Microstructured hollow core optical fiber using low chlorine concentration

Abstract

The invention relates to an optical fiber having an axial direction and a cross section perpendicular to said axial direction, and a method and preform for producing such an optical fiber. The optical fiber is adapted to guide light at a wavelength λ, and comprises a core region, an inner cladding region surrounding said core region, and at least one of a first type of feature comprising 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. A microstructured optical fiber adapted to guide light at a wavelength λ in the range from 800 nm to 2500 nm, said optical fiber having an axial direction and a cross section perpendicular to said axial direction, said optical fiber comprising: a hollow core region, an inner cladding region surrounding said core region, an end facet at an end of the optical fiber, and at least a portion of the optical fiber is comprised of a first type of feature comprising a void and a surrounding first silica material, said first type of feature extending along at least a part of said axial direction, said first silica material having a first attenuation coefficient, α.sub.1, at λ and a first chlorine concentration, c.sub.1, of about 300 ppm or less, providing that diffusion of chlorine to the end facet and resulting chlorine-induced degradation of the fiber end facet is mitigated.

2. The optical fiber according to claim 1, wherein said core region is formed by the void of one of said first type of feature.

3. The optical fiber according to claim 1, wherein said inner cladding region comprises a plurality of said first type of feature.

4. The optical fiber according to claim 1, wherein said first type of feature further comprises a second silica material with a second attenuation coefficient, α.sub.2, at λ and second chlorine concentration, c.sub.2.

5. The optical fiber according to claim 4, wherein said second attenuation coefficient is smaller than said first attenuation coefficient.

6. The optical fiber according to claim 4, wherein said first silica material is arranged to surround said second silica material.

7. The optical fiber according to claim 4, wherein said second silica material is arranged to surround said first silica material.

8. The optical fiber according to claim 4, wherein said first type of feature further comprises a third silica material arranged to surround said first and second silica materials, said third silica material having a third attenuation coefficient, α.sub.3, at λ and a third chlorine concentration, c.sub.3.

9. The optical fiber according to claim 8, wherein said first and third chlorine concentrations are smaller than said second chlorine concentration.

10. The optical fiber according to claim 4, wherein said first silica material is arranged to provide a diffusion barrier for Chlorine situated in said second silica material, thereby mitigating the diffusion of said Chlorine into said voids of said first type of feature.

11. The optical fiber according to claim 1, wherein said first silica material is arranged in a substantially annular region with a thickness in the range of about 10 nm to about 5000 nm.

12. The optical fiber according to claim 1, further comprising a second type of feature comprising a void and a silica material surrounding this void, said first and second type of feature being different in at least the silica material surrounding their respective voids.

13. The optical fiber according to claim 12, wherein said inner cladding region comprises a plurality of said first type of feature and a plurality of said second type of feature, wherein the first and second type of features are arranged so that the part of the inner cladding region closest to the hollow core region comprises a majority of said second type of feature.

14. The optical fiber according to claim 12, wherein said difference in the silica material relates to the composition of the silica material that is in direct contact with the voids of the first and second type of features.

15. The optical fiber according to claim 12, wherein said core region comprises at least one of said second type of feature.

16. The optical fiber according to claim 12, wherein said inner cladding region comprises a plurality of said first type of feature and a plurality of said second type of feature, wherein the first and second type of features are arranged so that the part of the inner cladding region closest to the core region comprises a majority of said first type of feature.

17. The optical fiber according to claim 1, wherein the voids in said first type of feature have a surface and said first silica material is arranged to reduce the content of Chlorine or Chlorine compounds at or near the surfaces of the voids in said first type of feature.

18. The optical fiber according to claim 1, wherein said fiber is adapted to guide light at a wavelength λ in the range from 1481 nm to 1654 nm.

19. The optical fiber according to claim 1, wherein the silica glass is substantially free of chlorine.

20. The optical fiber according to claim 1, wherein said inner cladding comprises a plurality of the first type of feature, said first silica material of the plurality of first type of feature mitigates diffusion of chlorine into the voids of the plurality of the first type of feature, thus reducing migration of the chlorine to the end facet.

21. A hollow core microstructured optical fiber adapted to guide light at a wavelength λ in the range from 800 nm to 2500 nm, said optical fiber comprising a plurality of voids extending in the longitudinal direction of the fiber, wherein said optical fiber is improved by having at least one of said voids surrounded by a first silica material having a first chlorine concentration, c.sub.1, of about 300 ppm or less.

22. The optical fiber according to claim 21, further comprising an end facet at an end of the optical fiber.

23. The optical fiber according to claim 22, wherein for at least a part of said voids, said first silica material is arranged between the void and a second silica material having a second chlorine concentration, c.sub.2, which is larger than said first chlorine concentration, so that said first silica material forms a Chlorine diffusion barrier arranged to reduce diffusion of Chlorine from said second silica material into the void, whereby diffusion of Chlorine to the fiber end facet and accordingly Chlorine induced end facet degradation is mitigated.

24. The optical fiber according to claim 21, wherein said fiber is adapted to guide light at a wavelength λ in the range from 1481 nm to 1654 nm.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

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

(3) 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,

(4) 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,

(5) FIG. 4 shows the end facet of an optical fiber according to the present invention,

(6) FIG. 5 shows the end facet of an optical fiber according to the present invention

(7) FIG. 6 shows the end facet of an optical fiber according to the present invention imaged over several hours,

(8) 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,

(9) FIG. 8 shows the measurement of HCl in the hollow core of a prior art fiber, and

(10) 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.

(11) The figures are schematic and may be simplified for clarity. Throughout, the same reference numerals are used for identical or corresponding parts.

(12) 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.

(13) 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.

(14) 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.

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

(16) 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.

(17) 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.

(18) 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 p.p.m.) 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.

(19) 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

(20) 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.

(21) The present inventors have realized methods for monitoring or characterizing the amount of gasses 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.

(22) 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 gasses having low N.sub.2 level in order to reduce or eliminate Ammoniumhydroxide at fiber end facets and/or inside fibers with holes.

(23) The reaction to generate Ammoniumhydroxide 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

(24) Notice that Ammoniumhydroxide has a boiling point of 38-100° C. (reference: Sigma-Aldrich).

(25) Typically, all substances are introduced or appearing during production of in HC fibers.

(26) 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).

(27) 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.

(28) 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.

(29) 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.

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

(31) The invention is applicable to transmission systems, gyroscopes, and sensors in general, gas lasers, and lasers and amplifiers in general, pulse compression, dispersion compensation, but it is not limited to such uses.

(32) 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