Optical Counting Using Lasers-Physics-Lab Report, Exercises of Physics

Download Optical Counting Using Lasers-Physics-Lab Report and more Exercises Physics in PDF only on Docsity! Introduction: This experiment is aimed to study the generation of Fourier transform of different diffraction gratings (objects) and then measure the intensities of the frequencies of different orders in the Fourier plane. The whole process is as under in chronological order. Generation of collimated beam: We know that laser beam is not a collimated beam and it diverges as it propagates due to diffraction. To get a collimated beam, two convex lenses are used. One lens converges the beam and the other lens then collimates the beam. But this will only happen if the distance between the two lenses is D=f1+f2 Where D= distance between the two lenses f1= focal length of one lens and f2= focal length of the second lens The arrangement is shown diagrammatically as under. The next task is to get a Fourier transform of a grating. Generating the Fourier transform: After getting a collimated beam we put a diffraction grating in front of the beam, and then place a Fourier transform lens to get a Fourier transform of the grating. The Fourier transform can be seen on a plane paper (called a Fourier plane). The arrangement is shown as under. docsity.com The Fourier transform pattern is bright spots separated by a distance proportional to the inverse of the spacing between the lines on the grating. The central spot is the brightest and is called zeroth order maxima. The other spots on either side of the central maxima are symmetrically distributed and decrease in intensity with increasing order. The pattern obtained will look like the following, Procedure: After making the laser on, I placed a converging lens of focal length f1 to focus the diverging beam. Then I placed another lens of focal length f2 longer than f1. This lens not only collimated the original laser beam but also increased its diameter by f2/f1 times. Now I placed diffraction gratings as an object inside this beam. Then with the help of a Fourier transform lens I got the Fourier transform of the grating on a white paper called the Fourier plane. This pattern of transform is in the form of bright spots. Then with the help of a photo-detector, I measured the intensity of the various orders of maxima. But due to the high intensity of the laser that we used, the photo-detector saturated not only for zeroth order but also for 1 st , 2 nd and 3 rd order maxima. docsity.com