EXPERIMENT No.
Aim: To measure the Beam intensity and beam divergence of a given laser beam
Apparatus: He-Ne laser, inch-tape, graph.
Significance of the experiment: One of the characteristic properties of laser is high
directionality. Thus the diameter of the laser at any position should be same. However, laser
has a small divergence due to diffraction effects. This experiment provides an easy and
accurate method to measure the divergence of a laser.
Theory: Laser is an extremely coherent, monochromatic, directional, focusable, polarized
and powerful light. These extraordinary features make it greatly applicable in day-to-day life,
science and technology. A few notable applications of laser include medical diagnosis and
treatments, fiber optic communications, CD-ROMS, CD players, laser printers, defense,
cutting, welding, drilling, surveying, aligning etc. Laser is produced due to stimulated
radiation; a process where a resonating photon stimulates the de-excitation of an excited
atom. This results in emission of two coherent photons, which are identical in all respects.
These photons further stimulate the de-excitation of other excited atoms and this continues to
generate an avalanche of coherent photons. For stimulated emission to take over spontaneous
emission and stimulated absorption, a few conditions are necessary. These are availability of
metastable state (life time ≈ 10 -3sec), population inversion (greater number of atoms in
metastable state than in lower energy state) and enough number of photons in the cavity
(mirrors).
He-Ne:-
Laser He-Ne laser is a low power, continuous gas laser, which is used in supermarket
scanners, student laboratories and holography. The active system is neon, which is pumped
electronically via helium in a resonant cavity made of discharge tube. The main lasing occurs
in neon between the levels E6 (metastable) and E3 which produces an intense coherent beam
of red color (wavelength 6328Aᵒ). The population of photons necessary for stimulated
emission is maintained by mirrors (one is semitransparent) on both sides. Brewster windows
are used to polarize the laser light. Power of He-Ne laser is 2mW.
Diagram:
Laser beam
Laser beam spot
Procedure:
(A)Beam Divergence
1. Paste a graph on a wall.
2. Place a laser about 5m away from the graph such the laser spot is formed clearly on
it. Note its diameter.
3. Take similar readings at 4m,3m, 2m and 1m.
4. Plot a graph between spot size and distance which shows that laser beam diverges as
move away from the graph paper.
(B)Beam Intensity
1. Find the area of laser beam spots for all the laser positions taken under (A).
2. Divide laser beam power by these spots areas for each laser position. This gives laser
beam intensity different positions of laser.
3. Plot a graph between beam intensity and distance which shows fall of beam intensity
as the laser is moved away from the graph paper.
Observations:
Sr.No Laser position(m) Spot diameter Spot area(m2)= Beam Intensity error
(mm) пr2 (mWm-2)
=2mW/area
1. 1 5
2. 2 6.5
3. 3 9
4. 4 10.5
5. 5 12
Calcultions:
Spot area=πr2
Beam intensity= laser power
Spot area
ERROR :
𝜃/2 = 𝑟/𝐷
Taking log on both sides
log 𝜃 = (log2+log 𝑟 − log𝐷)
𝑑𝜃 𝑑𝑟 𝑑𝐷
= ( − ) × 100%
𝜃 𝑟 𝐷
dr= 0.1 cm
dD=0.2 cm
Graphs:(A)Spot diameter vs divergence
(B)Beam intensity vs distance
Results:(A)Laser beam diverges with distance.
(B)Laser beam intensity falls with distance.
