ENERGY GAP OF A SEMI CONDUCTOR - anuraghyd.ac.in

Applied Physics Lab Manual

Anurag college of Engineering 1

ENERGY GAP OF A SEMI CONDUCTOR

Aim: To determine the energy band gap of a semiconductor material taken in the form of a p-n

junction diode.

Apparatus: 1)D.C.Power supply

2)semi-conductor diode

3)thermometer calibratedin degrees

4) Heating arrangement to heat the diode

5) Micro-ammeter

6)connecting wires,oil.

Formula: Energy gap = k x ln(𝑅1

𝑇

)

= 2.303 log10(R/1/T) joules slope=R/1/T from graph

Energy gap (Eg) = 2.303 𝑥 𝑘 𝑥 𝑠𝑙𝑜𝑝𝑒

1.6 𝑥 10−19 eV

Boltzmann constant (K) = 1.3806 X 10-23 oules/ Kelvin .

Description:

The experimental arrangement comprises of an oil bath which is provided with sockets at its

mouth. The sockets are used to insert the thermometer and the semiconductor diode in the oil

bath. A heating element is fixed inside the oil bath which is used to raise the temperature of

the oil bath by connecting to the A.C.main supply. The reverse biasing voltage can be adjusted

by means of the voltmeter and the reverse saturation current can be measured with the help of

a micro-ammeter.

Procedure:Connect the two terminals of the given semiconductor diode to the D.C.Power

supply and micro-ammeter in such a way that the diode is reverse biased. Immerse the diode

and thermometer in the oil . Switch on the D.C.Power supply and adjust the reverse voltage to

1.5 volt. Switch on the A.C. main supply, then the temperature of the oil gradually increases.

Consequently, the current through the diode also increases. When the temperature of the oil

bath reaches to about 75oc, then switch off the heater. Then, the temperature of the oil will



rise and stabilizes at about 85oc. Note the temperature of the oil and the corresponding current

values through the diode .similarly note the value of the current for every 5 oc decrease of the

temperature, till the temperature of the oil falls to the room temperature. Note the

observations in the tabular column.

Note:When the germanium diode is employed in the experiment, the temperature should not

be increased beyond 80oc. This is because at higher temperatures, the Ferni level moves

towards the centre of the forbidden energy gap and the junction properties are destroyed.When

silicon diode is employed, then the temperature should not exceed 125oc.GRAPH:Draw a

graph with 103/T on X-axis and log10RonY–axis. The graph will be straight line as shown in

the fig. from the graph, find the slope of the straight line. The energy band gap of the given

semiconductor can be calculated by substituting the value of the slope (m). The calculated

value of Eg should be compared with the standard value.

Precautions:The diode and the thermometer should be immersed at the same level in the oil

bath.The temperature and the current should be noted simultaneously.When the experiment is

performed with Germanium diode, then the temperature should not be increased beyond 80 oc,

whereas in the case of silicon diode, then the maximum temperature should not exceed 125oc.

The experiment should be performed by connecting the diode in the reverse biased position.

ῼ

K-1

Graph:



Circuit Diagram:

Circuit diagram for reverse biasing of p-n junction diode:



Observations:

To determine the reverse saturating current at different temperatures.

Biasing voltage = 1.5 volt Room temperature=

S.No.

Temperature Current R=𝑽

𝑰

(ῼ)

𝟏

𝑻

(K-1)

log10R

(ῼ) T (oc) T= t+273(oK) Is (μA)

1 80

2 75

3 70

4 65

5 60

6 55

7 50

8 45

9 40

10 35

Result: The energy gap of given semiconductor p-n junction diode is = ----------Energy gap of Si=1.1eV

and Energy gap of Ge=0.7eV



Viva Questions:

1) Define Semi Conductor and its type.

2) What is Energy Gap? And it importance.

3) What is direct and indirect Band gap semi conductor?

4) Why oil is used for heating of Diode?

5) State few application of P-N junction diode.

6) How charge carriers generation depends upon Temperature?



CHARACTERISTICS OF SOLAR CELL

Aim: To study the characteristics of Solar cell.

Apparatus:

1) Two meters,

2) Variable load,

3) Solar cell mounted on wooden base,

4) similar height single directional mercury coated variable intensity Energy gap (Eg) of

Silicon (Si) = 1.1 eV

Electronic symbol for Solar cell

Theory:

If the depletion of unbiased junction is illuminated, charge separation takes place resulting in

forward bias on the junction. Such device having large area junction very close to the surface is

capable of delivering power and is known as SOLAR CELL .This cell converts directly solar

energy into electricity.

The solar cell radiation is proportional to the delivered power of cell. The efficiency of a

cell is expressed in terms of electrical power, output compared with the power in the incident

photon flux. The efficiency of solar cell depends on the fraction of light reflected from the surface

and fraction absorbed before reaching the junction. Silicon is widely used for Solar Cells.

+ –

+

–

–

+



Graph:

Plot graph, between voltage and current at different intensities with & without load.

Table for Voltage variation with respect to Current in Solar Cell:

Procedure:

1. Place Solar Cell directly in front of variable light intensity source and connect output of Solar

Cell to voltmeter M1 on board.

2. Now gradually increase the intensity of light (bulb) and observe the output of Solar Cell to

voltmeter M1.

S.No. Voltage

(mv)

Current

(mA)



3. Then connect the circuit as shown in the diagram.

4. Vary the intensity, and note voltage and current on M1 AND M2 meters respectively and as well

as contacting load.

Precautions:

1. Connect the wires properly.

2. Focus the incandescent lamp towards the solar cell properly.

3. Observe the voltmeter and ammeter readings without errors.

Result:

I-V characteristics for a given solar cell show that as Voltage increases current decreases hence

voltage is inversely proportional to current.

Power (from Graph) = ––––––––––––––– mW

Power (from Table)= ––––––––––––––––––mW



Viva Questions

1) What is working principle of Solar Cell?

2) Define Photo voltaic cell.

3) State few application of Solar cell.

4) What is Fill factor?

5) What is Efficiency of Solar cell?

6) What are the advantages of Solar cell?



Light Emitting Diode Characteristics

Aim: To study the V-I characteristics of given LED.

Apparatus:

1) LED experiment kit consisting of LASER diode

2) Voltmeter

3) Ammeter

4) Regulating power supply

5) Connecting wires.

Theory: - A P-N junction diode, which emits light on forward biasing, is known as light

emitting diode. The emitted light may be in the visible range or invisible range and the intensity of

light depends on the applied potential.

Principle: - In a Pn junction charge carrier recombination takes place when the electrons cross

from the n-layer to the P-layer. The electrons are in the conduction band on the p-side while holes

are in the valence band on the p-side. The conduction band has a higher energy level compared to

the valence band and so when the electrons recombine with a hole the difference in energy is given

out in the form of heat or light. In case of silicon or germanium, the energy dissipation is in the

form of heat, whereas in case of gallium-arsenide and gallium phosphide, it is in the form of light.

But this light is in the invisible region & so these material cannot be used in the manufacture of

LED. Hence gallium – arsenide phosphide which emits light in the visible region is used to

manufacture an LED.

Construction: -An n-type layer is grown on a substance and a p-type layer is grown over it by

diffusion process. The P-layer is kept at the top because carrier recombination takes place in it. The

terminals anode and cathode are taken out of the n-layer and P-layer respectively. The anode

connections are made at the edge in order to provide more surface area for the emission of light. A

metal film is applied to the bottom of substance to reflect light to the surface of the device and also

to provide connection for the cathode terminal. Finally the structure are provided with an

encapsulated (cover) to protect them from destruction.



Advantage:

1. Works on low voltage and current and hence consumes less power.

2. Require no warm up time.

3. Can be switched ON and OFF at a faster rate.

4. Long lifetime.

5. Small size and less weight.

Applications:

1. Infra red LEDS are used in burglar alarms.

2. Used in solid state video displays.

3. Used in the field of optical communication.

4. Used in image sensing circuits.

5. Used in numerical displays like watches, pocket calculators etc.

Procedure:

Procedure for 𝑉

𝐼 characteristics of a Light emitting diode:

1. Connect the Light emitting diode circuit as shown below:

2. Slowly increase supply voltage using variable Power supply using coarse and fine knobs.

3. Note down current through the Light emitting diode at increasing values of Light emitting

diode voltage of 0.5V, 1.0V, 1.5V, 2.5 V.

4. Do not exceed current limit of 30mA else the Light emitting diode may get damaged.

5. Plot a graph of Light emitting diode voltage V/s Light emitting diode current .



Model Graph:

Table for Voltage and current variation for given LED:

S.No Voltage

(mV)

Current

(mA)

Result: Threshold voltage of given LED diode is V0= ––––––––– V beyond Threshold voltage

V & I are directly proportional to each other



Viva Questions

1) Define the LED?

2) What is working principle of LED?

3) Mention few applications of LED

4) What are the advantages of LED?

5) Mention few advantages of LED.



STEWART AND GEE’S EXPERIMENT

Aim: To find the magnetic induction at different points on the axis of a circular coil carrying

current using Stewart and Gee’s type of tangent galvanometer.

Apparatus:

1) Stewart and Gee’s type of tangent galvanometer

2) Battery eliminator

3) Commutator

4) Ammeter

5) Rheostat


Formula:

Theoretical value of magnetic induction can be calculated by below formula:

B = 𝝁𝟎𝒏𝒊𝒂𝟐

𝟐(𝒙𝟐+𝒂𝟐)𝟑𝟐

Tesla or 𝑤𝑏

𝑚2

n = number of turns of the coil

i = current flowing through the coil

a = radius of the coil

x = distance of magnetic needle from the centre of the coil towards east and west

μ0= permeability of free space = 4𝜋 X 10-7 H/m.

The experimental value of magnetic induction can be calculated by below formula :

B = Be Tan θ Tesla or 𝒘𝒃

𝒎𝟐

Be = horizontal component of the earth’s magnetic field = 0.38 X 10-4 Tesla.

θ = average angle of deflection of the magnetic needle

θe = average angle of deflection when the magnetic compass box is placed on east

of the coil

θw= average angle of deflection when the magnetic compass box is placed on west

of the coil



Circuit Diagram:

Observations:

Horizontal component of earth’s magnetic field Be = 0.38 X 10 -4 Tesla ( or Wb. m-2)

Circumference of the circular coil = 2Πa = ––––––– cm

a = –––––––– cm

Radius of the coil a = –––––––– m

Current carrying in the ammeter (i)= –––––––––– Amp

0 = 4 X 10-7H/m



Table for calculating angle of deflection (Ө)of magnetic needle:

Graph:-

tanӨW tanӨE

distance in cm 0 distance in cm

west east

To determine the radius of the circular coil:

Take a thread of about one meter long and wind it over the circumference of the coil.

Measure the length l of the thread. Then,

Circumference of the coil, 2πa = ––––– l cm

Radius of the coil, a = l /2π = ––––––– cm.

Distance

From the

Center of

coil(cm)

Deflection in East

direction

Mean E=

Ө1+Ө2+Ө3+Ө4

4

TanE

Deflection in

West direction

Mean W

=

Ө1+Ө2+Ө3+Ө4

4

TanW

𝜃

=Ө𝐸 + Ө𝑊

2

Tan

(degr

ees)

1 2 3 4 1 2 3 4



Precautions:

1. The ends of the connecting wires should be cleaned well with a piece of sand paper and the

connections should be tight.

2. Magnets, magnetic substances or current carrying conductors should be kept away from the

vicinity of the apparatus.

3. The plane of the coil should be set in the magnetic meridian properly; otherwise the

magnetic needle does not obey the tangent law.

4. Deflections should be noted without parallax between the aluminum pointer and its image.

5. The magnetic compass box and the coil should not be disturbed after setting the coil in the

magnetic meridian.

6. Readings should be noted after gently tapping the glass top of the compass box with fingers.

This eliminates the effects of friction at the pivot.

7. While shifting the magnetic compass box to various distances, the current should be

stopped.

8. The ammeter and rheostat should be kept well away from the magnetic compass box.

9. The curve should be drawn smoothly and the points of inflection should be located

carefully.

10. A storage battery should be employed so that the current remains constant throughout the

experiment.

Result: The magnetic field along the current carrying coils at different places theoretical &

practical values are given below .

Distance From the Center of coil

(m)

onia2

Theoretical B = ------Tesla.

2(x2+a2)3/2

Practical B= Be Tan (Tesla)



Viva Questions

1) Define Biot Servart Law.

2) Define Flemings Left hand rule.

3) How Magnetic field varies with current carrying coil?

4) What magnetic field intensity?

5) What is magnetic Flux?



LCR - Series Resonance

Aim: To study the frequency response characteristics of L.C.R. Series resonance circuit and to

determine the resonance frequency and quality factor.

Apparatus:

1) Signal generator

2) Vacuum tube voltmeter

3) Milli ammeter

4) Condenser

5) Inductance coil

6) Resistance box


Formula:

Theoretical Formula:

1. Resonance frequency of the series resonant circuit is,

fo = LC2

1

2. Quality factor of the circuit is,

Q = C

L

R

1

Where R = resistance of the resistor (ohm)

L = inductance of the coil (henry)

C = capacitance of the condenser (farad)

Experimental Formula:

1. Resonance frequency of the circuit, fo = Hz

(From the graph)

2. Bandwidth of the resonant circuit, ∆f = (f2- f1)

(From the graph)

Where f1 = lower half power frequency

f2 = upper half power frequency

3. Quality factor of the circuit, Q = f

fo

Hz



Circuit Diagram

Graph:



Observations:

Self inductance of the coil, L = ––––––– H

Capacity of the condenser, C = ––––––––F

Resistance of the resistor, R = ––––––––––– ῼ

Input voltage of the A.C, Vi= –––––––– volt

Table for Frquency and current variation of given LCR:

S.No. Frequency (f)

(KHz)

Current (i)

(mA)

Theory:

A coil of self inductance L, resistance R and condenser of capacitance C are connected in

series with a source of A.C.Supply as shown in the circuit. This circuit is known as L.C.R.circuit.

Where an alternating voltage of a pure sine waveform which is represented by,

E = Eo sin t

Is applied to the circuit, then an A.C. will flow through the circuit. The current flowing through the

circuit at any instant is given by,

I = Io sin t

Where E = instantaneous value of the applied voltage

Eo = maximum or peak value of the alternating voltage.



I = instantaneous value of the current

Io = maximum or peak value of the current

= angular frequency of the A.C supply

= 2 f

f = frequency of the alternating voltage

t = phase angle

= 2 f t

Due to the applied E.M.F i) an E.M.F ( L dt

di) is induced in the inductance which oppose the

applied E.M.F ii0 there is a potential drop (Ri) across the resistance and iii) a potential difference

(Q?C) is developed across the plates of the condenser, then, according to Ohm’s law,

Eo Sin RIC

Q

dt

dILt

tSinERIC

Q

dt

dIL o

Where Q = charge on the plates of the condenser at an instant

I = instantaneous value of the current in the circuit

The peak value of the current is given by

2

2 1

CLR

EI o

= 22 )( CL

o

XXR

E

Where, XL = fLL 2 inductive reactance of the circuit

XC =fcc 2

11 = capacitive reactance of the circuit

The total resistance offered to flow of current in the A.C.circuit is called the impedance (z) of the

circuit or effective resistance of the circuit.

The impedance of the circuit z=

2

2 1

CLR

Io = Z

Eo



The phase difference between the current in the circuit and the E.M.F applied to the circuit is given

by,

R

CL

Tan

1

From above equation, it is obvious that whether the current in the A.C.circuit leads the E.M.F or

lags behind the E.M.F. depends on the magnitude of the inductive reactance, L as well as on the

capacitive reactanceC.

Special case:When, C

L

1

, then Tanθ = 0, Z=R and I=R

Eo

In this case, the current and the applied E.M.F are in phase with each other. This condition is

known as resonance. So, the current in the circuit depends only on the value of R. If the value of R

is small, the impedance of the circuit is minium. Consequently, maximum A.C will flow through

the circuit and the circuit is said to be in tune or resonance with the applied voltage. The frequency

at which L =C

1

Is called the resonance frequency, fo and the circuit is known as the series resonance circuit.

L =C

1

orLC

12

LC

o

1 , = o at resonance

LC

fo

12

LCfo

2

1

Procedure:

A coil of self inductance L , a condenser of capacity C , a resistance R(10 ) and a

milliammeter a are to be connected in series with the signal generator as shown in the circuit.

Switch on the signal generator. Adjust the frequency nob of the signal generator so that the

frequency, f of the A.C.signal is 2 KHz. Adjust the amplitude of the input signal to a convenient



value by means of V.T.V.M. i.e., the input voltage Vi. Then, note the current I, in the circuit shown

by the milliammeter A.

Increase the frequency, f of the input signal in convenient steps (say 1 KHz), keeping input

voltage, Vi constant throughout the experiment. Then the current increases slowly at the beginning,

afterwards increases sharply and then reaches a peak value called the sharpness of the resonance.

This will happen when the frequency of the applied voltage is equal to the natural frequency of the

circuit i.e., resonance occurs. This frequency at which the current reaches a peak value is called the

resonance frequency, fo. At the resonance frequency, the current is maximum and the impedance is

minimum. Again increase the frequency of the input signal beyond the resonance frequency, and

then the current through the circuit gradually decreases. Note the observations in table.

Repeat the experiment b introducing different values of R in the circuit, for the same values

of L and C, keeping the input voltage Vi constant throughout the experiment. The theoretical values

of the resonance frequency, fo and the quality factor, Q can be calculated using the formulae.

Graph:

Draw a graph with the frequency, f on the x-axis and the current I on the y-axis. A sharp

resonance curve will be obtained. From the graph, note the maximum current, Io and

thecorresponding frequency at which the current is maximum. This frequency is called as the

resonance frequency f0.

To determine the bandwidth (∆f ) and quality factor Q:

From the graph, find the value of Io/ 2 . Mark the value on the y-axis. From the value draw

a line parallel to x-axis. This line cuts the curve at two points A,B< called half power points. From

the points A and B, draw lines parallel to y-axis, which meets the x-axis at two points

corresponding to two frequencies f1 and f2 called the half power frequencies, on either side of the

resonance frequency fo.

Bandwidth of the circuit, ∆f = (f2- f1) KHz

Quality factor of the circuit, Q = f

fo

Also draw a graph with frequency f, along the x-axis and the impedence Z along the y-axis.The

theoretical and experimental values of the resonance frequency fo and the quality factor Q are to be

compared.



Calculations:

1. Theoretical Values:

i) L = ----------henry C = --------farad R = -----------

Resonance frequency, fo = LC2

1 = --------------Hz

iii) Quality factors, Q = C

L

R

1= ---------

Experimental values (From graphs)

i)Resonance frequency,f0 = ------KHz = ------Hz

ii) Band width, f = (f2 – f1) = ------KHz = -------Hz

iii)Quality actor Q = f

f

0 = -----

PRECAUTIONS:

1. A fixed amplitude of voltage should be applied to the circuit for the selected values of L,C

and R at different frequencies.

2. The input voltage applied to the circuit should be checked at all the frequencies.

RESULT:

The theoretical and experimental values of resonance frequency, fo and the quality factor Q,

are calculated and compares. They are found to be equal.

S.No. Type of connection Parameter Theoretical Experimental

L1C1R1 L1C1R1

1 Series Resonance

Resonance frequency

foHz

2 Quality factor Q



Viva Questions

1) Define resonance frequency.

2) Define Quality Factor

3) How does resonance occurs in LCR series connection?

4) What is half power Frequency?

5) What is the effect of resistance on band width of the circuit?

6) What is an Acceptor Circuit?



C.R. Circuit

Aim: To determine the time constant of an given C.R. circuit.

Apparatus:

A network board consisting of

1) Capacitors

2) Resistors

3) Ammeter

4) Battery

5) Tap key

6) Stop clock


Formula:

Time constant of the C.R Circuit theoretically can be calculated by below formula i

Ӷ = RC sec

Where C = Capacitance in farads

R = Resistance in ohms

Time constant of RC circuit can be calculated practically by graph

Theory:

When a condenser ‘C’ is charged through a resistance ‘R’ then Voltage increases

exponentially in accordance with the formula.

V = Vo (1-e-t/RC)

Where V is the Voltage in time t and

V0 is the maximum Voltage

The product ‘CR’ is called time constant. It is the time taken to establish (1-e-t/RC) part of the

maximum charge in the condenser. It is equal to the time taken to establish 0.632 part of the total

charge.

When a condenser is discharged through a resistance, the voltage falls in accordance with

the formula.

V = Vo e-t/RC



The time constant in this case is equal to the time, taken to decrease the voltage of ‘e’ part

of the maximum voltage. It is equal to the time taken to discharge to a value of 0.368 part of

maximum voltage.

i.e. we can say that V α I

I = dq/dt = -t0 e-t/RC

When I = 0.36 I0, t = RC

Procedure:

The circuit is connected as the above circuit diagram, taking a set of R and C. The capacitor

C is charged by pressing the tap key for a short time till the deflection in the voltmeter ismaximum.

Then release the tap key. Now the capacitor starts discharging through R. The deflection decreases

rapidly in the beginning and then the rate of fall becomes slower as time passes. Note the voltmeter

reading at suitable regular intervals (5 sec). Note the observations in the table. Repeat the

experiment for different sets of R and C values.

Graph:

Draw a graph with time‘t’ along the X-axis and the Voltage ‘V’ along the Y–axis. Draw a

line parallel to X-axis at V= 0.36V0. This line meets the curve at one point. From this point draw a

line parallel to Y–axis which meets the X-axis at one point. The value of the time on the X-axis at

one point. The value of the time on the X-axis corresponding to V= 0.36V0gives the time constant,

tc of the C.R.Circuit. The theoretical value of the time constant can be calculated b using the

formula tc=RC. The theoretical and experimental values of the time constant are to be compared.

Precautions:

1. The connecting wires should be cleaned well sand paper.

2. The positive plate of the condenser should be connected to the positive terminal of the

battery during the charging of the condenser.

3. Before switching the power supply ensure that the voltmeter reads zero.

4. The condenser should be completely discharged before charging it.

5. The voltmeter reading should be noted quickly and accurately at every interval of



Circuit Diagram:

K

Model Graph:

μA

+ –

–

–

–

–

–

+

+

–

–

– R

Battery (5V) C

CHARGING OF CAPACITOR

DISCHARGING OF CAPACITOR



Table for : R1 = ---------------------- ῼ

C1 = ------------------------F

Ӷ1 = R1 X C1 = --------------------- sec

S.No. Time

(Sec)

Current

(A)

Result:

Time constants for given combination of R and C are

Theoretical value

R1 = ---------------------- ῼ

C1 = ----------------------- F

Ӷ1 = R1 X C1 = --------------------- sec

Practical value from graph : -------------------- sec

The theoretical value of the given R and C combination Is ------ and the practical value from the

graph is -------



Viva Questions

1) Define time constant.

2) What is Block Condenser?

3) What are the applications of RC circuit? And what factor does the rate of charging and

discharging of condenser depends?



Photoelectric Effect

Aim:

1. To understand the phenomenon Photoelectric effect as a whole.

2. To draw kinetic energy of photoelectrons as a function of frequency of incident radiation.

3. To determine the Planck's constant from kinetic energy versus frequency graph.

4. To plot a graph connecting photocurrent and applied potential.

5. To determine the stopping potential from the photocurrent versus applied potential graph.

Photoelectric Effect

The aim of the experiment is to study the emission of electrons by light. We also try and measure the

energy of the electrons emitted in the process. In addition to this, we will also observe the relation of

these electrons with the frequency of light used. To study the effect, we use an evacuated cathode ray

tube connected in a circuit as shown below:

Apparatus :

Near one of the plates inside the evacuated tube, there is present a small quartz window. The Quartz

window has two functions – it lets light in and it only lets the Ultra Violet light in. Hence by using a

Quartz window, we make sure that light of a specific frequency falls on the metal plate inside the

evacuated chamber.

Formula : Work function = hν

Theory:

During his experiments on electromagnetic radiation (to demonstrate light consists of e-m waves),

Hertz noticed a spark between the two metallic balls when a high frequency radiation incident on it.

This is called photoelectric effect. Photoelectric effect is the emission of electrons when

electromagnetic radiations having sufficient frequency incident on certain metal surfaces. We call

the emitted electrons as photoelectrons and the current they constitute as photocurrent. The

phenomenon was first observed by Heinrich Hertz in 1880 and explained by Albert Einstein in

1905 using Max Planck's quantum theory of light. As the first experiment which demonstrated the

quantum theory of energy levels, photoelectric effect experiment is of great historical importance.



1. The Photoelectric effect is an instantaneous phenomenon. There is no time delay between the

incidence of light and emission of photoelectrons.

2. The number of photoelectrons emitted is proportional to the intensity of incident light. Also, the

energy of emitted photoelectrons is independent of the intensity of incident light.

3. The energy of emitted photoelectrons is directly proportional to the frequency of incident light.

The basic experimental set up which explains Photoelectric effect is as given below,

It has been observed that there must be a minimum energy

needed for electrons to

escape from a particular metal surface and is called work

function 'W' for that

metal. The work function can be expressed in terms of

frequency as,

Where h is the Planck's constant and

V0 is the threshold frequency (minimum frequency for photoelectric effect).



The work function for some metals are listed in the table.

According to Einstein the Photoelectric effect should obey the equation,

From the above expression,

Which says the graph connecting the maximum kinetic energy of photoelectrons 'KEmax' and

frequency of incident radiation' 'will be a straight line with slope and Y-intercept = work

function.

Graph connecting 'KEmax' and frequency:

Maximum kinetic energy of photoelectrons versus frequency of incident radiation graph

Now, if we increase the reverse potential, the photocurrent gradually decreases and becomes zero

at a particular reverse potential. This minimum applied reverse potential is called stopping potential

V0. Hence the maximum kinetic energy of photoelectrons can be written as,



The graph between the photoelectric current and the intensity of light is a straight line when the

frequency of light used is above a specific minimum threshold value.

Graph connecting photocurrent and applied reverse potential:

For constant intensity and different frequencies

For constant frequency and different intensities



The circuit

We connect a voltmeter across the two plates. This measures the potential difference between the

plates. Moreover, we have a sensitive galvanometer in the circuit. This measures the photocurrent.

The Collector plate-C emits electrons which are then collected at the collector plate-A. These plates are

connected to the battery via the commutator. Let’s switch on this experiment and see what happens!

Well, in the beginning, let us have a zero potential. We open the quartz window and observe the

reading of the Voltmeter and the Ammeter. Both will give a non-zero reading (say for alkali metals),

proving the occurrence of the Photoelectric effect. As we increase the Voltage and change it again, we

will make the following observations:

The Effect of Intensity

The number of electrons emitted per second is observed to be directly proportional to the intensity of

light. “Ok, so light is a wave and has energy. It takes electrons out of a metal, what is so special about

that!” First of all, when the intensity of light is increased, we should see an increase in the photocurrent

(number of photoelectrons emitted). Right?



As we see, this only happens above a specific value of frequency, known as the threshold frequency.

Below this threshold frequency, the intensity of light has no effect on the photocurrent! In fact, there is

no photocurrent at all, howsoever high the intensity of light is.

The graph between the photoelectric current and the intensity of light is a straight line when the

frequency of light used is above a specific minimum threshold value.

The Effect of the Potential

Suppose you connect C to a positive terminal and A to a negative terminal. What do you expect will

happen to the photocurrent?

Since electrons are negatively charged, if we increase the negative potential at C, more and more

electrons will want to escape this region and run to the attractive plate A. So the current should

increase. Similarly, if we decrease the negative potential at C, removing electrons will become difficult

and the photocurrent will decrease. Hence the maximum current flowing at a given intensity of

incoming light is the saturation current.

As you can see in the graph, the value of saturation current is greater for higher intensities, provided

the frequency is above the threshold frequency. Imagine you are an electron and you just escaped the

metal surface. Now you are merrily accelerating towards A. What if we became mischievous and

increased the negative potential at A? You will feel a repulsion and consequently you will lose speed.

What if the potential is very strong? You will not be able to escape the metal surface at all! As a result,

we call this value of the potential for which the photocurrent becomes zero as the stopping potential or

the retarding potential. The more the negative potential of the collector plate, the more is the effort that

an electron has to make if it wants to escape successfully from the metal surface.



Thus we will get the following relationship between the stopping potential and the photocurrent.

Effect of Frequency

We see that for higher frequency values like ν3, stopping potential is more negative or greater than the

stopping potential for smaller frequencies like ν1. What does this mean? This means that there should

be a relationship between the frequency and energy.

In the End

We can sum up the observations as follows:

A. For a given metal (photosensitive material), the photoelectric current is directly proportional to

the intensity of the light used, above a minimum value of frequency called the threshold

frequency.

B. The saturation current depends on the intensity for a known value of frequency. At the same

time, we see that the stopping potential does not depend on the intensity over a specific value of

frequency.

C. The Photoelectric effect does not occur below a certain frequency. This is the threshold

frequency. If the frequency of light is above the threshold frequency, the stopping potential is

directly proportional to the frequency. In other words, to stop an electron emitted by a higher

frequency, we require more energy. The stopping potential provides this energy.

D. All of this happens instantaneously. As soon as we open the quartz window, electron emission

starts.



Precautions:

1. The high pressure mercury lamp also emits light in the UV range, and can thus damage the

eyes.

2. Never look into the direct or reflected beam of light from the high pressure mercury lamp.

3. Do not exert mechanical force on the vacuum cell, danger of implosions.

Result: Work function of a given material = __________



Viva Questions

1) Define Photo Electric Effect?

2) What is work function?

3) Emission of electrons depends upon which criteria ?



©

FIBER OPTICS CHARACTERISTICS USER MANUAL



NOTE: DONOT INTERCHANGE FIBER OPTICS EXPERIMENT ACCESSORIES WITH

ANY OTHER UNIT. STRICTLY ADHERE TO RESPECTIVE SERIAL NUMBERS PASTED

ON ALL PARTS/UNITS.

Manufactured by:

MIKRON INSTRUMENT INDUSTRIES



303, Moghuls Court

building, Basheerbagh

Hyderabad – 1.

Ph. 040 – 6450 890 / 51 . Fax. 040 – 6662 5145

email. [email protected] / [email protected]

1



EXPERIMENT: 1 LOSSES IN OPTICAL FIBER AT 660nm AND 850nm

1.1 Aim of the Experiment:

The aim of the experiment is to study various types of losses that occur in optical fibers and

measure losses in dBm of two optical fiber patch cords at two wavelengths. Viz. 660 nm and 850

nm.

Theory: Attenuation in an optical fiber is a result of following effects:

6. Losses at connectors and losses due to the length of the fiber cable. The optical power at a

distance L, in an optical fiber is given by PL α PoL, Where Po is the input power and α is

the attenuation coefficient in decibels per length. The typical attenuation coefficient value

for the fiber under consideration is 0.2dBm per meter at a wavelength 660nm.

7. Losses in fiber occur at fiber-fiber joints or splices due to axial displacement, Angular

displacement, separation (air core), mismatch of course diameter, mismatch of numerical

aperture, improper cleaving and cleaning at the ends. The loss equation for simple fiber

optic link is given by:

Pin(dBm)-Pout(dBm) = Lj1+LFIB1+Lj2+LFIB2+Lj3(dB): Where Lj1(dB) is the loss at LED

connector junction, LFIB1(dB) is the loss in cable1, Lj2(dB) is the insertion loss in a line

connector, LFIB2(dB) is the loss in cable2, and Lj3(dB) is loss at the connector detector

junction.

NOTE: In this particular experiment connections to Voltmeter (M1), Signal generator are

not required. Connect the circuit as mentioned below in the experiment procedure.

Experiment Procedure for IR L.E.D 850 nm ( Infra Red Light Emitting Diode ) :



We are mainly finding losses in optical fibers due to its length and bending.

Following procedure is for study of losses due to length of optical fibre using 850nm L.E.D.

Short M2 of I.R L.E.D ( +ve and –ve terminals ) on the Tx unit with patch cords ( for this

experiment M2 is not required ) .

Relieve all the twists and strains in the fiber cable, ensure that it is as straight as possible, as

shown in fig.1.1.

Connect one end of the cable 1 ( 1 meter ) to the 850nm LED FC ( Fiber Connector ) adapter

of the Tx unit and other end to the PIN diode FC ( Fiber Connector ) adapter of Rx unit as

shown in figure 1.1.

4. Move the S1 switch on Rx unit towards 850 nm IR LED.

5. Switch ON the AC mains of both Rx & Tx units..

6. Rotate Po ( Power Adjustment potentiometer ) pot to extreme clockwise direction to

set

maximum carrier power, note down the “Optical Power Meter Reading “ P1 in µW on Rx unit. 7.

Without altering the position of Po ( Power Adjustment potentiometer ) pot, Connect cable 2 (

3meters ) between 850nm FC adapter and PIN diode FC adapter of Rx Unit. Note/Tabulate

“Optical Power Meter Reading “, P2 on th Rx unit.

2



8. Without altering the position of Po ( Power Adjustment potentiometer ) pot, Connect cable 3 (

5 meters ) between 850nm FC adapter and PIN diode FC adapter of Tx Unit. Note/Tabulate

“Optical Power Meter Reading “, on the Rx unit, P3 ( As per table 1.1 ).

9. Convert the all power meter readings in µW to dBm using formula. Given below.

dBm=10*log (Power meter reading in mW/1mW)

In this case, dBm=10*log (Power meter reading in µW/1000)

10. Tabulate all the readings in table 1.1

11. Calculate average loss per meter at 850 nm. The loss per meter in the range of 2±0.5 dBm is

acceptable ( As per standards ) .

Experiment Procedure for L.E.D 660 nm ( Light Emitting Diode ) :

Following procedure is for study of losses due to length of optical fibre using 660nm L.E.D.:

1. Short M2 of 850nm LED ( +ve and –ve terminals ) on the Tx unit with patch cords ( for this

experiment M2 is not required ) .

2. Relieve all the twists and strains in the fiber cable, ensure that it is as straight as possible, as

shown in fig.1.1.

3. Connect one end of the cable 1 ( 1 meter ) to the 660nm LED FC ( Fiber Connector ) adapter

of the Tx unit and other end to the PIN diode FC ( Fiber Connector ) adapter of Rx unit as

shown in figure 1.1.

4. Move the S1 switch on Rx unit towards 660 nm L.E.D.

5. Switch ON the AC mains of both Rx & Tx units.

6. Rotate Po ( Power Adjustment potentiometer ) pot to extreme clockwise direction to



set

maximum carrier power, note down the “Optical Power Meter Reading “ P1 in µW on Rx unit.

7. Without altering the position of Po ( Power Adjustment potentiometer ) pot, Connect cable 2

( 3meters ) between 660nm FC adapter and PIN diode FC adapter of Rx Unit. Note/Tabulate

“Optical Power Meter Reading “, P2 on th Rx unit.

8. Without altering the position of Po ( Power Adjustment potentiometer ) pot, Connect cable 3

( 5 meters ) between 660nm FC adapter and PIN diode FC adapter of Tx Unit. Note/Tabulate

“Optical Power Meter Reading “, on the Rx unit, P3 ( As per table 1.1 ).




10. Tabulate all the readings in table 1.1

11. Calculate average loss per meter at 660 nm. The loss per meter in the range of 0.2 dBm is

acceptable ( As per standards ) .

Experimental Procedure for losses due to OF Bending using 850nm L.E.D:

STEP 1: Short the M2 +ve and –ve terminals of both LED’s on Tx unit, and Connect one end of

the cable 1mtr to the 850nm LED FC adaptar of the Tx unit and other end to the pin diode FC

adaptor of Rx unit as shown in figure 1.1. Move the S2 switches on Tx unit towards 850 nm IR

LED input. Then turn on the AC mains.

STEP 2: Relieve all the twists and strains in the fiber cable so that it is straight as shown in fig.1.1

STEP 3: Rotate P0 pot to the extreme clockwise direction to get maximum intensity and note

down the reading(P01) of power meter on Rx unit..



3



STEP 4: Now wing one turnt of he OF cable on mandrel and note down the new reading P02.

STEP 5:Repeat the experiment for 3meter and 5 meter cables with more turns and tabulate the

readings in the tabular column 1.3

STEP 6:Above experiment can be repeated for 660nm LED also.

Note: 1. Do not repeat the bending losses experiment more than 3 turns as it may cause

permanent damage to FO cable.

2. While doing experiment with 660nm LED move switch towards 660nm input.






TX Unit Rx Unit

A

660nm PT FC

S1

M2 850nm Pin FC PM

OF cable

Fig 1.1

1 meter cable

3 meter cable

5 meter cable



4



Table 1.1

Table of readings for 850nm

SI P1 Rd1 P2 Rd2 P3 Rd3 M0=Rd1-Rd2/2 M1=Rd2-Rd3/2 M2=R1-R3/4 L/M

No. in µW(dBm)in µW(dBm) in µW (dBm) (dBm) (dBm) (dBm) (dBm)


SI P1 R1 P2 R2 P3 R3 M0=R1-R2/2 M1=R2-R3/2 M2=R1-R3/4 L/M

No. in µW(dBm)in µW(dBm) in µW (dBm) (dBm) (dBm) (dBm) (dBm)

Where L= Mo + M1 + M2

3

Table 1.3


n=No of Turns on mandrel………



Cable P01 R01 P02 R02 R01-R02 P11 R11P12 R12 R11-R12

No of

L in µW in dBm in µW in dBm

n in µW in dBm in µW in dBm

Turns(n)

n

1 mtr

3 mtr

5mtr

5



EXPERIMENT 2: CHARACTERISATION OF 660 NM AND 850 NM L.E.D.

Aim of the experiment 2.1:

The aim of the experiment is to study the relationship between the LED dc forward

current and the L.E.D. optical power output and determine the linearity of the device at

660nm as well as 850 nm.

Theory:

L.E.D and Laser diodes are commonly used sources in optical communication systems,

whether the system transmits digital or analogue signals. In case of analogue transmission,

direct intensity modulation of optical source is possible, provided the optical out put from

the source can be linearly varied as a function of the modulating electrical signal

amplitude. L.E.D have a linear optical out put with relation to the forward current over a

certain region of operation. It may be mentioned that in many low cost, short-lenght and

small bandwidth applications, L.E.D at 660nm, 850nm and 1300nm are popular. While

direct intensity modulation is easy to realize, higher performance is achieved by FM

modulating the base band signal prior to intensity modulation.

The relationship between an L.E.D optical output Po and the L.E.D forward current Iғ is

given by Pо=K *Iғ (over a limited range) where K is constant.

Experiment Procedure:

1. Connect one end of the cable1(1mtr) to the 850nm L.E.D. FC adaptor of Tx unit and the

other end to the PIN photodiode FC adaptor of Rx unit.



2. Connect on board Ammeter to 850nm IR L.E.D M2 meter (to its respective +ve and –

ve terminals) provided on the circuit panel of the Tx unit as shown in fig 2.1, Move

the S2 switch on Rx unit towards 850 nm IR LED and Then turn on the AC mains.

3. Rotate Po pot to the extreme anticlockwise direction to reduce the Iғ (L.E.D. forward

current) to 0Amps and power meter should be zero.

4. Slowly rotate the Po knob in clockwise direction to increase Iғ. The power meter should

read 5µW approximately.

5. Now change Iғ in steps of 3mA by rotating Po in clockwise direction and note the

power meter readings.

6. Tabulate the readings as in Table 1.2. and Repeat the above complete experiment for

660nm L.E.D. also.

AC AC

Tx Unit Rx Unit

A660nm PT

P0 OF Cable

850nm Pin PM

6



M2

Fig 2.1

Table for 850nm LED

Table 1.2

SI IF in mA Power mtr reading Power mtr reading

In µW in(dBm)

1.

2.

3.

4.

Table 1.3

Table for 850nm LED

SI IF in mA Power mtr reading Power mtr reading

In µW in(dBm)

1.

2.

3.

4.



Note: Convert the power meter reading from µW to dB using formula. Given

below. dBm=10*log (Power meter reading in mW/1mW)

In this case, dBm=10*log(power mtr reading in µw/1000)

Conclusion:

From this experiment, It will be observed that the L.E.D. at 660nm and 850 nm have a

linear response for Po vs Iғ in a limited region of Iғ. By selecting this region of Iғ for

operation, a linear intensity modulation system for signal transmission can be designed.

EXPERIMENT 3: CHARACTERISATION OF A FIBER OPTIC PHOTO

TRANSISTOR.

7



Aim of the Experiment 3.1:

Aim of the experiment is to study the relationship between the optical input power on

photo detector and the resultant photo current Ip. The photo detector in this case is a

photo transistor.

Theory:

PIN photodiodes, avalanche photo diodes and photo transistors are commonly used photo

detectors used in optical communication systems.

In analogue signal reception, where intensity modulated light signal is to be received and

demodulated to give an electrical output, the linearity of the device is of paramount

importance. The relationship Between photo transistor photo current Ip and the optical

Power (Pin) input at a given wavelength is given by Ip=Q.Pin Where Q is a constant.


1. Short the M2 of I .R LED (+ve and –ve terminals) of Tx Unit and Connect the photo

transistor output terminals of Rx unit to M1 volt meter on Tx Unit as shown on fig 3.1,

2. Connect one end of OF cable1 to the 850nm L.E.D. FC adaptor of Tx unit and the

other end to PIN diode FC adaptor, Move the S1 switch on Tx unit towards 850nm IR

L.E.D. and Then turn on the AC mains.

3. Set the carrier power Po to approx,3µW value.

4. Remove the cable from PIN diode FC adaptor and Connect to Photo transistor FC

adaptor of Rx unit.

5. Rotate the Rin pot to the extreme clock wise direction to get minimum gain. Now

note down the Vout on the volt meter provided on Tx unit.



6. Photo transistor current can be calculated as Ip = Vout / (Rin*G). Where G is the

internal gain of the amplifier.

Hence G = For 660nm L.E.D. and G = For 850nm L.E.D.

7. Repeat the above steps by increasing in steps 3µW of Po values. And tabulate

the readings in table 3.1.

Entire experiment can be repeated for 660nm L.E.D, during this S1 switch should be

moved towards 660nm red L.E.D. throughout the experiment.

V660nm PT

OF Cable S1

850nm Pin PM

M2

Fig 3.1

8



Table 3.1:

Table for 850nm LED

SI Power meter P0 Vout Ip=Vout/Rin*G

NO. Reading in µW in (dBm) in Volts

1.

2.

3.

4.

5

Table 3.2:

Table for 660nm LED

SI Power meter P0 Vout Ip=Vout/Rin*G

NO. Reading in µW in (dBm) in Volts

1.

2.

3.



4.

5

Note: Convert the power meter reading from µW to dBm using formula.

Given below :

dB=10*log(power mtr reading in µw/1000)

Conclusion:

It will be observed from the above results that for the both 660nm and 850nm

wavelengths the Photo transistor responds linearly to the optical input over wide range of

Iғ values.

9



EXPERIMENT 4 : DESIGN AND STUDY OF A LINEAR INTENSITY

MODULATION SYSTEM FOR ANALOGUE TRASMISSION.

Aim of the experiment 4.1:

The aim of the experiment is to study the following AC characteristics of linear intensity

modulation systems.

(a). Gain characteristics of a Fo Linear intensity modulation system Vin(ac) vs. Vout(ac)

for fixed carrier power Po and signal frequency fo.

(b). Frequency Response of a Fo Linear Intensity Modulation System Vout (ac) vs. Fo at

fixed carrier power Po and Vin(ac)

(c). Waveform distortion in a Fo Linear intensity Modulation system Vin(ac)(Max)vs. P0

at fixed Fo.

(d) Gain bandwidth product of a Fo linear intensity modulation receiver Gain vs. Band

width for fixed Vin.

Experiment Procedure (4a)::

Gain characteristics of a Fo Linear intensity modulation system Vin(ac) vs. Vout(ac) for

fixed carrier power Po and signal frequency fo. With the Use of external dual trace CRO.

1. Connect the Sine wave frequency generator output terminals to the Vin and ground

terminals of the Tx unit using patch cords, Short the M2 of IR LED ( +ve and –ve

terminals ) on the Tx Unit.



2. Connect one end of cable 1 to the 850nm L.E.D. FC adaptor of Tx unit and the other

end to PIN diode FC adaptor of Rx unit and fix the carrier power Po at 5µW.

3. Remove FC connector from PIN diode FC adaptor and connect it to Photo

Transistor FC adaptor.

4. Connect the CH1 ( Channel 1 ) of CRO across the Vin +ve and Gnd terminal of the Tx

unit. And CH2 ( Channel 2 ) of CRO across the CRO inputterminals of the Rx unit. As

shown in fig.4.1(a)

5. Fix the input frequency at 500Hz using frequency setting pot on Tx unit from the sine

wave generator.

6. Now Increase the amplitude of the input signal in steps of 100mV, and

note the input and output signals Peak to Peak voltages on the CRO.

7. Tabulate the readings in Table 4.1

10



Fig 4.1(a)

Tx Unit Rx Unit

660nm PT

fin vin 850nm OF Cable

Pin

amp gnd M2 PM

CRO

Table 4.1

Fo=2KHz,P0=1µW

SI Vin(mVp-p) Vout(mVp-p) Gain=Vout/Vin

No

1.

2.

3.

4.



5.

11



Experiment Procedure(4b):

Frequency Response of a FO ( Fiber Optic ) Linear Intensity Modulation System, Vout

(ac) vs. Fo(out put frequency) at fixed carrier power Po and Vin(ac) With the Use of

external dual trace CRO.

1. Connect the frequency generator output terminals to the Vinput and ground terminals

of the Tx unit using patch cords, Short the M2 of IR LED ( +ve and –ve terminals ) of Tx

Unit.

2. Connect one end of cable 1 to the 850nm L.E.D. FC adaptor of Tx unit and the other

end to PIN diode FC adaptor of Rx unit and fix the carrier power Po at approx.

10µW.




unit. And CH2 ( Channel 2 ) of CRO across the output and Gnd terminals of the Rx

unit. As shown in fig.4.1(a)

5. Now fix input signal amplitude Vin(ac) at 500mv.

6. Increase the Frequency of the input signal in suitable steps of 100hZ.

and note the input frequency and output signal Vout Peak-Peak voltage on the

CRO channel 1 and channel 2 respectively.

7. Tabulate the readings in Table 4.2.

Table 4.2

P0=1µW; Vin (ac) =-500mVp-p



SI fo(Hz ) Vout SI F0(Hz) Vout(mv)

No. (Vp-Vp) No.

1. 100 6. 1000

2. 200 7. 1500

3. 300 8. 2000.

4. 400 9. 2500.

5. 500 10. 3000

12



Experiment Procedure(4c):

Waveform distortion in a FO Linear intensity Modulation system Vin(ac)(Max)vs. P0 at

fixed FO. With the use of dual trace CRO.

STEP 1: Connect the frequency generator output terminals to the Vin and ground

terminals of the Tx unit using patch cords, Short the M2 of IR LED +ve and –ve

terminals of Tx Unit.

STEP 2: Connect one end of cable 1 to the 850nm LED FC adaptor of Tx unit and the

other end to PIN diode FC adaptor of Rx unit and fix the carrier power Po at 5µW.

STEP 3: Remove FC connector from PIN diode FC adaptor and connect it to Photo

Transister FC adaptor.

STEP 4: Connect the CH1 of CRO across the Vin +ve and Gnd terminal of the Tx unit.

And CH2 of CRO across the output and Gnd terminals of the Rx unit. As shown in

fig.4.1(a)

STEP 5: Fix the input frequency at 800Hz using frequency setting pot on Tx unit.

STEP6:Now increase Vin(ac) gradually from 10mv(p-p) and Observe Vout on the

oscilloscope. Note the reading of Vin(p-p)max for which distortion sets in output voltage

Vout(p-p). Repeat this for other values of P0.

STEP 7: Tabulate the readings in Table 4.3.



Table 4.3.

Frequency fo=800Hz

SI.N0 Po(1µW) Vin(Vp-p) max

1. 5µW

2. 10µW

3. 15µW

4. 20µW

Experiment Procedure4(d):

Gain bandwidth product of a FO ( Fiber Optic ) linear intensity modulation receiver Gain

vs Band width for fixed Vin.

1. Connect the Sine wave frequency generator output terminals to the Vin and ground

13



terminals of the Tx unit using patch cords, Short M2 of IR LED( +ve and –ve terminals

) on Tx Unit with patch chords.

2. Connect one end of cable 1 ( 1 meter ) to the 850nm L.E.D. FC adaptor of Tx unit and

the other end to PIN diode FC adaptor of Rx unit and fix the carrier power Po at

approx. 10µW.




unit. And CH2 ( Channel 2 ) of CRO across the output and Gnd terminals of the Rx

unit. As shown in fig.4.1(a)

5. Now Fix input signal amplitude Vin(ac) at 500mv.

6. Increase the Frequency of the input signal to 800Hz observe the transmitted Vin and

Received Vout signals on the oscilloscope. Set the Rin such that the gain

(Vout/Vin)=1.0. Repeat the experiments for other gains(2,3,4 etc) and note the input,

output signal Pk-Pk voltages on the CRO channel 1 and channel 2 respectively.

7. Tabulate the readings in Table 4.4.

Table 4.4

Fin=800Hz,Vin(ac)=500mV,P0=5µW.

SI.N0 Gain Vin(Vp-p)

1. 0.25



2. 0.5

3. 1.0

.

4 2

5. 3



14



EXPERIMENT 5: DETERMINATION OF NUMERICAL APERTURE OF

OPTICAL FIBERS

Aim of the Experiment :

The aim of the experiment is to determine the numerical aperture of the optical fibers

available.

Theory:

Numerical aperture of any optical system is a measure of how much light can be

collected by the optical system. It is the product of refractive index of the incident

medium and sine of the maximum ray angle.

NA=ni.sinӨmax ni for air is 1, Hence NA=

NOTE: In this particular experiment connections to Voltmeter (M1),

Ammeter(M2),Signal generator are not required. Connect the circuit as mentioned below


The experimental set up for numerical aperture measurement system is as shown in

below figure 5.1.

AC mains NA Jig



Tx unit

Set P0

Fig 5.1

1. Connect one end of the cable one to the 660nm L.E.D. FO connector of Tx Unit and

other end to the NA jig as shown in the above figure 5.1.

2. Plug the AC mains Turn the Po knob to clockwise direction to set maximum P0.

The light intensity should increase at the end of the fiber on the NA Jig.

3: Hold the white scale-screen, provided in the Kit vertically at a distance of 15mm(L)

from the emitting fiber end and view the red spot on the screen (A dark room is

necessary to facilitate a good contrast).

Now measure the maximum diameter(W) of the spot.

4. Compute NA from the formula NA = sinӨmax = W / (4L²+W²)½, Repeat the

experiment at 10mm, 20mm an d 25mm distances and tabulate the readings in

the table 5.1

15



Table 5.1

SI N0 L (mm) W(mm) NA Ө(degrees)

1.

2.

3.

4.

5.

Specifications of PMMA Fiber optic cable



16

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