Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-04-20
2003-04-08
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06546177
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a dispersion shifted optical fiber and may be used singly or in combination with a dispersion compensating optical fiber and the like in a transmission path in an optical communication system that uses one type or two or more types of optical fibers, and may also be used for transmitting high power signal light and for performing wavelength multiplex transmissions in the above types of optical communication system.
BACKGROUND ART
The wavelength with the lowest loss in quartz based optical fibers is in the vicinity of 1.55 &mgr;m and, conventionally, this wavelength band has been used for long distance transmissions. As the transmission path (i.e. optical fiber) in this case, generally, a dispersion shifted fiber (DSF) is used that has been designed such that the absolute value of the chromatic dispersion values in the 1.55 &mgr;m wavelength band is small.
Moreover, optical communication systems that perform division multiplexing on the signal wavelength (WDM) and make use of high power signals utilizing light amplifiers such as EDFA (erbium doped optical fiber amplifiers) have appeared in recent years in response to the demands for even greater volumes in optical communication. In this case, because of the high intensity of the optical power transmitted through the fiber, the deteriorations in transmission due to non-linear optical effects cannot be ignored.
Furthermore, in conventional optical communication systems, the vicinity of 1530 to 1570 nm wavelength band has been used, however, recently, investigations have been progressing into the further expansion of the transmission volume in wavelength multiplex transmission systems. For example, devices in the range of 1570 to 1625 nm have been developed and results of investigations of the 1490 to 1530 nm wavelength band have also been made public. Currently, these wavelength bands that are in actual use or are under investigation are generally known by the following terms. Namely, the 490 to 1530 nm band is known as the S-band; the 1530 to 1570 nm band is known as the C-band; and the 1570 to 1630 nm band is known as the L-band. In actual practice, the used wavelength band (operating wavelength band) in an optical communication system may be appropriately selected from the range of 1490 to 1625 nm.
The non-linear optical effect of the transmission path is evaluated using a non-linear constant represented by n
2
/Aeff. n
2
is the non-linear refractive index of an optical fiber and Aeff is the effective core area (effective area) of the optical fiber.
In order to reduce the non-linear effect, it is necessary to reduce the non-linear constant n
2
/Aeff. Because n
2
does not change greatly once the material has been decided, conventionally, attempts have generally been made to reduce the non-linear constant by enlarging the Aeff.
The present inventors have proposed in, for example, Japanese Unexamined Patent Application, First-Publication (JP-A) Nos. 10-62640, 10-293225, and the like a dispersion shifted fiber with a far larger Aeff than a conventional dispersion shifted fiber as a dispersion shifted fiber suitable for long distance systems and wavelength multiplex transmissions.
In JP-A No. 11-119045, there is also proposed a dispersion shifted fiber that suppresses the enlargement of the Aeff and gives priority to reducing the dispersion slope.
The dispersion slope shows the wavelength dependency of the chromatic dispersion values and is a gradient of the curve when the chromatic dispersion values are plotted when the horizontal axis is set as the wavelength and the vertical axis is set as the chromatic dispersion values. In wavelength multiplex transmissions, the larger the dispersion slope of the transmission path, the larger the difference in the chromatic dispersion values between each wavelength, the more irregular the transmission state, and the more the overall transmission characteristics are deteriorated.
Further, what is known as NZDSF (non zero dispersion shifted fiber) has also been proposed. In NZDSF, because it is easy for four-wave mixing, which is one of the non-linear effects, to occur if the chromatic dispersion value is zero, chromatic dispersion values whose absolute value, although small, is not zero are set.
FIGS. 5A
to
5
C show examples of the refractive index distribution configuration (i.e. the refractive index profile) used in NZDSF and in the conventional proposed dispersion shifted fibers.
FIG. 5A
shows an example of a dual shape core type (step type) of refractive index profile. A core
4
is formed provided with a central core portion
1
and a step core portion
2
provided at the outer periphery of the central core portion
1
and having a lower refractive index than the central core portion
1
. In addition, cladding
7
having a lower refractive index than the step core portion
2
is provided at the outer periphery of the core
4
.
FIG. 5B
shows an example of a segment core type of refractive index profile. A core
24
is formed provided with a central core portion
21
having a high refractive index and an intermediate portion
22
having a low refractive index at the outer periphery of the central core portion
21
. A ring core portion
23
having a lower refractive index than the central core portion
21
and a higher refractive index than the intermediate portion
22
is further provided at the outer periphery of the intermediate portion
22
. In addition, cladding
27
provided with a first cladding
25
having a lower refractive index than the intermediate portion
22
and a second cladding
26
having a higher refractive index than the first cladding
25
and a lower refractive index than the intermediate portion
22
is formed at the outer periphery of the ring core portion
23
.
FIG. 5C
shows an example of an O ring type (i.e. a convex type) of refractive index profile. A core
34
having a two layer structure is formed with a central core portion
31
having a low refractive index in the center thereof and a peripheral core portion
32
having a high refractive index provided at the outer periphery of the central core portion
31
. A three layer (including the cladding
37
) structure refractive index profile is formed by providing cladding
37
having a lower refractive index than the peripheral core portion
32
at the outer periphery of the core
34
.
Conventional dispersion shifted fibers and the like that have these refractive index profiles are advantageous with regard to the design of the system, in view of the transmission speed and accumulated dispersion (the chromatic dispersion accumulated by the transmission) when transmitting over long distances, because the chromatic dispersion values in the used wavelength band (operating wavelength band) are small.
When a chromatic dispersion value is set as a negative value, then it is possible to build a system that comparatively easily compensates the chromatic dispersion value in combination with a typical 1.3 &mgr;m single mode optical fiber (1.3 SMF).
Namely, a 1.3 &mgr;m single mode optical fiber has a zero dispersion wavelength (i.e. when the chromatic dispersion value is zero) of approximately 1.3 &mgr;m and hitherto has been well used. Moreover, it has a comparatively large positive value (for example, slightly less than 17 ps/km
m) as the chromatic dispersion value in the 1.55 &mgr;m band. Therefore, it is possible to reduce the chromatic dispersion of the overall system by connecting a 1.3 &mgr;m single mode optical fiber to the output side of a dispersion shifted fiber having a negative chromatic dispersion value, and by compensating the negative chromatic dispersion accumulated due to transmission through the dispersion shifted fiber with the positive chromatic dispersion of the 1.3 &mgr;m single mode optical fiber.
However, because conventionally proposed dispersion shifted fibers and the like are used as general transmission paths, a small chromatic dispersion is required. For example, there are many cases in which the absolute value of the chromatic dispersion v
Abiru Tomio
Matsuo Shoichiro
Tanigawa Shoji
Bell Boyd & Lloyd LLC
Bovernick Rodney
Fujikura Ltd.
Pak Sung
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