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Feasibility Testing for Open-Path Cavity Ring-Down Spectroscopy Instrument
Alexis Hurstell, Tanner Fretthold, Joel Schulz, Justin Weinmeister
Colorado State University
Dr. Azer Yalin
Laurie McHale
April 18th, 2015
Abstract
Over the past two decades the number of active well sites
has doubled to meet natural gas demands [1]. Natural gas
is a highly efficient form of energy comprised primarily of
Methane, but as a Greenhouse gas, methane has 21 times
the global warming potential of CO2 [2]. Previous studies
investigating the process of obtaining natural gases have
reported conflicting results on the amount of gas leaked
into the atmosphere due to the process of drilling,
transportation, and burning of this fuel. Traditional
continuous wave cavity ring down spectroscopy (CRDS)
instruments are capable of measuring gas species down to
the parts per trillion level, and an available unit was used
for this study. The aim of this study was to investigate the
possibilities of mounting an open-path CRDS laser system
on an unmanned aerial vehicle (UAV) to measure leaks of
methane during extraction and transportation. Open path
CRDS systems are underdeveloped and steps were taken to
develop novel technologies to utilize a traditional CRDS
instrument in an open-path configuration. A series of tasks
were carried out to understand negative effects on the ring
down signal once the cavity was removed from the
instrument; mirror cleanliness in an exposed environment,
particulate induced scattering, and the instrument’s ability
to record data while moving were all key tasks required in
characterizing the open path system. It was found that
mirror cleanliness is not an issue for further development;
however, particulate scattering and data recording during
movement still pose development issues in dirty
atmospheres and high speeds, respectively. Despite the
observed influence of aerosols and pressure density
gradients, our research demonstrated that open-path
CRDS seems to be feasible for this use.
1 Introduction
With expanded use of hydraulic fracturing
techniques in the recovery of natural gas, the number of
wells in the United States has grown rapidly. The Denver-
Julesburg basin of Colorado is a local area that has seen
this rapid growth.
When comparing the energy production through
burning fuels to the greenhouse gas emissions produced
through combustion, natural gas is found to be a better
alternative to other conventional energy sources like coal
and oil. The primary component of natural gas is methane,
which in its non-combusted form is a more potent
greenhouse gas than carbon dioxide.
However if the natural gas is combusted, the
resulting net greenhouse gas emissions are comparatively
less than the initial methane. Because of this, natural gas
is a potent energy source which requires development of
monitoring infrastructure to ensure that the methane leaks
are contained and the greenhouse benefits of natural gas is
preserved.
Due to the risk of greenhouse gas pollution, the
development of an effective way to detect and locate leaks
is required. Conceptually it is possible to triangulate the
leak using concentration measurements taken from a
variety of locations on a mobile platform such as a UAV.
Such a feat requires equipment sensitive enough to detect
concentrations in the parts per billion scale, a fast data
collection rate to gain sufficient resolution for a reasonable
triangulation, and must maintain power and weight limits
such that it can be mounted on a mobile platform.
A measurement technique known as Cavity Ring-
Down Spectroscopy (CRDS) is capable of making these
measurements both with the required precision and
resolution. Current CRDS sensors do not meet all
requirements for implementation of a practical system
though. In addition to a set of highly reflective mirrors
(>99.99%) typical CRDS sensors use a partially evacuated
sample cavity which requires both a vacuum pump and
cavity structure which increase weight and energy
requirements of the system, and particulate free gas
samples, all things which lead to difficulty when
converting the system to a mobile operation.
Our sponsor gave our group the goal of working
through some of the initial trial working sensor
development, such as understanding the basics optics
utilized in the CRDS sensors, and characterizing the nature
of an open path device. All of which provided us grounds
to assess the feasibility of such a sensor system. An
explanation on the basic function of a CRDS sensor can be
found in Appendix A.
Until now, there has been limited development of
open-path CRDS, providing few references in the
literature. The most notable was a paper written by

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Pettersson [3]. Pettersson wasn’t actually working in open-
path CRDS but was specifically investigating a CRDS
sensor to measure aerosol particles. This gave us a good
idea of the magnitude of extinction to expect from typically
aerosol particles and acted as a guide for directing our
efforts. Thus the first course of action was to remove the
cavity from the system and identify the sensors
performance and any short comings that require
development. Following this, results from the initial
experiment, maintaining and characterizing mirror
cleanliness in an open path cavity were investigated, and
then finally mobilization of the system in an outdoor
environment was carried out. Utilizing data from these
experiments, a feasibility analysis and research direction
for future work can now be established.
2 Test Setup
We conducted experiments to determine if
ambient air degraded the signal versus a pure Nitrogen
environment due to Rayleigh scattering. This test was
conducted by constructing a large rectangular enclosure
made of polycarbonate that contained the sensor sample
path and provided a controlled environment for testing.
Data was collected using filtered ambient air and pure
nitrogen. The ambient air test was run by purging the
sealed cavity with air pumped through a HEPA filter. The
purge was run for one minute at 0.8 lpm and then turned
off and the system allowed to rest for one hour sealed. At
this point the stabilized Tau values (see Appendix A for a
detailed breakdown of Tau values relation to signal quality)
were recorded, and the test was ran again using 99.98%
pure Nitrogen. The second test of this experiment was used
to determine if particulates in the air were causing the
degradation. The polycarbonate cavity was placed on the
system and data was collected. The system was then
allowed to rest for 24 hours and data was taken again.
The second experiment was conducted to
determine if the mirrors on the CRDS system would get
dirty over the system’s use and degrade the mirrors
reflectivity. Reductions in reflectivity could be expected if
exposure to ambient dust, moisture and other unforeseen
operating conditions were not properly shielded. Initial
testing involved constructing solid models of the current
mirror holders and future possibilities that could be used in
flow analysis. Solid models were constructed in
Solidworks and then flow simulations were carried out.
This testing was then used to design and evaluate a purge
system that could be run on the mirror holders to actively
clean any settling particulate, or evaporate condensation.
The system was designed to be small and low power. A
micro pump and HEPA filter were used to create this purge
system which was used in later test setups. Finally tests
were run where the mirrors were placed in a dirty
environment. In this test setup the same polycarbonate
cavity was set on the system with a high resolution particle
counter connected. This setup is shown in Figure 1. Before
the test began a particle counter was placed in the box to
determine the amount of particulates in the ambient air.
The particle counter and CRDS system recorded the
number of particles and Tau values continually throughout
the entire experiment and were averaged into fifteen
second intervals. In this experiment two lit candles were
placed in the polycarbonate cavity while the environment
was enclosed. Once the oxygen was depleted in the
enclosure from the burning candles, the candles
extinguished, and the environment was left untouched for
five minutes. After the five minute period was over, a
nitrogen purge of 0.80 lpm and 1.00 lpm suction from a
vacuum pump simultaneously were placed in the encasing
to create a negative pressure. This continued for ten
minutes until the particulate count reached the count
recorded before the test was conducted.
Figure 1. Set-up for candle particulate testing
The third experiment aimed to test the unknown
limits of a mobile open-path cavity ring down system in the
field. This was done by creating a custom rig to make the
system mobile. This was then driven around while
collecting data to better characterize challenges that will be
faced in designing a UAV scale system.
The mobilization of the CRDS setup required
addressing some main requirements: securely mounting
the large optical cart and maintaining alignment of optical
components, reducing vibrations on the system, providing
a portable power supply/inverter capable of powering the
sensitive electronics, and shielding ambient light sources
from the systems photodetector. The cart was mounted to
a truck by placing the entire cart in a pickup truck bed and
secured with ratchet straps. The cart was also mounted on
rubber dampers used for washing machines to help isolate
road vibrations to the cart. The power supply for the system
was a 120V pure sinusoidal wave inverter. Due to the clean
powers need for the laser driver, AOM controller, and for
the photodetector’s amplifier to function properly, a pure
sinusoidal inverter was needed to replicate standard grid

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power for the system. The pure sinusoidal inverter utilizes
an inverting circuit to turn 12V DC into 120V AC, and due
to this the inverter creates a remarkable amount of EMF
noise. The computer system was initially stored inside the
truck with the inverter, but the EMF effects from the
inverter were found to cause remarkable amounts of noise
in the base signal. For this reason the inverter needed to be
shielded from the rest of the system. This was done by
creating a Faraday cage around the inverter, and moving
the tower portion of the PC to the cart stored in the truck
bed. All of the peripheral devices, e.g. keyboard, mouse,
and screen then were wired into the cab of the truck for safe
use. The photodetector was shielded from ambient light
sources by using a simple box sealed to the table of the
system. Additional protection was provided by using an
off-axis parabolic mirror to turn the incoming laser 90
degrees before hitting the photodetector. The entire system
was then driven to an airstrip away from town and
measurements were taken driving into and against the wind
at interval speeds between 5 - 40 mph. The wind speed was
approximately 5 mph throughout the test.
Figure 2. Above shows the test set up used for
experiment 3, with the system mounted in the bed of
the truck.
3.1 Results Experiment 1
Signal degradation of an open path CRDS system
was tested utilizing two tests. The first test aimed at
determining if Rayleigh scattering accounted for a
substantial portion of signal loss. Tau values were
recorded and averaged from two air samples, one
comprised of N2 (99.98% pure), and the other was HEPA
filtered ambient air.
The data shows the Tau values of both the N2 and
filtered Atmosphere to be identical, showing no difference
within the measuring capabilities of the CRDS.
Table 1. Rayleigh scattering
Averaged Tau (µsec) Gas Type
92.6 Nitrogen
92.1 Ambient Air
Atmosphere (N2 - 75.47%, O2- 23.20% by
weight) and Nitrogen (N2- 99.98% by weight) both
showed Tau values of 92 microseconds. These recordings
matched the predicted results using calculations done using
equation 1.
𝝉 =
𝑳
𝑪
×
𝟏
𝟏−𝒓+𝒍𝒐𝒔𝒔𝒆𝒔
Equation 1
Where L is the length of the cavity, c is the speed
of light, and R is the reflectivity of the mirrors at the
wavelength of the laser. Losses were calculated using
equation 2.
𝑙𝑜𝑠𝑠𝑒𝑠 = 𝜎 × 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠/𝑐𝑚3
× 𝐿 Equation 2
Where σ is the cross sectional area of the species
and L is the length of the cavity. Particles were calculated
from the ideal gas law. These equations gave values of
166.119 and 166.107 microseconds for atmospheric air and
N2, respectively. The 0.012 microsecond difference was
within the error range of our average trials. As these values
were much higher than those found in practice, the test was
run to confirm that the equations were giving accurate
results for our instrument. Additional tests were run to
confirm the accuracy of the equation used. The tests
compared data from when the instrument was a closed cell
CRDS system operating with a known gas sample to
calculated results. The equation was then suitable to use for
estimating extinction of different air samples with varying
quantities of particulates, used in experiment 2.

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Figure 3. Ring-down signal has created stability after a
24 hour period. The signal returns to the initial noise
state when the box is removed.
The second test used in characterizing the loss
mechanisms of the open path cavity required allowing the
system to sit in an isolated environment for 24 hours, thus
allowing potential particulate to settle and comparing the
noise in the signal between a settled and unsettled state.
The result of this test is shown in Figure 3. This shows a
stabilized Tau value of 125 microseconds, which returns to
the initial open path Tau value of 95 microseconds when
the box is removed.
3.2 Results Experiment 2
The second experiment simulated dust particulate
similar to the sizes that the open-path CRDS system would
encounter on the UAV 10 meters to 100 meters above the
surface and tested to see if the mirrors would get dirty over
the system’s use. After utilizing flow analysis software, a
purge system was designed and constructed to be tested
with the system. The CRDS system was isolated in a
controlled environment with the purge system running.
Smoke particulate were then introduced and then removed
over a 17 minute time period. The tau values were recorded
continually through the smoke injection and removal
processes to identify if the mirror purge system was
satisfactory.
The test results showed the effect of particulate
count on the recorded ring down of the open path CRDS
system.
Figure 4. Ring-down signal being extinguished from
smoke particulates then recovering as particulate
numbers decrease.
Above, the second experiment maintained a
stabilized tau value of 42 microseconds highlighted in blue.
At T=6325 second, the candles extinguished, the number
of particles in the environment were measured at 7450
particles/cm3
when the signal initially extinguished.
After the large spike in the particle count, the
signal was regained after thirteen minutes of purging the
chamber. The ring-down signal was regained at 2300
particles/cm3
. The signal successfully regained strength,
eventually restoring itself to the baseline obtained from
before the candles were introduced.
3.3 Results Experiment 3
The third experiment explored the limits of a
mobile open-path cavity ring down system. This
experiment breaks down into two main tests, first the
system integration of the sensor and vehicle, and secondly
the nature of sampling in a mobile system.
The first test required mounting the sensor and
characterizing system integration issues. Due to electrical
power being sourced off of a pure sinewave inverter wired
to the vehicles alternator, electromagnetic noise caused
disturbances with the CRDS sensors auxiliary electronics
and thus required shielding which was carried out by
mounting all sensitive components outside of the cab area.
In addition, the sensor diode used to read the laser
measurements also had to be shielded from external light
sources due to exposure, particularly the sun. If not
shielded a DC offset was created on the sensor system and
created false data readings. Figure 5 captures both the DC
offset due to lack of solar shielding and EMF noise created
due to proximity between the computer and inverter.

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Figure 5. Top, signal distortion due to EMF noise can
be seen creating disorder band voltages and weakening
signal strength overall. The bottom image shows a
typical strong clear signal which is comprised with
distinct evenly spaced interval bands and very little
noise in between.
The second test required actually driving at various speeds
both against and into the wind to determine the limitations
on collection. (Wind Speed was measured at 10 MPH
utilizing Colorado State University’s atmospheric science
weather monitoring data). Data in the form of Tau values
was recorded from these trips and plotted against speed and
direction. These plots are shown in the figures below.
Figure 6. More data can be seen collected with the wind
than against the wind at a 20 Mile Per Hour Driving
Speed
Figure 7. More data can be seen collected with the wind
than against the wind at a 30 Mile per Hour Driving
Speed

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Figure 8. Data can only be seen while traveling with the
wind at 40 Mile per Hour Driving Speed
Figure 9. More data can be seen collected with the wind
than against the wind at a 20 Mile per Hour Driving
Speed
4.1 Experiment 1 Discussion
An open-path CRDS system loses sensitivity
versus a closed path system as losses in the cavity reduce
the tau values. The two tests conducted roughly account for
Rayleigh and Mie scattering of the light due to ambient
aerosol particles present in the laboratory.
Tau value differences for both a N2 and
atmospheric air environment were not significant for our
tests. This confirms calculations done earlier to
approximate tau, using equation 1. The calculations
predicted a change of 0.012 microseconds between the
environments, below the detectable threshold for our
testing procedure. More rigorous tests may be run to
confirm the precision of this calculation. Currently there is
no need for this number to further test the efficacy of an
open-path CRDS instrument.
Tau values from an environment in which
particles were allowed to settle vs non-settled showed
significant deviation and further research is needed to try
to reduce these effects. The particles airborne in the
ambient air of the laboratory reduced the tau value from
approximately 160 microseconds to an unmeasurable value
shown with sharp dropouts. This loss is due to Mie
scattering of the laser light as it impacts particulates in the
cavity. This loss can be reduced by using the instrument on
days with fewer particulates in the air. Possible future
improvements to the system to reduce the effect of
particulates could include leading airfoils designed to
redirect airflow, or development of a DAQ algorithm to
filter out bad data points.
This experiment isolated the losses seen in the
open cavity to those from Mie scattering and not Rayleigh.
No future system will need to take into account the gas
composition of the sample when determining ring-down
times. The test does not address related losses that could be
due to mirage effects of a fast moving air sample.
4.2 Experiment 2 Discussion
The second experiment tested the effects of mirror
dirtiness on the CRDS system. The test results show that
even under extreme conditions the mirrors will stay clean
in their current holders with a lightweight purge system.
The mirrors stayed clean under multiple cycles of these
extreme particulate tests and under direct flow of
particulates. Future testing will need to be conducted on
individual instruments to determine their maintenance
schedule in their standard operating environment.
4.3 Experiment 3 Discussion
The mobile testing of the CRDS system shows
there is great potential for an open-path instrument to be
placed on a UAV for mobile data collection. Limitations
were found in the system’s ability to work at high velocities
or in environments with excessive vibration or EMF
radiation. The sensor is sensitive to transportation and
vibrations during operation. These challenges can be
overcome with secure mountings and vibration reducing
pads used on mounting points. These issues must be dealt
with separately when the system is designed for UAV
integration.
The power system used was specific to our system
but presented similar design issues that will be found in
future instruments. The EMF radiation produced from high
currents will pose problems in future systems and will
require shielding in the form of Faraday cages and physical
isolation of components on a system that may make the

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instrument’s footprint large. Additional noise from
ambient light can cause issues but is easily controlled.
The majority of work still needed to create an
open-path CRDS system for a UAV deals with signal loss
at high velocities. The system operates best in still air, and
as currently configured, does not operate effectively above
speeds of 30 miles per hour, which is on the lower side of
average flight velocities required for UAV flight.
Additionally, this includes wind airspeed, and so the
system cannot operate in significant winds. Losses from
these increased velocities do not greatly affect the average
ring down time of the signal but instead limit the sampling
rate. Future possibilities to reduce the effects of high
velocity include a leading airfoil to redirect incoming
particles or delayed sampling methods.
5 Conclusions
With the current lack of research pertaining to
open-path cavity ring down spectroscopy, the overarching
goal of this research was to explore the limitations and
possible capabilities of an open path CRDS system. The
largest obstacle with removing the cavity to the CRDS
system was shown to be limiting the decrease in both the
amplitude of the Tau signal along with the clarity of the
signal. Once the environment was sufficiently controlled,
such as in experiment 2, both the Tau and signal clarity
signal increased. Still, particulate induced scattering can
degrade collection rates, proving the need for a purge
system. Finally, the CRDS system was tested for its mobile
functionality. The preliminary groundwork using the
mobile CRDS system helps to show what the data output
and design challenges may be like for the open path CRDS
if it was implemented onto a UAV. The preliminary data
shows that there is a need to modification the system since
the open path is sensitive to velocity as well as wind
direction. The preliminary experimentation performed by
our group on this novel system will provide the
groundwork for future open path CRDS research and
development.
6 References
[1] (2015, February 27). Number of Producing Gas Wells
[Online]. Available:
http://www.eia.gov/dnav/ng/ng_prod_wells_s1_a.htm
[2] (2014, July 2). Overview of Greenhouse Gases
[Online]. Available:
http://epa.gov/climatechange/ghgemissions/gases/ch4.htm
[3] A. Pettersson. (2004, August). Measurement of
aerosol optical extinction at 532nm with pulsed cavity
ring down spectroscopy [Online]. Available:
http://www.sciencedirect.com

8. Hurstell Page 8
Appendix A
Basic CRDS Operation
CRDS sensors work by repeatedly reflecting a
laser pulse between two 99.99% or greater reflective
mirrors in a setup also known as a resonant cavity. The
light reflects many times between these two mirrors until it
has travelled up to several kilometers through the gas
sample which fills the space within the resonant cavity.
With each travel between the mirrors, the laser pulse is
minutely absorbed by the gas sample species. Each species
has a unique absorption spectrum that can be used to
identify the species interfering with the laser. Due to the
large number of reflections, the travel distance through the
sample becomes relatively large, and thus amplifies the
characteristic absorption phenomena that occur in the
cavity. With each reflection, small portions of the laser
pulse are decoupled from the cavity due to the imperfect
reflectivity of the mirror. This escaped light is then
externally captured and focused utilizing additional optics.
This focused light forms the laser output signal, and its
intensity is detected on the outside of the cavity using a
highly sensitive tuned optical sensor.
Figure 10. Utilizing a specific wavelength laser and
analyte gas species pairing, a concentration data can
be collected. An initial decay rate with a clean baseline
sample is recorded, τo. Then, a gas sample is
introduced and a second decay rate, τ, is collected.
Through comparing these decay rates, the absorption
phenomenon characterizes concentration of the
specified analyte.
The decay rate of the laser’s intensity depends on
how effective the gas sample is at absorbing the specific
wavelength of light used in the cavity. This process is then
repeated, using a scanning feature on the laser to shift its
wavelengths slightly above and below certain known
absorption features. Certain species in the gas sample will
absorb the laser light at very specific wavelengths, so that
the decay rate of the wavelength is greater compared to
neighboring ones. The increase can be related to
concentrations of the species down to parts per trillion
depending on how accurate the sensor is. The “ring down”
is the time required for the peak laser intensity to decay to
zero and it is this time can be referred to as the τ value. The
higher τ, or longer lifetime of the laser pulse in the cavity,
the more sensitive the instrument is.