To determine the numerical aperture of the given optical fiber

Aim

To determine the numerical aperture of the given optical fiber.

Apparatus Required:

One or two meters of a step index optical fiber, digital multimeter, Adaptors, Connectors, D.C. Power supply, Fiber optic trainer module,N.A. measurement jig.

Description:

The schematic diagram of the fiber optics trainer module is shown in figure1.

The circuit comprises of three parts.

1. Electrical to optical (E/O) converter
2. Optical to Electrical (O/E) converter and
3. Optical power meter

The E/O converter converts an input voltage to an optical output, P0, by driving the fiber optic light emitting diode (Fo – LED) current linearly using a negative feedback operational amplifier circuit. Direct current LED (DC – LED) current setting is done by rotating the knob SET / PO The optical power is coupled to the optical fiber through the connector. The LED current can be measured by monitoring by voltage with a digital millimeter. A.C. input is given to the Vin (Vr / 100) given the LED current in milliamp.

The optical to electrical converter accepts the input optical power (Pin) from the optical fiber connected through the connector and provides an output voltage (Vo). For D.C. measurements, a multi meter may be used. For A.C.measurements an oscilloscope may be used. In the experiment only D.C. measurements are made.

The optical power meter converts the optical power coupled to it through an SMA terminated optical fiber and facilitates display of the optical power Po in dB. The optical power in dB, is given by the (multimeter reading/10) in dB referred to 1 mw.

The numerical aperture of an optical system is a measure of the light collected by an optical system. It is the product of the refractive index of the incident

Numerical Aperture (????, ????) = ???? ???????????????? θ

For air = ???????? = 1

∴(????, ????) = ???????????? θ max------------------------------------------(1)

For a step index fiber, the N.A is given by :

∴(????,????) = (n²_core-n²_Clading) ^1/2 ------------------------------------------(2)

For small differences in refractive indices between the core and cladding, equation (2) reduces to:

N.A. = n _core2Δ ^1/2 1/ 2------------------------------------------(3)

Where Δ is the fractional differences in the refractive indices of the core and the cladding; i.e.,

[n_core - n_clade/n_core]

Light from the fiber end A falls on the screen BD. Let the diameter of the light falling on the screen = BD = W.

Let the distance between the fiber end and the screen = AO = L

Therefore from geometry,BD = W, AO = L , BO = OD = W/2

AB = (????² + ????²)/4 ^1/2

= [(4L² +W²)^1/2]/2

Therefore ???????? = W/√4L ²+W ²

Knowing the values of W & L the N.A. can be calculated.

Procedure:

The twists or the micro bends on the fiber, if any are to be removed first. In order to remove the twists, the optical fiber is wound on a mandrel. An adhesive tape may be used to hold the windings on the mandrel in the proper position. The experimental set up for the N.A.measurement is shown in figure 2 .

One end of the optical fiber is connected to the N.A. jig through the connector, and the other end of the fiber is connected to the power out Po of the N.A.module. The A.C. mains is switched ON and the light passing through the cable at the other end (coming to the N.A.jig) of the fiber is observed to ensure proper coupling is made or not. The “Set Po” knob is turned in the clock wise direction to get maximum intensity of light through the fiber. The “Set Po” is to be left free at this stage. A screen with concentric circles of known diameter is kept vertically at a distance (L) from the fiber end and the red spot is seen on the screen. The diameter of the red spot can be varied by varying the distance (L)

The experiment is repeated for the subsequent diameter of the circles by adjusting the length L. The diameter of the circle is determined using a travelling microscope. For each set of observations, the N.A. is calculated using the equation (4). Using the value of N.A. from (4) in equation (1), the acceptance angle is calculated. The results are tabulated in table1.

Observations:

Precautions:

1. Optical fiber cables should be connected appropriately.
2. Power meter values should be increased gradually.

Result:

Numerical Aperture of the given optical fiber is ……….

Reference Books:--

1. Practical Physics”,G.L.Squires,Cambridge University Press,1988
2. ”Fundamentals of Physics”,4th edition ,F.A.Jenkins and H.E.White, 1981
3. ”Physics”, M.Alonso and E.J.Finn, Addison Wesley, 1992
4. ”Laboratory Manual In Applied Physics”-Second edition- H.Sathyaseelam-New age International