laser beam manipulation

8/12/2019 Laser beam manipulation

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3. Laser Beam Manipulation

In this lab we will learn to manipulate laser beams using prisms and lenses. A laser

beam can be steered, split, expanded or collimated using these optical elements.

1) Beam steering using prisms [Room 402D]

As in the previous labs, please first make sure that the laser beam is parallel to the

center line of the bench. This will make our experiment much easier. Position a 45-45-

90 prism on the bench as shown in the figure below so that the laser beam is redirected

at right angles to its original direction. You may need a metal plate to raise the prism.

Please handle the prism with care, and never touch its optical surfaces. According to your

observation, how does the intensity of the redirected beam compare to that of the original?

Why isnt there a beam exiting from the hypotenuse side of the prism? Try rotating theprism and see if you can find other prism orientations that permit a beam to exit from the

hypotenuse side. Draw a figure of the laser beams you have observed, and show their

paths.

Fig. 1Redirection of a laser beam using a prism

Now use a second prism to redirect the beam parallel to its original direction as

shown below. What are the advantages and disadvantages of this method of displacing a

laser beam compared to the parallel plate method used in the previous lab?


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Fig. 2Displacing a laser beam using two prisms

2) Beam steering using lenses [Room 402D]

Remove the prisms from the bench. Place thef = -22 mm negative lens on an elementholder and position it in the laser beam close to the laser output window. Note that this

lens causes the laser beam to expand. Move the lens slightly in one direction (up, down,

right, left). What happens to the beam does it move in the same direction as the lens,

or in the opposite direction? This is called beam steering and can be used to aim the beam.

Please show a ray diagram to explain this result.

Replace the -22 mm lens with the f = +18 mm lens. Note that the beam is again

expanded. Move the lens in various directions and observe the effect on the beam. Is the

beam steered in the same direction or the opposite direction? Please show a ray diagram

to explain this result.

3) Beam splitting [Room 402D]

Sometimes it is necessary to split a laser beam into two or more parts. This is done

using a beam splitter. Position the thick glass plate so that the laser beam strikes its face

at a 45 angle. Note that the beam is split into several reflected and refracted beams.

Sketch the directions of the original beam and of all the reflected and refracted beams.

Note on your sketch the relative intensities of each beam. Can you determine the origin of

each reflected and refracted beam?

What happens when a thin glass plate is used?

4) Beam expansion [Room 402D]

With no lenses in the path of the laser beam, measure the beam diameter of the

unexpanded beam close to the laser (it should be approximately 1-2 mm). Also measure


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the beam diameter where the laser strikes the wall of the room. Is the beam collimated

(i.e., does its diameter remain constant)?

Now place the -22 mm focal length lens in the beam close to the laser. What is the

beam diameter at the wall? Repeat using the +18mm lens, the +136 mm lens, and the

+252 mm lens. What do you conclude about the expanding power of these lenses? Do

negative lenses work better than positive lenses? Does the expanding power of a lens

depend on its focal length? Explain your conclusions using ray diagrams.

5) Beam expansion and collimation [Room 402D]

You found in the previous part that the laser beam with no lenses in its path was not

collimated. (A laser beam is collimated when its diameter is constant.) However, we can

use two methods, i) two positive lenses, or ii) a negative lens and a positive lens, to

produce an expanded and collimated beam. Please first discuss with your partners on how

you can realize this. Please draw a sketch on how you are going to place the two lenses in

each case, and how many times the beam will be expanded. Please let the instructor have

a look at the sketch.

Please choose the two lenses that will produce the maximum expansion in each

method. Adjust the position of the lenses so that the beam is steered directly down the

center line of the bench. To ensure that the beam is collimated, you must measure its

diameter just after it passes through the second lens and then again at the wall. By

comparing the two diameters, you will be able to determine if the beam is collimated. If it

is not, you will have to fine tune the distance between the lenses.

Record the focal lengths of each lens you used, the distance between them, and thediameters of the expanded and collimated beam. You also need to measure the diameter

of the initial unexpanded laser beam. Does the magnitude of expansion agree with your

theoretical predictions?

laser beam manipulation

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