Neodymium-doped Gain Media
Definition: laser gain media containing laser-active neodymium ions
More general term: solid-state gain media
German: Neodym-dodierte Verstärkermedien
Categories: optical materials, lasers
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Author: Dr. Rüdiger Paschotta
Neodymium (chemical symbol: Nd) is a chemical element belonging to the group of rare earth metals. In laser technology, it is widely used in the form of the trivalent ion Nd3+ as the laser-active dopant of gain media based on various host materials, including both crystals and glasses.
The usual pump wavelength is 808 nm (for Nd:YAG; wavelengths for other host materials can somewhat differ), but a higher slope efficiency can be achieved by directly pumping into the upper laser level 4F3/2 with 869-nm light. The strongest laser transition is that from 4F3/2 to 4I11/2 for 1064 nm, but other transitions are available with longer or shorter wavelengths (see Figure 1). In order to achieve lasing on those, lasing at the 1064-nm line needs to be suppressed by inserting an appropriate wavelength filter (usually consisting of one or more dichroic mirrors) into the cavity. Via multi-phonon emission, the populations in levels 4I11/2 to 4I15/2 are quickly transferred to the ground-state manifold 4I9/2. (The lower-state lifetime is much smaller than the upper-state lifetime.) Hence, there is normally negligible population in all these levels, so that neodymium-doped gain media exhibit pure four-level behavior. The exception is the case where the lower level is the ground-state manifold 4I9/2: 946-nm Nd:YAG lasers (and other Nd-based lasers emitting between 900 and 1000 nm) are quasi-three-level lasers, exhibiting a fairly high threshold pump power.
For high excitation densities, as can occur particularly in Q-switched lasers, but also in lasers operating on the weaker laser transitions, there can be significant energy losses due to energy transfer (→ upconversion) to higher-lying levels with small lifetimes.
The most common neodymium-doped gain media are:
- Nd:YAG = Nd:Y3Al5O12 (yttrium aluminum garnet, → YAG lasers): the classical choice for 1064 nm, but also usable at 946 and 1319 nm (and a few other lines); isotropic; still very common particularly for high-power lasers and Q-switched lasers
- Nd:YVO4 (yttrium vanadate, → vanadate lasers) for 1064, 914 and 1342 nm: very high pump and laser cross sections and larger gain bandwidth, compared with Nd:YAG, hence particularly attractive for low-threshold lasers; also good properties for high-power operation with good beam quality (low dn / dT); birefringent
- Nd:YLF = Nd:YLiF4 (yttrium lithium fluoride → YLF lasers) for 1047 and 1053 nm: birefringent, long upper-state lifetime, weak thermal lensing; useful for, e.g., high-power Q-switched lasers
- Nd:glass: various glasses, mostly silicates and phosphates; often used for neodymium-doped optical fibers, e.g. in fiber lasers and amplifiers (→ laser crystals versus glasses)
Less common neodymium-doped gain media are:
- Nd:GdVO4 (gadolinium vanadate) for 1064 and 1341 nm: similar to Nd:YVO4, but having a larger gain bandwidth
- Nd:GDD (gadolinium gallium garnet): used for high-power heat capacity lasers
- the tungstates Nd:KGW = Nd:KGd(WO4)2 and Nd:KYW = Nd:KY(WO4)2: birefringent, large gain bandwidth, large Raman cross sections
- Nd:YALO = Nd:YAlO3 for 1079 and 930 nm: birefringent
- Nd:YAP = Nd:YAlO3 for 1079 or 1340 nm: high thermal conductivity, birefringent
- Nd:LSB = Nd:LaSc3(BO3)4 for 1062, 905 and 1348 nm: birefringent; allows very high neodymium concentration
- Nd:S-FAP = Nd:Sr5(PO4)3F for 1059, 923 and 1328 nm: birefringent
In all these media (except for some glasses), the neodymium dopant ions replace other ions (often yttrium) of the host medium, which have about the same size.
Neodymium-doped gain media face competition from ytterbium-doped media in the 1-μm spectral region. The latter have a smaller quantum defect, usually a higher emission bandwidth and a higher upper-state lifetime, also a simpler energy level structure which avoids various quenching processes. However, they exhibit quasi-three-level behavior, which tends to lead to a higher threshold, so that the power efficiency is not necessarily better than for neodymium-doped media.
Suppliers
The RP Photonics Buyer's Guide contains 44 suppliers for neodymium-doped gain media. Among them:
EKSMA OPTICS
High optical quality Nd:YAG laser crystals – rods with high damage threshold AR@1064 nm coatings. Custom crystals can be manufactured on request.
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Bibliography
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[7] | S. Kück et al., “Excited state absorption and stimulated emission of Nd3+ in crystals. Part 1: Y3Al5O12, YAlO3, and Y2O3”, Appl. Phys. B 67 (2), 151 (1998), doi:10.1007/s003400050486 |
[8] | L. Fornasiero et al., “Excited state absorption and stimulated emission of Nd3+ in crystals. Part 2: YVO4, GdVO4, and Sr5(PO4)3F”, Appl. Phys. B 67, 549 (1998), doi:10.1007/s003400050543 |
[9] | J. L. Blows et al., “Heat generation in Nd:YVO4 with and without laser action”, IEEE Photon. Technol. Lett. 10 (12), 1727 (1998), doi:10.1109/68.730483 |
[10] | N. Hodgson et al., “High power TEM00 mode operation of diode-pumped solid-state lasers”, Proc. SPIE 3611, 119 (1999), doi:10.1117/12.349265 |
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[12] | Y. Sato and T. Taira, “The studies of thermal conductivity in GdVO4, YVO4, and Y3Al5O12 measured by quasi-onedimensional flash method”, Opt. Express 14 (22), 10528 (2006), doi:10.1364/OE.14.010528 |
[13] | D. Krennrich et al., “A comprehensive study of Nd:YAG, Nd:YAlO3, Nd:YVO4 and Nd:YGdVO4 lasers operating at wavelengths of 0.9 and 1.3 μm. Part 1: cw-operation”, Appl. Phys. B 92, 165 (2008), doi:10.1007/s00340-008-3069-4 |
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See also: gain media, rare-earth-doped gain media, rare-earth-doped fibers, laser crystals, ytterbium-doped gain media, YAG lasers, YLF lasers, vanadate lasers, tungstate lasers
and other articles in the categories optical materials, lasers
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