Laser Pointer Education Kit #45-211

Laser Pointer Education Kit  #45-211
Laser Pointer
Education Kit
#45-211
INDUSTRIAL FIBER OPTICS
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Copyright © 2006
by Industrial Fiber Optics, Inc.
Revision - A
Printed in the United States of America
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All rights reserved. No part of this publication
may be reproduced, stored in a retrieval
system, or transmitted in any form or by any
means (electronic, mechanical,
photocopying, recording, or otherwise)
without prior written permission from
Industrial Fiber Optics, Inc.
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Introduction
This manual contains suggestions for activities,
demonstrations, experiments, and laboratory investigations that
can be performed using any laser pointer or source and
Industrial Fiber Optics’ Education Kit #45-211. This education
kit contains selected optic components to help perform most of
the activities and experiments described in this manual.
Industrial Fiber Optics makes every effort to incorporate
state-of-the-art technology, highest quality, and dependability in
its products. We constantly explore new ideas and products to
best serve the rapidly expanding needs of industry and
education. We encourage comments that you may have about
our products, and we welcome the opportunity to discuss new
ideas that may better serve your needs. For more information
about our company and products refer to http//www.ifiberoptics.com on the Worldwide Web.
Thank you for selecting this Industrial Fiber Optics product.
We hope it meets your expectations and provides many hours of
productive activity.
Sincerely,
The Staff at Industrial Fiber Optics
- i -
TABLE OF CONTENTS
INTRODUCTION .............................................................i
KIT COMPONENTS ..................................................... iv
SAFETY NOTES ............................................................1
UNPACKING AND PREPARATION FOR USE ........2
APPLICATIONS AND DEMONSTRATIONS
Pointing at a Projected Image on a Screen....................3
Pointing at Objects Indoors and Outdoors .....................3
Making the Laser Beam Visible .....................................4
Viewing Imperfections in an Ice Cube............................4
Using Color Filters to Absorb Laser Light ......................4
Scanning Bar Codes ......................................................5
Reflection and Refraction at a Water’s Surface .............5
Observing Internal Reflections in a Test Tube ...............5
Observing Internal Reflections in a Curved Water Jet...6
Viewing Frosted Light Bulb Filaments............................6
Curving a Laser Beam ...................................................7
Deflecting the Laser Beam with Voice ...........................8
Laser Light Music Show.................................................8
Cutting the Laser Beam with a Comb ............................9
Exercising Your Dog or Cat ...........................................9
EXPERIMENTS
Experiment 1 – Investigating Paired Muscle Balance..10
Experiment 2 – Opthalmology................................ 11-12
Experiment 3 – Specular and Diffused Reflection........13
- ii -
TABLE OF CONTENTS
(CONTINUED)
Experiment 4 – Reflection and Absorption………….…14
Experiment 5 – Verifying Law of Reflection……….15-16
Experiment 6 – Measuring the Index of Refraction of
Glass………………………………………………..…17-18
Experiment 7 – Measuring the Speed of Light in
Water………………………………………………………19
Experiment 8 – Measuring Laser Beam Wavelength...21
Experiment 9 – Polarization Effects ....................... 22-23
THEORY OF LASER POINTER OPERATION
Properties of Semiconductors in the Laser Chip..........24
Photon Production .......................................................24
Index-Guided Photon Enhancement............................25
LASER BEAM CHARACTERISTICS
Beam Shape ................................................................25
Beam Visibility .............................................................25
Beam Coherence Length .............................................26
Wavelength..................................................................26
Laser Beam Power ......................................................26
SHIPPING AND RETURN POLICY
Damage in Shipment and Return Policy ......................27
- iii -
KIT COMPONENTS
Qty.
Name
Description
1
Holographic
Diffraction
Grating
Holographic diffraction grating has 750 lines/mm. It is
used to measure the wavelength of the laser beam.
2
Polarized
filters
Transmits light waves that vibrate in one plane and
attenuates the intensity of light waves vibrating in other
planes.
2
Front
Surface
Mirrors
Changes direction of incident laser beam. It is used for
light shows and experiments involving laws of reflection.
1
Lens, Long
Focal
Length
Converges laser beam to a sharp focus.
1
Initially converges, and then diverges laser beam. It is
used together with the long focal length lens to collimate
Lens, Short
Focal Length the laser beam. This minimizes the beam divergence over
long distances.
1
Black Vinyl
Cap
Used to mount lenses to the front of the laser pointer.
1
Cylindrical
Lens
The purpose of this lens is to change the shape of the
beam spot to a straight line.
1
Solar Cell
The solar cell generates a small voltage that is proportional
to the intensity of light that illuminates the dark side of the
cell.
1
Rectangular
Prism
Used for splitting the laser beam into two parts. It is also
used for index of refraction experiments.
1
Fitted Case
Plastic case with hinged cover.
1
Color Filter
Set
Contains three color filters for color transmission and
absorption experiments (red, blue, and green filters).
1
Manual
Laser Pointer Education Kit
#45-211
- iv -
SAFETY NOTES
Unlike high-power industrial lasers, low-power laser pointers are
incapable of burning, cutting, or welding. However, because the
beam is so intense and concentrated it should always be treated
with common sense and caution.
The beam does not contain any invisible, exotic, or harmful
radiations. Never the less, long-term exposure to any bright light
can injure the delicate tissues of your eyes. Just as you should
never deliberately stare directly into the sun or the bright beam
coming from a classroom projector for a prolonged time, you
should never deliberately stare directly into the concentrated
beam coming from your laser pointer. See the back cover for
general safety practices.
Table 1. Common abbreviations used in this manual.
Abbr.
Long version
Numerical representation
mW
milliwatts
1 x 10-3 watts
µW
microwatts
1 x 10-6 watts
nW
nanowatts
1 x 10-9 watts
mm
millimeters
1 x 10-3 meters
µm
micrometers
1 x 10-6 meters
nm
nanometers
1 x 10-9 meters
- 1 -
UNPACKING AND PREPARATION FOR USE
As soon as you unpack this kit, it is important that all items are
checked in accordance with the instructions given here.
Spending a few minutes now can avoid many problems and
greatly extend the useful lifetime of the kit.
1.
Check that none of the items are missing from your kit.
Refer to the kit component list on page iv.
2.
The diffraction grating in the kit is a holograph. Be careful
not to rub or scratch its surface. Clear acetate sheets or
glass plates can protect the holograph from damage. These
protective coverings can be removed when precise
measurements are required.
3.
A thin semi-transparent film protects both sides of the
Polarized filters. Carefully peel off and discard the protective
films. If desired, the Polarized filter can be protected from
scratches and fingerprints by covering it with transparent
acetate or glass plates. Tape the edges to hold the
assembly together.
4.
The reflective surfaces of the two front-surface mirrors are
protected with a thin, blue film covering. Very carefully peel
off and discard the protective films, when precise
measurements are required. Careful handling is necessary
to avoid damaging the mirrors with dirt and fingerprints after
the protective films have been removed.
5.
The cylindrical lens is cemented to a vinyl cap that fits over
the front of your laser pointer. Be very careful when
mounting or removing this cap. Excessive stresses can
break the cement bond.
6.
An extra vinyl cap is provided for mounting a lens over the
front of your laser pointer. First, place the lens inside the
cap. Then press the cap over the front end of the laser
pointer.
- 2 -
UNPACKING AND PREPARATION FOR USE
(CON’T)
7.
Two terminals are provided on the back of the solar cell.
Solder a length of wire to each of the terminals. Connect the
other ends of these two wires to a plug that fits into the
microphone jack of your audio amplifier. For further details,
see page 9.
8.
Preserve the fitted case that comes with this kit. After each
use, check that every part is returned to its proper place.
APPLICATIONS AND DEMONSTRATIONS
Here are a few suggestions for using your laser pointer for
everyday applications in many environments including the
classroom and office.
Pointing at a Projected Image on a Screen
One of the most obvious uses for a
laser pointer is to call attention to a
portion of a projected image during a
presentation, lecture, or slide show.
The small red spot cast by the laser
beam can also be used to highlight a
feature on a movie screen, television
screen, or even a computer monitor.
For instruction, it is much more
effective than having observers try to
follow the movements of a cursor on
the computer display screen.
Figure 1.
Pointing at Objects Indoors and Outdoors
Use the laser pointer to call attention to details on charts, artwork,
displays and special features in the classroom, museum, and art
gallery. At night, use it outdoors to spot owls and communicate
with prearranged signals. Do not point directly at animal’s eyes
or face.
- 3 -
APPLICATIONS AND DEMONSTRATIONS
Making the Laser Beam Visible
A laser beam is invisible unless
there are objects in its path that
reflect or scatter the light toward
your eyes. In a dark room, turn the
laser pointer on and shake some
chalk dust along the beam path.
Each speck of chalk dust becomes
a reflector so the laser beam can
be seen and photographed.
Figure 2.
Caution: Avoid using chalk dust in the vicinity of
computers or other sensitive electronic
equipment.
Viewing Imperfections in an Ice Cube
In a darkened room, direct the
laser beam through an ice cube.
The light is scattered when the
beam encounters imperfections in
the crystalline structure of the ice.
Figure 3.
Using Color Filters to Absorb Laser Light
Shine the laser beam through the
color filters included in the
education kit. Observe how the
color of the filter affects the laser
beam. Try experimenting with
colored cellophane, colored
plastics, and colored liquids.
Notice how easily the laser beam
travels through a jar of cranberry
juice but how it can be completely
absorbed by some bottles
containing clear ginger ale.
- 4 -
Figure 4.
APPLICATIONS AND DEMONSTRATIONS
Scanning Bar Codes
Bar code data on grocery products can be
decoded because the bar code reflects laser
light in a distinctive pattern from the
contrasting bars and spaces. The reflected
light is received by a photodetector and the
signals are converted into digital data that
can be processed by a computer. Move the
beam of the laser pointer across the black
and white bars in Figure 5. Notice the
differences in the reflected light when
different bars are selected.
Figure 5.
Reflection and Refraction at Water’s Surface
Fill a florence flask halfway with water
and add a small amount of milk powder it
to make the water slightly cloudy. Then
fill the top half of the flask with smoke.
Aim the laser pointer’s beam through the
bottom of the flask. Observe the changes
in the beam’s intensity as the angle
between the laser beam and the water’s
surface is decreased.
Figure 6.
Observing Internal Reflections in a Test Tube
Fill a test tube with water and add a small
amount of milk powder to make the water
slightly cloudy. In a darkened room, aim
the laser beam so it enters the bottom of
the test tube and exits at the top. Then,
slowly change the angle of the test tube.
Notice that instead of exiting out the side
of the test tube the beam reflects in a
zigzag pattern until it emerges from the
top of the water. This illustrates the
internal reflections that take place in a
fiber optics cable.
- 5 -
Figure 7.
APPLICATIONS AND DEMONSTRATIONS
Observing Internal Reflections in a Curved Water Jet
Punch or drill a 5 mm hole near the
bottom of an empty 1 liter clear plastic
soda bottle. Aim the beam of a laser
pointer so it goes into the bottle and out
the hole. Fill the bottle with a mixture of
water and milk powder. Notice that as
the water emerges from the hole, the
laser beam will follow the water jet as it
arcs downward. This illustrates the
internal reflections that take place in a
fiber optics cable.
Figure 8.
Viewing Frosted Light Bulb Filaments
Aim the laser pointer through the side of
a frosted incandescent light bulb. The
frosting of the bulb scatters the laser
beam and a clear shadow of the filament
appears on the opposite side of the bulb.
Any break in the filament of a defective
bulb will be clearly visible using this
technique.
It is also interesting to try the same thing
with a fluorescent lamp tube. Aim the laser
pointer beam through the phosphors near
the ends of the tube. The shadows of the
internal electrodes are projected on the
tube by the scattered light.
- 6 -
Figure 9.
APPLICATIONS AND DEMONSTRATIONS
Curving a Laser Beam
A laser beam can be made to curve if its transmission medium
has gradual changes in its optical density (refractive index). This
can be observed by partially filling a fish tank with clear water and
adding a few lumps or cubes of sugar. Allow the sugar to
dissolve undisturbed. After a few hours you will have a sugar
solution that is dense at the bottom, but gradually becomes less
dense toward the surface. A horizontal beam from the laser
pointer will bend into a curve as it encounters changes in the
index of refraction of the transmission media.
Figure 10.
This principle is used in the construction of fiber optics cables.
The inner core of the fiber has a greater index of refraction than
the outer layers. Light rays that try to leave the fiber optics cable
through the sides of the cable are bent back toward the central
core.
The same principle is also used in the construction of your laser
pointer. Photons are produced when electrons meet holes in the
active layer of the laser diode chip. The semiconductors that are
above and below the active layer have decreasing indexes of
refraction. Any light rays that attempt to escape are bent back
into the main channel where they reinforce the laser action.
- 7 -
APPLICATIONS AND DEMONSTRATIONS
Deflecting the Laser Beam with Voice
Remove the top and bottom of a coffee can. Stretch a section of
balloon rubber over one end of the can and secure it with a
rubber band. Glue a piece of lightweight mirror near the center of
the rubber. Adjust the position of the can so the mirror reflects
the beam toward a distant wall. Speaking or singing into the
open end of the can vibrates the stretched rubber causing beam
deflections on the wall.
Figure 11.
Laser Light Music Show
Connect an external speaker to a radio. This can be done by
removing the internal speaker and reconnecting it to the original
terminals of the radio with longer wires.
Glue a lightweight mirror near the center of the speaker cone.
Then aim your laser pointer at the mirror so that the beam’s
reflection falls on a distant wall. When the radio is tuned to a
lively station, the speaker vibrations will form patterns on the wall.
These patterns keep time with the music.
Figure 12.
- 8 -
APPLICATIONS AND DEMONSTRATIONS
Cutting the Laser Beam with a Comb
To create a sound that mimics the sawing of wood, first aim the
laser pointer at the solar cell included in the education kit. Then
set up an audio amplifier and speaker near the solar cell.
Connect two short wires to the terminals of the solar cell.
Connect the other ends of the wires to a plug that fits in the
microphone jack of the amplifier.
Moving the teeth of the comb back and forth across the laser
beam interrupts the light reaching the solar cell. This will cause
sounds to be emitted from the speaker. The faster the movement
of the comb, the higher the pitch will be of the sound emitted from
the speaker.
Figure 13.
Exercising Your Dog or Cat
This application is a bit frivolous but it usually works and might
possibly be useful to someone. After the sun has set, aim your
laser pointer so it makes a small moving spot on the grass or on
some bushes. When you move the beam about, many dogs (and
some cats) will attempt to catch the spot.
Caution!
Never aim the laser beam directly into an animal’s
eyes.
Figure 14.
- 9 -
EXPERIMENTS
Experiment 1 – Investigating Paired Muscle Balance
Discussion
It is difficult to extend your arm horizontally and hold it steady.
When you try, paired flexor and extensor muscles in your arm
take turns contracting and relaxing. Although it is impossible to
hold your arm absolutely steady, some people are much better at
it than others.
Figure 15.
Procedure
1.
Extend your arm horizontally and aim the laser pointer at the
bull’s eye of a small target that is at least 7 meters away.
2.
Ask someone to watch the target and measure the longest
time that the laser beam could stay inside the bull’s eye of
the target. If the time is only a second or less, try measuring
the longest time for the beam to stay within the inner two
circles or the inner three circles of the target.
3.
Compare the scores achieved by your classmates or others
who volunteer to take the steadiness test. See if there is
any correlation of scores on the steadiness test with age,
sex, time of day, or other factors that you can think of that
might affect a person’s steadiness.
- 10 -
EXPERIMENTS
Experiment 2 – Ophthalmology
Discussion
The spot produced by an enlarged laser beam appears to be
filled with many small dots or grains. This granular appearance is
an interference pattern created in space by reflections from the
illuminated area. When the spot is observed closely and you shift
your head to the left, the grains appear to move toward the left or
the right. The directions that the grains appear to move may be
used to diagnose certain eye defects.
Figure 16.
Procedure
1.
In a room with subdued lighting, aim the laser beam at a
piece of white paper a few meters away. Expand the laser
beam using the lens furnished with the laser pointer
education kit.
2.
Observe the illuminated area and notice that there are many
small dots or grains in the spot. Move your head very slowly
from side to side while observing the spots. If you are
farsighted, or if your eyes are normal, the small dots will
appear to move in the same direction as your head. If you
are nearsighted, the dots will appear to move in a direction
opposite to that of your head. In nearsighted people, the
eye tends to focus the pattern a short distance in front of the
retina. Therefore, the parallax caused by the head
movement results in an apparent motion of the dots in the
opposite direction.
3.
Demonstrate the parallax effect by holding your fingers a few
centimeters apart in front of your eyes. Look at an object on
a distant wall. Your fingers represent the interference
pattern and the distant object represents the laser beam
spot. When your head is moved slowly from side to side,
your fingers appear to be moving in the opposite direction.
- 11 -
EXPERIMENTS
Experiment 2 – Ophthalmology
4.
Simulate myopia (nearsightedness) with the aid of the long
focal length lens from your laser pointer education kit. With
the lens held in front of one eye, move your head slowly
from side to side while observing the movement of the dots
produced by the laser beam. Record the results.
5.
Simulate hyperopia (farsightedness) by observing the laser
beam dots through the short focal length lens of the laser
pointer kit. Move your head slowly from side to side and
record your results.
6.
If you normally wear eyeglasses, check your eyes in the
manner described above with and without your corrective
lenses. Record your results.
- 12 -
EXPERIMENTS
Experiment 3 – Specular and Diffused Reflection
Discussion
In this experiment two kinds of laser beam reflections, specular
and diffused will be investigated. Specular reflections are
continuations of the original light rays, but in a different direction.
Diffused reflections are scattered and travel in many different
directions after they strike the reflecting object.
Figure 17.
Procedure
1.
In a darkened room, aim your laser pointer at an object with
a rough texture such as a woolen cloth or rough piece of
wood. Observe the spot of light that appears on a sheet of
white paper that is held to catch the reflections.
2.
Repeat Step 1 substituting an object of the same color but
with a smoother texture for example a silk cloth or very
smooth piece of wood.
3.
Substitute a variety of other objects that are readily
available. Observe the light that is reflected by each.
4.
Record your observations and conclusions. In each case
record the type of material and the appearance of the
reflected laser light.
- 13 -
EXPERIMENTS
Experiment 4 – Reflection and Absorption
Discussion
Whenever red laser light strikes an object, some of its energy will
be reflected and some will be absorbed. In this investigation, you
will observe how red laser light is reflected off a glossy page
printed in four colors.
Procedure
1.
Select a large four-color picture. Aim your laser pointer at a
white portion of the illustration. Note the intensity of the
reflected light from this illuminated area. Repeat this by
aiming your laser pointer at different portions of your
illustration (such as a red area, blue area, yellow area, and
black area). In each case record the relative intensity of the
reflected laser beam.
2.
When you have observed each area of the illustration
several times, summarize your observations and state your
conclusions.
3.
For research projects to determine the reflectivity of different
surfaces or colors use a calibrated laser power meter such
as Industrial Fiber Optics’ Laser Power Meter 45-545A.
- 14 -
EXPERIMENTS
Experiment 5 – Verifying Law of Reflection
Discussion
The law of reflection states that the angle of incidence is always
equal to the angle of reflection. Verify this with your laser pointer
and optics components in your laser pointer education kit.
Figure 18.
Procedure
1.
Fasten one of your front surface mirrors to the side of a
wood block as shown in the diagram. Make sure that the
mirror’s surface is kept very clean.
2.
Place the wood block near the top of a sheet of paper then
place a protractor on the paper so the base line of the
protractor is directly under the bottom edge of the mirror.
3.
On the paper, construct a normal to the mirror surface. This
is shown as a dotted line that makes an angle of 90 degrees
with the mirror.
4.
Aim the beam of your laser pointer at the mirror. Insert the
cylindrical lens in the laser beam so it spreads out the light.
Red lines showing the directions of the incident and the
reflected rays will appear on the paper.
- 15 -
EXPERIMENTS
Experiment 5 – Verifying Law of Reflection
5.
Measure and record the size of the angle of incidence (i).
This is the angle between the incident ray and the normal.
6.
Measure and record the size of the angle of reflection (r).
This is the angle between the reflected ray and the normal.
7.
Change the positions of your laser pointer to create several
additional angles of incidence. For each angle of incidence,
measure and record the corresponding angle of reflection.
Conclusions
Examine your data. It is likely that your data will show that the
corresponding angles of incidence and refection are not exactly
equal in every case. Account for your experimental errors.
- 16 -
EXPERIMENTS
Experiment 6 – Measuring the Index of Refraction of Glass
Discussion
When a beam of light crosses the boundary between air and
glass, its speed and direction change. The change of direction,
called refraction, depends on the relative speed of light in the two
media. By measuring the change in the direction of a laser beam
as it travels from air to glass, we can calculate its index of
refraction. This index is based on the ratio of the speed of light in
glass with respect to that in a vacuum. However, since the speed
of light in air is very close to that in a vacuum, there is only a
small error in using air rather than a vacuum as the reference
medium.
Figure 19.
Procedure
1.
Place a rectangular glass prism in the center of a sheet of
paper and carefully trace its outline on the paper.
2.
Aim your laser pointer so its beam just grazes the surface of
the paper and enters the glass at an angle as shown in the
diagram above. If this is done correctly, a bright red line will
appear on the paper as the laser beam enters the glass. A
second line will appear as the beam leaves the glass.
3.
With the aid of a sharp pencil and a straight edge, trace the
path of the laser beam on the paper. Be especially careful
to mark the points where the beam enters and leaves the
glass.
- 17 -
EXPERIMENTS
Experiment 6 – Measuring the Index of Refraction of Glass
4.
Remove the glass prism and draw a line on the paper to
show the path of the laser beam when it was inside the
prism.
5.
With the aid of a protractor, construct a dotted line on the
paper to show the normal to the glass edge at the point of
beam entry.
6.
On your paper, label the incident and refracted rays at the
point of beam entry.
7.
At the point of beam entry, measure the angle of
incidence θi. This is the angle between the incident ray and
the normal. Also, measure the angle of refraction θr the
angle between the refracted ray and the normal.
8.
Refer to the Snell’s Law equation below. Knowing that the
index of refraction of air (nair) is 1.00, solve for the index of
refraction of glass (nglass).
nairsinθi = nglasssinθr
9.
Using a slightly different angle of incidence, repeat Steps 1
through 8.
- 18 -
EXPERIMENTS
Experiment 7 – Measuring the Speed of Light in Water
Discussion
In a vacuum all light travels at the same speed, 3.00 x 108 m/s.
In other media light travels slower than it does in vacuum. Its
speed depends on the color of the light and the optical density of
the medium.
In this experiment, you will determine the speed of red laser light
as it travels through water. This can be done by comparing the
distance that beams from two laser pointers cover during the
same interval. One, as it is traveling through air, and the other,
through water. Recall that the speed of the light in air is almost
the same as that in vacuum, 3.00 x 108 m/s.
Figure 20.
Procedure
1.
Place a transparent plastic box over a sheet of graph paper
on a table. Partially fill the box with water and add a pinch of
powdered milk to make the water a bit cloudy.
- 19 -
EXPERIMENTS
Experiment 7 – Measuring the Speed of Light in Water
2.
Position two laser pointers about 5 cm apart on the table.
Aim them so the beams are parallel and enter the side of the
box at an angle of about 20 degrees. Observe how the
direction of the laser beams change as the light enters the
box and as the light leaves the box.
3.
On your graph paper, place four dots (A, B, C, and D) to
mark the path of the first laser beam. Place four additional
dots (E, F, G, and H) to mark the path of the second laser
beam.
4.
Remove the box and connect the dots on the graph paper to
show the zigzag paths of the beams from the time that they
leave the laser apertures until they exit the box.
5.
On your graph paper construct a perpendicular from point F
to line BC. Place a dot (J) where the perpendicular
intersects the line.
6.
Construct a second perpendicular from point B to line EF.
Place a dot (K) where the perpendicular intersects the line.
7.
Calculate the speed of light in water (vwater) using the ratio:
8.
Change the angle that the laser beams enter the water.
Repeat steps 2-7.
Note: BJ is a distance that one beam traveled in water.
During the same interval, light from the second laser
was able to travel a longer distance (KF) through air.
- 20 -
EXPERIMENTS
Experiment 8 – Measuring Laser Beam Wavelength
Discussion
The red light that is emitted by your laser pointer consists of
waves that are shorter than a millionth of a meter. Because of
microscopic differences in laser diode crystals, the wavelength of
a particular laser pointer can only be specified to within about
± 10 nanometers. Most common red diode lasers emit laser light
at around (to within ± 10) 635, 650 or 670 nanometers.
In this experiment, you will measure the precise wavelength of
your laser pointer. It will be done using the classic interference
technique developed by Thomas Young in 1801.
Figure 21.
Procedure
1.
Aim the laser pointer so its beam is perpendicular to the
center of a screen.
2.
Place a diffraction grating in front of the laser. This
produces a series of dots (interference antinodes) on each
side of the centerline.
3.
Measure the distance X This is one-half the distance
between the two dots closest to the centerline.
4.
Measure the distance L. This is the distance between the
point where the beam leaves the diffraction grating and a dot
closest to the centerline.
5.
Record distance d, the known distance between slits on your
diffraction grating (1.33 x 10-6 m for grating supplied with kit).
6.
Calculate the wavelength lambda (λ) using the following
equation:
- 21 -
EXPERIMENTS
Experiment 9 – Polarization Effects
Discussion
Unlike the light from the sun or from an incandescent lamp, the
beam of your laser pointer is partially polarized. A measurable
portion of the laser light is vibrating in one plane while the
vibrations of the rest of the light are distributed among other
planes perpendicular to the laser beam path. The presence of
polarized light can be detected with a polarizing filter. It has a
crystalline structure that allows waves vibrating in line with the
crystals to pass while restricting waves that vibrate in other
planes.
Figure 22.
Procedure
1.
Aim a flashlight beam through a Polarized filter at a white
screen. Rotate the Polarized filter and observe that there is
no perceptible difference in the brightness of the spot on the
screen. Substitute the laser pointer for the flashlight and
repeat the procedure. Observe the variations that occur in
the spot’s intensity, as the filter is rotated 360 degrees.
Maximum intensity is observed when the plane of the laser
beam vibration is aligned with the crystals in the filter. When
the alignment is offset by 90 degrees, the intensity of the
spot is reduced to a minimum.
- 22 -
EXPERIMENTS
Experiment 9 – Polarization Effects
2.
Aim the laser pointer at a white screen through a Polarized
filter. Rotate the filter until the beam intensity is at its
maximum. Then rotate a second Polarized filter in the laser
beam and observe the intensity variations that occur.
Repeat this procedure using a flashlight in place of the laser
pointer. Compare your observations with those in Step 1
above.
3.
Fill a test tube with water and add a pinch of milk powder to
scatter any light that enters the tube. Aim your laser pointer
so its beam enters the mouth of the tube and continues
through its center. Observe the intensity of the scattered
light by looking at a side of the tube through a Polarized
filter. Observe the intensity variations in the scattered light
as the laser pointer is rotated slowly about its long axis.
- 23 -
THEORY OF LASER POINTER OPERATION
Figure 20. Index-Guided Visible Laser Diode (VLD), Schematic Structure
Properties of Semiconductors in the Laser Chip
Laser action, which produces the red beam of the laser pointer, is
generated in a tiny channel sandwiched between
semiconductors. Typical dimensions of the laser chip are roughly
one or two millimeters. The active layer where light is emitted is
only a few microns thick.
The laser chips are built on a slice of gallium arsenide crystal that
is about 0.5 mm thick. This gallium arsenide is an n-type
semiconductor that has a surplus of mobile electrons within the
crystal lattice.
Impurities are added in sequence to the surface and diffuse into
the gallium arsenide crystal to form p-type semiconductor layers.
These impurities capture electrons and leave holes in their place.
We may think of the holes as carriers of positive charge because
they move in the same direction as positive charges under the
influence of an electric field. The schematic structure diagram
shows the arrangement of the semiconductor layers that
comprise the laser.
Photon Production
Applying a forward bias of 2-3VDC to the laser electrodes creates
an electric field across the semiconductors. With sufficient
energy both electrons and holes are injected into the active layer.
- 24 -
THEORY OF LASER POINTER OPERATION
When an electron and a hole meet, they annihilate each other
and release energy to produce a photon of red light. By itself, a
photon does not create much light. But, together with the
combined energy of many others of the same wavelength, an
intense laser beam is produced.
Index-Guided Photon Enhancement
Photons, created in the active area of the laser, stimulate other
electron-hole pairs to meet, annihilate, and produce additional
photons of the same wavelength, phase, and direction. Upon
reaching the ends of the crystal, most of the photons emerge to
form the laser beam. However, about 36% of the photons are
reflected back into the active layer by a pair of flat cleaved ends
at opposite sides of the laser diode. They enhance further
stimulation of electron-hole pairs.
Other photons may travel toward the sides of the diode crystal
instead of going in the desired direction toward the laser aperture.
To minimize this loss, the semiconductors that comprise the laser
are arranged so their indices of refraction decrease with distance
form the main channel. As the index of refraction becomes lower,
stray photons are bent away from the normal and many are
guided back into the main channel.
LASER BEAM CHARACTERISTICS
The beam emitted from the aperture of a diode laser pointer
differs from that of a helium-neon laser in three significant ways;
the shape, visibility, and coherence length.
Beam Shape
Because the beam comes from an end of a rectangular slab
rather than a round capillary tube, the beam is emitted with an
elliptical cross section. In a diode laser pointer this is corrected
with collimating optics.
Beam Visibility
A wavelength of 670 nm lies in the deep red portion of the
spectrum. The human eye is much less sensitive to this
wavelength than it is to the shorter wavelength of bright red light.
- 25 -
LASER BEAM CHARACTERISTICS
Therefore, the beam from a 670 nm diode laser appears to be
only one-quarter as bright as that of a 635 nm diode laser or a
helium-neon (632.8 nm wavelength) beam. However, a silicon
diode solar cell is more sensitive to the deep red light from a 670
nm pointer than it is to the bright red beam of the HeNe laser.
Beam Coherence Length
Beam coherence length is the distance that the laser beam can
travel while its photons remain in phase with each other. A long
coherence length is essential for alignments in interferometry or
holography set-ups.
Many laser pointers have coherence lengths that are 5 cm or
more. A longer coherence length makes it possible to use a laser
pointer for holography and interferometry without worrying too
much about highly precise placements of optical components.
Wavelength
Red diode laser pointers produce beams at wavelengths of
approximately (to within ± 10 nanometers) 635, 650 or 670
nanometers. The wavelength depends on the geometry and
composition of the individual laser crystal and also upon the
operating temperature to some extent. The crystals are mounted
on metallic heat sinks for thermal stability and are surrounded by
metal shielding to resist damage from rough handling and static
electricity.
Laser Beam Power
Power is related to the intensity of the laser beam and is
measured in milliwatts. The output power of a laser pointer
usually ranges between 0.8 to 0.9 mW (Class II) and 2.5 to 3 mW
(Class IIIa) when a set of fresh alkaline batteries is first installed.
As the batteries age, the output power of the laser beam can be
expected to gradually decline about 30 percent during the first
four hours of constant operation and another 20 percent during
the next eight hours. Thus, after 12 hours of constant operation
you can expect the output power of the laser beam to be reduced
by approximately 50 percent.
- 26 -
DAMAGE OR LOSS IN SHIPMENT
If damage to an Industrial Fiber Optics product should occur
during shipping, it is imperative that it be reported immediately,
both to the carrier and the distributor or salesperson from whom
the item was purchased. DO NOT CONTACT INDUSTRIAL
FIBER OPTICS.
Time is of the essence because damage claims submitted more
than five days after delivery may not be honored by the carrier. If
damage has occurred during shipment, please do the following:
•
Make a note of the carrier company; the name of the
carrier employee who delivered the damaged product; the
date; and the time of the delivery.
•
Keep all packing material.
•
In writing, describe the specific nature of damage to the
product.
•
In cases of severe damage, do not attempt to use the
product (including attaching it to a power source).
•
Notify the carrier immediately of any damaged product.
•
Notify the distributor from whom the purchase was made.
RETURN POLICY
Before returning any item to Industrial Fiber Optics, you must first
contact our office to get a Return Merchandise Authorization
(RMA) number. A copy of the RMA Invoice will then be faxed to
you. Returns must be shipped freight prepaid and be well
packed. We are not responsible for items damaged in transit
back to us. Include a copy of the faxed RMA Invoice, along with
a brief explanation as to the reason for the return. Please state
whether you are requesting an exchange, a repair, or a refund.
- 27 -
12 0092
Rules for Laser Safety
•
Lasers produce a very intense beam of light. Treat them with
respect. Most educational lasers have an output of less than 3
milliwatts, and will not harm the skin.
•
Never look into the laser aperture while the laser is turned
on! PERMANENT EYE DAMAGE COULD RESULT.
•
Never stare into the oncoming beam. Never use magnifiers
(such as binoculars or telescopes) to look at the beam as it
travels — or when it strikes a surface.
•
Never point a laser at anyone's eyes or face, no matter how
far away they are.
•
WHEN using a laser in the classroom or laboratory, always
use a beam stop, or project the beam to areas which people
won't enter or pass through.
•
NEVER leave a laser unattended while it is turned on — and
always unplug it when it's not actually being used.
•
REMOVE all shiny objects from the area in which you will be
working. This includes rings, watches, metal bands, tools,
and glass. Reflections from the beam can be nearly as
intense as the beam Itself
•
Never dissemble or try to adjust the laser's Internal
components. Electric shock could result.
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