Beam Divergence from non-circular laser beam

[nordmoon]

Homework Statement

The laser beam is not a point source. It is known that it has a rectangular shape with a divergence of 30 mrad x 1 mrad. I would like to know how large my laser lobe will be at a distance of 250 mm from the laser source.

Homework Equations

I think you can use trigonometri for a triangle. tan(A) = opposite/adjacent

The Attempt at a Solution

The angle of the rectangle will be half of angle of the divergence spread since the divergence is linear.

opposite = adjacent * tan(A), thus

opposite_height = 250 mm * tan(15mrad) = 3.75 mm
opposite_width = 250 mm * tan(15mrad) = 0.125 mm

Thus, the laser beam or lobe should have the size of 2*3.75 mm x 2*0.125 mm = 7.5 mm x 0.25 mm at a distance of 250 mm from laser source.

Correct?

 

[mfb]

Only if the spatial width of the laser beam at the exit is much smaller than those values. Otherwise you have to take this into account. If your laser exit has a width of several millimeters, a small divergence won't mean your beam magically shrinks to 0.125 mm.

What do you mean by "angle of the rectangle"?

 

[nordmoon]

Well, ok.. so a laser diode is coupled into an optical waveguide that is very elliptical in shape, giving a lobe shaped beam with divergence of 70 mrad x 1 mrad.