Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-10-04
2003-02-18
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06522438
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical communications, and in particular, to optical duobinary modulation schemes.
2. Description of Related Art
The ever-increasing demand for telecommunication systems has led to the development of high-bandwidth optical transmission systems having multi-gigabit data rates. As the bandwidth of a telecommunication system increases, previously unimportant system factors become critical to the successful operation of the system.
One factor which has become critical is the chromatic dispersion of an optical data signal, where the broadening of the digital pulses as the signal is propagated through the transmission system degrades the system performance and causes increased bit-error rates. Since the chromatic dispersion is essentially a function of the propagation velocity in a fiber and the wavelength of that light, signals with a broader optical spectrum will be more severely corrupted by inter-symbol interference effects than signals with a narrower spectral content. The broadening of the optical spectral content of a given signal is usually referred to as the “chirp” of the signal.
The use of dc-biased narrow-linewidth laser sources in conjunction with zero-chirp external modulators has resulted in the information bandwidth itself being the limiting and controlling factor of the spectral content of a signal, and hence the severity of the inter-symbol interference at a receiver. If all other parameters remain constant for such a system, a reduction in transmission bandwidth should result in a reduction of the chromatic dispersion penalty for the system.
The use of so-called duobinary codes is one way of reducing the information bandwidth, since a duobinary coded signal (i.e., duobinary modulated signal) has half the-transmission bandwidth of a binary signal. A duobinary modulated signal is in fact a ternary signal having three levels: −1, 0, 1.
As shown in
FIG. 1
, in a traditional optical duobinary modulation scheme, first an electrical signal
101
a
is input to an exclusive-or gate
103
. Exclusive-or gate
103
receives input electrical signal
101
a
as well as a feedback electrical signal
101
b
. Feedback electrical signal
101
b
is created by taking an output signal
105
(i.e., output from binary encoder
103
), adding a delay to this signal via a delay-block
107
, and then providing this signal back to exclusive-or gate
103
as feedback electrical signal
101
b
. Usually the added delay equals one time slot. Exclusive-or gate
103
and delay block
107
together are known as binary encoder
108
.
Exclusive-or gate
103
receives input electrical signal
101
a
and feedback electrical signal
101
b
, and creates an encoded binary signal
109
. This encoded binary signal
109
is then input to a low-pass filter
111
. Low-pass filter
111
receives encoded binary signal
109
and generates a duobinary signal
113
. Duobinary signal
113
typically comprises three levels, e.g., 0, 1, 2 or −1, 0, 1. The levels of duobinary signal
113
are induced by choosing appropriate bandwidth of low-pass filter
111
.
Duobinary signal
113
is then input to an optical modulator
115
. Optical modulator
115
receives duobinary signal
113
as well as an optical input
117
. Optical input
117
is an optical beam generated by a traditional light source
121
, e.g., a laser diode. Optical modulator
115
modulates optical beam
117
with duobinary signal
113
and creates a modulated optical duobinary signal
119
. Modulated optical duobinary signal
119
has the same line-rate as the input electrical signal
101
a
but has optical properties.
FIG. 2
illustrates a timing chart corresponding to the traditional scheme of FIG.
1
. The timing chart has been provided to illustrate processing of signals in the traditional scheme. In
FIG. 2
, graph
201
depicts timing of input electrical signal
101
a
, and graph
203
depicts timing of feedback electrical signal
101
b
. As illustrated, electrical signals
101
a
and
101
b
have the same line-rate.
Graph
205
depicts timing of encoded binary signal
109
, wherein encoded binary signal
109
has been created by inputting electrical signals
101
a
and
101
b
to exclusive-or gate
103
. Graph
207
depicts timing of duobinary signal
113
wherein duobinary signal
113
has been created by passing encoded binary signal
109
through low-pass filter
111
. In graph
207
, duobinary signal
113
is shown to have three levels: 0, 1, −1.
Graph
209
depicts timing of output optically modulated duobinary signal
119
wherein optically modulated duobinary signal
119
has been created by modulating optical beam
117
with duobinary signal
113
.
The modulation scheme described in
FIGS. 1 and 2
has many advantages including high fiber dispersion tolerance. However, this scheme requires a binary encoder
108
operating at the line-rate of input electrical signal
101
a
. As the line-rate increases, for example, 40 Gb/s or higher, it becomes increasingly difficult to implement binary encoder
108
operating at such a high line-rate. For example, there are no commercial binary encoders available in the market that function at a line-rate of 40 Gb/s or higher.
SUMMARY OF THE INVENTION
A method and apparatus for generating high-speed modulated optical duobinary signals are provided.
In accordance with the principles of the present invention, two input electrical signals are respectively input to two binary encoders. The binary encoders encode the input electrical signals and generate a pair of encoded binary signals which are then input to an analog amplifier. The analog amplifier amplifies the encoded binary signals and generates a pair of duobinary signals. Then, the duobinary signals are input to a dual-electrode modulator which modulates a pair of optical beams with the duobinary signals to generate a pair of modulated optical duobinary signals. Both modulated optical duobinary signals are then combined to generate the desired high-speed modulated optical duobinary signal.
The principles of the present invention provide a low-cost solution because two low-speed electrical signals may be used as input electrical signals to generate a high-speed modulated duobinary signal having twice the line-rate of the input signals, e.g., if the desired line-rate of the output duobinary signal is 40 Gb/s, the input electrical signals may have a line rate of 20 Gb/s.
Similarly, the principles of the present invention permit the use of low-cost binary encoders. For example, if the desired line-rate of the output duobinary signal is 40 Gb/s, each of the binary encoders of the present invention may operate at only 20 Gb/s (half of the line-rate of the desired high-speed optical duobinary signal). This is a substantial improvement over the prior art, which required binary encoders to operate at the same line-rate as the desired output line-rate.
In one embodiment, the present invention is a method for generating an optical duobinary signal, comprising the steps of (a) converting a first electrical signal into a first encoded binary signal; (b) converting a second electrical signal into a second encoded binary signal; (c) inputting the first and second encoded binary signals to an analog amplifier to create a first duobinary signal and a second duobinary signal; and (d) applying the first duobinary signal and a first optical beam to an first optical modulator, and applying the second duobinary signal and second optical beam to a second optical modulator to generate an output optical duobinary signal.
In another embodiment, the present invention is an optical duobinary signal generator, comprising (a) a first binary encoder configured to generate a first encoded binary signal from a first electrical input signal; (b) a second binary encoder configured to generate a second encoded binary signal from a second electrical input signal; (c) an analog amplifier coupled to the first and second binary encoders and configured to generate first and secon
Chan Jason
Gruzdkov Yuri
Lucent Technologies - Inc.
Mendelsohn Steve
Payne David
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