CA2168902C - Optical tapping filters employing long period gratings - Google Patents
Optical tapping filters employing long period gratingsInfo
- Publication number
- CA2168902C CA2168902C CA002168902A CA2168902A CA2168902C CA 2168902 C CA2168902 C CA 2168902C CA 002168902 A CA002168902 A CA 002168902A CA 2168902 A CA2168902 A CA 2168902A CA 2168902 C CA2168902 C CA 2168902C
- Authority
- CA
- Canada
- Prior art keywords
- cores
- optical
- core
- grating
- long period
- 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 - Fee Related
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
- G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
- G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
- G02B6/02095—Long period gratings, i.e. transmission gratings coupling light between core and cladding modes
-
- 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
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
- G02B6/29331—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by evanescent wave coupling
- G02B6/29332—Wavelength selective couplers, i.e. based on evanescent coupling between light guides, e.g. fused fibre couplers with transverse coupling between fibres having different propagation constant wavelength dependency
- G02B6/29334—Grating-assisted evanescent light guide couplers, i.e. comprising grating at or functionally associated with the coupling region between the light guides, e.g. with a grating positioned where light fields overlap in the coupler
Abstract
An optical filter comprises a plurality of optical fibers having a coupling region where the axially extending cores are closely spaced within a common cladding. The coupling region includes a long period grating for selectively shifting light of selected wavelengths from guided modes into non-guided modes. These non-guided modes are picked up by an adjacent core and light of the selected wavelengths is thus shifted from one core to another. The result is an optical filter particularly useful as a demultiplexer or a tapping device. In one embodiment the grating is formed in one of the cores. In an alternative embodiment, it is formed in the common cladding.
Description
OPTICAL TAPPING FILTERS EMPLOYING
- LONG PERIOD GRATINGS
Field of the Invention This invention relates to optical communications devices and, in particular to 5 optical filters employing long period gratings.
Back~round of the Invention Optical fiber communications systems are becoming increasingly important in the high speed tr~n~mi~.~ion of large amounts of information. A typical fiber communications systems comprises a source of optical input signals, a length of 10 optical fiber coupled to the source, and a receiver for optical signals coupled to the fiber. In essence, an optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are 15 transmitted with minimum attenuation in guided modes along the axis of the fiber.
Optical filters are used for shaping the spectral features of transmitted optical pulses, and multiplexers are used for transmitting a multiplicity of different signals at different wavelengths. The present invention relates to devices which can be used as filters and as demultiplexers to separate multiplexed signals in optical communications 20 systems.
Summar,v of the Invention An optical fiber comprises a plurality of optical fibers having a coupling region where the axially extending cores are closely spaced within a common cl~(l(1ing. The coupling region includes a long period grating for selectively shifting light of selected 25 wavelengths from guided modes into non-guided modes. These non-guided modes are picked up by an adjacent core and light of the selected wavelengths is thus shifted from one core to another. The result is an optical filter particularly useful as a demultiplexer or a tapping device. In one embodiment the grating is formed in one of the cores. In an alternative embodiment, it is formed in the common cl~(l(lin~.
In accordance with one aspect of the present invention there is provided an optical filtering device comprising: a plurality of optical waveguiding cores extending side-by-side in a common cl~l(1ing, and a grating for shifting light in a guided mode in one of said cores to a non-guided mode, said grating comprising a plurality of index perturbations spaced apart by a periodic distance ~, where 50 ~Lm ~ 1500 ~lm.
35 Brief Description of the Drawin~s FIG. 1 is a schematic view of an optical filter in accordance with one embodiment of the invention;
FIGs. 2, 3 and 4 are qualitative spectral diagrams of an optical pulse at various locations in the device of FIG. l;
FIG. 5 is a graphical plot of center wavelength versus period useful in making the device of FIG. 1; and FIG. 6 is a second embodiment of a filter in accordance with the invention.
Detailed D~,;~ n Referring to the drawings, FIG. 1 is a schematic cross section of a first embodiment of an optical filtering device 10 comprising a plurality of optical fibers 7 and 8 joined along a coupling region 9. Within the coupling region, theaxially extending waveguiding cores 11 and 12 are closely spaced within a commoncladding 13. Preferred center-to-center spacing between the cores is less than 10 core 15 diameters (or less than 10 mean core diameters if the cores have unequal diameters).
One of the cores, here core 12, includes a long period grating 14 for selectively shifting light of selected wavelengths from guided modes into non-guided modes.
Because of the coupling between the two cores, much of the light shifted from guided modes in core 12 is coupled to core 11 where it is absorbed into guided 20 modes. Advantageously, the device is provided with at least three ports 15, 16 and 17. In typical operation, optical input pulses will enter the device via port lS which has a core region continuous with grating core 12. One output can be taken from port 16 which is continuous with core 11 and another can be taken from port 17 which is continuous with core 12. While a two-core device is shown in FIG. 1, it25 will be appreciated that similar devices can be made employing three or more cores.
The long period grating 14 comprises a plurality of index pe,lull,ations of width w spaced apart by a periodic distance A where, typically, 50 llm < A S
1500 ~lm. Advantageously, llSA<w<415A and preferably w=l/2A. The perturbations are formed within the glass core of the fiber and preferably form an 30 angle 0(2~~90~) with the longitudinal axis of the fiber. The fiber is designed to transmit broad band light of wavelength centered about ~O.
The spacing A of the pe,lu~bations is chosen to shift transmitted light in the region of a selected wavelength ~p from the guided mode into a non-guided mode, thereby reducing in core 12 the intensity of light centered about ~p. In 35 contrast with conventional short period gratings which reflect light, these long period devices remove the light without reflection by converting it from a guided mode to a 2 1 6g902 non-guided mode. A substantial portion of the non-guided light couples into core 11 where it excites guided modes, producing in 11 a narrow pulse of light centered about ~p.
FIGs. 2, 3 and 4 illustrate the operation of the device. FIG. 2 shows the 5 spectrum of a relative broad pulse of light centered about ~O entering core 12 via port 15. FIG. 3 shows the spectrum of the relatively narrow pulse of light centered about ~p which is coupled from core 12 to core 11 and exits via port 16. FIG. 4 shows the spectrum of the pulse on core 12 at port 17 after the light has passedthrough the long period grating 14. The pulse is ~limini~hed in a region near ~p.
FIG. S is a graph useful in designing the long period grating 14.
Specifically, FIG. 5 plots for a communications optical fiber with core ~=.3% and core diameter of 8~Lm the periodic spacing A for shifting to an unguided mode, light centered about a wavelength ~p. Thus, for example, to make a device for shiftinglight centered around 1540 nm, one chooses a spacing of about 760 llm. Similar 15 curves can be empirically determined for other specific fibers.
Preferably the device is made by fusing together in the coupling region, two separate optical fibers. One fiber can have a core, such as germanosilicate glass, which is sensitive to UV radiation, and the other can have a core, such as aluminum or phosphorous doped silica, which is in.cen~itive to UV radiation. The cl~d~ing~ can be fused, as by the application of heat, and UV sensitivity can be enhanced by diffusing H 2 or D 2 into the glass. Alternatively, a single fiber having a plurality of cores and a common cladding can be drawn from preform cont~ining a corresponding plurality of core rods.
The long period grating can be written by exposing the fused region to U V radiation, e.g. 248 nm radiation from a KrF laser, through a slit or a mask. If a slit is used, the fiber is moved to successive exposure sites. The preferred exposure dosage for each slit is on the order of 1000 pulses of > 100mJ/cm2 fluence/pulse, and the number of index perturbations is in the range 10-100.
FIG. 6 illustrates the coupling region of an altemative embodiment of an 30 optical filtering device similar to FIG. l wherein the long period grating 60 is formed in the cladding common to a plurality of cores rather than in one of the cores. With this arrangement, the cores are advantageously of different diameter or different index so that light at the mid-band ~p for the grating is preferentially coupled into one of the cores.
,, 21~q~2 A convenient way to make the FIG. 6 device is to provide fibers with W sensitive cladding (e.g. germanosilicate glass) and UV incen~itive cores (e.g.aluminum or phosphorous doped silicates). The grating can be written as described above, but will form only in the UV sensitive cladding.
- LONG PERIOD GRATINGS
Field of the Invention This invention relates to optical communications devices and, in particular to 5 optical filters employing long period gratings.
Back~round of the Invention Optical fiber communications systems are becoming increasingly important in the high speed tr~n~mi~.~ion of large amounts of information. A typical fiber communications systems comprises a source of optical input signals, a length of 10 optical fiber coupled to the source, and a receiver for optical signals coupled to the fiber. In essence, an optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are 15 transmitted with minimum attenuation in guided modes along the axis of the fiber.
Optical filters are used for shaping the spectral features of transmitted optical pulses, and multiplexers are used for transmitting a multiplicity of different signals at different wavelengths. The present invention relates to devices which can be used as filters and as demultiplexers to separate multiplexed signals in optical communications 20 systems.
Summar,v of the Invention An optical fiber comprises a plurality of optical fibers having a coupling region where the axially extending cores are closely spaced within a common cl~(l(1ing. The coupling region includes a long period grating for selectively shifting light of selected 25 wavelengths from guided modes into non-guided modes. These non-guided modes are picked up by an adjacent core and light of the selected wavelengths is thus shifted from one core to another. The result is an optical filter particularly useful as a demultiplexer or a tapping device. In one embodiment the grating is formed in one of the cores. In an alternative embodiment, it is formed in the common cl~(l(lin~.
In accordance with one aspect of the present invention there is provided an optical filtering device comprising: a plurality of optical waveguiding cores extending side-by-side in a common cl~l(1ing, and a grating for shifting light in a guided mode in one of said cores to a non-guided mode, said grating comprising a plurality of index perturbations spaced apart by a periodic distance ~, where 50 ~Lm ~ 1500 ~lm.
35 Brief Description of the Drawin~s FIG. 1 is a schematic view of an optical filter in accordance with one embodiment of the invention;
FIGs. 2, 3 and 4 are qualitative spectral diagrams of an optical pulse at various locations in the device of FIG. l;
FIG. 5 is a graphical plot of center wavelength versus period useful in making the device of FIG. 1; and FIG. 6 is a second embodiment of a filter in accordance with the invention.
Detailed D~,;~ n Referring to the drawings, FIG. 1 is a schematic cross section of a first embodiment of an optical filtering device 10 comprising a plurality of optical fibers 7 and 8 joined along a coupling region 9. Within the coupling region, theaxially extending waveguiding cores 11 and 12 are closely spaced within a commoncladding 13. Preferred center-to-center spacing between the cores is less than 10 core 15 diameters (or less than 10 mean core diameters if the cores have unequal diameters).
One of the cores, here core 12, includes a long period grating 14 for selectively shifting light of selected wavelengths from guided modes into non-guided modes.
Because of the coupling between the two cores, much of the light shifted from guided modes in core 12 is coupled to core 11 where it is absorbed into guided 20 modes. Advantageously, the device is provided with at least three ports 15, 16 and 17. In typical operation, optical input pulses will enter the device via port lS which has a core region continuous with grating core 12. One output can be taken from port 16 which is continuous with core 11 and another can be taken from port 17 which is continuous with core 12. While a two-core device is shown in FIG. 1, it25 will be appreciated that similar devices can be made employing three or more cores.
The long period grating 14 comprises a plurality of index pe,lull,ations of width w spaced apart by a periodic distance A where, typically, 50 llm < A S
1500 ~lm. Advantageously, llSA<w<415A and preferably w=l/2A. The perturbations are formed within the glass core of the fiber and preferably form an 30 angle 0(2~~90~) with the longitudinal axis of the fiber. The fiber is designed to transmit broad band light of wavelength centered about ~O.
The spacing A of the pe,lu~bations is chosen to shift transmitted light in the region of a selected wavelength ~p from the guided mode into a non-guided mode, thereby reducing in core 12 the intensity of light centered about ~p. In 35 contrast with conventional short period gratings which reflect light, these long period devices remove the light without reflection by converting it from a guided mode to a 2 1 6g902 non-guided mode. A substantial portion of the non-guided light couples into core 11 where it excites guided modes, producing in 11 a narrow pulse of light centered about ~p.
FIGs. 2, 3 and 4 illustrate the operation of the device. FIG. 2 shows the 5 spectrum of a relative broad pulse of light centered about ~O entering core 12 via port 15. FIG. 3 shows the spectrum of the relatively narrow pulse of light centered about ~p which is coupled from core 12 to core 11 and exits via port 16. FIG. 4 shows the spectrum of the pulse on core 12 at port 17 after the light has passedthrough the long period grating 14. The pulse is ~limini~hed in a region near ~p.
FIG. S is a graph useful in designing the long period grating 14.
Specifically, FIG. 5 plots for a communications optical fiber with core ~=.3% and core diameter of 8~Lm the periodic spacing A for shifting to an unguided mode, light centered about a wavelength ~p. Thus, for example, to make a device for shiftinglight centered around 1540 nm, one chooses a spacing of about 760 llm. Similar 15 curves can be empirically determined for other specific fibers.
Preferably the device is made by fusing together in the coupling region, two separate optical fibers. One fiber can have a core, such as germanosilicate glass, which is sensitive to UV radiation, and the other can have a core, such as aluminum or phosphorous doped silica, which is in.cen~itive to UV radiation. The cl~d~ing~ can be fused, as by the application of heat, and UV sensitivity can be enhanced by diffusing H 2 or D 2 into the glass. Alternatively, a single fiber having a plurality of cores and a common cladding can be drawn from preform cont~ining a corresponding plurality of core rods.
The long period grating can be written by exposing the fused region to U V radiation, e.g. 248 nm radiation from a KrF laser, through a slit or a mask. If a slit is used, the fiber is moved to successive exposure sites. The preferred exposure dosage for each slit is on the order of 1000 pulses of > 100mJ/cm2 fluence/pulse, and the number of index perturbations is in the range 10-100.
FIG. 6 illustrates the coupling region of an altemative embodiment of an 30 optical filtering device similar to FIG. l wherein the long period grating 60 is formed in the cladding common to a plurality of cores rather than in one of the cores. With this arrangement, the cores are advantageously of different diameter or different index so that light at the mid-band ~p for the grating is preferentially coupled into one of the cores.
,, 21~q~2 A convenient way to make the FIG. 6 device is to provide fibers with W sensitive cladding (e.g. germanosilicate glass) and UV incen~itive cores (e.g.aluminum or phosphorous doped silicates). The grating can be written as described above, but will form only in the UV sensitive cladding.
Claims (6)
1. An optical filtering device comprising:
a plurality of optical waveguiding cores extending side-by-side in a common cladding, and a grating for shifting light in a guided mode in one of said cores to a non-guided mode, said grating comprising a plurality of index perturbations spaced apart by a periodic distance .DELTA., where 50 µm ~ .DELTA. < 1500 µm.
a plurality of optical waveguiding cores extending side-by-side in a common cladding, and a grating for shifting light in a guided mode in one of said cores to a non-guided mode, said grating comprising a plurality of index perturbations spaced apart by a periodic distance .DELTA., where 50 µm ~ .DELTA. < 1500 µm.
2. The device according to claim 1 wherein said grating is in one of said cores.
3. The device according to claim 1 wherein said grating is in said common cladding.
4. The device according to claim 1 wherein said cores are spaced apart by less than 10 mean core diameters.
5. The device according to claim 1 wherein said index perturbations have a width w, where 1/5 .DELTA. ~w ~4/5 .DELTA..
6. The device according to claim 1 wherein said pair of optical cores in a common cladding comprises a pair of optical fibers fused together.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/393,754 US5550940A (en) | 1995-02-24 | 1995-02-24 | Optical tapping filters employing long period gratings |
| US393,754 | 1995-02-24 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2168902A1 CA2168902A1 (en) | 1996-08-25 |
| CA2168902C true CA2168902C (en) | 1999-03-30 |
Family
ID=23556104
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002168902A Expired - Fee Related CA2168902C (en) | 1995-02-24 | 1996-02-06 | Optical tapping filters employing long period gratings |
Country Status (4)
| Country | Link |
|---|---|
| US (1) | US5550940A (en) |
| EP (1) | EP0729043A3 (en) |
| JP (1) | JPH08248229A (en) |
| CA (1) | CA2168902C (en) |
Families Citing this family (18)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB9324456D0 (en) | 1993-11-29 | 1994-01-12 | Univ Southampton | Waveguide coupler |
| GB9503781D0 (en) * | 1994-05-19 | 1995-04-12 | Univ Southampton | External cavity laser |
| US5768452A (en) * | 1996-04-17 | 1998-06-16 | Lucent Technologies Inc. | Radiolytic method for trimming planar waveguide couplers |
| US5778119A (en) * | 1996-10-08 | 1998-07-07 | Jds Fitel Inc. | In-line grating device for forward coupling light |
| US6631224B2 (en) * | 1997-06-06 | 2003-10-07 | Novera Optics, Inc. | Tunable filter with core mode blocker |
| US6801686B2 (en) * | 1997-06-06 | 2004-10-05 | Novera Optics, Inc. | Methods and apparatus for measuring the power spectrum of optical signals |
| US6850655B2 (en) | 1997-06-16 | 2005-02-01 | Novera Optics, Inc. | Optical apparatus with faraday rotator, static gain flattening filter and variable optical attenuator |
| US6058226A (en) * | 1997-10-24 | 2000-05-02 | D-Star Technologies Llc | Optical fiber sensors, tunable filters and modulators using long-period gratings |
| WO1999030445A1 (en) | 1997-12-08 | 1999-06-17 | Sumitomo Electric Industries, Ltd. | Dispersion compensation module |
| US6011881A (en) * | 1997-12-29 | 2000-01-04 | Ifos, Intelligent Fiber Optic Systems | Fiber-optic tunable filter |
| US6222973B1 (en) | 1999-01-15 | 2001-04-24 | D-Star Technologies, Inc. | Fabrication of refractive index patterns in optical fibers having protective optical coatings |
| US6528239B1 (en) * | 1999-01-15 | 2003-03-04 | Sabeus Photonics, Inc. | Method of forming a grating in a waveguide |
| US6360038B1 (en) | 1999-05-12 | 2002-03-19 | Sabeus Photonics, Inc. | Wavelength-selective optical fiber components using cladding-mode assisted coupling |
| US6850665B2 (en) * | 1999-05-12 | 2005-02-01 | Sabeus Photonics | Wavelength-selective optical fiber components using cladding-mode assisted coupling |
| US6445855B1 (en) * | 1999-05-25 | 2002-09-03 | Thomas & Betts International, Inc. | Method of fabricating fused and tapered couplers from optical fibers having photosensitive cladding regions |
| US6529676B2 (en) | 2000-12-08 | 2003-03-04 | Lucent Technologies Inc. | Waveguide incorporating tunable scattering material |
| US6826343B2 (en) | 2001-03-16 | 2004-11-30 | Cidra Corporation | Multi-core waveguide |
| WO2013134575A1 (en) * | 2012-03-07 | 2013-09-12 | Ofs Fitel, Llc | Grating-based sensor |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4737007A (en) * | 1986-02-24 | 1988-04-12 | American Telephone And Telegraph Company, At&T Bell Laboratories | Narrow-band wavelength selective optical coupler |
| JP3005996B2 (en) * | 1989-07-14 | 2000-02-07 | 日本電気株式会社 | Waveguide optical demultiplexer and method of manufacturing the same |
| CA2022367C (en) * | 1990-07-31 | 2002-12-17 | Kenneth O. Hill | Simplified wdm fused fiber coupler design |
-
1995
- 1995-02-24 US US08/393,754 patent/US5550940A/en not_active Expired - Lifetime
-
1996
- 1996-02-06 CA CA002168902A patent/CA2168902C/en not_active Expired - Fee Related
- 1996-02-14 EP EP96301029A patent/EP0729043A3/en not_active Ceased
- 1996-02-23 JP JP8036015A patent/JPH08248229A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPH08248229A (en) | 1996-09-27 |
| EP0729043A2 (en) | 1996-08-28 |
| CA2168902A1 (en) | 1996-08-25 |
| US5550940A (en) | 1996-08-27 |
| EP0729043A3 (en) | 1997-01-29 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKLA | Lapsed |