Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02368970 2001-10-31
REFRACTOMETER WITH BLAZEh l~~tAGC~ c3RRTINGB
pESCRI~'TIaN
T~'CH~IICAT.~ F'IELcD
The present invention re~.ates to a re~ractometer, that is to
say a system for measuring refractive indices.
1.0 It is applicable especially to measuring the .refyacti~re
index of a liquid or of a gas or of any other product or
chemical compound which is ct~rltact with a waveguide, in
pa.xt~.cular deposited on this waveguide. The latter may for
examp~,e be an optical. fibre.
1~
The refractometer comprises one or more Bragg grating
transducers formed orr such a waveguzde.
DESCRIPTION O~ T~iE PRIOR ART
~0
A Bragg grating, photo--inscribed in an optical fibre, is a
periodic strur_ture formed by modulating the refractive index
of the corm of the f ~.k~re _
This structure behaves in practice like a. m~.rro.r for a very
narrow spectral band around a characteristic wave l.encsth 7~,H
(wavelength for wh:i.ch Chere is phase matching between the
multiple ref~.ections within the grating) and remains
transparent fox all other wavelengths. This is because,
since the multiple waves ref7.ected at these other
wa~relengths are not in phase, they interfere destructively
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and axe therefore transmitted because of the conservation of
energy.
'fhe characteristic wavelength, called the "~ragg
wavelength° , is def:~r~.ed by the equation 7~$ _ 2 x n,~f~ x
where A i.s the pitch of the Bragg gra.t~.x~g {about 0.5 ~Ixn for
a standard grating? arid n,~EE is the effective index of the
fundamental guided mode incident on the grating.
Long Period Fibre Bragg Gratings {T.,PFG) are also formed by a
periodic modulation of the refractive index of the core of a
generally monornode optical fibre. Hawetrer, the value of the
' period A of this modulation is then typically greater than
10 0 ~,m .
When light from a broadband source is injected inCO a fibre
containing such a grating, a number of resonant bands are
observed, with widths at half maximum Hrhich are much greater
than that o~ a conventional Bragg grating (several
nanometres instead cf a few hundred picometres). Each of
these resonant bands corresponds to a coupling between the
guided light wave incident on the grat~.r:g azld. a mode called
"claddi.rlg mode" which is coprdpagati.ng (also called
cod.irect~.onal), this mode being propagated in the same
direction as the incident wave.
'The energy coi~tained i.n these modes decreases rapidly during
propagation through the fibre, because of the high losses at
r.he interface between. the opt~.cal cladding and the coating
protecting this fibre.
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- ~ _ _
Since the coupling takes place to the codirectTOnal modes,
the resonant bands appeax only in the form of absarpt.ion
bands on the transmission spectrum such that no energy is
obser~cred in reflection.
The wavelengths for which the phenomenon of coup a.ing to the
cladding modes occur$ deperi.d on the period 11 of the long-
period grating, on the amplitude of the photo-induced
1D modulation, denoted An, and on the opto-geometrical
characteristics of the optical fibre. They are given by the
condition called phase matching, as follo~r~rs:
(301 - ~ma~ = ~ ( ~. l
1.5 where ~3~1 and (3~~,,~~ represent the propagation constants of the
fundamental. guided mode arid o~ a c~.adding made,
xesgectively. This equation can be rewritten substituting
the etfectz.ve indices of the modes:
a F ~ 2 . 7s ria ~ ~ ._., ~.~.._.n ~. 2
Brat ~ ~t71 y ~ gra.t glad ,~
~grat = ( 1l~ T ~ '_ xlø~. d ~ . It
where ~.,~r~r denotes the centra:L wavelength of the resor~ar~t
band.
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_.
The Bragg gratings, called "blazed, tilted or slanted fibre
Bragg gratings t' , result from a photo-~,nduced modulation of
the index, the period of which is also about 0.5 J.un.
However, this, modulation has the specific feature o~ being
bla2ed, with respect ~o the longitudinal axis of the optical
fibre, by an angle 8 wha.ch is called the "blaze angle".
This pEriodiaity and the blaze of the index mcdulation
constitute the two key parameters making it possible to
explain the very particular spectral response of these
components arid the considerable differences between the
latter and the ronveritional. Bra.gg gratings togethex w~.th the
long period Bragg gratings.
1. 5
Figure l schematically represents a, blazed 8x~agg gratirzg ~
inscribed in the core 4 of an optical fibre 6, the optical
cladding of which has the xefersnce 7. A guided mode 8
incident on the grating can be coup7.~d either to a d~.screte
set of cladding modes 14 which are counter-propagating, or
to what is called a continuum of radiative modes 12 or both
to these cladding rrtQdes arid this continuum of radiati~cre
modes.
The discretization of the coupling to the counter-
propagating cladding modes ~.s conditioned by the finite
Grarlsverse dimexzsions of the optical fibre cladding. From
the spectx-al point of ~criew, the result thereof is
succession of resonant bands which have widths at half
3o maximum similar to those of a standard Bragg grating (width
at half maximum oL about '~00 pm) and are typically spaced
apart by about one nariometre_
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.. ~ ..
These resonant bands are present over a narrow spectral
range (a few tens of nanometres) which depends on the blaze
angle and on the opto-geometrical characteristics of the
fibre and of the grating (modulation period and amplitude).
Coupling to the radiative moc'les_ can only take place if the
cladding of the optical fibre is very la.~ge compared to the
wavelength.
This configuration can be si.mu3.ated by using an index-
matching liquid which a.s deposited aro~znd the fibre arid the
refractive index of which :~s virtually identical: to that of
the optical_cladding.
Figure 2 shows a transmission spectrum of a blazed Bragg
crating, which is 8 mm long with a blaze angle of 16°, when.
this grating is imair with a, refractive index next of of 1 - 0
(curve ~) and when it is in an index matching liguid for
which the vriluc of n~Xt is x..43 at 1550 nm (curve I~) . The
wavelength ~, (in nm) is plotted. on the x-axis and the
normalized . transmission TL~T o.n the y-axis. Where cot.~p~.ing is
to the cladding modes, coupling to the families of modes
called LPo" arid LPlrt is mainly observed.
~'or the blazed gratings, the phase-matching condition giving
2S the value of the various resor~.ant wavelengths takes the
form:
a'grat - ~ Ilp ~ ~ ';" ~a1 d
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- ~ ~ ._
where J~.~r.~r. denotes a resonant wavelength, A the modulata.on
period, 8 Ghe blaze angle, n01~ and n~lad rh~ effective
inde~c of the guided mode and the effective index of a
cladding mode, respectively. The ~- symbol. arises from the
S fact that counter-propagating modes rather than
codirectional modes are involved.
Tntrinsi,c optical fibre sensors (flu's?, sensors for which one
or more opti.cal~ properties of the fibre depend directly. for
examples, on chemical andlor biochemioa~. phezzomena which it
is desired to deterzr~ine, axe cox~.sidered below. The optical.
fibre then constiC.utes the transducer element of the sensor.
Tx~ particular, evanescent-wave intrinsic sensors and surface
1S plasmon sensors are known.
pevices usa.ng standard Bragg gra.ta.ngs which are photo-
irlsaribed in the monomode optical fibres for the purpose of
applicarions to refxactomeGry, are also known.
Furthermore, refractome~.ry systems which use .long period
Bragg gratings are known. For such gra~,ings, the rESOnant
wavelength associated w~.zh a given c~.adding mode depends on
the refractive index of the medium which is located beyond
the opti.eal. cladding of the fibre i,n which these gratings
are formed. Any change in this refractive index results in a
shift of the resonant wavelength.
The known sensors ox systems, ment~.oned above, have
drawbacks.
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With regard to evanescent-wave sensors, the Following wall
mainly be noted:
the ageing and the deG~rioration of the sensitive pa.~t
of such sensors, for example formed by the mediating
agent deposited on the optical fibre of these sensors,
which necessitates frequent recalibrations thereof;
the diffi.cul.ty in developing methods to compensate far
the degradation in the performance of these sensors,
the intensity measurement on which the use of the
latter is based at~d which is therefore lens hive to any
intensity fluctuation of the associated l~.ght source
and to modification in the conditions for injecting the
l~.ght into the fibre, hence a deterioration ire the
resolution and in the accuracy of measurements, and
25 the need for mechanically or chemically removing ~.he
cladding Pram the Qptical fibre in order to have
sufficient access to the evanescent fiEld, which is a
comp~.ax operation, is difficult to control and which
weakens the optical fibre.
Among the drawbacks of surface plasmon sensors, let us
mention:
the d.iff.iculty in forming all-fibre miniaturized
systems since systems using such sensors generally
employ bulky components around azz architecture which is
difficult to convert into an industrial system, and the
need to control perfectly the profile lma.inly th.e
thickness) of the metal layer used in such sensors and
the attachment of this layer.
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Among the drawbacks o~ de~cr~.ces using standard Bragg
gratings, there are problems similar to those presented lay
the evanescent wave sensors, namely:
the need to chemically ox mechanically attack the
cladding of the optical fibre at the measur~.n,g Bragg
grating,.
a selectivity pxoblern, since the Bragg peak is
sensitive to parameters other tha~z the inde~c o~ the
1.0 exterr~al medium (for example temperature and strainl,
which requires the use of compensation arid correction
techniques empl.oyirlc~, for example, reference sensors,
the relaCive weakness of the final measuring head,
the difficulty in producing the transducer, requiring
the optical cladding to be attacked, and r~lati~crely low
sensitivity.
With regard to refractometxy systems ueirlg the long period
8ragg gratings, the main drawbacks are as follocvrs:
great sensitivity of the long period grating resonant
to parameters other than the refractive index (for
example temperature and deformations), hence the reed
to use compensat~.on and correction techniques,
high non-linearity of the transducer sensiti~rity,
extremely limited multiplexing capacities since a very
sensitive sensor monopolises a ~.arge spectxa7. range, o~
at least 100 nm, and
large width of the resonant band, making it difficult
to determine the peak of the latter accurately.
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s~Y of SHE zrrv~r~zoN
The aim of the present inventir~n is to cwercome the
abovementioned drawbacks.
The abject of the ~.nvention is a system for measuring the
refractive index of at least one medium, this system being
characteri2c~d iri that it comprises:
a waveguide comprising at least one transducer farmed,
in the part of the wavegui.de brought into coxxtact with
the medium, by a blazed Bragg grating, she spectral
response of which depends on the refractive index of
the medium by rnee~ns of ex~ergy oomplirig becweeri the
guided made and cladding modes andlor a continuum of
radiative modes,
a light source optically coupled to the waveguide in
order to direct. this light the.r_ei,n and ~.o make it
interact witkz the grat~.ng,
spectral analysis means provided to analyse the light
2a which has interacted with the grating and to pro~ride a
spectrum corresponding to this grating,
acciuisit~.on means provided to recover this spectrum,
and
electronic processing means provided to correlate, from
~5 the spectrum thus recovered, Ghe spectral response of
. the grating with a value of the retracti~re index of the
medium arid to provide this value.
30 According to a first preferred embodiment of the system
which is the subject of the invention, the electronic
processing mear~~ are pxovi.ded in order- to determa.ne the
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lower and upper envelope curves of the normalized spectrum
and the normalized area between these two curves.
The waveguide, fvr example an optical fibre, may comprise a
s~.ng~.e blazed Bxagg' grating or, iri contrast, a plurality of
such gratings. ~n the latter case, the spectral ar~alysis
means are provided in oxder to analyse the light whych has
interacted with the gratir~gs and to provide the spectra
corresponding respectively to these gratings; the
acquisition means are provided in order to demultiplex, in
an optical: or digiral manner, the spectxa thus prov~.ded and
to discriminate the respective spectral responses of the
gratlxlgs and the electronic prvcessi.rxg means are provided in
order to cc~rreLate the spectral response of each grating
with the value of the refractive index of the medium
corresponding to 0'115 grating .
In all cases, the light souxoe may be a broadband source.
However, it i.s also possible to use a narrow spectrum
2~ source, the waver ength of which can be tuned, and the
spectral analys~.s means may then comprise a single
photodetector.
According to a first particular embodiment of the system
which i.s the subject of the irwent~.on, the light source is
optically coupled to a first end o~ the waveguide and the
spectx~a.l axzal.ysi.s means are optioal~.y co~.7.pled to a socond
end of this waveguide, for the purpose o~ measuring the
refractive index by transmisszon.
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According ro a second particular embodiment, the lz.ght
source and the spectral az~.~.lysis means are opt~.cally coupled
to a first end off' the wa~creguide and means of reflecting the
light axe prav~.~led at the second end of the wavegui.de, for
the purpose of measuring the refractive index by ref7.ectian.
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BRIEF DESCR~pTIt7N OF T~3E DR.AWI~GS
The present ~.nvention will be better understood on reading
the description of e~~mpJ.ax-y embodiments given below, purely
by way o~ example arad in no way limiting, with reference to
the appended drawings in which:
Figure 1 is a schematic view c~f a blazed Bragg grating
and has already been described,
Figuz~e 2 shows a transmission spectrum of a blazed
8ragg gratir~g and has already been described" and
Figures 3 to 6 are schematic °criews of various
particular embodiments of the system for measuring
refractive indices, which is the subject of the
inver~tion .
pFTAIIrED SUMMARY O~' PARTIGUTrAR ~:MHUDIMFNTS
First of all, let us cbnsider the transducers used izz the
present invention to measure refractive indioes, that is to
2Q say blazed Bxagg gratings, for example photo-inscribed i.n
the core of optical fzbres, and let us start by sGUdying the
spectral sensitivity of, such a grating with any modification
of the refractive index by an external medium with which the
waveguide comprising this grating ~.s in contact.
Let us therefore consider an optical fibre, or any c~tner
waveguide, in wh~.ch a b~.azed B;ragg gratyng has been
inscribed. This grating may have been formEd according to
any one o~ the known photo-inscript~.on ~rtethods, for example
the "phase mask" or "Lloyd mirror" techniques.
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... 1. ~ .~ _
In the rest of the present description, the numerical values
are given only by way of illustration and are not lirr~iting
i.n an~r case . They relate to a monomode optical, Fibre having
the following characteristics: core arid cladding indices
having the values of 1.62 and 1.457, respectively, at
1550 nm, core and cladding radii having the values of
4.125 dim and 62.5 Vim, respecti.vezy.
When light i.s injected into such a waveguide, it interacts
with t;he blazed grating. Tt is then coupled to a number of
cladding modes. This coupling only takes place far incident
wavelengths which comply r~r'i~h. a cond~.tion called phase
matching between the guided mode and any ane of the c~.adding
~.5 modes .
This condition is only complied wa.th by a discrete number of
wavelengths, which resuJ.ts from the ~xistence of discrete
resonant bands,
The location and the amplitude of these various spectral
resonants depend not on~.y 01~ the r~pto-~geometr~.cr.~1 parameters
of the guide tespecia~.ly indices and dimensions of the core
and of the optical cladding) but a~.so on the refractive
~5 index of the exter~.al medium, a Fnedium which surrounds the
optical cladding of the guide.
When this refractive index is modified, the various resonant
bands shift spectra~.ly and change amplitude.
Let us take the case o~ a grating havixlg a. blaze ang7.e 8 0~
15°. When the retractive index n~Xt of the external medium
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14 " _.
changes from 2.0 (index of air? to 1.3, the spectral
resonant bands shift towards long wavelengths, on average by
COQ pm, without sign~.f~.ca.ric change in their attenuation.
In contrast, when r~.~;xr. goes from 1. 3 to 1. ~3 , a phenomenon of
progressive disappearance of the resonant bands is observed
together with a s light spectral shift, until obtaining a
perfectly smooth arr.d cont~.nuous loss spectrum.
Figure 2 already described shows the spectrum of such a
gracing in air and in a med~.um of index 1.43.
The phsnoanenon mentioned above may be explained ae follows.
With each resonant wavelength ~,i it is possible to assacia,te
a cladding mode of effective z.ndex nzff, ~ which dECreases
wi rh ~.;, ,
Whex~ the xe~ractive index o~ the e~cLernal medium increases
until reaching the value n~=f,i, the cladding mode is
progressively less guided because of the decrease in the
ove~~.ap integral between the guided core mode and this
cladding mode. The result of this is a reduction in the
amplitude of the correseondi.ng resonant band.
When rl~Xr is egual to n~xf.i, the cladding mode is no longer
gu~.ded; hovrew~er, coupling takes plane with the continuum of
radiative modes.
In the present invention, ~.n order to profit from this
phenomenon, an analysis technique is used which consists in
determining the lower envelope E~ o~ the riormal~.zed loss
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_ 15 - _
spectrum of the blazed Bragg transducer grating (passing
through the maxima of the spectrum) and the upper envelope
E" of the same spectrum (passing through the Minima of the
spectrum) then the r~vrmalized area A between these twc~
envelopes .
'fhe determination of the envelopes takes place, for example,
through the deCe:~mination of the troughs and peaks of the
various resonant bands or, which is equ~.valent, through
1Q determining the minima and maxima of the transmission
spectrum.
These minima and maxima can be located by a direct method of
detecting extremes yr using a dex-ivation operation, leading
to a derived cur-cre, thewdetecting the zeros of this curve.
k'inally, the lower enveJ.vpe is obtained by interpo7.ation of
the set of maxima, tt~r example using spline furtctiox~s .
The upper envelope i.s also obtained by interpolation, using
such functions, of the set of the minima.
Trfotead of measuring the variation ixi the refractive index
of the external medium in the form of a shift in wavelength
of a resonant band, the change in the normalized area A is
~5 followed, which is defined as follows:
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CA 02368970 2001-10-31
_ ~s _ ._
~'~x ~xa! ~ ~~.at ~,
a ~~~) ~l ( ) d,~.
~i~
Where su (~,) and s1 (~.) are respectively the upper and lr~wer
envelopes of the ndrmali.zed loss spectrum of the blazed
~ragg transducer grating, ~,~in and ~."~X axe the limits of the
spectral window comprising all the spectral resonances of
the grstix~g (here, 1495 nrn and 157 nm respect~.vely).
r:y"' and ~iY~~ are two envelopes which are taken as a referer~ce
to and which correspond to the blazed grating spectrum placed
in an external medium of refractive index beyond which only
a spectra. shift can be observed there, nr,;f = n~xr = 1.296) .
When the refractive index Qf the external medium increases
beyond 1.3, the progressi~re smoothing of the spectrum is
equivalent to progressively bx-inging the two emcrelopes
together and, consequent~.y, to a decrease in the norm«lized
araa A.
2~ The benefit o~ the definition of A above is to make the
measurement independent of any fluctuation in intensity of
the eoux~Ce which em~.es the light injected ~.x~to the
wavegu~.de_ This is important for any industrial application
of the in.~rent ion .
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Let us specify that the resohution and the repeatability of
the measurements made with the blazed Bragg gratings and the
analysis technique described above have a value of ab4ut 10
',
Zn the invention, at least one blazed 8ragg grating is
therefore used in order to measure the refrac~.ive index ndxc
of a medium iri contact with the optica2 fibre in which this
grating is photo-inscribed. The sensitivity of such a
grating to the refractive index of the medium resu3.ts in a
progressive smoothing of the set o~ resonant bands present
i.n the transmission spectrum where next increases - The method
of analysing this spectrum may consist in following the
change in the area between the envelope passTrig through the
minima of the resonant bands and the envelope passing
through the maxima of these bands. Tt is thus possible to
parry out measuremexits with a resolution an,d a repeatability
of about 10-5. Mc~~'eover, it is possible to adapt the
dynamics of mEasuremen~s by altering the blaze angle 0. A
va.~.ue of about 16° for the latter makes it possible to cover
the ref~'acciw'e index range from 1.32 to 1.42 (values given
for a r~ravclengt~h of 1550 nm) .
In the foregoing, the transmission spectra of blazed
gratings have been used in order to carry out ~efractometry.
:Ln fact, it i.~ possib7-a to work in reflection. In ordex to
do th~.s, a mirrox' sending the light back in the reverse
direction is placed at the end of the fibre.
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_ 18 - _
~n this case, the l~.ght, which is propagated through the
core of the fibxe, interacts twice with the transducer
grating. The resulting spectrum, which can be observed at
the input by means of an optical Coupler, corresponds simply
to the square of the transmission spectrum.
Tk~e analysis method exp~.ained above for operating in
transmission is strictly identical when operating in
reflectipn, except that all the processing is carried out on
the square o~ the transmission spectrum.
Next, examples of a system for measurirsg the refractive
index according to the invention, which uses at least one
blazed 8ragg grating operating in transmiss~.on, will be
considered. It is necessary to obtain the spectra from this
transducer grating. Given the width of the resonant bands
and their spectxaJ. spacing, ~.t ~.e therefore necessary to
obtain there spectra with suff~.caenL resolution if it is
desired to optimize the resolution of the refractive index
measurements,
In order to be able to detect index variations of about 10-
5, it is necessary to acquire the spectra with wavelength
pitches of a.bQUt 10 picametres. With lower-resolution
spectra (for example with p~.tches of a few tens of
p~.cometres) the reso~.ution of the measurements would not be
as good.
3Q
Let us specify that the spectral range that it is cl.es~.red to
analyse may go from a few nanometres to several tens of
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namometres. This mainly depends on the dynamics of
measurement that it is desired to obtain.
A fsrst example of the refractive index measurement system,
rahich is the subject of- the invention, is schematically
shown iri Figure 3 and comprises an optical ffibre ~.~1 on which
a blazed Hragg grating 15 is formed constituting a
transducer.
14 The protective cladding of the fibre is not shown but it has
been, remo~red ovex the portion of fibre where the grating 16
is farmed. This portion of fibre is placed a.n the medium,
the re~.rrxctive i.r~dex o~ which it is desired to measure and
which is symbolized by the curve 18.
The system also comprises a broad spectxum apticai source
~0, the fight of which is injected inr.o one end of the
optical f~.bre . 'this source may be a~.:1 ~ilare ox ~,ot . when it
is not all fibre, a means of injecting the ~.ight into the
fibre is provided.
At the other end of the latter, a. spectrum ra.nal~ser 22,
wh~.ch matches r.he spectral range covered by the source 20
and the transducer gxating 16, is connected.
This spectrum analyser '22 is connected to a diga.ta7.
acquisa.tion device 2~ ir~.tended to convert the ar~al.ogue
signals provided by the spectrum analyser into digital
signals exploitable by an electronic processing dev~.ce 26
(computer).
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!.. ~ ~'
The analysis techn~.c~ue described abo~re is implemented (in
the form of software) in the electronic processing device 2&
which, furthermore, is ti.tted w~.th means (riot shown) for
displaying the results provided by the c4mputer.
Another example of the system which. is the subject of the
invention is illustrated schematica~.ly in Figure 4. ~n this
other example, the broad spectrum source 20 is replaced with
a laser scarce ~8 with a very narrow spectrum, and wh~.ch can
be spectrally tuned.
In this case, it is no longer necessary to place a spectral
analyser r~.t the oue.put of the fibre 1~: it is enough to use
a single photodetector ~0.
Again, the analysis technique mentioned above is employed,
by means of azz acquisition dev3.ce 32 connected to the
photadetector 30 and a computer 34 cor~nected to the devic~
32 and pxovzdecl tee employ the technique of analysing the
2b response from the lalazed gratings to the refractive index of
the external. medium.
Ar~ather example of the system wh~.ch is the subject of the
invention is schematically illustrated iz~. Figure ~. Unlike
the systems of Figures 3 and 4 which are operated in
transmission, the system of Figure 5 is operated in
reflection, in odder to dv this. a ma.z~ror 36 is placed at
one of the ends of the fibre 14.
Advantageously, this mirror ~36 is obtained by placing a
metal or dielectric coating at this end. The characteristics
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~~ _ _
of this coating depend on the spectral region i.n which the
operati.ox~ takes p~,ace .
Rn optical coupler 38 of the 1 x ? type is connected to the
other end of the fibre 24 and,-as can be seen, conriected via
an optical fibre 44 to the broad spectrum light source 20
and, via another optical fibxe 42, to a unit for processing
light signals successively comprising a spectrum analyser
44, are aaqu~.sation device 46 and a computer 48.
1~
The light emitted by the source 20 passes successi~crely
through the fibre 4~, the coupler 38 and the fibre 14, is
xe~fl~cted on the mi.rrox 36, then passes back through the
fibre 19 then through the fibre 42 after haring crossed the
coupler 38.
The spectrum ana~.yser 4~ , the acguisition dev~.ce 45 and the
computer 48 cooperate ~.n order to pro~ride measurem~ants o~
the refractive inde.~ of the medium 18 surrounding the
portion of fibre which contains the grating 16 whip taking
account of the fact: that, in this case; the operatz.or~ takes
plt~.c~ in refa..ection.
The person skilled in the art can adapt the e~ampLe of
Figure 5 to the. case whexe the laser source 28 of Figure 4,
with a very marrow spectrum ~.nd which can be spectrally
tuned, is used instead of the broad spectrum source 2Q,
The examples of F~.gures 3 to 5 comprise on.~,y a single
transducer gratiz~g. Figure 6 illustrates schematzca:Lly
another system according to the invention, operating in
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CA 02368970 2001-10-31
,.. ~ ~ _ °
transmission, a.n which a plurality of blazed transducer
gratings, for example N gx'atings R~., R2, ..., RN, are
respect~.ve~.y formed in po,rt~.ons of the same optics.l ~~.bre
54. The protective cladding on Chew portions is dispensed
wi th and they are placed in med~.a Mi , M2 , . . . , Min,
respectively, the respective refractive indices of which it
is desired to measure.
The broad spectrum light source 20 is again used in the
~.0 exsmp~.e of Figure 6 and its ~,~.ght is ir~,jected into the fi.b,re
54.
Such a configuration corn~spands t,o a multiplexed system. A
spectral. reg~.on A~.;_ ( 1 <_ I ~ n? ar chazznel is al located
specifica~.ly to each transducer grating R.;. These various
channels axe demultiple~c~d (by an. e~.ectronic, optical or
purely digital method) and the refxact~.ve index o~ the
medium surrounding each of the gratings is determined_
In order to do this, in the example of Figure 6, the fibre
50 zs again connected to a spectrum ana~.yser 5? prov~,ded in
order to acquire the transmission spectrum of the set of
transducer gratings R;..
This spectrum analyser 52 is connected to an acquisition and
demulti.pl.exing Device 54 provided in order to transform the
analogue signals provided by the. spectrum analyssx 52 into
digital s~.gnals and r_o isolate the spectral region
corresponding to each transducer grating.
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CA 02368970 2001-10-31
This acquisition and demultipl~xing device 54 is connected
to a computex 56 which is Fitted with display means (not
shown) and which is provided to employ the analysis
technique on each of the various spectral regions separated
by the acquisition and demultiplexa.ng device 54.
The person skilled in the art can adapt the example of
Figure 6 to operation in reflection, from the example of
Figure 5.
The use of blazed gratings for re~ractametry has the
following advantages:
a very low sensitivity to temperature and strain (for
example much smaller than that of the long period
gratings),
a suitable multiplexing capacity,
a response time of about 1 second, limited only by the
computing time of the computer and not by the
transducer grating,
the possibility of adapting the measurement dynamics
and the sensitivity by choosing the grating parameters,
in particular the blaze angle,
the possibility o~ attaining resolutions of about 10-5,
and
the possibility of making the transducer paxt operate
in reflection.
~o
In addiCiori, it should be hated that the spectral analysis
technique of the blazed gratings, explained above, makes it
possible to overcome problems of power fluctuation of the
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CA 02368970 2001-10-31
2~ - _
light sources or optical sources, of a~.~. accidental losses
in the blaaed grating sensor and of the sensitivity o~ this
sensor, that i.s to say of the r~,iriale intensity transform
function of the measurement system. It is a problem that. the
techniques of the prior art, which are based on an intensity
measurement; come up against. It is therefore a determining
advantage over the refractometry techniques using evanescent
waves..
Moreover, it is riot necessary to attack, chemically or
mechanically, the initial structure of the wa eguid.e in
order to obtain satisfactory serisiti~crities. It is in fact
difficult to car~.tx~a~, the repx~aduc:ibilit~ of such pz~ocesseg
which furthermore have the major drawback of weakening the
waveguide.
This last poil~t is also an advantage to the credit of the
present invention rruith respect to systems using standard
8ragg gratings. The latter systems furthermore ha~tre a weaker
metrological performance (in particular resolutions).
Compared to surface plasmon sensors, the use o~ blazed
gratings allows the simpler use of all-fibre sensors. This
is because the manufacturer of a surface plasmon sensor irl
an optical fibr8 requ~.res pxoduczn,g a metal coatirig
(typically made of silver) dirECtly on the core of the
fibre. It is there:~ore necessary to remove beforehand Che
optical. cladding of the fibre then to deposit a homogeneous
coating right around the latter. Furthermore, techn~.cal
difficulties ~.n attachi.rig the si~.ver layer to the silica (df
H 13555.3 PV
CA 02368970 2001-10-31
which the core aF the fibre is generally made), are often
encountered.
The technique cl.osest.to the present invention is that which
uses long period Hragg g~ati.ngs or LPFG. However, the two
types of gratings are very different. Although both produce
coupling to the cladding modes of a wa~creguide, the blazed
grat~.ngs produce Counter direetiorlal coupling, connected to
much smaller grating periods than those of the LPFG.
Furthermore, instead of ana.lys~.ng a, single resonant, the
present invention uses all res4nax~ts presented by the
trarmmi.sei,ora spectrum of the blazed gratings .
3.5 Moreover, the latter are clearly less sensitive to other
physical parameters of the external medium such as
temperature arrd strain. Th~.s makes it possible to avoid
resorting to compAnsation teehnigues.
Furthermore, they occupy a smaller spectral range, which
improves the multiplexing Capacities of the measurement
system.
Finally, the lengths of the blazed gratings are less than
those of the long period gratings, name7,y they a.re about a
few mzl~.i.metres compared to 20 Ca 30 mm for e,ho LPFG. This
makes it possible to make quasi-discre~:e measurements.
3~
Preferably, the acquisition and spectral s,nalysis means used
~.n the i.nveriti.on are prqvided in order to acquire each
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CA 02368970 2001-10-31
26 _ ._
spectrum, with as small. a wazrelength pitch as allowed by the
ana~.~rsis technique mentioned above.
In addition, the ~.riverWion car be implemented wi~,h
wavegui.des or,her th~.n the opt~.cal .fibres, for example with
one ox more plana.x waveguides.
8 7.3655.3 pV
