Recoatable temperature-insensitive long-period gratings

<i><u>FIELD OF THE INVENTION</u></i>

0001This invention relates to long-period gratings and, in particular, to long-period gratings recoated with polymer for enhanced temperature stability.

<i><u>BACKGROUND OF INVENTION:</u></i>

0002Long-period gratings are in-fiber devices that can couple light between two dissimilar copropagating spatial modes in a fiber. These devices typically comprise a length of optical fiber with a long-period grating which is formed by a plurality of refractive index perturbations spaced along the fiber by a periodic distance Λ. In general, long-period grating devices are those in which the period is at least 10 times larger than the wavelength of the input light. Typically Λ is in the range 15-1500 µm. The modes being coupled by the grating are either different modes guided by the core or are those guided by the cladding (with the next surrounding region, say air or a coating, acting as the effective cladding for these modes).

0003The major functional difference between long-period gratings and conventional short-period Bragg gratings (periods Λ &lt; 1 µm) is that there is no backreflected mode in a long-period grating. As an example, the light can be coupled out of the core from a fundamental mode and into the cladding of a fiber on a wavelength selective basis.

0004In the past, these devices have been used as simple mode-converters (Hill et al., U.S. Patent 5,104,209, Method of Creating an Index Grating in an Optical Fiber and a Mode Converter Using the Index Grating), as wavelength-dependent loss elements in broadband amplifiers (Vengsarkar <i>et al.,</i><u>Opt. Lett.</u> Vol. 21, p. 336, 1996) in high-power fiber lasers, (Grubb and Stentz, <u>Laser Focus World,</u> February 1996, p. 127), and as band-rejection filters (Vengsarkar <i>et al.,</i><u>Journal of Lightwave Technology,</u> vol. 14, p. 58, 1996). See also U.S. Patent No. 5,430, 817 issued to A. M. Vengsarkar on July 4, 1995.

0005While these devices provide an elegant solution to many problems in fiber communications, they exhibit temperature sensitivity. The peak wavelength λ_p shifts by 4-10 nm per 100° C change in temperature. For some applications where ambient temperatures can fluctuate between 5° and 45° C, such variations are not acceptable and temperature compensation schemes are desirable. The simplest solution is to place the grating device in a controlled temperature chamber; this solution however adds temperature controllers, thereby adding cost and increasing reliability concerns.

0006Several other solutions have also been suggested. For example, US-A-5703978 demonstrates a method of changing the fiber composition and profile such that the temperature dependence of the peak wavelength can be reduced below 4 nm per 100°C change in temperature. This solution, while useful for many applications, places the burden of stabilization on accurate preform and fiber manufacture. It further eliminates the possibility of using standard telecommunication fibers as the grating host, raises the necessity of splicing and introduces added insertion losses.

0007Another method described in US-A-5757549 uses a specifically tailored grating where the dependence of the peak wavelength is opposite in polarity for strain and temperature. Such a specially designed grating is packaged in a material with a specific thermal expansion and leads to a packaged temperature-insensitive device. This method, while attractive, places the burden of stabilization on the packaging material and, by its operating principle, requires that the fiber grating be placed under strain during its entire lifetime.

0008In addition, in prior efforts to compensate for temperature instability, the grating portion of the fiber is bare and not coated. The fiber grating is uncoated because once the temperature-insensitivity has been accomplished, the additional step of recoating the grating would change the spectral and thermal properties of the device, thus rendering it unstable to temperature variations. Leaving the grating section uncoated raises issues of reliability and packaging stability over the lifetime of the device. There is therefore a need for a recoated temperature insensitive grating that can use conventional communication grade fiber (which helps lower splice losses) and that does not need to be strained during its lifetime.

<i><u>SUMMARY OF THE INVENTION:</u></i>

0009Applicants have determined that the temperature sensitivity of long period grating devices is substantially affected by the provision of a polymeric coating over the conventional glass cladding and, in particular, that by providing a polymer overcoating with an appropriately selected index of refraction, one can minimize temperature sensitivity. In a preferred embodiment, the temperature sensitivity of a long period grating written in conventional dispersion-shifted fiber is reduced belowto 0.40 nm/100° C.

<i><u>BRIEF DESCRIPTION OF THE DRAWINGS:</u></i>

0010The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the accompanying drawings. In the drawings: <ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic view of a long period grating in accordance with one embodiment of the invention;</li><li>Fig. 2 is a graphical illustration of the index profile of the device of Fig. 1;</li><li>Fig. 3 is a graphical plot of center wavelength λ_p versus period Λ useful in making the device of Fig. 1.</li><li>Fig. 4 is a graphical plot showing the relationship between n_p and T for typical polymeric coatings.</li><li>Fig. 5 is a graphical plot showing the reduced temperature sensitivity of an exemplary long period grating in accordance with Fig. 1.</li><li>Fig. 6 shows an optical transmission system using the device of Fig. 1 to remove unused pump energy.</li><li>Fig. 7 shows an optical transmission system using the device of Fig. 1 to flatten the gain of an optical amplifier; and</li><li>Fig. 8 is a graphical plot showing the typical shift of peak wavelength due to the change of the index of refraction of the surrounding medium.</li></ul>

0011It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale.

<i><u>DETAILED DESCRIPTION:</u></i>

0012Referring to the drawings, Fig. 1 is a schematic cross section of a long-period grating device having reduced temperature sensitivity. The device comprises a length of optical fiber 10 for transmitting light in a guided mode having a core 11 surrounded by a composite cladding comprising a low index portion 12 and a higher index portion 13. Both portions of cladding 12, 13 have indices of refraction lower than the index of core 11. In accordance with the invention, a polymeric overcoat 20 of selected optical properties surrounds the composite cladding.

0013A preferred materials system is germanium-doped silica for the core 11, fluorine-doped silica for the low index portion of the cladding 12, and undoped silica for the higher index portion 13. The polymer overcoating is preferably a fluorinated acrylate with a photoinitiatior. Fig. 2, which is a plot of refractive index versus distance from the core center for an exemplary fiber, shows exemplary dimensions and refractive index profile for this application.

0014The core 11 includes one or more long-period gratings 14, each comprising a plurality of index perturbations 15 of width w spaced apart by a periodic distance Λ where, typically, 15 µm ≤ Λ ≤ 1500 µm. Advantageously, 1/5 Λ ≤ w ≤ 4/5 Λ and preferably w = 1/2 Λ. The perturbations are formed within the glass core of the fiber and preferably form an angle θ (2° ≤ θ ≤ 90°) with the longitudinal axis of the fiber. The fiber is designed to transmit broad band light of wavelength centered about λ_p.

0015The spacing Λ of the perturbations is chosen to shift transmitted light in the region of a selected wavelength λ_p from the guided mode into a non-guided mode, thereby reducing in intensity a band of light centered about λ_p. A non-guided mode is a mode which is not propagated coherently in the core, and is typically a cladding mode, a radiation mode, or, in the case of multilayer profiles, a ring mode.

0016Fig. 3 is a graph illustrating for the fiber of Fig. 2 the periodic spacing Λ for removing light centered about a wavelength λ_p. Thus, to make a device for removing light centered around λ_p = 1540 nm, one chooses a spacing of Λ = 330 µm as shown in Fig. 3.

0017The long-period grating can be formed by selectively exposing the core to beams of intense light of width w at locations separated by the distance A. Preferably, as a preliminary step, the fiber is loaded with molecular hydrogen or deuterium to enhance the photosensitivity of the core. The preferred exposure source is UV radiation from a KrF excimer laser. Proper spacing can be effected by exposing through a slit of width w and then moving the fiber to the next exposure site. Alternatively, the fiber can be exposed to a wide beam from the laser through an amplitude mask providing a plurality of transparent slits at spacing Λ and opening widths w. Preferably the exposure dosage for each slit is on the order of 1000 pulses of > 100 mJ/cm² fluence/pulse, and the number of perturbations is in the range 10-100.

0018In operation, light of wavelength λ_p is transmitted down the fiber in a core mode The long period grating shifts λ_p from the core mode to a higher order mode. The fiber at the grating has one effective index n₀₁ for the core mode and another effective index n_lm for the higher order mode. The n₀₁ is determined primarily by the geometry and index of the core. The n_lm is determined by the indices and geometry of the cladding and, significantly, by the index n_p of the polymeric overcoating. In accordance with the invention, the temperature sensitivity of the grating is minimized by choosing n_p such that the effect of temperature on n_lm closely approximates the effect of temperature on n₀₁. The specific chemical composition of the polymer is not relevant as long as it meets the n_p and ∂<i>n</i>_<i>p</i> /∂<i>T</i> criteria herein described.

0019The polymer can be applied onto the glass fiber containing the grating as a melt or be cast from solution. A preferred process is to apply a liquid monomer or oligomer mixture that can be polymerized <i>in situ.</i> This polymerization can be initiated simply by mixing reactive co-monomers, by moisture, by air, by heat, or by the action of actinic radiation (preferred). Examples of suitable polymers include fluorinated polymers and silicon-based polymers (siloxanes). The former include partially or fully fluorinated polymers, such as copolymers of perfluoro-2,2-dimethyldioxole and tetrafluoroethylene sold under the tradename TEFLON AF® by duPont; ring-cyclized homopolymers of perfluoro(allyl vinyl ether) sold under the tradename CYTOP® by Asahi Glass Co.; terpolymers of tetrafluoroethylene, hexafluoroethylene, and vinylidene fluoride sold under the tradename THV FLUOROPLASTIC® by 3M; copolymers of perfluoro-2,2-dimethyldioxole and chlorotrifluoroethylene; and terpolymers of perfluoro-2,2-dimethyldioxole, tetrafluoroethylene and chlorotrifluoroethylene. Suitable fluorinated polymers further comprise fluoroacrylates and/or their copolymers with hydrocarbon-based (non-fluorinated) acrylates (and/or methacrylates), fluoromethacrylates and/or their copolymers with hydrocarbon-based (non-fluorinated) acrylates (and/or methacrylates), fluorinated acrylamides, fluorinated urethanes, fluorinated epoxies, fluorinated vinyl ethers, and fluorinated vinyl esters. Mixtures of any of these fluorinated polymers, copolymers or terpolymers further may be used. Fluoroacrylates comprise esters of acrylic acid and perfluorinated or partially fluorinated alcohols, diols, or polyols. Fluoromethacrylates comprise esters of methacrylic acid and perfluorinated or partially fluorinated alocohols, diols, or polyols.

0020Suitable silicon-based polymers include polydimethylsiloxanes, polydiphenylsiloxanes, polymethylphenylsiloxanes, polysilanes, and polysilazanes. The silicon-based polymers further comprise polysiloxanes and polysilsesquioxanes having one or more substituents selected from the group consisting of acrylate, methacrylate, epoxy. amine, vinyl, hydride, silanol, carbinol, carboxylic acid, alkoxysilane having from 1 to 18 carbon atoms, and mercaptan. Copolymers or mixtures of any of these silicon-based polymers also may be used.

0021In the case of fluorine-containing polymers, the refractive index can be adjusted by changing the relative fluorine content in the polymer. In the case of siloxanes, the refractive index can be adjusted by changing the ratio of methyl to phenyl groups attached to the siloxane chain. A preferred polymer comprises at least 70% by weight of a perfluoropolyether diacrylate. Using this approach, one can make a recoated long period fiber grating with reduced temperature sensitivity below 1 nm/ 100° C and preferably below 0.40 nm/ 100° C.

0022The nature and advantages of the invention can now be better understood by reference to the following specific example.

<i><u>EXAMPLE</u>:</i>

0023In this example, we describe a specific design of a long-period grating written in a conventional dispersion-shifted fiber that will give a <i>dλ</i>_<i>p</i><i>/dT</i> approximately equal to zero. The fiber is commercially available, has a mode-field diameter of 8.5 <i>µm</i>, a cladding diameter <i>of 125 µm,</i> a core Δ of approximately 1 % (10 mole % germania in the core) and is singlemode at 1550 nm. The desired effect is achieved using a low index (lower than silica) polymer coating material with a well-designed negative <i>dn</i>_<i>p</i><i>/dT.</i> For amorphous polymers in general, the relationship between the refractive index and temperature dependence is shown in Figure 4 for glassy polymers and rubbery polymers. A long-period grating of periodicity Λ = 260 <i>µm</i> was written in the dispersion-shifted fiber, which led to a peak wavelength of coupling between the fundamental <i>LP</i>_<i>01</i> mode and the <i>HE</i>_<i>5,1</i> cladding mode of 1457 nm (at room temperature). With no coating, the temperature dependence of the peak wavelength was measured to be 8 nm/100°C (curve 1 of Fig. 5). This grating was recoated, using a perfluoropolyether diacrylate polymer with a refractive index of 1.32 and ∂<i>n</i>_<i>p</i>/∂T= - 3.2 x 10^-4/°C. The recoated grating was then temperature cycled between 20 and 80 °C, giving a temperature dependence of -0.48 nm/100°C (curve 2 of Fig. 5). This reduction of the temperature dependence by more than a factor of ten and the reversal of polarity is a clear indication that the polymer can be designed to completely eliminate the temperature dependence of a grating written in a conventional communication grade fiber. The procedure for extending this design to other fibers is set forth in Part II below.

0024These temperature insensitive long period gratings are useful in optical fiber telecommunications as spectral shaping devices. Fig. 6 illustrates an optical transmission system 50 using a temperature insensitive long period spectral shaping device to remove unused pump energy. Specifically, the system 50 comprises a transmitter source 51 of optical signals such as a digitally modulated 1.55 µm signal, an optical signal path comprising a length of optical fiber 52 for transmitting the signal, and a receiver 53 for receiving and demodulating the signal. An optical amplifier such as an erbium-doped fiber amplifier 54 is disposed in the optical signal path for amplifying the transmitted signal. The amplifier is pumped by pump sources 55, 56 of optical energy of pump wavelengths λ_p1 and λ_p2. Unused pump energy of each pump wavelength will pass through amplifier 54. The energy is advantageously removed from the system so that it will not deteriorate the performance of the pump sources 55, 56 and transmission and receiving equipment 51, 53. To remove unused pump energy, a long-period spectral shaping device 57 is disposed in the path of the energy from pump 55 after it has passed through amplifier 54. Specifically, in the dual-pumped laser of Fig. 6, device 57 has its spacing Λ chosen to remove energy of wavelength λ_p1. A second long-period grating 58 has its spacing chosen to remove energy of wavelength λ_p2. In a typical application, λ_s is 1.55 µm, λ_p1 is 0.978 µm and λ_p2 is 0.98 µm. Thus, for example, device 57 could comprise a hydrogen-loaded germanosilicate fiber with core index and diameter chosen such that it allows the propagation of only the fundamental mode at λ ≥ 0.97 µm. For this application the perturbations should be exposed by a dosage ≥ 100 mJ/cm² and there should be at least 20 perturbations in each grating it.

0025Fig. 7 illustrates an optical transmission system 70 using a temperature insensitive long period shaping device 72 to shape the spectral gain characteristics of a rare-earth doped optical fiber amplifier. Specifically it is here used to reduce the spectral dependence of an erbium-doped fiber amplifier 44. The device 72 is disposed in the path of the amplifier 54. The shaping device 72 has one set of spacings Λ chosen to remove energy of wavelength 1.53 µm corresponding to the gain peak wavelength of the amplifier and another set of spacings to remove energy of wavelength 1.56 µm at the other gain peak. By proper choice of the number of perturbations and the dosage of exposure, the gain spectrum of the amplifier/device combination can be made substantially flat over a range of wavelengths 1530 to 1560 nm. For a typical erbium amplifier, the shaping device exposed by a dosage ≤ 100 mJ/cm², 1000 pulses per slit will produce a more uniform gain response over the range of wavelengths 1530-1560 nm.

0026Advantageously, system 70 can be a WDM system using a plurality of different wavelength signals, e.g. λ_s1 and λ_s2. It should be understood that the shaping device 72 need only be in the optical path of the amplifier. It can be before, after or within the optical amplifier cavity comprising the rare-earth-doped fiber.

<i><u>PRINCIPLES OF DESIGN:</u></i>

0027Gratings operate on the principle that the induced periodic index changes match the phase difference between two spatial modes in a fiber. This coupling takes place at a peak wavelength λ_p usually defined in vacuum. A long period grating with period Λ will couple light between two co-propagating modes <i>(LP</i>_<i>01</i><i> and LP</i>_<i>lm</i><i>)</i> that satisfy the condition,<maths id="math0001" num="(1)"><math display="block"><mrow><msub><mrow><mtext>λ</mtext></mrow><mrow><mtext mathvariant="italic">p</mtext></mrow></msub><mtext> = </mtext><msub><mrow><mtext mathvariant="italic">(n</mtext></mrow><mrow><mtext mathvariant="italic">01</mtext></mrow></msub><mtext> - </mtext><msub><mrow><mtext mathvariant="italic">n</mtext></mrow><mrow><mtext mathvariant="italic">lm</mtext></mrow></msub><mtext mathvariant="italic">)</mtext><mtext> Λ,</mtext></mrow></math><img file="EP0909965A1_D0001.tif" /></maths> where Λ is the period of the grating (Λ is usually > <i>10 µm)</i>, and <i>n</i>_<i>lm</i> are the effective indices of the LP_<i>lm</i> modes. An important parameter in the above equation is the effective index of a mode. The effective index of a mode is related to the waveguiding condition that supports the mode in a fiber and is determined by the refractive indices of the central core and all adjacent cladding layers. The coating that surrounds the fiber can be considered to be one of the cladding layers and is an important parameter when analyzing cladding modes. Note that the refractive indices of the different radial layers of a fiber are material properties and can be changed by choosing different material compositions. The effective indices are waveguiding properties of the modes and are directly dependent on the physical dimensions (e.g., diameter) of the fiber and the refractive indices of all layers.

0028We now see how the peak wavelength changes if the temperature of the long-period grating is varied. By differentiating Eq. (1) with respect to temperature (T), we obtain<maths id="math0002" num=""><img file="EP0909965A1_D0002.tif" /></maths> where <i>dΛ/dT</i> is governed by the thermal expansion coefficient of the material. We see from Eq. (2) that to obtain a small change in peak wavelength with respect to temperature (small <i>dλ</i>_<i>p</i> /<i>dT</i>), we need to find materials such that the right hand side of Eq. (2) is approximately zero. Note that it is not very desirable to change <i>dn</i>_<i>01</i><i> /dT</i> since <i>n</i>_<i>01</i> is essentially controlled by the core material and the central core material has to satisfy other optical properties such as reactivity to uv-light (in order that the uv-induced grating may be written). This requirement entails that a photosensitive silicate glass is best-suited for the core. While germania, phosphorus and boron co-doping allows for photosensitive operation, the thermal dependence of the refractive indices of these glasses does not vary much. Further, the main point of this invention is that we use commercially available fibers with no changes made to the glass regions. We therefore focus on changing the material properties of the regions surrounding the core and the cladding. Rewriting Eq. (2) to separate the dependence of the higher-order LP_<i>lm</i>. mode on the polymer coating, we get,<maths id="math0003" num=""><img file="EP0909965A1_D0003.tif" /></maths>

0029The region surrounding the cladding contains some of the power of the LP_<i>lm</i> mode and both n_<i>lm</i>, <i>and dn</i>_<i>lm</i><i>/dT</i> are predominantly controlled by the material properties of this region. Let us assume that for a given fiber and given Λ which couples light between two prespecified modes, the first square bracket on the right hand side of Eq. (3) is equal to Γ. Our goal is to design a coating material that surrounds the fiber. with properties <i>n</i>_<i>p</i> and <i>dn</i>_<i>p</i><i>/dT (where n</i>_<i>p</i> is the refractive index of the polymer material), such that Λ x <i>dn</i>_<i>lm</i><i> /dT + n</i>_<i>lm</i> x <i>dΛ/dT</i> equals Γ. More specifically, we will work with the factor Λ x <i>∂n</i>_<i>lm</i><i>/∂n</i>_<i>p</i><i> x ∂n</i>_<i>p</i><i>/∂T,</i> where n_p is the index of the polymer material and <i>∂n</i>_<i>lm</i>/∂<i>n</i>_<i>p</i> denotes the dependence of the <i><b>effective</b></i> index of the <i>LP</i>_<i>lm</i> mode on the polymer <i><b>refractive</b></i> index. The term <i>∂n</i>_<i>lm</i><i> /∂n</i>_<i>p</i> can be modified by choosing a mode which spreads out to different levels into the coating and is thus variably affected by the coating index.

0030The choice of a suitable polymer material can lead to different values of ∂<i>n</i>_<i>p</i>/∂<i>T</i> The change of the polymer refractive index with temperature is given by the following expression [L. Bohn, in "Polymer Handbook," J. Brandrup and E H. Immergut, Eds., second edition, Wiley, 1975, p. 241]:<maths id="math0004" num=""><img file="EP0909965A1_D0004.tif" /></maths> where α is the volumetric thermal expansion coefficient of the polymer. Since α is positive for all polymers and n_p is > 1, ∂<i>n</i>_<i>p</i>/∂<i>T</i> is negative. The refractive index of the polymer <i>n</i>_<i>p</i><i>,</i> cannot be higher than the index of the glass cladding in order to permit propagation of cladding modes.

0031A suitable polymer for this application can be designed such that the condition of thermal insensivity is met. A general approach to designing the polymer is given below: <ul id="ul0002" list-style="none"><li>1. Long-period fiber gratings are fabricated in a conventional optical fiber.</li><li>2. The temperature dependence of the peak wavelengths corresponding to the different modes in the fiber are characterized.</li><li>3. The effect of recoating the grating with polymers of different refractive indices and different dn_p/dT is evaluated.</li><li>4. A plot of wavelength shift as a function of recoating index provides insight into the dn/dT requirements on the polymer. Such a plot is shown in Figure 8. Steps (1)-(4) allow us to evaluate the range of values for the terms Λ x dn_lm/dT, n_lm x dL/dT, and ∂n_lm/∂n_p. From Eqn. (3) and the conditions described above, we now need to pick a value for ∂n_p/∂T such that the condition Λ x dn_lm/dT + n_lm x dL/dT = Γ is met.</li><li>5. A suitable polymer is chosen using Eqn. (4) above. The emphasis is on picking the right combination of n_p and dn_p/dT to make dλ_p/dT zero.</li><li>6. The long-period grating is recoated and its temperature dependence is measured for the mode under consideration.</li></ul>

0032It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention.