U.S. patent application number 10/205643 was filed with the patent office on 2004-02-12 for arrangement for generating chirped return to zero (crz) optical data signals.
Invention is credited to -Jean Essiambre, Rene?apos, Winzer, Peter J..
Application Number | 20040028417 10/205643 |
Document ID | / |
Family ID | 31186610 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040028417 |
Kind Code |
A1 |
Essiambre, Rene?apos;-Jean ;
et al. |
February 12, 2004 |
ARRANGEMENT FOR GENERATING CHIRPED RETURN TO ZERO (CRZ) OPTICAL
DATA SIGNALS
Abstract
Optical chirped return-to-zero (CRZ) data signals are generated
without the need for a separate phase modulator, by using a
dual-drive Mach-Zehnder modulator for RZ pulse carving that is
driven with two typically sinusoidal signals of either unequal
amplitude or unequal relative phase, i.e. of non-vanishing phase
difference.
Inventors: |
Essiambre, Rene?apos;-Jean;
(Red Bank, NJ) ; Winzer, Peter J.; (Nuremberg,
DE) |
Correspondence
Address: |
Docket Administrator (Room 3J-219)
Lucent Technologies Inc.
101 Crawfords Corner Road
Holmdel
NJ
07733-3030
US
|
Family ID: |
31186610 |
Appl. No.: |
10/205643 |
Filed: |
July 25, 2002 |
Current U.S.
Class: |
398/188 |
Current CPC
Class: |
G02F 1/225 20130101;
G02F 2201/126 20130101; H04B 10/505 20130101; H04B 10/508 20130101;
H04B 10/5051 20130101 |
Class at
Publication: |
398/188 |
International
Class: |
H01S 003/10 |
Claims
1. Apparatus for generating optical chirped return-to-zero (CRZ)
data signals, said apparatus comprising a laser, a first modulator
for imprinting the output of said laser with a data signal to
produce a modulated optical data signal, and means for applying
said modulated optical data signal to a Mach-Zehnder modulator
having first and second arms that are driven by first and second
drive signals, u.sub.1(t) and u.sub.2(t), respectively, wherein
said first and second drive signals are imbalanced.
2. The invention defined in claim 1 where said drive signals have
imbalanced amplitudes.
3. The invention defined in claim 1 where said drive signals have
imbalanced phases.
4. The invention defined in claim 2 or 3 wherein said modulated
optical data signal is generated by a first modulator chosen from
the group consisting of an NRZ OOK modulator, a DPSK modulator, a
duobinary or modified duobinary modulator, a vestigial sideband
modulator and a QPSK modulator.
5. The invention defined in claim 2, wherein the pulse carver is
driven with two periodic signals of period 1/R or 2/R, where R is
the data rate, and RZ pulses with desired shape and duty cycle are
generated.
6. The invention defined in claim 5 wherein said periodic signals
are u.sub.1(t)=U.sub.1 cos[2.pi.Rt], and u.sub.2(t)=U.sub.2
cos[2.pi.Rt].
7. The invention defined in claim 6 wherein RZ pulses with duty
cycles between 50% and 33% are generated by selecting the
difference of the two drive amplitudes,
U.sub..DELTA.=U.sub.1-U.sub.2, in the range from V.sub..pi./2 to
0.
8. The invention defined in claim 5 wherein said periodic signals
are u.sub.1(t)=U.sub.1 cos[.pi.Rt], and u.sub.2(t)=U.sub.2
cos[.pi.Rt].
9. The invention defined in claim 8 wherein RZ pulses with duty
cycles between 67% and 50% are generated by selecting the
difference of the two drive amplitudes,
U.sub..DELTA.=U.sub.1-U.sub.2, in the range from V.sub..pi. to
0.
10. The invention defined in claim 3, wherein the pulse carver is
driven at the data rate R/2, and wherein the drive signals are
arranged such that u.sub.1(t)=U.sub.1 cos[.pi.Rt], and
u.sub.2(t)=U.sub.2 cos[.pi.Rt+.delta.], where .delta. represents a
non-zero phase angle between said two drive signals.
Description
FIELD OF INVENTION
[0001] 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
[0002] 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.
[0003] 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[j m cos(2.pi.Rt)], (1)
[0004] where m is called the modulation index.
[0005] 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.times.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 .about.0.5.pi.
without significantly degrading the data modulation.
SUMMARY OF THE INVENTION
[0006] 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).
BRIEF DESCRIPTION OF THE DRAWING
[0007] The present invention will be more fully appreciated by
consideration of the following detailed description, which should
be read in light of the drawing in which:
[0008] FIG. 1 is a block diagram of a prior art arrangement for
generating CRZ signals that uses three optical modulators;
[0009] FIG. 2 is a block diagram of an arrangement in accordance
with the present invention, for generating CRZ signals that does
not need a separate phase modulator, as required in the prior art
arrangement of FIG. 1;
[0010] FIG. 3 is a diagram illustrating the optical intensity and
chirped optical phase waveforms of signals generated using the
arrangement of FIG. 2, in which the pulse carver is sinusoidally
driven at the data rate R to generate RZ with around 50% duty
cycle;
[0011] FIG. 4 is similar to FIG. 3, but the pulse carver is
sinusoidally driven at half the data rate (R/2) and biased for zero
transmission, carrier-suppressed RZ (CSRZ), and
[0012] FIG. 5 is similar to FIG. 3, but the pulse carver is
sinusoidally driven at half the data rate (R/2) and biased at
maximum transmission to generate RZ with 33% duty cycle.
DETAILED DESCRIPTION OF THE INVENTION
[0013] In accordance with the present invention, we have eliminated
the need for a phase modulator for generating CRZ signals, by
imbalancing the drive signals of the Mach-Zehnder modulator used
for RZ pulse carving. As shown in FIG. 2, the light of a laser 201
is modulated with, for example, NRZ data, using a data modulator
202, to produce a modulated optical data signal. (It is to be noted
here that modulator 202 can be any known modulator arranged to
generate a modulated optical data signal, such as an NRZ OOK
modulator, an NRZ DPSK modulator, a duobinary or modified duobinary
modulator, a vestigial sideband modulator or a QPSK modulator,
etc.) The modulated optical data signal output from modulator 202
is applied to a RZ pulse carver 203, which includes a Mach-Zehnder
modulator 204 having first and second arms 205, 207, driven by
first and second drive signals applied via control inputs or
electrodes 206, 208, respectively. By arranging the drive signals
to be imbalanced in phase or amplitude, the output of pulse carver
203 is the desired CRZ signal.
[0014] The invention is based on the following principle: The
optical field e.sub.RZ(t) leaving the Mach-Zehnder pulse carver 203
is generally given by
e.sub.RZ(t)=e.sub.NRZ(t)exp[j.pi.(u.sub.1(t)+u.sub.2(t))/(2V.sub..pi.)]cos-
[.pi.(u.sub.1(t)-u.sub.2(t))/(2V.sub..pi.)+.phi..sub.bias]. (2)
[0015] where e.sub.NRZ(t) denotes the NRZ modulated field generated
by NRZ modulator 202 entering the pulse carver, and u.sub.1(t) and
u.sub.2(t) are the drive voltage signals 206, 208 of the two
modulator arms, 205, 207, respectively. These voltages are most
conveniently chosen sinusoidal, having a frequency of either the
data rate R or of R/2. The drive voltage required to achieve a
phase shift of .pi. in one of the modulator's arms is denoted
V.sub..pi., and .phi..sub.bias is the modulator bias.
[0016] There are basically three types of RZ pulse carver driving
to be distinguished, as shown in FIGS. 3, 4 and 5:
[0017] A. Pulse Carver Driven at the Data Rate R
[0018] If the pulse carver is sinusoidally driven at the data rate
R with
u.sub.1(t)=U.sub.1 cos[2.pi.Rt], and u.sub.2(t)=U.sub.2
cos[2.pi.Rt], (3)
[0019] and .phi..sub.bias is adjusted for maximum pulse extinction,
RZ pulses with duty cycles between 50% and 33% can be generated by
varying the difference of the two drive amplitudes,
U.sub..DELTA.=U.sub.1-U.sub.2- , from V.sub..pi./2 to 0. Usually,
U.sub.1=-U.sub.2=V.sub..pi./4 (or, equivalently,
U.sub..DELTA.=V.sub..pi./2) is chosen for chirp-free, 50% RZ pulse
carving. Imbalancing the drive amplitudes under the constraint that
their difference U.sub..DELTA. remains constant does not change the
intensity waveforms of the RZ pulses, but generates a
bit-synchronuous, sinusoidal phase modulation with maximum phase
excursions at the pulse center, as a consequence of the exponential
term in Equation (2), which then reads
exp[j.pi.(U.sub.1+U.sub.2)/(2V.sub..pi.)cos(2.pi.Rt)]. (4)
[0020] Comparing this expression to Equation (1), we identify the
phase modulation index m as
m=.pi.(U.sub.1+U.sub.2)/(2V.sub..pi.)=.pi.U.sub..SIGMA./(2V.sub..pi.).
(5)
[0021] By changing the sum U.sub..SIGMA.=U.sub.1+U.sub.2 of the
drive amplitudes while leaving their difference U.sub..DELTA.
constant, the amount of bit-synchronuous chirp can thus be easily
tuned to any desired value. Also, by either changing the modulator
bias .phi..sub.bias by .pi., or by exchanging the role of
u.sub.1(t) and u.sub.2(t), the sign of the chirp can be changed
from positive to negative. FIG. 3 illustrates this type of
operation in terms of intensity (upper graph) and phase (lower
graph) for three different amounts of chirp (m=0, m=.pi./2,
m=2.5.pi.).
[0022] B. Pulse Carver Driven at R/2, Biased at Minimum
Transmission
[0023] If the pulse carver is sinusoidally driven at half the data
rate (R/2) and biased for zero transmission, carrier-suppressed RZ
(CSRZ) is generated. The pulse duty cycle can be continuously
changed from 67% (shown in FIG. 4) to 50% by varying U.sub..DELTA.
from V.sub..pi.to 0. If, according to our invention, the amplitudes
of the carver are imbalanced, we achieve alternate-chirp CSRZ
(AC-CSRZ), as proposed in [R. Ohhira, D. Ogasahara, and T. Ono,
"Novel RZ signal format with alternate-chirp for suppression of
nonlinear degradation in 40 Gb/s based WDM", Proc. OFC'01, paper
WM2, 2001.] to mitigate nonlinear transmission effects. As for case
A, we can choose any phase modulation index m by setting
U.sub..SIGMA. to the desired value. FIG. 4 illustrates this type of
operation in terms of intensity (upper graph) and phase (lower
graph) for three different amounts of chirp (m=0, m=.pi./2,
m=2.5.pi.).
[0024] C. Pulse Carver Driven at R/2, Biased at Maximum
Transmission
[0025] If the pulse carver is sinusoidally driven at half the data
rate (R/2) and biased at maximum transmission, RZ with 33% duty
cycle is generated. As opposed to cases A and B, the pulse shape
cannot be tailored without degrading pulse extinction. Also, chirp
cannot be generated by imbalancing the drive amplitudes. However,
by imbalancing the relative phase between the two driving signals,
AC-CSRZ signals with variable phase modulation index can be
generated. Using the Ansatz
u.sub.1(t)=U.sub.1 cos[.pi.Rt], and u.sub.2(t)=U.sub.2
cos[.pi.Rt+.delta.], (6)
[0026] where .delta. represents a phase angle between the two drive
signals, we calculate the modulation index m from Equations (1) and
(2) as
m=.pi.U.sub.1/V.sub..pi. cos(.delta./2). (7)
[0027] Note that for this kind of pulse carver operation, the drive
amplitudes U.sub.1 and U.sub.2 have to be adjusted as
U.sub.1=U.sub.2=V.sub..pi. sqrt[m.sup.2/.pi..sup.2+0.25] to assure
good pulse extinction as well as no linear phase term that would
cause pulse frequency shifts. FIG. 5 illustrates this type of
operation in terms of intensity (upper graph) and phase (lower
graph) for three different amounts of chirp (m=0, m=.pi./2,
m=2.5.pi.).
[0028] Various modifications of this invention will occur to those
skilled in the art. Nevertheless, all deviations from the specific
teachings of this specification that basically rely upon the
principles and their equivalents through which the art has been
advanced are properly considered within the scope of the invention
as described and claimed. For example, although the imbalanced
signals have been described above as being sinusoidal, there may be
instances in which non-sinusoidal drive signals may be
preferred.
* * * * *