Optical modulator with programmable chirp

Details Optical modulator with programmable chirp Optical modulator with programmable chirp

2001-10-09

2004-09-21

Bovernick, Rodney (Department: 2874)

Optical waveguides

Directional optical modulation within an optical waveguide

C385S008000, C385S009000, C385S039000, C385S040000

Reexamination Certificate

active

06795595

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical based communications network, and more particularly to an optical modulator that is programmable in terms of the amount of chirp that is imparted on the modulated optical signal from the optical modulator.
Introduction
In a general fiber optical communication system, optical signals are sent along an optical fiber communication line to a desired location. One type of the fiber optical communication system that can handle optical signals of multiple channels through wavelength multiplexing is called a wavelength division multiplexed (WDM) system. Chirp is a typical problem in these type of systems. Chirp is the instantaneous broadening of the wavelength (and hence frequency) of the optical carrier. Depending on the particular systems application, chirp could either be desirable or undesirable. At the commonly used communications wavelength of 1550 nm, the conventional single mode fiber exhibits significant dispersion. An optical pulse that is broadened on account of chirp can interact with the dispersion in the fiber and impair the fidelity of data transmission. In this case, chirp is undesirable. However, a compressed (i.e. negative chirped) pulse can evolve over a length of fiber to restore its normal shape, thereby the enhancing fidelity for data transmission. In this case, negative chirp at the point of signal origination may be desirable.
In External Optical Modulators (EOMs), chirp &agr; can be set to a positive (+&agr;), negative (−&agr;) or zero (0) value. The amount of chirp required can be link specific. For example, optical links of different lengths or other physical differences between optical links often require different chirp values to counteract the effect of fiber dispersion. Therefore, it is desirable to design EOMs with an adjustable chirp value to provide for dispersion compensation as required in an arbitrary optical link.
2. Discussion of the Related Art
Previously, zero chirp, non-zero adjustable chirp, and fixed non-zero chirp modulators have been respectively demonstrated in U.S. Pat. Nos. 5,074,631, 5,303,079 and 5,408,544, all of which are expressly incorporated by reference, so-called, zero chirp (U.S. Pat. No. 5,074,631) and non-zero adjustable chirp (U.S. Pat. No. 5,303,079) EOMs have been based on a dual signal electrode design. Non-zero fixed chirp modulators (U.S. Pat. No. 5,408,544) have been implemented either in a single input signal electrode or a dual input signal electrode design. However, there are several drawbacks related to the foregoing related art design of modulators with adjustable or set chirp value.
For applications requiring a tunable chirp modulator, the tunable two-electrode design described in U.S. Pat. No. 5,303,079 is cumbersome to implement. In such a tunable chirp design, a second input signal is connected to a second input signal electrode and is typically derived from a signal that is input into a first input signal electrode. Such tunable designs require an increase in drive circuitry needed for operating an optical modulator because they require two signal electrode drivers, and thus cause an increase in device complexity, size and/or cost. Moreover, it is difficult to precisely set the phase and amplitude balance of the drive signals applied to the two input signal electrodes at high grade rates.
FIGS. 1 and 2
illustrate a zero chirp type optical modulator of the related art.
FIG. 1
is a plan view of a single input signal electrode related art EOM and
FIG. 2
is a cross-section of
FIG. 1
taken along line I-I′. As shown in
FIG. 1
, the related art EOM includes an optical modulation chip
1
having an electrooptical effect. The optical modulation chip
1
includes a waveguide, such as a Mach-Zehnder Interferometer (MZI), that runs from one to another end of the chip. The waveguide includes a first main channel
8
that branches into separate parallel paths along respective first and second waveguide arms
3
,
3
′ near one end of the chip
1
. Near the other end of the chip
1
, the arms
3
,
3
′ come back together as a second main channel
8
′ at the other end of the chip. Directly overlying the first waveguide arm
3
is a first coplanar-strip (CPS) electrode
4
for connecting an input signal. One end of the first CPS electrode
4
is connected to the input signal and the other end of the signal electrode is connected to a termination resistor. Alternatively, both ends of the first CPS electrode
4
can be connected to independent signal sources. Directly overlying the second arm
3
′ is a second CPS electrode
5
for connecting to ground. Both ends of the second CPS electrode
5
are connected to ground G.
FIG. 2
shows optical waveguide arms
3
,
3
′ that correspond to the two arms
3
,
3
′ of the interferometer shown in FIG.
1
. The waveguide arms are regions within an optical modulator chip
1
(e.g., LiNbO
3
). An insulating buffer layer
2
(e.g., SiO
2
) is provided on the optical modulator chip
1
between the CPS electrodes
4
,
5
and the waveguide arms
3
,
3
′. The electrode structure of
FIGS. 1 and 2
is a CPS electrode structure. In
FIG. 2
the dashed lines show a representation of how the electric field lines emanate from the signal CPS electrode
4
and are received by the ground CPS electrode
5
so as to interact with the optical signals as they pass through the optical waveguide arms
3
,
3
′. The electric field lines shown are not indicative of the actual path that electric field lines would take between the signal and ground CPS electrodes as the electric fields pass through the body of the optical modulator chip
1
. However, the electric field lines shown are generally indicative of electric field lines that go through the waveguide arm
3
from the signal CPS electrode
4
and out through waveguide arm
3
to the ground CPS electrode
5
.
As shown in
FIG. 2
, the CPS electrode structure has a symmetric electric field lines that interact with the waveguide branches in a push-pull manner, which results in a modulator output having no chirp. The chirp parameter |&agr;| of an EOM is directly proportional to the asymmetry in the V of the two arms of the interferometer. As depicted in
FIG. 2
, V is the potential across a cross-section of a waveguide arm. Chirp parameter |&agr;| is defined as a proportion of the V on one arm of the interferometer with respect to the V of the other arm of the interferometer as follows:
|
α
|
=
|
V
2
-
V
1
V
2
+
V
1
|
(
1
)
where V
2
is the potential across a waveguide arm
3
′ and V
1
is the potential across a waveguide arm
3
from the surface of the waveguide arm. The electric fields as shown in
FIG. 2
show the chirp &agr; to be zero because V
2
equals V
1
. It also can be seen from the equation (1) above that when V
2
does not equal V
1
, chirp is present in the modulator output.
FIG. 3
shows one related art approach for causing chirp in an EOM having a single input signal electrode by causing electric field asymmetry (i.e., a structure where V
2
≠V
1
) in the CPS structure. This can be accomplished with an asymmetric CPS structure by changing the width of the ground plane. As shown in
FIG. 3
, the width of the ground plane has been changed by widening the ground CPS electrode
5
′. As a result of the change in width of the ground CPS electrode
5
′, the electric field lines are more spread out with regard to arm
3
′. Therefore, V
2
has a smaller value than V
1
and results in the modulated optical output of the modulator having chirp.
While the related art modulator of
FIG. 3
can be designed with a desired chirp value, the chirp parameter of the modulator is fixed to a single value. In contrast to the above-described dual input signal adjustable chirp modulator, a non-zero fixed chirp modulator including only one input signal electrode cannot be tuned for different values of chirp, and hence is not an attractiv

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