WO2001055759A1

WO2001055759A1 - Optical waveguides and production thereof

Info

Publication number: WO2001055759A1
Application number: PCT/US2001/001263
Authority: WO
Inventors: Glenn E. Kohnke
Original assignee: Corning Incorporated
Priority date: 2000-01-31
Filing date: 2001-01-12
Publication date: 2001-08-02
Also published as: AU2001232803A1

Abstract

Gratings which have been annealed at low temperatures resulting in increased thermal stability. These may be made by a first UV or IR exposure followed by a low temperature annealing at 125 degrees for 24 hours and writing a grating (52) into the photosensitive core (36) of the fiber (40) as shown in figure 2. The use of these with temperature controllers is also disclosed.

Description

Optical waveguides and production thereof

BACKGROUND OF THE INVENTION The present invention relates generally to enhancing the photosensitivity of optical waveguides, and more particularly to increasing the thermal stability of gratings in optical waveguides.

While the present invention is subject to a wide range of applications, it is particularly well suited for providing persistent ultra-violet photosensitivity to optical waveguides and tailoring the refractive index profiles of such waveguides.

Gratings produced by ultra-violet (UV) induced refractive index changes in optical waveguides are well known in the art. Broadly speaking, such gratings generally fall into one of two categories. The first category of gratings known as a long period grating (LPG) is commonly formed by UV exposure through an amplitude mask. The second category known as a short period grating or fiber Bragg grating (FBG) is typically formed using two - beam interference. Generally speaking, the use of a phase mask to create the two - beam interference is currently recognized as a preferred method of "writing" a FBG into an optical waveguide. It is known that the strength of the FBG grating

(i.e., the change in refractive index resulting from UV exposure relative to the optical waveguide refractive index prior to UV exposure) is dependent upon, inter alia, the photosensitivity of the optical waveguide. FBG's are typically formed in doped silica glasses commonly used in both planar waveguides and optical waveguide fibers which in turn are commonly used in opto-electronic components and other devices of telecommunication systems. It has been found that the sensitivity of optical waveguides containing these doped silica glasses to UV radiation can be enhanced by a hydrogen treatment. Early proposed H sensitization treatments involved exposing a germanium doped silica glass to H₂ at a relatively high temperature, typically at least 400° C. Such high temperatures proved detrimental to germanium doped optical waveguide fibers, as well as to other optical waveguide fibers containing other dopants. In view of the fact that optical waveguide fibers are typically coated with a polymeric coating material as part of the draw process, such extreme temperatures were found to destroy or at least severely damage the fiber coatings. Moreover, such high temperature sensitization treatments typically increased the attenuation associated with the fiber and/or weakened the fiber itself. In view of these shortcomings, improved H₂ loading techniques have been developed. As a result, the photosensitive response of standard optical waveguide materials has been dramatically enhanced. In one such technique, an Si0₂ - based optical waveguide is exposed to an H₂ atmosphere (partial pressure greater than one atmosphere) at low temperature (at most 250° C) for a period of days or weeks, such that the H₂ diffuses into the optical waveguide. Such H₂ loaded optical waveguides are then exposed to UV radiation to increase the refractive index of the irradiated portion of the optical waveguide, thus forming a grating. Normalized index changes on the magnitude of 5 x 10^"5 or 10^"4 have been achieved for practical devices using this technique while index changes as high as 10^"2 have been reported for certain specialty fibers having less practical application. It was found that the index changes could persist substantially indefinitely provided the waveguide was not heated, and that at least a significant fraction of the change could survive moderate heating (e.g., less than or equal to 400° C). In a more recent development, it has been found that the index of refraction of a germania doped optical waveguide fiber can be increased by treating the fiber with hydrogen and applying heat. The glass is exposed to hydrogen at a pressure in the range 14-11 ,000 p.s.i. and a temperature in the range 21 ° to 150° C until sufficient hydrogen is diffused into the fiber. The fiber is then subjected to heat in excess of about 500° C using a flame or infrared radiation for a period of a second or less. This technique results in a substantial and long-lived increase in the normalized refractive index. Flame heating of a single mode germania doped optical waveguide fiber has produced normalized index changes (Δn/n) of 4 x 10^"3.

Both of the above-described techniques rely on hydrogen loading to provide the required optical waveguide photosensitivity. Once loaded, the waveguide is then exposed to either UV radiation or heat to change the refractive index of the waveguide. The gratings are subsequently annealed to stabilize the index change. Accordingly, devices produced by these techniques have refractive index characteristics which can vary substantially due to, among other things, variations in fiber geometry, and/or the intrinsic photosensitivity of the fiber. Thus, although the above-described techniques can produce significant refractive index changes, there is little control over the accuracy of those changes. As a result, only a small percentage of the devices produced by the above-described techniques are capable of adequately meeting the needs and requirements of the systems in which they will be used absent some additional processing.

One need, in particular, is the need for the grating to maintain its performance at temperatures over 70 ° C for lifetimes as long as twenty-five

(25) years. Gratings have been made stable and able to met the above need by annealing the grating. However, this annealing of the grating will reduce the change in refractive index associated with the grating. The specific annealing conditions are obtained by accelerated aging tests at temperatures significantly above the operating temperature. In view of the foregoing, there is a need for a method of enhancing the

UV photosensitivity of an optical waveguide, which simplifies the grating manufacture process while increasing the yield of acceptable gratings. In addition, there is a need for an optical waveguide which retains significant photosensitivity following out diffusion of the loading gas such that significant changes in refractive index can be achieved by exposing the waveguide to additional radiation treatments. Moreover, there is a need for a method of controlling the photosensitivity of an optical waveguide on a spatial scale on the order of microns. Furthermore, there is a need for an optical waveguide having a grating which is thermally stable.

SUMMARY OF THE INVENTION

The present invention is set forth in the appended claims. One advantage of the present invention is that an optical waveguide has a nonannealed grating with a decay of no more than 10%.

An advantage of the invention is that an optical waveguide has virgin grating. The grating includes a center wavelength having a negligible shift-after annealing the grating and a negligible change in transmission loss after annealing the grating.

A method of making the optical waveguide of the invention results in a number of advantages over other methods known in the art. For example, the method of the present invention produces an optical waveguide having persistent UV photosensitivity after the loading gas has diffused from the optical waveguide. Accordingly, it is possible to write a grating device, which is sufficiently strong and has the desired characteristics in an optical waveguide, which no longer contains the loading gas.

The grating is thermally stable and does not require subsequent heat treating such as annealing. As a result, the grating writing process is streamlined in that premature out diffusion of the loading gas is less problematic and the process step of annealing the grating has been eliminated. The elimination of annealing the grating also eliminates the requirement that the grating must be written to a high enough strength to account for grating decay that occurs during the annealing of the grating. The gratings of the invention have the advantage of eliminating the variability of the gratings properties associated with annealing. These properties include spectral width, center wavelength and dispersion.

Furthermore, the combination of H₂ or D₂ loading of optical waveguides followed by UV flood exposure has resulted in waveguides which retain a UV photosensitivity sufficient to produce refractive index changes greater than 10^"4. Moreover, in certain applications of the present invention, retained UV photosensitivity sufficient to produce refractive index changes of less than 10^"4 is considered very advantageous.

In addition, the method of the present invention provides a number of advantages not otherwise provided by other methods known in the art. Once a grating is written, UV flood exposure can be used to tune the grating wavelengths. In gain flattening applications, wavelength error of a gain flattening filter results in amplifier power variation as a function of wavelength, thus the tuning application of the present invention provides for a more consistent power level in the output of the amplifier. These capabilities make this aspect of the method of the present invention particularly well suited for such applications as gain flattening filters for use in amplifiers. In addition, the tuning aspects of the present invention provide a greater yield of gratings having the desired characteristics and capabilities prior to packaging. Heretofore, mechanical tuning techniques, such as tension tuning following packaging, among others, have been relied upon to tune grating strength and wavelength. Moreover, as grating device technologies mature, there is an increasing interest in chirped gratings and gratings with unusual apodization profiles. Generally, such gratings are fabricated by spatial control of the exposing beam and/or specialty phase masks. Until now, the spatial photosensitivity of the fiber has remained constant. The photosensitivity of an optical fiber can now be controlled on a spatial scale on the order of microns. Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description, or recognized by practicing the invention described in the written description and claims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of the flood exposure step of an optical waveguide in accordance with the invention.

Fig. 2 is a schematic view of an FBG writing step of an optical waveguide in accordance with the invention.

Fig. 3 shows the transmission spectrum of an FBG written in UV flood exposed CS-980 fiber in accordance with example of the present invention.

Fig. 4 shows the UV induced refractive index as a function of flood exposure time for an FBG written in CS-980 fiber in accordance with Example 1 of the present invention. Fig. 5 shows the UV induced refractive index as a function of flood exposure time for an FBG written in SMF-28 fiber in accordance with Example 1 of the present invention.

Fig. 6 shows the reflectance spectra of a sampled FBG grating device in accordance with Example 2 of the present invention.

Fig. 7 Illustrates the decay of transmission loss at 200° C of hydrogen loaded grating annealed at 125° C for 24 hours.

Fig. 8 illustrates the decay of transmission loss at 200° C of UV photosensitized grating without annealing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Generally speaking, the method of the present invention is directed to enhancing the photosensitivity of an optical waveguide by irradiating at least a portion of the optical waveguide with radiation. In an exemplary embodiment, the optical waveguide is hydrogen loaded prior to UV exposure. By "hydrogen" or "H₂", we mean herein, hydrogen and/or its isotope deuterium (D₂). Applicant expressly contemplates the manufacture of optical waveguide fibers, both single-mode and multi-mode, as well as planar waveguides, regardless of any specific description, drawings, or examples set out herein. In addition, it is anticipated that the present invention can be practiced in conjunction with any of the known optical waveguide processing techniques, including, but not limited to, the outside vapor deposition (OVD) technique, the modified chemical vapor deposition (MCVD) technique, the vertical axial deposition (VAD) technique, the plasma chemical vapor deposition (PCVD) technique, and sol-gel techniques. For the purposes of this specification, and to more clearly describe the various embodiments of the present invention, preferred embodiments of the method of enhancing the photosensitivity in optical waveguides, and optical waveguides made by the method, will be described herein and shown in the accompanying drawing figures as being optical waveguide fiber, and specifically optical waveguide fiber manufactured using the OVD technique.

An example of a method to enhance the photosensitivity of an optical waveguide fiber of the invention is shown, in part, in Fig. 1. Prior to UV flood exposure 20, optical waveguide fiber 22 is hydrogen loaded (100 atm at 25°C for 2 weeks). To inhibit out diffusion of the H₂, hydrogen loaded optical waveguide fiber 22 may be stored in a freezer until such time as UV exposure is desired. Hydrogen loaded optical waveguide fiber 22 includes an elongate core 24 having a relatively high refractive index, surrounded by a relatively low refractive index cladding 26. Cladding 26 is preferably bound by a polymeric coating 28. Preferably, the predominantly Si0₂ containing hydrogen loaded optical waveguide fiber 22 is doped with a dopant such as, but not limited to, germania (Ge0₂). A portion of coating material 28 is removed from the hydrogen loaded optical waveguide fiber 22 and the fiber 22 is positioned adjacent a source of UV radiation, preferably an excimer laser 30. Laser 30 delivers uniform UV light 32 over the exposed waveguide section 34 of hydrogen loaded optical waveguide fiber 22 for a period of time sufficient to increase the refractive index of the optical fiber 22. Typical exposure times range from minutes to several hours, depending upon the magnitude of the Δn desired. Generally speaking, the greater the flood exposure time, the greater the change in refractive index. Once the desired change in refractive index has been attained, optical waveguide fiber 22 is annealed at approximately 125°C for a period of about 24 hours. The annealing step (not shown) diffuses the hydrogen from the loaded optical waveguide fiber 22. It will be understood by those skilled in the art, however, that hydrogen will diffuse from fiber 22 without the application of heat over a longer time period. The exposed waveguide section 34 retains an UV photosensitivity sufficient to produce refractive index changes greater than 10^"4. Such waveguides are referred to as UV- photosensitized waveguides. The aforementioned method has a variety of uses. In one embodiment, it can be used to create refractive index gratings within the core 36 of a photosensitive optical waveguide fiber 40 as shown in Fig. 2. In standard H₂ loaded optical waveguide fiber, once the hydrogen has been removed from a hydrogen loaded optical waveguide fiber by annealing or otherwise, exposure to UV radiation has very little effect on the refractive index of the fiber.

However, as a result of the UV flood exposure step shown in Fig. 1 , the previously exposed section 42 of optical waveguide fiber 40 remains photosensitive following out diffusion of the hydrogen.

As shown in Fig. 2, a phase mask 44 is positioned between the laser 30 and the previously exposed section 42 of optical waveguide fiber 40.

UV radiation 32 is then delivered from the laser 30 through the phase mask 44. UV radiation 50 is passed through the phase mask 44 to produce two-beam interference, through the cladding 38, and into the core 36 which induces a change in refractive index within the core 36 of that area of the core 36 exposed to UV radiation 50. The two-beam interference produces an index modulation as a function of length, referred to as the modulated refractive index. The UV exposed portions of the core 36 form a fiber Bragg grating 52.

The strength of the index change is a function of the initial flood exposure time. In addition, it has been found that the UV photosensitivity effects scale with dopant concentration. Thus, the longer the exposure to UV radiation during flood exposure, and the greater the dopant concentration, the larger the induced change in refractive index within the core 36, and thus, the stronger the grating. Although the grating writing step shown in Fig. 2 incorporates a phase mask 44, it will be understood by those skilled in the art that other methods and devices used to create two-beam interference can be substituted for the phase mask 44 used in this embodiment of the present invention. Moreover, the present invention is not limited to UV exposure. Infrared radiation, such as that delivered by a C0₂ laser, or some other narrowly directed heat source can be used to flood expose the fiber or write the grating device in accordance with this embodiment of the present invention, and further can be used for tuning applications as will be described below. The waveguide exposed to such IR radiation are referred to as I R photosensitized waveguides. In addition, other gratings in addition to FBG devices can be written into the core of the optical waveguide fiber 20 of the present invention as will be more clearly described in the examples which follow below. Another exemplary use of the method of the present invention is the fine-tuning of grating wavelengths following the grating writing process.

The optical waveguide of the invention has a grating. The grating is a virgin grating. The term virgin is used to indicate that the grating has not been heated past its annealing point. Another way to describe this is that once the grating is written in the optical waveguide, the waveguide is not annealed or the grating is unannealed. The virgin grating in this example, of the invention has a decay of no more than 10% and preferably no more than 3%. The decay cited above is thermal decay. For further explanation on the issue of thermal decay, reference is made to U.S. Patent No. 5,620,496, which is incorporated herein by reference as though fully set forth in its entirety. The grating of the invention may also be described in terms of stability instead of decay.

It's preferred that the optical waveguide is a fiber, however the invention is not limited to an optical waveguide in the form of a fiber. It is preferred that the grating is a Fiber Bragg Grating. As shown in Fig. 3, the optical waveguide may have a transmission loss greater than approximately -40dB.

The optical waveguide of the invention will exhibit a change in modulated refractive index (Δn) that is at least about 10 ^~5. Preferably, the optical waveguide will exhibit a change in refractive index is at least 10 ^~4. One technique to determine if the grating is sufficiently stable is to anneal the optical waveguide with the virgin grating. An optical waveguide with a sufficiently stable grating will exhibit a transmission loss that is substantially the same before and after annealing the waveguide. The optical waveguide will also exhibit a center wavelength that is substantially the same before and after annealing the waveguide. Therefore, annealing the optical waveguide of the invention will not cause the center wavelength to shift, nor will the annealing step change the transmission loss. The change in transmission loss or the change in center wavelength may also be described as a negligible or minute change. A process for testing the virgin grating, for decoy performance described above, is to anneal the optical waveguide having the virgin grating at 125 ° C for twenty-four (24) hours.

For the purpose of comparison a hydrogen loaded grating is annealed under the same conditions. The hydrogen loaded gratings exhibit a significant change in transmission loss after annealing. The hydrogen loaded grating has a UV-induced index change of approximately 25%. In comparison, the change in grating of the invention is within experimental measurement error.

Also for the purposes of comparison, accelerated aging experiments were conducted on the grating of the invention (Fig. 8) and a hydrogen loaded grating (Fig. 7). Grating decay was measured at 200 ° C. The hydrogen loaded grating was annealed for twenty-four (24) hours at 125 °C. The hydrogen loaded grating lost strength (strength is defined as minimum transmission loss) at a significantly faster rate than the grating made in accordance with the invention (Fig. 8). The grating of the invention is therefore, more stable than the annealed hydrogen loaded grating.

An optical waveguide having a grating in accordance with the invention may have useful applications in the fields of pump stabilization gratings, filter gratings, and thermal tuning of gratings. The invention will be further clarified by the following examples, which are intended to be exemplary of the invention. EXAMPLE 1

A portion of a hydrogen loaded Corning CS-980 fiber (refractive index

Δ = 1.0%) was exposed to a uniform UV beam (approximately

60 mJ/cm²/pulse, 15 Hz, 32 mm) and was subsequently annealed at approximately 125°C for about 24 hours. An FBG device was then written using a frequency double dye laser operating at 240 nm, 10 Hz, with an intensity of approximately 50 mJ/cm²/pulse. The grating was approximately uniform along its length of approximately 3.9 mm. The transmission spectrum is shown in Fig. 3 for an exposure time of approximately 20 minutes. The grating has an average UV induced change in refractive index of 7.0 x 10^"4 and a modulated change in refractive index of 6.5 x 10^"4. The average UV induced index change does not include the increase from the baseline index due to the flood exposure process. To show that the strength of the index change is a function of the initial flood exposure time, a series of Corning CS-980 fibers were flood exposed for times varying between 15 and 180 minutes with the following exposure conditions: 240 nm, 60 mJ/cm²/pulse, 15 Hz, 32 mm length. Following annealing for about 24 hours at approximately 125°C, a FBG device was subsequently written into each of these fibers. The modulated Δn plotted in Fig. 4 is a function of the flood exposure time. The data is scaled to take into account changes in writing fluence.

To demonstrate that the above described effects scale with dopant concentration, data similar to that shown in Fig. 4 was collected for gratings written into Corning SMF-28™ fiber (refractive index Δ = 0.36%) with flood exposure times between 15 and 60 minutes. The modulated Δn is shown in Fig. 5. The difference between the SMF-28™ and CS-980 fiber results correlate roughly with the relative amounts of germanium in the two fibers.

EXAMPLE 2

A patterned photosensitivity was also written into the core of an optical waveguide fiber using a non-uniform exposure during the flood exposure step. This was demonstrated using a sampled grating. A sampled grating is an FBG device with a comb filter amplitude along its length. This type of grating has been made previously using a phase mask with an amplitude mask placed immediately before the phase mask. Only those areas through which the amplitude mask permits transmission of UV light are exposed to the sinusoidal intensity pattern to form the grating. Accordingly, this technique allows for the simplification of the FBG writing step. Following hydrogen loading, the fiber was exposed to UV light through an amplitude mask having 64μm windows on

564.7 μm centers. This fiber was then annealed prior to FBG writing. The result was a LPG device which, depending on the strength and location of the peaks, may or may not introduce losses of significant magnitude. An approximately 3.5 mm long FBG device was then written into the fiber using the standard phase mask process and an excimer laser pumped dye laser which was frequency doubled to operate at 248 nm. The exposure intensity was approximately 50 mJ/cm²/pulse. A strong grating was written where the fiber had been previously exposed and a relatively weak grating appears elsewhere due to the low intrinsic fiber photosensitivity. The resulting reflectance spectra shown in Fig. 6 depicts the expected comb-iike response. The central grating peak is stronger due to the weak grating written in the previously exposed region consisting of approximately 89% of the grating length.

The concept exemplified in example 2 above can similarly be extended to include various apodization profiles and may be extended to include phase- shifted gratings. Gratings having complex reflectivity and/or dispersion properties will result from this type of photosensitization process. Chirped gratings can be fabricated by tailoring the strength of the photosensitivity or the background refractive index. The flood exposure process produces an increasing refractive index change with increasing exposure time. This may be used to manufacture gratings having non-uniform peak wavelength but using a uniform period exposure process. For example, a linear chirped grating can be fabricated by a linear change in exposure dose along the grating. Such a dose can be fabricated either by scanning a small beam along the fiber length with variable speed, exposing the fiber through a mask with variable transmission, or some combination of the two. Subsequent exposure with a sinusoidal intensity pattern with uniform period results in an effectively chirped grating due to the linear change in resonant wavelength as a function of length. Since the grating strength also depends upon the flood exposure dose, it is preferable to change exposure dose beyond a minimum level that creates a grating of sufficient strength.

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

WE CLAIM:

1. An optical waveguide having a grating comprising: a virgin grating having a ratio of final modulated index to initial modulated index of no more than 3%.

2. The optical waveguide of claim 1 further comprising a transmission loss greater than approximately -40dB.

3. The optical waveguide of claim 1 further comprising a transmission loss that is substantially the same before and after annealing the waveguide.

4. The optical wave guide of claim 3 such that the absolute value of the transmission loss before annealing the waveguide to transmission loss after annealing the waveguide is less than 0.1.

5. The optical waveguide of claim 1 further comprising a change in refractive index that is at least about 10 ^~5.

6. The optical waveguide of claim 5 wherein the change in refractive index is at least 10 ^"4.

7. The optical waveguide of claim 1 wherein said grating is a fiber Bragg grating.

8. The optical waveguide of claim 1 wherein the grating is an amplifier pump stabilization grating.

9. The optical waveguide of claim 1 wherein the waveguide is a fiber.

10. An optical waveguide having a grating comprising: a nonannealed UV or IR photosensitized grating having a decay of no more than 10%.

11. The optical waveguide of claim 10 further comprising a transmission loss greater than approximately -40dB.

12. The optical waveguide of claim 10 further comprising a transmission loss that is substantially the same before and after annealing the waveguide.

13. The optical waveguide of claim 10 further comprising a change in refractive index that is at least about 10 ^~δ.

14. The optical waveguide of claim 9 wherein the change in refractive index is at least 10 ^~4.

15. The optical waveguide of claim 9 wherein said grating is a fiber Bragg grating.

16. The optical fiber of claim 9 wherein the grating is a pump stabilization grating.

17. An optical waveguide having a virgin grating comprising: an unannealed grating with a transmission loss having a negligible change after annealing the waveguide having the grating.

18. A method of producing a FBG grating comprising the steps of:

(i) photosensitizing a fiber;

(ii) annealing said photosensitized fiber; and

(iii) writing a grating in said annealed fiber.

19. Wherever said photosensitizing step includes said fiber with:

(i) loading said fiber with a gas selected from the group consisting of H₂ or D₂, and

(ii) treating said fiber with UV or IR radiation.

20. A thermally tuned grating comprising: a virgin grating having a ratio of final modulated index to initial modulated index of no more than 10%; and a temperature contoller controling temperature of said grating.

21. A grating according to claim 20, wherein said temperature controller includes resistive heating.