Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~2~
The invention relates to a sensor, and more
particularly an intrinsic fibre optic sen~or. One
favoured and more particularly envisaged applicatLon of
this sensor is as a gas sensor. However, the definition
S of the sensor of the invention allows it equally to be
used as, for example, a temperature sensor, or even as a
sensor of any physical or chemical parameter which would
be sensitive to the change in the environment within
which the sensor will be placed.
In particular in the field of opti~al gas
detectors or sensor~, it is currently known that their
advantages allow reduced maintenance and operation in an
oxygen-free atmosphere, their selectivity and their
re~i tance to the corrosive gases having furthermore
contributed to their succe~ in a number of high-
technology applications.
Completely optical apparatuRes, and more par-
ticularly fibre optic sensors, with ~ingle or networked
optical fibres, have in addition the following advan-
tages: intrinsic electrical safety (no requirement for
electrical insulation), possibility of distributed
measurements (combination in series, for example with
continuous sensitivity), insensitivity to electro-
magnetical perturbations, reduced weight and bulk.
With ~uch sensors, the optical detection in
particular of a gas i8 normally performed in an open or
closed cell, through which a light ray of determined
wavelength interacting with the gas to be detected
pas~es, in accordance with the principle of ~pectroscopic
absorption. The difference in intensity between the input
light signal and the output light signal of the cell
gives an indication of the concentration of thi~ gas.
Other research relates finally to fibre optic
chemical sensors which do not use a detection cell. Thus
35 Patent EP-A-0275275 gives an example of a fibre optic
sensor for detecting a physical or chemical parameter, or
alternatively a substance, using the combination of at
least one selective layer disposed on a surface of the
2~&~
fibre and an optical diffraction grating with which this
fibre is provided, the selective layer comprising, in its
composition, an optically conducting organic material
whose optical properties are sensitive to the said
substance or to the said parameter, in order consequently
to modify, by the intermediary of an optical diffraction
grating, the optical properties of a wave propagating in
the fibre.
However, this document EP-A-0275275 gives no
indication a~ to the structure of the fibre; it is simply
specified that it comprises an annular Bragg grating.
Difficulties in implementing this type of sensor
remain, in particular when the fibre is made with a
central part or core forming a waveguide, this core being
surrounded by at least one optically conducting cladding.
In practice, these difficulties are in particular linked
to the low proportion of light normally propagating
outside the core of the fibre (the evanescent field
proportion hitherto often being less than approxima~ely
1%), or to the increased fragility of the fibre when it
is treated so as to have a larger evanescent field
~because of a reduction in the diameter of the fibre, and
of the core in particular).
Furthermore, the selectivity or sensitivity of
existing sensors is not always suitable, and it is often
tricky to vary it.
Against this background, the invention provides
a fibre optic sensor, of the general type of that in
Patent EP-A-0275275, in which the fibre comprise~ a
waveguide core surrounded by an optically conducting
cladding, the selective layer occupying at least one
external portion of the said cladding and surrounding, at
the position of the diffraction grating, an internal
optically neutral portion of this cladding which comes
into contact with the core.
In this way it will be possible to obtain a
sensor ensuring a compromise (which may change~ between
the resilience of the fibre (mechanical strength of the
2 ~
-- 3 --
core) and the sensitivity of the sensor (linked to the
thickness of the selective layer). Alteration of this
selectivity will thus be possible, by adapting it as a
function of the types of detection to be performed
(temperature measurement, measuring the gas content of an
atmosphere, etc.).
The composition of the optical diffraction
grating may be obtained by various means.
It would for example be possible to choose to
make the grating with striae at the optical cladding.
This option would require deposition of a material rather
than making these ctriae optically. Research into this is
in progress.
However, the solution which the invention more
lS particularly provide~ consists in making this grating as
a Bragg grating, by creating it optically, for example by
optical pumping. For this purpose the fibre (and in
particular its core) will be exposed to an interference
field resulting from the mutual interference of two
ultraviolet radiation beams directed simultaneously
towards the fibre with different acute angles of
incidence with re~pect to the longitudinal axis of the
fibre, so that periodic variations in the refractive
index appear, at least in this core. The interference
field will thus have, at the engraved grating, fringes or
striae separated from each other and will be propagated
transversely in the fibre with an alteration (which can
be permanent if the intensity of the radiation is so
adapted~ in the refractive index of the zone of creation
of these striae, in correspondence with the interference
field.
Given that there ha~ recently been interest in
using fibre optical components which allow light to be
in~ected andtor extracted, if necessary selectively tsee
in particular FR-A-2,674,6Z9 or the corresponding
American Application US.87/058009 of March 26 1992), an
additional characteristic of the invention advantageously
provides for these periodic variations in the local
2 ~
refractive index to be engraved in order to be substan-
tially parallel to each other and have a normal which
makes an angle ~ with the axis of the fibre, such that
0 < ~ < 90~.
In this case, in order to make the fibre operate
in reflection, the cladding will then be preferentially
surrounded by a mirror having a reflecting concave
internal surface, directed to the fibre, in order to
return to it the light wave with which it will have been
illuminated.
A more detailed description of the invention will
now be given by way of non-limiting example, and with
reference to the attached accompanying diagrams in which:
Figure 1 is a view in longitudinal section of a
first type of sensor which can be l-sed in accordance with
the invention,
Figure 2 is a view which is comparable with that
of Figure 1 showing a first embodiment,
Figure 3 is, also along a view in section
identical to that in Figure 1, a representation of a
second embodiment,
Figure 4 is one embodiment of the cladding,
solely as a detailed view corresponding to the part
labelled IV in Figure 3,
Figure 5 i5 an enlarged plan view showing one
embodiment of construction of the mirror in particular in
Figure 3 according to the detail V, and
Figure 6 shows one way of optically creating the
striae.
Each of the first three figures therefore
illustrates the principle of an intrinsic fibre optic
~ensor, the fibre being monomode or multimode.
The following description will be lLmited, by way
of example and solely for clarity of explanation, to the
ca~e of a fluid sensor Quch a~ a gas sensor.
Each sensor illustrated comprises, here on the
basis of only one optical fibre, an optical cladding 3,
which is normally transparent, surrounding a transparent
2~?,~'$1.~
- s ~
core 5, forming a waveguide and extending longitudinally
along an axis 7 in order locally to include parts 9
having variations in refractive index (or optical thick-
ness) thus forming an optical diffraction grating.
It will be recalled that the optical thickness ~
of an optically homogeneous (possibly elementary) sheet
of thickness e and of refractive index n is such that:
= n x e.
For such a fibre to constitute an intrinsic
sensor, at least a part of the optically conducting
cladding 3, or even the parts 9 with variation in refrac-
tive index, CQmpriSeS in its composition an organic
material which is active with respect to the substance to
be detected and forms a selective layer whose optical
propertie~ can thus vary as a function of the change in
the parameter or in the substance P (in this case the gas
content), within the environment E in which the sensor is
placed.
In the examples illu~trated, the aforementioned
parts with variation in refractive index are here con-
stituted by zones or striae which are flat and parallel
to each other and periodic. These strlae preferentially
extend into the core 5 and can continue into the
optically neutral part of the cladding.
Such optical diffraction gratings are known in
the literature by the name Bragg Grating. The optical
fibre described in the aforementioned application
FR-A-2674639 is a good example of thi~, the base struc-
ture of this fibre being moreover quite usable within the
scope of the invention.
For any definition relating to these diffraction
5Or scattering) gratings, reference may if necessary be
made to the "Dictionary of Scientific and Technical
Term~", McGraw-Hill, pages 250 and 825 (see "Grating") or
to the "dictionnaire de physique" [Dictionary of Physics]
by E. Levy (Presse universitaire de France), page~ 109
and 685-686.
2 0 ~ 2 ~ "? ~j
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In order to create the striae 9 grating, optical
means are used, starting with a laser source 21 (such as
a laser diode) in order to transmit to the fibre, trans-
versely to its axis, an ultraviolet beam which will be
separated into two waves which will be made to converge
via two mirrors 25, 27, at a certain angle of incidence
(~1 or o2), towards the fibre (see Figure 6), as proposed
(for the grating) in Patent US-4,807,950 or alternatively
in Patent US-4,867,522.
As regards the active organic material, provision
may be in particular for using a heteropolysiloxane of
OH
chemical formula sl_o_ with in particular R being
OR
CH3 or C2H5, this material being able to be deposited in
a thin film (of a few microns).
In order to ensure resilience of the assembly,
the fibre will preferentially furthermore be coated with
one or more mechanical claddings tfor example made of
Kevlar~). So as not to complicate the figures further, no
mechanical cladding has however been represented.
After this overall presentation, we shall now
address ourselves to each figure independently.
It should first of all be noted that in Figure 1
the striae 9 of the Bragg grating are here made from the
said active material so as to be capable of absorbing the
ga~ to be detected whilst having optical characteristics,
more particularly reflective properties, which are
dependent on this gas concentration.
In the application adopted, the active material
con8tituting these striae will advantageously have a
refractive index n~ near to the refractive index nB of the
cladding 3 in the absence of gas, the value of na moving
away from n~ when the gas content in the active material
increases.
The Bragg grating, which is almost transparent in
the absence of gas, will therefore appear after the gas
has been absorbed.
The optical cladding 3 may be made over part of
its thickness from a base of the said active material.
This being so, it may in any case, like the external
protected mechanical cladd.ing, be made of a "porous" or
permeable material which allow~ the gas to be detected
to penetrate laterally into the sensor, and the fibre may
be made with a core and a cladding based on the same
material, the difference in refractive indices existing
between them being obtainable, by structural means, by
doping the base material of the cladding (normally
silica) with a metal oxide (such as germanium oxide GeO2).
In Figure 1 it will be noticed that the striae 9
which are parallel to each other and periodic are
inclined with respect to the axis 7 of the fibre with
their normal parallel to this axis. In other words, the
~triae are therefore perpendicular to the axis 7.
Thus the grating adopted will operate in
reflection.
Its principle of use will be the following,
irrespective of whether a single fibre or a set of
sensors placed end-to-end in series is involved.
Aft~r having manufactured the striae 9 made based
on the said active material embedded periodically in the
fibre, so that these striae can have at least two states
with different optical characteristics in the presence
and absence of gas, there wlll be transmitted axially to
the fibre in question, from one of its ends, an incident
luminous intensity I. having at lea~t one wavelength
corresponding to a resonant wavelength ~r f the optical
grating in order for the latter to reflect light having
the wavelength ~r~
At the same time, the luminous intensity Ir
reflected by the fibre at this wavelength ~r will be
measured.
The luminous intensities Ie and Ir will next be
compared, the ratio between them (in general Ir/Ie) being
representative of the variation in the parameter P
20(~ ?i~
analysed, and therefore in this case of the presence of
the gas to be detected, and even of its concentration,
inaQmuch a-q the reflected intensity Ir will increase with
the gas concentration.
The example of Figure 2 differs from that of
Figure 1 essentially in that here the striae 9 of the
optical grating are disposed with their normal inclined
by an angle ~ with respect to the axis 7 of the fibre,
such that 0 < ~ ~ 90.
In this case therefore, instead of measuring the
reflected luminous intensity, rather the output intensity
I, will be measured, at the opposite end of the fibre,
after the light has passed axially through the sensor in
question over the whole of its length.
Further, in order to be able to perform these
measurements, an incident luminous intensity I~ will be
transmitted axially to this fibre with at least one
predetermined wavelength chosen in order to make the
striae 9 appear as a function of their chromatic
signature, these representing, taking account of the
composition of the optical grating decided upon, quaii-
selective absorption.
In this case of course, the more the gas con-
centration increases in the sensor, the more the
intensity Ia decreases, if the incident intensity I~ is
taken to be constant, the reflective properties of the
striae increa~ing as does the difference between the
refractive indices n, of the latter and n6 cf the cladding
3.
Let us now consider the case of Figure 3, noting
that the optical fibre is here supplemented over at least
a part of the periphery of the cladding by a cylindrical
mirror 11 of circular cross-section whose purpose will be
better understood hereinbelow, the cladding itself here
being l'dividedl' into an internal part 3a, ad~acent to the
core 5 and made of an "inactive" optically neutral
material, that is to say one which is not sensitive to
the gas to be detected (for example made of doped silica)
2~ 3
_ g _
and an external active part 3b (in particular heteropoly-
siloxane) which ~urrounds it, at least at the striae 9,
and whose properties of variation in transparency as a
function of the gas content will essentially be u~ed. sy
S way of example, for a fibre having a core with a diameter
of the order of a few microns, or even a few tens of
microns, the radial thickness of the layer 3a can also be
a few microns, in general les~ than 10 microns. A thick-
ness of a few tenths of microns may even be envisaged.
The thickness of the selective layer 3b can be a few
micron.q .
As regards the mirror, its reflecting concave
internal surface lla which come~ into contact with the
external surface of the selective layer 3b can for
example be obtained by deposition of a thin layer of a
material such as gold, silver or aluminium.
As illustrated in Figure 4, the mirror 11 in its
entirety will advantageously have, at least opposite the
grating 10, a lacunary s~ructure with orifices 13 in
order to allow the substance to be detected to pass
through it. A material will thus be used which is porous
to this substance and has holes 13 whose sizes are
preferably markedly less than the wavelength of the
signal transmitted in the fibre (for example ~/10).
The cladding 3 may also have been made from an
inactive material and surrounded, as in Figure 5, with a
multilayer dielectric mirror 15, which is possibly porous
like the mirror 11 (cee arrows 19 in Figure 4) and the
optical thickness of at least the external layer 17 of
which varies as a function of the gas content of the
environment (E). This being so, the result would have
been comparable, the cladding 3 and the internal layers
of the mirror 15 then acting as the optically neutral
part 3a, and the external layer 17 replacing the
selective layer 3b.
AS for the optical grating, it will have been
noted that its ~triae 9 are, as in Figure 2, oriented so
that their normal makes an angle ~ (0 < Q < 90) with
2a~2~,g.~
-- 10 --
the axis 7 of the fibre, these striae here stopping at
the boundary between the parts 3a and 3b of the cladding.
In order for the sensor to operate efficiently,
its grating will resonate substantially around a deter-
mined and known wavelength ~r~
Once this has been produced, it will be pos~ible
to transmit axially into the fibre(s), still at one end,
an incident luminous intensity I~ having as its wavelength
or as one of its wavelengths the said resonant wavelength
10 ~r -
The luminous flux will then be extracted from the
fibre by the grating and reflected by the mirror 11 (or
15) in order thus to be rein~ected into the fibre in
que~2tion.
When the ga3 i5 present in the active material of
the selective layer, and when the waveguide used corre~-
ponds to the wavelengths of absorption of the gas, the
light pre~2ent in thi~2 material (and therefore in contact
with the gas) will thus be absorbed, causing a drop in
reflected intensity Ir.
Thus, in the presence of ga~, the absorption
intensity will be a function of the length of the grating
10, of the quantity of ga-2 ~ensed, and of the thickness
of the zone containing the active material.
With such a construction, it will in particular
be possible to obtain a selective sensor or a series of
selective sen~ors.
In fact, if the striae 9 are themselves produced
with the said active material and when a polychromatic
light (for example a white light) i~ transmitted to the
said fibre, the grating will then itself select the
resonant wavelength ~s which it will reflect, this wave-
length correspondinq to the absorption line of the sensed
gas, the analy~is being performed, a~ in the case in
Figure 1, by picking up the incident II~) and reflected
(Ir) luminou~ intensities and comparing them.
If conversely the striae of the Bragg grating are
not produced based on the active material, a
2~$2~ ,3 ~j
-- 11 --
monochromatic light covering the resonant wavelength of
this grating as well as the absorption line of the gas to
be detected will be transmitted into this fibre, the
selectivity being, in this case, performed "on input" by
the operator.
In general, as means for measuring the luminous
intensities a photomultiplier, a photoconducting detector
or a photodiode, possibly an avalanche photodiode will be
able to be used. In these four possibilities, the photo-
diode is the one most commonly used. It i5 fitted to mostreflectometer~ used for measuring the reflected luminous
flux (Ir)~ The incident luminous intensity (Ie) is
normally supplied by the manufacturer of the apparatus.
Its value may in any case be verified by one of the
aforementioned measurement means.
Another measurement method could moreover consist
in using a known reflectometric method to measure the
luminous intensity (Ir) reflected by the sensor or series
of sensors. For all information relating to such a
method, reference may be made in particular to the
publication "Opto No. 63 - September/October 1991" relat-
inq to optical fibre grating measurements. The following
publications will also be informative to the reader:
"Principles of Optical Fiber Measurements" (Academic
Press Inc., 111 Fifth Avenue, New York) and "Very High
Optical Return-Loss Measurement using the OTDR Technique"
(Symposium on Optical Fiber Mea~urements, Boulder,
Colorado, September 11-12 1990).
Of course, even if these methods for recording a
reflected luminous flux appear attractive, the solution
consisting in adopting a fibre structure comparable to
those in Figures 3 to 5, with or without a mirror and
with striae perpendicular to the axis of this fibre,
could be selected and does not, in any case, depart in
any way from the scope of the invention.
