Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-05-07
2004-03-02
Mack, Ricky (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S237000, C359S245000
Reexamination Certificate
active
06700691
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to devices for modulating optical signals in a telecommunications system. More particularly, the invention concerns electro-optic modulators that are made of lithium niobate (LiNbO
3
) and that have an increased bandwidth and lower drive voltage than conventional modulators.
Modern telecommunications have increasingly adopted fiber optics as the medium for transmitting signals. As with electrical telecommunication signals, optical telecommunication signals can carry larger amounts of information when subjected to intensity modulation. Because much of existing electronics technology uses electrical signals, it is desirable to encode information from an electrical signal onto an optical carrier signal used in a telecommunication system.
Two approaches are commonly used to perform such electro-optic modulation: direct modulation and external modulation. Direct modulation involves varying the optical signal by directly modulating the laser diode that originates the optical signal. External modulation involves varying the optical signal after it has left the laser diode through the use of an electro-optic modulator. Unlike direct modulation, external modulation can be provided with negligible chirp, which refers to a change in carrier frequency over time.
Electro-optic external modulators work by causing the index of refraction of an optical waveguide to change in response to an applied electromagnetic signal. As the optical waveguide undergoes a variation in time of its refractive index, the optical signal passing through it is phase modulated with the corresponding electromagnetic signal. For an external modulator to achieve such a performance, the optical waveguide is formed in a material that has a strong electro-optic effect, i.e., where its optical index of refraction is easily affected by an electromagnetic signal. Typically, that material is a crystal substrate such as lithium niobate (LiNbO
3
). The optical waveguide is formed in the lithium niobate substrate by photolithography and diffusion of titanium. The path of titanium will have a higher index of refraction than the surrounding substrate and will constrain an optical signal within the path.
The electromagnetic signal is imparted on the external modulator through electrodes formed on the surface of the lithium niobate substrate. The electrodes are usually made of gold or a similarly conductive material and are positioned in parallel to the optical path. A portion of the electromagnetic signal travels from a “hot” electrode to one or more ground electrodes by passing through the optical path within the lithium niobate substrate, which causes modulation of the optical signal within the path.
The positioning of electrodes with respect to optical path(s) within the modulator differs depending on whether the lithium niobate is an x-cut or a z-cut crystal. The z-axis of the lithium niobate crystal has the highest electro-optic coefficient. Consequently, the electrodes and the optical path(s) are positioned in the modulator such that the electromagnetic field passes through the optical path along the z-axis. Generally, in an x-cut substrate, the optical path(s) are positioned between the hot and ground electrodes. In a z-cut substrate, the optical path(s) are positioned directly under the electrodes.
FIGS. 1A and 1B
illustrate a cross-sectional view and a top view, respectively, of a conventional external modulator in the form of a Mach-Zehnder interferometer made from a substrate of x-cut lithium niobate. As shown in
FIG. 1A
, lithium-niobate substrate
102
forms the base of the modulator
100
and includes two optical paths
104
a
and
104
b
. An RF signal applied between hot conductor
106
and ground electrodes
108
a
and
108
b
will cause some of the electromagnetic field to pass through optical paths
104
a
and
104
b
, modulating the optical signals passing through those paths. Optical paths
104
a
and
104
b
are positioned in this x-cut modulator so that the electromagnetic field passes through them horizontally, i.e., along the z-axis.
FIG. 1B
shows the Mach-Zehnder format of the external modulator
100
. The incoming optical signal travels along optical path
122
beginning at one end of the modulator and then splits at junction
124
between paths
104
a
and
104
b
. Along paths
104
a
and
104
b
, the optical signal is phase modulated as it is subjected to the RF electromagnetic field passing between electrodes
106
and
108
a
and
106
and
108
b
. At junction
126
, the optical signals traveling on paths
104
a
and
104
b
are combined, and they exit modulator
100
via path
128
. X-cut modulators of this type have proven effective for digital modulation at 10 Gbits/sec.
FIGS. 2A and 2B
illustrate a cross-sectional view and a top view, respectively, of a conventional external modulator in the form of a Mach-Zehnder interferometer made from a substrate of z-cut lithium niobate. The same elements and references from FIG. IA apply to FIG.
2
A. As shown in
FIG. 2A
, optical paths
104
a
and
104
b
are positioned directly beneath hot electrode
106
and ground electrode
108
b
. The optical paths are located in this z-cut modulator so that the electromagnetic field passes through them along substantially vertical lines, i.e., in parallel to the z-axis.
FIG. 2B
shows a similar arrangement for the Mach-Zehnder interferometer as in
FIG. 1B
except that the optical paths
104
a
and
104
b
are positioned under hot electrode
106
and ground electrode
108
b.
The z-cut crystal results in a more concentrated flux of the RF field passing through the optical paths than in an x-cut crystal. However, the improved performance of the z-cut device are mitigated by intrinsic pyroelectric problems and by a chirp parameter of approximately −0.7, which is due to the difference in overlap between the two z-cut optical waveguides. See Wooten et al., “A Review of Lithium Niobate Modulators for Fiber-Optic Communications Systems,”
IEEE Journal of Selected Topics in Quantum Electronics
, vol. 6, no. 1, pp. 69-82 (January/February 2000).
Several performance criteria determine the quality of an external optical modulator. For example, an effective modulator must have a broad modulation bandwidth. The standard layout of an optical modulator, however, limits the modulation bandwidth because the electromagnetic signal travels more slowly through the electrodes than the optical signal travels through the optical path. This velocity mismatch introduces a phase error that is a function of the frequency of the electromagnetic signal and the length L of the interaction between the electromagnetic signal and the optical signal within the modulator. A common figure of merit for an optical modulator is the product of its −3 dB
e
modulation bandwidth, which is denoted f
3dB
, and its interaction length L. This figure of merit should be as high as possible.
To improve velocity matching between the RF signal and the optical signal in the modulator, conventional devices include a buffer layer
120
on the surface of the lithium niobate substrate. Generally comprising SiO
2
or BenzoCycloButene (BCB), buffer layer
120
lowers the dielectric constant of the material through which the RF electromagnetic field must pass, thereby increasing the velocity of the field.
Another performance concern for optical modulators is the voltage level required for the electromagnetic signal. The switching voltage necessary for a given amount of modulation should be as low as possible. The necessary voltage level is dictated in large measure by the gap G between the hot and ground electrodes and the electrode length, a small gap G decreasing the required switching voltage. A quality of modulation efficiency is expressed through the half-wave voltage V
&pgr;
. For a typical Mach-Zehnder interferometer made from lithium niobate, the half-wave voltage is given by the following:
V
π
=
λ
G
n
0
3
r
33
Γ
L
(
1
)
Nespola Antonino
Pensa Simone M.
Avanex Corporation
Mack Ricky
Suzue Kenta
Thomas Brandi N
Wilson Sonsini Goodrich & Rosati
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