GB2162836A - Optical fibre grating structures - Google Patents

Optical fibre grating structures Download PDF

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Publication number
GB2162836A
GB2162836A GB08420589A GB8420589A GB2162836A GB 2162836 A GB2162836 A GB 2162836A GB 08420589 A GB08420589 A GB 08420589A GB 8420589 A GB8420589 A GB 8420589A GB 2162836 A GB2162836 A GB 2162836A
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United Kingdom
Prior art keywords
grating
fibre
optical
porous glass
photosensitive
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GB08420589A
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GB2162836B (en
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Kevin Christopher Byron
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STC PLC
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STC PLC
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Priority to GB08420589A priority Critical patent/GB2162836B/en
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Publication of GB2162836B publication Critical patent/GB2162836B/en
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    • 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
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G02B6/02128—Internal inscription, i.e. grating written by light propagating within the fibre, e.g. "self-induced"
    • 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
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02171—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes
    • G02B6/02176—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes due to temperature fluctuations
    • G02B6/02185—Refractive index modulation gratings, e.g. Bragg gratings characterised by means for compensating environmentally induced changes due to temperature fluctuations based on treating the fibre, e.g. post-manufacture treatment, thermal aging, annealing

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Light Guides In General And Applications Therefor (AREA)
  • Optical Fibers, Optical Fiber Cores, And Optical Fiber Bundles (AREA)
  • Diffracting Gratings Or Hologram Optical Elements (AREA)

Abstract

A grating structure is produced in a short length of optical fibre consisting of porous glass impregnated with photosensitive material, by transmitting a laser beam in a standing wave in a Fabry-Perot resonator comprised by the short length of fibre. The laser beam serves to cause a periodic variation in refractive index of the optical fibre by photodissociation of the photosensitive material, which variation can be made permanent by a heat "fixing" process. The grating spacing can be increased or decreased following fixing, by pulling the fibre to a smaller diameter, or heating the fibre to reduce its porosity, respectively. The grating spacing can thus be matched to a required operating wavelength.

Description

SPECIFICATION Optical fibre grating structures This invention relates to optical fibres and, in particular to the induction of gratings in optical fibres, whereby to provide grating structures, for example, interference filters, and to methods of manufacturing such grating structures.
According to one aspect of the present invention there is provided a method of manufacturing a grating structure consisting of a periodic variation in refractive index in an optical fibre, including the step of transmitting a laser beam in a standing wave in a Fabry-Perot resonator comprising a short length of the optical fibre consisting of porous glass impregnated with photosensitive material, the laser beam serving to write the grating structure into the fibre by photodissociation of the photosensitive material.
According to another aspect of the present in vention there is provided a grating structure consisting of an optical fibre having a periodic variation in refractive index written therein by transmitting a standing wave in the optical fibre by a laser beam, the optical fibre being comprised of porous glass impregnated with photosensitive material, the grating spacing of which structure can be increased or decreased subsequent to said writing whereby to match it to an operating wavelength.
It has previously been reported that interference filters can be grown in short lengths of optical fibre, see for example "Fiber-optic integrated interference filters" by J. Lapierre et al. Optics Letters Vol.7 No.1 p37-38, Jan. 1982. A short length ( < lm) of quasi-monomode fibre (for example germaniumdoped silica core fibre) is illuminated at one end with a relatively high power density (=1mW cam~2) laser beam, the ends of the fibre being cut flat and parallel to form a low-contrast Fabry-Perot interferometer. Under the application of the laser beam, the fibre's reflectivity grows rapidly as a result of the production of a periodical structure along the axis of the fibre. The periodicity of the structure is equal to that of the standing-wave pattern characteristic of the Fabry-Perot interferometer.The fibre's exposure to the laser beam results in the production of a grating consisting of a periodic variation in refractive index which is obtained by a photo-refractive effect. This is termed "writing" of a grating into the fibre structure.
The photo-refractive induced gratings have only been obtained in the visible part of the spectrum and are thus not suitable for use in optical communications systems employing wavelengths in the infra-red part of the specification, particularly the 1.3 or 1.55 micron wavelengths which are currently favoured. In addition, in optical communications applications matching of the grating spacing to the operating wavelength is required and it is thus desirable to be able to manufacture gratings in a manner facilitating such matching.
It is thus proposed to induce gratings in optical fibres made of photosensitive impregnated porous glass, in which case writing of the grating is achieved by a photo-dissociation process rather than a photo-refractive effect. An advantage of use of photosensitive impregnated porous glass is that the grating spacing can be reduced or increased after the grating has been written in, in order to match it to a desired wavelength of operation.
Such a photosensitive impregnated porous glass may be porous glass impregnated with photosensitive organometallic compounds such .as transition metal carbonyls, as described in "Photosensitive impregnated porous glass" by N.F. Borrelli and D.L. Morse, Appl. Phys. Lett. 43(11) p.992-3, 1 Dec 1983. When exposed to light such impregnated porous glasses exhibit optical absorption and/or refractive index changes, which changes can be made permanent and insensitive to further light exposure by a fixing process comprising gently heating the glass after exposure in order to remove unreacted compound by volatilization and to stabilize of the photolyzed product through further oxidation.The stability of the photolyzed product is such that the porous glass can be heated to a temperature (near 1200"C) at which a consolidated states reached, that is the porous structure collapses. The induced optical changes are maintained but the consolidation produces a volume shrinkage.
Thus the grating spacing of a grating induced in a photosensitive impregnated porous glass can be reduced by elimination of the porosity of the glass.
Alternatively, the grating spacing can be increased if the grating is first written in fibre of a first diameter and subsequent to the heat "fixing" treatment, the fibre is pulled to a second, smaller diameter. In this case the fibre will initially be oversize in comparison with its required application.
Using this writing of gratings into lengths of optical fibres comprised of porous glass impregnated with photosensitive organometallic compounds gratings can be achieved which can be matched to the required operating wavelength as required by increasing or decreasing the grating spacing subsequent to the writing and fixing process. Typically such gratings may be employed as narrow band filters or selective reflectors for optical communications applications, but the gratings are not restricted to optical communication wavelengths and may be produced for use in systems employing other wavelengths.
A A method of manufacturing a grating struc- ture consisting of a periodic variation in refractive index in an optical fibre, including the step of transmitting a laser beam in a standing wave in a Fabry-Perot resonator comprising a short length of the optical fibre consisting of porous glass impregnated with photosensitive material, the laser beam serving to write the grating structure into the fibre by photodissociation of the photosensitive material.
2. A method as claimed in claim 1, wherein the written grating structure is fixed into the fibre by
**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

  1. **WARNING** start of CLMS field may overlap end of DESC **.
    SPECIFICATION Optical fibre grating structures This invention relates to optical fibres and, in particular to the induction of gratings in optical fibres, whereby to provide grating structures, for example, interference filters, and to methods of manufacturing such grating structures.
    According to one aspect of the present invention there is provided a method of manufacturing a grating structure consisting of a periodic variation in refractive index in an optical fibre, including the step of transmitting a laser beam in a standing wave in a Fabry-Perot resonator comprising a short length of the optical fibre consisting of porous glass impregnated with photosensitive material, the laser beam serving to write the grating structure into the fibre by photodissociation of the photosensitive material.
    According to another aspect of the present in vention there is provided a grating structure consisting of an optical fibre having a periodic variation in refractive index written therein by transmitting a standing wave in the optical fibre by a laser beam, the optical fibre being comprised of porous glass impregnated with photosensitive material, the grating spacing of which structure can be increased or decreased subsequent to said writing whereby to match it to an operating wavelength.
    It has previously been reported that interference filters can be grown in short lengths of optical fibre, see for example "Fiber-optic integrated interference filters" by J. Lapierre et al. Optics Letters Vol.7 No.1 p37-38, Jan. 1982. A short length ( < lm) of quasi-monomode fibre (for example germaniumdoped silica core fibre) is illuminated at one end with a relatively high power density (=1mW cam~2) laser beam, the ends of the fibre being cut flat and parallel to form a low-contrast Fabry-Perot interferometer. Under the application of the laser beam, the fibre's reflectivity grows rapidly as a result of the production of a periodical structure along the axis of the fibre. The periodicity of the structure is equal to that of the standing-wave pattern characteristic of the Fabry-Perot interferometer.The fibre's exposure to the laser beam results in the production of a grating consisting of a periodic variation in refractive index which is obtained by a photo-refractive effect. This is termed "writing" of a grating into the fibre structure.
    The photo-refractive induced gratings have only been obtained in the visible part of the spectrum and are thus not suitable for use in optical communications systems employing wavelengths in the infra-red part of the specification, particularly the 1.3 or 1.55 micron wavelengths which are currently favoured. In addition, in optical communications applications matching of the grating spacing to the operating wavelength is required and it is thus desirable to be able to manufacture gratings in a manner facilitating such matching.
    It is thus proposed to induce gratings in optical fibres made of photosensitive impregnated porous glass, in which case writing of the grating is achieved by a photo-dissociation process rather than a photo-refractive effect. An advantage of use of photosensitive impregnated porous glass is that the grating spacing can be reduced or increased after the grating has been written in, in order to match it to a desired wavelength of operation.
    Such a photosensitive impregnated porous glass may be porous glass impregnated with photosensitive organometallic compounds such .as transition metal carbonyls, as described in "Photosensitive impregnated porous glass" by N.F. Borrelli and D.L. Morse, Appl. Phys. Lett. 43(11) p.992-3, 1 Dec 1983. When exposed to light such impregnated porous glasses exhibit optical absorption and/or refractive index changes, which changes can be made permanent and insensitive to further light exposure by a fixing process comprising gently heating the glass after exposure in order to remove unreacted compound by volatilization and to stabilize of the photolyzed product through further oxidation.The stability of the photolyzed product is such that the porous glass can be heated to a temperature (near 1200"C) at which a consolidated states reached, that is the porous structure collapses. The induced optical changes are maintained but the consolidation produces a volume shrinkage.
    Thus the grating spacing of a grating induced in a photosensitive impregnated porous glass can be reduced by elimination of the porosity of the glass.
    Alternatively, the grating spacing can be increased if the grating is first written in fibre of a first diameter and subsequent to the heat "fixing" treatment, the fibre is pulled to a second, smaller diameter. In this case the fibre will initially be oversize in comparison with its required application.
    Using this writing of gratings into lengths of optical fibres comprised of porous glass impregnated with photosensitive organometallic compounds gratings can be achieved which can be matched to the required operating wavelength as required by increasing or decreasing the grating spacing subsequent to the writing and fixing process. Typically such gratings may be employed as narrow band filters or selective reflectors for optical communications applications, but the gratings are not restricted to optical communication wavelengths and may be produced for use in systems employing other wavelengths.
    A A method of manufacturing a grating struc- ture consisting of a periodic variation in refractive index in an optical fibre, including the step of transmitting a laser beam in a standing wave in a Fabry-Perot resonator comprising a short length of the optical fibre consisting of porous glass impregnated with photosensitive material, the laser beam serving to write the grating structure into the fibre by photodissociation of the photosensitive material.
  2. 2. A method as claimed in claim 1, wherein the written grating structure is fixed into the fibre by treatment serving to remove unreacted photosensitive material.
  3. 3. A method as claimed in claim 2, wherein said treatment is heat treatment serving to volatise said unreacted photosensitive -material.
  4. 4. A method as claimed in claim 2 or claim 3, wherein the grating spacing of the written and fixed grating structure is increased or reduced whereby to match it to a predetermined operating wavelength.
  5. 5. A method as claimed in claim 4, wherein the grating spacing is reduced by heating the grating structure whereby to eliminate or at least reduce the porosity of the glass of the fibre.
  6. 6: A method as claimed in claim 4, wherein the grating spacing is increased by pulling the fibre to a smaller diameter.
  7. 7. A method of manufacturing a grating structure as claimed in claim 1 and substantially as herein described.
  8. 8. A grating structure manufactured by a method as claimed in any one uf the preceding claims.
  9. 9. A-grating structure consisting of an optical fibre having a periodic variation in refractive index written therein by transmitting a standing wave in the optical fibre by a laser beam, the optical fibre being comprised of porous glass impregnated with photosensitive material, the grating spacing of which structure can be increased or decreased subsequent to said writing whereby to match it to an operating wavelength.
  10. 10. An optical communications system including a grating structure as claimed in claim~9, the grating spacing having been matched to the required operating wavelength by appropriate treatment of the fibre following grating writing.
GB08420589A 1984-08-14 1984-08-14 Optical fibre grating structures Expired GB2162836B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB08420589A GB2162836B (en) 1984-08-14 1984-08-14 Optical fibre grating structures

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB08420589A GB2162836B (en) 1984-08-14 1984-08-14 Optical fibre grating structures

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GB2162836A true GB2162836A (en) 1986-02-12
GB2162836B GB2162836B (en) 1988-05-18

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992008999A1 (en) * 1990-11-08 1992-05-29 British Telecommunications Public Limited Company Method of forming optical fibre gratings
US5530666A (en) * 1990-11-08 1996-06-25 British Telecommunications, Plc Optical memory

Non-Patent Citations (3)

* Cited by examiner, † Cited by third party
Title
APPLIED PHYSICS LETTERS VOL 43. NO. 11 (1983) PAGES 992-3, *
APPLIED PHYSICS LETTERS, VOL 32, NO. 10 (1978) PAGES 647-9 *
OPTICS LETTERS, VOL 7, NO. 1 (1982) PAGES 37-38 *

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992008999A1 (en) * 1990-11-08 1992-05-29 British Telecommunications Public Limited Company Method of forming optical fibre gratings
US5384884A (en) * 1990-11-08 1995-01-24 British Telecommunications Public Limited Company Method of forming optical fibre gratings
US5530666A (en) * 1990-11-08 1996-06-25 British Telecommunications, Plc Optical memory

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Publication number Publication date
GB2162836B (en) 1988-05-18

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