experiment to determine the numerical aperture of an optical fibre..

optical fibre's aperture

1. OBJETIVE OF THE EXPERIMENT To determine the numerical aperture of an optical fibre

2. THEORY Numerical aperture of an optical fibre is defined as the light gathering ability of the fibre. Numerical aperture also refers to the maximum angle at which the light incident on the fibre end is totally internally reflected and is properly transmitted along the fibre .

3. The light ray should strike the fibre end with in this cone of acceptance else it is refracted out of the fibre. Numerically, it is also defined as the sine of the acceptance angle.

4. WHAT IS OPTICAL FIBRE An optical fiber  ( or  optical fibre) is a flexible, transparent fiber made of glass ( silica ) or plastic, slightly thicker than a human hair.  It functions as a waveguide , or “ light pipe ”to transmit light between the two ends of the fiber .

5. PARTS OF OPTICAL FIBRE Core –thin glass center of the fiber where light travels. Cladding –outer optical material surrounding the core. BufferCoating –plastic : Coating that protects the fibre.

6. PRINCIPLE OF OPTICAL FIBRE It works on the principle of total internal reflection.

7. ADVANTAGES OF OPTICAL FIBRE Low Transmission Loss and Wide Bandwidth Immunity and Interference Small Size and Weights Signal Security Electrical Isolation Abundant Raw Material Less Expensive

8. MATERIAL REQUIRED An optical fibre cable Optical fibre trainer board Screen connecting wires

9. SCREEN

10. OPTICAL FIBRE TRAINER BOARD Fiber Optics Trainer Model is designed to learn the basics of fiber Optics. A large number of experiments.

11. PROCEDURE Make all the connections . Connect one end of the optical fibre cable to the optical fibre trainer and other end to the numerical aperture jig. Hold the white screen with four concentric circles (10, 15, 20 and 25 mm diameter) vertically at a suitable distance to make the red spot emitted from the optical fibre coincide with the 10 cm circle. Note that the circumference of the spot (outermost) must coincide with the circle.

12. Record L, the distance of the screen from the fibre end and note the diameter (W) of the spot. Compute the numerical aperture (NA) of the optical fibre by using the formula NA = Sin θ = W/(4L2+W2)1/2 , where θ is called as the acceptance angle is the maximum angle of incidence at the input end of the optical fibre so that he optical ray can just propagate within the optical fibre. Tabulate the reading and repeat the experiment for 15 mm, 20 mm and 25 mm diameter too.

13. OBSERVATION AND CALULATIONS S.No LENGTH(mm) DIAMETER OF THE SPOT(mm) NUMERICAL APERTUER(NA) ACCEPTANCE ANGLE( DEGREE) 1 9 10 0.4 23.5 2 14 15 0.4 23.5 3 19 20 0.4 23.5 4 24 25 0.4 23.5

14. RESULT Numerical aperture of optical fibre is 0.4mm.

15. PRECAUTIONS The optical fibre cable should be free from twists and folds so as to avoid the power loss. Connections should be proper and tight .

16. VIVA VOCE What do you mean by refractive index? What is difference between reflection and refraction? What is the basic principle of propagation of light in an optical fibre?

17. Define critical angle? Numerical aperture is (a) sine of critical angle (b) cos of acceptance angle (c) sine of acceptance angle (d) acceptance cone

18. Name the two types of optical fibres? What are the advantages of optical fibre? What are the applications of optical fibres? Name the two types of optical fibres?

19. Numerical aperture depends upon (a) Refractive indices of core and cladding (b) Material of core (c) Light source (d) Length of optical fibre cable.

20. REFERENCES http://www.labelectronics.com/index.php?option=com_virtuemart&page=shop.browse&category_id=22&Itemid=53&vmcchk=1&Itemid=53 http://www.trainer-kits.com/fibre-optical-trainer.html http://en.wikipedia.org/wiki/Optical_fiber Engineering physics by H.K MALIK