Optical device comprising a salt in liquid state

0001The present invention relates to the field of optical devices comprising an optical component and an optical medium of different optical indices, in which the optical properties of the optical component at least partially depend on this optical medium. The optical components may be bulk or embedded in waveguides such as optical fibers or planar waveguides. These waveguides, fibered or planar, may be photonic crystal. These optical components can be for example Bragg gratings, whose lines are straight or angled, with short or long steps, couplers, Mach-Zehnder interferometers based on couplers, tapering fibers ("tapers" in English) or sections of specific fiber such as sections of photonic crystal fiber. By way of illustration but without limitation, the invention is described below more specifically in the case of optical devices comprising, as optical components, so-called "long-distance" Bragg gratings (LPG) used to produce equalizers of dynamic gain, including equalizer filters.

0002A waveguide is conventionally composed of an optical core, whose function is to transmit and possibly to amplify an optical signal, surrounded by an optical cladding, whose function is to confine the optical signal in the core. For this purpose, the refractive indices of the heart n₁ and sheath n₂ are such that n₁> n₂. Thus, the heart and the sheath form a waveguide. As is well known, the propagation of an optical signal in a single-mode waveguide breaks down into a fundamental guided mode in the core and in secondary modes guided over a distance in the optical core-sheath assembly, called also sheath modes. The sheath is itself surrounded by an external medium of refractive index n₃ greater than or less than n₂. The core-sheath assembly associated with the external medium again forms a waveguide.

0003The core and / or the sheath of the guide may be doped so as to be made photosensitive for the inscription of a Bragg grating, for example with germanium (Ge). The networks conventionally used for gain equalization are angle networks or inclined gratings, known by the term SBG for "Slanted Bragg Grating" in English, or long-distance networks, known by the term LPG for "Long Period Grating "in English. Such networks are designed to allow coupling of the fundamental mode in cladding modes. They thus constitute filters without reflection at the filtering wavelengths.

0004In the case of a long-range grating (LPG), the spectral response of the filter depends on the effective index n_effCoeur fundamental mode guided in the heart as well as the effective index n_effGaine of the sheath mode with which there is coupling. Indeed, Bragg's relationship for long-distance networks is expressed as follows:<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>λ</mtext></mrow><mrow><mtext>B</mtext></mrow></msub><msub><mrow><mtext> = Λ (n</mtext></mrow><mrow><mtext>effCoeur</mtext></mrow></msub><msub><mrow><mtext> - not</mtext></mrow><mrow><mtext>effGaine</mtext></mrow></msub><mtext>), with the step of the LPG network.</mtext></mrow></math><img file="EP1605282A1_D0001.tif" /></maths>

0005This peculiarity of the long-range networks makes them particularly well suited to tunable filter applications because a localized action on the core, on the sheath and / or on the environment outside the cladding can induce a modification of one of the effective indices and hence the length of Bragg or the spectral form of the filter. In addition, the insertion losses in such a filter are low because it is easily solderable to another component fiber or directly to the transmission fiber, and the known manufacturing methods of such filters make it possible to obtain losses of polarization dependence. very weak.

0006Gain equalizing filters, also known by the acronym GFF for "Gain Flattening Filter" in English, are integrated in optical devices associated with optical amplifiers regularly distributed along transmission lines. Optical amplifiers generally do not provide equal amplification for all wavelengths, which is particularly troublesome in the case of wavelength division multiplexing (WDM) transmission for which several signals at different wavelengths are transmitted on the same fiber and amplified by the same amplifiers. It is therefore necessary to associate the on-line amplifiers with gain equalization filters which make it possible to eliminate, or at least reduce, the amplification disparities of the different transmission channels of an optical system.

0007An optical system is often scalable and it is not uncommon that the parameters of an optical component, such as a gain equalizer, are no longer adapted to the real transmission spectra. For example, such an evolution may be due to aging, temperature variation, interventions located on the line or additions of optical modules or transmission optical channels after installation of the line. The previously fixed parameters of the various optical components of the optical system then become unsuitable.

0008The properties of the optical component must therefore be able to be modified in order to adapt them to the evolution of the other components of the system. It is therefore necessary to produce optical components that dynamically tune their spectral response to correspond to changes in the operating conditions of the optical systems in which they are arranged over a wide spectral range and without losses.

0009Total or partial immersion of a waveguide portion in an external medium, whose refractive index is different from the index of the cladding, induces a modification of the interface conditions between the cladding and the medium. outside. However, an LPG is particularly sensitive to changes in the environment external to the sheath of the guide on which it is inscribed.

0010The document by H. LABIDI et al. titled "Dynamic gain control of optical amplifier using an all-fiber solution" presented at 28^th European Conference on Optics (8-12 September 2002) proposes a device based on the dependence of the optical properties of an LPG on its degree of immersion in a medium of given index different from that of the waveguide. In practice, at least one reservoir containing a liquid of given index slides along the network to control the length immersed in the liquid. Thus, it is possible to modify the spectral response of the filter according to the level of immersion of the network at long pitch in the external medium.

0011Of the optical media which may be used, liquid media are preferably employed. Solvent mixtures of the trademark "CARGILLE" are particularly known, but these are unsuitable for use in the field of telecommunications. On the one hand, they undergo rapid aging by evaporation (for example total evaporation at ambient temperature takes place in one week); for a telecommunication application, it is requested to ensure a service life of at least 15 years for the terrestrial network and at least 25 years for the submarine network. On the other hand, the liquid state of these solvents is not conserved at low and high temperatures (solidification type transitions at low temperatures and evaporation at high temperatures); or the optical telecommunication components are exposed to temperatures ranging from -40 ° C to + 85 ° C or more.

0012The object of the present invention is therefore to propose an optical device in which the optical properties, for example the spectral response, of the optical component can be at least partially controlled by modifying the optical index of the optical medium so as to adapt the optical properties of the optical medium. optical component to the evolution of the other components of the optical system.

0013The object of the present invention is an optical device comprising an optical component and an optical medium of different optical indices, said optical component being at least partially immersed in said medium, wherein said medium comprises at least one salt which is at the liquid state in the temperature range of use and storage of said optical component.

0014The temperature at which the salt is in a liquid state is preferably from -40 ° C to + 200 ° C. The operating temperature of the optical component most often ranges from -10 ° C to + 70 ° C, but the optical component is likely to be stored at temperatures between -40 ° C and + 85 ° C. The salt must therefore be in the liquid state in this range and the device must not be altered by a phase change of the salt, in particular by passing from the liquid phase to the solid phase.

0015The salts according to the invention are in the liquid state over a wide range of temperature including ambient temperature, and are called RTILs for "Room Temperature Ionic Liquid". They are also often called "ionic liquids" because they are compounds, or mixtures of several compounds, containing dissociated species in ionic form.

0016The salts used in the present invention have the advantage of their high electrical conductivity, their thermal stability, their resistance to aging. In addition to salts, they are neither flammable nor volatile. Indeed their boiling temperature is nonexistent as they generally degrade beyond 250 ° C without going into the vapor state.

0017Most of these salts have the advantage of having reduced toxicity and being miscible with the solvents usually used for cleaning during manufacture. Most of the salts according to the present invention are stable at high temperature in the presence of moisture (for example + 85 ° C. and 85% relative humidity) with regard to their refractive index (for example measured at 1500 nm and 589 nm). nm before and after the test) and their viscosity.

0018The optical medium according to the invention preferably contains a mixture of at least two salts. Mixing is understood to mean the addition of two salts each in the liquid state, a solution of at least one first liquid salt in which is dissolved and dissociates at least one second salt which is initially present in the liquid. solid state. The refractive index of the medium depends on the composition of the salt mixture. This property has the advantage of making it possible to obtain very precisely the desired index, in particular over a range of refractive index of 1.35 to 1.52 measured at 589 nm.

0019The refractive index of these salts varies with temperature, but very weakly. In some cases, this variation is too small to induce a variation in the response of the optical component. This is particularly interesting in the case where the variation of the spectral response of the optical component is obtained by a modification of the immersion zone of said optical component. This is the case in the example of the LPG network described more specifically in this text, in which the adaptation of the refractive index medium is induced by a displacement of the contact zone between the component and the index medium. of refraction. Indeed, in the case of LPGs, the variation of the refractive index with the temperature has no consequence on the shape of the filter under the conditions of use of the terrestrial optical component, namely from -10 ° C to 70 ° C (ie a range of 80 ° C). The spectral response of an LPG filter, inscribed in a fiber of adapted profile, surrounded by a medium of index 1.46 or 1.47 does not show any significant difference

0020On the other hand, in some cases, the variation of the refractive index of the optical medium with the temperature is sufficient to modify the response of the optical component. For example, when the variation in refractive index with temperature is considered to be high (for example from -20 ° C. to 120 ° C., ie a range of 140 ° C.), the profile of a photonic crystal fiber can be to be adjusted according to the temperature of the optical medium and spectral variations in transmission on the optical component can thus be induced. In the case of tunable optical components based on photonic crystal fiber, a significantly different spectral response has been observed for optical media whose refractive index varies between 1.46 and 1.48.

0021According to a preferred embodiment of the invention, the salt comprises an anion and a cation.

0022The anion is preferably chosen from a fluoride F^-Cl chloride^-Br bromide^-, an iodide I^-, a ClO perchlorate₄^-, an aluminochlorate AlClO₄^-, a BF tetrafluoroborate₄^-, a PF hexafluorophosphate₆^-, an HSO hydrogen sulphate₄^-, a CF trifluoroacetate₃COO^-, a heptafluoro-1-butanoate C₃F₇COO^-, a CF trifluoromethanesulfonate₃SO₃^- (or "triflate" TfO^-), a nonafluorobutylsulfonate C₄F₉SO₃^- (or "nonaflate" NfO^-), a bis-trifluoromethanesulfonimide (CF₃SO₂)₂NOT^-(or Tf₂NOT^-), and a bis-trifluoromethanesulfonemethide (CF₃SO₂)₂C^- (or Tf₂C-).

0023The cation is preferably chosen from ammonium NH₄⁺, the sulfonium HSO₃⁺Phosphonium PH₄⁺Li lithium⁺, pyridinium C₅H₅NH⁺, the imidazolium cations (C₃H₆NOT₂X¹X²)⁺, the ammonium cations (X¹X²X³X⁴NOT₄)⁺, sulfonium cations (X¹X²X³S)⁺phosphonium cations (X¹X²X³X⁴P)⁺, and pyridinium cations (X¹C₅H₄NX²)⁺, in which X¹, X², X³, X⁴ represent aromatic or aliphatic type groups.

0024According to a preferred embodiment of the invention, the salt is chosen from imidazolium salts. Preferably, the cationic salt is N, N 'dialkylimidazolium of general formula:<chemistry id="chem0001" num="0001"><img file="EP1605282A1_D0002.tif" /></chemistry> in which R¹ and R² are aliphatic or aromatic groups coordinated with a counterion, such as an anion. R¹ and R² may be the same or different and contain at least one anion chosen preferably from F^-, Cl^-, Br^-, I^-, BF₄^-, PF₆^-, ClO₄^-, AlClO₄^-, CF₃COO^-, C₃F₇COO^-, CF₃SO₃^-, C₄F₉SO₃^-, (CF₃SO₂)₂NOT^-, (CF₃SO₂)₂C^-As an example of an aliphatic group, mention may be made of the methyl group -CH₃, ethyl-C₂H₆, propyl -C₃H₇ and butyl -C₄H₉.

0025In this case, the salt contains an anion chosen preferably from Cl^-, Br^-, PF₆^-, BF₄^-, (CF₃SO₂)₂NOT^-, C₄F₉SO₃^-. According to the nature of groups R¹ and R² and the associated anion, the salt has a different liquefaction temperature. Thus it is possible to choose salt families that are liquid at the temperature of use and storage of the optical component.

0026In addition, the physicochemical properties, and in particular the refractive index of the salt can be granted by modifying the groups R¹ and / or R²and / or the associated anion. It is known in particular that the insertion of fluorine atoms into the structure of the cation and the anion decreases the refractive index. On the contrary the presence of halogenated anions such as Br^-, Cl^- and I^- increases the refractive index of salt. There is thus a wide choice of refractive indices by mixing at least two salts or by dissolving a solid salt in another liquid salt at the operating temperature. The reproducibility of the manufacture of such a mixture, as well as the homogeneity of the mixture obtained have been shown.

0027According to a particular embodiment, the optical component is a long-term Bragg grating, called "LPG". In this case the grating is preferably tuned by modifying the immersion zone. The optical response of the network, in contact with the medium according to the invention, is substantially constant in the range of use of the optical component, between -10 ° C and + 70 ° C.

0028According to another embodiment, the optical component is a section of photonic crystal fiber. In this case, the response of the fiber is preferably tuned by varying the refractive index of the medium by means of a temperature variation, preferably in a range between -20 and + 120 ° C.

0029Other features and advantages of the present invention will become apparent from the following description of embodiments given of course for illustrative and non-limiting purposes, and in the accompanying drawing in which the single figure represents<ul id="ul0001" list-style="dash" compact="compact"><li>FIG. 1 represents the variation of refractive index RI measured for a wavelength λ of 589 nm as a function of the mass M in% of one of the salts composing a mixture of two salts that are liquid at room temperature,</li><li>FIG. 2 represents the variation of refractive index RI measured for a wavelength λ of 589 nm as a function of the mass M in% of one of the salts composing a solution of a solid salt at ambient temperature in a liquid salt at room temperature,</li><li>FIG. 3 shows, for a same mixture, the variation of the refractive index RI measured for a wavelength λ of 589 nm as a function of the temperature T in degrees Celsius,</li><li>FIG. 4 is a simplified diagram of an exemplary embodiment of an LPG long network immersed in an adaptable index medium according to the invention,</li><li>FIG. 5 is the transmission spectral response of a long LPG pitch network as a function of its degree of immersion in an adaptable index medium according to the invention; the normalized power P in dBm is given on the ordinate, and on the abscissa the wavelength λ is given in nanometers nm.</li></ul>

EXAMPLES 1 TO 9

0030The following study concerns the influence of the composition of the mixture of two salts on the refractive index of this mixture.

0031Figure 1 shows the variation of refractive index (curve 10) that can be obtained with a mixture of variable composition of two liquid salts at room temperature. The mixture comprises a first salt HMIM, CI and a second salt BMIM, BF₄. The variation of the refractive index IR was represented as a function of the mass M of the first salt HMIM, CI expressed as a% by weight in the mixture.

0032This linear variation shows that the miscibility of the two salts is correct. These mixtures make it possible to meet an index demand of between 1.46 and 1.48 in a temperature range framing the ambient temperature, by selecting the composition of the mixture that is best adapted from these curves.

0033In the following Table 1, the values ​​of refractive index (index measured at 589 nm and 20.0 ° C.) of the mixture of two liquid salts HMIM, CI and BMIM, BF are presented.₄ according to the mass proportions of two constituents. The amount of each salt is expressed in mass%. Correct linearity is observed between the refractive index IR and the proportion of the constituents of the binary mixture.<tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="4" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="left">example</entry><entry namest="col2" nameend="col2" align="left">HMIM, CI in%</entry><entry namest="col3" nameend="col3" align="left">BMIM, BF₄ in %</entry><entry namest="col4" nameend="col4" align="left">IR</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">1</entry><entry namest="col2" nameend="col2" align="char" char=".">87.9</entry><entry namest="col3" nameend="col3" align="char" char=".">12.1</entry><entry namest="col4" nameend="col4" align="char" char=",">1.5027</entry></row><row><entry namest="col1" nameend="col1" align="left">2</entry><entry namest="col2" nameend="col2" align="char" char=".">74.5</entry><entry namest="col3" nameend="col3" align="char" char=".">25.5</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4932</entry></row><row><entry namest="col1" nameend="col1" align="left">3</entry><entry namest="col2" nameend="col2" align="char" char=".">62.1</entry><entry namest="col3" nameend="col3" align="char" char=".">37.9</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4831</entry></row><row><entry namest="col1" nameend="col1" align="left">4</entry><entry namest="col2" nameend="col2" align="char" char=".">50.5</entry><entry namest="col3" nameend="col3" align="char" char=".">49.5</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4714</entry></row><row><entry namest="col1" nameend="col1" align="left">5</entry><entry namest="col2" nameend="col2" align="char" char=".">50.6</entry><entry namest="col3" nameend="col3" align="char" char=".">49.4</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4711</entry></row><row><entry namest="col1" nameend="col1" align="left">6</entry><entry namest="col2" nameend="col2" align="char" char=".">40.1</entry><entry namest="col3" nameend="col3" align="char" char=".">59.9</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4623</entry></row><row><entry namest="col1" nameend="col1" align="left">7</entry><entry namest="col2" nameend="col2" align="char" char=".">32.4</entry><entry namest="col3" nameend="col3" align="char" char=".">67.6</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4559</entry></row><row><entry namest="col1" nameend="col1" align="left">8</entry><entry namest="col2" nameend="col2" align="char" char=".">22.8</entry><entry namest="col3" nameend="col3" align="char" char=".">77.2</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4463</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">9</entry><entry namest="col2" nameend="col2" align="char" char=".">12.3</entry><entry namest="col3" nameend="col3" align="char" char=".">87.7</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4345</entry></row></tbody></tgroup></table></tables>

EXAMPLE 10

0034The following example relates to the case of a mixture of an initially solid salt BMIM, Br dissolved in a liquid salt BMIM, BF₄. Curve 20 of FIG. 2 shows variation of the refractive index as a function of the composition of the binary mixture BMIM, BF₄ / BMIM, Br

0035Correct linearity is observed between the refractive index and the proportion of the constituents of the binary mixture.

EXAMPLES 11 AND 12

0036The following study concerns the influence of temperature on the refractive index of the mixture of two liquid salts.

0037FIG. 3 relates to two mixtures 11 and 12 of two liquid salts for which the variation of refractive index as a function of temperature has been noted.

0038The mixture 11 (curve 30) is composed of 48.2% by weight of a salt HMIM, CI and 51.8% by weight of a salt BMIM, PF₆ marketed by the company "SOLVENT INNOVATION". The mixture has now a refractive index of 1.4659 for a wavelength of 589 nm at a temperature of 20 ° C, and of 1.4773 at a temperature of 0 ° C.

0039The mixture 12 (curve 31) is composed of 50.5% by weight of a salt HMIM, CI and 49.5% by weight of a reference salt BMIM, BF4. Blend B has a refractive index of 1.4714 for a wavelength of 589 nm at a temperature of 20 ° C, and 1.4741 at a temperature of 0 ° C.

0040It is observed that, in both cases, this variation is linear decreasing with increasing temperature and slope dn / dT of the order of -3.10^-4.

EXAMPLES 13 TO 17

0041The stability of the wet heat refractive index of salts or salt mixtures according to the invention has been studied. The test was carried out at high temperature in the presence of humidity: temperature of + 85 ° C. and 85% relative humidity.

0042The results are grouped in the following table 2. The refractive index IR is measured before and after the test at 589 nm and 20.0 ° C. The weight of the sample is taken before and after the test, and this gravimetric follow-up is represented by the mass variation Δm in% defined by: Δm = [(mass of initial product - mass of the product after test) / mass of product initial] x 100.<tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="5" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="31.50mm" /><colspec colnum="2" colname="col2" colwidth="31.50mm" /><colspec colnum="3" colname="col3" colwidth="31.50mm" /><colspec colnum="4" colname="col4" colwidth="31.50mm" /><colspec colnum="5" colname="col5" colwidth="31.50mm" /><thead valign="top"><row rowsep="1"><entry namest="col1" nameend="col1" align="left">Example</entry><entry namest="col2" nameend="col2" align="left">formula</entry><entry namest="col3" nameend="col3" align="left">Dm</entry><entry namest="col4" nameend="col4" align="left">IR before test</entry><entry namest="col5" nameend="col5" align="left">IR after test</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">13</entry><entry namest="col2" nameend="col2" align="left">BMIM PF₆</entry><entry namest="col3" nameend="col3" align="left">0.3%</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4108</entry><entry namest="col5" nameend="col5" align="char" char=",">1.4103</entry></row><row><entry namest="col1" nameend="col1" align="left">14</entry><entry namest="col2" nameend="col2" align="left">BzMIM, BF₄/ BzBMIM, Br</entry><entry namest="col3" nameend="col3" align="left">0.7%</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4742</entry><entry namest="col5" nameend="col5" align="char" char=",">1.4744</entry></row><row><entry namest="col1" nameend="col1" align="left">15</entry><entry namest="col2" nameend="col2" align="left">BMIM, BF₄/ BzMIM, BF₄</entry><entry namest="col3" nameend="col3" align="left">0.3%</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4632</entry><entry namest="col5" nameend="col5" align="char" char=",">1.4683</entry></row><row><entry namest="col1" nameend="col1" align="left">16</entry><entry namest="col2" nameend="col2" align="left">BMIM, BF₄/ BMIM, Br</entry><entry namest="col3" nameend="col3" align="left">0.7%</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4610</entry><entry namest="col5" nameend="col5" align="char" char=",">1.4596</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">17</entry><entry namest="col2" nameend="col2" align="left">BMIM, NFO</entry><entry namest="col3" nameend="col3" align="left">-</entry><entry namest="col4" nameend="col4" align="char" char=",">1.4038</entry><entry namest="col5" nameend="col5" align="char" char=",">1.4034</entry></row></tbody></tgroup></table></tables>

0043It is observed that the refractive index remains constant before and after the test, as well as the surface tension and the viscosity of the salt or mixture of salts. In addition, in the presence of a humid environment and at high temperature, these ionic liquids show good stability since the weight gain remains very low (Δm &lt;1%). The absorption of water by these salts remains very moderate, and induces no modification of their physicochemical and optical properties.

EXAMPLE 18

0044According to a particular embodiment of the present invention, the optical medium may for example contain a salt of N, N 'dialkylimidazolium associated with a BF anion.₄^- whose groups R¹ and R² are respectively a methyl group and a butyl group. Its crystallization temperature is then -80 ° C. This salt has anti-electrostatic properties of particular interest for the intended telecommunications application. By way of comparison, if the methyl group is replaced by an ethyl or butyl group, the crystallization temperature of the salt becomes + 15 ° C. which makes it less easy to use.

EXAMPLE 19

0045The following study concerns the influence of the degree of immersion of a long-pitch LPG network in a liquid medium according to the invention. The degree of immersion of LPG in the medium can be varied, in particular by means of a device similar to that described in the document by H. LABIDI et al. titled "Dynamic gain control of optical amplifier using an all-fiber solution" presented at 28^th European Conference on Optics (8-12 September 2002).

0046FIG. 4 is a schematic and simplified representation of an optical fiber 40 on a portion 41 from which a long LPG grating has been inscribed. This portion 41 may be at least partially immersed in a medium according to the invention comprising a liquid salt, and contained in a reservoir 42 able to move along the fiber 40. The reservoir 42 is represented here in the initial position 43 in which the portion 41 carrying the network is not immersed in the medium (zero displacement). The tank 42 can be moved in the direction represented by the arrow 44 (forward), and the portion 41 is then gradually immersed in the medium contained in the tank 42. The reservoir 42 can also be moved in the opposite direction in the direction represented by the arrow 45 (return).

0047FIG. 5 represents the spectral response of the LPG long pitch grating, with a total length of 30 mm, immersed in a medium of index 1.404 at room temperature (20 ° C.). The curves 50 and 51 correspond to a zero displacement (initial position), the response being respectively measured on the outward and returnward directions. The curves 52 and 53 correspond to a displacement of 4 mm from the initial position, the response being measured during the passage in this position respectively during a movement to the outward and return. The curves 54 and 55 correspond to a displacement of 8 mm from the initial position, the response being respectively measured on the outward and returnward directions. By displacement of the immersed zone of the long-inscribed network, a continuous and reversible variation of the contrast is observed with a conservation of the Bragg wavelength.