relatively low brightness of pump.

There are many hundreds of solid-state media in which laser action has been achieved, but relatively few types are in widespread use. Of these, probably the most common type is neodymium-doped YAG. Neodymium-doped glass (Nd:glass) and ytterbium-doped glasses and ceramics are used in solid-state lasers at extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Titanium-doped sapphire is also widely used for its broad tunability.

The first material used for lasing was ruby. Ruby lasers are still used for some applications, but are not common due to their low efficiency. Er:YAG lasers lase in the mid-infrared.

Some solid-state lasers can also be tunable using several intracavity techniques which employ etalons, prisms, and gratings, or a combination of these.^[1]

Pumping

Solid state lasing media are typically optically pumped, using either a flashlamp or arc lamp, or by laser diodes. Diode-pumped solid-state lasers tend to be much more efficient, and have become much more common as the cost of high power semiconductor lasers has decreased.

Mode locking

Mode locking of solid state laser had wide applications in that large energy ultra-short pulse could be obtained out of it. Like its counterpart: fiber laser, there are three types of real saturable absorbers widely used as mode lockers: SESAM^[2][3][4] .^[5][6][7], SWCNT and Graphene^[8][9][10], which was highlighted in Nature Asia Materials ^[11].

Particularly, Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It is recently confirmed that the optical absorption from graphene could become saturated when the input optical intensity is above a threshold value. This nonlinear optical behavior is termed saturable absorption and the threshold value is called the saturation fluency. Graphene can be saturated readily under strong excitation over the visible to near-infrared region, due to the universal optical absorption and zero band gap. This has relevance for the mode locking of fiber lasers, where wideband tuneability may be obtained using graphene as the saturable absorber. Due to this special property, graphene has wide application in ultrafast photonics.^[12][13] Furthermore, comparing with the SWCNTs, as graphene has a 2D structure it should have much smaller non-saturable loss and much higher damage threshold. Indeed, with an erbium-doped fiber laser we self-started mode locking and stable soliton pulse emission with high energy have been achieved.^[14]

Applications

This section requires expansion.

Solid-state lasers are being developed as optional weapons for the F-35 Lightning II, and are reaching near-operational status,^[15][16][17], as well as the introduction of Northrop Grumman's FIRESTRIKE laser weapon system. ^[18][19]

See also

• Fiber laser
• Disk laser
• Laser construction
• soliton
• vector soliton
• dissipative soliton

Notes and references

• Koechner, Walter (1999). Solid-State Laser Engineering (5th ed. ed.). Springer. ISBN 3-540-65064-4. 

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