The document describes an experiment to determine the wavelength of a semiconductor laser using diffraction. A laser beam is directed at a metal scale with graduations. The diffraction patterns are observed on a screen and the distances between the direct beam and diffraction spots are measured. These measurements are used to calculate the path difference and apply the diffraction equation to determine the laser's wavelength. The experiment is repeated to obtain an average wavelength value.

1. EXPERIMENT NAME
Wavelength of a Semiconductor laser
BY
PRAVEEN N VAIDYA
SDM COLLEGE OF ENGIEERING AND TECHNOLOGY,
DHARWAD
DEPARTMENT OF PHYSICS
SUBJECT: ENGINEERING PHYSICS PRACTICAL

2. Laser
• Atoms of a material excite to higher energy state, if
absorb sufficient energy in ground state known as
Induced absorption.
• Atoms in excited state immediately return to ground
state by the emission of incoherent electromagnetic
radiation is called as spontaneous emission.
• Stimulated emission: If Excited atom interacts with
a photon of same wavelength which was responsible
for its excitation, then atoms return to ground state
with the emission of two photons same wavelength.
• Light Amplification – The photons emitted in above
condition are coherent photons and they emit with
zero path difference, hence superposition these two
constructively give resultant photon with maximum
energy or amplified light.
Highly monochromatic, coherent, Amplified and concentrated beam of light emitted by
a source.
Light Amplification by Stimulated Emission of Radiation
Induced Absorption. Spontaneous Emission and Stimulated emission:
Incident
photon
Spontaneous emission
Incident
Photon
Passing
photon
Coherent or
stimulated emission
Ground state
Excited state
Excited state
Ground state
Excited state
Ground state
Induced absorption

3. Process Interaction of light with matter

4. To get effective stimulation at least there should be three energy
level, the Metastable level where electrons stay more time to
coordinate effective stimulation process

5. Semiconductor Laser
Classification of Laser based on Active
medium:
Solid state laser:
Solid dielectric transparent crystals are
Active medium.
Ex.: Crystal of Ruby (Al2O3 : Cr3+) in Ruby
laser
Gas Laser:
The mixture of gas behave like active
medium.
Ex Mixture He and Ne in He-Ne laser
Liquid dye laser:
Organic dyes used as active medium.
Semiconductor laser:
Direct bandgap pn junction of
semiconductors used as lasing Medium.
Ex: Direct bandgap semiconductors like
AlGaAs
Gas Laser:
Semiconductor laser:
Solid state laser:

6. Band diagram of pn
junction to work as
laser
Structure of pn
junction to produce
laser
Semiconductor laser
p
n
Heavily doped direct BG p-n
junction diode connected in FB

7. Diffraction of Laser
• Diffraction is process of deviation of light radiation at
the geometrical shadow after passing through an
obstacle or an aperture.
• The Diffraction is effective when size of obstacle is
comparable with the wavelength of light.

8. Experiment
Aim of Experiment:
To determine the Wavelength of given semiconductor
laser by method of diffraction of laser at Graduation
of Metal scale.
Principle:
Diffraction at Grazing incidence (metal Roller)
Theory:
When light incident at an angle ‘α’ on the horizontal
metal Roller, the light diffracts from the various
graduations with angles like α, β, γ, δ etc. as in
below fig.

9. Diffraction at Grazing Incidence
•The diffraction (reflection between the graduations of the ruler) of
laser at the is called Grazing incidence. Where the Laser incident on
the scale at a small angle (α).
•The diffraction takes place at takes place as shown in fig the dots
represented are due to constructive interferences and place between
two dots due to destructive interferences.
•The distance between two bright dots is inversely proportional to
distance between any two consecutive graduations (d) of roller are
also called vernier divisions.
• α is angle between roller axis and distance between point of
incidence to first order diffraction spot (highest intensity point)
•β is angle between Roller axis and distance between point of
incidence to second order diffraction spot ( next to highest intensity
point) and so on.

10. Diffraction at the metallic Scale
X3
X0 X1
Metal Roller
with Graduations
Laser
Source Laser
light
Z
Direct beam
Ist order
IInd order
IIIrd order
α β γ
α
Y 0 = xo /2, Y1 = x1 – Yo , Y2 = x2 - Yo …………

11. Theory
• In such cases the path difference between any two
order of diffraction is given by,
mλ = d (cos α – cos β)------------------- 1
For small angles
And
From above figure
Therefore eqn. 1
becomes

12. Theory
Re arranging the terms,
with m = 1
In general we have the equation
Above equation gives the wavelength of laser.

13. Procedure.
• An half meter metal scale (Roller) with mm graduations is fixed on a perfectly
plane surface. Take care that the Roller is not bent.
• A laser source is kept opposite to a white board at a distance about three
meters.
• The direct light of laser source must fall on the board near the lower end of the
board almost at the centre.
• Now Place the surface with Roller between Laser source of board in such a way
the laser light falls on the graduations at a small angle (about 30o to 60o)
• Observe the diffraction spot on the screen. Mark on the diffraction spots
starting with highest intensity spot to above (about 10 spots).
• Mark the point of the direct spot (spot without placing the Roller).
• Measure the distance between the direct spot and to all the diffraction spots
(X0, X1, X2. X3,…… ) and determine Y0 = X0/2, Y1 = X1 – Y0 , Y2 = X2 – Y0 , and so on
and tabulate the reading.

14. Procedure
• Each time calculate the ym
2 – y0
2 and
• Determine the mean value of
• Hence determine the wavelength of laser is
•

15. Observations and tabular column
• Distance between the incident laser spot on
roller* to the board = Z =________________m
• The vernier division**= d =______________m
Sl.
No.
order Distance
between
direct spot
to
diffraction
spots (X)
Ym = Xm – Y0 Ym
2 Ym
2– Y0
2 (Ym
2– Y0
2)/m

16. AIM AND APPARATUS

17. RAY DIAGRAM OF DIFFRACTION OF LASER

18. Tabular column an Readings

20. Tabular column an Readings

21. Find the mean value of
Is the wavelength of laser.
Repeating the experiment another time the average of wavelength
determined both the time is determined below

22. Bibliography
1. https://docplayer.net/20869431-Diffraction-and-interference.html
2. https://www.slideshare.net/PraveenVaidya1/laser-42133146
3. https://www.slideshare.net/PraveenVaidya1/semiconductor-physics-for-
undergraduates.
4. https://www.researchgate.net/publication/354090959_LASER_and_Optical_fi
ber