Explanation of laser intensity

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Discussion Overview

The discussion revolves around the concepts of laser intensity and spot size, exploring the relationship between these two parameters in the context of classical and modern physics. Participants examine how spot size affects intensity and the factors that determine the spot size of a laser beam.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that with equal power output, increasing the beam diameter decreases the intensity, measured in [W/m^2], and clarifies the distinction between irradiance and intensity.
  • Another participant suggests that laser beams can be approximated as Gaussian beams, indicating that the minimum spot size is inversely proportional to the beam spread angle (divergence).
  • A participant expresses curiosity about the relationship between the particle interpretation of light and spot size, questioning if the probability of particle impact relates to the area of the spot size.
  • One participant affirms that diffraction can be analyzed in terms of particles, implying a connection to the discussion on spot size.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between beam diameter and intensity, but the discussion remains open regarding the implications of particle behavior and diffraction in relation to spot size.

Contextual Notes

There are unresolved aspects regarding the specific factors that determine the beam diameter at the laser aperture, as well as the implications of interpreting light as particles in relation to spot size.

Thermodave
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I've become interested in lasers and this question came to me today. Let's say we have two 10 mW lasers. One has a spot size of 1mm and one has a spot size of 10mm. Therefore, the first laser has a greater intensity. In classical physics we are told the wave amplitude increases. In modern physics we have more photons. Do either of these concepts relate to spot size (cross sectional area of a laser beam)? Also, what determines the spot size of a laser from a functional point of view? - Thanks
 
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Since the lasers have equal power output, increasing the beam diameter decreases the intensity (units [W/m^2]). Note that in photometry, the quantity of [W/m^2] is *irradiance* (or emittance, or exitance) while 'intensity' is [W/sr].

Laser beams are often approximated as "Gaussian beams", and so the minimum spot size is inversely proportional to the beam spread angle (divergence). I'm not sure what sets the beam diameter at the laser aperture, but it may be related to the resonator geometry and manufacturing constraints.
 
Andy Resnick said:
Since the lasers have equal power output, increasing the beam diameter decreases the intensity (units [W/m^2]). Note that in photometry, the quantity of [W/m^2] is *irradiance* (or emittance, or exitance) while 'intensity' is [W/sr].

Yes, I realize this. Let me clarify as my title was misleading. I interpret light as a particle traveling in the x-direction yet oscillating in the y-direction. The "particle" hits within an area of known probability. Do this thousands of times and you have a dot, right? I was wondering if this had anything to do with spot size.
 
Sure- that's how diffraction is analyzed in terms of particles.
 

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