PHYSICS EXPERIMENT 10

EXPERIMENT NO. 10

Object: To measure the numerical aperture of the given an optical fiber.

Apparatus Required: Diode Laser Source, Fiber Holders, Optical Fiber, and Base With Rotational Mount, Holders And Bases.

Theory: Numerical aperture is a basic descriptive characteristic of a specific fiber. It represents the size or degree of openness of the input acceptance cone. Mathematically it is defined as the sine half angle of the acceptance cone.

Using snell’s law, the maxima angle with in which light will be accepted into and guided through fiber is

NA=Sin(ө_a)= (n₁²- n₂²)^1/2

Where              ө_a is the numerical aperture and n₁ and n₂ are the refractive indices of the core and the cladding. If the incident angle ө < ө_a, the ray undergoes multiple internal reflections at core and cladding interface and it is called the guided ray. If ө_a < ө, the ray undergoes only partial reflection at core cladding interface.

In short length of straight fiber, ideally a ray launched at angle ө at the input end should come out at the same angle ө from output end. Therefore, the far field at the output end will also appear as a cone of semi angle ө_a emanating from the fiber end.

DIAGRAM:- 

Procedure:

1.    Mount Laser source, objective and detector on the respective holders.

2.    Mount both the ends of the optical fiber on the fiber holders.

3.    Align the difference objects as per the setup shown below.

4.    Couple the light from the laser source onto one of the fiber ends using a microscopic objective (provided with the kit).

5.    Place the screen (sheet having circular markings) at some distance from the output end of the fiber such that it is perpendicular to the axis of the fiber. Now move the screen towards or away from the output end of the fiber such that circular beam emanating from the fiber end covers the (1^st or 2^nd or 3^rd) circle on the screen.

6.    Measure the distance between the output end of optical fiber and screen. Let this be L, also measure the diameter of the circular spot formed on the screen. (Diameter is mentioned in mm). Let it be D.

7.    Use the formula

Observations table:

S.No.	Diameter of Laser Spot D mm	Distance between Optical Fiber and Screen L mm			NA=Sin ө

Result:

Numerical Aperture of given optical fiber is=……………………………………

Percentage Error:-

                                           (Standard value~ Observed Value) X 100

                  % ERROR =

                                                                    Standard Value

Precautions:

1.    Reading should be taken carefully.

2.    Direct viewing of laser light should be avoided.

3.    Laser light should be incident normally on the Screen.

Viva voce

   1. Define optical fiber.

        Ans. It is a cable which carries optical signal.

2. On which phenomenon light propagation take place in optical fiber

        Ans. Total internal reflection.

3.  What is total internal reflection?

      Ans. When light travels from denser medium to rarer medium and   angle of incidence is greater than critical angle then light reflects back into same medium this phenomenon is called Total internal reflection.

4.  Define  angle of acceptance

      Ans. Angle at which light enter into optical fiber and Total Internal Reflection   takes place.

5.  What is physical significance of numerical aperture.

    Ans. Light gathering ability of the fiber.

6.  What are the types of optical fiber

    Ans. There are two types of fiber (1) step index fiber (2) graded index fiber.

7.  What is fractional refractive index?

     Ans. It is the ratio of difference of the refractive indices of core and cladding to the refractive index of core i.e.,

8.  Which light source is used in experiment

    Ans. Diode Laser

9.  LASER stands for what?

    Ans. Light Amplification by Stimulated Emission of Radiation

10.     How many type of fiber losses are there?

        There are three types of fiber losses

(1)    Absorption

(2)   Rayleigh Scattering

(3)   Geometrical losses