Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-11-28
2003-03-18
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S127000
Reexamination Certificate
active
06535678
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical fiber used for relatively long-distance transmission, that is mainly applicable to super high-speed transmission or to multiplex transmissions with a high wavelength density.
BACKGROUND ART
Single mode fibers are familiar examples of conventional optical fibers used in high-speed transmission. These single mode fibers for high-speed transmission are usually formed of quartz glass, where the term as employed here shall mean quartz glass having silicon dioxide as the main component. In addition, the quartz glass forming the optical fiber core in this specification is understood to be quartz glass in which at least 50 wt % or more of the composition is silicon dioxide.
A single mode fiber of the simplest structure has a step-refractive index distribution. This step-index single mode fiber is designed with a cladding which is around and in contact with a core. The core has a uniform refractive index and the cladding has a lower refractive index than the core.
The electromagnetic field of a step-index single mode fiber can be determined by solving a Maxwell equation.
If the radius of the core in a cross-section of the optical fiber is designated as a, the refractive index of the core (peak refractive index) is designated as n
1
, and the refractive index of the cladding is designated as n
clad
then the core-cladding relative index difference of refraction (i.e., relative refractive index difference) &Dgr; can be expressed by the following Equation (1)
Δ
=
(
n
1
2
-
n
clad
2
)
/
(
2
n
1
2
)
≈
(
n
1
-
n
clad
)
/
n
1
≈
(
n
1
-
n
clad
)
/
n
clad
Equation (1)
Setting the light wavelength to &lgr;, the normalized frequency V can be expressed by the following Equation (2).
V
=(2&pgr;/&lgr;)
an
1
(2&Dgr;) Equation (2)
Single mode conditions enabling only a single LP mode to be propagated are assured provided that this normalized frequency V is below a given set value.
The LP mode (i.e., linearly polarized mode) will now be explained.
The mode which propagates through the optical fiber core is referred to as the “propagating mode” and the mode which propagates through the clad is referred to as the “cladding mode.” The cladding mode radiates to the outside as it propagates over a specific distance, and becomes attenuated.
Strictly speaking, the propagating mode consists of modes that have a variety of directional components in the form of electromagnetic field vectors like TE, TM, HE, EH, etc. In a given approximation, or more specifically, under the condition in which the core-cladding relative refractive index difference is small, when perpendicular axes are placed in the fiber's cross-sectional plane, it is possible to approximate the propagation state of the light using an LP mode which has an electromagnetic field vector in only one of the two perpendicular directions. In general, it is said that the relative refractive index difference between the core and the cladding is 1% or less. Provided that slight error is allowed, however, an approximation can be established in the case of a refractive index difference of up to 3%.
The correspondence between the LPmn mode and the strict field mode is as follows.
LP
01
mode=HE
11
mode
LP
11
mode=TE
01
mode, TM
01
mode, HE
21
mode
LP
21
mode=EH
11
mode, HE
31
mode
LP
02
mode=HE
12
mode
In a step-index single mode fiber, it is known that when V≦2.405, only the lowest order mode (the fundamental mode, i.e., the LP
01
mode) meets the single mode conditions for propagating through the core.
As may be understood from Equation (2) above, the disadvantage of this step-index single mode fiber is that, in order to fulfill single mode conditions for a given wavelength &lgr;, the product of core radius a (or core diameter
2
a
) and the square root of the relative refractive index difference &Dgr;
½
cannot be increased. In other words, in order to satisfy single mode conditions, the mode field diameter (MFD), which describes the region in which the mode is present, tends to become smaller in principle. When the MFD is small, however, it is not possible to satisfy the conditions for low-loss connection of plural optical fibers.
On the other hand, if an attempt is made to increase the MFD while maintaining the condition V≦2.405, it becomes necessary to expand core diameter
2
a
and thus decrease the relative refractive index difference &Dgr;.
When this type of design is executed, however, the refractive index difference is small, and the mode is large and spreads out from the core center. As a result, if only a slight bend (i.e., a microbend) is applied to in the fiber, loss readily occurs as the energy of the propagating mode passes through the cladding and is radiated to the outside.
Accordingly, as one countermeasure, rather than strictly maintaining the condition V≦2.405 shown in Equation (2), it is theoretically possible to set V so that a second order mode LP11 mode can be present.
In other words, if a design is provided that permits a value of about 3.0 for V, then there is strong containment of the electromagnetic field inside the core, even when setting the comparatively large MFD of the LP
01
mode. For this reason, even if a slight bending is applied to the fiber, the bending loss does not become very large, so that transmission is possible.
Since the LP
11
mode is only slightly contained within the core at this time, it does not propagate over long distances, but is attenuated quickly as it propagates over several to dozens of meters due to the large radiating losses from bending that the fiber incurs under the conditions in which it is actually employed. Thus, the LP
11
mode does not effect transmission.
However, in a design in which two or more modes propagate in this way, the following problems may occur if the higher order mode does not quickly attenuate.
In general, when there are multiple modes propagating through an optical fiber, the individual modes do not have equivalent propagation speeds. For this reason, when the energy of an optical signal is distributed to a plurality of modes and simultaneously propagated in an optical fiber communications system, the individual modes will arrive at different times following propagation over a long distance, and the signal waveform following demodulation will be distorted. Accordingly, the effective result is that high-speed transmission is not carried out. In recent years, optical communications typically have been carried out at a transmission speed of several Gb/s or more per one wave in the propagation wavelength, with 10 Gb/s being reported on the level of practical applications, and 20~100 Gb/s being reported experimentally. However, the wavelength dispersion (or more simply, “dispersion”) in an optical fiber is determined based on the sum of the following two components. Namely, the first component is the material dispersion, which is determined by the material forming the fiber. The second component is the waveguide dispersion (i.e., structural dispersion), which is determined by the structure of the optical fiber's refractive index distribution. In the 1.3~1.6 &mgr;m wavelength region which is important for optical fiber communications, the material dispersion of a quartz type optical fiber tends to increase as the wavelength becomes longer. In the above-described typical step-index single mode fiber, the waveguide dispersion contribution is small, with material dispersion dominating. Thus, total dispersion, i.e., the sum of material dispersion and waveguide dispersion, becomes zero near 1.3 &mgr;m.
The minimum loss wavelength of an optical fiber, particularly an optical fiber having quartz glass as the main component, occurs at around 1.55 &mgr;m. The loss in a quartz optical fiber is mainly due to Rayleigh scattering, and becomes minimal in the 1.55 &mgr;m band. Thus, in this wavelength band, a step-index single mode fiber in which V is 2.4~3.0 has a large dispersion and is not very
Abiru Tomio
Matsuo Shoichiro
Takahashi Koichi
Yamauchi Ryozo
Chadbourne & Parke LLP
Connelly-Cushwa Michelle R.
Fujikura LTD
Ullah Akm E.
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