Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2003-06-02
2004-11-16
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S285000, C385S002000
Reexamination Certificate
active
06819472
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator and a communications system including the optical modulator. More particularly, the present invention relates to an optical modulator for use to transmit an RF signal having a frequency of several GHz or more by a lightwave communications technique and also relates to a communications system including such an optical modulator.
2. Description of the Related Art
A system for exchanging or processing information by using an optical signal needs to modulate the phase or intensity of light by means of an electric signal (e.g., an RF signal falling within the microwave or milliwave band). Light can be modulated for that purpose either by a direct modulation technique or by an external modulation technique.
The direct modulation technique is a method of changing the intensity of light that has been emitted from a light source (e.g., a semiconductor laser diode) by directly changing the amount of drive current being supplied to the light source as shown in FIG.
1
A. The direct modulation technique contributes to reducing the overall size of a communications system because no modulators need to be provided outside of the light source. According to this method, however, it is difficult to modulate the light at a high frequency of several GHz or more. In addition, long distance fiber optics transmission can be carried out only under limited conditions due to a chirping phenomenon which is often observed in semiconductor laser radiation.
In the external modulation technique on the other hand, light that has been emitted from a light source such as a semiconductor laser diode (i.e., light with a stabilized output power) is input to an optical modulator, which modulates the phase or intensity of the light as shown in FIG.
1
B. In this technique, the light may be modulated by utilizing electro-optical effects, acoustooptical effects, magnetooptical effects or nonlinear optical effects.
As described above, it is difficult to achieve ultrahigh speed light modulation by the method of directly modulating the output of a semiconductor laser diode. Thus, an external modulator is currently under vigorous research and development because an element of that type normally achieves high speed light modulation. Among various types of external modulators, an electro-optical modulator, which uses dielectric crystals exhibiting Pockel's effect, can operate at such an extremely high speed and yet causes little disturbance in phase as a result of the modulation. For that reason, this electro-optical modulator can be used very effectively in high-speed data transmission, long distance fiber-optics communications and other applications. Also, if an optical waveguide structure is constructed using such an electro-optical modulator, the modulator may be implemented at a small size and can operate efficiently enough at the same time.
An electro-optical modulator usually includes: a transmission line, which is provided as a modulating electrode (or signal electrode) on electro-optic crystals to propagate a modulating signal therethrough; and an optical waveguide, which is provided near the transmission line. In this electro-optical modulator, the refractive index of the optical waveguide is changed by an electric field to be induced around the modulating electrode, thereby modulating the phase of the light wave being propagated through the optical waveguide.
Crystals that are normally used in such an electro-optical modulator have a relatively small electro-optic coefficient. The electro-optic coefficient is a parameter that forms the basis of optical modulation. Accordingly, an electro-optical modulator should apply an electric field to the optical waveguide as efficiently as possible.
FIG. 2
is a cross-sectional view showing the fundamental structure of an electro-optical modulator. As shown in
FIG. 2
, an optical waveguide is provided on the surface of a substrate that is made up of crystals exhibiting electro-optical effects (i.e., electro-optic crystals), and a modulating electrode is provided on the optical waveguide.
The electro-optic crystals have optical anisotropy and change their refractive indices substantially proportionally to the strength of the electric field applied thereto (i.e., exhibit the Pockel's effect). Thus, by adjusting the potential V applied to the modulating electrode, the refractive index n of the optical waveguide can be changed. The variation &Dgr;n in the refractive index of the optical waveguide is proportional to the strength of the electric field E applied to the optical waveguide. When the refractive index of the optical waveguide changes by &Dgr;n, the phase of the output light shifts by &Dgr;&phgr; as shown in FIG.
2
. The phase shift &Dgr;&phgr; is normally proportional to the product of the strength of the electric field E and the length L of the optical waveguide.
To create the electric field in the optical waveguide, a modulating signal is supplied externally (i.e., from outside of the optical modulator) to the electrode of the optical modulator by way of the input line. Thus, it is important to input the modulating signal as efficiently as possible.
Next, a specific configuration for a conventional optical modulator will be described in further detail with reference to FIG.
3
.
FIG. 3
is a plan view of a conventional optical modulator as disclosed in U.S. Pat. No. 5,400,416.
As shown in
FIG. 3
, the optical modulator includes a substrate
101
, which is made of an electro-optic material, and an optical waveguide
112
, which may be formed on the surface of the substrate
101
by thermally diffusing a metal element toward a portion of the substrate
101
, for example.
On the surface of the substrate
101
, a parallel coupled line structure
113
, obtained by patterning a metal film of aluminum, gold or other suitable metallic material, is provided on the right- and left-hand sides of the optical waveguide
112
. On the other hand, a ground plane
114
, also obtained by patterning a metal film, is provided on the back surface of the substrate
101
. The parallel coupled line structure
113
includes two lines
113
a
and
113
b
that extend parallelly to each other.
In the example illustrated in
FIG. 3
, the two lines
113
a
and
113
b
of the parallel coupled line structure
113
are coupled together by way of a single line
124
. However, the U.S. Pat. No. 5,400,416 identified above also discloses a structure in which the two lines
113
a
and
113
b
are not coupled together.
An input terminal
129
is further provided so as to be connected to a portion of the line
113
b
by way of a tap
128
. An RF signal source
119
is connected between the input terminal
129
and the ground plane
114
.
Incoming light is introduced through one end of the optical waveguide
112
, passed through a portion of the optical waveguide
112
in the gap
116
between the two lines
113
a
and
113
b
of the parallel coupled line structure
113
, and then output as outgoing light through the other end of the optical waveguide
112
. In the meantime, the input terminal
129
and the parallel coupled line structure
113
are magnetically coupled together. Thus, an RF signal, supplied from the RF signal source
119
, is propagated through the respective lines
113
a
and
113
b
of the parallel coupled line structure
113
to generate an electric field in the gap
116
between the lines
113
a
and
113
b
. According to the strength of that electric field, the refractive index of the optical waveguide
112
changes due to the electro-optical effects. As a result, the phase of the outgoing light is modulated. In this manner, the present optical modulator can operate as a phase modulator.
The parallel coupled line structure normally operates in either even mode or odd mode. In the odd mode, the voltages of the two lines included in the parallel coupled line structure have mutually opposite polarities, thus inducing a huge electric field in the gap between them. The
Enokihara Akira
Kosaki Masahiro
Yajima Hiroyoshi
Akin Gump Strauss Hauer & Feld & LLP
Hanig Richard
Matsushita Electric - Industrial Co., Ltd.
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