Why do people refer to the "fast axis" vs. "slow axis" of Diode Lasers?

[DaveE]

This is really a nomenclature question.
The radiation pattern of a typical laser diode has one axis with low divergence (the slow axis) compared to the other (the fast axis). I understand about antenna radiation patterns, spatial Fourier transforms (Fourier optics), etc. So I think I understand the relationship between the emitter aperture and the divergence. But I've never understood the "fast" vs. "slow" appellation. There must be a physical basis for this categorization. Why do people say that the wider divergence is "fast"?

 

[jedishrfu]

From a mathematical perspective:

Fast and slow refer to series that converge or diverge quickly or slowly as begin the process of computing them.

https://math.stackexchange.com/questions/772251/slow-and-fast-rates-of-convergence

 

[jedishrfu]

I also found this article which doesn’t answer your question specifically but does provide insight for why the axes exist and how they are collimated.

https://integratedoptics.com/laser-beam-collimation