Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2002-05-29
2004-11-30
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S037000
Reexamination Certificate
active
06826340
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to optical pulse transmission line parts which are particularly, but not exclusively, for use in high capacity, wavelength division multiplexed (WDM) optical waveguide transmission lines employing return-to-zero (RZ) optical pulses. The invention also relates to components for parts of an optical pulse transmission line, and to an optical pulse transmission line.
BACKGROUND TO THE INVENTION
At high bit rates optical communication systems suffer degradation of their transmission capacity. This is mainly due to the detrimental effects on the optical pulses of chromatic dispersion, fibre non-linearity and noise from optical amplifiers in the system.
In the transmission of soliton pulses, positive use is made of the fibre non-linearity by achieving a balance with the pulse broadening due to dispersion in the region of the anomalous chromatic dispersion. Conventional soliton transmission lines comprise optical fibre which has a constant, or slightly varying, dispersion coefficient. However, the distance over which soliton pulses may be transmitted along such a transmission line is limited by two contradicting requirements: the dispersion must be low in order to minimise Gordon-Haus timing jitter, which is driven by optical amplifier noise; and the dispersion must be high in order to suppress four-wave mixing in wavelength division multiplexed transmission systems. This contradiction is resolved by using a technique known as dispersion management in which the optical fibre transmission line has high local dispersion and low path-average dispersion.
One of the major factors limiting optical data transmission distance and capacity in WDM systems is the effect of dispersion slope, which is a wavelength dependent parameter. The dispersion slope of an optical waveguide causes a variation in the dispersion parameters of the waveguide as a result of a change in the wavelength of an optical signal propagating along the waveguide. That is to say, the dispersion experienced by an optical signal propagating along the waveguide is dependent upon the wavelength of the optical signal. Therefore each optical channel in a WDM system will experience a different dispersion parameter. As a result it is difficult to achieve system optimisation for all wavelength channels simultaneously.
Existing approaches to compensating for the effect of dispersion slope are rather complex. These include per channel dispersion slope compensation, the manufacture of pairs of optical fibres having opposite values of dispersion and dispersion slope, and the manufacture of compensating optical fibres capable of compensating for both the dispersion and dispersion slope of known transmission fibres, such as standard monomode fibre (SMF).
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an optical pulse transmission line portion having dispersion slope compensation, the transmission line portion comprising:
a first optical waveguide of a first length (L
1
) having a first dispersion parameter (D
1
) and a dispersion slope parameter (S
1
) of a first sign,
optically coupled to
first dispersion slope compensating means of a second length (L
2
) having a second dispersion parameter (D
2
) and a dispersion slope parameter (S
2
) of the opposite sign,
wherein the path average dispersion slope of the transmission line portion is substantially zero and is given by
<
S>=S
1
L
1
+S
2
L
2
and the path average dispersion (<D>) of the transmission line portion is generally non zero, and is given by
⟨
D
⟩
=
D
1
S
2
-
D
2
S
1
S
2
-
S
1
The path average dispersion of the transmission line portion is preferably wavelength independent.
The optical pulse transmission line portion is preferably for inclusion in a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero optical signal.
The first optical waveguide and the first dispersion slope compensating means are desirably arranged in a precompensating dispersion map configuration, or may alternatively be arranged in a postcompensating dispersion map configuration.
The transmission line portion preferably further comprises an optical amplifier. The optical amplifier may be optically coupled to the dispersion map at one end of the dispersion map or at a location along the dispersion map.
The transmission line portion may further comprise a second optical waveguide of a third length (L
3
) having a first dispersion parameter (D
1
) and a dispersion slope parameter (S
1
) of a first sign and a second dispersion slope compensating means of a fourth length (L
4
) having a second dispersion parameter (D
2
) and a dispersion slope parameter (S
2
) of the opposite sign.
The first and second optical waveguides and the first and second dispersion slope compensating means are desirably optically coupled together in a symmetric dispersion map configuration. The transmission line portion may further comprise a second optical amplifier. Desirably, one of the first and second optical amplifiers is provided at an end of the dispersion map and the other of the first and second optical amplifiers is provided towards the middle of the dispersion map.
The or each optical amplifier may be a fibre amplifier, such as an erbium doped fibre amplifier, or a fibre Raman amplifier. The or each optical amplifier may alternatively be a semiconductor optical amplifier device.
The first and second optical waveguides are preferably sections of a first optical fibre. The first optical fibre may be a transmission fibre such as standard monomode fibre, Lucent 2-wave fibre or Alcatel Terralight fibre.
The first and second dispersion slope compensating means preferably comprise sections of a third optical waveguide, which is most preferably a second optical fibre. The second optical fibre may be a compensating fibre, such as dispersion compensating fibre or reverse dispersion fibre.
The first and second dispersion slope compensating means may alternatively or additionally comprise sections of a fourth optical waveguide including an optical grating. The fourth optical waveguide is preferably a third optical fibre. The optical grating is preferably an optical fibre grating, such as a fibre Bragg grating.
According to a second aspect of the present invention, there is provided optical pulse transmission line portion components for incorporation into an existing terrestrial communication line which comprises a first optical waveguide of a first length (L
1
) having a first dispersion parameter (D
1
) and a dispersion slope parameter (S
1
) of a first sign, the components comprising:
first dispersion slope compensating means of a second length (L
2
) having a second dispersion parameter (D
2
) and a dispersion slope parameter (S2) of the opposite sign,
the first dispersion slope compensating means being connectable in optical communication with the first optical waveguide to form therewith an optical pulse transmission line portion having compensated dispersion slope,
wherein the path average dispersion slope of the transmission line portion is substantially zero and is given by
<
S>=S
1
L
1
+S
2
L
2
and the path average dispersion (<D>) of the transmission line portion is generally non zero, and is given by
⟨
D
⟩
=
D
1
S
2
-
D
2
S
1
S
2
-
S
1
The path average dispersion of the transmission line portion is preferably wavelength independent.
The optical pulse transmission line portion preferably forms part of a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero signal.
Desirably the first optical waveguide and the first dispersio
Bennion Ian
Turitsyn Sergei Konstantinovich
Aston University
Connelly-Cushwa Michelle R.
Ullah Akm Enayet
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