Download Laser Beam Waiste Measurement-Physics-Lab Report and more Exercises Physics in PDF only on Docsity! Introduction When a laser beam comes out of the laser, it spreads out due to diffraction. We often use a convex lens to converge it to a point. The beam has a minimum diameter at the converging point and this diameter is called Beam waist. The method for its calculation is straight forward. We exploit the property of diffraction of light from small holes called pin holes. The systematic procedure is given below. Focusing the Beam A laser beam after emitting has a trend to get spread due to diffraction, resulting in an increase in its diameter. A convex lens focuses it about its focal point. After focusing it again spreads as it propagate, thus its minimum diameter can be found at the point where it is focused by the lens. The beam will look like the following before focusing And after focusing by the convex lens, it will look like the following docsity.com APPARATUS 1. Laser gun 2. Lens of appropriate focal length 3. Pin hole 4. Rigid base where the whole apparatus have to be mounted 5. Stands for lens and sliding one for pin hole 6. Screen 7. Horizontal scale (micro screw gauge). Procedure I switched the laser on, and waited for 5 minutes. Then I focused the beam with a converging lens. The beam diameter is minimum near the focal point and it spreads after the focal point. A bright spot is obtained at the screen located at some distance. Now to find the location of the waist, I used several pin holes. I moved the pin hole away from the focusing lens while the beam was passing through it, there was airy diffraction pattern on the screen confirming that the beam diameter was bigger than the hole diameter at that point. Then I moved it farther, and a certain point reached where the airy pattern vanished. This point is near the point where the beam waist is minimum, but not exactly on that point. I moved the lens farther and again noticed that the airy pattern started forming on the screen. Disappearance of the airy pattern means that a single bright spot is formed on the screen without any tiny circles. The point where the airy pattern started vanishing is designated as X1 and the point at which the pattern again re-appeared is designated as X2. The beam waist lies at I used pin holes of several diameters and found different X1 and X2 for them but point X was the same for all of them. This point X is the point where the beam diameter is minimum and is called the beam waist. Now the beam waist is found by the following procedure. I kept changing the different diameter holes, and kept moving them while the beam was passing through them. The airy pattern due to diffraction kept disappearing and reappearing on the screen, until one pin hole of smallest diameter reached for which the airy pattern did not vanish and thus confirming that the beam diameter is bigger than its diameter along the whole length of the beam. Thus this hole has a diameter less than the beam, and the waist diameter is equal or nearly equal to the diameter of that pin hole whose diameter is just bigger than the diameter of this hole. This was around 50 mm. docsity.com
