Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-09-25
2003-03-18
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06535677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical fiber suitable for transmitting signals of a plurality of channels, a method of manufacturing the same, an optical communication system including the same and an optical fiber preform for obtaining the same in a wavelength division multiplexing (WDM) transmission.
2. Related Background Art
The WDM transmission system is an optical communication system which can realize a high-speed optical communication of a large capacity by transmitting signals of a plurality of channels. In the WDM transmission system, since the transmission loss of a silica-based optical fiber applied to an optical transmission line becomes small in the vicinity of the wavelength of 1.55 &mgr;m and an optical amplifier which amplifies signals at the wavelength band of 1.55 &mgr;m has been practically used, signals of a plurality of channels included in the wavelength band of 1.55 &mgr;m are available.
In the optical transmission line through which signals of a plurality of channels propagate, it is known that when the chromatic dispersion is generated at the signal wavelength band (the wavelength band of 1.55 &mgr;m), the pulse waveform of respective signals becomes broadened so that the transmission characteristics is deteriorated. Accordingly, from this point of view, it is desirable that the chromatic dispersion within the signal wavelength band is small. On the other hand, when the chromatic dispersion value within the signal wavelength band is approximately zero, a four-wave mixing which is one of the nonlinear optical phenomena is liable to be generated and hence, crosstalks and noises caused by the four-wave mixing are generated thus deteriorating the transmission characteristics. To suppress the generation of the four-wave mixing, the power of signals propagating through the optical transmission line may be reduced by making the repeater spacing short. However, it becomes necessary to install a large number of optical amplifiers along the whole optical transmission line thus pushing up a cost of the optical communication system as a whole.
To make the repeater spacing long while suppressing the occurrence of the above-mentioned nonlinear optical phenomenon, a dispersion-managed optical fiber in which portions having a positive chromatic dispersion and portions having a negative chromatic dispersion are alternately arranged, at a predetermined wavelength (for example, the wavelength being 1.55 &mgr;m=1550 nm), has been proposed. In the optical transmission line which has adopted such a dispersion-managed optical fiber, the mean chromatic dispersion (at the wavelength of 1.55 &mgr;m) from the viewpoint of the whole optical transmission line becomes approximately zero and hence, the deterioration of transmission characteristics caused by the generation of the chromatic dispersion can be effectively suppressed. Further, since the chromatic dispersion is generated in substantially all regions of the optical transmission line, the deterioration of transmission characteristics caused by the four-wave mixing can be effectively suppressed.
For example, Japanese parent Laid-open No. 201639/1996 discloses a dispersion-managed optical fiber which changes a sign (positive or negative) of the chromatic dispersion by changing the outer diameter of a core in the longitudinal direction. This publication also discloses a method of manufacturing such a dispersion-managed optical fiber. U.S. Pat. No. 5,894,537 discloses a dispersion-managed optical fiber which is designed such that signs (positive and negative) of the chromatic dispersions which are generated at respective portions are made different by changing the outer diameter of a core or the outer diameter of a cladding in the longitudinal direction, and it also discloses a method of manufacturing such a dispersion-managed optical fiber. The Japanese Patent Laid-open No. 318824/1997 discloses an optical fiber cable in which two kinds of optical fibers which differ from each other in their effective areas as well as in signs (positive and negative) of the chromatic dispersion are connected.
SUMMARY OF THE INVENTION
Upon reviewing the conventional dispersion-managed optical fiber and cable, the inventors of the present invention have found following problems. That is, the conventional dispersion-managed optical fiber disclosed in the Japanese Patent Laid-open No. 201639/1996 and U.S. Pat. No. 5,894,537 is manufactured by drawing the optical fiber preform which changes the outer diameter of the core or the outer diameter of the cladding along the longitudinal direction and hence, the manufacturing is not easy. Further, in the conventional dispersion-managed optical fiber, since the outer diameter of the core or the outer diameter of the cladding is changed along the longitudinal direction, it is difficult to connect this optical fiber with other optical fiber. Further, there arises a case that the connection loss becomes large. For example, the optical fiber cable disclosed in Japanese Patent Laid-open No. 318824/1997 connects two kinds of optical fibers which differ from each other in the effective area and hence, the connection loss becomes large.
The present invention has been made to solve the above-mentioned problems and it is an object of the present invention to provide a dispersion-managed optical fiber having a structure which facilitates the manufacturing thereof and the connection thereof with other optical fiber, a method of manufacturing such a dispersion-managed optical fiber, an optical communication system which adopts the dispersion-managed optical fiber as an optical transmission line, and an optical fiber preform for obtaining the dispersion-managed optical fiber.
A dispersion-managed optical fiber according to the present invention is a silica-based optical fiber ensuring its single mode at a predetermined wavelength within a signal wavelength band, that is, a continuous (including unitary) optical fiber in which one or more first portions having a positive chromatic dispersion at the predetermined wavelength and one or more second portions having a negative chromatic dispersion at the predetermined wavelength are arranged alternately and adjacent to each other.
This dispersion-managed optical fiber includes a plurality of glass layers which are sequentially laminated in a radial direction. Among the plurality of glass layers, a dopant concentration of a glass layer doped with a dopant for adjustment of refractive index is made uniform such that the maximum change along the longitudinal direction of the dispersion-managed optical fiber is not more than 20-30%, and preferably, not more than 10%. Further, a refractive index of a glass layer which does not substantially include GeO
2
as a dopant is changed along the longitudinal direction of the dispersion-managed optical fiber. Further, the dispersion-managed optical fiber according to the present invention may have a constitution in which the stresses remaining in the plurality of glass layers may be changed along the longitudinal direction of the dispersion-managed optical fiber. It is preferable that a core region in the dispersion-managed optical fiber includes a layer comprised of non-intentionally-doped glass (hereinafter called “pure silica glass”). This is because that since the viscosity of the pure silica glass layer is greater than that of the glass layer including a dopant, the adjustment of residual stress is facilitated. Further, even when a predetermined amount of GeO
2
is added into this pure silica glass layer unintentionally during the manufacturing, the relative refractive index difference of this layer, in which the residual stress is given, with respect to the pure silica glass is restricted to a value lower than the relative refractive index difference of the glass layer into which an amount of GeO
2
equal to that of the added GeO
2
is added, and therefore the influence of addition of GeO
2
can be effectively suppressed. In this specification, the relative refractive
Ishikawa Shinji
Kato Takatoshi
Nishimura Masayuki
Sasaoka Eisuke
Tanaka Shigeru
Bovernick Rodney
McDermott & Will & Emery
Pak Sung
Sumitomo Electric Industries Ltd.
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