Introduction:- Laser has a number of applications. It is used in imaging, medical
surgery, nuclear fusion, material processing, microscopy, laser communication and in
many more applications. One of its uses is in an important application that is range
detection. Military use it for ranging and target designation etc. There are many
experiments to measure some specific range using laser.
One of these is Lunar Laser Experiment. It also has use in LIDAR for range detection.
When the Apollo astronauts visited the moon, they planted retro reflector arrays to make
possible the Lunar Laser Ranging Experiment. Laser beams are focused through large
telescopes on Earth aimed toward the arrays, and the time taken for the beam to be
reflected back to Earth measured to determine the distance between the Earth and
Moon with high accuracy.
Another method used for range detection is laser pulse time-of-flight distance
measurement. Principle of this method is very simple. A laser pulse is projected in the
scene and the time the pulse takes to hit the target, reflect and reach the detector is
measured. If d is the distance to the target, t is the echo time, and c is the speed of light,
then 2d = ct
relation gives distance of detector and target. In this case time t should be greater than
pulse width.
There are many other methods which are used for distance or range finding. In our
experiment we used a simpler method which depends on intensity of reflected beam.
Apparatus:- Laser source, detector, beam splitter, some materials as targets, stands
etc,
Setup:- First step, important of all is alignment. Laser is alignment such that its beam
passes through beam splitter and lenses, for collimation and converging, and falls on
target. The alignment should be made such that reflected part of beam from target falls
on beam splitter and then on detector. Detector is placed on opposite side of beam
splitter, i.e. partial beam of laser beam, reflected from beam splitter is reflected on one
side while detector is on other side and reflected light from target falls on detector. A
converging lens is placed after beam splitter, which focuses light of laser.
After laser source collimation lenses are placed. First converging lens is of small focal
f1length so it diverge light after its focal point and a second lens of larger focal length f2
is placed at distance d= f1+f2 from first lens. This will produces collimated beam. Beam
splitter is placed between the collimation lenses. Another lens is used after the
collimation lenses which converge light. This lens should be of large focal point and
target is detected by this lens.
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