Fibre grating filter and method of producing the device

Abstract: The present invention relates to a fibre grating filter optical waveguide device, comprising an optical fibre consisting essentially of silica, whereby said optical fibre has an area with a diffractive grating region and wherein said area with a diffractive grating region is covered with a material having a negative thermal expansion coefficient α satisfying the following equation: α = - (dn eff /dT) / n eff wherein dn eff / dT is the thermo-optic coefficient of the fibre material and n eff is the effective refractive index. Furthermore, the present invention provides a method of manufacturing such a device.

Description: The present invention relates to a Fibre Grating Optical Waveguide Device and a method of manufacturing the same. Optical filters have numerous applications in optical communications and in particular, they can be used for providing wavelength selectivity and tuning in WDM (wavelength division multiplexing) and DWDM (dense wavelength division multiplexing) systems. For this purpose, transmission filters based on diffractive grating structures or diffractive fibre grating structures, offer near ideal filter response and high channel isolation. Furthermore they can be used in sensor applications. The term "grating structures" as used in the following and in the field of optics in general defines structures comprising for example FBG (fibre Bragg gratings), SBG (slanted Bragg gratings), LPG (long period gratings) and mode converters. Often the material of the optical fibre and consequently the wavelengths passing through the fibre is affected by thermal changes. For example, an uncompensated grating displays a shift in the so-called "Bragg wavelength" over a 100°C temperature range as large as 1 nm which leads to serious problems in some optics systems applications. An increase or decrease in temperature has to be balanced in order to maintain a good tuning and/or selection of the wavelengths in the optical fibre. The center wavelength (or Bragg-wavelength) λ B of a filter is given by the following equation: λ B = 2 Λ n eff Where Λ is the period of the grating and n eff is the effective refractive index. The change in the Bragg wavelength λ B for a temperature variation ΔT is given by : Δλ B /ΔT = 2 Λ (dn eff / dT + n eff α) whereby α is the linear thermal expansion coefficient of the fibre. The variation in the center of the fibre is strongly influenced by the thermo-optic coefficient dn eff / dT which is about 10-11·10 -6 / K for germania-silica or silica glasses. The expansion term contributes less than ten percent of the total variability. To avoid this undesired temperature dependency, two methods for athermalizing diffractive gratings are known which provide thermal control of the FBG in a device environment : The first athermalization method is conventionally termed as "active", providing a thermal control of the entire device environment. Active athermalization, however, needs an external power source to provide a constant and controlled temperature which is costly to elaborate and to power. Furthermore the compensating device, often termed as "package" or "packaging" is not compact enough for many applications. The second athermalization method is termed as "passive" which is conventionally achieved by two ways: The first way makes use of an arrangement, where the filter is maintained in a mechanical structure comprising two materials having different but positive thermal expansion coefficients (TEC). The design is such that the final dilatation behaviour results in a negative elongation of the fibre with an increasing temperature. The second way is to manufacture the diffractive grating directly into a structure comprising a single material (polymer or ceramic) with a negative TEC. The FBG is packaged into a structure in such a way that the strain is relieved as the temperature increases. The reducing strain compensates the increasing refractive index and holds λ B constant: Δλ B /ΔT = 0 ⇒ α FBG packaged = - (dn eff /dT)/n eff Passive athermalization is well known in the art : US 5,042,898 discloses a package which compensates the wavelength shift with temperature. This is achieved by a combination of two materials having different thermal expansion coefficients. This effect is being used to partially relieve the tension applied to the optical fibre with a temperature increase, and vice versa with a temperature decrease. To avoid optical fibre buckling when temperature increases, the optical fibre is always maintained under tension. The disadvantage of the known passive athermalization methods is that the optical fibre is fixed on the package at only two points by the help of clamps or glue. This packaged device is very sensitive towards ageing which results in an uncontrolled variation of fibre tension, therefore in an uncontrolled shift of λ B . The difficulty to ensure a stable and desired center wavelength is a major disadvantage of this method. Iwashima et al. in Electronics Letters 1997, Vol.33, p. 417-419 and furthermore in EP-A-0798 573 proposed a multilayer package around the fibre. The fibre is hold in a tube made of a liquid-crystall polymer and fixed into the tube by an epoxy glue or silicon. A major disadvantage of this method is the fact that obtaining perfectly concentrically surrounding layers requires a tedious and technically demanding process which leads often to non-reproducible results. EP 0 798 573 describes coating of a liquid crystal polymer by an extrusion technique This process leads to a coating of polymer on the fiber, where the polymer has a negative thermal expansion coefficient .The adhesive process on the fiber is not sufficient to achieve a good thermal compensation. The underlying problem of the invention was therefore to provide a fiber grating filter which is not influenced by variations in temperature. This problem is solved by a fiber grating filter with the features of claim 1. Accordingly, a fibre grating filter comprises an optical fibre consisting essentially of silica, whereby said optical fibre has an area including a diffraction grating and wherein said area with a diffraction grating is directly covered with a material having a negative thermal expansion coefficient α satisfying the following equation: α = - (dn eff /dT) / n eff wherein dn eff / dT is the thermo-optic coefficient of the fibre material and n eff is the effective refractive index. The material is a crosslinked polymeric material. Polymeric materials display an easily controllable behaviour, so that they can be chosen and tailored according to the specific needs of an application, so that the adhesion between a glassy material as for example a fibre and the polymeric material is optimized. Furthermore, they can be easily applied on the surface of Bragg gratings. The crosslinked polymeric material may display anisotropic mechanical and thermomechanical properties. It is preferred to use crosslinkable monomeric or oligomeric precursors which are generally easier to apply than the corresponding polymers. Advantageously, the monomeric and/or oligomeric precursor materials of said polymeric material display LC characteristics in the molten state. The problem underlying the invention is further solved by a method of manufacturing a fiber grating filter according to the invention comprising the following steps: forming a diffraction grating area along an optical axis of an optical fibre bringing in direct contact of at least said area of the optical fibre monomeric and/or oligomeric precursor materials forming a layer or a coating of said monomeric and/or oligomeric precursor materials on at least said area curing the layer of the monomeric and/or oligomeric precursor materials The curing according to the invention is carried out after applying the monomeric and/or oligomeric precursor materials onto the optical fibre with the diffractive grating region. This implies an excellent adhesion between the polymeric material and the material of the fibre surface is obtained. The curing is carried out preferably by temperature, UV, electron beam or gamma irradiation, but any other method known by a person skilled in the art is applicable as well. By deliberately choosing the curing temperature, the crosslinking degree can be adapted to the specific requirements of the polymeric coating. In a further advantageous embodiment, the monomeric and/or oligomeric precursor materials are aligned in a magnetic field along the fibre axis before or during curing. This leads to an uniaxial orientation of the corresponding monomers and/or oligomers, so that a controllable negative TEC of polymeric material in the fibre axis direction is obtained with the suitable value (-7 to -9.10 -6 /K). The negative TEC of the polymer coating, which is especially advantageous compensates the positive thermal sensitivity of the filter. It is understood that the afore mentioned advantages and the features of the invention explained in the following, are not only used in the specifically described combination, but can also be used by a person skilled in the art in other combinations or alone, without exceeding the scope of the invention as defined by the appended claims. In the following, the invention is described in detail. First of all, definitions of terms used herein are given in order to allow a better understanding of the present invention The term "covered" as used herein means that the material with said advantageous thermal expansion coefficient is arranged in direct contact without any interfaces, as for example local fixation means like clamps, resin or glue, on the surface of the area with a Bragg grating region of an optical fibre. The term "covered" means also that the material exhibits a good adhesion property on the optical fibre. In an advantageous embodiment, the area is wholly covered by said material Athermal "package" or "packaging" refers to any means capable of counterbalancing thermal effects on optical filter wavelengths. The diffractive grating, especially a Bragg grating, of an optical fibre is formed by any method known by a person skilled in the art, as for example disclosed in US 4,725,110 or in EP 0 730 172 A1. In the following, the working principle of the invention is explained in detail: The principle of the compensation of temperature influences on the Bragg wavelength in Fibre Bragg gratings according to the invention is based on a "passive" method. A coating of the polymeric material is concentrically surrounding the optical fibre having the grating area. This material is characterized by a negative thermal expansion coefficient α (TEC) equal to α FBG packaged (-7 to -9·10 -6 / K). Depending on the nature of the fibre, the values of the thermooptic coefficient and effective refractive index are variable. In most cases, a value in the range comprising 10-11.10 -6 /K and 1.45-1.47 will be sufficient. The polymeric material is made from suitable monomeric and/or oligomeric precursor materials. These precursor materials are preferably molecules comprising: mesogenic groups (rigid or semi-rigid core) which confer the precursors liquid-crystalline behaviour cross-linkable end groups which ensure the polymerization and the formation of the final polymeric material. Several classes of monomeric and/or oligomeric precursors can be used without exceeding the scope of the present invention. The cross-linking groups may be acetylene, maleimide, amide, cyanate, isocyanate, acrylate, methacrylate or epoxy groups, or other groups known to a person skilled in the art, alone or in combination with curing agents such as amine compounds. The rigid or semi-rigid rod mesogen groups comprise aromatic, cycloaliphatic or aliphatic groups (for example biphenyl, phenyl, benzoate, methyl-stilbene, naphthyl structures), and other groups known to a person skilled in the art, for instance heteroatoms containing groups such as ester, amide, azomethine. A non exhaustive list has been given by Shiota et al. in: Progr. Polym. Sci. 1997, vol.22, pp 975-1002. The polymerization produces advantageously no by-products. The monomeric and/or oligomeric precursor materials are preferably cured by using conventional methods. The monomers display a liquid crystalline behaviour in molten state and can be aligned in high magnetic or electric fields before or during curing. This results in uniaxial orientation of said precursors in the polymeric material and leads after curing to a polymeric material exhibiting a negative TEC along the fibre axis direction. An adjustment of the chemical composition and the polymerization parameters allow to obtain a polymeric material with the suitable thermal expansion so that the negative TEC of the coating compensates the positive thermal variation of the diffractive grating structures. The direct fixation of the coating on the optical fibre is ensured by adhesion between the polymeric material and the silica on all the surface of the diffractive grating. The curing temperature is preferably above the maximum use temperature. It is the condition to obtain a composite polymer-fibre with uniform thermomechanical properties in the use temperature range from -20°C to 80°C. The spectral response of the diffractive gratings during the packaging manufacturing (coating and curing) can be recorded. Thus, the desired Bragg wavelength is adjustable if necessary by applying an additional mechanical stress to the fiber during the process. The mounting principle is based on an one-step process and this simplification decreases the package cost.

Claims: Fibre grating filter, comprising an optical fibre consisting essentially of silica, whereby said optical fibre has an area including a diffraction grating, said area with a diffraction grating being in direct contact with a material having a negative thermal expansion coefficient α satisfying the following equation: α = -(dn eff /dT)/n eff wherein dn eff / dT is the thermo-optic coefficient of the fibre material and n eff is the effective refractive index characterized in that the material is a crosslinked polymeric material and the fixation of the material on said area is an adhesive process. Device according to claim 1, characterized in that the monomeric and/or oligomeric precursor materials of said polymeric material display an anisotropic behaviour. Device according to claim 2, characterized in that the monomeric and/or oligomeric precursor materials of said polymeric material display liquid crystalline behaviour in the molten state. Device according to claim 3, characterized in that the polymeric material displays anisotropic characteristics. Device according to claim 4, characterized in that the polymeric material exhibits a negative thermal expansion coefficient α along the fibre axis. Method of manufacturing a device according to one of the preceding claims, comprising the following steps: forming a diffraction grating area along an optical axis of an optical fibre bringing in direct contact of at least said area of the optical fibre with monomeric and/or oligomeric precursor materials forming a layer or a coating of said monomeric and/or oligomeric precursor materials on at least said area curing the layer of the monomeric and/or oligomeric precursor materials. Method according to claim 6, wherein that the curing is carried out by temperature, UV, electron beam or gamma irradiation. Method according to claim 6 or 7, wherein that the monomeric and/or oligomeric precursor materials are aligned by a magnetic field in the fibre axis direction before or during curing.

Inventors: FRADET ALAIN, GIRARDON VALERIE, RIANT ISABELLE, TESSIER MARTINE, FRADET, ALAIN, GIRARDON, VALERIE, RIANT, ISABELLE, TESSIER, MARTINE, Girardon, Valérie
Applicants: CIT ALCATEL, ALCATEL
Application year: 2001
Main classification: G02B6
IPC codes: G02B5, G02B6, C03C25