Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-07-12
2002-01-29
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06343175
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber suitable for an optical transmission line in a long-haul optical communication system.
2. Related Background Art
As optical transmission lines for transmitting light signals in an optical communication system, several types of optical fibers have been used or studied. For example, as the first prior art, a silica glass based fiber having a core region doped with germanium (to be referred to as a Ge-doped core optical fiber hereinafter) has been used. As the second prior art, a silica glass based fiber having a core region doped with no germanium and a cladding region formed around the core region and doped with fluorine (to be referred to as a pure silica core optical fiber hereinafter) has been used. As the third prior art, a silica glass based optical fiber is disclosed in Tanaka, et al., “High Silica Core Single-Mode Fibers for 1.55 &mgr;m Transmission”, Fujikura Technical Review, 1990, in which the core region is doped with chlorine to reduce residual stress (to be referred to as a chlorine-doped core optical fiber hereinafter) so as to effectively suppress deformation of the refractive index profile of the optical fiber obtained after drawing.
SUMMARY OF THE INVENTION
The inventors have found the following problems upon examining the above prior arts. An optical fiber for communication is required to have a small transmission loss. In consideration of the use of an optical fiber for a cable, it is also required that the bending loss be small. In order to reduce bending loss, the relative refractive index difference of a core region with respect to the refractive index of a cladding region must be increased to increase light confinement efficiency.
In the above Ge-doped core optical fiber as the first prior art, a reduction in bending loss can be attained by increasing the contents of germanium in the core region and increasing the relative refractive index difference between the core region and the cladding region. If, however, a large amount of germanium is added in the core region on which the optical power of incident light concentrates, the transmission loss increases because the Rayleigh scattering coefficient caused by the germanium is larger than that caused by pure silica. For this reason, in the Ge-doped core optical fiber as the first prior art, it is difficult to effectively reduce both the transmission loss and the bending loss.
In the pure silica core optical fiber as the second prior art, a reduction in bending loss can be attained by increasing the contents of fluorine in the cladding region and increasing the relative refractive index difference between the core region composed of pure silica and the cladding region. In the second prior art, however, even if the contents of fluorine in the cladding region increases, the transmission loss due to Rayleigh scattering is small because the core region is composed of pure silica. However, it is difficult from a production viewpoint to add a large amount of fluorine in the cladding region. In addition, since the differences in physical property value (e.g., viscosity upon heating) between the core region and the cladding region increase, the transmission loss due to structural mismatching at the interface between the core region and the cladding region increases. In the pure silica core optical fiber as well, therefore, it is difficult to effectively reduce both the transmission loss and the bending loss.
In the chlorine-doped core optical fiber as the third prior art, according to Tanaka, in laying the a submarine cable, the level of bending loss upon bending with a diameter of 20 mm needs to be 3 to 1 dB/m or less. Such a description, however, states only a required value of bending loss. However, there is no description in this reference about how to realize an optical fiber that can satisfy this required value of bending loss. In addition, there is no description about a specific level to which bending loss can be actually reduced.
The present invention has been made to solve the above problems, and has as its object to provided an optical fiber having a structure that effectively reduces both transmission loss and bending loss, and a method of manufacturing the optical fiber.
An optical fiber according to the present invention contains silica as a main component and includes a core region containing a predetermined amount of chlorine and a cladding region which is provided on the periphery of the core region and which contains a predetermined amount of fluorine. A characteristic feature of the optical fiber, in particular, is that the peak value of the relative refractive index difference of the core region with respect to the refractive index of pure silica is 0.05% or more.
In the optical fiber according to the present invention, since the Rayleigh scattering coefficient caused by chlorine added in the core region is small, the transmission loss due to Rayleigh scattering is small. In addition, since chlorine is added as a dopant in the core region, the differences in physical property value between the core region and the cladding region decrease, and the transmission loss due to structure mismatching at the interface between the core region and the cladding region decreases. Furthermore, since the peak value of the relative refractive index difference of the core region with respect to the refractive index of pure silica is 0.05% or more, the bending loss is sufficiently reduced.
The concentration of chlorine added in the core region preferably increases from a peripheral portion of the core region toward its center. By setting the concentration of chlorine in a peripheral portion of the core region to be lower than that in the center of the core region, the differences in physical property value between the core region and the cladding region further decrease. This further decrease the transmission loss due to structure mismatching at the interface between the core region and the cladding region.
The chlorine added in the core region diffuses into the cladding region in the process of manufacturing the optical fiber according to the present invention, and hence chlorine is contained in at least part of the cladding region.
In the optical fiber according to the present invention, an increase in transmission loss due to an OH-radical at a wavelength of 1.38 &mgr;m is 0.5 dB/km or less. Such a reduction in transmission loss can be attained by sufficiently performing dehydration using a halogen gas in the process of manufacturing the optical fiber. Chlorine to be contained in the core region is also introduced in this dehydration step. In the case shown in
FIG. 8
in the reference by Tanaka, an increase in transmission loss due to an OH-radical at a wavelength of 1.38 &mgr;m exceeds 0.6 dB/km. That is, dehydration is not performed or not sufficiently performed.
The optical fiber according to the present invention has a zero dispersion wavelength at 1.34 &mgr;m or more. By setting a zero dispersion wavelength to 1.34 &mgr;m or more, chromatic dispersion at a wavelength of 1.55 &mgr;m is reduced. This eliminates the necessity of dispersion compensation or allows optical transmission at a wavelength of 1.55 &mgr;m with a small amount of dispersion compensation. By setting a zero dispersion wavelength in this manner, the transmission loss due to an OH-radical at a wavelength of 1.38 &mgr;m can be suppressed to 0.5 dB/km or less. This allows the use of a 1.38 &mgr;m band as a signal wavelength band.
In order to shift the zero dispersion wavelength to a long wavelength side while the relative refractive index difference between the core region and the cladding region is maintained constant, the diameter of the core region must be decreased. If, however, the diameter of the core region decreases, the bending loss increases. The bending loss can effectively be reduced by increasing the relative refractive index difference between the core region and the cladding region. However, as the relative refractive index
Healy Brian
Wood Kevin S.
LandOfFree
Optical fiber with core containing chlorine and cladding... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical fiber with core containing chlorine and cladding..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical fiber with core containing chlorine and cladding... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2832749