FIELD

The present invention relates to optical fibers having high chlorine dopant levels in the core of the fiber.

TECHNICAL BACKGROUND

There is a continuing need for lower attenuation optical fibers. Low attenuation is one of the most critical properties in optical fibers. Most optical fibers use germania (GeO2) doped silica for the core region and pure silica for the overclad region. However, the Raleigh scattering due to germania doping limits low fiber attenuation to about 0.18 dB/km for practical fibers due to Rayleigh scattering related to dopant concentration fluctuation. To reduce dopant concentration fluctuation, relatively high silica core fibers have been made which utilize a fluorine doped cladding. These fibers sometimes include small amounts of chlorine. However, these high silica content core optical fibers have high viscosity that increases the Rayleigh scattering due to high fictive temperature in the core. In addition, the fluorine (F) doped cladding has a much lower viscosity, which results in high draw induced stress in the core region. The high stress in the core region reduces the glass relaxation, which increases the Rayleigh scattering loss. In addition, the stress effect reduces the core refractive index through stress-optic effects, making it difficult to achieve the core refractive index change required for making a single mode fiber, therefore even higher amounts (˜2×) of fluorine doping in the cladding is required. This higher F-doping makes the silica core and F-doped cladding have even higher viscosity and stress differences, resulting in the fibers being drawn at slow speeds to achieve low attenuation.

SUMMARY

Disclosed herein are optical waveguide fibers which comprise a core comprising silica and greater than or equal to 1.5 wt % chlorine and less than 0.6 wt % fluorine, the core having a refractive index Δ1MAX, and a cladding region having a refractive index Δ2MINsurrounding the core, where Δ1MAX>Δ2MIN. The fibers disclosed herein are preferably single moded at 1550 nm. In some embodiments, the fibers disclosed herein may exhibit a 22 m cable cutoff less than or equal to 1260 nm. In some preferred embodiments, the molar ratio of chlorine in the core to fluorine in the cladding is greater than 1, more preferably greater than 1.5. The fibers disclosed herein preferably contain less than 1 weight percent GeO2, and more preferably contain no GeO2. In some preferred embodiments, the core comprises chlorine in an amount greater than 2 weight percent, more preferably greater than 2.5 weight percent, and even more preferably greater than 3 weight percent.

In some embodiments, the core region may be doped with greater than 2.5 wt % chlorine with no fluorine doping in the cladding region. In yet another embodiment, the core region is doped with greater than 3 weight % chlorine with no fluorine doping in the cladding region.

The fiber designs disclosed herein provide a core with equal or lower viscosity than the cladding. This results in reduced stresses within the fiber and correspondingly reduced fiber attenuation, not only because of reduction in the viscosity mismatch but also reduction in the CTE (coefficient of thermal expansion) mismatch. Modeled examples of these fibers have attenuations of about 0.15 dB/Km at 1550 nm, even when drawn at high draw speeds.

The fiber designs disclosed herein are capable of resulting in fibers having optical properties that are G.652 compliant, MFD greater than 8.2 microns at 1310 nm, typically between 8.2 microns and 9.4 microns at 1310 nm, zero dispersion wavelength, λ0, of 1300≤λ0≤1324 nm, cable cutoff less than or equal to 1260 nm, and attenuation at 1550 nm≤0.18 dB/Km, more preferred ≤0.17 dB/Km, even more preferred ≤0.16 dB/Km at 1550 nm, and even more preferred ≤0.15 dB/Km at 1550 nm, and most preferably ≤0.14 dB/Km at 1550 nm.

The fiber designs disclosed herein also include optical fibers having effective area at 1550 nm of larger than 70 micron2. In some embodiments, the effective area at 1550 nm of disclosed fibers is larger than 90 micron2. In other embodiments, the effective area at 1550 nm of disclosed fibers is larger than 110 micron2. In still other embodiments, the effective area at 1550 nm of disclosed fibers is larger than 130 micron2. In some preferred embodiments, optical fibers having effective at 1550 nm area larger than 70 micron2have cable cutoff less than 1530 nm. In some other preferred embodiments, optical fibers having effective area at 1550 nm larger than 110 micron2have cable cutoff less than 1530 nm. In still other preferred embodiments, optical fibers having effective area at 1550 nm larger than 130 micron2have cable cutoff less than 1530 nm. In some other embodiments, optical fibers having effective at 1550 nm area between 70 micron2and 90 micron2and a bend loss at 1550 nm of less than 2 dB/turn on a 20 mm diameter mandrel. In some other embodiments, optical fibers having effective at 1550 nm area between 70 micron2and 90 micron2and a bend loss at 1550 nm of less than 1 dB/turn on a 20 mm diameter mandrel. In some other embodiments, optical fibers having effective at 1550 nm area between 70 micron2and 90 micron2and a bend loss at 1550 nm of less than 0.5 dB/turn on a 20 mm diameter mandrel. In some other embodiments, optical fibers having effective at 1550 nm area between 90 micron2and 120 micron2and a bend loss at 1550 nm of less than 3 dB/turn on a 20 mm diameter mandrel. In some other embodiments, optical fibers having effective at 1550 nm area between 120 micron2and 150 micron2and a bend loss at 1550 nm of less than 5 dB/turn on a 20 mm diameter mandrel. Also disclosed herein is a method of making an optical fiber, the method comprising providing an optical fiber preform having surface area greater than 10 m2/gm to a first furnace, and chlorine doping the preform in a first chlorine doping step comprising exposing the preform to an atmosphere containing chlorine at temperatures higher than 1300° C. The first chlorine doping step may comprise exposing the preform to an atmosphere comprising SiCl4, and said method further comprises exposing the preform to atmosphere containing water or oxygen after said first chlorine doping step to further activate the preform, and said method further comprises chlorine doping the preform in a second chlorine doping step in an atmosphere containing chlorine at temperatures higher than 1300° C.

Reference will now be made in detail to the present preferred embodiments, examples of which are illustrated in the accompanying drawings.

