METHOD OF DETECTING MOLECULES AND OPTICAL SENSOR
The invention relates to a method for detecting mole
cules of a particular substance as well as to an op
tical sensor designed for performing this method.
The document EP 2 270 478 Al describes an optical
sensor comprising an optical path, a light source for
generating light and for feeding this light into the
optical path, and a photo detector for detecting an
optical signal coupled out of the first optical path,
wherein the optical path comprises, amongst others,
an optical resonator covered at least partially with
an active layer of a covering material for selec
tively adsorbing molecules of a particular kind. A
measurement implemented with this sensor makes use o f
an influence of adsorbed molecules on an optical
length of the resonator which has the consequence
that resonance frequencies of the resonator shift,
which in turn can be detected by detecting the light
coupled out of the optical path. Thus, it is possible
to detect whether a fluid which is brought into con
tact with the active layer contains molecules of the
particular kind.
Even if very selective materials are used for the ac
tive layer, this layer will adsorb not only molecules
of exactly one substance but rather molecules of a
whole group of substance. This applies in particular
for substances which are chemically similar. For this
reason, ambiguities cannot be avoided in the state of
the art mentioned above so that it remains uncertain
whether the detected molecules really are of a par
ticular substance to be detected or only chemically
similar .
It is, therefore, an objective of the present inven
tion to suggest a more precise method and a corre
sponding sensor for detecting molecules of a particu
lar substance avoiding ambiguities. Furthermore, it
should be able to perform the method and to perform a
corresponding measurement using with little effort
and in a short time.
According to the present invention, this objective i
achieved by a method according to claim 1 and by a
sensor according to any of the claims 6 or 16. Advan
tageous embodiments of the invention can be realized
with the features of the dependent claims.
The suggested method for detecting molecules of a
particular substance by means of an optical sensor
comprises
- bringing the sensor into contact with a fluid to be
analyzed,
- coupling light of a first wavelength into an opti
cal resonator of the sensor, the resonator being cov
ered at least partially with an active layer of a
covering material for selectively adsorbing a group
of substances comprising the substance to be de
tected,
- coupling light of a second wavelength into the same
resonator or into a second optical resonator of the
sensor, the second resonator being covered at least
partially with an active layer of the same covering
material ,
- detecting, for each of the first and the second
wavelengths, an optical signal coupled out of an op
tical path containing the respective resonator,
- varying, for each of the first and the second wave
lengths, an optical length of the respective resona
tor or the respective wavelength such that an inter
val comprising at least one resonance o f the respec
tive resonator is scanned,
- detecting, for each of the first and the second
wavelengths, a broadening of this resonance indicat
ing an absorption of the light fed into the respec
tive resonator by molecules accumulated at the active
layer of this resonator.
Detecting the broadening of a resonance means, in
this context, detecting or measuring a width of the
corresponding resonance and comparing this width with
a reference value. Preferably, the reference value is
chosen to be the width of the same resonance in a
situation when the sensor is not in contact with and,
thus, not influenced by the fluid to be analyzed. It
may, e.g. be defined as the width of the respective
resonance measured when the sensor is brought into
contact with a reference fluid like, for example, wa
ter or with air. The broadening of the different
resonances may be detected in that the same measure
ments are preformed in addition before bringing the
sensor into contact with the fluid to be analyzed or
once more after removing the fluid to be analyzed
from the sensor and/or after bringing the sensor into
contact with the reference fluid.
Any usual measure may be used for defining the width
of the respective resonance. The width may, e.g., be
defined as full width at half maximum of or as a difference
between two wavelengths separated by a reso
nance wavelength of the respective resonance and cor
responding to inflection points or points of maximum
slope of two edges of this resonance.
In this context, the term fluid may denote any analyte
. In particular, this term may denote a liquid or
a gas .
By this method, information is obtained not only on
whether molecules of any substance contained in the
fluid have been adsorbed by the active layer or lay
ers or not. In addition, information about a light
absorbing behaviour of these molecules is obtained
for two different wavelengths as the first wavelength
and the second wavelength are different. This helps
to decide of which substance - amongst the group of
substances for which the active layer acts as a se
lective adsorber - the adsorbed molecules are. Of
course the method can be generalized by performing
the same analysis for at least one further wave
length .
In particular, it is possible, after performing the
steps described above, to decide whether the fluid
contains molecules of the substance to be detected
depending on whether a broadening of the resonance -
or a broadening exceeding a certain threshold - can
be detected only for the first wavelength or only for
the second wavelength or for both wavelengths. The
method may, thus, further comprise identifying
whether the fluid contains molecules of the substance
to be detected in a significant concentration depend
ing on whether a broadening exceeding a certain
threshold is detected only for the first wavelength
or only for the second wavelength or for both wave
lengths. To this end, the first and the second wave
length can be selected to be typical absorption wave
lengths of one or more but not all of the substances
of the group of substances.
A difference between the first and the second wave
length will usually be chosen at least one order of
magnitude larger - typically several orders of magni
tude larger - than a spacing between adjacent reso
nances of the resonator or of each of the resonators
at the respective wavelength. In this context, an or
der of magnitude is to be understood as a factor of
ten .
In a preferred embodiment of the method, a shift of
the resonance is determined, for at least one of the
first and the second wavelengths, in addition to the
broadening, the shift indicating a change of the op
tical length of the respective resonator caused by
the molecules accumulated at the active layer of this
resonator. This makes the analysis more precise as
the shift indicates that molecules have been adsorbed
by the active layer and can even be taken as a meas
ure of an amount of the adsorbed substance so that
the broadening can be evaluated taking into account
the amount. In particular, the afore -mentioned
thresholds for the broadening of the resonances ap
plied for deciding whether one or another of the sub
stances is contained in the fluid can be defined de
pending on the shift of one or the other of the reso
nances .
The broadening of the resonance can, in each case,
easily be detected by determining, within said inter
val, a derivative of the optical signal with respect
to the wavelength and by determining a spacing between
two extrema of the derivative at the resonance,
the spacing being a measure of the broadening. More
precisely, the two extrema will be two maxima of an
absolute value of the derivative. To this end, the
derivative can be determined by means of a lock- in
amplifier by modulating the optical length of the
resonator or the wavelength with a modulation signal,
feeding an output of a photo detector used for de
tecting the optical signal into the lock- in ampli
fier, and using the modulation signal as a reference
signal for the lock-in amplifier.
In order to detect molecules of a particular sub
stance as described here above, two similar optical
sensors are suggested, both realising the same idea.
In a first embodiment of the invention, the optical
sensor for detecting molecules of a particular sub
stance comprises
- a first optical path, a first light source for gen
erating light of a first wavelength, and a first
photo detector, the first light source being opti
cally coupled to the first optical path for feeding
the light of the first wavelength into the first op
tical path, the first photo detector being optically
coupled to the first optical path for detecting an
optical signal coupled out of the first optical path,
- the sensor further comprising a second optical
path, a second light source for generating light of a
second wavelength, and a second photo detector, the
second light source being optically coupled to the
second optical path for feeding the light of the sec
ond wavelength into the second optical path, the sec
ond photo detector being optically coupled to the
second optical path for detecting an optical signal
coupled out of the second optical path.
Each of the first and the second optical paths com
prises an optical resonator covered at least partially
with an active layer of a covering material
for selectively adsorbing a group of substances com
prising the substance to be detected, the covering
material being the same for the resonator of the
first optical path and the resonator of the second
optical path. The resonators of both optical paths
and/or the first and the second light sources are
tunable for varying an optical length of the resona
tors and/or the first and the second wavelengths, the
sensor further comprising a control unit for control -
ling the light sources and/or the resonators, the
control unit being configured for varying the optical
lengths of the resonators and/or the first and the
second wavelengths such that an interval comprising
at least one resonance of the respective resonator is
scanned.
The first and the second wavelengths are different.
Typically, a difference between the first wavelength
and the second wavelength is at least one order of
magnitude - preferably several orders of magnitudes -
larger than a spacing between adjacent resonances of
each of the resonators at the respective wavelength.
In each case, the interval scanned by varying the
first and the second wavelengths or equivalently the
wavelength interval scanned by shifting said resonance
by varying the optical length of the respective
resonator is understood to be at least one order of
magnitude smaller than the difference between the
first and the second wavelength. This also applies to
the second embodiment described here below. So, even
if the first and the second wavelengths are varied
they can be clearly distinguished from each other as
to different wavelengths or wavelength intervals.
In a second embodiment of the invention, the optical
sensor for detecting molecules of a particular sub
stance comprises an optical path, at least one light
source for generating light of a first wavelength and
of a second wavelength, and a photo detector, the at
least one light source being optically coupled to the
optical path for coupling the light of the first
wavelength and of the second wavelength into the same
optical path, the photo detector being optically cou
pled to the optical path for detecting an optical
signal coupled out of the optical path. In this case
too, the optical path comprises an optical resonator
covered at least partially with an active layer of a
covering material for selectively adsorbing a group
of substances comprising the substance to be de
tected. Again, a difference between the first and the
second wavelengths is typically at least one order of
magnitude or even several orders of magnitude larger
than a spacing between adjacent resonances of the
resonator at each of the first and the second wave¬
lengths. The resonator and/or the at least one light
source are tunable for varying an optical length of
the resonator and/or the first and the second wave
lengths, the sensor further comprising a control
unit, the control unit being configured
- for controlling the at least one light source such
that the light of the first wavelength and the light
of the second wavelength are successively fed into
the optical path,
- for varying the optical length of the resonator
and/or the first and the second wavelengths such
that, for each of the first and the second wave
lengths, an interval comprising at least on resonance
of the resonator is scanned,
- and for detecting, for each of the first and the
second wavelengths, a width and/or a broadening of
this resonance.
In both embodiments, the sensor can advantageously be
used for performing the detecting method described
above. At the same time it has a rather simply struc
ture and can be realized in a compact and robust
for .
To this end, the sensor or at least parts of it - in
particular the optical path or the optical paths in
cluding the resonator or resonators - can be realized
on a chip as a so called integrated optical circuit.
The optical path or each of the first and the second
optical paths may comprise one or two optical
waveguides for coupling the resonators of the optical
path or of the first and the second optical paths to
the respective light source and to the respective
photo detector in order to make sure that the sensor
is compact and robust. The waveguides can be designed
as photonic wires.
Each of the resonators can preferably be realized as
a ring resonator, in particular as a so called microring
resonator. The ring resonators can be coupled
to the respective waveguide or waveguides by evanes
cent fields. They are particularly well suited as
they show a very high sensitivity for molecules accu
mulated at a surface. This means that their optical
length depends very sensitively on an amount of molecules
adsorbed by the active layer. However, other
types of optical resonators may be used too instead
of ring resonators, for example Fabry- Perot resona
tors .
The sensor may, in addition, have a channel for con
ducting the fluid to be analyzed to the active layer
of the resonator of each optical path. The at least
one light source or the first and the second light
sources are preferably chosen as lasers which are appropriate
for their monochromatic light.
The sensor may comprise a signal processing unit for
analyzing an output of the first and the second photo
detectors or, in the second embodiment, the output of
the only photo detector. This signal processing unit
may be comprised by the control unit. The signal
processing unit can be configured for determining,
within the scanned intervals, a derivative of the op
tical signal with respect to the wavelength. To this
end, the control unit can be configured for modulat
ing, for each of the first and the second optical
paths or for the only optical path, the optical
length of the respective resonator or the respective
wavelength with a modulation signal. In this case,
the signal processing unit may comprise, for determining
said derivative, a lock- in amplifier, the con
trol unit being connected to the lock- in amplifier
for feeding the modulation signal as a reference sig
nal into the lock- in amplifier. The derivative is
hereby obtained as an output of the lock- in amplifier.
It is equivalent, of course, whether the light
source or the optical length of the resonator is
modulated and whether the light source or the resona
tor is varied in order to scan the interval around
the first and the second wavelengths. The most practical
solution will be to tune the light source for
scanning said interval and to modulate the respective
resonator - e.g. electro-optically or thermooptically
- for determining the derivative.
Determining the derivative of the respective optical
signal with respect to the wavelength is desirable
for easier determining a measure of the width and,
thus, of the broadening of the respective resonance.
In preferred embodiments the signal processing unit
of the sensor is configured for determining, for each
of the two wavelengths or for each of the resonators,
a measure of a width and/or of a broadening of the
resonance of the respective resonator comprised by
the scanned interval, If the derivative is determined
as described above, the signal processing unit can be
configured for doing so by determining a spacing be
tween two extrema of the derivative at the resonance,
the spacing being the measure of the width and/or the
broadening in this case.
In order to get more information about the substances
contained in the fluid to be detected and in order to
achieve more precise results, the signal processing
unit can be further configured for determining, for
at least one of the resonators, a shift of the reso
nance comprised by the scanned interval.
It is possible that the optical path or each of the
optical paths comprises at least one further optical
resonator covered at least partially with an active
layer of a further covering material for selectively
adsorbing molecules, the further covering material
being different from the aforementioned covering ma
terial or containing the same substance but in a different
concentration. Depending on how the selec
tively adsorbing covering materials are chosen, this
enables the sensor to be used for simultaneously de
tecting a presence or absence of molecules of differ
ent substances and/or for an even more precise detection
of molecules of the particular substance looked
for. In this case it is essential that the resonators
- i.e. their optical length - can be modulated in or
der to identify the resonances that can be attributed
to a particular resonator.
In the embodiment with the first and the second opti
cal paths, the sensor may optionally comprise at
least one further optical path, a further light
source for generating light of a further wavelength,
and a further photo detector, the further light
source being optically coupled to the further optical
path for feeding the light of the further wavelength
into the further optical path, the further photo de
tector being optically coupled to the further optical
path for detecting an optical signal coupled out of
the further optical path. In this case, the at least
one further optical path, too, may comprise an opti
cal resonator covered at least partially with an ac
tive layer of the same covering material as used for
the resonators of the first and the second optical
paths, the resonator of the further optical path
and/or the further light source being tunable for
varying an optical length of this resonator and/or
the further wavelength, the control unit also being
configured for varying the optical length of this
resonator and/or the further wavelength such that an
interval comprising at least one resonance of the
resonator of the further optical path is scanned.
Hereby, ambiguities, which are caused by the fact
that the active layers are not selective enough, can
be further reduced.
The covering materials used for the active layers of
the sensors described here may be, for example, mo
lecular imprinted polymers. The ambiguities which are
reduced by the suggested method and sensors are due
to the fact that not only molecules of one particular
substance but also similar substances which have,
e.g., certain structures in common with the substance
to be detected, may be adsorbed by these active layers
.
A resonance of the respective optical resonator is
broader at a wavelength at which the molecules accu
mulated at the resonator show a higher absorption
rate. Thus, additional information about the absorb
ing behaviour of the adsorbed molecules for at least
two different wavelengths is obtained by the method
and the sensors described here. This information
helps to reduce the afore-mentioned ambiguities as
some of the substances of the group of substances
which may be adsorbed by the active layer can be exeluded
if this substance has a high absorption rate
at the first or the second wavelength and if no
broadening of a resonance can be seen at this par
ticular wavelength.
Exemplary embodiments of the invention are explained
hereafter with reference to Figs. 1 to 7 .
Fig. 1 is a schematic top view of an optical sensor
in a first embodiment, this sensor com
prising two or more optical paths with sev
eral ring resonators.
Fig. 2 is a diagram showing, in a schematic way,
typical transmission spectra of three dif
ferent substances,
Fig. 3 is a diagram showing, in a schematic way, a
transmission spectrum of an optical path
comprising a ring resonator as contained in
the optical sensor of Fig. 1 ,
is a diagram showing, in a schematic way,
an output of one of several lock- in ampli
fiers contained in a signal processing unit
of the sensor of Fig. 1 , this output being
plotted for two different cases as a func
tion of a wavelength in a neighbourhood of
a resonance of one of the ring resonators,
is a table illustrating different possible
results obtained in a measurement performed
with the sensor of Fig. 1 ,
Fig. 6 is a schematic top view of an optical sen
sor in a second embodiment, and
Fig. 7 is a schematic top view of an optical sen
sor in a further embodiment only slightly
different to the embodiment of Fig. 6 .
Fig. 1 shows an optical sensor for analyzing fluids
and for detecting molecules of one or several par
ticular substances in a fluid to be analyzed. The
most important components of this sensor are realized
in planar technology on a chip 1 and form an inte
grated optical circuit. This integrated optical cir
cuit has a first optical path comprising an optical
waveguide 2 , an optical ring resonator 3 and a fur
ther optical ring resonator 4 as well as a second optical
path comprising a waveguide 2', an optical ring
resonator 3 and a further optical ring resonator 4 .
All these ring resonators 3 , 3', 4 , and 4 ' are micro
rings of a diameter of between 10 m and 200 m and
are realized, as the waveguides 2 , and 2', as
photonic wires. They are coupled to waveguide 2 or 2
of the respective optical path via evanescent fields.
A first light source 5 is optically coupled to the
waveguide 2 for feeding light of a first wavelength
l into the first optical path. In the same way, a
second light source 5 ' is coupled to the waveguide 2 '
for feeding light of a second wavelength l 2 into the
second optical path. Both light sources 5 and 5 ' are
tunable lasers so that the two wavelengths l and l2
can be varied to a certain extent.
At an opposite end of the two optical paths, the
waveguide 2 is optically coupled to a first photo de
tector 6 for detecting an optical signal coupled out
of the first optical path while the waveguide 2 ' is
optically coupled to a second photo detector 6 for
detecting an optical signal coupled out of the second
optical path. In this case, the photo detectors 6 and
6 ' are realized as phododiodes on the chip 1 .
Each of the resonators 3 and 3 is covered with an
active layer of a covering material for selectivelyadsorbing
molecules of a group of substances compris
ing a particular substance to be detected. The cover
ing material is the same for the resonators 3 and 3 '
and may be, e.g., an MIP. Similarly, the further
resonators 4 and 4 ' are covered with an active layer
of another covering material for selectively adsorb
ing molecules of another group of substances compris
ing the same or another substance to be detected,
this covering material being the same for the further
resonators 4 and but different from the covering
material of the resonators 3 and 3 '. The active lay
ers are visualized by shadings. An optical length of
each of the resonators 3 , 3', 4 , and 4 ' can be modulated
electro-optically or thermo-optically by means
of electrodes 7 .
As indicated in Fig. 1 by dashed lines, the sensor
may comprise a further optical path of the same
structure as the first and the second optical paths,
a further light source for generating light of a fur
ther wavelength l3, for feeding the light of the fur
ther wavelength l3 into the further optical path and
a further photo detector for detecting an optical
signal coupled out of the further optical path. In
this case, the further optical path comprises an op
tical resonator and a further optical resonator too,
each of them being covered with an active layer of
the same covering material as used for the resonators
3, 3 or the further resonators 4 , 4 ' of the first or
the second optical path, respectively. Furthermore,
the further light source is tunable as the first and
the second light sources 5 and 5 ', and the resonator
and the further resonator of the further optical path
can be modulated together with the resonators 3 and
3 ' and the further resonators 4 and 4 ' of the first
and the second optical paths.
The sensor comprises a control unit 8 for controlling
the light sources 5 and 5 ' and the resonators 3 and
3 ' as well as the further resonators 4 and 4 '. The
control unit 8 is configured for varying the optical
lengths of the resonators 3 and 3 ' by a modulation
signal of a frequency f i and to correspondingly modu
late the further resonators 4 and 4 ' by a modulation
signal of a different frequency f2 - Where applicable,
the same applies for the resonator and the further
resonator of the further optical path. Furthermore,
the control unit 8 is configured for varying the
first wavelength and the second wavelength l -
and such that an interval comprising at least one
resonance of the respective resonator 3 or 3 and of
the respective further resonator 4 or 4 ' is scanned.
Where applicable, the control unit 8 is in the same
way configured for additionally varying the further
wavelength l3 such that at least one resonance of
each of the resonator and the further resonator of
the further optical path are scanned.
It should be noted that each of the optical paths
might have a different waveguide for optically cou
pling the resonators 3 or 3 ' and 4 or 4 to the re
spective photo detector 6 or 6 ' respectively. In this
case, the waveguides 2 and 2 ' would only be used for
coupling them to the respective light source 5 or 5 '
respectively.
On top of the chip 1 , a microf luidic channel 9 is
provided for conducting the fluid to be analyzed tot
he active layers of the different resonators 3 , 3',
4 , and 4 '.
For analyzing an output of the first photo detector 6
and the second photo detector 6 ' and where applicable
of the further photo detector, the sensor comprises a
signal processing unit 10 with lock-in amplifiers 11
and an evaluation unit 12. The signal processing unit
10 is configured for determining, for each of the
resonators 3 , 3 ', 4 , and 4 ', a measure of a broaden
ing of the resonance of the respective resonator 3 ,
, 3 or 4 ' comprised by the respective wavelength
interval which is scanned by tuning the light sources
5 and 5 . To this end, the signal processing unit 10
is configured for determining, within each of the
scanned intervals, a derivative of the respective op
tical signal with respect to the wavelength. This is
done by means of the respective lock- in amplifier 11,
the control unit 8 being connected to the lock- in am
plifier 11 for feeding one of the modulation signals
as a reference signal into the lock-in amplifier. The
modulation signal of the frequency f i is used as reference
signal if the resonances of the resonators 3
and 3 ' are to be investigated, while the frequency f2
is chosen for the reference signal for investigating
the resonances of the further resonators 4 and 4 .
Depending on whether the reference signal is chosen
to have the frequency f or f2, the respective lockin
amplifier 11 filters a contribution of the resona
tor 3 or 3 ' or of the further resonator 4 or 4 ' out
of the respective optical signal. An output of the
respective lock- in amplifier 11 corresponds to the
derivative of this contribution to the optical signal
with respect to the wavelength.
At each resonance of the respective resonator 3 , 3',
' or 4 , an absolute value of this derivative shows
two maxima. The evaluation unit 12 is configured for
determining a spacing between these two maxima, this
spacing being a measure of a broadening of this reso
nance. In addition, the evaluation unit 12 is config
ured for determining, for each of the resonators 3 ,
3', 4 , and 4', a shift of the resonance comprised by
the scanned interval.
Hereafter, an analysis of the fluid conducted by the
channel 9 using the resonators 3 and 3 ' is described.
In the same way, the further resonators 4 and 4 ' can
be used for an additional analysis of this fluid in
order to get additional or more precise information
about what kind of substances are contained in the
fluid.
Fig. 2 shows, as an example, a transmission spectrum
I of a first substance, a transmission spectrum II of
a second substance II and a transmission spectrum III
of a third substance. We assume that these three sub
stances form the aforementioned group of substances
preferentially adsorbed by the active layers of the
resonators 3 and 3'. Reflecting a typical situation,
the first substance shows a high absorption at the
two wavelengths l i and l while the second substance
shows a high absorption only at the first wavelength
l and the third substance only at the second wave
length l2 . A difference between these two wavelengths
li and l2 may be something like 300 nm while a spac
ing between adjacent resonances of the ring resona
tors 3 , 3', 4 , and 4 ' is about two orders of magnitude
smaller and has a value of about 2 nm. Fig. 3
shows a transmission spectrum of one of the optical
paths. Some of the resonances of the respective reso
nator 3 or 3 ' - a contribution of the corresponding
further resonator 4 or 4 ' being neglected for sim
plicity - can clearly be seen in this diagram. The
light sources 5 and 5 ' are chosen and tuned to pro
duce light of the absorption wavelength or l2 re
spectively and to vary the respective wavelength
slightly so that a small interval comprising one of
the resonances of the respective resonator 3 or 3 ' is
scanned.
Fig. 4 shows an output of the lock- in amplifier 11
within the scanned wavelength interval. We assume
that the reference signal is chosen to have the frequency
f used for modulating the resonators 3 and 3 '
in this case. As explained above, the output of the
amplifier 11 corresponds to a wavelength derivative
of a contribution of the resonator 3 or 3 ' respec
tively to the optical signal coupled out of the respective
optical path. A solid line shows the output
in a situation when no molecules absorbing light of
the respective wavelength li or l are accumulated at
the active layer of the respective ring resonator. A
dotted line shows a corresponding signal after accumulating
molecules at the active layer which do ab
sorb light of the respective wavelength l or l2 . The
accumulation of light absorbing molecules results in,
both, a shift and a broadening of the resonance, the
enlarged spacing Dl ' between the two maxima of the
output shown in Fig. 4 - compared to the spacing Dl
before the accumulation of absorbing molecules - be
ing a measure of this broadening. The evaluation unit
12 is configured to detect the shift indicating that
a certain amount of molecules have been adsorbed by
the active layer and the enlarged spacing Dl ' indi
cating to what degree these molecules have an absorbing
behaviour for light of the respective wavelength
li or l2 .
By conducting the fluid to be analyzed through the
channel 9 , this fluid is brought into contact with
the resonators 3 , 3', 4 , and 4 ' and in particular
with the active layers thereon. If the fluid contains
any of the three substances mentioned above, mole
cules of the respective substance will be adsorbed by
and accumulated on the active layers of the resona
tors 3 and 3 '. A measurement of the shift of the
resonances caused by this accumulation alone indi
cates that any of the substances of said group of
substances is contained in the fluid. It does, however,
not answer the question yet whether the sub
stance contained in the fluid is - in our example -
the first, the second or the third substance. This
question, however, may be answered using the result
of the detection of the broadening of the resonances.
This is illustrated in the table of Fig. 5 . This ta
ble shows for the three substances mentioned above in
the context of Fig. 2 and for the two wavelengths l 1
and l whether a resonance at the respective wave
length l or l 2 will be broadened or not if the resonators
3 and 3 have been in contact with the respec
tive substance, an X indicating a broadening in each
case. If a broadening can be seen only for l , it can
be concluded that the fluid contains the second sub
stance. If a broadening can be seen only for l2, the
fluid contains the third substance. If a broadening
is detected for both wavelengths li and l , the fluid
contains the first substance or, both, the second and
the third substance.
In order to detect the broadening of the different
resonances, the same measurements are preformed not
only after bringing the fluid into contact with the
resonators 3 , 3', 4 , and 4', but in addition before
bringing the sensor into contact with this fluid or
once more after removing the fluid to be analyzed
from the sensor or after bringing the sensor into
contact with a reference fluid like, e.g., clean wa
ter or air. The respective broadening may then be de
fined as Dl '- Dl , wherein Dl is the width of the re
spective resonance measured when the sensor is not in
contact with the fluid to be analysed and Dl ' is the
width of the same resonance measured after bringing
the sensor into contact with this fluid.
The evaluation unit 12 is configured to perform this
analysis by a method for pattern recognition after
determining the shifts and broadenings of the reso
nances contained in the scanned intervals .
The Figs. 6 and 7 show two similar optical sensors.
The features explained above in the context the sen
sor shown in Fig. 1 are marked with the same refer
ence signs. The only difference between the sensor of
Fig. 6 and the sensor of Fig. 1 is that the sensor of
Fig. 6 has only one optical path. By means of a coupier
13, light of both wavelengths l and l2 gener
ated by the two light sources 5 and 5 ' can be fed
into the waveguide 2 of this optical path. The con
trol unit 8 is, in this embodiment, configured for
controlling the two light sources 5 and 5 ' such that
the light of the first wavelength l and the light of
the second wavelength l2 are successively fed into
the waveguide 2 via the coupler 13. Thus, the method
for analyzing the fluid described above can be per
formed analogously with this sensor.
The sensor of the embodiment shown in Fig. 7 differs
from the example shown in Fig. 6 only by the fact
that this sensor has only one light source 5 which
is, in this case, tunable over a range which is large
enough to cover both wavelengths and l so that no
second light source is needed to perform the method
described above. The signal processing unit 10 may of
course, in the embodiment of Fig. 7 as well as in the
embodiments of Fig. 1 and Fig. 6 , be comprised by or
being understood as part of the control unit 8 .
Claims
A method for detecting molecules of a particular
substance by means of an optical sensor, the
method comprising,
- bringing the sensor into contact with a fluid
to be analyzed,
- coupling light of a first wavelength (l i ) into
an optical resonator (3) of the sensor, the
resonator (3) being covered at least partially
with an active layer of a covering material for
selectively adsorbing a group of substances com
prising the substance to be detected,
- coupling light of a second wavelength (l2)
into the same resonator (3) or into a second op
tical resonator (3') of the sensor, the second
resonator (3') being covered at least partially
with an active layer of the same covering mate
rial ,
- detecting, for each of the first and the sec
ond wavelengths (l l2), an optical signal cou
pled out of an optical path containing the re
spective resonator (3, 3'),
- varying, for each of the first and the second
wavelengths (l , l ), an optical length of the
respective resonator (3, 3') or the respective
wavelength (l , l2) such that an interval com
prising at least one resonance of the respective
resonator (3, 3') is scanned,
- detecting, for each of the first and the sec
ond wavelengths (l , l2), a broadening of this
resonance indicating an absorption of the light
fed into the respective resonator (3, 3') by
molecules accumulated at the active layer of
this resonator (3, 3').
The method of claim 1 , characterised in that the
method further comprises identifying whether the
fluid contains molecules of the substance to be
detected in a significant concentration depend
ing on whether a broadening exceeding a certain
threshold is detected only for the first wave
length (l ) or only for the second wavelength
(l2) or for both wavelengths (li , l2).
The method of any of the claims 1 or 2, charac
terized in that the broadening of the resonance
is detected by determining, within said inter
val, a derivative of the optical signal with re
spect to the wavelength and by determining a
spacing (Dl , Dl ' ) between two extrema of the de
rivative at the resonance, the spacing being a
measure of the broadening.
The method of claim 3 , characterized in that the
derivative is determined by means of a lock- in
amplifier (11) by
- modulating the optical length of the resonator
(3, 3') or the wavelength (l , l2) with a modu
lation signal,
- feeding an output of a photo detector (6, 6')
used for detecting the optical signal into the
lock-in amplifier (11) , and
- using the modulation signal as a reference
signal for the lock-in amplifier (11) .
The method of any of the claims 1 to , charac
terized in that, for at least one of the first
and the second wavelengths (l i , l2), a shift of
the resonance is determined in addition to the
broadening, the shift indicating a change of the
optical length of the respective resonator (3,
3 ) caused by the molecules accumulated at the
active layer of this resonator (3, 3').
An optical sensor for detecting molecules of a
particular substance,
the sensor comprising a first optical path, a
first light source (5) for generating light of a
first wavelength (l ), and a first photo detec
tor (6) , the first light source (5) being opti
cally coupled to the first optical path for
feeding the light of the first wavelength (l )
into the first optical path, the first photo de
tector (6) being optically coupled to the first
optical path for detecting an optical signal
coupled out of the first optical path,
the sensor further comprising a second optical
path, a second light source (5') for generating
light of a second wavelength (l2), and a second
photo detector (6 ) , the second light source
(5') being optically coupled to the second opti
cal path for feeding the light of the second
wavelength (l2) into the second optical path,
the second photo detector (6') being optically
coupled to the second optical path for detecting
an optical signal coupled out of the second op
tical path,
wherein each of the first and the second optical
paths comprises an optical resonator (3, 3')
covered at least partially with an active layer
of a covering material for selectively adsorbing
a group of substances comprising the substance
to be detected, the covering material being the
same for the resonator (3) of the first optical
path and the resonator (3') of the second opti
cal path,
and wherein the resonators (3, 3') of both opti
cal paths and/or the first and the second light
sources (6, 6') are tunable for varying an opti
cal length of the resonators (3, 3') and/or the
first and the second wavelengths (l l2),
the sensor further comprising a control unit (8)
for controlling the light sources (5, 5') and/or
the resonators (3, 3'), the control unit (8) be
ing configured for varying the optical lengths
of the resonators (3, 3') and/or the first and
the second wavelengths (l l2) such that an in
terval comprising at least one resonance of the
respective resonator (3, 3') is scanned.
The sensor of claim 6 , characterized in that a
difference between the first wavelength (l ) and
the second wavelength (l2) is at least one order
of magnitude larger than a spacing between adja
cent resonances of each of the resonators (3,
3') at the respective wavelength (l c , l2).
The sensor of any of the claims 6 or , charac
terized in that each of the first and the second
optical paths comprises one or two optical
waveguides (2, 2') for coupling the resonators
(3, 3') of the first and the second optical
paths to the respective light source (5, 5') and
to the respective photo detector (6, 6').
The sensor of any of the claims 6 to 8, charac
terized in that the resonators (3, 3') are ring
resonators .
The sensor of any of the claims 6 to 9 , charac
terized in that it comprises a signal processing
unit (10) for analyzing an output of the first
and the second photo detectors (6, 6'), the sig
nal processing unit (10) being configured for
determining, for each of the resonators (3, 3'),
a measure of a width and/or of a broadening of
the resonance of the respective resonator (3,
3') comprised by said interval.
The sensor of claim 10, characterized in that
the signal processing unit (10) is configured
for determining, within said interval, a deriva
tive of the optical signal with respect to the
wavelength and by determining a spacing between
two extrema of the derivative at the resonance,
the spacing being the measure of the width
and/ or the broadening .
The sensor of claim 11, characterized in that
the control unit (8) is configured for modulat
ing, for each of the first and the second opti
cal paths, the optical length of the respective
resonator (3, 3') or the respective wavelength
, l2) with a modulation signal, and in that
the signal processing unit (10) , for determining
said derivative, comprises a lock-in amplifier
(11) , the control unit (8) being connected to
the lock- in amplifier (11) for feeding the modulation
signal as a reference signal into the
lock-in amplifier (11) .
The sensor of any of the claims 10 to 12, char
acterized in that the signal processing unit
(10) is further configured for determining, for
at least one of the resonators (3, 3'), a shift
of the resonance comprised by said interval .
The sensor of any of the claims 6 to 13, charac
terized in that
it comprises at least one further optical path,
a further light source for generating light of a
further wavelength (l ), and a further photo de
tector, the further light source being optically
coupled to the further optical path for feeding
the light of the further wavelength (l3) into
the further optical path, the further photo de
tector being optically coupled to the further
optical path for detecting an optical signal
coupled out of the further optical path,
wherein also the at least one further optical
path comprises an optical resonator covered at
least partially with an active layer of the same
covering material,
wherein the resonator of the further optical
path and/or the further light source are tunable
for varying an optical length of this resonator
and/or the further wavelength (l3),
and wherein the control unit (8) is further con
figured for varying the optical length of this
resonator and/or the further wavelength (l3)
such that an interval comprising at least one
resonance of the resonator of the further opti
cal path is scanned.
The sensor of any of the claims 6 to 14, charac
terized in that each of the optical paths com
prises at least one further optical resonator
(4, 4') covered at least partially with an ac
tive layer of a further covering material for
selectively adsorbing molecules, the further
covering material being different from the
aforementioned covering material or having a
different concentration.
An optical sensor for detecting molecules of a
particular substance, the sensor comprising
an optical path, at least one light source (5,
5') for generating light of a first wavelength
( and of a second wavelength (l2), and a
photo detector (6) , the at least one light
source (5, 5') being optically coupled to the
optical path for coupling the light of the first
wavelength (l ) and of the second wavelength
(l2) into the same optical path, the photo de
tector (6) being optically coupled to the opti
cal path for detecting an optical signal coupled
out of the optical path,
wherein the optical path comprises an optical
resonator (3) covered at least partially with an
active layer of a covering material for selec
tively adsorbing a group of substances compris
ing the substance to be detected,
wherein the resonator (3) and/or the at least
one light source (5, 5') are tunable for varyi
an optical length of the resonator (3) and/or
the first and the second wavelengths ( l , l 2 ) ,
the sensor further comprising a control unit
(8) , the control (8) unit being configured
- for controlling the at least one light source
(5, 5') such that the light of the first wave
length ( l i ) and the light of the second wave
length ( l 2 ) are successively fed into the opti
cal path,
- for varying the optical length of the resona
tor (3) and/or the first and the second wave
lengths ( l 1 l 2 ) such that, for each of the
first and the second wavelengths ( l , l 2 ) , an
interval comprising at least on resonance of the
resonator (3) is scanned,
- and for detecting, for each of the first and
the second wavelengths ( l i , l ) , a width and/or
a broadening of this resonance .