Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-06-15
2003-06-10
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S127000
Reexamination Certificate
active
06577800
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to an optical communication system for signal transmission. More particularly, the invention relates to an optical transmission fiber for use in a metropolitan or access network.
The optical transmission networks use optical communication lines composed of a series of spans of optical fibers to connect a transmitter to a receiver. The optical networks within the optical system can be classified on the basis of the distance covered by the network. The network covering the greatest distance is known as a transport network. The transport network is typically used to provide a point to point connection between cities and is usually composed of 80 km fiber spans. Often signal amplifiers are connected between the fiber spans to account for power losses in the transmission line.
The transport networks are generally connected to smaller networks called metropolitan networks. The metropolitan networks provide a backbone structure used to distribute the signals received from the transport network. The distance covered by a metropolitan network is typically equivalent to a single span of the transport network. However, this distance can in general be as high as 150 km to cover large metropolitan areas. The metropolitan networks are used to collect and distribute signals coming from and going to the city. The metropolitan networks are best viewed as the interface between long straight transport and the shorter distribution networks connected to the end receiver.
The shorter distribution networks are commonly referred to as distribution or access networks. For purposes of simplicity, the term access networks is intended to include the distribution and access networks, as well as any other network that accomplishes the same purpose. The access networks are the shortest of all networks and serve to connect the end receiver to the metropolitan network.
Because each category of network is designed to fulfill a different purpose, the transmission characteristics of the optical fiber used in each network is preferably different. For example, the primary purpose of a transport network is to carry a signal over a long distance. Thus, the optimal optical fiber for a transport network should have a low power loss, or low attenuation. Having a low attenuation will decrease the number of amplifiers required to send the signal over the transmission length and increase the overall efficiency of the network.
The primary purpose of the metropolitan and access networks is to distribute the signal received from the transport network. Since both networks focus on distributing the signal, the optimal fiber for both networks will have similar transmission characteristics. More particularly, the optimal fiber for both networks should be capable of handling a large number of signals at a high data transmission rate. The fiber should also allow the signals to be easily split. In addition, the fiber should have a low attenuation (as with the transport network fiber) to avoid the need for excessive amplifications and a quite large effective area to facilitate coupling (e.g., by splices and/or connectors).
The so called effective core area, or briefly, effective area, is given by
A
eff
=
2
π
[
∫
0
∞
&LeftBracketingBar;
F
(
r
)
&RightBracketingBar;
2
r
ⅆ
r
]
2
∫
0
∞
&LeftBracketingBar;
F
(
r
)
&RightBracketingBar;
4
r
ⅆ
r
.
(
1
)
where r is the radial coordinate of the fiber and F(r) is the fundamental mode radial distribution.
Other characteristics desired in a metropolitan network fiber include the ability to handle a large amount of optical power and the presence of a low dispersion slope. The frequency of splitting of the optical signals traveling in the metropolitan network fiber requires that signals with a large amount of power are coupled into the beginning of the fiber. Consequently, the metropolitan fiber should have a low attenuation and should have a quite low nonlinearity coefficient r to cope with nonlinear effects induced by the high power signal. A low dispersion slope helps to equalize the dispersion among WDM channels.
The strength of non-linear effects acting on pulse propagation in optical fibers is linked to the product of the non-linearity coefficient &ggr; and the power P. The definition of the non-linearity coefficient, as given in the paper “Nonlinear pulse propagation in a monomode dielectric guide” by Y. Kodama et at., IEEE Journal of Quantum Electronics, vol. QE-23, No. 5, 1987, is the following:
γ
=
1
λ
n
eff
∫
0
∞
n
(
r
)
n
2
(
r
)
&LeftBracketingBar;
F
(
r
)
&RightBracketingBar;
4
r
ⅆ
r
[
∫
0
∞
&LeftBracketingBar;
F
(
r
)
&RightBracketingBar;
2
r
ⅆ
r
]
2
.
(
2
)
where n
eff
is the effective mode refractive index, &lgr; is a signal wavelength, n(r) is the refractive index radial distribution, and n
2
(r) is the non-linear index coefficient radial distribution.
Applicants have identified that equation (2) takes into account the radial dependence of the non-linear index coefficient n
2
which is due to the varying concentration of the fiber dopants used to raise (or to lower) the refractive index with respect to that of pure silica.
If we neglect the radial dependence of the non-linear index coefficient n
2
we obtain a commonly used expression for the coefficient &ggr;.
γ
=
2
π
n
2
λ
A
eff
(
3
)
The approximation (3), in contrast to the definition (2) does not distinguish between refractive index radial profiles that have the same effective core area A
eff
value but different &ggr; values. While 1/A
eff
is often used as a measure of the strength of non-linear effects in a transmission fiber, &ggr; as defined by equation (2) actually provides a better measure of the strength of those effects.
Furthermore, the fiber used in the metropolitan and access networks must be compatible with the fiber used in the transport networks and with currently installed systems. The majority of currently installed systems have operating wavelengths within a band of wavelengths surrounding either 1310 nm or 1550 nm. Generally, long distance transmissions require low fiber attenuation, which can be obtained at larger wavelengths. To take advantage of the low attenuation, the current trend in optical amplifiers is to allow the amplification of larger wavelengths. New generation amplifiers are expanding the amplification wavelength band surrounding 1550 nm to extend up to and include 1625 nm as a possible operating wavelength. The access networks typically operate in the wavelength band around 1310 nm and a number of components have been developed to also operate at this wavelength. In addition, CATV systems generally operate around 1550 nm but may include a service channel operating at around 1310 nm. Moreover, optical amplification in the wavelength band around 1310 nm is being developed.
To account for these considerations, the optimal metropolitan or access fiber should be capable of operating within the wavelength bands surrounding both the 1310 nm and 1550 nm wavelengths and supporting both positive and negative dispersion systems. By operating successfully in these wavelength bands, the metropolitan network fiber will support currently available components installed for 1310 nm systems and also adapt to future generations of components operating at wavelengths up to 1625 nm.
To meet the high capacity requirement, metropolitan and access networks will likely take advantage of Wavelength Division Multiplexing (WDM) technology to increase the number of transmission channels. WDM technology is limited by the phenomenon of Four Wave Mixing (FW
Roba Giacomo Stefano
Sarchi Davide
Bovernick Rodney
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Cavi e Sistemi S.p.A.
Stahl Michael J.
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