Optical transmitter system and method

Details Optical transmitter system and method Optical transmitter system and method

1998-08-19

2002-01-08

Negash, Kinfe-Michael (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C375S291000

Reexamination Certificate

active

06337756

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical duobinary transmitter system and method using optical intensity modulation.
DESCRIPTION OF RELATED ART
At high bit rates, the chromatic dispersion in standard single mode fibers (SSMF) limits the transmission distance in the 1550 nm window. There has been a number of different methods proposed to overcome this limitation of which the most common are pre-chirped modulators, dispersion compensating fibers, chirped Bragg gratings, mid-span spectral inversion, and special signal formats such as dispersion supported transmission and duobinary transmission.
Duobinary transmission has been investigated for modulators showing no or very little chirp, i.e. &agr;≈0, see, e.g. Gu et al., 10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fiber, Electron. Lett., Vol. 29, No. 25, 1993, pp. 2209-2211 and May et al., Extended 10 Gbit/s fiber transmission distance at 1538 nm using a duobinary receiver, IEEE Phot. Technol. Lett., Vol. 6, No. 5, 1994, pp 648-650. The chirp parameter &agr; is defined as
α
=
∂
ϕ
∂
t
1
2
⁢
P
⁢
∂
P
∂
t
where &phgr; is the phase and P the intensity of the optical signal.
The duobinary signal is DC-free and its transmission spectrum is narrower than the spectrum of the binary signal. If the duobinary signal is modulated on a carrier, the modulated signal will behave as a double sideband signal with suppressed carrier.
The main benefit with duobinary transmission is that the transmission spectrum is reduced compared to ordinary binary transmission. In a dispersion limited system, the transfer length is inversely proportional to the square of the bandwidth of the transmission spectrum. This means that if the transmission spectrum is reduced to one half the transfer length is quadrupled.
Further, since the carrier frequency is suppressed in the duobinary transmission spectrum, the limitation for the output optical power due to stimulated Brillouin scattering in the fiber can be relaxed.
Optical duobinary transmission can be considered as a three-level signaling scheme which can be detected with an ordinary binary receiver. The normal marks in binary transmission are “0” and “1”, whether the marks in duobinary transmission are “−1”, “0”, and “1”, In the optical case, the duobinary marks are modulated as “−P”, “0”, and “P”, where P is the optical peak power. These will be interpreted as “P”, “0”, and “P” in an ordinary opto-electric quadratic detector.
A common way to construct an optical duobinary transmitter is to make use of a double-electrode Mach-Zehnder (DEMZ) modulator, see, e.g. the U.S. Pat. No. 5,543,952 or the international application WO 95/29539. The DEMZ-modulator has also been proposed for adjustable chirp applications, see A. H. Gnauck et al., Dispersion penalty reduction using an optical modulator with adjustable chirp, IEEE Phot. Technol. Lett., Vol. 3, No. 10, 1991, pp 916-918, as well as simultaneous 2:1 multiplexing and modulation, see P. B. Hansen et al., A dual-drive Ti:LiNbO
3
Mach-Zehnder Modulator used as an optoelectric logic gate for 10 Gbit/s simultaneous multiplexing and modulation, IEEE Phot. Technol. Lett., Vol. 4, No. 6, 1992, pp 592-593.
A typical optical duobinary transmitter based on a DEMZ-modulator according to prior art is explained with reference to the layout as shown in FIG.
1
.
The input signal of the transmitter is an electrical binary signal S
1
and its complement S
2
={overscore (S
1
)}. Each of these signals is fed through a binary-to-duobinary encoder
1
,
3
and an AC-amplifier
5
,
7
. The resulting duobinary, i.e. three-level, signals S
3
, S
4
are amplified and then used as driving signals of the electrodes of the modulator
9
.
Continuous light from a laser diode
11
is coupled into the modulator
9
and split into two components in the Y-junction
9
a
of the left part of the modulator. The light in the two branches
9
b
,
9
c
of the modulator will then undergo positive or negative phase shift in the middle part of the modulator, the phase shift being controlled through the linear electro-optic effect by the applied voltage, i.e. the duobinary driving signals S
3
, S
4
, of the electrodes of the modulator. The phase shift in the upper branch is controlled by the upper electrode, and the phase shift in the lower branch is controlled by the lower electrode. The electrodes are supplied by bias voltage
13
in order to obtain the same phase shift in the two branches when no driving signals are applied to the electrodes.
The light in the two branches are then combined coherently in the Y-junction
9
d
in the right part of the modulator. If there is a 0° phase shift between the components, all light will be injected in the outgoing optical waveguide. If there is a 180° phase shift, no light will be injected in the outgoing waveguide. In the latter case, the light will be radiated into the modulator.
The coding procedure for duobinary transmission is very simple. In
FIG. 2
is shown the binary-to-duobinary encoder
1
which converts the binary signal S
1
into a duobinary signal S
3
by using two flip-flops
15
,
17
and a clock pulse
19
. The flip-flops have binary output signals S
5
,
56
, which are equal to the input binary signal but shifted one bit and two bits, respectively. The binary output signals S
5
, S
6
are then fed through an adder
21
with the following function
S
3
=S
5
+S
6
−1
thus, generating the duobinary signal S
3
. In
FIG. 3
is shown an example of the output signal- S
3
and the encoding intermediate signals S
5
, S
6
for duobinary modulation of the binary signal S
1
. It may be observed that a direct transition between the marks “−1” and “1” never occurs in duobinary modulation. The binary-to-duobinary encoder
3
is constructed and functioning likewise with the only difference that the input signal S
2
is the complement of the binary signal S
1
.
The introduced phase shift in the upper and in the lower branch of the optical duobinary modulator for each of the marks are indicated in
FIG. 4
a
. The logical “1” mark corresponds to a light pulse with full amplitude and a 0° phase shift, the “0” mark corresponds to no light pulse at all as the two components are opposite in phase and cancel each other out, and the “−1” mark corresponds to a light pulse with full amplitude and a 180° phase shift.
FIG. 4
b
shows a polar graph (amplitude vs phase) of the locus of the optical output signal (thick solid line) and the location of each of the duobinary marks (dots). The phase of the optical output signal does not vary on its way between the marks. Therefore, d&phgr;/dt=0 and &agr;=0 according to the formula presented above.
The main problem with a duobinary transmitter as described is that the chromatic dispersion still limits the transmission distance and may be a problem for long haul fiber transmission systems.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical duobinary transmitter with an improved performance in terms of dispersion immunity.
This object among others is fulfilled by an inventive optical duobinary transmitter system and method, which introduces a blue-shift frequency chirp.
The inventive system and method comprises an input terminal, a driving circuit, a double electrode optical modulator, particularly of the Mach-Zehnder type, and an output terminal.
The input terminal is arranged to receive a first binary signal and the driving circuit, which is connected to said input terminal, is arranged to convert the first binary signal into a second and a third binary signal. The double electrode optical modulator is connected to the driving circuit in such a way that its upper and lower electrode may be driven by said second and third binary signal, respectively, said modulator being further arranged to modulate the amplitude and phase of an optical carrier according to the binary driving signals so as to provide an optica

Affiliated with

Also associated with

Rating

Optical transmitter system and method does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical transmitter system and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical transmitter system and method will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2852062