Measuring the Diameter of Laser Beam by Knife Edge Technique-Physics-Lab Report, Exercises of Physics

Download Measuring the Diameter of Laser Beam by Knife Edge Technique-Physics-Lab Report and more Exercises Physics in PDF only on Docsity! Introduction The intensity of a laser beam is not uniform along its cross section, but it is distributed obeying Gaussian distribution. Our main aim here in this experiment is to find the diameter of the beam. The method we employ is simple, a photo-detector scans the whole beam across, and then the region of space over which a reasonable intensity part of the beam is distributed is named the diameter of the beam. Theoretically the beam spreads up to infinity but we are going to find the dimensions of that region of space in which the intensity is of practical importance. Procedure First I switched on the He-Ne laser. Then I used two convex lenses to get a collimated beam. The collimation is already discussed in optical computing experiment. Then I aligned the beam. After alignment I focused the beam on the photo-detector. I measured the maximum intensity of the beam. This maximum intensity is the center of the beam. Then I block the laser light so that no light can fall on the detector, and measured the background intensity. Then I subtracted the background intensity from the maximum intensity. This intensity is called net maximum intensity. The detector is very sensitive to the slightest changes in the environment and hence great care is required. It is also required that the lights should be turned off inside the lab to avoid and stray light from entering the detector. The following fig. depicts the arrangement. docsity.com Now I took a knife edge sharp metal strip, and put it in the path of the beam so that it allows only a small part of the beam inside the detector. This will enable us in scanning the whole beam intensity profile. The metal strip is very sharp with no fine structures such as dents and bending, thus it gets the name knife edge. Now I moved away the knife edge from the beam so that all the light enters the detector. Then I noted the reading for this. Then I started moving the knife edge towards the beam very slowly, and I reached a point at which the intensity value on the detector started changing. I marked this point as X1 on the micrometer attached to the knife edge. I noted this point and also the intensity on this point. Then I moved the knife edge so much such that the whole detector is covered and no light is entering it. Then I started moving it back and a point reached such that some intensity was registered on the detector. I called this point X2 on the micrometer and noted the intensity on this point. Then I calculated the distance X2-X1 and divided it by 30 so that I can scan this distance in 30 steps hence taking 30 readings. The following graph is depicting the whole story. Also d=X2-X1 and I found the step after which I had to take reading by the formula S=d/30 Where 30 are the no. of readings. docsity.com Saboohi readings IB=0.25 Imax=2.06 Distance mm I Inet=I- IB Inor=Inet/Imax 0 2.31 2.06 1 0.5 2.29 2.04 0.99 1 2.28 2.03 0.98 1.5 2.26 2.01 0.97 2 2.19 1.94 0.94 2.5 2.08 1.83 0.88 3 1.93 1.68 0.81 3.5 1.85 1.6 0.77 4 1.74 1.49 0.72 4.5 1.3 1.05 0.51 5 1.04 0.79 0.38 5.5 0.81 0.56 0.27 6 0.59 0.34 0.16 6.5 0.42 0.17 0.08 7 0.3 0.05 0.02 7.5 0.25 0 0 D=6.3 mm here. docsity.com Irfan bhai reading IB=0.7 Imax =2.65 Distance mm I Inet=I- IB Inor=Inet/Imax 0 3.35 2.65 1 0.5 3.35 2.65 1 1 3.33 2.63 0.99 1.5 3.32 2.62 0.98 2 3.3 2.6 0.98 2.5 3.21 2.51 0.94 3 3.04 2.34 0.88 3.5 2.82 2.12 0.8 4 2.58 1.88 0.71 4.5 2.25 1.55 0.58 5 1.91 1.21 0.45 5.5 1.64 0.94 0.35 6 1.35 0.65 0.24 6.5 1.11 0.41 0.15 7 0.93 0.23 0.08 7.5 0.72 0.02 0.007 8 0.7 0 0 D=6.6 mm here. docsity.com Combining the result into a single result by taking the average of the three readings we get D= (6.6+6.3+6.6)/3 =6.5 mm Thus the beam diameter is 6.5 mm. docsity.com