The term YLF lasers are commonly used for lasers based on neodymium-doped YLF (Nd3+:YLF) crystals, although there are other rare-earth-doped YLF crystals, such as ytterbium, erbium, thulium, holmium, or praseodymium. YLF is an acronym for yttrium lithium fluoride (YLF4). Due to their similar size, yttrium ions can be replaced with laser-activated rare earth ions without strongly affecting the lattice structure.
YLF is birefringent, which eliminates heat-induced depolarization losses. There are stronger 1047 nm wires in π polarization and weaker 1053 nm wires in σ polarization. The 1053 nm spectral line is in good agreement with the gain peak of neodymium glass, which makes the Nd:YLF seed laser and preamplifier suitable for neodymium glass amplifier chains. There are other transitions at 1321nm (π) and 1313nm (σ), which allow, for example, the generation of red light by frequency doubling.
The negative thermo-optical coefficient dn / dT causes the thermal lens to be defocused, which can be roughly compensated for by focusing the lens from the raised position of the end face if a suitable design is chosen. Nd:YLF lasers can be diode-pumped or lamp-pumped. Compared to Nd:YAG (YAG lasers), Nd:YLF has a lower thermal conductivity, but still exhibits a weaker thermal distortion (due to the weak negative value of dn/dT), so better beam quality can be obtained, with significant anisotropic thermal expansion and lower resistance to breaking (which limits output power), as well as a long upstate lifetime (which is advantageous for diode-pumped Q-switched lasers with high pulse energy, for example). Another distinguishing feature is the high UV transparency, which facilitates the use of xenon flash lamps for pumping.
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