Neodymium-doped Laser 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 direct pumping into the upper laser level ⁴F_3/2 with 869-nm light (which is sometimes called in-band pumping, although this is quite accurate). The strongest laser transition is that from ⁴F_3/2 to ⁴I_11/2 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 laser resonator. Via multi-phonon emission, the populations in levels ⁴I_11/2 to ⁴I_15/2 are quickly transferred to the ground-state manifold ⁴I_9/2. (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 ⁴I_9/2: 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.

Overview on Neodymium-doped Gain Media

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 <$\partial n / \partial T$>); 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 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. Those 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 pump power, so that the power efficiency is not necessarily better than for neodymium-doped media.

More to Learn

Encyclopedia articles:

• laser gain media
• rare-earth-doped laser gain media
• rare-earth-doped fibers
• laser crystals
• ytterbium-doped laser gain media