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

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SUMMARY

The terms "fast axis" and "slow axis" in diode lasers refer to the divergence characteristics of the laser beam. The fast axis exhibits a wider divergence, while the slow axis has low divergence, which is crucial for understanding laser beam collimation. This nomenclature is rooted in mathematical concepts of convergence rates, where "fast" indicates a rapid divergence in beam width. The physical basis for these terms is linked to the emitter aperture and its impact on the radiation pattern of laser diodes.

PREREQUISITES
  • Understanding of laser diode radiation patterns
  • Familiarity with spatial Fourier transforms in optics
  • Knowledge of laser beam collimation techniques
  • Basic mathematical concepts of convergence and divergence
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  • Research laser beam collimation techniques using specific tools like beam expanders
  • Study the principles of spatial Fourier transforms in optics
  • Explore the mathematical foundations of convergence rates in optics
  • Investigate the impact of emitter aperture on laser beam divergence
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Optical engineers, laser physicists, and anyone involved in the design and application of diode lasers will benefit from this discussion.

DaveE
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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"?
 
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