CA2115906C - Photosensitization of optical fiber and silica waveguides - Google Patents
Photosensitization of optical fiber and silica waveguides Download PDFInfo
- Publication number
- CA2115906C CA2115906C CA 2115906 CA2115906A CA2115906C CA 2115906 C CA2115906 C CA 2115906C CA 2115906 CA2115906 CA 2115906 CA 2115906 A CA2115906 A CA 2115906A CA 2115906 C CA2115906 C CA 2115906C
- Authority
- CA
- Canada
- Prior art keywords
- waveguide
- photosensitive
- comprised
- weakly
- flame
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
- G02B6/02114—Refractive index modulation gratings, e.g. Bragg gratings characterised by enhanced photosensitivity characteristics of the fibre, e.g. hydrogen loading, heat treatment
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Optical Integrated Circuits (AREA)
- Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
- Optical Couplings Of Light Guides (AREA)
Abstract
A method of fabricating a strongly photosensitive optical waveguide is comprised of locally heating at least a portion of a weakly photosensitive optical waveguide for a period of time sufficient to increase the density of defects associated with photosensitivity in the heated portion of the waveguide core.
Description
__ _1_ FIELD OF THE INVENTION: 2 1 1 5 9 0 6 This invention relates to optical waveguides and particularly to a method of fabricating a strongly photosensitive waveguide from a weakly photosensitive S waveguide.
BACKGROUND TO THE INVENTION:
Optical waveguides can exhibit the property of photosensitivity manifest as a permanent, light-induced refractive index change. Initial experiments as described in U.S. Patent 4,474,427 issued October 2nd, 1984, invented by K.O. Hill et al demonstrated the phenomenon in optical fibers but recently it has been detected as well in planar glass structures, including, for example, silica-on-silicon and silica-on-silica planar waveguides.
Photosensitivity can be used to make retro-reflecting Bragg gratings, mode convertor gratings and rocking rotators in optical waveguides; to fabricate such devices, a permanent, spatially periodic refractive index modulation is impressed with light along the length of the photosensitive core of the optical waveguide.
Methods of fabricating such Bragg gratings are described in U.S. Patent No. 5,367,588 granted November 22, 1994, invented by K.O. Hill, B. Malo, F. Bilodeau and D. Johnson and entitled METHOD OF FABRICATING BRAGG
GRATINGS USING A SILICA GLASS PHASE GRATING MASK.
The near UV absorption spectrum of the Ge-doped core of optical fiber, Ge:Si02-on-Si or Ge:Si02-on-Si02 waveguide is influenced strongly by the type and concentration of in-core defects. It has been found that photosensitivity in Ge-doped core waveguides is linked with absorption due to oxygen vacancy defects in the 240 nm UV region, as described by G. Meltz et al in Optical Letters 14, 823 (1989). High quality optical B
BACKGROUND TO THE INVENTION:
Optical waveguides can exhibit the property of photosensitivity manifest as a permanent, light-induced refractive index change. Initial experiments as described in U.S. Patent 4,474,427 issued October 2nd, 1984, invented by K.O. Hill et al demonstrated the phenomenon in optical fibers but recently it has been detected as well in planar glass structures, including, for example, silica-on-silicon and silica-on-silica planar waveguides.
Photosensitivity can be used to make retro-reflecting Bragg gratings, mode convertor gratings and rocking rotators in optical waveguides; to fabricate such devices, a permanent, spatially periodic refractive index modulation is impressed with light along the length of the photosensitive core of the optical waveguide.
Methods of fabricating such Bragg gratings are described in U.S. Patent No. 5,367,588 granted November 22, 1994, invented by K.O. Hill, B. Malo, F. Bilodeau and D. Johnson and entitled METHOD OF FABRICATING BRAGG
GRATINGS USING A SILICA GLASS PHASE GRATING MASK.
The near UV absorption spectrum of the Ge-doped core of optical fiber, Ge:Si02-on-Si or Ge:Si02-on-Si02 waveguide is influenced strongly by the type and concentration of in-core defects. It has been found that photosensitivity in Ge-doped core waveguides is linked with absorption due to oxygen vacancy defects in the 240 nm UV region, as described by G. Meltz et al in Optical Letters 14, 823 (1989). High quality optical B
2 fibre such as Corning ~MF-28* fibre and NTT silica-on-silicon planar guides contain comparatively Low concentratz.ons of defects. As a result both types of waveguide are relatively transparent in the near UV and are characterized as being weakly photosensitive.
SUMMARY OF THE PRESENT INVENTION:
We have discovered an effective and simple method l0 for augmenting substantially the photosensitivity of weakly photosensitive high silica Ge-doped core optical waveguides to ultraviolet light. Different dopants can result in photosensitivity at different wavelengths. It appears that the method may also work to photosensitize other types of optical waveguide at different wavelengths, for example those doped with cerium, erbium or europium and co-doped with alumina, for example.
In contr«st to ether methods, as described by G.D. Maxwell et al in Electronic Letters 20, 2106 (1992), photosensitization in our case is achieved with a negligible increase in loss at the three principal optical communication windows for typical bragg retro-reflector device lengths. Further, our technique permits localized photosensitization of commercially available high-quality optical waveguides.
In one aspect, the invention provides a method of fabricating a strongly photosensitive optical waveguide comprising locally heating while maintaining the structure integrity of a portion of a weakly photosensitive optical waveguide to an approximately white hot temperature for a period of time sufficient to increase the concentration of defects associated with photosensitivity in the heated portion of said wa~e~~uide.
* - Trade Mark - 2a -In a second aspect, the invention provides a method of fabricating a strongly photosensitive optical S waveguide comprising placing a portion of a weakly photosensitive optical waveguide in a sufficiently high temperature and pressure plasma of hydrogen or deuterium gas for a period of time sufficient to increase the concentration of defects associated with photosensitivity in said portion of the waveguide, said portion being heated to approximately white heat.
In a third aspect, the invention provides a method of photosensitization comprising locally heating to approximately white heat at least a portion of a weakly sensitive si7_ica optical waveguide doped with phosphorous for a period of time sufficient to increase the concentration of defects associated with photosensitivity in the heated portion of the waveguide, and irradiating said heated portion with actinic radiation in order to cause a refractive index change in the waveguide.
In accordance with an embodiment of the invention, a method of fabricating a strongly photosensitive optical waveguide is comprised of locally heating a portion cf a weakly photosensitive optical waveguide for a period of time sufficient to increase the density of defects in the waveguide to a level required to enhance the waveguide's photosensitive response.
In accordance with another embodiment, a length of weakly photosensitive optical waveguide has a localized section which is significantly photosensitive.
21159x6 -3-BRIEF INTRODUCTION TO THE DRAWINGS:
A better understanding of the invention will be obtained by reference to the detailed description below, in conjunction with the following drawings, in which:
Figure 1 illustrates an optical fiber being flame brushed, Figure 2 is a plot of index change as a function of flame brush processing time as observed in i0 an optical fiber, Figure 3 curve (a) illustrates the dispersion in absorption coefficient of a Ge-doped slab waveguide layer measured normal to the substrate prior to treatment, Figure 3 curve (b) illustrates the dispersion in absorption coefficient of the slab waveguide layer after flame brush treatment, Figure 3 curve (c) illustrates an experimental measurement of the photo-induced change in 2o the dispersion of the absorption coefficient with wavelength for the photosensitized Ge-doped slab waveguide layer caused by UV light irradiation, Figure 4 illustrates a measurement of the wavelength response of a Bragg grating in the core of a planar three dimensional waveguide after processing in accordance with the present invention, Figure 5 is a cross-section of a portion of an optical fiber containing a Bragg grating, and Figure 6 is a cross-section of a fragment of a grating face mask.
DETAILED DESCRIPTION OF THE INVENTION:
Figure 1 illustrates a section of waveguide in the form of an optical fiber comprised of core 1 surrounded by cladding 3. A region of the waveguide is immersed in a flame 5, which is brushed "back and forth"
2115906 _4_ repeatedly in a localized region with the flame along the region in the directions shown by the arrows.
However, it could be locally heated by some other suitable source of heat such as C02 laser or an electric arc. Brushing is used to increase the size of the treated region. Otherwise, brushing need not be used.
In a preferred embodiment of the invention, the region of weakly (including negligibly) photosensitive optical waveguide that is to be photosensitized is "brushed" repeatedly by flame 5 fueled by a fuel such as hydrogen or propane but to which a small amount of oxygen is sometimes added (approximate flame temperature is 1700°C). It has been found that photosensitization typically takes approximately 20 minutes to complete for maximum effect under the conditions used in our experiments (Corning SMF28 fiber waveguide), although we have operated the process successfully using 10 minutes of flame brushing. Because a relatively small flame can be used to "brush" the waveguide, the method provides highly localized photosensitization.
The temperature that the glass of the waveguide is heated is to approximately white heat (approximately the fusion point of glass).
We have used the flame brush method to increase the photosensitivity of standard (Ge-doped core Corning SMF28) telecommunications fiber by a factor greater than ten (photoinduced refractive index charge > 10-3 at ~, = 1530 nm) .
Figure 2 is a plot of the maximum (saturated) photoinduced index change observed in Corning SMF -28 fiber as a function of processing time under the flame brush. The W light exposure conditions used were: ~, = 249 nm, fluence = 300 mj/cm2/per pulse, pulse repetition frequency (PRF) - 50 Hz, exposure 2115906 _5_ time = 15 min and pulse duration = 12 nsec. In a fiber sample that is not flame brushed we observed a photoinduced index change of only 1.2x10'4; after 20 minutes of flame brush processing, the photoinduced refractive index change that was observed in a similar fiber sample peaked at 1.4x10'3.
We found that the flame brush method also rendered strongly photosensitive the waveguide core of our samples of high quality Ge:Si02-on-Si three-dimensional waveguides and of Ge:Si02-on-Si02 two-dimensional planar waveguides that were negligibly photosensitive prior to treatment. Experimentally, we observed a larger photosensitization effect in these planar waveguides than we did in the SMF-28 fiber: the photoinduced refractive index change in the waveguide core of unprocessed planar waveguides was below our detection threshold, but with a flame brush treatment became larger than the photoinduced refractive index change in similarly flame brushed SMF-28 fiber for the same W exposure conditions.
The planar slab film waveguides were fabricated by flame hydrolysis deposition of Ge-doped Si02 on a silica substrate. The index step, 0n, of the 5 ~Cm thick Ge-doped layer on the Si02 was measured at a wavelength ~, = 633 nm to be 1.14 ~ 0.04x10'2 prior to treatment and increased to 1.32 + 0.04x10'2 after 10 minutes of flame brush treatment. Associated with this change in refractive index was an increase in the absorption coefficient of the waveguide core in the ultraviolet spectral region.
Curve (a) of Figure 3 illustrates the dispersion in absorption coefficient (multiplied by 200) of the waveguide layer measured normal to the substrate prior to flame brush treatment.
Curve (b) illustrates the dispersion in absorption coefficient for the waveguide layer measured after flame brush treatment. The enormous increase in W absorption of the waveguide layer that results from flame brush treatment is apparent: approximately 1000 dB/mm at 240 nm. Kramers-Kronig causality predicts that an increase in absorption at short wavelengths causes an increase in refractive index at long wavelengths, as observed.
to Curve (c) illustrates an experimental measurement of Via, the photoinduced change in the dispersion of the absorption coefficient for the flame photosensitized waveguide layer, caused by 249 nm W
light irradiation for 40 minutes with fluence per pulse of 112 mJ/cm2 at 50 Hz PRF. The effect of the irradiation is to bleach out the absorption at 240 nm and simultaneously to increase it on both sides of the band (at 213 nm and at 281 nm). The result is that W
irradiation increases the net W absorption of the sample and thus concomitantly increases, as observed, the refractive index at longer wavelengths.
For comparison, we measured the change in absorption due to flame brush treatment of a homogeneous silica substrate. The substrate was identical to that which we used for the flame hydrolysis deposition of the Ge:Si02 planar optical waveguide described above. The absorption of the substrate was basically unaffected by the processing (less than 2% change in absorption at 240 nm). Therefore we attribute the increase in 3o absorption in samples processed by flame brush entirely to the effect of processing on the optical properties of the Ge:Si02 waveguide layer.
We postulate on this evidence that flame brushing of a waveguide affects preferentially the optical properties of the Ge-doped silica core and ._ 21~59os leaves unaffected the properties of the cladding. In this sense it is an ideal photosensitization process because the cladding can remain transparent, thereby enabling the activating light to reach the core of the waveguide unattenuated. At the same time, processing creates in the Ge-doped core very strong absorption, rendering the core highly photosensitive so that W
light can affect the change in its refractive index.
For reasons of flexibility, grating devices 1o are usually written in a photosensitive waveguide by illuminating the waveguide from the side with W light.
Typically, the optical waveguide core is surrounded by cladding material several times the transverse dimension of the core. For successful side-writing not only must the activating W light be transmitted by the cladding, but it must also be absorbed sufficiently by the core to enable the light to photoinduce efficiently an index change there. Therefore, because the transverse dimensions of an optical waveguide are the order of only 2o a few microns, as a primary requirement for high photosensitivity, the absorption due to the presence of the core dopant must be as large as possible. Flame brushing has been found to create these advantageous conditions for effective side-writing.
In optical waveguides photosensitized by our method we have written strong Bragg gratings 10 as shown in Fig. 5 in three-dimensional planar and optical fiber waveguides, with KrF (249 nm) radiation incident on the samples through a zero-order-nulled photolithographic grating phase mask 12 as shown in Fig. 6 patterned with a pitch required for optical waveguide Bragg resonance at about 1540 nm.
Figure 4 illustrates the measured wavelength response of a 1 mm long Ge:Si02-on-Si planar waveguide Bragg grating. The 81% reflectivity corresponds to a 2'~ 1 5906 core refractive index modulation amplitude of 8.9x10'4 for each of the two principal polarization states. To write the Bragg grating, the waveguide was photosensitized by 10 minutes of flame brush processing prior to irradiation by a KrF excimer laser beam which was incident on the waveguide at 50 PRF far 15 minutes through the photolithographic grating phase mask.
The writing of Bragg gratings is very useful for characterizing the photosensitive response of optical waveguides. Both the average change in index with exposure and the corresponding depth of modulation of the index can be monitored. The shift of the Bragg resonance wavelength as a function of photolithographic exposure dose to W light provides a measure in real time of the average change in the refractive index caused by the exposure. The strength of the Bragg resonance yields the depth of the photoinduced spatial modulation. With phase mask photolithography on fiber, the ratio of the modulation to the average index change that we attained consistently was about 0.4. At the high fluence per pulse levels needed to write the in-core Bragg grating, we wrote simultaneously a high quality surface relief grating at the silicon-silica interface which we attributed to light-induced melting of the silicon, together with stress relaxation at the interface.
We carried out temperature stability measurements on the photoinduced Bragg grating whose response is shown in Figure 4. After the sample was held at 500°C for 17 hours, the refractive index modulation stabilized at 3.6x10'4, representing a decrease of 40% of its original value.
Photosensitized waveguide could simplify single pulse writing of in-fiber Bragg gratings and yield useful sensor devices.
__ X115906 -9-The present invention provides an effective method for photosensitizing optical waveguides with high spatial selectively (only the selected portion of the waveguide need be photosensitized) and for fabricating efficient Bragg gratings in at least Ge:Si02-on-Si and Ge-doped core optical fiber waveguides.
Localized heating with a flame has been found to be a simple and effective method for augmenting substantially the photosensitivity of high silica to optical waveguides to ultraviolet light. The method increases the photosensitivity of standard (Ge-doped core) telecommunications fiber by a factor greater than ten (photoinduced ~n > 10'3) and renders strongly photosensitive high quality Ge:Si02-on-Si, Ge:Si02-on-Si02 and Ge:Si02-on-A1203 planar waveguides that were negligibly photosensitive prior to treatment.
Using waveguides photosensitized by the present invention strong Bragg gratings have been written in fiber and planar optical waveguide, with KrF (249 nm) radiation incident on the waveguides through a zero-order-nulled phase mask. It is noteworthy that photosensitization by our method is achieved with a negligible increase in loss at the three principal optical communication windows.
While the above-described flame brushing method is described as preferably using a flame fueled by a fuel such as hydrogen or propane, but to which a small amount of oxygen is sometimes added, it has been found that an improvement is achieved using a flame 3o fueled by deuterium rather than with hydrogen. It is found that the resulting photosensitized optical waveguide has lower losses than a hydrogen fueled flame brushed optical waveguide at the wavelength of about 1550 nm, and at other wavelengths of interest in optical communications and in sensing applications.
11~g06 -i0- _ It has also been found that the method described in this specification will render a weakly photosensitive waveguide strongly photosensitive where the waveguide is comprised of silica doped with phosphorous. Bragg gratings, for example, can be made with optical fibers having a phosphorous doped core, photosensitized using either flame brushing or hydrogen loading and irradiated with actinic radiation of, for example, 193 nm, which can be produced with an XCIMER
laser. The cladding of the optical fiber or other waveguide should be essentially transparent to the actinic radiation.
In accordance with another embodiment of the invention, instead of flame brushing the weakly photosensitive optical waveguide, it should be placed in a plasma, wherein the plasma is generated in hydrogen or deuterium gas. The plasma maintains the waveguide at a lower temperature than the temperature experienced locally in a brushing flame, which advantageously reduces the likelihood of diffusion of dopant during the photosensitized process.
It should be noted that the invention is applicable to various optical waveguides, such as planar or channel waveguides, and is not restricted to optical fiber waveguides.
A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above. All of those which fall within the scope of the claims appended hereto are considered to be part of the present invention.
SUMMARY OF THE PRESENT INVENTION:
We have discovered an effective and simple method l0 for augmenting substantially the photosensitivity of weakly photosensitive high silica Ge-doped core optical waveguides to ultraviolet light. Different dopants can result in photosensitivity at different wavelengths. It appears that the method may also work to photosensitize other types of optical waveguide at different wavelengths, for example those doped with cerium, erbium or europium and co-doped with alumina, for example.
In contr«st to ether methods, as described by G.D. Maxwell et al in Electronic Letters 20, 2106 (1992), photosensitization in our case is achieved with a negligible increase in loss at the three principal optical communication windows for typical bragg retro-reflector device lengths. Further, our technique permits localized photosensitization of commercially available high-quality optical waveguides.
In one aspect, the invention provides a method of fabricating a strongly photosensitive optical waveguide comprising locally heating while maintaining the structure integrity of a portion of a weakly photosensitive optical waveguide to an approximately white hot temperature for a period of time sufficient to increase the concentration of defects associated with photosensitivity in the heated portion of said wa~e~~uide.
* - Trade Mark - 2a -In a second aspect, the invention provides a method of fabricating a strongly photosensitive optical S waveguide comprising placing a portion of a weakly photosensitive optical waveguide in a sufficiently high temperature and pressure plasma of hydrogen or deuterium gas for a period of time sufficient to increase the concentration of defects associated with photosensitivity in said portion of the waveguide, said portion being heated to approximately white heat.
In a third aspect, the invention provides a method of photosensitization comprising locally heating to approximately white heat at least a portion of a weakly sensitive si7_ica optical waveguide doped with phosphorous for a period of time sufficient to increase the concentration of defects associated with photosensitivity in the heated portion of the waveguide, and irradiating said heated portion with actinic radiation in order to cause a refractive index change in the waveguide.
In accordance with an embodiment of the invention, a method of fabricating a strongly photosensitive optical waveguide is comprised of locally heating a portion cf a weakly photosensitive optical waveguide for a period of time sufficient to increase the density of defects in the waveguide to a level required to enhance the waveguide's photosensitive response.
In accordance with another embodiment, a length of weakly photosensitive optical waveguide has a localized section which is significantly photosensitive.
21159x6 -3-BRIEF INTRODUCTION TO THE DRAWINGS:
A better understanding of the invention will be obtained by reference to the detailed description below, in conjunction with the following drawings, in which:
Figure 1 illustrates an optical fiber being flame brushed, Figure 2 is a plot of index change as a function of flame brush processing time as observed in i0 an optical fiber, Figure 3 curve (a) illustrates the dispersion in absorption coefficient of a Ge-doped slab waveguide layer measured normal to the substrate prior to treatment, Figure 3 curve (b) illustrates the dispersion in absorption coefficient of the slab waveguide layer after flame brush treatment, Figure 3 curve (c) illustrates an experimental measurement of the photo-induced change in 2o the dispersion of the absorption coefficient with wavelength for the photosensitized Ge-doped slab waveguide layer caused by UV light irradiation, Figure 4 illustrates a measurement of the wavelength response of a Bragg grating in the core of a planar three dimensional waveguide after processing in accordance with the present invention, Figure 5 is a cross-section of a portion of an optical fiber containing a Bragg grating, and Figure 6 is a cross-section of a fragment of a grating face mask.
DETAILED DESCRIPTION OF THE INVENTION:
Figure 1 illustrates a section of waveguide in the form of an optical fiber comprised of core 1 surrounded by cladding 3. A region of the waveguide is immersed in a flame 5, which is brushed "back and forth"
2115906 _4_ repeatedly in a localized region with the flame along the region in the directions shown by the arrows.
However, it could be locally heated by some other suitable source of heat such as C02 laser or an electric arc. Brushing is used to increase the size of the treated region. Otherwise, brushing need not be used.
In a preferred embodiment of the invention, the region of weakly (including negligibly) photosensitive optical waveguide that is to be photosensitized is "brushed" repeatedly by flame 5 fueled by a fuel such as hydrogen or propane but to which a small amount of oxygen is sometimes added (approximate flame temperature is 1700°C). It has been found that photosensitization typically takes approximately 20 minutes to complete for maximum effect under the conditions used in our experiments (Corning SMF28 fiber waveguide), although we have operated the process successfully using 10 minutes of flame brushing. Because a relatively small flame can be used to "brush" the waveguide, the method provides highly localized photosensitization.
The temperature that the glass of the waveguide is heated is to approximately white heat (approximately the fusion point of glass).
We have used the flame brush method to increase the photosensitivity of standard (Ge-doped core Corning SMF28) telecommunications fiber by a factor greater than ten (photoinduced refractive index charge > 10-3 at ~, = 1530 nm) .
Figure 2 is a plot of the maximum (saturated) photoinduced index change observed in Corning SMF -28 fiber as a function of processing time under the flame brush. The W light exposure conditions used were: ~, = 249 nm, fluence = 300 mj/cm2/per pulse, pulse repetition frequency (PRF) - 50 Hz, exposure 2115906 _5_ time = 15 min and pulse duration = 12 nsec. In a fiber sample that is not flame brushed we observed a photoinduced index change of only 1.2x10'4; after 20 minutes of flame brush processing, the photoinduced refractive index change that was observed in a similar fiber sample peaked at 1.4x10'3.
We found that the flame brush method also rendered strongly photosensitive the waveguide core of our samples of high quality Ge:Si02-on-Si three-dimensional waveguides and of Ge:Si02-on-Si02 two-dimensional planar waveguides that were negligibly photosensitive prior to treatment. Experimentally, we observed a larger photosensitization effect in these planar waveguides than we did in the SMF-28 fiber: the photoinduced refractive index change in the waveguide core of unprocessed planar waveguides was below our detection threshold, but with a flame brush treatment became larger than the photoinduced refractive index change in similarly flame brushed SMF-28 fiber for the same W exposure conditions.
The planar slab film waveguides were fabricated by flame hydrolysis deposition of Ge-doped Si02 on a silica substrate. The index step, 0n, of the 5 ~Cm thick Ge-doped layer on the Si02 was measured at a wavelength ~, = 633 nm to be 1.14 ~ 0.04x10'2 prior to treatment and increased to 1.32 + 0.04x10'2 after 10 minutes of flame brush treatment. Associated with this change in refractive index was an increase in the absorption coefficient of the waveguide core in the ultraviolet spectral region.
Curve (a) of Figure 3 illustrates the dispersion in absorption coefficient (multiplied by 200) of the waveguide layer measured normal to the substrate prior to flame brush treatment.
Curve (b) illustrates the dispersion in absorption coefficient for the waveguide layer measured after flame brush treatment. The enormous increase in W absorption of the waveguide layer that results from flame brush treatment is apparent: approximately 1000 dB/mm at 240 nm. Kramers-Kronig causality predicts that an increase in absorption at short wavelengths causes an increase in refractive index at long wavelengths, as observed.
to Curve (c) illustrates an experimental measurement of Via, the photoinduced change in the dispersion of the absorption coefficient for the flame photosensitized waveguide layer, caused by 249 nm W
light irradiation for 40 minutes with fluence per pulse of 112 mJ/cm2 at 50 Hz PRF. The effect of the irradiation is to bleach out the absorption at 240 nm and simultaneously to increase it on both sides of the band (at 213 nm and at 281 nm). The result is that W
irradiation increases the net W absorption of the sample and thus concomitantly increases, as observed, the refractive index at longer wavelengths.
For comparison, we measured the change in absorption due to flame brush treatment of a homogeneous silica substrate. The substrate was identical to that which we used for the flame hydrolysis deposition of the Ge:Si02 planar optical waveguide described above. The absorption of the substrate was basically unaffected by the processing (less than 2% change in absorption at 240 nm). Therefore we attribute the increase in 3o absorption in samples processed by flame brush entirely to the effect of processing on the optical properties of the Ge:Si02 waveguide layer.
We postulate on this evidence that flame brushing of a waveguide affects preferentially the optical properties of the Ge-doped silica core and ._ 21~59os leaves unaffected the properties of the cladding. In this sense it is an ideal photosensitization process because the cladding can remain transparent, thereby enabling the activating light to reach the core of the waveguide unattenuated. At the same time, processing creates in the Ge-doped core very strong absorption, rendering the core highly photosensitive so that W
light can affect the change in its refractive index.
For reasons of flexibility, grating devices 1o are usually written in a photosensitive waveguide by illuminating the waveguide from the side with W light.
Typically, the optical waveguide core is surrounded by cladding material several times the transverse dimension of the core. For successful side-writing not only must the activating W light be transmitted by the cladding, but it must also be absorbed sufficiently by the core to enable the light to photoinduce efficiently an index change there. Therefore, because the transverse dimensions of an optical waveguide are the order of only 2o a few microns, as a primary requirement for high photosensitivity, the absorption due to the presence of the core dopant must be as large as possible. Flame brushing has been found to create these advantageous conditions for effective side-writing.
In optical waveguides photosensitized by our method we have written strong Bragg gratings 10 as shown in Fig. 5 in three-dimensional planar and optical fiber waveguides, with KrF (249 nm) radiation incident on the samples through a zero-order-nulled photolithographic grating phase mask 12 as shown in Fig. 6 patterned with a pitch required for optical waveguide Bragg resonance at about 1540 nm.
Figure 4 illustrates the measured wavelength response of a 1 mm long Ge:Si02-on-Si planar waveguide Bragg grating. The 81% reflectivity corresponds to a 2'~ 1 5906 core refractive index modulation amplitude of 8.9x10'4 for each of the two principal polarization states. To write the Bragg grating, the waveguide was photosensitized by 10 minutes of flame brush processing prior to irradiation by a KrF excimer laser beam which was incident on the waveguide at 50 PRF far 15 minutes through the photolithographic grating phase mask.
The writing of Bragg gratings is very useful for characterizing the photosensitive response of optical waveguides. Both the average change in index with exposure and the corresponding depth of modulation of the index can be monitored. The shift of the Bragg resonance wavelength as a function of photolithographic exposure dose to W light provides a measure in real time of the average change in the refractive index caused by the exposure. The strength of the Bragg resonance yields the depth of the photoinduced spatial modulation. With phase mask photolithography on fiber, the ratio of the modulation to the average index change that we attained consistently was about 0.4. At the high fluence per pulse levels needed to write the in-core Bragg grating, we wrote simultaneously a high quality surface relief grating at the silicon-silica interface which we attributed to light-induced melting of the silicon, together with stress relaxation at the interface.
We carried out temperature stability measurements on the photoinduced Bragg grating whose response is shown in Figure 4. After the sample was held at 500°C for 17 hours, the refractive index modulation stabilized at 3.6x10'4, representing a decrease of 40% of its original value.
Photosensitized waveguide could simplify single pulse writing of in-fiber Bragg gratings and yield useful sensor devices.
__ X115906 -9-The present invention provides an effective method for photosensitizing optical waveguides with high spatial selectively (only the selected portion of the waveguide need be photosensitized) and for fabricating efficient Bragg gratings in at least Ge:Si02-on-Si and Ge-doped core optical fiber waveguides.
Localized heating with a flame has been found to be a simple and effective method for augmenting substantially the photosensitivity of high silica to optical waveguides to ultraviolet light. The method increases the photosensitivity of standard (Ge-doped core) telecommunications fiber by a factor greater than ten (photoinduced ~n > 10'3) and renders strongly photosensitive high quality Ge:Si02-on-Si, Ge:Si02-on-Si02 and Ge:Si02-on-A1203 planar waveguides that were negligibly photosensitive prior to treatment.
Using waveguides photosensitized by the present invention strong Bragg gratings have been written in fiber and planar optical waveguide, with KrF (249 nm) radiation incident on the waveguides through a zero-order-nulled phase mask. It is noteworthy that photosensitization by our method is achieved with a negligible increase in loss at the three principal optical communication windows.
While the above-described flame brushing method is described as preferably using a flame fueled by a fuel such as hydrogen or propane, but to which a small amount of oxygen is sometimes added, it has been found that an improvement is achieved using a flame 3o fueled by deuterium rather than with hydrogen. It is found that the resulting photosensitized optical waveguide has lower losses than a hydrogen fueled flame brushed optical waveguide at the wavelength of about 1550 nm, and at other wavelengths of interest in optical communications and in sensing applications.
11~g06 -i0- _ It has also been found that the method described in this specification will render a weakly photosensitive waveguide strongly photosensitive where the waveguide is comprised of silica doped with phosphorous. Bragg gratings, for example, can be made with optical fibers having a phosphorous doped core, photosensitized using either flame brushing or hydrogen loading and irradiated with actinic radiation of, for example, 193 nm, which can be produced with an XCIMER
laser. The cladding of the optical fiber or other waveguide should be essentially transparent to the actinic radiation.
In accordance with another embodiment of the invention, instead of flame brushing the weakly photosensitive optical waveguide, it should be placed in a plasma, wherein the plasma is generated in hydrogen or deuterium gas. The plasma maintains the waveguide at a lower temperature than the temperature experienced locally in a brushing flame, which advantageously reduces the likelihood of diffusion of dopant during the photosensitized process.
It should be noted that the invention is applicable to various optical waveguides, such as planar or channel waveguides, and is not restricted to optical fiber waveguides.
A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above. All of those which fall within the scope of the claims appended hereto are considered to be part of the present invention.
Claims (43)
1. A method of fabricating a strongly photosensitive optical waveguide comprising locally heating while maintaining the structure integrity of a portion of a weakly photosensitive optical waveguide to an approximately white hot temperature for a period of time sufficient to increase the concentration of defects associated with photosensitivity in the heated portion of said waveguide.
2. A method as defined in claim 1 in which the waveguide is an optical fiber.
3. A method as defined in claim 2 in which said weakly photosensitive waveguide is comprised of one of Ge:SiO2 on a Si substrate, Ge:SiO2 on a SiO2 substrate, and Ge:SiO2 on a A1 2O3 substrate.
4. A method as defined in claim 2 in which said weakly photosensitive waveguide is comprised of silica doped with one of germanium, cerium and europium.
5. A method as defined in claim 1 in which the waveguide is a planar waveguide.
6. A method as defined in claim 5 in which said weakly photosensitive waveguide is comprised of one of germanium doped silica or a silicon, silica and sapphire substrate material.
7. A method as defined in claim 5 in which said weakly photosensitive waveguide is comprised of silica doped with one of germanium, cerium and europium.
8. A method as defined in claim 1 in which said defects are oxygen deficiency defects.
9. A method as defined in claim 1 in which said waveguide is fabricated strongly photosensitive to wavelengths of approximately 248 nm.
10. A method as defined in claim 1 in which the weakly photosensitive optical waveguide local heating is conducted by moving a local heating source along said waveguide to increase a size of a region containing said defects.
11. A method as defined in claim 1 in which the local heating source is a flame, used to brush the optical waveguide, the flame being fueled by a gas.
12. A method as defined in claim 11 in which the temperature of the flame is approximately 1700°C.
13. A method as defined in claim 12 in which the gas is hydrogen.
14. A method as defined in claim 12 in which the flame brushing is continued for approximately 10-20 minutes.
15. A method as defined in claim 5, including the initial steps of fabricating the planar waveguide by flame hydrolysis of Ge doped SiO2 on a silica substrate to form a layer approximately 5 µm thick.
16. A method as defined in claim 1 in which said weakly photosensitive waveguide is comprised of silica doped with one of germanium, cerium and europium.
17. A method as defined in claim 1 including the further steps of creating at least one of a Bragg and surface relief grating comprising positioning a grating phase mask at the side of the strongly photosensitized portion of the waveguide with its plane parallel to the axis of the waveguide, and irradiating said strongly photosensitized portion of the waveguide through the phase mask with monochromatic light at a wavelength at which said waveguide is strongly photosensitive.
18. A length of weakly photosensitive optical waveguide having a localized section which is ten times more photosensitive.
19. A waveguide as defined in claim 18 in the form of an optical fiber.
20. A waveguide as defined in claim 18 in the form of a planar waveguide.
21. A waveguide as defined in claim 18 comprised of one of Ge:SiO2 on a Si substrate, Ge:SiO2 on a SiO2 substrate, and Ge:SiO2 on a A1 2 O 3 substrate.
22. A waveguide as defined in claim 21, in the form of a slab waveguide.
23. A waveguide as defined in claim 21, in the form of an optical fiber.
24. A waveguide as defined in claim 18 in which said localized section has significantly more oxygen deficiency defects than adjacent sections of the waveguide.
25. A waveguide as defined in claim 18, in which said localized section is strongly photosensitive to wavelengths of approximately 240 nm.
26. A waveguide as defined in claim 18, in which said waveguide is a slab waveguide comprised of silica doped with one of germanium, cerium, erbium and europium.
27. A waveguide as defined in claim 18 comprised of at least one of a Bragg and a surface relief grating contained in said localized section.
28. A method as defined in claim 2, wherein the waveguide is codoped with alumina.
29. A method as defined in claim 7, wherein the waveguide is codoped with alumina.
30. A method as defined in claim 11 in which oxygen is added to the gas.
31. A method as defined in claim 16, wherein the waveguide is codoped with alumina.
32. A waveguide as defined in claim 21, in the form of a planar waveguide.
33. A waveguide as defined in claim 18, in which said waveguide is a planar waveguide comprised of silica doped with one of germanium, cerium, erbium, and europium.
34. A method as defined in claim 26, wherein the waveguide is codoped with alumina.
35. A method as defined in claim 26, wherein the waveguide is codoped with alumina.
36. A method as described in claim 1, in which the heating step is comprised of placing the weakly photosensitive optical waveguide in a flame fueled by hydrogen or deuterium.
37. A method of fabricating a strongly photosensitive optical waveguide comprising placing a portion of a weakly photosensitive optical waveguide in a sufficiently high temperature and pressure plasma of hydrogen or deuterium gas for a period of time sufficient to increase the concentration of defects associated with photosensitivity in said portion of the waveguide, said portion being heated to approximately white heat.
38. A method as defined in claim 1 in which said weakly photosensitive waveguide is comprised of silica doped with one of germanium, cerium, europium and phosphorous.
39. A method as defined in claim 38 in which the waveguide is one of an optical fiber, a channel and at least part of a planar layer.
40. A method of photosensitization comprising locally heating to approximately white heat at least a portion of a weakly sensitive silica optical waveguide doped with phosphorous for a period of time sufficient to increase the concentration of defects associated with photosensitivity in the heated portion of the waveguide, and irradiating said heated portion with actinic radiation in order to cause a refractive index change in the waveguide.
41. A method as defined in claim 40 in which a waveguide core is covered by a cladding essentially transparent to active radiation, and passing said radiation through the cladding to the waveguide core.
42. A method as defined in claim 41 in which the heating step is comprised of retaining said weakly sensitive waveguide in a plasma fueled by hydrogen or deuterium.
43. A method as defined in claim 40 in which the waveguide is codoped with a laser active dopant.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US1851193A | 1993-02-17 | 1993-02-17 | |
| US08/018,511 | 1993-02-17 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2115906A1 CA2115906A1 (en) | 1994-08-18 |
| CA2115906C true CA2115906C (en) | 2001-12-25 |
Family
ID=21788303
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2115906 Expired - Lifetime CA2115906C (en) | 1993-02-17 | 1994-02-17 | Photosensitization of optical fiber and silica waveguides |
Country Status (5)
| Country | Link |
|---|---|
| JP (1) | JP3011308B2 (en) |
| CA (1) | CA2115906C (en) |
| DE (1) | DE4404874C2 (en) |
| FR (1) | FR2704323B1 (en) |
| GB (1) | GB2275350B (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| IT1282113B1 (en) * | 1994-02-16 | 1998-03-12 | Ca Minister Energy | PHOTOSENSITIZATION OF FIBER OPTIC AND SILICA WAVE GUIDES |
| GB2302413B (en) * | 1995-06-20 | 1997-11-12 | Northern Telecom Ltd | Bragg gratings in waveguides |
| FR2738353B1 (en) * | 1995-08-31 | 1997-11-21 | France Telecom | PROCESS FOR PHOTOSENSITIZING AN ALUMINOSILICATE OPTICAL WAVEGUIDE AND WAVEGUIDE OBTAINED BY THIS PROCESS |
Family Cites Families (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4474427A (en) * | 1979-05-07 | 1984-10-02 | Canadian Patents & Development Limited | Optical fiber reflective filter |
| JPS5863902A (en) * | 1981-10-12 | 1983-04-16 | Fujitsu Ltd | Controlling method of optical waveguide |
| GB2189901B (en) * | 1986-04-25 | 1989-12-06 | Stc Plc | Laser induced optical fibre grating devices. |
| JP2588710B2 (en) * | 1987-04-06 | 1997-03-12 | 日本電信電話株式会社 | Method for manufacturing quartz-based optical waveguide film |
| JPH0711606B2 (en) * | 1988-02-01 | 1995-02-08 | 日本電信電話株式会社 | Quartz optical waveguide film |
| JP2623308B2 (en) * | 1988-08-19 | 1997-06-25 | 日本電信電話株式会社 | Manufacturing method of glass optical waveguide film |
| JPH0353202A (en) * | 1989-07-21 | 1991-03-07 | Hitachi Cable Ltd | Production of waveguide added with rare earth element |
| JP2667264B2 (en) * | 1989-12-08 | 1997-10-27 | 日立電線株式会社 | Active optical multiplexer / demultiplexer |
| FR2657967B1 (en) * | 1990-02-06 | 1992-05-22 | Bertin & Cie | METHOD FOR PRODUCING AN INDEX NETWORK IN AN OPTICAL FIBER, AND NEARLY DISTRIBUTED SENSOR NETWORK FORMED IN THIS FIBER. |
| JP2599488B2 (en) * | 1990-02-26 | 1997-04-09 | 日本電信電話株式会社 | Method for adjusting characteristics of optical waveguide circuit and optical waveguide circuit used in the method |
| JP2842954B2 (en) * | 1991-06-05 | 1999-01-06 | 日立電線株式会社 | Rare earth element doped optical waveguide and method of manufacturing the same |
-
1994
- 1994-02-17 JP JP6020261A patent/JP3011308B2/en not_active Expired - Lifetime
- 1994-02-17 DE DE19944404874 patent/DE4404874C2/en not_active Expired - Lifetime
- 1994-02-17 CA CA 2115906 patent/CA2115906C/en not_active Expired - Lifetime
- 1994-02-17 GB GB9403040A patent/GB2275350B/en not_active Expired - Lifetime
- 1994-02-17 FR FR9402018A patent/FR2704323B1/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| GB9403040D0 (en) | 1994-04-06 |
| DE4404874A1 (en) | 1994-09-08 |
| GB2275350B (en) | 1997-06-18 |
| JP3011308B2 (en) | 2000-02-21 |
| FR2704323B1 (en) | 1997-09-05 |
| FR2704323A1 (en) | 1994-10-28 |
| GB2275350A (en) | 1994-08-24 |
| CA2115906A1 (en) | 1994-08-18 |
| DE4404874C2 (en) | 2001-03-01 |
| JPH07196334A (en) | 1995-08-01 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US5495548A (en) | Photosensitization of optical fiber and silica waveguides | |
| EP0569182B1 (en) | Photoinduced refractive index change in hydrogenated germano-silicate waveguide | |
| JP3371048B2 (en) | Method of forming photo-induced Bragg grating | |
| US20190193208A1 (en) | Femtosecond laser inscription | |
| EP1460459A1 (en) | Bragg grating and method of producing a bragg grating using an ultrafast laser | |
| EP3417247B1 (en) | Low insertion loss high temperature stable fiber bragg grating sensor and method for producing same | |
| JP2000504853A (en) | Method of manufacturing a symmetric optical waveguide | |
| US6233381B1 (en) | Photoinduced grating in oxynitride glass | |
| US8402789B2 (en) | High temperature stable fiber grating sensor and method for producing same | |
| US7515792B2 (en) | Method of increasing photosensitivity of glasses to ultrafast infrared laser radiation using hydrogen or deuterium | |
| CA2115906C (en) | Photosensitization of optical fiber and silica waveguides | |
| EP0622343A2 (en) | Method for increasing the index of refraction of a glassy material | |
| Malo et al. | Photosensitivity in optical fiber and silica-on-substrate waveguides | |
| EP2136227B1 (en) | High temperature stable fiber grating sensor and method for producing same | |
| Chen et al. | Large photosensitivity in germanosilicate planar waveguides induced by 157-nm F2-laser radiation | |
| Chen et al. | Photosensitivity and application with 157-nm F2 laser radiation in planar lightwave circuits | |
| ITMI950272A1 (en) | PHOTOSENSITIZATION OF FIBER OPTIC AND SILICA WAVE GUIDES | |
| Chen et al. | F2-laser photosensitivity applications in germanosilicate fiber and planar waveguides | |
| Chen et al. | 157-nm F2 laser photosensitivity and photosensitization in optical fibers | |
| CA2287137C (en) | Polarization insensitive grating in a planar channel optical waveguide and method to achieve the same | |
| Mihailov et al. | Fabrication of fiber Bragg gratings (FBG) in all-SiO2 and Ge-doped core fibers with 800 nm picosecond radiation | |
| Zhang | Photosensitivity and Bragg gratings in optical fibers | |
| Slattery et al. | High-intensity UV laser inscription of fibre Bragg gratings and comparison with other fabrication techniques | |
| Chen | Silica Optical Waveguides | |
| CA2363719A1 (en) | An apparatus and a method for changing refractive index |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKEX | Expiry |
Effective date: 20140217 |