KR20020038756A - Method of forming a grating in an optical waveguide - Google Patents

Abstract

The present invention relates to placing fibers in a tube (34), sealing the tube, and writing a lattice to the fibers in the tube (38).

Description

Method of forming a grating in an optical waveguide

Since the late 1970s, the sensitivity of optical waveguide fibers to light of a particular wavelength and intensity has been known. It has been found that the loss characteristics and refractive index of the waveguide fibers can be changed continuously by exposing the waveguide to light of a given wavelength and intensity and allowing periodic changes in the refractive index to be formed over the length of the optical fiber. The periodic change in the refractive index of the waveguide along the long axis of the waveguide is commonly known as an optical waveguide grating. Fiber Bragg gratings are optical waveguide gratings in waveguide fibers that selectively filter out propagated light having a wavelength that is twice the grating period. The fiber Bragg grating is useful as a waveguide filter.

Fiber Bragg gratings may be formed by a multi-step process that includes writing with actinic radiation, etching, or other mechanism to produce periodic perturbation. Lateral writing is a technique for forming a lattice in an optical waveguide fiber, where light such as actinic radiation causes a periodic arrangement of alternating light / dark fringes along the long axis of the waveguide. An example of such a periodic arrangement is an interference pattern formed along the side of the waveguide fiber and along a portion of the long axis of the waveguide fiber. The periodic light intensity pattern generated by the light interference induces a periodic change in refractive index along a portion of the long axis of the waveguide fiber.

During the fiber grating processing step, exposure of the bare fibers to contaminants can lead to errors and degraded reliability of the fiber grating device. In addition, due to fiber slippage, it may be difficult to make the fiber have sufficient stability to prevent lattice degradation. Such slippage can occur because it is essential to maintain the fiber by its polymer coating using a small amount of tension.

Therefore, it would be very beneficial to provide a process for making a fiber Bragg grating that protects the stripped portion of the optical fiber during the process of writing the grating.

Summary of the Invention

The present invention provides a preferred method of forming a grating in an optical waveguide. By placing the photosensitized optical waveguide in a package prior to writing the grating to the optical waveguide, the present invention allows the optical waveguide to be kept safe and protected from contamination during the manufacturing process step.

According to one aspect of the present invention, the present invention provides a method of positioning a light waveguide in an enclosing structure, sealing the structure such that the waveguide is firmly positioned within the structure, and forming a grating in a portion of the waveguide. It comprises the step of.

In another aspect, embodiments of the present invention provide a method of photosensitizing the optical waveguide, testing the spectral performance of the grating, tuning the grating within the sealed structure, and annealing the grating and the structure. It includes.

The optical waveguide may have various specific forms, including, for example, single mode or multimode optical fiber, multicore optical fiber, channel waveguide, or planar waveguide.

A more complete understanding of the invention and other features and advantages will be apparent from the following detailed description and the accompanying drawings.

The present invention relates generally to the manufacture of fiber optic components. More specifically, the present invention relates to a method of forming a Bragg reflector grating in an optical waveguide, whereby the optical fiber is protected from contamination during manufacture.

1 is a cross-sectional view of a grid package according to the present invention,

2 is a flow chart of a method of forming a fiber grating according to the present invention,

3 is a graph of the reflection curve of a fiber grating formed in accordance with the present invention,

4 is a graph of the permeation curve of a fiber grating formed in accordance with the present invention,

5 is a graph of the transmission spectrum of a fiber grating at multiple time intervals formed in accordance with the present invention.

The invention will be more fully described with reference to the accompanying drawings, in which several preferred embodiments of the invention are shown. However, the invention may be embodied in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these representative embodiments are described in detail so that this disclosure will be thorough in its entirety and will fully implement the scope, structure, operation, function, and applicability of the invention to the prior art.

In the drawings, FIG. 1 is a cross-sectional view of a grating package 10 according to the invention formed by the method described below. For technical purposes, the waveguide is an optical waveguide fiber and an embodiment in which the package is generally tube-shaped is shown and discussed below. However, it will be appreciated that the method of the present invention can also be used with various types of waveguides with appropriate modifications of package form and process steps. The optical fiber 12 is partially sealed by a tubular structure 14 formed of a material that is transparent to actinic radiation such as ultraviolet (UV) light. Boron-doped silica or other glass that is transparent to UV radiation is a suitable material for the structure 14. The tubular structure 14 has an inner diameter “a” (eg 255-1000 μm), an outer diameter “b” (eg 3.0 mm) and a length “c” (eg 70 mm). The optical fiber 12, including its coating 16, has an outer diameter “d” (eg, 250 μm). The coating 16 is stripped along the length of the optical fiber 12 contained in the hollow tube 14.

Two seals 20, 21 disposed at each end 22, 23 of the hollow tube 14 hold and support the area of the optical fiber 12 including the fiber grating 18. The seals 20, 21 may be frits, which include copper glass or other suitable material. The package 10 also includes two plugs 24, 25 of epoxy or other suitable material disposed at each end 22, 23 of the tubular structure 14. The ends 22, 23 of the structure 14 are funnel-shaped, for example, at an angle of 45 ° to facilitate the arrangement of the plugs 24, 25 and the insertion of the optical fiber 12. to be.

While preferred materials and dimensions of the present invention are described herein, the grating package 10 of the present invention may include a variety of materials and sizes, and is not limited to the embodiments described herein. A US patent application filed on Sep. 16, 1999 entitled "Methods and Apparatus for Packaging Long-Period Fiber Lattice", which details of other grating packages and packaging methods suitable for the present invention are incorporated herein by reference. The number is given to Carberry 6).

2 illustrates a method 30 of forming a waveguide grating in a package (such as the grating package 10 above) according to the present invention. In the sensitizing step 32, a waveguide, such as an optical fiber, is sensitized. Examples of suitable optical fibers for the present invention are high-delta, germanium doped, step-index fibers having a delta index of substantially 2%. As used herein, the term delta index indicates the difference in relative refractive index between the core and cladding of the optical fiber and is expressed as a percentage. Examples of suitable processes for photosensitive the optical fiber include exposing the optical fiber to 100 atmospheres of hydrogen atmosphere for two weeks. A section of the fiber is exposed to ultraviolet light. UV lasers operating at 248 nm pulsed at 15 Hz were found to be suitable for this flode exposure. The exposure can be performed at a pulse fluence of 75 milli Joules / cm 2 for 30 minutes. The optical fiber is annealed at 125 ° C. for 24 hours. Other processes suitable for the photosensitization of the present invention are described in US patent application Ser. No. 09 / 252,151, filed Feb. 18, 1999, entitled "Light Waveguide Sensitization," which is hereby incorporated by reference in its entirety.

Next, in packaging step 34, the optical fiber is placed in a hollow tube (such as the hollow tube 14) and sealed to form a package. The package firmly supports and protects the optical fiber from contamination during the processing step. In the lattice writing step 36, a lattice is written into the optical fiber. Various lateral writing techniques can be used to write the grating to the optical fiber. In a technique suitable for the present invention, an excimer-pumped, frequency-doubled dye laser system operating substantially at 240 nm (nanometer) is used as the source of the ultraviolet light. The 240 nm beam generated by the laser first passes through the silica slit. Suitable silica slits are disclosed in US patent application Ser. No. 09 / 081,912, filed May 19, 1998 entitled "Spatial Filters for High Pressure Laser Beams", which is hereby incorporated by reference in its entirety. After the 240 nm beam passes through the silica slit, it passes through an image mask and an optical fiber in the silica tube. The image mask may be a transmission diffraction grating, the structure and properties of the elements being known in the art. The image mask may also be a substrate having consecutive numerical apertures at periodic intervals. During the lattice writing step 36, the tube 14 in the embodiment described above is located approximately 4 millimeters from the image mask. Exposure pulsed from the beam of the excimer laser is present at a repetition rate of 10 Hz for 25 minutes. The laser fluence or intensity at the location of the optical fiber is approximately 75 mill Joules / cm 2. Exemplary reflection and transmission curves of the resulting grating are shown in FIGS. 3 and 4, respectively. The average refractive index change during exposure to the laser beam is approximately 2 × 10 ⁻⁴ .

Next, in a first test step 38, the spectral performance of the grating is tested. The spectral performance is adjusted or tuned in the first tuning step 40. In the first tuning step 40, the grating is flood exposed to UV rays provided by an excimer laser system operating at substantially 248 nm, for example, for 5 minutes to meet the spectral target required for the grating. At the position of the optical fiber of the above-described embodiment, the laser fluence is approximately 75 mill Joules / cm 2, and the repetition rate is 15 Hz. 5 shows an exemplary transmission spectrum of the fiber grating at multiple time intervals during the float exposure. During this exposure time, a total wavelength shift of approximately 0.15 nm occurs. As shown in Figure 5, the transmission minimum increases with exposure time because of a decrease in the amplitude of the grating modulation. The decrease in grating modulation is the result of an increase in the refractive index in the trough previously exposed to light of the grating. In the embodiment described above, this is followed by a first annealing step 42. In this step 42 the package is annealed at 125 ° C. for 24 hours.

In one embodiment, the present invention can be performed with other testing, tuning and annealing steps. In a second test step 44, the spectral performance of the grating is tested to see if the grating meets the spectral target. If the grating does not meet the spectral target, in a second tuning step 46, the grating is flood exposed to UV radiation by an excimer laser system operating at substantially 248 nm to tune the grating. As described above, at the location of the optical fiber, the laser fluence is approximately 75 mill Joules / cm 2 and the repetition rate is 15 Hz. In the final annealing step 48 the package is annealed at 125 ° C. for 24 hours.

It is clear that various modifications and changes can be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims (22)

Positioning an optical waveguide of a predetermined length within the hermetic structure; Sealing the hermetic structure to form a package to protect the length of the optical waveguide in the package; And Forming a grating within a portion of the length of the optical waveguide. The method of claim 1 wherein the structure is substantially transparent to ultraviolet light. The method of claim 2, wherein the structure is boron-doped silica glass. 3. The method of claim 2, wherein the optical waveguide comprises an optical fiber. 3. The method of claim 2, wherein the optical waveguide comprises a planar waveguide. 3. The method of claim 2, wherein the optical waveguide comprises a channel waveguide. The method of claim 1 wherein the optical waveguide is photosensitized. 5. The method of claim 4, wherein the hermetic structure is generally a cylindrical tube. The method of claim 1, wherein the package protects the optical waveguide of the length from contamination. The method of claim 1, wherein forming the grating, Exposing a portion of the length of the optical waveguide to actinic radiation in the package. The method of claim 10, wherein the ultraviolet light comprises a wavelength of substantially 240 nm. The method of claim 10, wherein the ultraviolet light passes through the silica slit. The method of claim 10, wherein the ultraviolet light passes through the image mask. The method of claim 10, wherein the ultraviolet light is pulsed at a repetition rate of substantially 10 Hz for substantially 25 minutes. The method of claim 1, wherein the method comprises forming the grating: Tuning the grating to adjust the spectral characteristics of the grating. The method of claim 15, wherein tuning the grating comprises flooding the grating with ultraviolet light. The method of claim 15, wherein the method comprises, after tuning the grating: Annealing the grating. 18. The method of claim 17, wherein after the annealing the grating, Further tuning the grating to adjust the spectral characteristics of the grating. 19. The method of claim 18, wherein secondary tuning the grating comprises flooding the grating with ultraviolet light. 20. The method of claim 19, wherein the ultraviolet light comprises a wavelength of substantially 248 nm. 21. The method of claim 20, wherein the ultraviolet light is pulsed at a repetition rate of substantially 15 Hz for substantially 5 minutes. 19. The method of claim 18, wherein after the second tuning of the grating, Further annealing the lattice.