AU7471100A - Method of forming a grating in an optical waveguide - Google Patents

Description

WO 01/21538 PCTIUSOO/23501 METHOD OF FORMING A GRATING IN AN OPTICAL WAVEGUIDE FIELD OF THE INVENTION The present invention relates generally to the production of fiber optic 5 components. More specifically, the present invention relates to methods for forming a Bragg reflection grating in an optical waveguide whereby the optical fiber is protected from contamination during fabrication. BACKGROUND OF THE INVENTION 10 The sensitivity of optical waveguide fibers to light of certain wavelength and intensity has been known since the late 1970's. It was found that the loss characteristic and refractive index of a waveguide fiber could be permanently changed by exposing the waveguide to light of a given wavelength and intensity, allowing periodic variations in the refractive index of a length of optical fiber to be formed. A periodic variation in 15 refractive index of the waveguide along the long axis of the waveguide is commonly known as an optical waveguide grating. A fiber Bragg grating is an optical waveguide grating in a waveguide fiber which will selectively filter propagated light having a wavelength which is twice the period of the grating. Such a fiber Bragg grating is useful as a wavelength filter. 20 Fiber Bragg gratings may be formed by a multiple step process which includes writing with actinic radiation, etching, or other mechanisms for making periodic perturbations. Side writing is a technique for forming a grating in an optical waveguide fiber wherein light, such as actinic radiation, is caused to form a periodic series of alternating light and dark fringes along the long axis of the waveguide. An example of 1 WO 01/21538 PCTIUSOO/23501 such a periodic series is an interference pattern formed on the side of a waveguide fiber and along a portion of the long axis of a waveguiding fiber. The periodic light intensity pattern, produced by the light interference, induces a periodic change in refractive index along a portion of the long axis of the waveguide fiber. 5 It is recognized that during the fiber grating processing steps, exposure of the bare fiber to contaminants can lead to failure of the fiber grating device and reduced reliability. Additionally, there may be some difficulty holding the fiber with sufficient stability to prevent grating degradation due to fiber slippage. Such slippage may occur because it is necessary to hold the fiber by its polymer coating utilizing a small amount 10 of tension. Accordingly, it would be highly advantageous to provide a process for making a fiber Bragg grating which protects and secures the stripped portion of the optical fiber during the process of writing the grating. 15 SUMMARY OF THE INVENTION The present invention provides an advantageous method for forming a grating in an optical waveguide. By placing a photosensitized optical waveguide into a package prior to writing the grating into the optical waveguide, the present invention allows the optical waveguide to be securely held and protected from contamination 20 during the fabrication process steps. According to one aspect of the invention, the method includes placing an optical waveguide within an enclosing structure, sealing the structure so that the waveguide is secured within the structure, and forming a grating within a portion of the waveguide. In alternative aspects, an embodiment of the invention may include the steps of 25 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. The optical waveguide may have many specific forms including that of, for example, a single mode or a multimode optical fiber, a multicore optical fiber, a 30 channel waveguide, or a planar waveguide. 2 WO 01/21538 PCT/USOO/23501 A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following detailed description and the accompanying drawings. 5 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view of a grating package in accordance with the present invention; Fig. 2 is a flowchart of a method of forming a fiber grating in accordance with the present invention; 10 Fig. 3 is a graph of a reflectance curve of a fiber grating formed in accordance with the present invention; Fig. 4 is a graph of a transmittance curve of a fiber grating formed in accordance with the present invention; and Fig. 5 is a graph of a transmission spectrum of a fiber grating at multiple time 15 intervals formed in accordance with the present invention. DETAILED DESCRIPTION The present invention now will be described more fully with reference to the accompanying drawings, in which several currently preferred embodiments of the 20 invention are shown. However, this invention may be embodied in various forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these representative embodiments are described in detail so that this disclosure will be thorough and complete, and will fully convey the scope, structure, operation, functionality, and potential of applicability of the invention to those skilled in the art. 25 Referring to the drawings, Fig. 1 shows a cross-sectional view of a grating package 10, formed by methods described below, in accordance with the present invention. For purposes of illustration, an embodiment in which the waveguide is an optical waveguiding fiber and the package is generally tube-shaped is shown and discussed. It will be understood, however, that the method of the invention may also be 30 used with other types of waveguides, with suitable modification of package shapes and process steps. An optical fiber 12 is partially enclosed by a tube-shaped structure 14 formed of material that is transparent to actinic radiation, such as ultraviolet (UV) light. 3 WO 01/21538 PCT/USOO/23501 Boron-doped silica or other glass which is transparent to UV light are suitable materials for the structure 14. The tube-shaped structure 14 has an inner diameter "a" (e.g., 255 1000 ptm), an outer diameter "b" (e.g., 3.0 mm) and a length "c" (e.g., 70 mm). The optical fiber 12 including its coating 16 has an outer diameter "d" (e.g., 250 Im). The 5 coating 16 has been stripped from a length of the optical fiber 12 which is contained within the hollow tube 14. The optical fiber 12 has written into it a fiber grating 18 along a portion of the length which has been stripped of the coating 16. Two seals 20, 21 disposed at each end 22, 23 of the hollow tube 14 tensionally maintain and support the region of the optical fiber 12 containing the fiber grating 18. 10 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 tube-shaped structure 14. The ends 22, 23 of the structure 14 are funnel-shaped at an angle of, for example, 450 to facilitate placement of the plugs 24, 25 and insertion of the optical fiber 12. 15 While presently preferred materials and dimensions are disclosed herein, one skilled in the art would appreciate that the grating package 10 of the present invention may include a variety of materials and sizes, and should not be construed as limited to the embodiments or dimensions shown and described herein, which are exemplary. Further details of other grating packages and packaging methods suitable for use with 20 the present invention are provided in U.S. Patent Application (Attorney Docket No. Carberry 6), filed on September 16, 1999, entitled "Method And Apparatus For Packaging Long-Period Fiber Grating" which is incorporated by reference herein in its entirety. Fig. 2 shows a method 30 of forming a waveguide grating in a package (such as 25 the grating package 10) in accordance with the present invention. In a sensitizing step 32, a waveguide such as, for example, an optical fiber is photosensitized. An example of an optical fiber suitable for use with the present invention is a high-delta, germanium doped, step-index fiber with an index delta of substantially 2%. As used herein, the term index delta refers to the relative refractive index difference between the core and 30 the cladding of the optical fiber and is expressed as a percentage. An example of a process suitable for photosensitizing the optical fiber includes exposing the optical fiber to a hydrogen atmosphere at 100 atmospheres of pressure for two weeks. A section of 4 WO 01/21538 PCTIUSOO/23501 the optical fiber is then flood exposed to ultraviolet light. A UV laser operating at 248 nm pulsed at 15 Hz has been found suitable for this flood exposure. The exposure may be at a pulse fluence of 75 millijoules/cm2 for 30 minutes. The optical fiber is then annealed for 24 hours at 1250 C. Another process suitable for photosensitizing for use 5 with the present invention is described in U.S. Patent Application No. 09/252,151, filed on February 18, 1999 entitled "Optical Waveguide Photosensitization" which is incorporated by reference herein in its entirety. Next, in a packaging step 34, the optical fiber is placed within a hollow tube (such as the hollow tube 14) and sealed to form a package. The package securely holds 10 and protects the optical fiber from contamination during the process steps. In a grating writing step 36, a grating is written onto the optical fiber. Any of a variety of side writing techniques may be used to write the grating into the optical fiber. In one exemplary technique suitable for use with the present invention, an excimer-pumped, frequency-doubled dye laser system operating at substantially 240 nm (nanometers) is 15 used as the source of the ultraviolet (UV) light. The 240 nm beam produced by the laser is first passed through silica slits. Suitable silica slits are described in greater detail in U.S. Patent Application Serial No. 09/081,912, filed on May 19, 1998, entitled "Spatial Filter For High Power Laser Beam" which is incorporated by reference herein in its entirety. After the 240 nm beam has passed through the silica slits, it passes 20 through a phase mask and then onto optical fiber within the silica tube. The phase mask may be a transmission diffraction grating, a component whose structure and characteristics are known in the art. A phase mask may also be a substrate having a series of periodically spaced openings. During the grating writing step 36, the tube 14 in the illustrated embodiment is located approximately 4 millimeters from the phase 25 mask. The pulsed exposure from the beam of the excimer laser is at a repetition rate of 10 Hz for 25 minutes. The laser fluence, or intensity, at the position of the optical fiber is approximately 75 millijoules/cm2. Exemplary reflectance and transmittance curves of the resulting grating are shown in Figs. 3 and 4, respectively. The average refractive index change during exposure to the laser beam was approximately 2 x 10-4. 30 Next, in a first testing step 38, the spectral performance of the grating is tested. Spectral performance is adjusted, or tuned, in a first tuning step 40. In the first tuning step 40, the grating is flood exposed to UV light provided by, for example, an excimer 5 WO 01/21538 PCT/USOO/23501 laser system operating at substantially 248 nm for 5 minutes in order to meet the spectral target required for the grating. At the position of the optical fiber in the illustrated embodiment, the laser fluence is approximately 75 millijoules/cm2 and the repetition rate is 15 Hz. Fig. 5 shows an exemplary transmission spectrum of the fiber 5 grating at multiple time intervals during the flood exposure. During the exposure time, a total wavelength shift of approximately 0.15 nm occurs. As seen in Fig. 5, the transmission minimum increases as exposure time increases due to a decrease in the amplitude of the grating modulation. The decrease in grating modulation is the result of an increase of the refractive index in the previously light exposed troughs of the 10 grating. In the illustrated embodiment, this is followed by a first annealing step 42. In this step 42, the package is annealed for 24 hours at 1250 C. In one embodiment, this may be followed by further testing, tuning and annealing steps. In a second testing 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 15 spectral target, then in a second tuning step 46 the grating is flood exposed to UV light by an excimer laser system operating at substantially 248 nm in order to tune the grating. As illustrated, at the position of the optical fiber, the laser fluence is approximately 75 millijoules/cm 2 and the repetition rate is 15 Hz. In a final annealing step 48, the package is annealed for 24 hours at 125'C. 20 It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the present invention. Thus, 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. 6

Claims (22)

1. A method of forming a grating in an optical waveguide comprising the steps of: placing a length of said optical waveguide within an enclosing structure; 5 sealing said enclosing structure to form a package such that said length of optical waveguide is secured within said package; and forming a grating within a portion of said length of said optical waveguide.

2. The method of claim 1 wherein the structure is substantially transparent to ultraviolet radiation. 10

3. The method of claim 2 wherein the structure is boron-doped silica glass.

4. The method of claim 2 wherein the optical waveguide includes an optical fiber.

5. The method of claim 2 wherein the optical waveguide includes a planar waveguide. 15

6. The method of claim 2 wherein the optical waveguide includes a channel waveguide.

7. The method of claim 1 wherein the optical waveguide is photosensitized.

8. The method of claim 4 wherein the enclosing structure is a generally 20 cylindrically shaped tube.

9. The method of claim 1 wherein the package protects the length of optical waveguide from contamination.

10. The method of claim I wherein the step of forming the grating includes a sub-step of: 25 exposing a portion of said length of the optical waveguide within the package to actinic radiation.

11. The method of claim 10 wherein the ultraviolet light includes a wavelength of substantially 240 nanometers.

12. The method of claim 10 wherein the ultraviolet light passes through 30 silica slits.

13. The method of claim 10 wherein the ultraviolet light passes through a phase mask. 7 WO 01/21538 PCT/IUSOO/23501

14. The method of claim 10 wherein the ultraviolet light is pulsed at a repetition rate of substantially 10 Hz for substantially 25 minutes.

15. The method of claim I further comprising, after the step of forming the grating, the step of: 5 tuning the grating to modify the spectral characteristics of the grating.

16. The method of claim 15 wherein the step of tuning the grating includes flood exposing the grating to ultraviolet light.

17. The method of claim 15 further comprising, after the step of tuning the grating, the step of: 10 annealing the grating.

18. The method of claim 17 further comprising, after the step of annealing the grating, the step of: second tuning the grating to modify the spectral characteristics of the grating.

19. The method of claim 18 wherein the step of second tuning the grating 15 includes flood exposing the grating to ultraviolet light.

20. The method of claim 19 wherein the ultraviolet light includes a wavelength of substantially 248 nanometers.

21. The method of claim 20 wherein the ultraviolet light is pulsed at a repetition rate of substantially 15 Hz for substantially 5 minutes. 20

22. The method of claim 18 further comprising, after the step of second tuning the grating, the step of: second annealing the grating. 8