Pag

e1

Tunable Diode Laser Absorption Spectroscopy of Oxygen

Matthew Karam

Physics Senior Thesis

Dr. Jonathan Weinstein

University of Nevada, Reno

May, 2010

Pag

e2

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.

Pag

e3

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.

Pag

e4

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

Pag

e5

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.

Pag

e6

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

Pag

e7

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

Pag

e8

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.

Pag

e9

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.

Pag

e10

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

Pag

e11

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

Pag

e12

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

Pag

e13

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

Pag

e14

Figure 9

Absorption at various pressures with frequency offset of 13091.8 cm-1

Pag

e15

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.

Pag

e16

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

Pag

e17

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)

Pag

e18

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

Pag

e19

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

Pag

e20

Front Side

Figure 12 Gas handling system block diagram

Pag

e21

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

Pag

e22

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.

Pag

e23

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

Pag

e24

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.

Pag

e25

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.