Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1997-10-17
2001-09-18
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S037000
Reexamination Certificate
active
06292601
ABSTRACT:
The invention relates to dispersion compensation in optical fibre transmission.
Data transmission in optical fibres is generally limited by power loss and pulse dispersion. The advent of erbium-doped fibre amplifiers (EDFAs) has effectively removed the loss limitation for systems operating in the third optical communication window (around a wavelength of about 1.55 &mgr;m (micrometer)), leaving pulse dispersion as a serious limitation, especially in future high-capacity multi-wavelength optical networks.
More importantly, most fibre which has already been installed for telecommunication links (ie. standard non- dispersion shifted fibre) exhibits a dispersion zero around 1.3 &mgr;m and thus exhibits high (about 17 ps
m.km (picosecond per nanometer-kilometer)) dispersion 1.55 &mgr;m. Upgrading this fibre to higher bit rates involves the use of EDFAs and a shift in operating wavelength to 1.55 &mgr;m where dispersion-compensation becomes a necessity.
Several techniques have been demonstrated including laser pre-chirping (reference 1—below), mid-span spectral-inversion (phase-conjugation) (reference 2—below), the addition of highly-dispersive compensating fibre (reference 3—below) and chirped fibre gratings (references 4 to 7—below). Chirped fibre gratings are of particular interest, since they are compact, low-loss and offer high negative-dispersion of arbitrary and tunable profile. In separate experiments 450 fs (femtosecond) pulses have been successfully reconstructed after transmission through 245m of fibre (reference 4—below), and gratings with dispersion equivalent to 20 km and 1 km of standard fibre have been fabricated (references 5 and 6—below). Whilst more recently a grating has been employed to compensate the dispersion of 160 km of standard fibre in a 10 Gbits
−1
(gigabits per second) externally modulated experiment (reference 7—below) although no information of the grating strength was given in this case.
It is a constant aim to improve dispersion compensation techniques in optical fibre transmission systems.
The article in IEEE Photonics Technology Letters, April 1993, USA, vol. 5, no. 4, pages 425-427, Farre J et al: “Design of bidirectional communication systems with optical amplifiers”, discloses the use of optical amplifiers at various positions in an optical fibre link.
The article in Optics Letters, vol. 19, no. 23, Dec. 1, 1994, Washington US, pages 2027-2029, Lauzon et al: “Implementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradient”, discloses (as the title suggests) the manufacture of chirped fibre gratings by imposing a temperature gradient onto a fibre grating.
GB-A-2 161 612 discloses a chirped fibre grating for dispension compensation in an optical fibre link.
This invention provides an optical transmitter for use with an optical fibre transmission link, the transmitter comprising:
a light source capable of direct or indirect modulation; and
an optical amplifier;
characterised by:
a chirped grating to provide compensation for the dispersion characteristics of the link over the range of wavelengths of the modulated light source.
Preferably the optical amplifier is operable in a saturation mode.
It is advantageous to position the compensating grating at the input end of the link, since in this position the optical input signal is still relatively large and thus a relatively insignificant noise penalty is incurred. In addition, if the grating's (compensated) output is then routed to an optical amplifier operating in saturation, the amplifier's output power will be effectively unaltered by the presence of the compensating grating.
The skilled man will appreciate that the dispersion compensation in this context need not be complete, but simply that the non-linear response of the grating acts against the dispersion characteristics of the transmission link.
This invention also provides an optical fibre transmission system comprising:
an optical fibre transmission link; and
an optical amplifier disposed at an input end of the link;
characterised by:
a chirped grating disposed at the input end of the link, the chirped grating providing compensation against the dispersion characteristics of the link.
The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:
FIG. 1
is a schematic diagram of a dispersion compensating optical fibre transmission system;
FIG. 2
schematically illustrates the spectrum of a DFB (distributed feedback) laser transmitter;
FIGS. 3
a
to
3
c
schematically illustrate the reflectivity spectra of a fibre grating as written (
FIG. 3
a
), with a temperature gradient set to add to the existing chirp (
FIG. 3
b
) and with a temperature gradient set to reverse the existing chirp (
FIG. 3
c
).
FIGS. 4
a
to
4
c
schematically illustrate the time delay of the gratings of
FIGS. 3
a
to
3
c
respectively;
FIGS. 5
a
and
5
b
schematically illustrate sampling oscilloscope traces of an approximately 10 ps, 0.318 nm spectral halfwidth signal after propagation through 50 km of standard fibre without compensation (
FIG. 5
a
) and with compensation (
FIG. 5
b
);
FIG. 6
schematically illustrates bit error rate (BER) curves for the system of
FIG. 1
;
FIG. 7
schematically illustrates a transmission penalty at a 10
−9
BER as a function of span length with and without dispersion compensation; and
FIGS. 8
a
to
8
f
schematically illustrate eye diagrams showing the different results obtained without (
FIGS. 8
a
to
8
c
) and with
FIGS. 8
d
to
8
f
) dispersion compensation.
REFERENCES:
patent: 5673129 (1997-09-01), Mizrahi
patent: 5701188 (1997-12-01), Shigematsu et al.
patent: 5867304 (1999-02-01), Galvanuskas et al.
patent: 2161612 (1986-01-01), None
Lauzon et al: “Implementation and characterzation of fiber Bragg gratings Linearly chirped by a temperature gradient”. Optics Letters, vol. 19, No. 23. pp. 2027-2029, Sep. 1994.*
Garthe et al. Proceedings of ECOC' 94. 20th European Conference on Optical Communications, Firenze, Italy. pp. 11-14, Sep. 1994.*
Farre et al. IEEE Photonics Technology Letters. vol. 5, No. 4. pp. 425-427, Apr. 1993.*
B. Wedding et al., “Dispersion Supported Transmission at 10Gbit/s Via Up To 253km Of Standard Single-Mode Fibre,” Proc., ECOC '93, Vol. 2, Paper TuC4.3, pp. 101-104 (1993).
R. I. Laming et al., “Transmission of 6ps Linear Pulses Over 50km of Standard Fibre Using Mid-Point Spectral Inversion to Eliminate Dispersion,” IEEE Journal of Quantum Electronics, Vol. 3, No. 9, pp. 2114-2118 (1994).
M. Onishi et al., “High Performance Dispersion-Compensating Fiber and its Application to Upgrading of 1.31 &mgr;m Optimized System,” Proc., ECOC '93, Vol. 2, paper WcC8.5, pp. 357-360 (1993).
R. Kashyap et al., “30ps Chromatic Dispersion Compensation of 400fs Pulses at 100Gbits/s in Optical Fibres Using an All Fibre Photoinduced Chirped Reflection Grating,” Electronics Letters, Vol. 30, No. 13, pp. 1078-1080 (1994).
K. O. Hill et al., “Aperiodic In-Fiber Bragg Gratings For Optical Fiber Dispersion Compensation,” OFC'94, Optical Fiber Communication, Technical Digest, PD2, pp. 17-20 (1994).
J.A.R. Williams, et al., “Fibre Dispersion Compensation Using a Chirped In-Fibre Bragg Grating,” Electr. Lett., Vol. 30 (12), pp. 985-987 (1994).
B. Malo et al., “Dispersion Compensation of a 100 km, 10Gbit/s Optical Fiber Link Using a Chirped In-Fiber Bragg Grating With a Linear Dispersion Characteristic,” Proceedings, ECOC '94, Vol. 4, pp. 23-26 (1994).
Cole Martin
Ian Laming Richard
Reekie Laurence
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Ngo Hung N.
Pirelli Cavi E Sistemi S.p.A.
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