cryogenic lasers (original) (raw)

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

Alternative term: cryogenically cooled lasers

Category: article belongs to category laser devices and laser physics laser devices and laser physics

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The Concept of Cryogenic Laser Operation

The idea of operating lasers at very low temperatures is not exactly new: the second laser in history was already 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.

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.

Some kinds of semiconductor lasers, for example quantum cascade lasers, can also benefit from operation at cryogenic temperatures. In some cases, continuous-wave operation can be achieved only under such conditions, or at least the achieved performance becomes much better.

Advantages of Cooling

In high-power solid-state 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 thermal conductivity of the gain medium is strongly increased, mainly because the mean free path length of phonons is increased. Therefore, temperature gradients are strongly decreased. As an example, the thermal conductivity of YAG increases by a factor of 7 when the temperature is reduced from 300 K to 77 K.
The thermal expansion coefficient is also strongly reduced. This together with the reduced temperature gradients reduces thermal lensing from bulging and stress, and of course the tendency for stress fracture.
The thermo-optic coefficient (($\partial n / \partial T$)) is also reduced, further reducing thermal lensing.
The laser and absorption cross-sections of rare earth ions are increased, essentially because thermally induced broadening is reduced. As a result, saturation powers are reduced, and the laser gain is increased. Consequently, the threshold pump power is reduced, and shorter pulses can be obtained in Q-switched operation. The slope efficiency can be increased by increasing the output coupler transmission, so that parasitic resonator losses become relatively less important.
The thermal population in the lower laser level in quasi-three-level laser gain media is reduced, which again reduces the threshold pump power and leads to laser designs with increased power efficiency [5]. For example, Yb:YAG for 1030-nm emission behaves as a quasi-three-level system at room temperature, but as a four-level system at 77 K. The same is true for Er:YAG lasers emitting at 1.6 μm [8].
Depending on the gain medium, the strength of certain quenching processes may be reduced.

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.

Possible Disadvantages

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.

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 elsewhere.

Used Cryogens

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 with optical windows.

Frequently Asked Questions

What is cryogenic laser operation?

It is the practice of operating a laser's gain medium at a very low temperature, such as that of liquid nitrogen (77 K), to significantly improve the laser's performance, particularly at high power levels.

What are the main benefits of cooling a laser crystal to cryogenic temperatures?

Cryogenic cooling drastically improves the thermal properties of the gain medium. It increases thermal conductivity while reducing thermal expansion and the thermo-optic coefficient, which strongly mitigates performance-limiting effects like thermal lensing.

How does cryogenic cooling improve laser efficiency?

It increases the laser cross-sections, boosting gain. For quasi-three-level media like Yb:YAG, it depopulates the lower laser level, reducing the pump power needed to reach the threshold and thus increasing overall efficiency.

Are there disadvantages to cryogenic laser operation?

The primary disadvantage is the increased system complexity, as it typically requires a cryocooler and a vacuum chamber to house the laser crystal. It can also narrow the gain bandwidth, potentially limiting wavelength tunability.

Bibliography

[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
[11]	D. C. Brown et al., “The application of cryogenic laser physics to the development of high average power ultra-short pulse lasers”, Appl. Sci. 6 (1), 23 (2016); doi:10.3390/app6010023
[12]	Y. Wang et al., “1.1 J Yb:YAG picosecond laser at 1 kHz repetition rate”, Opt. Lett. 45 (24), 6615 (2020); doi:10.1364/OL.413129
[13]	U. Demirbas et al., “Highly efficient cryogenic Yb:YLF regenerative amplifier with 250 W average power”, Opt. Lett. 46 (16), 3865 (2021); doi:10.1364/OL.430651
[14]	L. E. Zapata et al., “One-joule 500-Hz cryogenic Yb:YAG laser driver of composite thin-disk design”, Opt. Lett. 47 (24), 6385 (2022); doi:10.1364/OL.476964

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