Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1998-03-20
2001-01-23
Ngo, Hung N. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S127000
Reexamination Certificate
active
06178279
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a dispersion compensating optical fiber which is connected to a single-mode optical fiber having zero dispersion in the wavelength band of 1.3 &mgr;m, so as to perform a function of compensating dispersion in an optical signal in the wavelength band of 1.55 &mgr;m which is transmitted through the single-mode optical fiber, while serving as a transmission optical fiber. The present invention also relates to a wavelength division multiplex light transmission line using such a dispersion compensating optical fiber.
BACKGROUND OF THE INVENTION
As a transmission network for optical communication, single-mode optical fibers having zero dispersion in the wavelength band of 1.3 &mgr;m are installed all over the world. In recent years, as the information-oriented society has developed, the quantity of communication information has been dramatically increasing. The explosion of the quantity of information has made wavelength division multiplex transmission (WDM transmission) widely accepted in the field of communication. Now, the world is in the age of wavelength division multiplex transmission. Wavelength division multiplex transmission is a method in which a wavelength for optical communication is divided into a plurality of wavelengths to transmit a plurality of optical signals instead of using only one wavelength for optical communication, and therefore, is a light transmission method which is suitable to large-capacity high-speed communication.
However, in general, the installed single-mode transmission optical fibers which have zero dispersion at 1.31 &mgr;m have a large transmission loss. Hence, when wavelength division multiplex optical communication is to be realized using the wavelength band of 1.3 &mgr;m, since the wavelength range of the installed single-mode transmission optical fibers does not match a wavelength band of 1.55 &mgr;m which is a gain bandwidth of a regular light amplifier, the light amplifier can not be used and long-distance optical communication is troubled accordingly. Due to this, in recent years, wavelength division multiplex optical communication is conducted in the wavelength band of 1.55 &mgr;m using the installed single-mode transmission optical fibers which have zero dispersion in the wavelength band of 1.3 &mgr;m.
However, when optical communication is conducted in the wavelength band of 1.55 &mgr;m using a single-mode transmission optical fiber which has zero dispersion at 1.31 &mgr;m, since the installed single-mode transmission optical fibers have positive dispersion and a positive dispersion slope in the wavelength band of 1.55 &mgr;m, as optical signals propagate through the single-mode transmission optical fiber, the signal at each one of the wavelength division multiplexed wavelengths disperse largely. This makes it difficult to separate the signals at a receiving end, degrades the quality of the optical communication, and impairs the reliability of the optical communication.
To solve such problems, dispersion compensating optical fibers have been developed recently. A dispersion compensating optical fiber has negative dispersion. If a dispersion compensating optical fiber is connected to a receiving end of a single-mode transmission optical fiber, positive dispersion of optical signals which propagate through the single-mode transmission optical fiber is reduced by the negative dispersion of the dispersion compensating optical fiber so that the optical signals are received at the receiving end as they have substantially zero dispersion. As a dispersion compensating optical fiber is connected to a single-mode transmission optical fiber in this manner, it is possible to separate the respective wavelength division multiplexed optical signals at a receiving end, and therefore, a dispersion compensating optical fiber is expected to realize large-capacity high-speed communication of a high quality.
This type of a dispersion compensating optical fiber is formed as a module. It is a common practice to connect a dispersion compensating optical fiber which is formed as short as possible to a receiving end of a single-mode transmission optical fiber for the purpose of compensation of dispersion.
OBJECT AND SUMMARY OF THE INVENTION
However, to form a dispersion compensating optical fiber as a module and compensate dispersion with a short fiber length, it is necessary that the module of the dispersion compensating optical fiber has high negative dispersion and negative dispersion slope.
Despite this, a dispersion compensating optical fiber can not have high negative dispersion and a negative dispersion slope unless very strict conditions regarding various parameters are met which define a distribution of the refractive index of the dispersion compensating optical fiber, which makes it difficult to fabricate the dispersion compensating optical fiber. Further, when a dispersion compensating optical fiber has a refractive index configuration which enables high negative dispersion and a negative dispersion slope, a non-linear phenomena easily occurs, whereby a mode field diameter (MFD) of the optical fiber is reduced. If the non-linearity occurs, the waveforms of signals deform, which is a new problem against an effort to increase the speed and the capacity of wavelength division multiplex light transmission.
If the specific refraction index difference of the center core becomes too large in line with such a structure where the mode field diameter of optical fibers is reduced, a lowering of the transmission quality results from a non-linearity, and a large transmission loss is created when the optical fiber is bent.
Meanwhile, other proposed method is to conduct light transmission using a dispersion shifting optical fiber whose zero dispersion wavelength is shifted to 1.55 &mgr;m from 1.31 &mgr;m. While signal transmission without dispersion is possible when a dispersion shifting optical fiber which has zero dispersion at the wavelength of 1.55 &mgr;m is used for transmission of optical signals without dispersion by means of the wavelength of 1.55 &mgr;m, if wavelength division multiplex light transmission is conducted using a signal which is in the wavelength band of 1.55 &mgr;m, signals at other wavelengths near 1.55 &mgr;m disperse although no dispersion occurs at the wavelength of 1.55 &mgr;m. Thus, this light transmission method is not suitable to large-capacity high-speed wavelength division multiplex optical communication. Moreover, this type of a dispersion shifting optical fiber has a further problem that the non-linearity easily occurs.
Conversely, a single-mode optical fiber which has zero dispersion at the wavelength of 1.31 &mgr;m is excellent in terms of low non-linearity. Hence, it is ideal to use the installed single-mode transmission optical fibers which have zero dispersion at the wavelength of 1.31 &mgr;m as light transmission lines, connect dispersion compensating optical fibers which have low non-linearity and a large mode field diameter to the single-mode transmission optical fibers and conduct wavelength division multiplex light transmission using the wavelengths in the bandwidth of 1.55 &mgr;m with substantially zero dispersion.
Noting this, the inventor of the present invention has conceived the present invention from the conventional idea of simply forming a dispersion compensating optical fiber as a module and using the module as an optical fiber which only compensates dispersion. A first object of the present invention is to provide a dispersion compensating optical fiber which is formed in substantially the same length as a single-mode light transmission optical fiber and is connected to the dispersion compensating optical fiber so that the dispersion compensating optical fiber performs a function of transmitting optical signals over a long distance while compensating for dispersion of the optical signals which propagate through the single-mode transmission optical fiber, i.e., so that the dispersion compensating optical fiber has both the function of compensation of dispe
Mukasa Kazunori
Suzuki Yoshihisa
Lacasse & Associates
Ngo Hung N.
The Furukawa Electric Co. Ltd.
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