Laser beam divergence

Object:

Measuring the beam divergence angle and compare between laser diode and He-Ne

laser .

Apparatus:

He Ne laser, pin hole, screen, beam expander, diode laser.

Theory

one of the string features of most laser in that the output is in the form of in almost

parallel beam this is a very useful features for a number of application and can be

define it is the full angle of the opening beam it is measure in rad.

The relation between degree and rad. is 2n = 360 degree. And | rad = 57.3,

I mrad = 0.057 degree.

Can be measured the spot size from this relation:

Tan (0/2) =(R-1r)/L.............Q))

Where R= radius of spot

And r= radius of aperture

And__L = the distance from laser and screen

The other feature of the laser is brightness it can be define as the power emitted per unit

solid angle. The emission sold angle (w) is:

waa. (2)

The beam brightness (B) is given by:

B = power /( area xsolid angle )

Since it mean that is very easy to collect emitting radiation and focus it on to a small area

using a fairly simple lens system a laser beam is not perfectly parallel this is not because

of a fault in laser design but is due to diffraction caused by the wave nature of light.

procedure

1- Determine (r) which represent the aperture of He Ne laser.

2- Place the screen supported by the magnetic mount directly in front of He Ne laser so

that the light from laser is perpendicularly incident on the screen.

3- Put He- Ne laser on the table make sure the area where the laser beam is pointing is

clear

4-increce the distance (L) from (1-6) M

5- Measure and record (on the data sheet) the distance of screen from laser (L)

6- Measure and record (on the data sheet) the distance of the beam (R) on the screen in

each mater.

7- Determine the exact diameter of the beam is difficult. Because the beam intensity is

reduced with 1/ (distance) 7. And over the beam across section it decay almost

exponentially (Gaussian shape) toward the edges. more accurate measurement is by

subtracting the laser beam diameter at the laser out pout ( d ) from the measure value

and call the new value ( D*)

8- Draw the graph of (D*) VS (L) a

9- Calculate (on the data sheet) the slope of the graph.

10- Calculate (on the data sheet) the angle divergence of the laser.

The slope = angle of the divergence (rad)

11- Repeat all steps with the beam expander.

12- Repeat all steps with the diode laser

L(m)	R(mm)	r(mm)	0(rad)
1	3	2.5	0.928
2	3.5	2.5	0.926
3	3.75	2.5	0.76
4	4.5	2.5	0.934
5	4.5	2.5	0.76
6

Discussion

1- What the reason for the laser divergence?

2- Calculate the brightness of 3mw He-Ne laser and | cm in diameter?

3- The divergence of the laser beam after sending it through a telescope is 10 ~ rad

What is the diameter of the spot formed on the moon surface if the laser is directed

towards the earth? (The distance from earth to the moon is (3.8 10 *km)

(the laser beam diameter is 2mm).

R 0/2 R L 0/2 Fig (1) divergence angle R 0/2 R L 0/2 Fig (1) divergence angle