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Cryogenic Lasers

Definition: lasers where the gain medium is operated at cryogenic temperatures

Alternative term: cryogenically cooled lasers

German: tieftemperaturgekühlte Laser

Category: lasers

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The idea of operating lasers at low temperatures is not exactly new: the second laser in history already was a cryogenic one [1]. While this concept was originally used just because room-temperature operation was hard to achieve, a renewed interest in cryogenic operation for high-power lasers and amplifiers developed in the 1990s.

In high-power laser sources, thermal effects such as depolarization loss, thermal lensing or even fracture of the laser crystal can be a real problem limiting the performance. A number of the detrimental thermal effects can be effectively suppressed by cryogenic cooling, meaning cooling of the gain medium to low temperatures such as 77 K (the temperature of liquid nitrogen) or even 4 K (liquid helium). The main effects of such cooling are:

The combination of these factors allows for strong improvements in laser performance. In particular, cryogenically cooled lasers have the potential for generating much higher output powers without excessive thermal effects, i.e. with good beam quality.

A possible concern is that the bandwidth of both the emission and absorption of the cryo-cooled laser crystal may be reduced, which may lead to a narrower range for wavelength tuning and to more stringent requirements on the linewidth and wavelength stability of the pump laser. However, this effect does not necessarily occur.

Cryogenic cooling may be achieved with a cryogen such as liquid nitrogen or helium, ideally circulating through channels in a cooling finger which is attached to the laser crystal. The cryogen may be taken from some supply, which is refilled from time to time, or recycled in a closed loop, containing e.g. a Stirling engine. To avoid condensation, one usually has to operate the laser crystal in a vacuum chamber.

Of course, the concept of operating the laser crystal at a very low temperature can also be applied to amplifiers. It is used e.g. to build regenerative amplifiers based on Ti:sapphire with average output powers of tens of watts.

Although cryogenic cooling arrangements certainly add to the complexity of such a laser system, more conventional cooling systems are also not always very simple, and the great effectiveness of cryogenic cooling may allow for a reduction in complexity at other places.

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[1]P. P. Sorokin and M. J. Stevenson, “Stimulated infrared emission from trivalent uranium”, Phys. Rev. Lett. 5 (12), 557 (1960), doi:10.1103/PhysRevLett.5.557 (the second laser, which also was the first cryogenic laser)
[2]H. Glur et al., “Reduction of thermally induced lenses in Nd:YAG with low temperatures”, IEEE J. Quantum Electron. 40 (5), 499 (2004), doi:10.1109/JQE.2004.826448
[3]D. C. Brown, “The promise of cryogenic solid-state lasers”, JSTQE 11 (3), 587 (2005), doi:10.1109/JSTQE.2003.850237
[4]I. Matsushima et al., “10 kHz 40 W Ti:sapphire regenerative ring amplifier”, Opt. Lett. 31 (13), 2066 (2006), doi:10.1364/OL.31.002066
 [5]T. Y. Fan et al., “Cryogenic Yb3+-doped solid-state lasers”, JSTQE 13 (3), 448 (2007), doi:10.1109/JSTQE.2007.896602
[6]R. L. Aggarwal et al., “Measurement of thermo-optic properties of Y3Al5O12, LuAl5O12, YAlO3, LiYF4, LiLuF4, BaY2F8, KGd(WO4)2, and KY(WO4)2 laser crystals in the 80–300 K temperature range”, J. Appl. Phys. 98 (10), 103514 (2005), doi:10.1063/1.2128696
[7]S. Tokita et al., “Sapphire-conductive end-cooling of high power cryogenic Yb:YAG lasers”, Appl. Phys. B 80, 635 (2005), doi:10.1007/s00340-005-1779-4
[8]N. Ter-Gabrielyan et al., “Temperature Dependence of a Diode-Pumped Cryogenic Er:YAG Laser”, Opt. Express 17 (9), 7159 (2009), doi:10.1364/OE.17.007159
[9]S. Banerjee et al., “High-efficiency 10 J diode pumped cryogenic gas cooled Yb:YAG multislab amplifier”, Opt. Lett. 37 (12), 2175 (2012), doi:10.1364/OL.37.002175
[10]D. E. Miller et al., “Cryogenically cooled, 149 W, Q-switched, Yb:LiYF4 laser”, Opt. Lett. 38 (20), 4260 (2013), doi:10.1364/OL.38.004260

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See also: lasers, solid-state lasers, laser cooling units, thermal lensing, depolarization loss
and other articles in the category lasers


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