Group velocity of laser beams in free space: comparison between models.

The precise knowledge of group velocity of laser pulses in free space is important for free-space laser communications [1-3]. There are three different approaches to evaluate the group velocity of a laser pulse [1-3]. One of them is dedicated to the study of so-called spatio-temporal wave packets [1] and the other two are more focused on the spatial structure of the beams [2,3]. These last two approaches lead to different predictions of the spatial distribution of the group velocity [2,3]. This difference is being studied in this work. The first of these two approaches results in good agreement with experiments but there are concerns about energy conservation. The second approach satisfies energy conservation, but it does not predict some experimental results. This suggests that the measured signal speed is not equal to the energy transfer speed. The results of this work are useful to define areas of applicability of these two approaches.

[1] Kondakci H.E., Abouraddy A.F. // Nature Communications. 2019. V. 10. P. 929.

[2] Bouchard F., Harris J., Mand H., Boyd R.W., Karimi E. // Optica. 2016. V. 3. № 4. P. 352.

[3] Galinsky M.K., Rumyantsev V.V. // Problems of Artificial Intelligence. 2018. V. 3 № 10. P. 14.