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Laser beam divergence and spot size
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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:

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