E9
Introduction to Laser
C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
E9 - Introduction to Lasers
The word LASER is an acronym for Light Amplification by Stimulated Emission of
Radiation. The technological and research applications of lasers are due to the unique properties
of laser light. This lab consists of a series of experiments which will familiarize you with these
unique properties of laser light namely, that laser light is bright (intense), of one color
(monochromatic), directional (collimated) and in phase (coherent). The experiments you will
perform take advantage of these properties of laser light.
You will find in Appendix B a section from reference 1 titled “Electronic Structure of
Matter” which is a brief introduction to the interaction of light with matter and the operating
principles of lasers1.
Please read this section carefully and familiarize your self with the
characteristics of laser light.
Also review the PowerPoint presentation titled “lasers.ppt”
available on the course home page.
SAFETY: You must wear laser goggles when performing any of the experiments described
below. Never look directly into the beam of a laser, and when using portable lasers (like
the helium-neon laser) make sure you do not point the laser towards any one.
Experiment 1: Comparison of light from a Helium-Neon laser with that from a bulb
Turn on the Helium-Neon (HeNe) laser and project the output on the wall. Turn on the
flashlight and project the output on the wall. Visually compare the output of the two light
sources in terms of intensity (brightness), collimated (directional), and monochromaticity (of
single color). To compare the directionality of the two beams measure the spot size of the laser
and flashlight beams a few feet away from the source. Compare the monochromaticity (of single
color or wavelength) light beams by placing a diffraction grating (which, like a prism, separates
light into its component wavelengths) in the path of the two light sources.
Experiment II: Diffraction of light.
Read in Appendix B the section from reference 1 titled “Diffraction: Light Scattering from
Ordered Systems”. As described in this reference, diffraction is a manifestation of the wave
properties of light. While diffraction occurs with light from any source, it is easier to illustrate
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C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
diffraction with monochromatic, collimated light.
Hence in this experiment you will take
advantage of the light output of a HeNe laser. As a comparison, after you have completed this
experiment, shine a flashlight through the grating, and consider if you could have performed this
experiment with a flashlight. Discuss this in your lab report.
In this experiment you will experimentally determine the wavelength of the HeNe laser
using a diffraction grating. A diffraction grating consists of equally spaced lines or grooves on
an optical surface. Light transmitted through or reflected off a diffraction grating is separated
into its component wavelengths much like a prism disperses incident light into its component
wavelengths. Light travelling through a prism is separated into its component wavelengths
because of refraction of light and the fact that the angle of refraction varies with wavelength.
Light transmitted or reflected off a grating separates into its component wavelengths due to
diffraction off the equally spaced lines (see Appendix B). Most spectrometers (like absorption
and fluorescence spectrometers) use diffraction gratings to separate light into its component
wavelengths.
The experimental set up for this experiment is shown below. Light from a HeNe laser
transmits through the grating and is incident on a board with graph paper. The light diffracts as
it traverses through the grating resulting in a diffraction pattern on the graph paper. The
positions of the diffracted spots obey Bragg’s equation (equation 3-2 in Appendix B)
n  = d sin n
where
n is the diffraction order
l is the wavelength of the HeNe laser light
d is the distance between two lines or grooves on the grating
n is the diffracted angle of order n
Measure the distance between the zero order spot and the 1st and 2nd order diffracted spots on
either side of the zero order spot (see figure below). The groove density of the grating you will
use is 63 lines/mm. Use Bragg’s equation to calculate the wavelength of the HeNe laser light in
nm.
When you have completed your measurements, do not forget to try the same experiment with a
flashlight as the source of light.
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C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
board
diffraction grating
2nd order diffracted beam
2
incident beam
1
1
 2
1st order diffracted beam
zero order diffracted beam
1st order diffracted beam
2nd order diffracted beam
board
diffraction grating
incident beam
measure distances between zero
order spot & 1st & 2nd order spots
measure distance between
front surface of grating and board
Experiment III: Refractometry (adapted from Reference 2)
Read the attached section in Appendix B from Reference 1 titled “Refraction of Light”.
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C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
Refractometry is an analytical chemistry technique based on the measurement of the refractive
index of a material.
In this experiment you will measure the amount light refracts when
travelling through liquids of different refractive indices. You will then use this information to
determine the refractive index of two unknown compounds. While refraction occurs with light
from any source, you will take advantage of the unique properties of laser light, namely the fact
that laser light is collimated and monochromatic to perform these experiments. As a comparison,
after you have completed this experiment, shine a flashlight through the beaker filled with any
one of the solvents, and consider if you could have performed this experiment with a flashlight.
Discuss this in your lab report.
The experimental set up is shown below.
1
measure distance between
3 spots 2 & 3
2
1 un-refracted spot
2 refracted spot (water)
3 refracted spot (solvent)
beaker
incident beam
With the beaker empty note the position of the HeNe laser beam traversing through the beaker on
the graph. Make a mark on the paper indicating this spot. Fill the beaker with distilled water,
making sure the level of the water in the beaker is above the point at which the laser enters and
leaves the beaker. Mark the position of the laser beam on the graph paper after it has traveled
through the beaker filled with water. Remove the water with a Pasteur pipette and refill the
beaker. Make sure that the laser beam returns to the same spot.
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C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
Now remove the water from the with a Pasteur pipette. Rinse the beaker out with the first
solvent listed below. Use Pasteur pipettes to rinse the beaker. MAKE SURE YOU DO NOT
MOVE THE BEAKER. After rinsing out the beaker a couple of times with the first solvent, fill
the beaker up with this solvent making sure the level of the water in the beaker is above the point
at which the laser enters and leaves the beaker. Measure the distance between the refracted laser
spot traveling through this solvent to the refracted spot when the laser beam travels through
water. Remove the solvent with a Pasteur pipette and refill the beaker with the same solvent.
Repeat so that you have three readings of the position of the refracted spot with this solvent to
the refracted spot with water in the beaker.
Repeat the same process for all solvents listed below and in the order listed below, three times
for each solvent. Each time you use a new solvent, make sure you rinse the beaker well. ALSO
MAKE SURE YOU DO NOT MOVE THE BEAKER.
†
Solvent
Refractive index (n) @ 20 to 25 oC
Water
1.3325
Ethanol
1.3611
Acetone
1.3588
Hexane
1.37506
1-Propanol
1.3850
1-Butanol
1.3993
Cyclohexane
1.4266
Toluene
1.4961
(† Reference for refractive indices – Handbook of Chemistry and Physics, CRC Press)
After you have completed measurements with the solvents above, repeat the same procedure for
the sample with an unknown refractive index.
When you have completed your measurements, do not forget to try the same experiment with a
flashlight as the source of light. You can try this with any one of the solvents you used.
Data Analysis: Plot the refractive index of each solvent in the list versus the position of the
refracted beam through the solvent with respect to the position of the refracted beam through
water. You should obtain a straight line. Fit the data points to a straight line. Use your fit and
your measurement of the position of the refracted beam through the solvent with unknown
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C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
refractive index with respect to the position of the refracted beam through water to determine the
refractive index of the solvent.
Experiment IV: Measure the speed of light. (adapted from Reference 3)
The lasers you have been using for the other experiments in this lab were continuous.
This last experiment will introduce you to pulsed lasers – lasers that put out light in short,
repetitive bursts. The laser you will use is a Nd: YAG (which stands for Neodymium: Yttrium
Aluminum Garnet) laser. The Nd:YAG laser puts out 500 mJ of light at 1.064 (1micron or  =
10-6 m) in a pulse duration of 8nsecs at a repetition rate of 10Hz. Calculate the peak power
(energy in one 8 nanosecond pulse in Joules/sec) at 1.064 of the Nd:YAG laser and compare
the peak power to that of a standard 100W light bulb. The infrared output at 1.064m passes
through a non-linear crystal that doubles the frequency of the 1.064m light, producing laser light
at 532nm. About 45% of the 1.064m light is converted to 532 nm light. Optics separate the 532
nm light from the 1.064 nm light, and the 532 nm light is used for the experiments in this lab.
Since the peak power of a laser such as the pulsed Nd:YAG laser in the lab is so high,
lasers such as these are used to study chemical and physical processes which depend on the
presence of light but which have a very low probability of occurring. Since these processes are
dependent on light intensity, the high intensity output of lasers can drive more atoms or
molecules to undergo the chemical or physical process, allowing measurements of these
processes. In this experiment you will measure the pulse duration (width of a laser pulse in time)
of the 532 nm output from the Nd:YAG laser. You will then use these short bursts of light to
measure the speed of light in air.
The set up for this experiment is shown below. While a very small amount of the 532 nm
light is used for these experiments, please make sure that you are wearing your laser goggles
when performing this experiment.
Simplified Schematic for “Measuring Speed of Light” experiment
A small portion of the doubled output (532 nm) of the Nd:YAG laser is partially reflected
and partially transmitted through an optical window. The transmitted portion strikes an optical
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C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
detector called a photodiode. The output of this photodiode is proportional to the intensity of
light striking the detector. In addition the response of the photodiode is fast enough to faithfully
represent the time (temporal) profile of the light from the laser pulse striking it. The output of
the photodiode is visible on the oscilloscope. Look at the oscilloscope trace and determine the
pulse width at half height of the laser light. The reflected beam is reflected off a few mirrors,
and after travelling a certain distance, strikes another photodiode. The signal from this second
photodiode is also visible on the oscilloscope. Look at the temporal profile of this signal.
Determine the time it takes for the reflected light beam to reach the second photodiode.
Measuring the distance between the two photodiodes and the time difference between the two
signals. Use these measurements to calculate the speed of light. In your report comment on the
temporal profile of the Nd:YAG pulsed laser and why this is an important factor in determining
the speed of light.
Discussion
In the Discussion section of the lab report, explain clearly the differences between laser light and
light from a flashlight and how you observed these differences. Also explain how the properties
of laser light allowed the experiments you performed possible.
References
1 “Laser Experiments for Beginners”, R. N. Zare, B. H. Spencer, D. S. Springer, M. P. Jacobson;
1995, University Science Books.
2. “The Use of an Inexpensive Laser Pointer to Perform Qualitative and Semi-Quantitative Laser
Refractometry”, A. Neder et. al., J. Chem Ed., 78, 1481 (2001)
3. “Physical Chemistry: Developing a Dynamic Curriculum”, Eds. R. W. Schwenz, R. J. Moore,
1993, American Chemical Society
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