Neodymium-doped Gain Media
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.
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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
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This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!
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