OPTICAL FIBER WITH LOW CHLORINE CONCENTRATION IMPROVEMENTS RELATING TO LOSS AND ITS USE, METHOD OF ITS PRODUCTION AND USE THEREOF

Abstract

An optical fiber having an axial direction and a cross section perpendicular to the axial direction, and a method and preform for producing such an optical fiber. The optical fiber is adapted to guide light at a wavelength ?, and includes a core region, an inner cladding region surrounding said core region, and at least one of a first type of feature including a void and a surrounding first silica material. The core, the inner cladding region and the first type of feature extends along said axial direction over at least a part of the length of the optical fiber. The first silica material has a first chlorine concentration of about 300 ppm or less.

Claims

1-57. (canceled)

58. A preform configured for fabricating an optical fiber adapted to guide light at a wavelength ?, the preform comprising a hollow core part and an inner cladding part arranged to provide a hollow core region and a surrounding inner cladding region, respectively, in a fabricated optical fiber, the inner cladding part comprising one or more precursor elements, wherein the one or more precursor elements comprises at least one of a first type of precursor element comprising a void and a surrounding first silica material, said first silica material having a first chlorine concentration, c.sub.1, of about 300 ppm or less.

59. The preform according to claim 58, wherein said first silica material is selected from the group of natural occurring quartz and thermal oxide glass.

60. The preform according to claim 58, wherein at least one of said one or more precursor elements comprises a silica capillary tube.

61. The preform according to claim 58, wherein the first chlorine concentration, c1, is about 200 ppm or less.

62. The preform according to claim 58, wherein the first chlorine concentration, c1, is about 100 ppm or less.

63. The preform according to claim 58, wherein the first chlorine concentration, c1, is about 10 ppm or less.

64. The preform according to claim 58, wherein the silica glass is substantially free of chlorine.

65. The preform according to claim 58, wherein said inner cladding region comprises a plurality of said first type of precursor element.

66. The preform according to claim 65, wherein said plurality of said first type of precursor element is arranged in a substantially periodic arrangement.

67. The preform according to claim 65, wherein at least part of the plurality of first type of precursor elements are arranged in the inner cladding part.

68. The preform according to claim 66, wherein said substantially periodic arrangement comprises a closed packed hexagonal structure.

69. The preform according to claim 58, wherein said inner cladding part comprises a plurality of said first type of precursor element.

70. The preform according to claim 58, wherein the first silica material is arranged to reduce the content of chlorine or chlorine compounds at or near the surfaces of the void in said first type of precursor element.

71. The preform according to claim 58, wherein the first silica material is arranged in a substantially annular part of the first type of precursor element which annular part has a thickness in the range of about 10 nm to about 5000 nm in the fabricated optical fiber.

72. The preform according to claim 58, wherein the preform comprises an outer cladding part surrounding said inner cladding part, said outer cladding part comprising an outer cladding silica material.

73. The preform according to claim 58, wherein the wavelength ? is in the range from 800 nm to at least 2500 nm.

74. A preform configured for fabricating an optical fiber, the preform comprising a hollow core part and an inner cladding part surrounding said hollow core part, said preform comprising a plurality of tubes arranged in a stack, wherein said tubes comprise a silica material with Chlorine content below 300 ppm.

75. A hollow core optical fiber adapted to guide lights at a wavelength in the range of 800 nm to 2500 nm, the hollow-core optical fiber comprising a hollow core region and an inner cladding region surrounding said hollow core region, wherein the inner cladding region comprises one or more of a first type of feature comprising a void and a surrounding first silica material, said first silica material having a first chlorine concentration, c1, of about 300 ppm or less, wherein said hollow core optical fiber is drawn from a perform according to claim 58.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0061] The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

[0062] FIG. 1 shows end facet degradation in a Hollow-Core PCF (HC-PCF),

[0063] FIG. 2 shows a schematic illustration of a section of a fiber preform with a plurality of first type of precursor elements in a closed-packed hexagonal arrangement,

[0064] FIG. 3 shows a schematic illustration of a preform for the production of a HC-PCF comprising a stack of capillary tubes and an overcladding tube,

[0065] FIG. 4 shows the end facet of an optical fiber according to the present invention,

[0066] FIG. 5 shows the end facet of an optical fiber according to the present invention

[0067] FIG. 6 shows the end facet of an optical fiber according to the present invention imaged over several hours,

[0068] FIG. 7 shows the end facet of an optical fiber according to the present invention and the attenuation of a mode propagating in the core,

[0069] FIG. 8 shows the measurement of HCl in the hollow core of a prior art fiber, and

[0070] FIG. 9 shows a schematic illustration of a section of a fiber preform with a plurality of first type of precursor elements in a closed-packed hexagonal arrangement, together with two different embodiments of the first type of precursor element.

[0071] FIG. 10 shows a schematic illustration of a laser based gyroscope that includes a laser light source and a laser system.

[0072] The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

DETAILED DESCRIPTION

[0073] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

[0074] The invention is defined by the features of the independent claim(s). Preferred embodiments are defined in the dependent claims. Any reference numerals in the claims are intended to be non-limiting for their scope.

[0075] Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims.

[0076] In the following examples to further illustrate preferred embodiments of the present invention(s) are described.

[0077] FIG. 3 shows a schematic illustration of a preform 20 for the production of a HC-PCF comprising a stack of precursor elements 22 each comprising a void 23, and an overcladding tube 25. The core part comprises a precursor element 24 with a void 21. Both the precursor elements of the core and the inner cladding region may be of the first type of precursor element. Additional elements may be included in the stack, such as for example solid rods at interstitial sites in the cladding, a hollow tube (in the center) and filling rods to for example assist mechanically stability of the stacked perform. The figure is schematic and simplified for clarity, and just shows details, which ease the understanding of the invention, while other details are left out.

[0078] The use of silica material with a Chlorine content of about 300 ppm or less drastically reduces the amount of contamination as illustrated using FIG. 4 and FIG. 5.

[0079] In FIG. 4 was taken 4 days after the cleaving of the fiber and only sub-micron sized droplets are forming on the glass part of the end-facet. The end facet contamination is clearly mitigated when using silica materials with a very limited content of Chlorine (<300 ppm) for the precursor elements. This could for example be F100/F110 glass from HERAEUS or crystalline SiO.sub.2 from commercial vendors. On FIG. 5 is shown a microscope image of the end facet of a HC fibers build form capillary tubes drawn from F110 glass taken 4 days after the cleaving of the fiber. The use of this material effectively cancels the building up of end facet contamination over time. As it was also concluded above, it is highly plausible the facet contamination is related to the presence of Cl in the glass material constituting the fiber.

[0080] FIG. 6 shows measurement results for another optical fiber produced using a preferred embodiment of the present invention, where images of the end facet are taken several hours after cleaving the optical fiber. The largest image showing the end facet 100 hours after cleaving. The inventors have shown that PCFs can be produced in silica material with a low Chlorine content. FIG. 7 shows the end facet of an optical fiber according to the present invention and the attenuation of a mode propagating in the core. At 1550 nm the attenuation 14.0+/?0.7 dB/km, while the lowest attenuation of 13.2+/?0.4 dB/km is obtained at 1504 nm. The 10 dB Band gap Width is 173 nm (1654-1481), and the number of scatter points is 1 km.sup.?1.

[0081] The present inventors have realized methods for monitoring or characterizing the amount of gases in hollow core fibers. Preferably, the methods are used for sensor and/or measuring applications. In FIG. 8 is shown a measurement of the HCl content in the hollow core clearly indicating the this Chlorine compound is present in the core region of the fiber and diffusion of Chlorine to the fiber and may be the reason for end fact degradation in optical fibers with a Chlorine content of more than 300 ppm. The absorption peaks measured on the hollow core fibres most probably stems from the first overtone of the HCl rotations and vibrations.

[0082] The present inventors have realized that further improvements to HC fibers may be obtained using for example bake out of preforms and/or improvements to the fiber drawing process. These improvements include flushing preform with gasses, such as Oxygen, Ar, or other types of inert gasses. In particular, flushing preforms before, during or after fibers drawing is advantageous. In particular, it is preferred that flushing with gases having low N.sub.2 level in order to reduce or eliminate Ammonium hydroxide at fiber end facets and/or inside fibers with holes.

[0083] The reaction to generate Ammonium hydroxide may be written as:

NH.sub.3+H.sub.2O.fwdarw.NH.sub.4.sup.+OH.sup.?(water solution)

NH.sub.4.sup.+OH.sup.?+HCl.fwdarw.NH.sub.4Cl+H.sub.2O

[0084] Notice that Ammonium hydroxide has a boiling point of 38-100? C. (reference: Sigma-Aldrich). Typically, all substances are introduced or appearing during production of in HC fibers.

[0085] In some embodiments, the Chlorine level in the silica glass is kept at a low level by avoiding the used of Chlorine cleaning and/or using silica that has not undergone Chlorine cleaning steps at manufacture, Nitrogen during pressure control is avoided (Helium may be used as flush gas on stack to avoid Nitrogen in the stack), Argon gas may be used for pressure control in process steps, such as fiber drawing process step, and the water content in the silica is kept at a low level by controlling gas composition during process steps (sealing and flushing prior to heating steps).

[0086] A gas flow, such as Argon flow, through the cane may be used. Optionally, a bake-out before drawing a fiber from the preform is made.

[0087] In further embodiments, cold traps are used. For example, peltier elements and/or dry ice is preferred to liquid Nitrogen to avoid pressure instabilities in pressure control using either Nitrogen or Argon.

[0088] The method according to the present invention may furthermore comprise the steps of a Bake-out and/or a flushing with a gas selected from the group of Ar, O.sub.2, He, Ne, Kr, or Xe.

[0089] The present invention is not limited to specific PCF designs, but may be utilized in general to produce any kind of optical fiber comprising one or more voids. The various preferred embodiments and improvements may be used independently or in any combination.

[0090] The invention is applicable to a laser system 2, transmission systems, gyroscopes 1, a pulsed laser light source 3, and sensors in general, gas lasers, and lasers and amplifiers in general, pulse compression, dispersion compensation, but it is not limited to such uses.

[0091] Some embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject-matter defined in the following claims. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined in the appended claims