WO2003078338A2 - Preforme, son procede de production et son utilisation pour produire des fibres optiques microstructurees - Google Patents

Preforme, son procede de production et son utilisation pour produire des fibres optiques microstructurees Download PDF

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Publication number
WO2003078338A2
WO2003078338A2 PCT/DK2003/000182 DK0300182W WO03078338A2 WO 2003078338 A2 WO2003078338 A2 WO 2003078338A2 DK 0300182 W DK0300182 W DK 0300182W WO 03078338 A2 WO03078338 A2 WO 03078338A2
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WIPO (PCT)
Prior art keywords
preform
holes
rods
length
gel
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PCT/DK2003/000182
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English (en)
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WO2003078338A8 (fr
Inventor
Christian Jakobsen
Guillaume Vienne
Theis Peter Hansen
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Crystal Fibre A/S
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Application filed by Crystal Fibre A/S filed Critical Crystal Fibre A/S
Priority to AU2003226893A priority Critical patent/AU2003226893A1/en
Publication of WO2003078338A2 publication Critical patent/WO2003078338A2/fr
Publication of WO2003078338A8 publication Critical patent/WO2003078338A8/fr

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/016—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD] by a liquid phase reaction process, e.g. through a gel phase
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/01205—Manufacture of preforms for drawing fibres or filaments starting from tubes, rods, fibres or filaments
    • C03B37/01225—Means for changing or stabilising the shape, e.g. diameter, of tubes or rods in general, e.g. collapsing
    • C03B37/01228—Removal of preform material
    • C03B37/01231—Removal of preform material to form a longitudinal hole, e.g. by drilling
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/01446—Thermal after-treatment of preforms, e.g. dehydrating, consolidating, sintering
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/01486—Means for supporting, rotating or translating the preforms being formed, e.g. lathes
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/01486—Means for supporting, rotating or translating the preforms being formed, e.g. lathes
    • C03B37/01493—Deposition substrates, e.g. targets, mandrels, start rods or tubes
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/018—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD] by glass deposition on a glass substrate, e.g. by inside-, modified-, plasma-, or plasma modified- chemical vapour deposition [ICVD, MCVD, PCVD, PMCVD], i.e. by thin layer coating on the inside or outside of a glass tube or on a glass rod
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/018—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD] by glass deposition on a glass substrate, e.g. by inside-, modified-, plasma-, or plasma modified- chemical vapour deposition [ICVD, MCVD, PCVD, PMCVD], i.e. by thin layer coating on the inside or outside of a glass tube or on a glass rod
    • C03B37/01884—Means for supporting, rotating and translating tubes or rods being formed, e.g. lathes
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B37/00—Manufacture or treatment of flakes, fibres, or filaments from softened glass, minerals, or slags
    • C03B37/01—Manufacture of glass fibres or filaments
    • C03B37/012—Manufacture of preforms for drawing fibres or filaments
    • C03B37/014—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD]
    • C03B37/018—Manufacture of preforms for drawing fibres or filaments made entirely or partially by chemical means, e.g. vapour phase deposition of bulk porous glass either by outside vapour deposition [OVD], or by outside vapour phase oxidation [OVPO] or by vapour axial deposition [VAD] by glass deposition on a glass substrate, e.g. by inside-, modified-, plasma-, or plasma modified- chemical vapour deposition [ICVD, MCVD, PCVD, PMCVD], i.e. by thin layer coating on the inside or outside of a glass tube or on a glass rod
    • C03B37/01884—Means for supporting, rotating and translating tubes or rods being formed, e.g. lathes
    • C03B37/01892—Deposition substrates, e.g. tubes, mandrels
    • 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
    • G02B6/02295—Microstructured optical fibre
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/10—Internal structure or shape details
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/10—Internal structure or shape details
    • C03B2203/14—Non-solid, i.e. hollow products, e.g. hollow clad or with core-clad interface
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/10—Internal structure or shape details
    • C03B2203/14—Non-solid, i.e. hollow products, e.g. hollow clad or with core-clad interface
    • C03B2203/16—Hollow core
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2203/00—Fibre product details, e.g. structure, shape
    • C03B2203/42—Photonic crystal fibres, e.g. fibres using the photonic bandgap PBG effect, microstructured or holey optical fibres

Definitions

  • the present invention relates to preforms for microstruc- tured optical fibres, methods for producing such preforms, and use thereof in production of microstructured optical fibres.
  • Microstructured optical fibres also known as photonic crystal fibres, photonic band gap fibres, holey fibres and hole-assisted fibres have undergone a very rapid development in their short existence.
  • the usual way of building preforms from stacked capillary tubes (see for example "2D Photonic band gap structures in fibre form", T. A. Birks et al., Photonic Band Gap Materials, Kluwer, 1996) is now being challenged by new schemes.
  • the introduction of new schemes is inevitable in order to deal with issues such as impurity induced loss, PMD, reducing operator dependence, method suitability for large scale production etc. This should not be looked upon as a problem but rather as a natural consequence of the group of microstructured optical fibres being on the brink of be- coming commercially interesting.
  • microstructured fibres Due to the diversity of microstructured fibres and their applications, there will be a need for different fabrication schemes. Besides depending on the experience within each manufacturer also the suitability and feasibility of each scheme strongly depend on the specific fibre design. The vast number of published fibre designs clearly displays the variety of complexity within microstructured optical fibres that will favour individual choices for best practice in production.
  • the present inventions provides range of new methods of making preforms for micro- structured optical fibres, including designs of such preforms, and designs of optical fibres that are produced using such preforms.
  • Circular structures are particularly interesting and have several advantages over the hexagonal structures of the usual stacking technique. For example they are expected to have reduced polarization mode dispersion (PMD) as well as to better match the mode structure of conventional fibre (an important issue for connectorisation) .
  • PMD polarization mode dispersion
  • the stability observed ⁇ in this circular type of micro- structured fibres is further advantageous in order to provide low PMD values due to the strong uniformity.
  • a method of making a preform for a microstructured optical fibre comprising the steps: a) providing a tubular vessel having a length; b) providing a plurality of elongate elements extending at least a portion of said length and being maintained in a predetermined spatial arrangement with respect to the vessel; c) filling at least a portion of said vessel with a silica-containing sol; d) permitting or causing said sol to gel to thereby form a wet gel body; and e) drying said wet gel body, such that a dried or at least partly dried gel body comprising said plurality of elongate elements results.
  • step c) is performed before step b) .
  • step e) includes or further includes sintering of the gel body.
  • the sintering process is performed after the drying process, such that a dried or at least partly dried and sintered gel body comprising said plurality of elongate elements results.
  • step e) further includes purifying of the gel body, such that, a dried or at least partly dried, purified and sintered gel body comprising said plurality of elongate elements results.
  • the elongate elements are maintained in the predetermined arrangement by holding fixtures.
  • the vessel is removed after the gelation process of step d) and before the drying process of step e) .
  • the holding fixtures are removed after the gelation process of step d) and before the drying process of step e) .
  • the sintering process is performed at a temperature in the range of 1100-1600 °C or in the range of 1300-1500 °C.
  • the sintering process is performed at a temperature in the range of 1200-1750 °C.
  • the elongate elements comprise one or more capillary tubes and/or one or more rods.
  • At least part of the one or more capillary tubes is sealed before the filling of at least a portion of said vessel with the sol.
  • At least a part of the sealed capillary tubes are unsealed after the drying or sinter- ing process.
  • the one or more rods comprise one or more doped rods.
  • the dried or sintered body is sleeved in an overclad tube.
  • a compound glass is added to one or more of the capillary tubes after the drying or sintering process.
  • the compound glass contains less than 70% of silica.
  • a gas is added to one or more of the capillary tubes.
  • said gas filled capillary tubes are sealed after the adding of gas.
  • the gas is added to the capillary tubes after the drying or sintering process.
  • the added gas is containing chlorine or fluorine or an inert gas.
  • the gel body has a diameter or largest cross-sectional dimension in the range of 10- 150 mm or in the range of 20-100 mm.
  • the gel body has a length larger than 100mm or in the range of 150-1200 mm.
  • said method comprising using a preform as previously described in one or more of the preferred embodiments.
  • a gas is flowing through a number of open capillary tubes during at least part of the drawing process.
  • the flowing gas is containing chlorine or fluorine or an inert gas.
  • said covering of the outer and/or inner grooves thereby providing a number of elongated holes extending in the direction of the length of the preform elements.
  • said at least one sol-gel element comprises an inner sol-gel preform element having a number of grooves being arranged on the outer surface of the inner sol-gel element.
  • said at least one sol-gel preform element comprises one or more tubular formed sol- gel elements having a number of grooves being arranged on the outer surface of the element and/or the inner surface of the element.
  • said at least one sol-gel preform element comprises a first number of curve-shaped elements with a number of outer and/or inner grooves, said first number of curve-shaped elements being arranged so as to form a first compound tube with outer and/or inner grooves.
  • At least one of said one or more tubular formed sol-gel elements is used as an outer or inner cover preform element.
  • said first compound tube formed element is used as an outer or inner cover preform element.
  • the casting of a sol-gel preform element comprises:
  • step d) includes or further includes sintering of the gel body.
  • the sintering process is performed after the drying process, such that a dried or at least partly dried and sintered gel body comprising said number of grooves results.
  • step d) further includes purifying of the gel body, such that a dried or at least partly dried, purified and sintered gel body comprising said number of grooves results.
  • the sol-gel preform element has a diameter or largest cross-sectional dimension in the range of 10-150 mm or in the range of 20-lOOmm.
  • the sol-gel preform element has a length larger than 100 mm or in the range of 150- 1200 mm.
  • a preform for a microstructured optical fibre comprising:
  • said deposition process comprises a Modified Chemical Vapor Deposition, MCVD, process or a plasma Chemical Vapor Deposition, plasma CVD, process.
  • the elongate elements are maintained in the predetermined arrangement by holding fixtures.
  • the holding fixtures comprises two discs, each disc having a first number of holes for placement of the elongate elements and a second number of holes for allowing a flow of process gas.
  • the holding fixtures are made of quartz or silica.
  • the holding fixtures are fused to the deposition tube by heating.
  • the elongate elements comprise one or more capillary tubes and/or one or more rods.
  • the elongate elements comprise one or more doped or un-doped silica rods.
  • the elongate elements comprise one or more doped or un-doped capillary silica tubes.
  • the preform body has a diameter or largest cross-sectional dimension in the range of 10-150 mm or in the range of 20-100 mm.
  • the preform body has a length larger than 100 mm or in the range of 150-1200 mm.
  • CVD Chemical Vapor Deposition
  • the deposition process comprises a VAD or an OVD deposition process.
  • said method further comprising one or more sintering processes during the deposition of the doped glass material.
  • said method further comprising the removal of the deposition rod after the deposition of the doped glass material.
  • the elongate elements are maintained in the predetermined arrangement by holding fixtures.
  • the holding fixtures comprises two discs, each disc having a first number of holes for placement of the elongate elements and a rod hole for placement of the deposition rod.
  • the holding fixtures are made of quartz or silica.
  • the holding fixtures are fused to the deposition tube by heating.
  • the elongate elements comprise one or more capillary tubes and/or one or more rods.
  • the elongate elements comprise one or more doped or un-doped silica rods.
  • the elongate elements comprise one or more doped or un-doped capillary silica tubes.
  • the preform body has a diameter or largest cross-sectional dimension in the range of 10-150 mm or in the range of 20-100 mm.
  • the preform body has a length larger than 100 mm or in the range of 150-1200 mm.
  • a method of making a preform for a microstructured fibre comprising:
  • step c) includes sintering of the body.
  • said porous or non-solid silica containing body is provided by use of a tubular vessel having a length, filling at least a portion of said vessel with a silica-containing sol, and permitting or causing said sol to gel to thereby form a gel body.
  • said porous or non-solid silica containing body is provided by use of a deposition rod having a length, performing a chemical vapour deposition process to thereby deposit doped glass on the deposition rod until at least a part of the deposition rod is covered with doped glass.
  • said deposition process comprises a so-called VAD or a so-called OVD deposition process.
  • the body is removed from the vessel before the formation of the elongate holes or voids.
  • the body is at least partly dried without being sintered before the formation of the elongate holes or voids.
  • the body is purified the sintering process.
  • the elongate holes or voids are formed by a drilling process.
  • the drilling process comprises a rotary or an ultra sonic drilling process.
  • the formed holes or voids are washed and etched.
  • the elongate holes or voids are formed by use of laser.
  • a C0 2 laser is used for said formation of holes or voids.
  • one or more rods are 1 inserted in the formed holes or voids after the solidification.
  • one or more tubes are inserted in the formed holes or voids after the solidification.
  • one or more of said rods and/or tubes are made of a doped material.
  • the solidified body has a diameter or largest cross-sectional dimension in the range of 10-150 mm or in the range of 20-100 mm.
  • the solidified body has a length larger than 100 mm or in the range of 150-1200 mm.
  • the elongate holes or voids formed in step d) have a diameter or largest cross sectional dimension larger than 0.8 mm. In a preferred embodiment, the elongate holes or voids formed in step d) have a length larger than 50 mm.
  • a method of making a preform for a microstructured optical fibre comprising:
  • the electrically resistive elements are arranged substantially parallel along said longitudinally direction.
  • said method further comprising removal of the electrically resistive elements after said local solidification, whereby the created gaps leaves a number of elongate holes or voids extending in said longitudinally direction.
  • said method further comprising permitting or causing said silica containing body to solidify after the removal of the electrically resistive elements. In a preferred embodiment., said method further including sintering of the body.
  • said porous or non-solid silica containing body is provided by use of a tubular vessel having a length and surrounding the electrically resistive elements, filling at least a portion of said vessel with a silica-containing sol, and permitting or causing said sol to gel to thereby form a gel body at least partly surrounding said electrically resistive elements .
  • said porous or non-solid silica containing body is provided by use of a deposition rod having a length and being surrounded by the electrically resistive elements, performing a chemical vapour deposition process to thereby deposit doped glass around the deposition rod until at least a part of the deposition rod is covered with doped glass and until the electrical- ly resistive elements are at least partly buried by the deposited material.
  • said deposition process comprises a so-called VAD or a so-called OVD deposition pro- cess.
  • the body is purified before the sintering process.
  • one or more rods are inserted in the formed holes or voids.
  • one or more tubes are inserted in the formed holes or voids.
  • one or more of said rods and/or tubes are made. of a doped material.
  • the silica containing body has a diameter or largest cross-sectional dimension in the range of 10-150 mm or in the range of 20-100 mm.
  • the silica containing body has a length larger than 100 mm or in the range of 150- 1200 mm.
  • the formed gaps have a diameter or largest cross sectional dimension larger than 0.2 mm or larger than 0.8 mm.
  • the formed gaps are extending in all the length of the silica containing body.
  • a preform for a microstructured optical fibre comprising:
  • the deposition process is plasma Chemical Vapor Deposition, PCVD, process.
  • a plasma coil is arranged around the silica tube or rod for use during the plasma deposition process.
  • the elongate holes or voids are formed by a drilling process.
  • the drilling process comprises a rotary or an ultra sonic drilling process.
  • the formed holes or voids are washed and/or etched.
  • the elongate holes or voids are formed by use of laser.
  • the doped glass material comprises a dopant selected from the group of dopants comprising: Ge, P, Sn, N, B and F.
  • the silica tube or rod has a diameter or largest cross-sectional dimension in the range of 10-150 mm or in the range of 20-100 mm.
  • the silica tube or rod has a length larger than 100 mm or in the range of 150-1200 mm.
  • the elongate holes or voids formed in step b) have a diameter or largest cross sectional dimension larger than 0.2 mm or larger than 0.8 mm.
  • the elongate holes or voids formed in step b) have a length larger than 50 mm or larger than 100 mm.
  • the elongate holes or voids formed in step b) are extending in all the length of the silica tube or rod.
  • the elongate holes may be formed in several ways, but according to one embodiment it is preferred that the elongate holes or voids are formed by a drilling process.
  • the drilling process may comprise a rota'ry or an ultra sonic drilling process. It is preferred that the formed holes or voids are washed and etched.
  • the elongate holes or voids may be formed by use of laser.
  • a C02 laser may be used for the formation of holes or voids.
  • the rods or capillary tubes are made of a doped material.
  • the doped material may be a doped silica material.
  • Such doped silica material may comprise a dopant selected from the group of dopants comprising: Ge, P, Sn, N, B and F and rare-earths.
  • the glass containing body may have a diameter or largest cross-sectional dimension larger than 5 mm, such as in the range of 5-150 mm or in the range of 20-100 mm. It is also preferred that the glass containing body has a length larger than 20 mm, such as in the range of 20-500 mm or in the range of 150- 1200 mm. Preferably, the glass containing body is cylin- drically shaped. For the elongate holes or voids formed in step b) it is preferred that they have a diameter or largest cross sectional dimension larger than 0.5 mm or larger than 0.8 mm, or in the range of 0.5-10 mm.
  • the elongate holes or voids formed in step b) it is preferred that they have a length larger than 20 mm, larger than 50 mm, or larger than 100mm, or in the range of 20-500 mm. It is also within a preferred embodiment that the elongate holes or voids formed in step b) are extending in all the length of the glass containing body.
  • the glass body or glass containing body may be made of different glass materials, but according to a preferred embodiment the glass containing body is a synthetic silica body.
  • the glass containing body may also or alternatively be a doped silica body comprising a dopant selected from the group of dopants comprising: Ge, P, Sn, N, B and F and rare-earths.
  • a preform for a microstructured optical fibre comprising a number of cylindrically shaped bodies having substantially equal radial dimensions, each said body having a number of spaced apart through-holes extending in a longitudinal direction along the axis of the body, and the radial position of at least part of said through-holes being substantial equal for each of the bodies.
  • the bodies may be stacked so as to align at least part of the through-holes of the bodies in the axial direction, and an outer tube may be contact- ingly surrounding at least part of the stacked bodies.
  • the bodies are stacked so that one or more elongate holes extending through several of the bodies are obtained via the aligned through-holes.
  • at least two or three of said elongate holes may be obtained, but it is also within the present invention to have a larger number of such elongate holes, and the number of elongate holes extending through several of the bodies may be in the range of 3-300.
  • said one or more elongate holes extend through at least two or three bodies.
  • the number of stacked bodies is at least two or at least three, but the present invention also covers a larger number of stacked bodies such as four or larger.
  • each or at least part of the cylindrically shaped bodies has a substantially planar front surface and/or back surface.
  • each or at least part of the substantially planar front surface (s) and/or back surface (s) may have been grinded and/or polished.
  • the ninth aspect of the present invention also covers an alternative embodiment in which the diameter of one or more of the bodies is larger than the length of the corresponding body.
  • the preform further comprises one or more rods and/or capillary tubes being inserted in the aligned through- holes of at least two successively arranged bodies.
  • one or more of said rods or capillary tubes may be made of a doped material, such as a doped glass, or silica material.
  • Such doped glass or silica material may comprise a dopant selected from the group of dopants com- prising: Ge, P, Sn, N, B and F and rare-earths.
  • the preform may further comprise a cylindrically shaped dummy body arranged at the top of the stacked bodies having through-holes.
  • the top dummy body should preferably have substantially equal radial dimensions as said stacked bodies with through-holes, and the top dummy body may be contactingly surrounded by the outer tube.
  • the preform may also or alterna- tively comprise a cylindrically shaped dummy body arranged at the bottom of the stacked bodies with through- holes.
  • the bottom dummy body should have substantially equal radial dimensions as said stacked bodies with through-holes, and the bottom dummy body may be con- tactingly surrounded by the outer tube.
  • the top and/or bottom dummy body need not to have any aligned through- holes, and thus may have no through-holes or the number of through-holes in a dummy body may be smaller than the number of through-holes in any of the stacked bodies with through-holes.
  • the stacked bodies are aligned by use of one or more alignment rods and/or tubes inserted in one or more of the through-holes of the bodies to be aligned.
  • the stacked bodies may be made of different materials, but it is preferred . that they are made of a glass material. Similarly, it is preferred that the dummy body or bodies are made of a glass material.
  • Such glass material may be a synthetic silica material, or the glass material may be a doped silica material.
  • the doped silica material may comprise a dopant selected from the group of dopants comprising: Ge, P, Sn, N, B and F and rare-earths.
  • the cylindrically shaped bodies may have a diameter or largest cross-sectional dimension larger than 5 mm, such as in the range of 5-150 mm or in the range of 20-100 mm. It is also pre- ferred that the cylindrically shaped bodies have a length larger than or equal to 20 mm, larger than or equal to 50 mm, or larger than or equal to 100 mm. For the through-holes of the bodies it is preferred that they have a diameter or. largest cross sectional dimension lar- ger than 0.5 mm or larger than 0.8 mm, or in the range of 0.5-10 mm.
  • a drilling process similar to the drilling process used when making a preform according to an embodiment of the eighth aspect of the invention may be used.
  • the through-holes may be formed by use of a laser such as a C0 2 laser.
  • an inner tube or inner rod having a length
  • a first plurality of cladding' tubes and/or rods arranged around said inner tube or rod and extending at least a portion of said length
  • a first outer tube surrounding said first plurality of cladding tubes and/or rods.
  • a preform or part thereof for a microstructured optical fibre comprising:
  • an inner tube or inner rod having a length
  • a first grooved tube having elongated slits and/or grooves extending at least a portion of said length, said first grooved tube surrounding said inner tube or rod
  • a first outer tube surrounding said first grooved tube
  • a number of pluralities of cladding tubes and/or rods extending at least a portion of said length are arranged in corresponding outer tubes concentrically about the inner tube or inner rod.
  • one or more of the correspond- ing outer tubes comprises a number of slits and/or grooves extending at least a portion of said length.
  • a number of grooved tubes having elongated slits and/or grooves extending at least a portion of said length are arranged concentrically about the inner tube or inner rod.
  • each of said number of grooved tubes is surrounded by a corresponding solid outer tube.
  • the number of pluralities of cladding tubes and/or rods is at least two, with the second plurality being surrounded by a second outer tube.
  • said second plurality of cladding tubes and/or rods is arranged around the first outer tube.
  • the number of pluralities of cladding tubes and/or rods is at least three, with the third plurality being surrounded by a third outer tube.
  • said third plurality of clad- ding tubes and/or rods is arranged around the second outer tube.
  • the number of pluralities of cladding tubes and/or rods is at least four, with the fourth plurality being surrounded by a fourth outer tube.
  • said fourth plurality of cladding tubes and/or rods is arranged around the third outer tube.
  • said number of pluralities of cladding tubes and/or rods is at least five, six or seven.
  • one or more plurality of cladding rods comprise one or more doped or un-doped silica rods.
  • one or more plurality of cladding tubes comprise one or more doped or un-doped silica tubes.
  • the first plurality of clad- ding tubes and/or rods is arranged in a single layer only around said inner tube or, rod.
  • the first plurality of cladding tubes and/or rods is arranged in two layers around said inner tube or rod.
  • the first plurality of cladding tubes and/or rods is arranged in three, four, five or six layers around said .inner tube or rod.
  • the second plurality of cladding tubes and/or rods is arranged in a single layer only being surrounded by said second outer tube.
  • the second plurality of cladding tubes and/or rods is arranged in two layers only being surrounded by said second outer tube.
  • the second plurality of clad- ding tubes and/or rods is arranged in three, four, five or six layers only being surrounded by said second outer tube. In a preferred embodiment, . one or more plurality of cladding tubes and/or rods is arranged in a single layer only being surrounded by the corresponding outer tube.
  • one or more plurality of cladding tubes and/or rods is arranged in two layers only being surrounded by the corresponding outer tube.
  • the present invention provides a microstructured fibre made from a preform or parts thereof according to the invention.
  • Fig. la shows a sol-gel vessel 13 similar to that described in prior art. However, the elements here 11 become elements in the final fibre and hence will not be removed from the sol-gel.
  • Fig. lb shows an example showing the principle in a preform.
  • a doped rod 14 and a sealed tube 15 is embedded in the sintered sol-gel body 16.
  • Fig. lc shows the main steps in a sol-gel process.
  • the solution containing Si is mixed with water in a container together with additives.
  • the solution is loaded into the vessel. After formation of the wet gel body, it is gently removed from the vessel to dry.
  • the porous gel body is treated with gasses in order to remove remaining substances such as water and additives. Finally, the preform is sintered to dense glass.
  • Fig. 2a shows the principle of a multiple piece sol-gel preform 25.
  • One central element 21 and the first outer element are shown 22.
  • Several of such outer elements are of course possible.
  • four of the . grooves 23 form holes in the final preform and the extra feature (shown as a groove in the outer element 27 and a moulding 28 on the inner element serves to ensure proper angular alignment) .
  • Fig. 2b shows an alternative version where the outer element is split. Additional features for ensuring proper alignment can also be made here (not shown) .
  • Fig. 3 shows an assembly of rod and/or capillary tube elements 31 are placed in a couple of discs 32 (prefer- ably of quartz) for ensuring their positions. This again is placed in a deposition tube 33. Process gasses are let through the tube as a standard MCVD or PCVD process, and the elements are embedded in the final preform.
  • Fig. 4 an assembly of rod and/or capillary tube elements 41 are placed in a couple of discs 42 (preferably of quartz) for ensuring their positions.
  • the discs are placed on a deposition rod or tube 43 and the deposition process can be OVD.
  • the element array is embedded in the final preform.
  • Fig. 5 shows a C0 2 laser "drilling" of a sol-gel body 52 before the sintering step.
  • the holes can be placed arbitrarily.
  • the method has the advantage of. being highly clean.
  • Fig. 6 shows a vessel for casting a sol-gel body 63.
  • a top plate 62 and a bottom plate 64 that hold high temperature electrical conducting wires 61.
  • a quartz rod/cylinder 71 (could alternatively be a tube) is depicted with holes that have been drilled mechanically. By letting process gas 73 in through the holes and placing a plasma coil 74 around the cylinder plasma, CVD can be carried out.
  • the holes can have individual dopants in the deposited glass.
  • Fig. 8 is a schematic illustration of a prior art preform made by hexagonal stacking of capillary tubes and doped rods.
  • Fig. 9 is a picture of a cane obtained from pulling the preform illustrated in Fig. 8.
  • Fig. 10 is a schematic illustration of a preform manufactured according to an embodiment of the present invention, in which holes have been drilled in a quartz cylinder with doped rods arranged in part of the holes.
  • Fig. 11 is a schematic illustration of a preform according to another preferred embodiment of the present invention, in which glass discs or rods with holes are stacked in a sleeving tube so as to align the holes of the discs or rods.
  • Fig. 12a illustrates the stacking technique employed for the first realization of photonic crystal fibres.
  • Fig. 12b illustrates the stacking technique for commercially available photonic crystal fibres.
  • Fig. 12c illustrates the difficulty of stacking in a tube 122 when a larger element 121 is inserted between the elements 120.
  • Fig. 13a is a schematic illustration of one of the proposed schemes for concentric stacking.
  • Fig. 13b is an- other schematic illustration of one of the proposed schemes for concentric stacking.
  • Fig. 14a is a circular preform made of concentric embedded tubes with grooves 302 in which elements 300 are in- serted.
  • Fig. 14b is a circular preform made of concentric embedded tubes with slits 312 in which elements 310 are inserted.
  • Fig. 15a is a schematic illustration of a preform based on the concentric stacking scheme illustrated in Fig. 13 or Fig. 13b.
  • Figs. 15b and 15c are schematic illustration of fibres obtained from pulling the preform illustrated in Fig. 15a.
  • Fig. 16a and 16b show circular structures consisting of an inner tube 501 surrounded coaxially by an outer tube 502, the gap between the tubes 503 being filled with unsealed 504 and sealed elements 505, respectively.
  • Fig. 17a shows a preform 600 consisting of a concentric stack of circular holey elements 601,602, and 603.
  • Fig. 17b shows different rods, tubes, or slitted-tubes (doped or undoped) ' or Combinations' of these for filling of a gap between concentric tubes. As it can be seen a great number of combinations are possible.
  • Fig. 18a and 18b are pictures of a structure at cane (5 mm diameter) and fibres (200 ⁇ m diameter) dimensions, respectively, obtained by pulling a preform assembled as illustrated in Fig. 16b with a solid rod instead of the tube 501.
  • Fig. 19a and 19b show examples of fibres which can be obtained from concentric designs.
  • Fig. 20a and 20b shows two preform assemblies for canes.
  • Fig. 20c shows a concentric stack of the two canes obtained from pulling the elements of Fig. 20a and 20b.
  • Fig. 20d is an example of fibres obtained from pulling the stack shown in Fig. 20c.
  • Fig. 21 is a schematic illustration of one of the proposed schemes for concentric stacking based on ade- quate choice of the radii of the elements for each concentric layer.
  • Fig. 22b is a schematic illustration of a fibres obtained from pulling the preform illustrated in Fig. 22a.
  • Fig. 22c is similar to Fig. 22a but the capillaries 1100 are replaced by the rods 1120.
  • Fig. 22d is a schematic illustration of a fibre obtained from pulling the preform illustrated in Fig. 22c.
  • Figs. 23b, 23c, 23d, and 23e are examples of fibre structures that can be obtained from the basic structure shown in Fig. 23a using suitable choice of tube or rod for the various elements.
  • Fig. 24a shows a fibre based on the concentric stacking principle illustrated in Fig. 21. The core radius and normalized hole size are indicated.
  • Fig. 24b shows the fibre structure from Fig. 24a with an up-doped rod added to the core region.
  • Fig. 24c is a schematic illustration of a fibre with multiple doped regions.
  • An up-doped rod has been added to the core, as in Fig. 24b, and an up-doped ring has been added, corresponding to stacking the second ring of holes from up-doped capillary tubes.
  • Figs. 25a, 25b, and 25c show cut-off properties for fibre structure shown in Fig. 24a for hole sizes 20%, 30%, 60%, respectively.
  • Fig. 26a shows cut-off and dispersion properties for fibre structure shown in Fig. 24b (doped core) for a hole size of 20%.
  • Fig. 26b shows cut-off properties for fibre structure shown in Fig. 24b (doped core) for a hole size of 30%.
  • the first aspect of the present invention relates to a microstructured optical fibre preform produced by a sol- gel method.
  • elements such as undoped and/or doped glass rods (e.g. silica based glass) and/or capillary tubes (for example sealed and/or unsealed at one or more ends) may be held in a fixture according to the design of interest (see schematic illustration in fig. 1) .
  • Fig. la shows sol-gel vessel 13 similar to that described in prior art. How- ever, the elements 11 here become elements in the final fibre and hence will not be removed from the gel.
  • the sol may be added as described in prior art (EP1172339) .
  • the fixture 12 may be removed from the elements when the gel body is removed from the vessel 13.
  • the difference compared to prior art is that the elements remain in the gel during drying, purification and sintering, see Fig. lb, where a doped rod 14 and a sealed tube 15 are embedded in the sintered sol-gel body 16.
  • a silicon alkoxide is mixed with water.
  • Silica particles generally produced by flame hydrolysis and other additives may also be mixed in the solution, see US 5,240,488 for more details.
  • the solution is then poured in a mould containing the added elements.
  • the solution undergoes gelation, resulting in a porous body with embedded elements. Drying and purification of the gel body is carried out in furnaces, followed by the sinter- ing to the preform, a solid body with embedded elements.
  • the hereby formed preform may be drawn to fibre - as possible by a person skilled in the art of drawing microstructured fibres - or the preform may be stretched prior to fibre drawing.
  • the preform may also be sleeved prior to any of the two afore-mentioned steps or during these.
  • Capillary tubes might be unsealed prior to draw and etched or flushed with dry gasses that will avoid hydrogen to enter. The unsealed capillary tubes might be pressure controlled during draw if necessary.
  • Some of the advantages of the present invention as compared to the prior art preform fabrication using sol-gel include • The gel will not suffer from the load that is inevitably applied to it in attempts of removing elements. In prior art this load is claimed to have a tendency to form cracks, • No contaminating lubricants are added as in the prior art for facilitating removal of elements.
  • Softer glasses may for example be added in capillary tubes after the sintering step if desired.
  • the present invention relates to casting a microstructured optical fibre preform in a number of elements.
  • a microstructured optical fibre preform can be cast in a number of elements (see figs. 2a and 2b) .
  • Fig. 2a depicts the principle of a multiple piece sol-gel preform.
  • One central element 21 and the first outer element 22 are shown.
  • Several of such outer elements are of course possible.
  • four of the grooves 23 form holes 24 in the final preform 25 and the extra feature 26 (shown as a groove 27 in the outer element 22 and a moulding 28 on the inner element 21 serves to ensure proper angular alignment) .
  • Fig. 2b shows an alternative version, where the outer element is split 2100, 2110.
  • Features for ensuring proper alignment can also be made here (not shown) .
  • the casting of a preform according to the invention will result in a larger surface to volume ratio easing the drying step and reducing the risk of crack formation.
  • only grooves or grooves in combination with holes are cast. This ensures access to the inner of what will be holes' in the assembled preform allowing polishing, deposition of film on the groove surface etc.
  • the preform may be assembled in a sleeving tube and fused by drawing it to a cane on a drawing tower (or directly to fibre) . Subsequently it may be drawn to fibre while controlling the pressure in the holes. Alternatively, prior to drawing, the cane may act as an inner element for a larger preform though still allowing pressure control of the holes.
  • the preform may for example consist of an inner element cast separately surrounded by a number of ring-like elements.
  • a guiding feature may be made in order to ensure proper alignment of the elements in the assembly.
  • the elements can also be made with outer elements being split as schematically shown in Fig. 2b.
  • the present invention relates to microstructured optical fibre preform fabrication by in- build rods and tubes in chemical vapour deposition (CVD) processes.
  • CVD chemical vapour deposition
  • the present inventors have realized an improved scheme that involves doped or undoped silica rods or capillary tubes. These elements may be held in a fixture in order to preserve their positions during processing.
  • the first variant is combined with MCVD or plasma CVD (see schematic illustration in Fig. 3) .
  • Fig. 3 shows an assembly of rod and/or capillary tube elements 31 placed in a couple of discs (preferably of quartz) 32 for ensuring their positions. This again is placed in an MCVD tube 33.
  • MCVD process gasses 34 such as SiCl 4 , GeCl , P0C1 3 , BBr 3 , or SiF 4 are let through the tube as a standard MCVD pro- cess and the elements are embedded in the final preform.
  • Two discs 32 preferable from silica have been, prepared with holes for placing the elements 31 and one or more holes 35 for allowing a flow of process gas 34. through the arrangement.
  • the arrangement may be placed in an MCVD or PCVD deposition tube 33 and the discs are kept in place by heating the tube till it fuses to the discs.
  • the MCVD or PCVD process is carried out in a usual manner as known to a person skilled in the art, thereby causing an incorporation of the arrangement in the deposited glass.
  • the same scheme may be carried out using the VAD or the OVD process by placing the elements in a similar structure around the deposition rod thereby burying the arrangement in the soot during the deposition process (see schematic illustration for the OVD process in Fig. 4) .
  • the soot may be sintered at regular interval during the process to avoid the formation of bubbles.
  • the elements may be placed on the surface of the deposition rod or tube in order to ensure an accurate radial position in the final preform. This process may be repeated to embed several layers of elements.
  • the present invention relates to deposition of doped and/or undoped glass materials around a rod or tube until the rod or tube (these being referred to as elongated elements) are at least partly buried by the deposited material, such that a preform body comprising a plurality of elongate elements results.
  • Fig. 4 shows an assembly of rod and/or capillary tube elements 41 placed in a couple of discs (preferably of quartz) 42 for ensuring their positions.
  • the discs are placed on a deposition rod or tube 43 and the deposition process can be either VAD or OVD.
  • the element array is embedded in the final preform.
  • a heating source 44 is indicated.
  • the present invention relates to form- ing holes in a porous glass body.
  • VAD OVD
  • sol-gel Most preform fabrication processes (VAD, OVD, sol-gel) involve as an intermediate step, the formation of a porous body, which is later sintered to a solid body forming the finished preform.
  • the porous body is more amenable to processing than the solid.
  • the present inventors have found methods to form structures in the porous body that remain in the final solidified preform.
  • the holes - or voids - may be made in a number of ways, such as for example by traditional drilling (rotary or ultra sonic drilling) , or for avoiding contamination the holes may be "drilled" by a C0 2 laser (see schematic illustration in Fig. 5) .
  • Fig. 5 illustrates the C0 laser "drilling" 51 of a sol- gel body 52 before the sintering step.
  • the holes 53 can be placed arbitrarily. This method provides means for a clean process that will allow a wide range of preform designs, and consequently fibre designs, to be produced. After solidifying the processed gel, the holes may remain as they are, or doped rods and/or tubes may be inserted according to the fibre design.
  • the present invention relates to solidification of a porous body around electrically re- sistive elements.
  • Fig. 6 illustrates an alternative method to drilling to form holes in a porous body.
  • the holes are created by electrically resistive filaments (or rods) placed in the porous body.
  • the array of filaments forming the desired structure is maintained in the similar manner as the elements 11 or 41 in figure 1 or 4.
  • the porous body containing the filaments is locally solidified in vicinity of the filaments when current is flown through so as to reach a temperature in the range of around 1300 to 2000 °C (for instance tungsten, tantalum, rhenium are suitable for this temperature range) . Other temperature ranges may apply depending on the preform material.
  • the solidification creates a gap around the filaments. This gap enables removal of the filaments, leaving an array of holes in the porous body.
  • the porous body is solidified while the holy structure is retained.
  • Fig. 6 illustrates this method when a sol-gel process is used to form the porous body.
  • a vessel for casting sol- gel body is shown 63. There is a bottom 64 and a top fixture plate 62 in the vessel. High temperature electrical conducting wires 61 are held between the two plates. The solution of silica alkoxide, water and relevant additives is poured into the vessel. The sol is brought to gel. The wires are detached from the top and bottom plates and the gel body 65 is gently removed from the vessel. The body undergoes the usual drying and purification steps as is described in reference to Fig. 1. Due to the reactive gasses involved in the purification step, the wires can be coated prior to the placement in the vessel. One type of coating is silica.
  • the wires After drying and purification of the porous gel body, the wires are heated up by letting through a strong current. This is to take place in an oxygen free atmosphere.
  • the silica will solidify around the wires and hence causing a gap around the wire. If a relative displacement, e.g. circular, between the gel body and the wire is made at a slow speed the holes can be enlarged.
  • the filaments need not be straight. For instance, spiral-shaped filaments can be placed in the gel body. When translated, it will act as a filament having the same diameter as the spiral. After the holes have been formed and the filaments have cooled off, the filaments can be removed from the gel body. The gel body is then solidified to a dense silica glass preform containing an array of holes.
  • the present invention relates to microstructured optical fibre preform fabrication from deposition in machined bulk glass.
  • the seventh aspect of the invention is a scheme based on drilling holes in a quartz tube or rod by mechanical drilling (rotary or ultra sonic drilling) , where plasma deposition is carried out in the holes.
  • the deposition may be carried out in the holes individually such that each hole may have a different content than its neigh- bour, or it may be carried out on a number of holes simultaneously (see schematic illustration in Fig. 7) .
  • Fig. 7 shows a quartz cylinder (could also be a tube) 71 with holes 72 that have been drilled mechanically. By letting in process gas 73 through the holes and placing a plasma coil 74 around the cylinder, plasma CVD can be carried out.
  • the holes 72 can have individual dopants in the deposited glass.
  • the here-disclosed method is versatile, thus allowing a great degree of freedom for designing fibres.
  • the plasma deposition technique itself is known to be robust and well proven, and very low impurity levels have been reached by this method.
  • MCVD modified chemical vapour deposition
  • the heat is applied from the inside, where the deposition process occurs. Therefore the applied heat is modest, hence reducing the risk of deforming the preform structure.
  • the present invention relates to the fabrication of a microstructured optical fibre preform from machined bulk glass.
  • the present invention covers a scheme that is based on drilling holes in a glass tube or rod, e.g. silica or doped silica, by mechanical drilling (for example rotary or ultra sonic drilling) .
  • the dopants may for example be Ge, Sn, B, N, P F or a rare earth.
  • the length of element is typically from 20 mm to 500 mm and the diameter is typically in the range of 5 mm to 150 mm.
  • fabrication of microstructured optical fibre preforms by drilling holes in bulk silica (quartz) glass is disclosed. The focus is on ways of drilling holes in silica.
  • the present inventors have realized that it is an advantage to add various glass rods or tubes in one or more of the holes.
  • Fig. 8 shows a schematic il- lustration of a prior art preform made by an established preform production method using hexagonal stacking of capillary tubes and doped rods. As opposed to the established stacking technique (see for instance Russell et al. Optical Fiber Communication Conference, tutorial Session, TuL, 2001) dopants and holes can be placed at any position in the fibre using the here-disclosed method.
  • this process is more amenable to automation (for instance by use of a CNC machine) .
  • impurities may be introduced.
  • the processed glass may be washed, preferably by using an acid (such as for example a 5% HF, 45% H2S04, 50% deionised water solution) .
  • the drilling may leave a scratched surface that is not wanted due to potential scattering loss in the fibre. Therefore, polishing of the holes before draw may be needed for low loss applications.
  • An alternative to polishing is to insert a sealed capillary tube in a drilled hole, where a hole is wanted in the preform. After drawing to a cane, the capillary tube may have become a hole in the preform pro- viding a surface quality superior to that of a drilled hole.
  • the here-disclosed method is applicable for a great variety of designs.
  • a potential limitation is related to how dense holes may be placed.
  • the method may be pre- ferred for designs with a limited number of holes/voids.
  • One of the advantages of this scheme is when the glasses involved in a preform differ in softness. When stacking capillary tubes in the traditional manner, misplacement during heat treatment (for example during drawing) easily occurs in regions of capillary tubes with mixed softness (see doped regions 93 in Fig. 9) .
  • Fig. 9 is a microscope picture of a cane 91 made from the prior art preform schematically illustrated in Fig. 8. It consists of glasses with a difference in softness. Outside the ring with 6 holes 92, 6 softer rods 93 are placed, but it is evident from comparing Fig. 8 and Fig. 9 that they have shifted from their original posi- tion in the preform. Furthermore, the doped regions 93 have been deformed by the collapse of the interstitial gaps of the preform.
  • Fig. 10 shows a design of a drilled preform according to the present invention.
  • 13 holes are drilled in a silica rod 301.
  • the central hole and the six outer holes are filled with the doped rods 3000 and 3003, respectively.
  • the doped rods will keep their positions in the drilled bulk silica 3001.
  • the fabrication of preforms according to the present invention does not only apply for silica based glasses, but can be applied to other glasses, which e.g. are potential better candidates for fibres with high non- linearities. Tubes of compound glass with smooth surfaces and low geometrical tolerances are not readily available. It is therefore an advantage over the stacking technique that the preforms of the present invention do not require tubes but simply bulk samples, preferably cylindrical in shape.
  • the present invention also relates to a ninth aspect of fabrication of a preform for a microstructured optical fibre from machined bulk glass. This is illustrated in Fig. 11, where, due to. limitation on uniformity and directionality on drilling long holes in glass (e.g.
  • silica it is suggested to drill in shorter glass elements 4001, discs or rods, which can be stacked later on in a sleeving tube 4005.
  • the size of the glass elements is typically in the range of 25-500mm for the length and 5- 150mm for the diameter.
  • the holes 4002 are typically from 0.5-lOmm in diameter.
  • Rods or tubes 4006 can be inserted in one or more holes in order to ensure alignment of the elements 4001 so that the holes run continuously through what is to become the preform, see Fig. 11.
  • the alignment rods and tubes 4006 can be left in the holes as elements in the final fibre.
  • capillary tubes for lining holes 4002 may reduce the potential problems of rough and scattering surfaces of the drilled holes, and ' hence alleviate the need for polishing the drilled holes.
  • the sleeving tube 4005 can be closed in the lower end and a dummy glass rod can be the first element for drop-off at drawing.
  • a dummy rod can be placed as the last element also, ensuring a constant heating of the elements during the pull.
  • the alignment rods or tubes 4006 can be removed before draw.
  • the preform can be drawn to a cane, which can be drawn to a fibre later on. In the transition from one element to another there may be an area of imperfection in the caned preform. When this is the case, the length of preform cane between two such areas may give the maximum length of a flawless preform cane. Alternatively, the preform cane is drawn to fibre and the defect fibre is simply cut out. Ensuring a smooth and even surface at the ends of the elements 4001 (e.g. by grinding and polishing) may minimize the effects of going from one disc to another during draw.
  • the top dummy ele- ment may have taken the shape of a draw cone, and in case of a long sleeving tube the remains of the dummy element can act as the drop-off element for yet another such preform.
  • the preforms according to embodiments of the ninth aspect are not only limited to silica based glasses for the same arguments that was discussed above in the eighth aspect of the invention.
  • Fig. 12. a illustrates the stacking technique employed for the first realization of photonic crystal fibres, a process described by Birks et al. in Photonic Band Gap Materials, Kluwer, 1996.
  • the elements 100 had a hexagonal cross-section and a well-defined size.
  • the cladding was formed by elements having a central air hole (capillaries) while the core was formed by an element with no air hole (a rod) .
  • the hexagonal elements 100 were bundled by tantalum wires (not shown here) , forming a hexagonal preform for fiber pulling.
  • Fig. 12b illustrates a latter version of hexagonal stacking. It was realized that the capillaries 110 did not need to be hexagonal. The capillary holes 111 could be maintained while the interstitial holes 113 were I collapsed during fibre pulling (see for example US5802236 for fabrication of such fibre - further information on fabrication of microstructured fibres may for example by found in EP1172339 or WO0060388) . That alleviated the need for milling the capillaries' preform to a hexagonal tube, thus reducing processing time and surface contamination. The hexagonal stack was placed in a sleeving tube 112 along with supporting elements 114.
  • Fig. 12c illustrates the case where a larger element 121 is inserted between smaller elements 120 in a sleeving tube 122. Close pack- ing is not achieved.
  • the interstitial holes 123 between elements 120 vary in size.
  • the number of elements per layer is determined by the radial position of the layers: one element is placed in the centre of the preform, six elements are placed in the first layer surrounding the centre element, twelve in the second layer, and 6*n in the n'th layer.
  • Fig. 13b further shows examples of the flexibility of a preferred embodiment of a method for producing a preform.
  • fillings 1004 of the gap between tubes 1001, 1002 are used to form hybrid elements. These elements may be stretched, drawn or otherwise processed to form hybrid elements or parts for making preforms for optical fibres. As it can be seen a great number of combinations are possible using tubes, rods, other types of elements and/or combinations of these.
  • the elements to be stacked are substantially circular in shape and will typically be a few hundreds of microns to a few millimetres in diameter.
  • the central element 201 could even be a few tens of millimetres in diameter.
  • the elements would in most cases be pulled on a tower so their length can easily be varied from a few centimetres to a few meters. Typically they would be longer than ten centimetres and shorter than two meters to yield sufficient yet manageable stack preform length for subsequent pulling to fibre.
  • the elements are in most cases made of glass, but other materials such as polymers, metal or combinations of these materials may be used.
  • the glass generally would contain more than 70 mol% of silica but compound glasses (here defined as containing less than 70 mol% silica) can also serve as material for microstructured fibres, as demonstrated by Hewak et al. in Optical Fiber Communication Conference 2001 (OFC'01), TuC4. Compound glass microstructured fibres are particularly interesting for their nonlinear properties. But when low losses and robustness are sought, silica, and in particular synthetic silica, is the material of choice.
  • An element can also be doped entirely or in part in order to change its refractive index or/and its viscosity. Typically the doped regions may contain a combination of Ge, P, Al,' N, Sn, F, B.
  • the dopants may also be rare-earths such as for example Yb, Er, Nd.
  • An element can also be hollow. It can be a capillary (having a central air hole) or a microstructure.
  • the tubes 202 can either be made of pure silica, preferably synthetic silica, or of doped silica.
  • the dopants may be present throughout the whole tube volume or in one or many annular regions. They can for instance be deposited as layers in a Chemical Vapor Deposition process. Typically the doped regions may contain a combination of Ge, P, Al, N, Sn, F, B, and rare earths.
  • rod and/or capillary tubes 200 can be chosen regardless from the choices of element sizes in the other rings 202.
  • the elements in the displayed design can also be rods 13007 or doped tubes 13008 as indicated in Fig. 13b.
  • Each ring can be connected to a pressure chamber, allowing to control structural dimensions and parameters such as air/void fraction in each ring.
  • Figs. 14a and 14b show a modified version of concentric preform construction scheme outlined above.
  • the elements 300 are placed in grooves 301 formed in a tube 302 or a rod (for the central layer) , which is tightly sleeved by a larger tube 303.
  • the grooves can be formed at any position on the tube, and many layers can be stacked.
  • Fig. 14.b shows ⁇ l similar method were the elements 310 are placed in slits 311 formed through the whole thickness of a tube. 312.
  • a particularly effective way of forming the grooves and the slits is to focus the light of the C0 2 laser on the surface of the tube to be processed, as described in PCT/DK02/00154, included here by . reference.
  • the focusing, the optical power, and the movement of the laser beam give excellent control on the geometry of the grooves 301 or slits 311.
  • Fig. 15a is a schematic illustration of a. preform based on the concentric stacking scheme illustrated in Fig. 13. An alternate pattern of capillaries 401 and rods 402 is used.
  • Figs. 15b and 15c are schematic illustrations of fibres obtained from pulling the preform illustrated in Fig. 15a.
  • the size of one or more of the holes may gene- rally be controlled during the pull by pressurizing, see for example US4551162 that also describe methods of controlling interstitial holes.
  • a particularly interesting feature of the concentric stacking is that the azi- muthal position of the elements can be chosen freely.
  • the fibres of Fig 15.b&c show an alternate pattern of five high index and five low index sectors. This structure is a microstructured fibre that may allow single-mode operation over a wide wavelength range with a large core diameter.
  • Figs. 16. a and 16.b show a preform for such a cane. It consists of an inner tube 501 surrounded coaxially by an outer tube 502, where the gap 503 between the tubes is filled with elements 504 or 505. Alternatively the elements can be inserted in grooved or slits as illustrated in fig. 14. The same elements as described in relation to fig. 13 can be used. As an example the elements . are unsealed capillaries 504 in fig. 16a and sealed capillaries 505 in fig. 16b.
  • Fig. 17b shows other preferred embodiments of the present invention, to further illustrate the concepts of Fig. 16a and Fig. 16b.
  • Fig. 17b shows schematically hybrid preform elements in the form of ring-shaped elements comprising at an outer element 10001 (typically a tube) , a number of filling elements 10004 (such as tubes, rods, and/or other types of elements) , and an inner element 10002 (typically a tube or a rod) .
  • the fillings elements are preferably made of doped and/or undoped silica, such as silica doped with one or more rare earths. As it can be seen a great number of combinations are possible.
  • a hybrid preform element (also referred to as a basic element) may be drawn to cane of desired di- mensions.
  • the diameter of an element may be control.
  • Pressurizing the capillaries or the slits or grooves from the inside when pulling or stretching, the dimensions of a basic element may also be controlled.
  • Ap- plying vacuum between the capillaries when pulling may also serve, for example, to collapse interstitial holes or control their size - as described for Fig. 16a and Fig. 16b.
  • a set of canes drawn from basic elements may be produced in a manner so that they fit into each other to form a preform (see schematic illustration in Fig. 17a) .
  • 17a shows a preform 600 consisting of a central element 601 surrounded by circular rings 602, 603.
  • the thickness in a ring is independent of the choice made for the neighbour rings.
  • Each ring may be connected to a pressure chamber, allowing control of the air fraction (or another gas than air) in each ring in a simple manner.
  • the present inventors have realized that the scheme using building blocks (such as the canes from hybrid preform elements) as described above, is advantageous for large- scale production, since the preform may be easy to assemble from the caned elements. Another advantage is that the preforms may be round. From the standard hexagonal stacking of capillary tubes the present inventors have observed that problems may arises at the outer corner elements of the hexagonal stack where the stack touches the inside of a sleeving tube. Using circular designs, as covered by the present invention, the problems at corners are eliminated and more stable preforms may be fabricated. Hence, the here-disclosed scheme provides improvements by ensuring a circular structure.
  • Fig. 18a and 18b show an example of circular structure on cane level and on fibre level, respectively.
  • Fig. 18a shows the cane 701 where some imperfections 702 of the holey features 703 are present.
  • Oh Fig. 18b a fibre 705 drawn from the cane of fig. 18a is shown to have a more uniform structure of the microelements 706. It seems that the fibre structure is locked to a stable condition with strong angular uniformity and a locked radial position. For standard hexa- gonal stacking it is usually the case that imperfections on cane level may get worse on the fibre level.
  • Figs. 19a and 19b show two examples of fibres which can be realized from concentric designs.
  • the fibre illustrat- ed in fig. 19a has the basic structure of a dispersion compensating fibre, relying on the optical coupling between the two guiding regions 800 and 802, separated by a lower index holey region 801.
  • the fibre illustrated in figure 19.b has the basic structure of a photonic band gap fibre. In this fibre light can be guided in the holey centre 810. If the bridges between holes 811 are substantially thinner than the wavelength of the guided light their influence on the optical properties of a holey layer 812 can be neglected.
  • the thickness of each layer can conveniently be adjusted during the pull as explained above in relation to fig. 17a and Fig. 17b.
  • Fig. 20d is an example of fibre obtained from pulling the stack shown in cross-section in Fig. 20c, where two microstructured canes are assembled one into the other, as in the assembly scheme illustrated in Fig. 17a and Fig. 17b.
  • the two canes are obtained from pulling the preforms shown in Figs. 20a and 20b.
  • pressurizing the holes of the capil- laries 901 and 911 more than the air gaps present in the preforms allows for the capillaries' holes to be maintain while the gaps collapse fully, resulting in the two holey canes shown in fig. 20c.
  • the two canes fuse together while pulled to fibre. To facilitate this process a low pressure may be apply in the sleeving gap 922.
  • the fibre shown in fig. 20d presents the structure of a double clad fibre.
  • the advantage of using a holey structure is that very high numerical apertures can be obtained, an attractive feature for coupling the pump power into the first cladding 933, as for example Danish Patent Application PA 200200158 included here by reference.
  • the shape of the inner cladding 933 can be chosen by selectively collapsing some of the holes 921 or by replacing some of the capillaries 911 by rods in the outer cane's preform of Fig. 20b.
  • a non-circular inner cladding 933 is desirable to favour pump absorption in the core 930.
  • the presence of holes 931 in the first cladding 933 can serve to modify the propagation of the signal in the core 930, in particular to favour single mode operation.
  • the stacking scheme illustrated in Fig. 21 consists in concentric layers around the centre element 1001, each with a given number N of elements (N>2) .
  • the size of the elements in each layer is chosen so that each element of a given layer is in contact with its two neighbours within the layer. If we call Ri the radius of the centre element 1001, R the radius of an element 1002 of layer
  • R k The outer radius of the layer k, R k , is given by:
  • the radii of the elements will show some deviation from the target radii calculated.
  • the deviation will typically be +1 to -1% of the target radius but up to +5 to -5% may be tolerated.
  • the elements to be stacked are as described in relation to Fig. 13a and Fig. 13b. Note that the preform fabrication of Figs. 13a ⁇ and 13b, where one or several layers of elements are placed between two interfacial tubes 202, may be best used in combination with the stacking technique described here.
  • Fig. 22b is a schematic illustration of a fibre obtained from pulling the preform illustrated in Fig. 22a.
  • the interstitial gaps 1103 have been collapsed while the holes 1102 have been maintained to obtain the holes 1112 in the fibre.
  • Fig. 22c is a similar to Fig. 22a but the capillaries 1100 are replaced by the rods 1120.
  • Stacking rods may be preferable to stacking capillaries in order to reduce scattering losses in the fibre because cleaning the outer surface of rods is easier than cleaning both the outer and inner surfaces of capillary tubes.
  • Fig. 22d is a schematic illustration of a fibre obtained from pulling the preform illustrated in Fig. 22c. Contrary to Fig. 22b, the interstitials gaps 1123 have been maintained to obtain the holes 1133 in the fibre. This is usually done in two steps. First, the preform is pulled to cane with sufficiently low heating to avoid the collapse of the interstitial gaps 1123, but with suf- ficient heating to fuse each element 1120 to its neigh- bour. In a second step, the resulting cane, usually inserted in a tube, may be sealed or pressurized in an end opposite to the pulling end in order to control the hole size(s) when pulling to fibre.
  • capillary tubes may be introduced in the interstitial gaps 1123 of the preform illustrated in Fig. 22c. (Fig. 20a shows an example of such an insertion of capillaries) and pressurized to maintain the capillaries' holes during the pull while all the gaps, maintained at a lower pressure, collapse.
  • Fig. 23d shows a fibre with an elongated core. This geometry is desirable to obtain a large birefringence.
  • prior art birefringent fibres based on hexagonal stacking and reported by Hansen et al. in IEEE Photonics Technology Letters Vol. 13, No. 6 (June 2001) the core presented a central waist detrimental to efficient light coupling. It is an advantage that of the present stacking technique will not favour the formation of a waist.
  • Figs. 24a and 24b show the actual structures used in the simulations.
  • the core radius is denoted ⁇ , and is defined as the distance from the centre of the fibre to the centres of the holes in the inner-most ring.
  • the holes are characterized by a relative hole size, defined as the ratio between the hole radius 1301 and the distance between two neighbouring holes in a ring 1302. This relative hole size is constant throughout the fibre.
  • the core area is defined as the area of the circle 1303 with a radius equal to the core radius as defined above.
  • Fig. 24b is a schematic of a fibre with multiple up-doped regions: The up-doped element in the core is maintained from Fig. 24b, while an up-doped ring 1307 is added by stacking the second ring of up-doped capillary tubes.
  • Fig. 25a shows calculations of the effective mode area, ⁇ ff , for the 10 lowest order eigenmodes, using the equa- tion (see e.g. G.P. Agrawal: Nonlinear Fiber Optics - Third edition, Academic Press, 2001)
  • H(r) is the magnetic field.
  • the first two eigen- modes corresponds to the two degenerated polarizations of the fundamental mode.
  • the next four corresponds to the second order mode, etc.
  • the fibre simulated is the structure shown in Fig. 24a with 16 holes in each ring, 4 rings and a relative hole size of 20%.
  • the core and fibre areas are indicated on the figure, the latter is defined as the area of the region inside the truncation as shown on Fig. 24a.
  • the effective area of the fundamental mode 1401 is seen to be comparable to or smaller than the core area, which indicates a strong confinement to the core.
  • the fibre In this region ( ⁇ / ⁇ 0.6) the fibre is expected to provide single mode guidance of light with a low sensitivity to bending.
  • the higher order modes are seen to have effective areas that are significantly larger than the core for all wavelengths. This indicates that no higher order modes are supported by this structure. Therefore this fiber exhibits endlessly single mode operation, as it is known from photonic crystal fibers with e.g. a triangular cladding geometry. In the long wavelength region the fundamental mode extends further into the cladding region, but remains guided by the core.
  • Example: A fiber designed to guide light with a wavelength of ⁇ 1.55 ⁇ m.
  • Radius of the holes in the four inner most rings are: 0.302 ⁇ m, 0.426 ⁇ m, 0.600 ⁇ m, and 0.844 ⁇ m.
  • Fig. 25b shows calculations for the same basic design as in Fig. 25a, but with larger holes: (Hole size: 0.30).
  • the fiber is seen to support a second order mode at short wavelengths.
  • Fig. 25c an even larger hole size, 60%, is used.
  • the fibre design remains the same as in Figs. 25a and 25B.
  • the fibre is found to be multi mode for normalized wavelengths shorter than ⁇ / ⁇ l.l supporting both a second 1404 and third 1405 order mode.
  • Figs. 26a and 26b' show calculations . performed on the structure shown in Fig. 24b with, hole sizes of 20% and 30%, respectively.
  • the up-doped region has a parabolic index-profile with a peak value 2% larger than the background material, which is silica in this case.
  • the radius of the doped region is 0.25 ⁇ .
  • Fig. 26a calculations of the dispersion in the fundamental mode has been included.

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Abstract

L'invention concerne un procédé de production d'une préforme pour fibres optiques microstructurées. Dans un mode de réalisation, ce procédé se présente sous la forme d'un procédé sol-gel amélioré. Selon ce procédé, des éléments allongés tels que des barres et/ou des tubes s'étendent à travers un récipient tubulaire et sont maintenus selon une configuration spatiale prédéterminée par rapport audit récipient, ces éléments allongés faisant partie d'un corps de gel final résultant du procédé sol-gel. L'invention se rapporte également à une préforme ou à une partie de préforme servant à produire des fibres optiques microstructurées. Cette préforme comprend des tubes concentriques et une pluralité de tubes et/ou de barres placé(e)s entre les tubes concentriques. L'invention concerne en outre des fibres optiques microstructurées produites au moyen desdites préformes.
PCT/DK2003/000182 2002-03-18 2003-03-18 Preforme, son procede de production et son utilisation pour produire des fibres optiques microstructurees WO2003078338A2 (fr)

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US7526165B2 (en) 2004-03-19 2009-04-28 Crystal Fibre A/S Optical coupler devices, methods of their production and use
US7787729B2 (en) 2005-05-20 2010-08-31 Imra America, Inc. Single mode propagation in fibers and rods with large leakage channels
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