Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Reexamination Certificate
2000-02-02
2002-09-24
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
C385S037000
Reexamination Certificate
active
06456771
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the field of optical fibers. More particularly, the present invention pertains to an optical fiber, including a pure silica core, having a Bragg grating formed in the core.
BACKGROUND OF THE INVENTION
According to the prior art, a Bragg grating in an optical fiber is usually created in the core region of the optical fiber, a core region typically of silica and containing germanium or other dopants so as to impart to it, by exposure to ultraviolet (UV) light, the photo-refractive structure known as a Bragg grating. See, e.g. K. O. Hill et al., “Photosensitivity In Optical Fiber Waveguides: Application To Reflection Filter Fabrication”, Appl. Phys. Lett. 32, 1978, pp. 647-649; and G. Meltz et al., “Formation Of Bragg Gratings In Optical Fibers By A Transverse Holographic Method”, Opt. Lett. 14, 1989, pp. 823-825.
Bragg gratings are generally produced in such a doped silica core of an optical fiber by laterally exposing the optical fiber to a three dimensional fringe pattern created by interfering holographically two coherent high intensity UV beams or by exposing the optical fiber to UV light passed through a diffractive optical element called a phase mask. The doped glass, by virtue of the lattice defects (in this case point imperfections) associated with the dopants, interacts with the bright portions of a UV pattern to produce light-absorbing color centers. Either technique produces a pattern of UV light consisting of alternating bright and dark regions. The doped glass interacts with the bright portions of the UV pattern in such a way that its refractive index is modified leaving a refractive index modulated according to the UV pattern.
Bragg gratings have been demonstrated in commercial telecommunications-grade optical fibers that contain germanium in the core (Corning SMF-28 for example). However, high levels of photosensitivity of these optical fibers is needed to achieve high reflectivity gratings and compatibility with manufacturing processes within reasonable exposure parameters. This has led to the use of hydrogenation, along with the development of special optical fibers containing high levels of germanium or other photosensitizing dopants, as a means to increase the number of defect centers to promote optical fiber photosensitivity. Pure silica core optical fibers, which contain little or no such defects, have been found unsuitable as a host material for forming Bragg gratings using UV exposure processes.
Pure silica core optical fibers however, are attractive in a number of applications due to their superior resistance to hydrogen-induced attenuation and to nonlinear effects. The hydrogen-induced attenuation is attributed to changes over time of energy levels of the doped glass structure; it is well known that UV-induced refractive index changes in “color center” Bragg gratings decay over time, at a rate depending on temperature, corresponding to the thermal depletion of lower energy trap states. (See, e.g. T. Erdogan et al., “Decay Of Ultraviolet-Induced Bragg gratings”, Appl. Phys. Lett. 76, 1994, pp. 73-80.) Accelerated aging techniques (thermal annealing) are routinely employed as a means of providing stable grating performance. But if a Bragg grating could be provided in pure silica, aging would not be necessary. What is needed is a way of providing a Bragg grating in a pure silica core of an optical fiber, which would then not suffer from the drawbacks of Bragg gratings as provided by the prior art, namely by relying on doping the core.
The present invention builds on another approach besides accelerated aging to provide a stable Bragg grating. It forms a Bragg grating by providing in a target length of an optical fiber a periodic depletion (a sequence of alternating high and low concentrations) of an index-varying dopant in the core within the target length, thus providing an index modulation and corresponding Bragg resonance. As opposed to the “color center” type gratings whose index modulation is subject to decay across a wide temperature range, the gratings of the invention are stable to temperatures in excess of 500° C., where the glass network becomes mobile. (See M. A. Fokine et al., “High Temperature Resistant Bragg Gratings Fabricated In Silica Optical fibers”, ACOFT, 1996.)
These gratings, called chemical gratings (as opposed to color center gratings formed by exposure of doped silica to UV light), rely on the presence of (point) defect sites that lead to hydroxyl formation. Chemical gratings are formed in a three-step process:
Step 1:
(Hydrogenation)
≡Ge—O—Ge≡ + H
2
≡Si—O—Si“ + H
2
Step 2:
(UV Exposure)
≡Ge—O—Ge≡ + H
2
→ ≡Ge—O—H +
H—Ge≡
Step 3:
(Heating)
≡Si—O—H + F—Si≡ ←→
≡Si—O—Si≡ + HF↑
In the first step, a special fluorine-codoped germanium silicon (Ge—Si) single-mode optical fiber is hydrogenated. The hydrogen loaded optical fiber then undergoes a typical UV exposure used to write traditional Bragg gratings, i.e. it is exposed to an interference or diffraction pattern of UV light. In those portions of the optical fiber exposed to UV light, as opposed to those portions exposed to only low intensity UV light (or to no UV light at all), the UV light causes a photochemical reaction in which the free hydrogen migrates to defect sites (primarily from the Ge dopant) and forms hydroxyl. The optical fiber is then heated causing atomic fluorine (F) in the presence of hydroxyl (OH) to form (volatile) hydrogen fluoride (HF), which then thermally diffuses rapidly out of the optical fiber. Because the optical fiber is exposed to spatially varying amounts of UV radiation, the hydroxyl formation, and thus HF concentration, will also vary according to the UV exposure pattern. Therefore, the fluorine depletion from the out-diffusion of HF will follow the pattern, leading to a refractive index modulation where fluorine depleted sections exhibit a higher refractive index (by virtue of fluorine being an index-lowering dopant). This reaction can be observed by monitoring the optical spectrum of the grating during these steps. After the UV exposure, a “first” grating appears. Upon the heating step and the subsequent chemical reaction, the “first” grating is erased prior to the formation and appearance of the “chemical” grating.
Fluorine in low concentrations is commonly used by the optical fiber industry as an index lowering dopant in providing cladding glasses. Recently, chemical gratings have been formed in commercial optical fibers, by thermally diffusing the fluorine present in the optical cladding region into the core region of the optical fiber. Applying this diffusion technique can eliminate the requirement for a special fluorine-doped core optical fiber. A process for making chemical gratings in commercial optical fibers with fluorinated cladding regions has been demonstrated ( M. A. Fokine and R. Stubbe, Private Correspondence, Sep. 30, 1998) that is identical to the original process, except for the addition of a fluorine diffusion step, as shown below.
Step 1:
(Fluorine In-Diffusion)
Si—F (cladding) → ≡Si—O—Si≡ + Si—F (core)
Step 2:
(Hydrogenation)
≡Ge—O—Ge≡ (core) + H
2
≡Si—O—Si≡ (core) + H
2
Step 3:
(UV Exposure)
≡Ge—O—Ge≡ + H
2
→ ≡Ge—O—H + H—Ge≡
Step 4:
(Heating)
≡Si—O—H + F—Si≡ ←→ ≡Si—O—Si≡ + HF↑
In this modified process, a commercial Ge-doped optical fiber is used. The presence of the Ge dopant provides a suitable population of defect sites to form hydroxyl in the presence of hydrogen, during UV exposure.
This process, without more, is not suitable for providing chemical gratings in pure silica core optical fibers because of the lack of a suitable number of defect sites in pure silica. What is needed is a way of providing a suitable number of defect sites in pure silica, without doping, and so adapting a pure silica material so that it i
Cherry Euncha
CiDRA Corporation
Spyrou Cassandra
Ware, Freesola, Van Der Sluys & Adolphson LLP
LandOfFree
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