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United States Patent |
6,061,483
|
Laming
,   et al.
|
May 9, 2000
|
Dispersion compensation in optical fibre transmission
Abstract
An optical fiber transmission system comprises an optical signal source
operable to generate an optical signal at a predetermined bit rate and at
a signal wavelength; an optical fiber transmission link connected at a
first end to the signal source, the link having dispersion characteristics
at the signal wavelength; an optical amplifier serially disposed in the
link; and a signal receiver connected at the second end of the Lank; in
which a grating is connected in the link, the grating being chirped by an
amount providing at least partial compensation of the dispersion
characteristics of the link, the compensation such as to provide a signal,
at the second of the link, compatible with the sensitivity requirements of
the receiver at the second end of the link.
Inventors:
|
Laming; Richard Ian (Southampton, GB);
Cole; Martin (Southampton, GB);
Reekie; Laurence (Southampton, GB)
|
Assignee:
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Pirelli Cavi E Sistemi S.p.A. (Milan, IT)
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Appl. No.:
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860996 |
Filed:
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October 17, 1997 |
PCT Filed:
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January 29, 1996
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PCT NO:
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PCT/GB96/00189
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371 Date:
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October 17, 1997
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102(e) Date:
|
October 17, 1997
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PCT PUB.NO.:
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WO96/23372 |
PCT PUB. Date:
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August 1, 1996 |
Foreign Application Priority Data
Intern'l Class: |
G02B 006/28 |
Field of Search: |
359/188,173,127,130,134,161
385/15,24,37
|
References Cited
U.S. Patent Documents
5673129 | Sep., 1997 | Mizrahi | 359/124.
|
5701188 | Dec., 1997 | Shigematsu et al. | 359/161.
|
5867304 | Feb., 1999 | Galvanuskas et al. | 359/333.
|
Foreign Patent Documents |
2161612 | Jan., 1986 | GB.
| |
Other References
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.
|
Primary Examiner: Ngo; Hung N.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner, L.L.P.
Claims
What is claimed is:
1. 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;
characterized by:
a chirped grating to provide compensation for the dispersion
characteristics of the link over the range of wavelengths of the modulated
light source.
2. A transmitter according to claim 1, in which the amplifier is disposed
between the grating and the transmission link.
3. A transmitter according to claim 2, in which the amplifier is operable
in a saturation mode.
4. A transmitter according to any one of claims 1 to 3, in which the
grating is a fibre grating.
5. A transmitter according to claim 4, in which the grating is a reflection
fibre grating.
6. A transmitter according to claim 4, in which the chirped grating is
formed by applying a temperature gradient to a portion of optical fibre on
which a non-linear grating is impressed, the variation induced by the
temperature gradient acting against the non-linear variation of the
impressed grating.
7. A transmitter according to claim 6, in which the temperature gradient at
least negates the non-linear variation of the impressed grating, thereby
generating a grating having a non-linear variation in the opposite sense
to the impressed grating.
8. A transmitter according to any one of claims 1-3, comprising an optical
circulator for receiving the modulated optical output of the light source,
the circulator being connected to route the modulated optical output to
the chirped grating and to route optical signals from the chirped grating
to an output port;
the optical amplifier being connected to receive optical signals from the
output port of the optical circulator.
9. An optical fibre transmission system comprising:
an optical transmitter according to any one of claims 1-3, the transmitter
being operable to generate optical signals in dependence on input data;
an optical fibre transmission line for propagating optical signals
generated by the transmitter; and
an optical receiver for converting optical signals output from the
transmission link into corresponding electrical data signals.
10. An optical fibre transmission system comprising:
an optical fibre transmission link; and
an optical fibre amplifier disposed at an input end of the link;
characterized by:
a chirped grating disposed at the input end of the link, the chirped
grating providing compensation against the dispersion characteristics of
the link.
11. A system according to claim 10, in which the optical signal at a distal
end of the link has a dispersion causing a penalty in sensitivity at the
receiver of less than 8.5 decibels at a bit error rate of 10.sup.-9 in a
link longer than 200 kilometers.
12. A system according to claim 10, in which the optical amplifier is
operable in a saturation mode.
13. A system according to claim 10, comprising a variable attenuator
connected between the output of the transmission link and the optical
receiver.
Description
This 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 .mu.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 .mu.m and thus exhibits high (about
17 ps/nm.km (picosecond per nanometer-kilometer)) dispersion around 1.55
.mu.m. Upgrading this fibre to higher bit rates involves the use of EDFAs
and a shift in operating wavelength to 1.55 .mu.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 245 m 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.sup.-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 dispersion
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. 3a to 3c schematically illustrate the reflectivity spectra of a fibre
grating as written (FIG. 3a), with a temperature gradient set to add to
the existing chirp (FIG. 3b) and with a temperature gradient set to
reverse the existing chirp (FIG. 3c).
FIGS. 4a to 4c schematically illustrate the time delay of the gratings of
FIGS. 3a to 3c respectively;
FIGS. 5a and 5b 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. 5a) and with
compensation (FIG. 5b);
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.sup.-9 BER
as a function of span length with and without dispersion compensation; and
FIGS. 8a to 8f schematically illustrate eye diagrams showing the different
results obtained without (FIGS. 8a to 8c) and with (FIGS. 8d to 8f)
dispersion compensation.
Referring now to FIG. 1, in this embodiment a chirped fibre grating 10 was
incorporated into a 2.5 Gbits.sup.-1 directly-modulated system operating
at 1536 nm. (However, in other embodiments and in the description below,
an indirectly modulated transmitter could be used instead). As a
consequence of the direct modulation the output of the DFB laser
transmitter 20 was chirped and exhibited a 3 dB optical bandwidth of 0.1
nm and a 10 dB (decibel) bandwidth of 0.24 nm, ie equivalent to a 10
Gbit/s modulation signal.
The transmitter 20 was supplied with data from a commercial multiplexer
(not shown) from a Phillips SDH (synchronous digital hierarchy) 2.5
Gbits.sup.-1 system. The multiplexer combines 16 channels of data at 140
Mbits.sup.-1 (megabits per second) up to a line rate of 2.5 Gbits.sup.-1.
In the absence of data on any channel, the multiplexer generates random
data. Random data was input to several of the channels whilst, on the test
channel, pseudorandom data at 140 Mbits.sup.-1 with a 2.sup.23 -1 pattern
length (generated by a BER test set 110) was employed. However, in a real
application, it will of course be appreciated that real input data would
be supplied to the transmitter instead of the pseudorandom data from the
BER test set.
The transmitter 20 consisted of a directly-modulated DFB laser with
wavelength centred at 1536 nm and whose chirped output had a 3 dB
bandwidth of 0.108 nm and 10 dB bandwidth of 0.24 nm. The spectral
characteristics of the transmitter output are illustrated schematically in
FIG. 2. As a consequence of this chirp (and the fibre dispersion) a
penalty was observed for transmission distances in standard fibre in
excess of a few tens of km.
The transmitter was followed by a single-stage, 980 nm-pumped erbium-doped
power-amplifier 30 giving an output power of +12 dBm (decibels relative to
1 milliwatt) which was transmitted through standard fibre having lengths
of 100, 143 and 200 km. In the latter case (as illustrated in FIG. 1), a
dual-stage 980 nm-pumped line amplifier 40 giving an output power of +13
dBm was incorporated between two series connected 100 km lengths of fibre
50, 60.
The output of the link was coupled via a variable attenuator 70 to a
commercial, Phillips, receiver and demultiplexer 80, the output of which
was in turn passed to the BER test set 110 for BER measurement (by
comparison with the test data supplied to the transmitter 20 by the BER
generator 110).
Dispersion-compensation of the link was provided by incorporating the
chirped fibre grating 10 between the transmitter 20 and power amplifier
30. Since the grating 10 operates in reflection, an optical circulator 90
was included to convert it to a transmission device. The grating was
connected to the circulator using so-called NTT FC/PC compatible
connectors (not shown). However, to ensure successful operation, index
matching liquid (not shown) was inserted in the connection to minimise
reflections.
Power levels in the link are such that it is operating in the so-called
linear regime, thus the dispersion compensation could in theory be
performed at any location in the link. However it is advantageous to
incorporate the grating in its present location (before the fibre lengths
50, 60) since the input signal to the power amplifier is still relatively
large and thus a relatively insignificant noise penalty is incurred. In
addition, since this amplifier 30 is then operating in saturation the
output power will be effectively unaltered. Alternatively, if the
dispersion compensation had been included immediately prior to the
receiver a penalty would have been incurred due to its insertion loss.
The fibre grating was written using standard techniques with a
frequency-doubled excimer laser in a germania-boron co-doped fibre (0.1 NA
(numerical aperture), 1 .mu.m .lambda..sub.cutoff (cutoff wavelength)).
The grating was about 20 mm in length with an approximately Gaussian
strength profile and about 70% peak reflectivity. In its "as-written"
state it had some residual chirp and a measured bandwidth of about 0.2 nm.
The grating was further chirped to a 3 dB bandwidth of about 0.3 nm by
applying a linear temperature gradient. The temperature gradient could be
applied to either add to or reverse the existing chirp.
Surprisingly superior performance was obtained when the temperature
gradient was applied to reverse the existing chirp of the grating. This
was due to the slightly non-linear characteristics of the existing chirp.
In other words, the chirped optical fibre grating is formed by applying a
temperature gradient to a portion of optical fibre on which a non-linear
grating is impressed, the variation induced by the temperature gradient
acting against the non-linear variation of the impressed grating, and in
particular where the temperature gradient at least negates the non-linear
variation of the impressed grating, thereby generating a grating having a
non-linear variation in the opposite sense to the impressed grating.
FIGS. 3a to 3c show the grating spectral response as written (FIG. 3a),
with the temperature gradient set to add to (FIG. 3b) and reverse (FIG.
3c) the existing chirp. A slight dip is noted in FIG. 3b.
FIGS. 4a to 4c show the time delay of the gratings, measured using an
standard interferometric set up, corresponding to the respective cases
illustrated in FIGS. 3a to 3c. (Since the measurements were performed on
different instruments there is a slight mismatch in indicated wavelengths.
Also, all three measurements were taken from the same end of the grating
and thus in the grating of FIG. 4b the grating was tested by the
interferometer in the opposite direction to its direction of use in the
embodiment of FIG. 1).
As stated, case b (i.e. as shown in FIGS. 3b and 4b) did not tend to give
stable link performance and thus a temperature profile 100 indicated in
FIG. 1 (case c, i.e. as indicated in FIGS. 3c and 4c) was employed. The
centre wavelength of the chirped grating was also tuned to match the laser
wavelength of 1536 nm. Once chirped, the grating reflectivity reduced and
thus the circulator-grating combination exhibited an insertion loss of 3.5
dB, but owing to its location at the output of the transmitter 20 and
before the amplifier 30, this had a negligible effect on the link's
power-budget.
Separate measurements involving compensation for the propagation of about
10 ps pulses over 50 and 100 km using this grating showed structure in the
compressed pulses, indicating phase-distortion of the pulses and thus
non-perfect compensation of the dispersion. However, owing to the
non-transform-limited data (chirped source) an improvement in system
performance was nevertheless obtained.
FIG. 5a shows a sampling oscilloscope trace of an approximately 10 ps,
0.318 nm spectral halfwidth pulse after propagation through 50 km of
standard fibre. The pulse can be seen to have broadened to about 281 ps.
After recompression with the grating, FIG. 5b, the pulse width is seen to
be reduced to about 39 ps. However, structure can be seen, particularly on
the leading edge of the pulse which might be detrimental at higher bit
rates.
Bit-error-rate (BER) curves for the system are shown in FIG. 6. Data are
given for back-to-back and direct transmission through 100, 143 and 200 km
of standard fibre. Dispersion-equalised curves, with the chirped grating
included, are given for back-to-back and transmission through 100 and 200
km of fibre.
In the case of direct transmission, a back-to-back sensitivity of -32.7 dBm
at a 10.sup.-9 BER is observed. At this error rate a penalty of 1.3 dB was
found at 100 km, increasing to 3.3 and 8.5 dB at 143 and 200 km,
respectively.
The increase in penalty with distance is shown again in FIG. 7. With the
grating incorporated, the back-to-back sensitivity is actually improved by
1.2 dB, since the grating compresses the chirped-source pulses. The
grating virtually eradicates the penalty at 100 km (0.5 dB) and
significantly reduces the penalty at 200 km to only 3 dB. No floor in the
error-rate curves was observed when using the grating.
The increase in penalty with distance in this case can be compared with the
direct result in FIG. 7, where it can be seen that the grating dispersion
is equivalent (but opposite in sign) to around 60 km of standard fibre.
This result is in substantial agreement with the delay data illustrated in
FIG. 4c.
Receiver eye diagrams are shown for various points in the system in FIG. 8.
Although the interpretation of the eye diagrams is always subjective, the
skilled man will appreciate that the beneficial effect of using the
grating 10 can be seen.
In summary, dispersion-compensation using a chirped fibre grating has been
successfully demonstrated in a 200 km standard-fibre transmission
experiment using a 2.5 Gbits.sup.-1 1.55 .mu.m directly-modulated
transmitter. The about 20 mm long, 0.3 nm chirped grating 10 effectively
compensated for about 60 km of standard fibre (i.e. fibre having a
dispersion zero around 1.3 .mu.m and about 17 ps/nm.km dispersion around
1.55 .mu.m), as anticipated. These results demonstrate that a
non-uniformly chirped grating could provide significant improvements in
current, directly modulated commercial systems.
Thus, an approximately 20 mm (millimeter) long grating with substantially
linear chirp, to give a 0.3 nm 3 dB bandwidth, substantially negates the
dispersion of about 60 km of standard fibre. This allowed transmission
through 200 km of standard fibre with a 3 dB penalty, which compares with
an approximately 8.5 dB penalty without the compensation.
In summary, therefore, embodiments of the invention make use of a grating
is connected (in a dispersion compensating fashion e.g. using an optical
circulator) in an optical fibre link, where the grating is chirped by an
amount providing at least partial compensation of the dispersion
characteristics of the link, to provide an output signal from the link
compatible with the sensitivity requirements of the receiver at the second
end of the link. In particular, in embodiments of the invention, the
optical signal at the output end of the link can be made to have a
dispersion causing a penalty in sensitivity at the receiver of less than
8.5 decibels at a bit error rate of 10.sup.-9 in a link longer than 200
kilometers.
Publication References
1. B. Wedding, B. Franz and B. Junginer, "Dispersion supported transmission
at 10 Gbit/s via up to 253 km of standard single-mode fibre", Proc. ECOC,
Sep. 12-16, 1993, paper TuC4.3.
2. R. I. Laming, D. J. Richardson, D. Traverner and D. N. Payne,
"Transmission of 6 ps linear pulses over 50 km of standard fibre using
mid-point spectral inversion to eliminate dispersion." IEEE Jnl. of
Quantum Electronics, Vol. 30, 1994. pp.2114-2119.
3. M. Onishi, H. Ishikawa, T. Kashiwada, K. Nakazato, A. Fukuda, H.
Kanainori and M. Nishimura. "High performance dispersion-compensating
fiber and its application to upgrading of 1.31 .mu.m optimized system",
Proc. ECOC, Sep. 12-16, 1993, paper WoC8.5.
4. R. Kashyap, S. V. Chernikov, P. F. McKee and J. R. Taylor, "30 ps
chromatic dispersion compensation of 400 fs pulses at 100 Gbits/s in
optical fibres using an all fibre photoinduced chirped reflection
grating", Electronics Letters, Vol. 30, No. 13, pp. 1078-1080, 1994.
5. K. O. Hill, F. Bilodeau, B. Malo, T. Kitagawa, S. Theriault, D. C.
Johnson, J. Albert and K. Takiguchi, "A periodic in-fibre Bragg gratings
for optical fibre dispersion compensation", Proc. OFC'94, PD2, pp. 17-20.
6. J. A. R. Williams, I. Bennion, K. Sugden and N. J. Doran, "Fibre
dispersion compensation using a chirped in-fibre grating", Electr. Lett.,
Vol.30(12), 1994, pp.985-987.
7. D. Garthe, W. S. Lee, R. E. Epworth, T. Bricheno and C. P. Chew,
"Practical dispersion equaliser based on fibre gratings with a bit-rate
length product of 1-6 TB/s.km" Proc. ECOC Vol. 4 (Postdeadline papers),
pp.11-14, Sep. 25-29, 1994,
8. B. Malo, K. O. Hill, S. Theriault, F. Bilodeau, T. Kitagawa, D. C.
Johnson, J. Albert, K. Takiguchi, T. Kataoka and K. Hagimoto, "Dispersion
compensation of a 100 km, 10 Gbit/s optical fiber link using a chirped
in-fiber Bragg grating with a linear dispersion characteristic", ibid.,
pp. 23-26.
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