. .
.
Laser beam divergence and spot size
.
.

### Performing the real lab:

• Arrange the laser and detector in an optical bench arrangement.

• The laser is switched on and is made to incident on the photodiode.

• Fix the distance, z between the detector and the laser source.

• By adjusting the micrometer of the detector, move the spot in the horizontal direction, from left to right.

• Note the output current for each distance, x from the measuring device.

• Then the beam profile is plotted with the micrometer distance along the X-axis and intensity of current along Y-axis. We will get a gaussian curve (see Fig.1).

Fig.1 beam profile of Laser beam divergence.

• The experiment is repeated for different detector distances.

• Note the points in the graph where the intensity equals 1/e2 of the maximum intensity, say it as Ie (see Fig.1).

• Find the micrometer distance across the beam corresponding to these points ( B-A from the Fig.1) for a pair of detector distances z1 and z2. Half of this distance is noted as w1and w2.

• Then the divergence and spot size of the laser beam can be calculated from the equations.

### Observation and calculation

To find the Least Count of Screw gauge

One pitchscale division ( n) = .............. mm

Number of divisions on head scale (m) = ...............

Least Count (L.C) = n/m = ......................

 z1 = ....................... cm z2 = ................ cm 1/e2 of maximum intensity,Ie =................ mA  Diameter of the beam corresponds to Ie,  d1= ..................mm 1/e2 of maximum intensity,Ie =................ mA  Diameter of the beam corresponds to Ie,  d2= ..................mm

Divergence angle(Θ) = (d2-d1)/(z2-z1) = ........................ mrad

### Performing the simulator:

• The experimental arrangement is shown in the simulator. A side view and top view of the set up is given in the inset.

• The start button enables the user to start the experiment.

• From the combo box, select the desired laser source.

• Then fix a detector distance, say 100 cm, using the slider Detector distance, z.

• The z distance can be varied from 50 cm to 200 cm.

• For a particular z distance, change the detector distance x, from minimum to maximum, using the slider Detector distance, x. The micrometer distances and the corresponding output currents are noted. The x distances can be read from the zoomed view of the micrometer and the current can be note from the digital display of the output device.

• Draw the graph and calculate the beam divergence and spot size using the steps given above.

• Show graph button enables the user to view the beam profile.

• Using the option Show result, one can verify the result obtained after doing the experiment.

Cite this Simulator:

.....
..... .....
 Copyright @ 2020 Under the NME ICT initiative of MHRD