Definition: lasers based on YLF (yttrium lithium fluoride) crystals, usually Nd:YLF
More specific term: solid-state lasers
Categories: optical materials, laser devices and laser physics
Author: Dr. Rüdiger Paschotta
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The term YLF laser is usually used for lasers based on neodymium-doped YLF (Nd3+:YLF) crystals, although there are other rare-earth-doped YLF crystals, e.g. with ytterbium, erbium, thulium, holmium or praseodymium doping. YLF is the acronym for yttrium lithium fluoride (YLiF4). Due to the similar size, yttrium ions can be replaced with laser-active rare earth ions without strongly affecting the lattice structure.
YLF is birefringent, which eliminates thermally induced depolarization loss. Also, the gain and the emission wavelength of Nd:YLF are polarization-dependent: there is the stronger 1047-nm line for <$\pi$> polarization, and a weaker one at 1053 nm for σ polarization. The 1053-nm line fits well to the gain peak of Nd:glass, which makes Nd:YLF seed lasers and preamplifiers suitable for Nd:glass amplifier chains. There are additional transitions at 1321 nm (<$\pi$>) and 1313 nm (σ), which allow for, e.g., red light generation via frequency doubling.
The negative thermo-optic coefficient <$\partial n / \partial T$> leads to a defocusing thermal lens, which may be approximately compensated by the focusing lens from bulging of the end faces, if a suitable design is chosen. Nd:YLF lasers can be diode-pumped or lamp-pumped. Compared with Nd:YAG (→ YAG lasers), Nd:YLF has a lower thermal conductivity, but nevertheless exhibits weaker thermal distortions (due to the weakly negative <$\partial n / \partial T$>), thus allows a better beam quality, has significantly anisotropic thermal expansion and a lower fracture resistance (limiting the output power), and a longer upper-state lifetime (which is favorable for, e.g., diode-pumped Q-switched lasers with high pulse energy). Another remarkable feature is the high UV transparency, which is favorable for pumping with xenon flashlamps.
See also: YAG lasers, vanadate lasers, laser crystals, neodymium-doped laser gain media, rare-earth-doped laser gain media, solid-state lasers, polarization of light
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What is the exact reason why the laser wavelength depends on the polarization?
The author's answer:
YLF is a non-isotropic medium due to its crystal structure. As a consequence of that (and not surprisingly), various optical properties depend on the propagation direction and on the polarization.