Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-01-17
2002-06-11
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C359S199200
Reexamination Certificate
active
06404967
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical fiber which is suitable for wavelength multiplexed optical transmissions in, for example, a wavelength band of a 1.55 &mgr;m wavelength and a wavelength multiplexed optical transmission line using the same optical fiber.
BACKGROUND ART
As a transmission network for optical transmissions, a single-mode optical fiber having zero dispersion in a wavelength band of a 1.3 &mgr;m wavelength has been laid across the world. Recently, in accordance with development of the information society, information transmission capacities tend to significantly increase. In accordance with such an increase in information, wavelength multiplexed transmissions (WDM transmissions) are widely accepted in the transmission field, and now the age of wavelength multiplexed transmissions has been entered. Wavelength multiplexed transmissions are the method for transmitting a plurality of light signals which are not on one wavelength but are divided into a plurality of wavelengths, which is suitable for large-capacity and high-rate transmissions.
However, in the case where an existing single-mode optical fiber in operation for transmissions which has zero dispersion near 1.3 &mgr;m is used and a wavelength band near 1.3 &mgr;m is used to carry out wavelength multiplexed optical transmissions, this wavelength band does not coincide with the 1.55 &mgr;m wavelength band (for example, 1530 nm to 1570 nm) of the gain bandwidth (including 1500 nm to 1650 nm) of a general optical amplifier using an erbium doped optical fiber. Therefore, in the case where optical transmissions are carried out by using a wavelength band near 1.3 &mgr;m, the optical amplifier cannot be used, and therefore, trouble may occur in long-distance optical transmissions. In order to solve this problem, recently, wavelength multiplexed optical transmissions in a wavelength band of 1.55 &mgr;m are carried out by using an optical amplifier and an existing single-mode optical fiber having zero dispersion in a wavelength band of 1.3 &mgr;m.
However, when optical transmissions are carried out in a wavelength band of 1.55 &mgr;m by using the single-mode optical fiber having zero dispersion near 1.3 &mgr;m, the existing single-mode optical fiber has positive dispersion of approximately 17 ps
m/km in a central wavelength of 1.55 &mgr;m of the wavelength band of 1.55 &mgr;m, and furthermore, the single-mode optical fiber has a positive dispersion slope of approximately 0.06 ps
m
2
/km in a wavelength band of 1.55 &mgr;m. Therefore, distortion in waveform of the light signals of the respective multiplexed wavelengths increases as the light signals are transmitted in the single-mode optical fiber, separation and distinction of the signals at the receiver side become difficult, the quality of optical transmissions deteriorates, and the reliability of optical transmissions is lost.
Furthermore, as a transmission network for optical transmissions, a dispersion shifted optical fiber whose wavelength of zero dispersion is shifted to be close to 1.55 &mgr;m which is the gain bandwidth of an optical amplifier has been proposed. When dispersion in wavelength in optical transmissions becomes close to zero, since a non-linear phenomenon called four wave mixing becomes easy to generate, in particular, in wavelength multiplexed transmissions, a dispersion shifted optical fiber having minute dispersion of a degree at which a non-linear phenomenon is not generated in the wavelengths for optical transmissions has been demanded.
However, if a dispersion shifted optical fiber having the abovementioned minute dispersion is used for long-distance optical transmissions, since the influence of the minute dispersion cannot be ignored, it is difficult to dependently use the dispersion shifted optical fiber having minute dispersion for long-distance large-capacity and high-rate transmissions.
Therefore, in order to solve such a problem, a method has been proposed in which, to compensate for dispersion in the wavelength band of 1.55 &mgr;m of a 1.3 &mgr;m zero dispersion single-mode optical fiber, an optical fiber having great negative dispersion in the wavelength band of 1.55 &mgr;m is inserted into the single-mode optical fiber transmission line, whereby positive dispersion in the wavelength band of 1.55 &mgr;m of the single-mode optical fiber is compensated, and deterioration in transmission signals due to chromatic dispersion is suppressed.
As an example of the optical fiber for compensating the dispersion, for example, an optical fiber having a refractive index profile of a single-peak form as shown in
FIG. 6
has been proposed. The optical fiber having a refractive index profile of a single-peak form is formed by covering the circumference of center core
1
with a refractive index greater than that of the silica level with outer cladding
5
. The optical fiber of the proposed example is formed so that the refractive index of the outer cladding
5
is smaller than that of the silica glass.
However, the dispersion value of the optical fiber having a refractive index profile of a single-peak form in the wavelength band of 1.55 &mgr;m is approximately −80 ps
m/km at most as a limit value in practical use, and therefore, an optical fiber having a smaller dispersion value (absolute value of negative dispersion is great) cannot be realized by means of a refractive index profile of a single-peak form. Therefore, in order to compensate for the positive dispersion of the single-mode optical fiber by an optical fiber with a refractive index profile of a single-peak form, the length required for the optical fiber for dispersion compensation increases, so that it is difficult to reduce the size of an optical fiber for dispersion compensation in which the abovementioned optical fiber is coiled and housed.
Furthermore, in the optical fiber with a refractive index profile of a single-peak form, the dispersion slope in the wavelength band of 1.55 &mgr;m is positive, so that it is difficult to compensate for the chromatic dispersion of the single mode optical fiber over a broadband of a 1.55 &mgr;m wavelength band.
Therefore, an optical fiber having a W-formed refractive index profile as shown in
FIG. 7
has been proposed. The optical fiber having a W-formed refractive index profile is formed so that the circumference of center core
1
with a refractive index greater than that of the cladding level is covered by side core
12
having a refractive index smaller than that of the cladding level, and normally, the circumference of the side core
12
is covered by outer cladding
5
having a refractive index which is almost equal to that of the silica level.
In the optical fiber having the W-formed refractive index profile, the dispersion value in the wavelength band of 1.55 &mgr;m can be smaller (absolute value of negative dispersion can be made greater) than that of the optical fiber having a refractive index profile of a single-peak form, whereby an optical fiber whose dispersion value at the wavelength of 1.55 &mgr;m is approximately −120 ps
m/km has become practicable. Furthermore, in the optical fiber having the W-formed refractive index profile, the dispersion slope in the wavelength band of 1.55 &mgr;m can be made negative, whereby the positive dispersion slope of the single-mode optical fiber can be compensated to a degree for practical use, so that dispersion over a broadband of a 1.55 &mgr;m wavelength can be compensated more than in the case of the optical fiber having a refractive index profile of a single-peak form.
Moreover, for example, in Japanese Laid-Open Patent Publication No. 313750 of 1996, a method is proposed in which an optical fiber having a W-formed refractive index profile whose detailed structure is properly determined is used to compensate for the chromatic dispersion and dispersion slope in the wavelength band of 1.55 &mgr;m of the single-mode optical fiber, whereby the chromatic dispersion and dispersion slope in the wavelength band of 1.55 &mgr;m are compensated to be almost zero. I
Aiso Keiichi
Arai Shinichi
Sugizaki Ryuichi
The Furukawa Electric Co. Ltd.
Ullah Akm E.
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