f) Photo-Acoustic Sensors for Environmental and Medical Applications
An
increasing
number
of
environmental
problems
has
initiated
the
development
of
sensitive
detec
tors
for
monitoring
pollutants
in
the
atmosphere.
Particularly
sensors
based
on
the
photo-
acoustic
effect
in
conjunction
with
lasers
have
the
potential
of
monitoring
trace
gases
at
con
-
centra
tions
down
to
the
ppt
(parts
per
trillion)
region.
Also
in
medical
diagnostics
photo-acoustic
sensors find successful applications as breath test analyzer.
Photo-Acoustic Effect
Photo-acoustic
detection
utilizes
the
fact,
that
the
excitation
energy
of
light
absorbing
molecules
is
essentially
transferred
into
kinetic
energy
of
the
surrounding
molecules
via
inelastic
collisions.
This
causes
a
local
pressure
increase
in
the
absorbing
gas.
If
the
excitation
source
is
modulated,
a
sound
wave
is
generated,
that
can
be
detected
by
a
mi
-
crophone.
This
signal
is
di
-
rectly
proportional
to
the
con
centration
of
the
absorb
-
ing
molecules
in
the
sample.
Therefore
a
calibrated
set-up
allows
to
measure
directly
the
absolute
concentration
of a gas.
Photo-acoustic
spectroscopy
(PAS)
has
the
advantage
of
pro
ducing
a
signal
only
when
light
is
absorbed.
Hence,
contrary
to
transmission
spectroscopy,
PAS
is
an
offset-free
technique,
and
it
is
possible
to
replace
relatively
long
absorption
cells
by
much
smaller
ones.
In
addition,
acoustic
cell
resonances
can
successfully
be
used
to
further
enhance
the
signal
and
there
-
by to further increase the detec
tion sensitivity.
The
PAS
technique
is
primarily
used
in
con
-
junction
with
high
power
gas
lasers,
since
the
photo-acou
stic
signal
is
proportional
to
the
intensity
of
the
light.
However,
the
disadvan
-
tage
of
these
lasers
is
often
their
non-tuna
bi
-
lity
and
therefore
the
re
quirement
of
a
co
in
-
cidence
with
an
absorp
tion
line.
Usually,
the
modulation
of
these
lasers
is
per
formed
with
a
mechanical
chopper,
which
may
cause
coher
-
ence
noise
by
the
rotating
blade
and
an
additional
photo-acoustic
signal
by
absorption
of
the
cell windows. This deteriorates the sensiti
vity of a photo-acoustic detector.
Most
of
these
disadvantages
can
be
avoided
by
a
photo-acoustic
spectrometer,
which
is
based
on a single frequency diode laser for the selective excitation of the trace gases.
In
the
meantime
the
available
emission
wavelength
of
these
lasers
covers
the
entire
near
IR
and
mid
IR
spectral
range
which
is
important
for
trace
gas
detection,
since
most
molecules
have
their strong fundamental vibration absorption bands in this region.
Detection of Toxic Industrial Gases
An
example
for
high
selective
and
sensible
detection
of
a
toxic
gas
is
a
photo-acoustic
sensor
for
hydrofluoric
acid
(HF).
This
gas
is
widely
used
in
industrial
processes
as
a
chemical
intermediary
for
purification,
cleaning,
and
synthesis.
It
is
also
released
to
the
atmosphere
at
garbage
incinerators,
at
power
plants
or
in
the
metal
and
chemical
industry.
Since
HF
is
highly
toxic,
already
smallest
con
centrations
must
be
monitored
to
register
any
leakage
in
a
gas
pipe
system
or
closed
compart
ment
and
to
give
alarm
at
dangerous
concentration
levels.
HF
has
a
Threshold
Limit
Value
of
3
ppm
(by
volume)
and
is
generally
measured
with
relatively
expensive
sensors
like chromatographs, ion mo
bility spectrometers or electro-chemical cells.
A
sensor
based
on
a
room
temperature
distributed
feed¬back
(DFB)
diode
laser
operating
on
a
wavelength
of
2.476
μm
in
combination
with
a
photo-acoustic
resonance
cell
represents
a
much
cheaper,
more
compact
and
much
more
sensitive
detector
allowing
measurements
of
HF
con
-
cen
trations of a few ppb.
13
C-Breath-Test-Analyzer
Medical
breath
tests
are
well
established
diagnostic
tools,
predominantly
for
gastroenterological
in-spections,
but
also
for
many
other
examinations.
Since
the
composition
and
concentration
of
ex
haled
volatile
gases
reflect
the
physical
condition
of
a
patient,
a
breath
analysis
al
-
lows
to
recog
nize
an
infectious
disease
in
an organ or even to identify a tumor.
One
of
the
most
prominent
breath
tests
is
the
13
C-urea-breath
test,
applied
to
ascer
-
tain
the
pres
ence
of
the
bacterium
helico
-
bacter
pylori
in
the
stomach
wall
as
an
indi
-
cation of a gastric ul
cer.
For
such
a
breath
test
a
patient
orally
re
-
ceives
isotope-marked
urea,
where
the
most
common
isotope
12
C
is
replaced
to
a
large
extent
by
the
isotope
13
C
(natural
abundance
is
1.1
%).
In
the
stomach
the
substrate
is
metabolized
to
iso
tope
marked
carbon
dioxide
which
in
the
presence
of
helicobacter
bacteria
is
absorbed
by
the blood and finally released in the patient’s breath.
For
a
reliable
diagnosis
already
changes
of
the
13
CO
2
concentration
of
1%
have
to
be
detected
at
a
concentration
level
of
this
isotope
in
the
breath
of
about
400
ppm.
The
total
CO
2
con
centration
in the exhaled breath is about 4%.
Generally
this
concentration
is
determined
by
a
highly
sophisticated
and
expensive
gas
ana
ly
-
zer
like
a
differential
mass
spectrometer
or
a
specially
adapted
optical
gas
detector.
An
at
-
tractive
and
rela
tive
cheap
alternative
is
a
photo-acoustic
ana
lyzer
operating
with
an
am
-
plitude
modulated
se
micon
ductor
laser
on
a
wavelength of 2.744 µm.
The
concentration
ratio
of
the
CO
2
isotopo
-
logues
is
determined
by
measuring
the
integral
absorption
over
a
13
CO
2
line
in
comparison
to
a
12
CO
2
line.
In
a
specially
selected
spectral
range
the
lines
have
similar
strengths,
although
the
concentrations
dif
fer
by
a
factor
of
90.
Therefore,
the
signals
are
well
comparable.
The
laser
wavelength
is
accu
rately
controlled
and
tuned
across
the
absorption
lines
by
changing
the
laser
temperature.
Since
it
is
much
easier
to
monitor
any
variations
of
the
isotopic
abundance
with
respect
to
a
refer
ence
than
to
detect
an
absolute
concentration,
the
laser
radiation
is
split
into
two
beams
which
then
pass
through
two
identical
photo-acoustic
cells
containing
different
breath
samples,
one taken before (used as reference) and one after capturing the isotope-marked substrate.
Such
a
set-up
gives
excellent
signal-to-
noise
ratios
and
allows
to
resolve
isotopic
concentration
dif
ferences of 1‰ or even less, which is quite sufficient for a reliable breath test analysis.
Doctoral Theses
L. Pfeifer
Photoakustische Untersuchungen am TNT in der Dampfphase
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1992
M. Wolff
Photoakustischer Nachweis von Spurengasen - Realisierung kompakter Gassensoren
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 1997
M. Germer
Photoakustischer Stickstoffmonoxid-Detektor auf Basis eines Quantenkaskadenlasers
School of Electrical Engineering, Helmut-Schmidt-Uiversity, Hamburg 2010
Refereed Publications in Journals and Conference Digests
M. Wolff, H. Harde
Photoacoustic Spectroscopy with a Frequency-Modulated DFB Diode Laser
Conference on Lasers and Electro-Optics, Vol. 11, 1997 OSA Technical Digest Series (Optical Society of
America), Washington, D.C.,1997, p. 518 (1997)
M. Wolff, H. Harde
Photoacoustic Spectrometer Based on a DFB-Diode Laser
Infrared Physics and Technology, 41 (5), pp. 283-286 (2000)
M. Wolff, H. Harde
Photoacoustic Spectrometer Based on a Planckian Radiator with Fast Time Response
Infrared Physics and Technology 44, pp. 51-55 (2003)
M. Wolff, H. Harde, H. Groninga
Photoacoustic Sensor for Medical Applications
Photonics East – Smart Medical and Biomedical Sensor Technology and Applications, Providence/RI,
Proceedings of SPIE, Vol. 5261, pp. 5261–09 (2003)
M. Wolff, H. Groninga, H. Harde
Photoakustischer Sensor für Medizinische Diagnostik
Innovationsforum Photonik, Tagungsband, Goslar (2003)
M. Wolff, H.G. Groninga, H. Harde,
Photoacoustic Distributed Feedback Laser Spectroscopy on Hydrogen Fluoride,
Appl. Spectrosc., 58 (6), pp. 552-554 (2004)
M. Wolff, H. Harde, H.G. Groninga
Isotope-Selective Sensor based on PAS for Medical Diagnostics
13th International Conference on Photoacoustic and Photothermal Phenomena (ICPPP), 13P-15,
Rio de Janeiro/Brazil (2004)
M. Wolff, H. Groninga, H. Harde
Photoacoustic Sensor for Medical Diagnostics
Conference Digest of Conference on Bio-Medical Optics 2004, Rio de Janeiro (2004)
M. Wolff, H.G. Groninga, M. Dressler, H. Harde
Photoacoustic Sensor for VOC’s: First Step towards a Lung Cancer Breath Test
European Conference on Biomedical Optics (ECBO), Munich/Germany, 12.-16. September 2005,
pp. 110-112 (2005)
M. Wolff, H.G. Groninga, M. Dressler, H. Harde
Photoacoustic Sensor for VOC’s: First Step towards a Lung Cancer Breath Test
Proc. SPIE, Vol. 5862, pp. 5862-16 (2005)
M. Wolff, H.G. Groninga, H. Harde
Resonance Investigations using a Photoacoustic Multi-Microphone System
Gordon Research Conference on Photoacoustic & Photothermal Phenomena (GRC PPP), A26,
Trieste/Italy (2005)
M. Wolff, H. Groninga, H. Harde, B. Baumann, B. Kost
Resonance Investigations using PAS and FEM
Forum Acusticum, Budapest, FA-7760 (2005)
M. Wolff, H. Groninga, B. Baumann, B. Kost, H. Harde
Resonance Investigations using PAS and FEM
Acta Acustica, Vol. 91, Suppl. 1, 99 (2005)
M. Wolff, H. Harde, H.G. Groninga
Isotope-Selective Sensor Based on PAS for Medical Diagnostics
J. Phys. IV France, 125, 773-775 (2005)
M. Wolff, H. Groninga, H. Harde
Photoacoustic CO
2
Detection at 2.7 μm
Photonics Europe – Optical Sensing II, Strasburg, 5. April 2006, Proceedings of SPIE, Vol. 6189,
pp. 220-225 (2006)
M. Wolff, H.G. Groninga, H. Harde
Isotope-Selective CO2 Detection at 2.7 µm
Proceedings of the 9th International Conference on Infrared Sensors & Systems Nürnberg (2006)
H. Harde, J. Pfuhl, M. Wolff, H. Groninga
MolExplorer: A New Tool for Computation and Display of Spectra from the HITRAN Database
Ninth HITRAN Conference, Harvard University, Cambridge MA, USA, 25.-28. Juni 2006
http://cfa-www.harvard.edu/hitran/HITRAN_conf06_presentations/Session5/5.5-Harde.pdf
M. Wolff, M. Germer, H. Groninga, H. Harde
Photo-Acoustic CO
2
Sensor based on a DFB Diode Laser at 2.7 µm
14th International Conference on Photoacoustic and Photothermal Phenomena (ICPPP), P12.2, 185, Cairo/Egypt,
Technical Digest and Journal de Physique IV (2007)
M. Wolff, M. Germer, H.G. Groninga, H. Harde
Photo-Acoustic CO
2
Sensor based on a DFB Diode Laser at 2.7 µm
Eur. Phys. J. Special Topics, Vol. 153 (1), 409-413 (2008)
H. Harde, M. Dressler, G. Helmrich, M. Wolff, H. Groninga
New Optical Analyzer for
13
C-Breath Test
Proc. SPIE 6991, Biophotonics: Photonic Solutions for Better Health Care, 69910T (2 May 2008); Strasburg,
France, doi: 10.1117/12.781271; https://doi.org/10.1117/12.781271
M. Dressler, G. Helmrich, Hermann Harde
Photoacoustic HF-Sensor
Proc. SPIE 7003, Optical Sensors 2008, Strasburg, 70031F (28 April 2008);
doi: 10.1117/12.781679 (2008)
H. Harde, G. Helmrich, M. Wolff
Opto-Acoustic
13
C-Breath Test Analyzer
BiOS, Proc. SPIE 7564, Photons Plus Ultrasound: Imaging and Sensing 2010, 75641E (24 Feb. 2010),
San Francisco, https://doi.org/10.1117/12.841660
Contributions on National Conferences and Meetings
L. Pfeifer, H. Harde
Photoakustische Untersuchungen an Explosivstoffen in der Dampfphase
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachausschuss Kurzzeitphysik, Hannover,
27. März 1992, Verhandl. DPG (VI) 27, K 10.2, 1278 (1992)
M. Wolff, U. Selbach, H. Harde
Isotopenselektiver Nachweis von CO
2
für medizinische Anwendungen mit Hilfe der photoakustischen
Spektroskopie
Poster Lasertag 1992, Hannover, 2. Dezember 1992
M. Wolff, U. Selbach, H. Harde
Isotopenselektiver Nachweis von CO
2
für medizinische Anwendungen mit Hilfe der photoakustischen
Spektroskopie
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Arbeitsgemeinschaft Quantenoptik, Berlin,
17. März 1993, Verhandl. DPG (VI) 28, Q 31.5, 429 (1993)
M. Wolff, H. Harde
Photoakustische Spektroskopie am HF mit Hilfe eines Halbleiterlasers
Poster Lasertag 1995, Hannover, 6. Dezember 1995
M. Wolff, H. Harde
Spektroskopische Untersuchungen an der ersten Oberschwingung von HF
Frühjahrstagung der DPG, Sektion Quantenoptik, Jena, 14. März 1996,
Verhandl. DPG (VI) 31, Q 28.3, 255 (1996)
M. Wolff, H. Harde
Photoakustischer Sensor für Halogen-Wasserstoffe
Poster Lasertag 1996, Hannover, 11. Dezember 1996
M. Wolff, H. Harde
Photoakustische Spektoskopie mit Hilfe einer frequenzmodulierten DFB-Laserdiode
Frühjahrstagung der DPG, Arbeitsgemeinschaft Quantenoptik, Mainz, 3. März 1997,
Verhandl. DPG (VI) 32, Q 12.7, 318 (1997)
M. Dreßler, M. Wolff, H. Groninga, H. Harde
Photoakustischer Sensor für medizinische Anwendungen
2. Hamburger Studententag zur Medizin- u. Biotechnologie 2005
M. Germer, M. Wolff, H.G. Groninga, H. Harde
Photoakustische NO-Detektion mittels gepulstem Quantenkaskadenlaser
Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Sektion Quantenoptik,
Verhandl. (DPG) (VI) 43, Q-Laser (2008)
Patents
M. Wolff, H. Harde, Vorrichtung und Verfahren zur Messung der Konzentration eines Gases in einem
Gasgemisch,
DE 44 46 723 (1994)
M. Wolff, H. Harde, Gasdetektor zur Messung der Konzentration eines Gases in einem Gasgemisch,
DE 196 38 761 (1996)
M. Wolff, H. Harde, H. Groninga, Verfahren zur Messung von Gaskonzentrationen oder dem Verhältnis von
Gaskonzentrationen mit potentiellen Anwendungen in der Atem-Analyse, DE 103 08 409 (2003)
Physics & Climate