This document describes infrared absorption spectroscopy and a new dispersion technique called CLaDS. CLaDS relies on measuring the dispersion of a dual beam laser rather than direct absorption. This provides several advantages over other techniques including high sensitivity down to parts per billion, a linear response over a wide dynamic range, and immunity to fluctuations in light intensity. Potential applications of CLaDS include atmospheric monitoring, combustion analytics, medical diagnostics, and remote open path detection due its unique capabilities.

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2. Intro: Infrared Absorption Spectroscopy
Infrared (IR) electromagnetic radiation causes vibrations in molecules. This allows
the identification and quantification of an unknown molecule in a sample.
• Emission wavelength of the laser is tuned over the characteristic absorption lines
of a species in the path of the laser beam.
• Reduction of the measured signal intensity used to determine the gas
concentration  typically a small intensity change over a large background of
the total transmitted laser intensity
Absorption

3. Absorption Spectroscopy: Limitations
• Signal dependant on light intensity, which is affected various factors in real life
scenarios (dirt, water vapour, snow, etc..). Baseline fluctuations can be
misinterpreted as a molecular absorption.
• Any noise introduced by light source or transmission through the optical system
will deteriorate sensitivity
• Absorption nonlinear above a certain limit, thus limiting the dynamic range (e.g.
300 ppm* to 3 ppm as in the stack analyser from Cascade: http://www.cascade-
technologies.com/Products/Gas-Sensing/CT2100-onstack-multi-gas-analyser/)
*ppm – parts per million (e.g. the unit “1 ppm” can be used for a mass fraction if a water-borne pollutant is present at one-
millionth of a gram per gram of sample solution.

4. C La D S: Dispersion Technique
Direct absorption techniques measure
change in intensity of light passing
through a sample  disadvantages
discussed in previous slides.
When the frequency of an electromagnetic wave coincides with a resonance of the irradiated
medium (e.g. rotation, vibrational or electronic), both absorption and dispersion of the
transmitted light wave occurs. Dispersion refers to different refractive indices experienced by
waves with different frequencies.
Absorption Dispersion
CLaDS relies on dispersion of
transmitted light wave to derive
concentration of the target molecule.
CLaDS uses a novel configuration to
detect the much weaker dispersion
signal.

5. CLaDS: How it Works
Note: Simplified version.
SAMPLE
When the dual beam interacts
with a molecular transition –
both wavelengths experience
slightly different refractive
indices.
PHOTO DETECTOR
The photodetector measures
the difference in refractive
index between the two optical
waves allowing the
concentration of the species to
be derived.
SOURCE
A laser emits light within a
narrow band of infrared
wavelengths, this is coupled
to a device which produces
a two coherent beams with
a slight frequency offset.
1 2 4

6. CLaDS: Unique Features
• High sensitivity (parts per billion)
• Linear response, so dynamic range extends from
the detection limit (in the ppb range) to nearly
full concentration.
• No baseline-reading required, without the need
for complicated and expensive systems
• Output data immune from source intensity (see
right hand side)  outperforms competing
technologies in dirty environments.
• Fast response time (frozen atmosphere) 
immune to mechanical vibrations, allows
monitoring in turbulent conditions.
• Low power consumption and compact (shoebox
size).
Figure above shoes CLaDS’s immunity to detected
power fluctuations.

7. CLaDS: Comparison
Technology Type
CLaDS CEAS (QCL based) Mass Spectrometry FTIR
High Dynamic Range P P
Compact P P
Selective P P
Sensitive P P P P
Fast response P P
Portable P P
Immunity to source
intensity P P
Baseline Free
Measurements P P
Measurements in Situ P P P
Wide Range of Molecules P P

8. Potential Applications
Defence / Military and Law Enforcement Identification of hazardous materials
Chemical weapons, explosives and their precursors
Automotive Industry Analysis of car exhaust gases
Manufacturing Industry / Power
Generation
Monitoring of CO2 and other greenhouse gas emission
Assessing process efficiency (by analysing exhaust gases)
Analysis of headspace gases in pipes and barrels
Semiconductor device manufacture exhaust gases
Environmental Protection Atmosphere monitoring (pollution)
Crop monitoring (e.g. ground cover, soil chemistry)
Medical Diagnostics Breath testing for various diseases
Scientific Astronomical study of atmospheres
Monitoring of chemical reactions

9. Example: Remote open path detection
ReflectorDetector
Immunity to light intensity. This feature is particularly important in open-path remote sensing
application where the amount of light that reaches the photodetector can strongly fluctuate
due to transmission variations or turbulence.
Reason

10. Example: Combustion analytics
Reason
In a combustion process, concentration of target molecules may change rapidly by several
orders of magnitude. This requires sensing methods that provide high dynamic ranges, a key
feature of CLaDS. In contrast to standard techniques that can measure effectively only samples
that absorbs up to approximately 20% of light, CLaDS can be used also for higher
concentrations at which sample becomes nearly opaque

11. Development
CLaDS is a technique that can use different light sources:
• A quantum cascade laser (QCL), a semiconductor injection laser which emits light
within a narrow band of mid-infrared wavelengths. Many gas species strongly
interact within this wavelength region (see below). This allows high sensitivities
to be achieved however at much higher costs.
• A tuneable diode laser operating in the near-IR. These light sources are far
cheaper however weaker signals as they operate in the near-IR.
Currently the system is set up to test single species. Simultaneous multispecies and
heavy molecules are possible with emerging widely tuneable sources (e.g. External –
Cavity QCLs and Optical Parametric Oscillators).

12. How can you help?
We are looking to gain insight into various sectors in order to find the right
place for CLaDS. We’re especially looking for applications exploiting the high
dynamic range, insensitivity to fluctuations in source intensity, sensitivity
and mechanical vibration.
• What applications can you think of and why is CLaDS the ideal solution?
• Is there a capability gap that this technique lends itself to?
• Can it replace existing instruments which lack the features CLaDS can
offer?