Laser-Based Range Detection: Experiment and Results, Exercises of Physics

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Laser Based Specific Range Detection

Submitted to:

Dr. Asloob Ahmed Mudassar

Submitted by:

Yasir Ali

M.Phil. Physics DPAM

PIEAS

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 f 1 length so it diverge light after its focal point and a second lens of larger focal length f 2 is placed at distance d= f 1 +f 2 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.

Intensity measured at different position of target aluminum

Position Intensity Position Intensity Position Intensity Position Intensity

48 85 38 123 28 132 18 97 45 104 36 126 27 126 16 92 42 107 34 138 24 114 41 110 32 143 23 106 40 115 30 138 20 102

2. Wood:- Second target used was wood. Its reflected intensity was measured for different positions. Normalized intensity plot for this case is shown. It again shows that high reflected intensity is observed at focal point.

Intensity measured at different position of target wood

Position Intensity Position Intensity Position Intensity Position Intensity

50 66 40 80 30 120 20 87

48 65 38 87 28 118

46 67 36 93 26 107

44 70 34 104 24 100

42 78 32 115 22 92

3. Paper:- Third material used as target was paper. Its normalized intensity plot against position of paper is shown. This plot again shows maximum intensity at focal point of lens.

Intensity measured at different position of target paper

Position Intensity Position Intensity Position Intensity Position Intensity

44 50 34 66 24 79 14 52

42 51 32 82 22 68

40 55 30 99 20 61

38 56 28 102 18 57

36 59 26 96 16 53

specific range detection. A system like this experiment can be fixed on tip of missile which will collect data for itself more correctly than previous system. This system in connection with electronic system can work more significantly.

Precautions:-

1. Apparatus should be aligned perfectly so that any variation in target position will give some variation in intensity.
2. Detector, target, beam splitter and laser should be on same height from ground or board i.e. they should be on same level.
3. Partial beam reflected by beam splitter can reflect back and give some variations to intensity. Some that beam should not reflect back on detector.