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Tunable Diode Laser Absorption Spectroscopy of Oxygen
Matthew Karam
Physics Senior Thesis
Dr. Jonathan Weinstein
University of Nevada, Reno
May, 2010
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Abstract
Laser absorption spectroscopy of oxygen is performed at room temperature and at
atmospheric pressure with air, and in a pressure-controlled cell using pure oxygen. A tunable
laser diode is used to probe oxygen via absorption spectroscopy. The laser frequency scans
across an oxygen absorption line and is tuned using a piezoelectric device driven by a custom
scanbox controller which is discussed in an appendix. The experiment is repeated in a pressure
controlled pure oxygen cell and a gas handling system is constructed to evacuate the cell while
minimizing vibration noise; detailed in an appendix. The oxygen line full-width at half-
maximum (FWHM) is measured in open atmosphere in relation to laser output power, and in a
pressure-controlled cell in relation to pure oxygen pressure. The FWHM is measured and
analyzed for Doppler, pressure and power broadening. We were unable to saturate the oxygen
into the excited state in open atmosphere. The goal is to identify broadening factors and
determine the effects relating to pressure and laser output power. An appendix is included
discussing an imaging system for a cryocell as supplemental lab infrastructure used in other
experiments.
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Introduction
Tunable diode absorption spectroscopy
Quantum mechanics explains that molecules are more likely to absorb energy from
radiation at certain wavelengths which coincide with the energy levels of the molecule. Using
lasers adds control to this process because the emitted radiation is coherent at the specific
wavelength. Tunable diode absorption spectroscopy uses a laser which is tuned to a desired
wavelength (or range) in order to target a specific transition, which will appear as a line centered
over the transition energy level. An absorption line indicates the presence of specific molecules.
These lines can be visualized using detectors to compare initial versus final characteristics of the
laser. For example, comparing the initial emitted power to the transmitted power through a
sample can indicate concentration while the shape of the line can indicate temperature.
Additional applications
This research group studies cold atomic collisions and interactions. They would like to
study interactions with molecular oxygen. In order to do that, we need to be able to detect
oxygen concentrations in the cell. However, since the laser setup is in open atmosphere it will
also travel through atmospheric oxygen. This experiment demonstrates our ability to identify
oxygen in the atmosphere and in the cell as well as identify line broadening factors.
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Experimental Setup
Laser system and characteristics
The laser being used is a laser diode with an anti-reflection coating, Eagleyard Photonics
part number EYP-RWE-0790-04000-0750-SOTO1-0000, installed in a custom tunable housing
(Hardman, 2008) which uses feedback from a diffraction grating to stabilize the frequency. The
laser is tuned close to the desired wave number of 13091.7 cm-1
by using manual fine-thread
screw adjustments to adjust the grating position and its frequency is monitored on a Burleigh
WA-20VIS wave meter. The target wave number is taken from an oxygen line image which
appears to have the two closest distinct peaks within the capability of the laser when queried in
the HITRAN2004 database at spectralcalc.com, as shown in figure 1. Once the laser has been
generally tuned to the desired wavelength, a piezoelectric ramp oscillates the diffraction grating
in the laser causing a scan over wavelengths. A specialty scanbox was designed (see appendix)
and built to control the amplitude and frequency of the piezo oscillation. At the proper
wavelengths, oxygen absorption occurs and can be observed on the oscilloscope which monitors
photodiodes.
Figure 1
Oxygen spectra calculated at a temperature of 347.15 K, a volume mixing ration (VMR) of 0.18 and a length of 100cm
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The laser used was not able to automatically scan across any two oxygen spectral lines in a
single pass so we center on the line at 13091.7 cm-1
. Because we cannot scan over two oxygen
lines, we use a well known rubidium measurement (Steck, 2009) to determine the frequency
scale of our observations, as our measurements are relative. The oxygen transition we are
observing is from the X3∑ (v’’=0) state to the b
1∑g (v’=0) state (NIST Chemistry Web Book,
2009) and particularly the P9 rotational transition (HITRAN, 2004).
Optic table setup for open atmosphere
The optics are setup as shown in Figure 2. The laser is tuned to the same wave number of
13091.7 cm-1
because the spectral line remains in effectively the same location under the
proposed conditions of this experiment. The emitted beam is split with a wedge. One reflected
beam is directed to the Burleigh WA-20 VIS wave meter and the other passes through a
rubidium cell and into a photodiode. The transmitted beam is directed through a neutral density
filter (labeled ND) to decrease the beam power and then a polarizer to more finely adjust the
beam power at different polarizations. The beam is passed through another wedge and the
transmitted beam is blocked. Of the two reflected beams from the wedge, one is directed at one
receptor of a linked photodiode constructed by Muir Morrison which measures the difference in
the photocurrents of two photodiodes. The second reflected beam travels to a mirror (at position
M) and then is reflected into the second receptor of the linked photodiode. Because this
experiment is conducted in atmospheric air, absorption cannot be measured by one incident
beam. Oxygen is present throughout the entire beam path. To overcome this problem we must
measure the laser power of two separate beam paths having a known difference in path length.
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Figure 2
Setup for open atmosphere absorption as described in text
By this method, the two beams will have interacted with different amounts of oxygen and thus
will have different resulting energies when they reach the photodiodes. To initially calibrate the
zero absorption level of the photodiodes, the laser is tuned off resonance for oxygen absorption
and scanning is disabled so the emitted beam stays on one wavelength. Ideally this wavelength
should be off-resonance, but that was not verified in this experiment which may account for
some data inaccuracies. The two beams are aligned into the center of the photodiodes using a
viewfinder and then the polarizing filter is adjusted until the power reading is equal on both
photodiodes producing an initial “zero” reading on the oscilloscope.
Optic Table setup for vacuum cell
The experiment is repeated using an evacuated cell which is then filled with 99.995%
pure O2, as shown in figure 3. The optics table is arranged as shown having a beam split using a
glass wedge with one beam passing through the cell and another through the atmosphere. The
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emitted beam first travels through a glass wedge with one reflected beam entering a wave meter.
The transmitted beam passes through a neutral density filter (labeled ND) which is used to
modify the beam intensity as well as an adjustable polarizing filter to finely adjust the intensities
of the two beams. After it is attenuated, the beam is split using two reflections from a glass
wedge (the transmitted beam is terminated).
Figure 3
Setup for vacuum cell absorption as described in text
One of the reflected beams travels through a telescope to adjust the collimation and then through
a cell of varying pressures of oxygen and is finally focused onto one receptor of a linked
photodiode. The second reflected beam from the wedge is reflected to a mirror (labeled M)
which has an adjustable position in order to make the atmospheric beam paths have the same
length, less the distance through the cell. By this method, an atmospheric reading can be taken
and subtracted from the cell reading to isolate the effects in the cell. The beams are focused onto
a linked photo diode system as in the open-atmosphere setup. In order to evacuate the cell and
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fill it with oxygen, a gas handling system is designed and built. By using pure oxygen in the cell,
broadening effects from other atmospheric gasses are removed.
Recording Data
In both the atmospheric and evacuated cell experiments, both of the split beams terminate
onto the other one of two receptors of a linked photodiode. The output difference in voltage
enables us to capture the oxygen absorption line despite the fact that both beams travel through
atmospheric oxygen. The data from open-atmosphere experiment is recorded using a digital
oscilloscope. The oxygen line measurements from the evacuated cell setup are recorded using a
computer data recorder.
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Results and Discussion
In the open-atmosphere setup, the experiment was conducted with the Mirror “M” placed
such that the difference between the two beam paths was approximately one meter. The laser was
tuned to 13091.7 cm-1
and then the piezo scan was activated. The observed oxygen absorption
line can be seen in figures 4 and 5. The green line is the piezo ramp, the blue is the oxygen
absorption and the red is the fitted curve to the absorption line produced using Igor Pro.
Figure 4 Atmospheric O2 absorption, frequency offset 13091.7 cm
-2
Figure 5 Lorentzian fit, negative v corresponds to more absorption
The Lorentzian curve appears a better fit by sight and the chi2
value is about half that of the
Gaussian though the asymmetry of the line may disrupt the curve analysis. The data may be
adjusted because the photo diode voltage was calibrated too near an absorption line, making the
zero value inaccurate. This may slightly affect the full width at half maximum (FWHM), which
appears by eye to be approximately 1 GHz. 1 GHz is significantly wider than the natural
linewidth of oxygen; and if the Doppler broadening were the only factor we would expect a
width of approximately 600 MHz (see below) and a Gaussian fit . Consequently we assume that
pressure broadening has a significant effect on the observed line width. We then attempted to
saturate the oxygen by increasing the beam intensity but were unable to do so. We suspect this is
due to collisional quenching with nitrogen or water vapor but we also had a limited intensity
output on our laser due to mode hopping at higher energies.
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The experiment is repeated in the evacuated cell to enable absorption measurements at
varying oxygen pressures. Additionally, the lack of nitrogen collisions and other atmospheric
effects may have lead to less noise in the data recordings. Using an ND1 filter, we block one
incident beam on the detector and measure that 100% absorption is approximately 2 volts on the
oscilloscope. This is used below to calibrate absorption percentage. The total beam power is
approximately 54 microwatts measured from the front of the telescope. The mirror at location M
is placed such that the total beam path distance is approximately 173 cm. We then filled the cell
with approximately 55 torr of oxygen and began pumping the oxygen out of the cell while
recording the absorption data. As expected, higher oxygen densities yielded deeper absorption
peaks. Figure 6 shows an absorption line at 51.4 torr and the piezo scan line in volts versus
milliseconds. The data viewed in this way confirms that we have not fallen victim to the same
issue as our readings in open atmosphere where our absorption line tails trail off the scan. Both
tails of the spectral line clearly are included in this reading.
Figure 6
Piezo scan in red with absorption in blue, showing full line profile and piezo scan slope
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The data recordings are then converted into a more useful format. Because we cannot
visualize multiple oxygen absorption lines simultaneously, we use the spectra of rubidium to
calibrate the laser scan and convert from time to frequency, in GHz. From the measurements on
rubidium taken by Kyle Hardman, we know that a change in the scan voltage of 1V corresponds
to a change in the frequency of 1.855 GHz. Then, using the known 1.951 V/ms slope of our
piezo scan in figure 6, we determine that each data point (ten microseconds) corresponds to
3.6187x10-4
GHz for the scan conditions to convert to frequency in figure 7. Then we subtract
the data recording from our lowest measured pressure of 1.19 torr to effectively remove
background noise. Additionally, the voltage measurement in the range is divided by the 100%
absorption value measured previously. The resulting data depicts percent absorption in the
vertical and GHz line width in the horizontal. We suspect that the shift in absorption line center
and zero-absorption values in figure 7 is due to the laser drifting rather than pressure shift
changing the line.
Readings in the figure below correspond to oxygen pressures and their absorption percentages:
51.4 torr 0.87% 20.7 torr 0.39% 3.48 torr 0.15%
41.2 torr 0.78% 10.8 torr 0.25% 2.27 torr 0.11%
30.4 torr 0.60% 6.11 torr 0.18% 1.19 torr Low Ref
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Figure 7
Absorption at all pressures measured in vacuum cell, frequency offset 13091.7 cm-1
As expected, as seen in figure 8, the maximum absorption percentage is linearly related to the
pressure of pure oxygen; however the fitted line does not intersect the origin. This may be due to
imperfect measurements in pressure and voltage, or an incorrect calibration of the zero-value
point.
Figure 8
% Absorption is linear with pressure
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Each absorption line is fitted with a Gaussian and a Lorentzian using Igor and shown in figure 9.
In each case, a Gaussian yields a better or equal goodness of fit based on the chi2 value. This
result confirms Demtroder’s assertion that Doppler width will mask the natural line width at
room temperature (Demotroder, 2003). As expected, the signal to noise ratio gets much worse as
the pressure of oxygen in the cell is reduced, making the chi2 value larger for both the Gaussian
and Lorentzian fit tests and making the two fits less distinguishable. The FWHM is calculated for
each of the fitted Gaussian curves using Igor. Each FWHM is approximately the same, indicating
that the FWHM is pressure independent.
O2 @ ND1 FWHM
51.4 torr 0.584 GHz 10.8 torr 0.564 GHz
41.2 torr 0.581 GHz 6.11 torr 0.568 GHz
30.4 torr 0.574 GHz 3.48 torr 0.547 GHz
20.7 torr 0.564 GHz 2.27 torr 0.632 GHz
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Figure 9
Absorption at various pressures with frequency offset of 13091.8 cm-1
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Using the formula 𝜕𝜔𝐷 = (2𝜔0
𝑐) 2𝑅𝑇 𝑙𝑛2/𝑀 , where R is the gas constant 8.3145x10
7 erg K
-1
mol-1
and M is 32, the molar mass of oxygen; the expected Doppler line width is approximately
0.629 GHz (Demtroder, 2003). Our results are consistent with the expected linewidth within
approximately 10% which is common for the shift in our laser scan linearity.
Two measurements are taken with an ND2 filter in the place of the original ND1 filter.
The beam power is measured to be approximately 5.4 microwatts (one tenth the ND1 value).
One measurement at very low pressure is subtracted from another at 17.2 torr. The resulting line
is analyzed for its percent absorption, line profile and FWHM. As with all previous profiles, a
Gaussian fit yields the best Chi2
value. The absorption is approximately 0.385%, approximately
the same as the 20.7 torr reading taken at ND1. This result implies that absorption percentage is
effectively independent of the intensity of the beam at this power level.
Figure 10
Absorption percentage is independent of beam intensity
The FWHM is measured to be 0.553 GHz which is consistent to the measurements at ND1.
Therefore the FWHM is also effectively independent of the beam intensity.
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Combined cell and atmospheric absorption
In an attempt to isolate atmospheric broadening effects, two readings are taken at ND0
and 17.2 torr. For the second reading, then a mirror is removed such that atmospheric oxygen
effects are not cancelled out on the linked photo diode. The absorption profile with the mirror is
a subtracted from the profile without a mirror in order to obtain an atmospheric oxygen line. The
result is similar to the oscilloscope reading taken in the previous open-atmosphere experiment.
The atmospheric line profile is not fitted as well with a Gaussian as the pure oxygen which
implies additional broadening factors convoluting the Gaussian such as pressure broadening with
other atmospheric gasses like nitrogen (Demtroder, 2003, p 73).
Figure 11
Combined absorption for cell and open atmosphere
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References
W. Demtröder (2004). “Laser Spectroscopy Basic Concepts and Instrumentation, Third Edition”,
Springer-Verlag Berlin Heidelberg, New York
K. Hardman (2008). “Laser Design for Wide Scan Atom and Molecule Spectroscopy”,
University of Nevada, Reno
K.P. Huber and G. Herzberg (data prepared by J.W. Gallagher and R.D. Johnson, III) "Constants
of Diatomic Molecules" in NIST Chemistry WebBook, NIST Standard Reference Database
Number 69, Eds. P.J. Linstrom and W.G. Mallard, National Institute of Standards and
Technology, Gaithersburg MD, 20899, http://webbook.nist.gov, (retrieved April, 2010)
A. Roth (1990). “Vacuum Technology Third, Updated and Enlarged Edition”, Elsevier Science
B.V., Amsterdam, The Netherlands
L.S. Rothman, D. Jacquemart, A. Barbe, D. Chris Benner, M. Birk, L.R. Brown, M.R. Carleer,
C. Chackerian, Jr., K. Chance, L.H. Coudert, V. Dana, V.M. Devi, J.-M. Flaud, R.R. Gamache,
A. Goldman, J.-M. Hartmann, K.W. Jucks, A.G. Maki, J.-Y. Mandin, S.T. Massie, J. Orphal, A.
Perrin, C.P. Rinsland, M.A.H. Smith, J. Tennyson, R.N. Tolchenov, R.A. Toth, J. Vander
Auwera, P. Varanasi and G. Wagner (2004) “The HITRAN 2004 Molecular Spectroscopic
Database”, Harvard-Smithsonian Center for Astrophysics, Atomic and Molecular Physics
Division, Cambridge, MA 02138, USA
Daniel A. Steck, “Rubidium 87D Line Data,” available online at http://steck.us/alkalidata
(revision2.1.2, 12 August 2009)
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Appendix 1: A Low Vibration Gas Handling System
Many atomic experiments must be conducted in low pressure or single-gas environments.
This is traditionally accomplished using a vacuum system to remove the undesired gasses and
then adding a pure gas from a separate supply. Typical gas handling systems require an initial
vacuum stage using a roughing pump to lower the pressure enough for a second stage to begin
pumping. This application uses a mechanical rotary vane pump (RVP). The problem posed is
that mechanical roughing pumps such as RVPs generate vibration noise. This is undesirable in an
experiment where scanning lasers may be susceptible to interference and disruption due to the
vibrations. This appendix outlines a design for a mobile vacuum station which mitigates
vibration noise and easily enables introducing gas supplies.
The entire system is designed to fit within a half-height PC rack enclosure. An enclosure
on wheels is selected for mobility. The system is also designed to implement components already
in stock or readily available to the lab. In particular, an RVP is used to create the initial vacuum
and a mag-lev turbo pump is used to evacuate the remaining gasses. The central vacuum
chamber is made from a Conflat (CF) “cross” which provides enough access for this application.
To prevent vibrations from the RVP from affecting the laser the RVP is suspended from
springs attached to the rack. The springs effectively decouple the pump from the ground but the
vibration must be dampened to prevent excessive bouncing on the spring suspension. To
accomplish this, the RVP is connected to the system using thick flexible tubing. The tubing is
sufficiently soft to mitigate vibration transfer but firm enough to dampen the vibrations of the
RVP. The trap includes a heating element and is filled with specialized pellets which absorb any
oil which travels up the vacuum piping. If the pellets become contaminated with oil, the heating
element can be used evaporate the contaminants. After the trap is an automatic valve which is
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connected to the same power supply as the RVP so that if the RVP power fails, the valve will
close. Not only does this prevent oil and other contaminants from entering the rest of the system
but it also maintains the vacuum to protect the mag-lev pump which is connected to the
automatic valve. The mag-lev pump will fail if it is run at too high pressure. On the opposite side
of the mag-lev pump is a valve which allows the vacuum chamber and manifold to be completely
separated from the pumps. A simple cross is used as a vacuum chamber both to save on cost and
because minimal extensions will need to be connected to the pumping station. Of the three
outputs of the cross, one is used for a small diameter connection to the gas manifold, one may be
used for large diameter connections to speed up evacuation of larger chambers and the third may
be connected to a residual gas analyzer (RGA) to rest for leaks and verify cleanliness of the
system.
The system has an integrated MKS Series 910 micro-pirani pressure gauge integrated into the
manifold. Because the manifold is made on low internal-diameter tubing and has a number of
bends, there will be a pressure difference between the measured manifold pressure and the
pressure in the vacuum chamber or the evacuated cell. The conductance is calculated (Roth,
1990) and is determined for the following criteria:
SS tube inner diameter 0.4 cm, Length from gauge to CF 93 cm
310875.7 xP torr.
310875.7 xP torr.
)(1.123
LDCair P
LDCair )(182
4
( P in torr.)
s
LxCair
31033.8 s
LPxC air )105( 2
)10777.2( 5
1
xPP
201
2
1
P
PP
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Front Side
Figure 12 Gas handling system block diagram
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Appendix 2: A PC Controlled Imaging System
The transmitted light through a target can indicate the concentration of material. Using an
image array rather than a photodiode can show the dimensionality of how the material is
dispersed over larger areas and give a more accurate result. A PC controller system and a custom
LabView program were created to record the images from the Point Grey Research Flea 2
camera. A dedicated computer was built for this purpose using the following components:
The image is captured at 648x488 resolution in 16-bit grayscale at 80 frames per second, which
requires the use of a high-speed firewire interface. The operating system on the PC was
Windows XP SP2; and in SP2 full firewire speeds had been disabled. This limitation has been
lifted in Windows XP SP3 and all subsequent windows versions. At the time the system was
constructed, manual Windows registry changes were necessary to enable full firewire speeds but
now a patch can be applied pursuant to Microsoft KB885222 for SP2 or KB955408 if the system
is upgraded to SP3. Once full speed was restored we attempted to use a PCI firewire card for the
image capture but the system crashed every time the camera was enabled. We suspected that the
camera feed was stressing the PCI bus and crashing the machine and so we switched to a firewire
interface on the PCI express bus which provided adequate throughput for our desired frame rate,
size and bit depth.
We also determined the quantum efficiency of the camera CCD, the shot and read noise for
the system using the formulae 𝐵 = 𝐺 × 𝑄 × 𝑃 + 𝑆 + 𝑅 + 𝑂 and 𝐸 = 𝑄 × 𝑃 + 𝑆 where B is
the total number of bits recorded by the CCD, G is the gain, Q is the quantum efficiency, P is the
number of photons, S is the shot noise, R is the read noise, O is the offset, and E is the number of
electrons on the CCD. First we assign an offset so that no CCD wells have zero values by
default; using a “black” image with a lens cap on we increase the offset until all CCD wells read
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positive values. An AOM is used as a shutter to precisely control the amount of time the beam is
on the detector; but the AOM “leaks” some laser light through its aperture in the “closed” state
so we also need to determine how much light only hits the detector during the “open” state.
We can determine the effective offset, O, by taking a “black” image and then averaging
all CCD bit values.
To get the read noise, R, subtract the values of two “black” images then record 𝜎
2.
We can find the shot noise, S, expecting that it is has a standard deviation of 𝐸. Then
determine the sum of the noise values, S x G + R by taking two “identical” images taken
one immediately after the other under the same conditions. Take the difference between
the two images, then locate regions in the image of equal light intensity and record 𝜎
2,
subtract the known value of R to get S x G.
Take the total bit value of that same region, subtract the offset, O, multiply by the number
of pixels in the region. Divide by S x G, and square the result to find the number of
electrons, E.
Divide E by the quantity of the total bits minus the offset to find gain, G.
Get the average bits through the “leaky” AOM image then subtract this from the bits of a
normal image to get the total bits, B.
We used a calibrated photocell to determine 𝑃ℎ𝑜𝑡𝑜𝑛𝑠
𝑆𝑒𝑐𝑜𝑛𝑑 then multiply by the AOM shutter
time to get total photons, P.
Divide total bits, B, by total photons, P, to get Q*G. Divide by G to determine quantum
efficiency, Q.
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The quantum efficiency of the CCD will change at different wavelengths. Our calculation was
performed at 25106.7 cm-1
and the quantum efficiency was determined to be approximately 0.51,
which was in line with the advertised specs of this camera’s Sony ICX424 CCD.
Figure 13
Beam image (right) and shot noise (below)
note the higher noise values in the beam area
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Appendix 3: A Custom Laser Scanbox
This circuit controls the scan of the piezo in the tunable laser. Potentiometers are used to
adjust frequency and amplitude for a triangle wave generator output. The triangle wave output is
connected to the input of our piezo driver. An internal jumper toggles operation between low and
high voltage for use with different piezo drivers with or without internal amplifiers.
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Conclusion
The development presented in this thesis, including all appendices, is in pursuit of
enabling or supporting the group’s research. While few “new scientific discoveries” were made,
we successfully explored the capabilities of our equipment and learned to develop new methods
and tools for our existing research. For example, we implemented solutions using purpose-driven
parts such as a specific CCD camera or a conflat cross instead of a traditional vacuum chamber.
One motivation for the work performed was to implement appropriate systems at a lower cost
and to demonstrate their suitable functionality for our use. Additionally, probing atmospheric
oxygen at the conditions in our laboratory outlines the capabilities of our laser and potential
sources of problems in future experiments. I also validated data from tables and formulae that
will be used in our research; demonstrating that the results of our laser and measuring systems
can produce results in agreement with the existing theories.