Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
2002-07-25
2004-02-17
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Modulation
C372S031000, C372S029023
Reexamination Certificate
active
06693929
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to the field of optical telecommunications, and in particular, to a method and apparatus for generating optical chirped return-to-zero (CRZ) data signals.
BACKGROUND OF THE INVENTION
Optical CRZ signals, i.e., signals in which (a) the optical intensity returns to zero during each bit period, and (b) the optical phase varies (by a desired amount) within each bit period, are important for ultra long distance optical fiber communications. Such signals can be advantageously used e.g., in undersea lightwave systems. For this class of systems, CRZ has been shown to yield high tolerance towards impairments caused by fiber nonlinearities. See, for example, N. S. Bergano, “Undersea Communication Systems”, in
Optical Fiber Telecommunications IV
B, Academic Press, 2002.
One known arrangement for generating CRZ signals uses three optical modulators, as shown in FIG.
1
. The first optical data modulator
102
imprints data modulation (among many other possible formats, e.g., non-return-to-zero on-off keying (NRZ-OOK), or NRZ-differential phase shift keying (NRZ-DPSK)) on the light emitted by a continuously operating laser
101
. The second modulator
103
is a return-to-zero (RZ) pulse carver that carves RZ pulses out of the optical (usually NRZ) data signal. As indicated in
FIG. 1
, modulator
103
is frequently realized as a dual-drive Mach-Zehnder modulator using sinusoidal drive signals at either the data rate or at half the data rate. The third modulator
104
is a sinusoidally driven phase modulator that generates the desired amount of bit-synchronuous phase modulation, i.e. a phase term
exp[
jm
cos(2
&pgr;Rt
)], (1)
where m is called the modulation index.
Another known way of generating CRZ can be found in a paper by F. Liu et al., “Chirped return-to-zero source used in 8×10 Gbit/s transmission over 2000 km of standard singlemode fiber”,
Electron. Lett
. 36 (16), 1399-1400, 2000, where a single Mach-Zehnder modulator is used to generate the data modulation, carve RZ pulses, and generate chirp. However, the maximum phase excursion obtainable from that scheme is limited to ~0.5 &pgr; without significantly degrading the data modulation.
SUMMARY OF THE INVENTION
In accordance with the present invention, optical chirped return-to-zero (CRZ) data signals are advantageously generated without the need for a separate phase modulator
104
, as required in the prior art, by using a dual-drive Mach-Zehnder modulator (such as modulator
103
in
FIG. 1
) for RZ pulse carving that is driven with two typically sinusoidal signals of either unequal amplitude or unequal relative phase (i.e. with some non-vanishing phase difference).
REFERENCES:
patent: 2003/0053728 (2003-03-01), Yang
patent: 2003/0133652 (2003-07-01), Andrekson et al.
N. S. Bergano, “Undersea Communication Systems”,Optical Fiber Telecommunications, vol. IVB, Elsevier Science (USA) 2002, pp 154-197.
F. Liu et al, “Chirped Return-To-Zero Source Used In 8×10 Gbit/s Transmission Over 2000 km of Standard Singlemode Fiber”,Electron. Lett.36, (16), 2000, pp 1399-1400.
R. Ohhira et al, “Novel RZ Signal Format With Alternate-Chirp For Suppression of Nonlinear Degradation in 40 Gb/s Based WDM”,Proc. OFC '01,Paper WM2, 2001.
Essiambre Rene'-Jean
Winzer Peter J.
Freedman Barry H.
Jr. Leon Scott
Sasso David
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