Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2002-06-27
2004-11-09
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000
Reexamination Certificate
active
06816659
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reconfigurable unit for compensating the chromatic dispersion, and optionally, the chromatic dispersion slope, comprising a holey optical fibre and a temperature adjusting device.
Moreover, the present invention relates to an optical communication line and an optical communication system comprising such unit, and a method for compensating chromatic dispersion, and optionally, the chromatic dispersion slope, in a reconfigurable manner.
In the present description and claims, the expression:
“chromatic dispersion coefficient D” is used to indicate the dependence at the first order of group velocity on the wavelength. More in particular, the chromatic dispersion coefficient D is expressed by the following relation (Govind P. Agrawal, “Nonlinear Fiber Optics—Second Edition”, Academic Press, pages 8-10)
D
=
ⅆ
β
1
ⅆ
λ
=
-
2
π
c
λ
2
β
2
where &bgr;
1
and &bgr;
2
are the propagation constants of the first and second order, respectively, and D is expressed in ps/(nm*Km). Moreover, the chromatic dispersion coefficient D can have a positive or negative value based on the sign of the propagation constant &bgr;
2
;
“chromatic dispersion D*L” is used to indicate the chromatic dispersion, expressed in ps
m, accumulated along an optical transmission fibre having a chromatic dispersion coefficient D, expressed in ps/(nm*Km), and a length L, expressed in Km (such product D*L can have a positive or negative value according to whether the chromatic dispersion coefficient D is positive or negative);
“chromatic dispersion slope s” is used to indicate the derivative, with respect to the wavelength, of the chromatic dispersion coefficient D, is expressed in ps/(nm
2
*Km) and can have a positive or negative value; and
“optical transmission fibre” is used to indicate an optical fibre used in an optical communication line or system for transmitting optical signals from a point to the other, located at a considerable distance (for example, at at least some km or tenths of km).
2. Description of the Related Art
In the field of optical telecommunications and optical signal propagation in an optical transmission fibre, chromatic dispersion (or second order dispersion), defined by the above chromatic dispersion coefficient D, is a phenomenon for which different spectral components of a light pulse propagating in an optical fibre travel at different speeds, causing a temporal broadening of the pulse.
In an optical communication system, chromatic dispersion thus limits the maximum data transmission speed (that is, the bit rate) or the maximum connection length without electrical signal regeneration.
In order to compensate chromatic dispersion Dt*Lt accumulated along an optical transmission fibre having a chromatic dispersion coefficient Dt and a length Lt, there are known devices comprising, for example, an optical fibre specifically designed to have a very high value of the chromatic dispersion coefficient Dc, with opposed sign with respect to that of the optical transmission fibre, and such length Lc as to satisfy the relation Dt*Lt=−Dc*Lc.
However, a compensator device of this type, designed so as to compensate a certain value of chromatic dispersion Dt*Lt, is not suitable for compensating the chromatic dispersion of another optical transmission fibre characterised by a different value of product Dt*Lt with respect to that for which the compensator device has been designed.
For example, a compensator device designed to compensate exactly the chromatic dispersion accumulated along a span of 100 Km (Lt=100 Km) of a conventional single mode optical fibre (or SMF) having a value of the chromatic dispersion coefficient Dt equal to about 17 ps/(nm*Km) is not capable of compensating exactly the chromatic dispersion of spans of the same SMF fibre having, however, a length different from 100 Km (for example, 70, 80, 90 or 110 Km).
Moreover, a compensator device designed to compensate exactly the chromatic dispersion accumulated along a span of 100 Km of a conventional SMF fibre is not capable of compensating exactly the chromatic dispersion accumulated along such span of 100 km in case of variations of the chromatic dispersion coefficient Dt of the SMF fibre with respect to the nominal value due, for example, to the variation of system parameters, such as temperature.
Even though in this latter case the variation of the value of chromatic dispersion Dt*Lt of the optical transmission fibre generally is irrelevant in an optical communication system with moderate bit rates, it becomes very important at high bit rates (2,5, 10, 40, 80 Gbit/s), at which an increasingly higher precision of the chromatic dispersion compensation is required.
There is thus the need for a unit for compensating the chromatic dispersion, suitable to be reconfigured so as to compensate, according to requirements, different values of chromatic dispersion Dt*Lt accumulated along an optical transmission fibre.
B. J. Eggleton et al. (“
Tunable dispersion compensation in a
160-
Gb/s TDM system by a voltage controlled chirped fiber Bragg grating
”, IEEE Photonics Technology Letters, Vol. 12, No. 8, August 2000, pages 1022-1024) describe an integrated chirp-tunable Bragg grating for compensating chromatic dispersion in a dynamic manner and capable of recovering 2 ps pulses over a 50 ps
m tuning interval, with a system penalty that is less than 1.3 dB.
S. T. Vohra et al (“
Dynamic dispersion compensation using bandwidth tunable fiber Bragg gratings
”, ECOC 2000) describe a device for compensating chromatic dispersion in a tunable manner, realised with a fibre Bragg grating and capable of compensating chromatic dispersion in a tuning interval from −150 ps
m to −3500 ps
m.
However, the above devices are not capable of compensating also the third order dispersion or chromatic dispersion slope (or slope) according to which light pulses at different wavelength propagate in an optical fibre with different dispersions.
This phenomenon, caused by the chromatic dispersion being a phenomenon depending on the wavelength, is a problem in wavelength division multiplexing (or WDM) optical communication systems, where information is carried along the same optical fibre by a plurality of optical signals at different wavelength.
Thus, in WDM optical communication systems, it is necessary to compensate not only chromatic dispersion but also chromatic dispersion slope in the interval of wavelengths of interest.
Devices for compensating both chromatic dispersion and chromatic dispersion slope of a conventional single mode fibre (or SMF) are known.
For example, to compensate both a chromatic dispersion coefficient Dt and a chromatic dispersion slope st, there are known devices comprising an optical fibre specifically designed to have very high values of the chromatic dispersion coefficient Dc and of chromatic dispersion slope sc, with opposed sign with respect to those of the SMF optical fibre of which dispersion is to be compensated, so that relation Dt/st=Dc/sc is satisfied (T. Kashiwada et al., “
Broadband dispersion compensating module considering its attenuation spectrum behavior for WDM system
”, OFC '99, WM12, pages 229-231; G. E. Berkey et al., “
Negative slope dispersion compensating fibers
”, OFC '99, WM14, pages 235-237 and L. Gruner-Nielsen et al., “Design and manufacture of dispersion compensating fibre for simultaneous compensation of dispersion and dispersion slope”, OFC '99, Technical Digest WM13, pages 232-234).
In fact, it is known that both the following relations must be satisfied for compensating both the chromatic dispersion coefficient Dt and the chromatic dispersion slope st
Dt*Lt+Dc*Lc
=0
st*Lt+sc*Lc
=0
that is, the Dt/st ratio must be equal to the Dc/sc ratio.
Moreover, the dispersive properties of a holey optical fibre have been studied in recent years.
A holey optical fibre typically consists of a single material in
Gorni Giacomo
Romagnoli Marco
Socci Luciano
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Knauss Scott Alan
Sanghavi Hemang
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