C2507 Intensive General Chemistry – Spring 02 – E9: Introduction to Laser
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 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 l = d sin qn
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
qn 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.
Experiment III: Refractometry (adapted from Reference 2)
Read the attached section in Appendix B from Reference 1 titled “Refraction of Light”.
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.
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.
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 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 1.064m (1micron or m = 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 1.064m 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 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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