Physics Experiment

PH YSICS EXPER IME NT AIM: To measure the numerical aperture (NA) of the given optical fibre.

APPARATUS: Scale, LED (Light Emitting Diode), Optical fibre, Screen. THEORY: Numerical aperture accounts for the light gathering ability of the fibre and it amounts the light accepted by the fibre. NA refers to the maximum angle at which the light incident on the fibre end is totally internally reflected and is transmitted properly along the fibre. NA depends only on the refractive indices of the material of the core and the cladding. Mathematically, NA is defined as sine of angle of acceptance. Thus, if θ0 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 fibre. The cone formed by the rotation of angle of acceptance along the axis of the fibre is the cone of acceptance of the fibre. The

light should strike the fibre end within its cone of acceptance else it is refracted out of the fibre. For short length of straight fibre, light ray incident at angle θ at the input end and must come out at same anglerom th output end. So, at the output end of the fibre also a cone of the same angle appears. Hence, NA can be determined by making measurements on the cone at the output end for fibre. therefore

NA = W / (4L2 + W2)1/2

where W = diameter of circular spot formed on the screen L = distance between output end of the fibre and screen

PROCEDURE:1. Connect the power to the board. 2. Connect one end of the fibre cable to the outut socket of the emitter and other end to the numerical aperture measurement jig. 3. Hold the white screen facing fibre such that its cut face is perpendicular to the axis of the fibre. Hold the white screen with two concentric circles (0.8cm & 1.6cm diameter) Vertically to a suitable distance to make the red spot coincide with small circle. 4. Record the distance of the screen from the fibre end and the diameter of the spot. 5. Compute NA from the formula given above. 6. Vary the distance between the screen and the fibre optic cable so that the sot coincides with the bigger circle. Note the distance of the screen from the fibre end.

OBSERVATIONS:S.No. Diameter of spot (Cms)

Distance between screen and fibre (Cms)

NA = W / (4L2 + W2)1/2

1. 2.

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 fibre provided should be handled carefully so as to prevent cracks. RESULT: Numerical aperture of the given optical fibre is _________.

VIVA VOICE 1. Define the following terms:i) Optical fibre ii) Core & Cladding iii) Acceptance angle & Acceptance cone iv) Normalized frequency or V-Parameter 2. Which phenomena takes place at the core-cladding interface. Define Total Internal . reflection and give its essential conditions.