# Intensity of a Gaussiam Beam

## Main Question or Discussion Point

Hi!.

I´m modeling a gaussiam beam. I found that considering the electric field distribution of Gaussian laser pulses along the axis of propagation, we can write the intensity as: Where $y = I_l$
R is the power is the power reflection coefficient
$y = w_0$ is the beam waist radius at $y = z=0$
$t_l$ is the laser pulse duration

But, I don´t now what represents $r$ in the formula. I find that r is the distance from the center axis of the beam, but i don´t understand this explanation.

Thank you very much.

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UltrafastPED
Gold Member
The Gaussian decay is symmetric about the axis of propagation ... the further from the axis, the weaker the Gaussian beam. Thus r is the radial distance from the axis of propagation. Cylindrical coordinates, with z the axis of propagation, is a convenient coordinate system.

Thank you!

I am implementing this equation with numerical values, so, is r a number or is a variable in the equation?

Redbelly98
Staff Emeritus
Homework Helper
r is a variable, representing (transverse) distance from the beam's central axis. So, the intensity has a maximum on the axis (r = 0), and becomes weaker at greater distances from the central axis.

Therefore, r is the distance between the axis of propagation and what the variable is?

Redbelly98
Staff Emeritus
Homework Helper
No, it's the distance between the axis of propagation and the location where you are trying to calculate the intensity.

I don't know what you mean by "distance between the a.o.p. and what the variable is", since "what the variable is" does not describe an object or a location in space (as far as I can tell).

EDIT: maybe these images will help your understanding? (Don't worry too much about the captions included with the figures, I'm posting these mainly for illustration and have added some explanatory notes of my own) The dotted line is the central axis of the beam.
"r" represents the distance above or below that line. Here "x" is what we are calling "r".
The circular-shaped figure represents the beam intensity, with the beam directed out of the plane of the page.​

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Sorry, I´m from Colombia, so I have to improve my English. It is more clear for me. Therefore. I am trying to evaluate the Intensity of the Gauss Beam at the beam waist in (z=0), so what is the correct value of r?

Additionaly, The beam waist is in z=0, so what is the correct value of r in z=0?

Redbelly98
Staff Emeritus
Homework Helper
The intensity at the beam waist is when z=0, and is given by the equation you originally posted.
The maximum intensity occurs for r=0 and t=0, again in the equation you originally posted.

So, if you want the intensity as a function of r, there is no "correct" value of r. It is a variable that can be any value between 0 and infinity.

But if you want to know the maximum value of the intensity, then use r=0. And t=0 also.

I hope that helps.

It is more clear for me, but if I want to calculate the intensity at the beam waist, then, is there a specific value of r?

Redbelly98
Staff Emeritus