Solid-state laser

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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 Laser diode).

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 ytterbium. Many of the common dopants are rare earth elements, because the excited states of such ions are not strongly coupled with the thermal vibrations of their crystal lattices (phonons), and their operational thresholds can be reached at relatively low intensities of laser pumping.

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 is neodymium-doped yttrium aluminum garnet (Nd:YAG). Neodymium-doped glass (Nd:glass) and ytterbium-doped glasses or ceramics are used at very high power levels (terawatts) and high energies (megajoules), for multiple-beam inertial confinement fusion.

The first material used for lasers was synthetic ruby crystals. Ruby lasers are still used for a few applications, but they are not common because of their low power efficiencies. At room temperature, ruby lasers emit only short pulses of light, but at cryogenic temperatures they can be made to emit a continuous train of pulses.[1]

Some solid-state lasers can also be tunable using several intracavity techniques which employ etalons, prisms, and gratings, or a combination of these.[2] Titanium-doped sapphire is widely used for its broad tuning range, 660 to 986 nanometers. Alexandrite lasers are tunable from 700 to 820 nm, and they yield higher-energy pulses than titanium-sapphire lasers because of the gain medium's longer energy storage time and higher damage threshold.

Pumping

Further information: Laser 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 lasers has wide applications as large energy ultra-short pulses can be obtained. Like its counterpart, the fiber laser, there are three types of real saturable absorbers widely used as mode lockers: SESAM,[3][4][5] SWCNT and graphene.[6][7][8][9][10]

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 fluence. 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.[6][7] Further, 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, self-started mode locking and stable soliton pulse emission with high energy have been achieved.[8]

History and applications

Solid-state lasers are being developed as optional weapons for the F-35 Lightning II, and are reaching near-operational status,[11][12][13] as well as the introduction of Northrop Grumman's FIRESTRIKE laser weapon system.[14][15] In April 2011 the United States Navy tested a high energy solid state laser. The exact range is classified, but they said it fired "miles not yards".[16][17]

Uranium-doped calcium fluoride was the second type of solid state laser invented, in the 1960s. Peter Sorokin and Mirek Stevenson at IBM's laboratories in Yorktown Heights (US) achieved lasing at 2.5 µm shortly after Maiman's ruby laser.

See also

Notes and references

  1. ↑ "Continuous solid-state laser operation revealed by BTL". Astronautics: 74. March 1962. 
  2. ↑ N. P. Barnes, Transition metal solid-state lasers, in Tunable Lasers Handbook, F. J. Duarte (Ed.) (Academic, New York, 1995).
  3. ↑ H. Zhang et al, “Induced solitons formed by cross polarization coupling in a birefringent cavity fiber laser”, Opt. Lett., 33, 2317–2319.(2008).
  4. ↑ D.Y. Tang et al, “Observation of high-order polarization-locked vector solitons in a fiber laser”, Physical Review Letters, 101, 153904 (2008).
  5. ↑ L.M. Zhao et al, “Polarization rotation locking of vector solitons in a fiber ring laser”, Optics Express, 16,10053–10058 (2008).
  6. ↑ 6.0 6.1 Qiaoliang Bao, Han Zhang, Yu Wang, Zhenhua Ni, Yongli Yan, Ze Xiang Shen, Kian Ping Loh,and Ding Yuan Tang, Advanced Functional Materials, "Atomic layer graphene as saturable absorber for ultrafast pulsed lasers"
  7. ↑ 7.0 7.1 H. Zhang, D. Y. Tang, L. M. Zhao, Q. L. Bao, K. P. Loh. "Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene" (free download pdf). Optics Express 17: P17630. arXiv:0909.5536. Bibcode:2009OExpr..1717630Z. doi:10.1364/OE.17.017630. 
  8. ↑ 8.0 8.1 Han Zhang,Qiaoliang Bao,Dingyuan Tang,Luming Zhao,and Kianping Loh. "Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker". Applied Physics Letters 95: P141103. arXiv:0909.5540. Bibcode:2009ApPhL..95n1103Z. doi:10.1063/1.3244206. 
  9. ↑ Graphene: Mode-locked lasers
  10. ↑ Diagnostic and therapeutic applications of plasmonic nanobubbles
  11. ↑ Fulghum, David A. "Lasers being developed for F-35 and AC-130." Aviation Week and Space Technology, (8 July 2002). Access date: 8 February 2006.
  12. ↑ Morris, Jefferson. "Keeping cool a big challenge for JSF laser, Lockheed Martin says." Aerospace Daily, 26 September 2002. Access date: 3 June 2007.
  13. ↑ Fulghum, David A. "Lasers, HPM weapons near operational status." Aviation Week and Space Technology, 22 July 2002. Access date: 8 February 2006.
  14. ↑ "Northrop Grumman Press Release". Retrieved 2008-11-13. 
  15. ↑ "The Register Press Release". Retrieved 2008-11-14. 
  16. ↑ "US Navy's laser test could put heat on pirates". Fox News. April 13, 2011. 
  17. ↑ Kaplan, Jeremy A. (April 8, 2011). "Navy shows off powerful new laser weapon". Fox News. 
  • Koechner, Walter (1999). Solid-State Laser Engineering (5th ed.). Springer. ISBN 3-540-65064-4. 
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