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JIMS-TEC, Greater Noida Engineering Physics Lab Manual JIMS – TEC Greater Noida Engineering Physics Lab Manual B.Tech. 1st year Faculty Name: Mr. Nafees Udin &Ms. Jyoti Bala Applied Physics Lab 1
JIMS-TEC, Greater Noida Engineering Physics Lab Manual EXPERIMENT 1 AIM: To determine numerical aperture of an optical fiber. APPARATUS: A fiber optic, 20X microscopic objective, He-Ne laser, screen, graph paper and measuring scale THEORY: Numerical aperture accounts for the ligth gathering ability of the fiber and it amounts the light accepted by the fiber. NA refers to the maximum angle at which the light incident on the fiber end is totally internally reflected and is transmitted properly along the fiber. NA depends only on the refractive indices of the material of the core and the cladding. Mathematically, NA is defined as a sine of angle of acceptance. Thus if θo is the angle of acceptance then: NA = sin (θ0) or NA = (n12 – n22)1/2/n0 NA is also called the figure of merit of optical fiber. The cone formed by the rotation of angle of acceptance along the axis of the fiber is the cone of acceptance of the fiber. The light should strike the fiber end within its cone of acceptance else it is refracted out of the fiber. Acceptance angle Fig.1 Fig.1. For short length of straight fiber, light ray incident at angle θ at the input end and must come out at same angle from out put end. So, at the output end of the fiber a cone of the same angle appears. Hence NA can be determined by making measurements on the cone at the output end of fiber. NA = D / (4L2 + D2)1/2 where D = diameter of circular spot formed on the screen L = distance between output end of the fiber and screen Applied Physics Lab 1
JIMS-TEC, Greater Noida Engineering Physics Lab Manual PROCEDURE: Fig.2. 1. Connect the power to the source. 2. Connect one end of the fiber cable to the output socket of the source through microscopic objective and other end to the numerical aperture measurement jig as shown in Fig.2. 3. Hold the white screen facing fiber such that its cut face is perpendicular to the axis of the fiber. 4. Record the distance (L) of the screen from the fiber end and the diameter (D) of the spot. 5. Compute NA from the formula given above. 6. Repeat step 4 and 5 with varying distance. 7. Calculate the NA for each value of L and take the mean. OBSERVATIONS & CALCULATIONS: S.No Distance between output end of Diameter of circular spot the fiber and screen (L) cm formed on the screen (D) cm NA = D / (4L2 + D2)1/2 1 …. 7 RESULT: Mean Numerical aperture of Optical fiber = ……………… Applied Physics Lab 1
JIMS-TEC, Greater Noida Engineering Physics Lab Manual PRECAUTIONS: 1. Optical source should be properly aligned with the cable. 2. Distance of the launch point from cable should be properly selected to ensure that maximum amount of optical power is transferred to the cable. 3. The optical fiber provided should be handled carefully so as to prevent cracks. Applied Physics Lab 1

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