Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-02-08
2002-08-06
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S276000, C359S279000, C385S002000, C385S008000, C385S131000
Reexamination Certificate
active
06429959
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulation system, and more particularly, to the interconnection of external electrodes to an optical modulator so as to minimize loss of signal energy and to prevent the introduction of spurious modes into the signal within the optical modulator.
2. Discussion of the Related Art
In a general fiber optical communication system, optical signals are sent along an optical fiber communication line to a desired location. Optical modulators with performance in the 40 GHz frequency range and beyond, are critical components in optical communication systems.
To achieve high-frequency operation in LiNbO
3
, the electrical and optical velocity of the modulating and modulated signal must be matched. This is achieved by employing thick (>10 &mgr;m) electrodes in conjunction with a buffer layer (typically SiO
2
). The buffer layer is deposited directly on the LiNbO
3
and the electrode structure is delineated on the buffer layer. While the buffer layer facilitates velocity matching, it also results in decreased modulation efficiency because the applied voltage is partially dropped across the buffer layer. LiNbO
3
is an anisotropic material, with the following dielectric constants:
∈
extra-ordinary
≈26, ∈
ordinary
≈43
Thus, planar and uni-planar transmission lines such as microstrip, coplanar waveguide (CPW) and coplanar strip (CPS) tend to be very dispersive when built directly on LiNbO
3
. As the frequency increases, the fields become more concentrated in the regions below the metal strips, where the substrate permittivity has already resulted in a relatively larger electric displacement since the fields are forced into the LiNbO
3
to an increasing extent as the frequency increases. Therefore, a frequency-dependent effective permittivity can be defined for the transmission line.
FIG. 1
illustrates an optical modulator of the prior art. The modulator has a mounting base
1
that is typically conductive or a non-conductive material covered with a conductive layer. The mounting base
1
is typically at the ground potential of the device and will herein be referred to as the grounded base
1
. The optical modulator has an optical modulator chip
2
, for example a LiNbO
3
chip covered with an insulating buffer layer, mounted on the grounded base
1
. The grounded base
1
includes input/output optical terminals
345
and input/output electrical terminals
71
. The optical modulator chip
2
has two ground electrodes
3
/
3
′ and a signal electrode
4
mounted on top of the buffer layer above the waveguide
34
of the optical modulator chip
2
. This electrode configuration is known as the coplanar-waveguide (CPW). When the electrode structure of the optical modulator chip
2
comprises just one signal electrode, and one ground plane, it is known as the coplanar-strips (CPS) configuration.
The optical modulator chip
2
is comprised of active
6
and non-active sections
5
. The active section
6
of the device is the section of the optical modulator chip
2
wherein the electrical and optical signals interact to cause optical modulation. Typically, the electrode dimensions, such as the width of the signal electrode
4
, and the electrode gap tend to be very narrow (5-25 microns) in the active section
6
. These dimensions are prohibitively small to facilitate direct connection of the device to standard electrical connectors. Hence, the electrodes
3
/
4
/
3
′ for the active section
6
are flared
31
/
41
/
31
′ in the non-active section
5
to facilitate external connection to the signal electrode line
4
and the ground electrodes
3
/
3
′. The flared electrodes
31
/
41
/
31
′ do not take part in the process of optical modulation, but are required to facilitate connection of the active section of the modulator to standard electrical interface media. External electrical connection to the flared electrodes
31
/
41
/
31
′ of the optical modulator chip
2
is facilitated by either a transition chip
7
having leads
23
/
24
/
23
′ connected to the flared electrodes
31
/
41
/
31
′ of the optical modulator chip
2
via wires or a direct external connection to the flared electrodes
31
/
41
/
31
′ of the optical modulator chip
2
via wires from the electrical terminals
71
.
FIG. 2
illustrates a side view of the optical modulator in the direction shown as A—A in FIG.
1
.
FIG. 2
shows electrodes
31
/
41
/
31
′ on a buffer layer
8
terminating on the top surface edge of the optical modulator chip
2
and the grounded base
1
underlying the optical modulator chip
2
. Although the ground electrodes
31
/
31
′ of
FIGS. 1 and 2
are shown as single lines, the ground electrodes may be ground planes which cover most of the top surface of the optical modulator chip
2
except for the signal electrode
4
and an area just outside the signal electrode
4
. For example, there can be ground planes that cover most of the top surface of the optical modulator chip
2
but are no closer to the signal electrode than the ground electrodes
3
/
3
′ shown.
The intended electrical guided mode for an optical modulator contains the frequency of an input or frequencies of input on the optical modulator for operating the optical modulator. Typically, an optical modulator has a range of sets of frequencies that can be used as electrical inputs to modulate an optical signal. For proper operation of the modulator, the intended electrical guided mode of the device must be such that the electric fields originating on the signal electrode must properly terminate on the adjacent ground electrodes without straying elsewhere in the modulator chip or package. The intended electrical guided mode of the optical modulator will herein after be referred to as the dominant CPW mode of the optical modulator.
Once the electric fields of the signal electrode penetrate through the buffer layer into the optical modulator chip, several other effects could occur. Depending on frequency, a CPW mode may couple with other extraneous electrical modes that the structure of the optical modulator can support. These modes could either be highly dispersive slab modes, or could be zero-cut-off modes. Examples of extraneous modes are: transverse-electric (TE) or transverse magnetic slab modes, slot-line mode (that could occur between the two ground planes of the CPW structure), parallel-plate modes (that could be excited between the electrodes on the top surface and the grounded base), and microstrip mode (between the top electrodes and the grounded base). When coupling to extraneous modes occurs, there is a loss of power for the dominant CPW mode. Such a power loss degrades the optical modulator's modulation performance and the clarity of the output modulated optical signal is degraded. The amount of power lost to spurious or other extraneous modes depends on the field overlap between the dominant CPW mode and the other extraneous modes supported by the device.
One approach to avoid coupling to spurious or other extraneous modes in CPW structures is by reducing the cross-sectional dimension of the CPW transmission line. Referring to
FIG. 1
, by decreasing (S+
2
D), which is the width of electrode
4
plus twice the distance that one of the optical modulator grounds
3
/
3
′ is located from the signal electrode
4
, there is less field penetration into the optical modulator chip
2
and hence less of an opportunity for overlap between the guided CPW mode and other extraneous modes that can be supported by the device. Since there is less overlap in structures with smaller (S+
2
D), between the CPW mode and other extraneous modes, there is less of a power loss from the CPW mode and hence less degradation of the outputted modulated optical signal.
However, a CPW transmission line with a smaller cross-sectional dimension is not very practical because the device still requires external electrical connection. Typically, in the nonactive se
Blanchard Richard C.
Donovan Kevin G.
Gopalakrishnan Ganesh K.
Codeon Corporation
Epps Georgia
Morgan & Lewis & Bockius, LLP
Spector David N.
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