Download Laser Beam Divergence-Physics-Report and more Study Guides, Projects, Research Physics in PDF only on Docsity!
Laser Beam Divergence
Introduction
Divergence means “Departure from norm or Deviation”. The beam divergence of laser beam is the measure of increase of diameter or radius. A laser beam consists of very nearly parallel light rays, the beam diameter increases far more slowly with distance from the light source as compare to the light beam from a other light sources.
Figure 1
In order to determine the divergence, or spreading rate of a laser, we will measure the beam diameter as a function of distance from the laser. The best place for this experiment is probably in the hallway outside the laboratory. We can see that the diameter of laser beam increases with the increase in distance from laser source. We can plot graph between distance and diameter of beam and we can check that graph is straight line and distance and beam diameter are perpendicular to each other. We will note that the edge of the beam is somewhat fuzzy, and we will need to determine some criterion for the "beam Divergence." We know that the change in diameter divided by the corresponding increase in distance from the laser is the tangent of the spreading angle. If the angle Ө is measured in radians (2p radians = 360°), it is very nearly true for small angles that tan Ө = Ө. Therefore the beam divergence conventionally expressed in units of mill radians.
Application
Lasers and diffraction techniques are widely used in research laboratories and industry. A particularly useful property of laser beams is the small amount of spreading even after very large distances. We can measure the rate of spreading of a typical laser beam in today's laboratory activity and reduce its divergence using collimation techniques and used it in many applications.
Another important property of a laser beam is the well-defined wavelength value of the laser light. We will make use of this property to measure very small diameters by a diffraction technique as an illustration of an application.
Because laser beam have parallel light rays and the beam diameter increases more slowly with distance from the light source as compare to the light beam from another light sources. This makes laser beams very useful for alignment applications in large construction projects, surveying, monitoring seismic activity, etc. By reflecting a laser beam off a mirror placed on the moon, it is even possible to monitor lunar seismic activity.
Apparatus
Helium Neon Laser Beam fixed on optical bench, Diverging lens of size 20/0.35 and 160/0.17, Optical bench, White page fixed on card board, measuring tape and precise ruler
Figure 2
Helium Neon Laser Source Measuring tape
Card board
Optical Bench^ Diverging^ Lens
D 2 = 0.012m
Ө 1 = 0.7684mRad
Third reading
D 1 = 0.004m L = 3.231m
D 2 = 0.009m
Ө 1 = 0.773m rad
ӨAvg = =
ӨAvg =0.7761m rad
Uncertainty = 0.7761 – 0.7684 = ±0.0077m rad
Percentage uncertainty =
Uncertainty = = 1%
Divergence = Ө =0.7761* ± 1% rad
Readings with lens
Frist reading
D 1 = 0.003m L = 0.106m
D 2 = 0.019m
Ө 1 = 0.0753rad
Second reading
D 1 = 0.002m L 1 = 0.027m
D 2 = 0.006m
Ө 2 = 0.074rad
Third reading
D 1 = 0.003m L 1 = 0.060m
D 2 = 0.011m
Ө 3 = 0.066rad
ӨAvg =
ӨAvg = 0.07177rad
Uncertainty = 0.07177 - 0.074 = ±0.
Uncertainty = = ±
Divergence = Ө = 0.07177 ± 3 rad
Focal length of lens
Now we want to measure the focal length of lens using formula
Where f = Focal length of lens
r = radius of lens
= divergence angle
Precautions
(i) The direction of laser device should be perpendicular to the card board. (ii) Optical devices should be dust free or clean. (iii) Dark room should be used for doing laser experiments. (iv) Mark the straight lines on cardboard of different sizes and adjust the position of spot of laser between lines precisely.
Safety
(i) Safe your eyes from the direct interaction with Laser light because laser light is highly intense and unidirectional so it can damage eyes. (ii) Safe optics form the dust particles and cover the apparatus after doing experiments because due to dust particles there may be two beams emit or unclean beam emit. (iii) 5000V DC power supply is present in the electrodes of He-Ne gas tube so be careful while touching the gas tube.
