Measurement of Numerical Aperture of Fiber optic cable

Applied Instrumentation Lab

8EI7

Experiment No.  –  3  3 Object: - Measurement of Numerical Aperture of Optical Fiber . Apparatus: - Optical Fiber Cable, NA Measurement setup Theory: - When a ray of light inside a fiber core strikes the outside wall of the fiber at an angle of incidence (to the fiber axis) less than the critical angle θc. it will be totally internally reflected and contribute to the propagation of light along the fiber. If it strikes at greater than the critical angle some or all of the light will be refracted into the cladding material and be lost.

Fig. 1.1 Total internal reflection At the critical angle qc, the ray will enter the cladding at an angle of 00, or parallel to the interface. Substituting this limit in Snell's Law give the critical angle as cos θ c =

n2/n1 = √      

where n1 and n2 are the indices of refraction of the core and the cladding respectively. When the light is launched into a properly cut normal fiber end, it also experiences refraction at the air/core interface. Applying Snell's Law to this interface with the critical angle ray gives a critical entry angle qa (to the fiber axis) where any rays with entry angles less than this critical angle value will propagate while those greater will not propagate. The critical entry angle is found to be

n0 sin θ a = n1 sin θ c

Substituting n0 = 1 for air (or other if launching from a different medium), and for sin θ c

 NA = Sin θ a = √n1  –  n2 2

Sin θ a

Global Technical Campus, Jaipur

2

= D/2L

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Applied Instrumentation Lab

8EI7

Where L = Distance between the cable end and the measurement bench D = Diameter of the acceptance cone

Fig. 1.2 Numerical Aperture Measurement Set up

Procedure:1. Insert one end of the cable into the laser source (660nm) and other end into the measuring  bench. 2. Activate the laser source 3. Evaluate the diameter of the lightened area 4. As we move from cable 3 to cable 5 the brightness of the light point decreases as it is the function of the core diameter and the light become focused at single point

Observation: Diameter (D)

Cable Length (L)

NA

Result: - It has been observed that as the diameter of the core decreases the NA also decreases as the light gathering capability is the function of core diameter.

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