What is a cryogenic laser

The concept of lasers operating at low temperatures is not new: the second laser in history was cryogenic. Initially, the concept was difficult to achieve room temperature operation, and the enthusiasm for low-temperature work began in the 1990s with the development of high-power lasers and amplifiers.

In high power laser sources, thermal effects such as depolarization loss, thermal lens or laser crystal bending can affect the performance of the light source. Through low temperature cooling, many harmful thermal effects can be effectively suppressed, that is, the gain medium needs to be cooled to 77K or even 4K. The cooling effect mainly includes:

The characteristic conductivity of the gain medium is greatly inhibited, mainly because the mean free path of the rope is increased. As a result, the temperature gradient drops dramatically. For example, when the temperature is lowered from 300K to 77K, the thermal conductivity of the YAG crystal increases by a factor of seven.

The thermal diffusion coefficient also decreases sharply. This, together with a reduction in the temperature gradient, results in a reduced thermal lensing effect and therefore a reduced likelihood of stress rupture.

The thermo-optical coefficient is also reduced, further reducing the thermal lens effect.

The increase of absorption cross section of rare earth ion is mainly due to the decrease of broadening caused by thermal effect. Therefore, the saturation power is reduced and the laser gain is increased. Therefore, the threshold pump power is reduced, and shorter pulses can be obtained when the Q switch is operating. By increasing the transmittance of the output coupler, the slope efficiency can be improved, so the parasitic cavity loss effect becomes less important.

The particle number of the total low level of the quasi-three-level gain medium is reduced, so the threshold pumping power is reduced and the power efficiency is improved. For example, Yb:YAG, which produces light at 1030nm, can be seen as a quasi-three-level system at room temperature, but a four-level system at 77K. Er: The same is true for YAG.

Depending on the gain medium, the intensity of some quenching processes will be reduced.

Combined with the above factors, low temperature operation can greatly improve the performance of the laser. In particular, low temperature cooling lasers can obtain very high output power without thermal effects, that is, good beam quality can be obtained.

One issue to consider is that in a cryocooled laser crystal, the bandwidth of the radiated light and the absorbed light will be reduced, so the wavelength tuning range will be narrower, and the line width and wavelength stability of the pumped laser will be more stringent. However, this effect is usually rare.

Cryogenic cooling usually uses a coolant, such as liquid nitrogen or liquid helium, and ideally the refrigerant circulates through a tube attached to a laser crystal. Coolant is replenished in time or recycled in a closed loop. In order to avoid solidification, it is usually necessary to place the laser crystal in a vacuum chamber.

The concept of laser crystals operating at low temperatures can also be applied to amplifiers. Titanium sapphire can be used to make positive feedback amplifier, the average output power in tens of watts.

Although cryogenic cooling devices can complicate laser systems, more common cooling systems are often less simple, and the efficiency of cryogenic cooling allows for some reduction in complexity.