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Tungstate Lasers

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Definition: solid-state lasers based on tungstate crystals as gain media

German: Wolframat-Laser

Categories: lasers, optical materials

How to cite the article; suggest additional literature

Before the development of Nd:YAG, Nd3+:CaWO4 was a very important rare-earth-doped laser gain medium. It was in fact used in the first continuously operating crystal laser. However, once Nd3+:YAG became commercially available, the interest in CaWO4 dropped sharply. In later years, certain other tungstate crystals started to attract a lot of attention.

Tungstate Crystal Materials

Monoclinic Double Tungstates

Yb:KGW crystals

Figure 1: A block of Yb3+:KGW and some laser crystals cut from this material. The photograph has been kindly provided by EKSMA OPTICS.

Various ytterbium-doped monoclinic double tungstates Yb3+:KGd(WO4)2 (Yb:KGW, potassium gadolinium tungstate) and Yb3+:KY(WO4)2 (Yb:KYW, potassium yttrium tungstate) have become particularly popular. Also under consideration is Yb3+:KLu(WO4)2 (Yb:KLuW, also called Yb:KLW). Other rare earth dopants such as neodymium (Nd3+), erbium (Er3+), thulium (Tm3+), and praseodymium (Pr3+) can also be used in tungstates.

Tetragonal Double Tungstates

Another interesting group of crystals is that of tetragonal double tungstates such as Yb3+:NaGd(WO4)2 (Yb:NGW) [7] and Yb3+:NaY(WO4)2 (Yb:NYW). Unlike their monoclinic cousins, the tetragonal tungstates are disordered crystal materials, where e.g. the Na+ and Gd3+ ions are randomly distributed on the same type of lattice sites. As the dopant ions (e.g. Yb3+) replace the other rare earth ions (e.g. Gd3+), this disordered structure results in inhomogeneous broadening and thus to a broader gain bandwidth. Some of these crystals can be grown with good quality only when using sophisticated (and often slow) growth methods, whereas others (e.g. the two mentioned above) are suitable for the common Czochralski method.

Properties of Rare-earth-doped Tungstates

Rare-earth-doped tungstate crystals have a number of special properties:

For some applications, tungstates offer a combination of various useful properties. In particular for passive mode locking, ytterbium-doped double tungstates are often superior to other media in terms of the combination of high gain bandwidth, large cross sections, and relatively good thermal properties – even though in each particular discipline better materials could be found. Even for continuous-wave high-power thin-disk lasers, tungstates offer advantages over Yb:YAG in terms of efficiency, although the fabrication of thin disks from such materials is not easy (Yb:YAG is certainly easier to handle). A passively mode-locked thin-disk Yb:KYW laser [4] has probably generated the highest output power in sub-300-fs pulses which has been achieved with a bulk laser with no amplifier.

Bibliography

[1]F. Brunner et al., “Diode-pumped femtosecond Yb:KGd(WO4)2 laser with 1.1-W average power”, Opt. Lett. 25 (15), 1119 (2000)
[2]H. Liu et al., “Diode-pumped Kerr-lens mode-locked Yb:KY(WO4)2 laser”, Opt. Lett. 26 (21), 1723 (2001)
[3]P. Klopp et al., “Passively mode-locked Yb:KYW laser pumped by a tapered diode laser”, Opt. Express 10 (2), 108 (2002)
[4]F. Brunner et al., “240-fs pulses with 22-W average power from a passively mode-locked thin-disk Yb:KY(WO4)2 laser”, Opt. Lett. 27 (13), 1162 (2002)
[5]M. Rico et al., “Tunable laser operation of ytterbium in disordered single crystals of Yb:NaGd(WO4)2”, Opt. Express 12 (22), 5362 (2004)
[6]S. Biswal et al., “Thermo-optical parameters measured in ytterbium-doped potassium gadolinium tungstate”, Appl. Opt. 44 (15), 3093 (2005)
 [7]C. Cascales et al., “Structural, spectroscopic, and tunable laser properties of Yb3+-doped NaGd(WO4)2”, Phys. Rev. B 74 (17), 174114 (2006)
[8]J. Liu et al., “Efficient high-power laser operation of Yb:KLu(WO4)2 crystals cut along the principal optical axes”, Opt. Lett. 32 (14), 2016 (2007)
[9]V. Petrov et al., “Growth and properties of KLu(WO4)2, and novel ytterbium and thulium lasers based on this monoclinic crystalline host”, Laser & Photon. Rev. 1, 179 (2007)
[10]S. Rivier et al., “Thin-disk Yb:KLu(WO4)2 laser with single-pass pumping”, Opt. Lett. 33 (7), 735 (2008)
[11]R. Peters et al., “Power scaling potential of Yb:NGW in thin disk laser configuration”, Appl. Phys. B 91, 25 (2008)

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