Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-12-20
2003-06-24
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S254000, C385S003000, C385S001000, C385S002000, C385S008000, C385S031000, C385S039000
Reexamination Certificate
active
06583917
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to optical intensity modulators, more especially, but not exclusively, to optical modulators suitable for high bit rate optical communication systems using return-to-zero (RZ) data modulation.
The Mach-Zehnder interferometer (MZI) is perhaps the most common solution for realizing integrated optical intensity modulators. An optical signal is split from an input waveguide by a Y-splitter (or 2-way coupler) into two waveguide arms. Electrodes are placed along the two arms to control the optical path length difference between the two arms, e.g. using the electro-optic effect. The two arms rejoin at another Y-splitter (or 2-way coupler) so that light components carried by the two arms interferometrically recombine. Data carried by an electrical signal connected to the electrodes can be used to control the interferometric recombination of the light components at the output Y-splitter, thereby impressing the data onto an optical signal traversing the device. A favored technology for implementing MZI's is a lithium niobate crystal substrate with diffused titanium waveguides (LiNbO
3
:Ti technology). A variety of other technologies can also be used, for example GaAs.
FIG. 1
of the accompanying drawings illustrates a conventional balanced MZI
110
comprising two arms
118
and
120
. The device is fabricated in an x-cut lithium niobate substrate
100
, as described in reference [
1
] and reference [
2
]. In use, a phase shift is induced between the two arms
118
and
120
, trough the electro-optic effect. The phase shift is induced by applying an RF data modulation signal across a signal electrode
114
and two ground electrodes
112
and
116
, suitably arranged in respect of the arms
118
and
120
. As well as AC (i.e. RF) electrodes, there are also provided DC electrodes (not shown) for biasing the modulator at a desired working point. A desired intensity variation at the interferometer output is thus obtained to provide data modulation. In the figure, there is also shown an output side phase modulator
125
suitable for chirping the signal output from the MZI
110
using a pair of electrodes
130
and
132
. In this way, chirped RZ (CRZ) data modulation can be achieved.
The MZI modulator is easy to realize, is simple to use and achieves high performance. It has thus become a key component in optical communication systems. However, the relatively weak electro-optical coefficient of the available materials used for realization of the waveguides means that the modulator electrodes need to be relatively long in order to induce the necessary phase shift to the light. Long electrodes are undesirable, because they limit the frequency bandwidth of the modulator.
With the continuously increasing bit rate of modem optical communication systems and the interest in RZ data modulation for soliton and other non-linear transmission systems, there is a need for an intensity modulator capable of generating still shorter optical pulses.
In principle, to provide shorter optical pulses, it is sufficient to feed the AC electrodes of a simple MZI with shorter RF pulses. However, in this case, the bandwidth of the modulator (i.e. the bandwidth of the RF drivers and the RF electrodes of the modulator) must be at least equal to the bandwidth of the RF pulses or even higher, that is higher than the inverse of the bit rate. This approach is generally avoided because of the large bandwidth required by the modulator.
FIG. 2
of the accompanying drawings shows an approach designed to solve this problem, as disclosed in references [
1
], [
2
] and [
3
]. The proposed approach is to cascade two MZI's
10
and
150
. The illustrated device is fabricated in x-cut lithium niobate. In use, one of the MZI's, for example MZI
110
, is modulated via its electrodes
112
,
114
and
116
with non-return-to-zero (RZ) modulated data, while the other MZI
150
is modulated via its electrodes
152
,
154
,
156
with a clock signal for converting the NRZ optical signal into an RZ optical signal. Generally, the clock-signal receiving MZI has travelling wave electrodes resonating at the clock frequency, which equals the bit rate of the NRZ modulated data
If a chirp is also desired, another phase modulator must be cascaded to the two NZI's. In other words an additional stage similar to the phase modulator shown in
FIG. 1
would be added to the device of FIG.
2
. As a result, the device would become even longer than the two-stage device of FIG.
2
. In addition, it would be difficult to synchronize the two, or even three, modulating RF signals applied to the electrodes.
SUMMARY OF THE INVENTION
The invention is based on the idea of dispensing with the conventional two-arm MZI (2aMZI) used in the prior art for RZ data modulation and instead using a suitably biased and configured three-arm MZI (3aMZI).
More especially, according to one aspect of the invention, there is provided a device for modulating an optical signal comprising:
an interferometer comprising: an optical signal divider; first, second and third arms; and an optical signal combiner arranged respectively to divide, convey and interferometrically recombine components of the optical signal into, along and from the arms, wherein the optical signal divider is structured so that the components of the optical signal coupled into the first and third arms are of approximately equal power;
a DC electrode structure for biasing the interferometer at a working point of the device; and
an AC (i.e. RF) electrode structure for applying an electrical modulation signal to the interferometer, wherein the AC electrode structure is configured so that the electrical modulation signal induces phase shifts of opposite sign and approximately equal magnitude in the first and third aims.
Although it is preferred that the three-arm interferometer is of the Mach-Zehnder type, in principal other interferometer configurations could be used, such as Michelson. However, in the following, it is assumed that the three-am interferometer is a 3aMZI. In addition, it may be possible to add further arms to the interferometer.
The 3aMZI modulator can be used as a data modulator or a pulse generator, for example.
In preferred embodiments, the 3aMZI has a symmetric power split between the first and third arms, with no more than 50% of the total power coupled into the second arm, and an antisymmetric phase shift between the first and third arms.
The second arm may be arranged as a central waveguide between the first and third arms which are arranged as two external waveguides extending either side of the central waveguide. Typically, the three waveguides will be joined at the input side of the device by some form of input signal divider and at the output side by some form of output signal combiner. The signal divider and combiner can be fabricated using splitters or couplers. Various alternative implementations are discussed further below.
As mathematically described further below, the 3aMZI may provide the same form of intensity (or power) transfer function as two cascaded 2aMZI's (2×2aMZI—see FIG.
2
), but has a number of advantages over the 2×2aMZI modulator of reference [
1
], and more generally over 2aMZI modulator designs.
One advantage is that with a 3aMZI modulator, only one modulation source is required, that is only one RF driver is required. By contrast, each stage of the 2×2aMZI modulator requires a separate RF driver. This is of benefit for achieving high data rates, since there is no requirement to synchronize different RF drivers (e.g. one for the clock and another for NRZ data).
Another related advantage is that a 3aMZI modulator is inherently more compact than the 2×2aMZI modulator, since it has only one stage rather than two. This is also of benefit for achieving high data rates.
A further advantage is that, in comparison with a 2aMZI modulator, the 3aMZI modulator can provide a larger notch width, which makes the device general
Martinelli Mario
Melloni Andrea
Epps Georgia
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pirelli Cavi e Sistemi S.p.A.
Tra Tuyen
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