U.S. patent application number 16/665222 was filed with the patent office on 2020-10-01 for optical modulator and optical transmission apparatus using the same.
The applicant listed for this patent is SUMITOMO OSAKA CEMENT CO., LTD.. Invention is credited to Norikazu Miyazaki, Toru Sugamata.
Application Number | 20200310218 16/665222 |
Document ID | / |
Family ID | 1000005087273 |
Filed Date | 2020-10-01 |
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United States Patent
Application |
20200310218 |
Kind Code |
A1 |
Miyazaki; Norikazu ; et
al. |
October 1, 2020 |
OPTICAL MODULATOR AND OPTICAL TRANSMISSION APPARATUS USING THE
SAME
Abstract
An optical modulator includes an optical modulation element
having a plurality of signal electrodes; a plurality of signal
input terminals each of which inputs an electrical signal to be
applied to each signal electrode; a relay substrate on which a
plurality of signal conductor patterns electrically connecting the
signal input terminals and the signal electrodes, and a plurality
of ground conductor patterns are formed; and a housing that houses
the optical modulation element and the relay substrate, in which
the relay substrate has at least one groove extending from the
signal input side on which the signal input terminal is connected
to the signal conductor pattern, in at least one ground conductor
pattern formed between adjacent signal conductor patterns, and the
groove is formed such that a length extending from the signal input
side is longer than a length of the signal input terminal extending
on the signal conductor pattern.
Inventors: |
Miyazaki; Norikazu; (Tokyo,
JP) ; Sugamata; Toru; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO OSAKA CEMENT CO., LTD. |
Tokyo |
|
JP |
|
|
Family ID: |
1000005087273 |
Appl. No.: |
16/665222 |
Filed: |
October 28, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02F 1/2255 20130101;
G02B 26/02 20130101; G02B 6/2935 20130101; G02F 1/2252
20130101 |
International
Class: |
G02F 1/225 20060101
G02F001/225; G02B 26/02 20060101 G02B026/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2019 |
JP |
2019-061560 |
Claims
1. An optical modulator comprising: an optical modulation element
having a plurality of signal electrodes; a plurality of signal
input terminals each of which inputs an electrical signal to be
applied to each of the signal electrodes; a relay substrate on
which a plurality of signal conductor patterns that electrically
connect each of the signal input terminals to each of the signal
electrodes, and a plurality of ground conductor patterns are
formed; and a housing that houses the optical modulation element
and the relay substrate; wherein on a signal input side of the
relay substrate where the electrical signal from the signal input
terminal is input to the signal conductor pattern, the signal input
terminal is disposed to extend from the signal input side onto the
signal conductor pattern, wherein the relay substrate has at least
one groove extending from the signal input side, in at least one
ground conductor pattern formed between the signal conductor
patterns adjacent to each other, on a front surface on which the
signal conductor pattern is formed, and wherein the groove is
formed such that a length of the groove extending from the signal
input side is longer than a length of the signal input terminal
extending from the signal input side,
2. The optical modulator according to claim 1, wherein the groove
extends up to a signal output side of the relay substrate, where an
electrical signal is output from the signal conductor pattern to
the signal electrode of the optical modulation element,
3. The optical modulator according to claim 1, wherein the groove
is formed such that a depth of an end of the groove measured from
the front surface at the signal input side is deeper than a depth
of the groove measured from the front surface at the other end of
the groove.
4. The optical modulator according to claim 3, wherein the groove
is formed such that the depth measured from the front surface is
deepened stepwise or continuously from the other end of the groove
toward the signal input side.
5. The optical modulator according to claim 2, wherein the groove
is formed up to a rear surface of the relay substrate facing the
front surface at the signal input side, or is formed up to the rear
surface of the relay substrate facing the front surface within a
range of a predetermined distance from the signal input side.
6. The optical modulator according to claim 1, wherein a metal film
is formed on an inner surface of the groove, or on the inner
surface and a bottom surface of the groove.
7. The optical modulator according to claim 1, wherein a metal film
is formed on an inner surface and a bottom surface of the groove,
wherein a ground conductor is formed on a rear surface of the relay
substrate facing the front surface, and wherein a via that connects
the metal film on the bottom surface and the ground conductor on
the rear surface is formed on the bottom surface of the groove.
8. The optical modulator according to claim 1, wherein the groove
does not extend up to a signal output side of the relay substrate,
where the electrical signal is output from the signal conductor
pattern to the signal electrode of the optical modulation element,
and the entire groove is formed so as to penetrate to a rear
surface of the relay substrate facing the front surface.
9. The optical modulator according to claim 8, wherein a metal film
is formed on an inner surface of the groove.
10. The optical modulator according to claim 1, wherein the groove
is formed such that a length of the groove extending from the
signal input side is longer than a width of the groove measured
along a direction orthogonal to a direction of the extension.
11. An optical transmission apparatus comprising: the optical
modulator according to claim 1; and an electronic circuit that
outputs an electrical signal for causing the optical modulator to
perform a modulation operation.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Japanese Patent
Application No. 2019-061560 filed Mar. 27, 2019, the disclosure of
which is herein incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to an optical modulator
including a relay substrate that relays propagation of an
electrical signal between a signal input terminal and an optical
modulation element electrode, and an optical transmission apparatus
using the optical modulator.
Description of Related Art
[0003] In a high-speed/large-capacity optical fiber communication
system, an optical modulator incorporating a waveguide type optical
modulation element is often used. Among these, optical modulation
elements in which LiNbO.sub.3 (hereinafter, also referred to as LN)
having an electro-optic effect is used for substrates has a small
light loss and can realize a wide band optical modulation
characteristic, so the optical modulation elements are widely used
for high-frequency/large-capacity optical fiber communication
systems.
[0004] In an optical modulation element using the LN substrate, a
Mach-Zehnder type optical waveguide and a signal electrode for
applying a high-frequency electrical signal as a modulation signal
to the optical waveguide are provided. The signal electrodes
provided in the optical modulation element are connected to lead
pins or connectors which are signal input terminals provided on the
housing, through a relay substrate provided in the housing of the
optical modulator accommodating the optical modulation element.
Since the lead pins and connectors that are signal input terminals
are connected to a circuit board on which an electronic circuit for
causing the optical modulator to perform a modulation operation, an
electrical signal output from the electronic circuit is applied to
the signal electrodes of the optical modulation element through the
relay substrate.
[0005] Due to the increasing transmission capacity in recent years,
the main stream of modulation methods in optical fiber
communication systems is multi-level modulation and the
transmission format adopting polarized wave multiplexing for
multi-level modulation, such as Quadrature Phase Shift Keying
(QPSK) and Dual Polarization-Quadrature Phase Shift Keying
(DP-QPSK), which are used in fundamental optical transmission
networks and is also being introduced into a metro networks.
[0006] An optical modulator that performs QPSK modulation (QPSK
optical modulator) and an optical modulator that performs DP-QPSK
modulation (DP-QPSK optical modulator) each include a plurality of
Mach-Zehnder optical waveguides in a nest structure called
so-called nested type, each of which includes at least one signal
electrode. Therefore, these optical modulators are provided with a
plurality of signal electrodes, and the above-described DP-QPSK
modulation operation is performed in cooperation with
high-frequency electrical signals applied to these signal
electrodes.
[0007] In the optical modulator in which high-frequency electrical
signals applied to the plurality of signal electrodes cooperate,
all the high-frequency electrical signals are required to be input
to the signal electrodes of the optical modulation element without
being affected by noise or the like. However, on the other hand,
the demand for downsizing of the optical modulator is unchanged,
and the downsizing of the relay substrate is progressing with
downsizing of the housing of the optical modulator. As a result, a
plurality of different high-frequency signals propagate in close
proximity to a narrow relay substrate, and electrical crosstalk
between high-frequency signal lines formed on the relay substrate
is becoming impossible to ignore.
[0008] In addition, commercial DP-QPSK modulators are often used at
a transmission rate of 100 Gb/s at present, but development for
increasing the transmission rate to 400 Gb/s is also in progress.
If the transmission rate is increased in the future, the problem of
crosstalk between the high-frequency signal lines generated in the
relay substrate may become a more serious problem.
[0009] As a method of suppressing the crosstalk, it is conceivable
to increase the distance between adjacent high-frequency signal
lines, but this method is contrary to the demand for downsizing of
the optical modulator as described above, and is difficult to
adopt. Therefore, a method of providing, for example, vias in the
ground electrodes provided between the high-frequency signal lines
and connecting them to the ground layer on the rear surface of the
relay substrate, and strengthen the ground electrodes to increase
the shielding effect between the high-frequency signal lines is
adopted (see, for example, Japanese Laid-open Patent Publication
No. 2012-156947).
[0010] However, in the DP-QPSK modulator having a high transmission
rate of 400 Gb/s or more, there is a problem that the crosstalk
cannot be sufficiently suppressed only by the vias as described
above.
[0011] According to experiments conducted by the inventor of the
present invention, in the DP-QPSK modulator having the high
transmission rate as described above, with respect to the above
crosstalk, the influence of leakage of the high-frequency signal at
the input portion accompanying not only the direct transfer of
signal energy between adjacent high-frequency signal lines, but
also the occurrence of signal propagation mode conversion
(hereinafter referred to as propagation mode conversion) mainly at
the high-frequency signal input part (signal input part) of the
relay substrate cannot be ignored.
[0012] That is, since a connector, a lead pin, or the like is
generally used for inputting a high-frequency signal to the optical
modulator, the high-frequency signal propagates in a coaxial mode
until it is input to the relay substrate. On the other hand, the
high-frequency signal line provided on the optical modulation
element substrate or the relay substrate is generally a coplanar
line, and the propagation mode in the line is a coplanar mode
(hereinafter, referred to as CPW mode).
[0013] Therefore, in the signal input part of the relay substrate,
propagation mode conversion from the coaxial mode to the CPW mode
(that is, different mode conversion) occurs, and a part of the
energy of the high-frequency signal propagating in the coaxial mode
is released in a radiation mode to the inside or outside (to the
air) of the relay substrate. A part of the high-frequency signal
energy released to the inside or outside (to the air) of the relay
substrate additionally acts on the occurrence of the crosstalk.
SUMMARY OF THE INVENTION
[0014] From the above background, in an optical modulator including
a relay substrate that electrically connects each of the signal
electrodes of the optical modulation element and each of the signal
input terminals, it is required to effectively suppress the
increase in crosstalk between the signal conductor patterns on the
relay substrate due to an increase in a transmission rate, and
realize good optical modulation characteristics.
[0015] According to an aspect of the present invention, there is
provided an optical modulator including: an optical modulation
element having a plurality of signal electrodes; a plurality of
signal input terminals each of which inputs an electrical signal to
be applied to each of the signal electrodes; a relay substrate on
which a plurality of signal conductor patterns that electrically
connect each of the signal input terminals to each of the signal
electrodes, and a plurality of ground conductor patterns are
formed; and a housing that houses the optical modulation element
and the relay substrate, in which on a signal input side of the
relay substrate where the electrical signal from the signal input
terminal is input to the signal conductor pattern, the signal input
terminal is disposed to extend from the signal input side onto the
signal conductor pattern, in which the relay substrate has at least
one groove extending from the signal input side, in at least one
ground conductor pattern formed between the signal conductor
patterns adjacent to each other, on a front surface on which the
signal conductor pattern is formed, and in which the groove is
formed such that a length of the groove extending from the signal
input side is longer than a length of the signal input terminal
extending from the signal input side.
[0016] According to another aspect of the present invention, the
groove extends up to a signal output side of the relay substrate
where an electrical signal is output from the signal conductor
pattern to the signal electrode of the optical modulation
element.
[0017] According to another aspect of the present invention, the
groove is formed such that a depth of an end of the groove measured
from the front surface at the signal input side is deeper than a
depth of the groove measured from the front surface at the other
end of the groove.
[0018] According to another aspect of the present invention, the
groove is formed such that the depth measured from the front
surface is deepened stepwise or continuously from the other end of
the groove toward the signal input side.
[0019] According to another aspect of the present invention, the
groove is formed up to a rear surface of the relay substrate facing
the front surface at the signal input side, or is formed up to a
rear surface of the relay substrate facing the front surface within
a range of a predetermined distance from the signal input side.
[0020] According to another aspect of the present invention, a
metal film is formed on the inner surface of the groove or on the
inner surface and the bottom surface of the groove.
[0021] According to another aspect of the present invention, a
metal film is formed on the inner surface and the bottom surface of
the groove, a ground conductor is formed on a rear surface of the
relay substrate facing the front surface, and a via for connecting
the metal film on the bottom surface and the ground conductor on
the rear surface is formed on the bottom surface of the groove.
[0022] According to another aspect of the present invention, the
groove does not extend up to a signal output side of the relay
substrate where an electrical signal is output from the signal
conductor pattern to the signal electrode of the optical modulation
element, and the entire groove is formed so as to penetrate to a
rear surface of the relay substrate facing the front surface.
[0023] According to another aspect of the present invention, a
metal film is formed on an inner surface of the groove.
[0024] According to another aspect of the present invention, the
groove is formed such that a length of the groove extending from
the signal input side is longer than a width of the groove measured
along a direction orthogonal to a direction of the extension.
[0025] Another aspect of the present invention is an optical
transmission apparatus including any one of the optical modulators
described above and an electronic circuit that outputs an
electrical signal for causing the optical modulator to perform a
modulation operation.
[0026] According to the present invention, in an optical modulator
provided with a relay substrate, it is possible to effectively
suppress an increase in crosstalk between signal conductor patterns
on the relay substrate due to an increase in a transmission rate,
and to realize good optical modulation characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a plan view of an optical modulator according to a
first embodiment of the present invention.
[0028] FIG. 2 is a side view of the optical modulator shown in FIG.
1.
[0029] FIG. 3 is a detailed view of part A of the optical modulator
shown in FIG. 1.
[0030] FIG. 4 is a perspective view of a front surface of a relay
substrate used in the optical modulator shown in FIG. 1 as viewed
from a side where signal input terminals are disposed.
[0031] FIG. 5 is a diagram illustrating a first modification
example of the relay substrate used in the optical modulator
according to the first embodiment.
[0032] FIG. 6 is a diagram illustrating a second modification
example of the relay substrate used in the optical modulator
according to the first embodiment.
[0033] FIG. 7 is a diagram illustrating a third modification
example of the relay substrate used in the optical modulator
according to the first embodiment.
[0034] FIG. 8 is a diagram illustrating a fourth modification
example of the relay substrate used in the optical modulator
according to the first embodiment.
[0035] FIG. 9 is a diagram illustrating a fifth modification
example of the relay substrate used in the optical modulator
according to the first embodiment.
[0036] FIG. 10 is a diagram illustrating a configuration of an
optical transmission apparatus according to a second embodiment of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0037] In order to solve the above-described problems, in the
following embodiments and modification examples thereof, an
increase in crosstalk between adjacent signal conductor patterns on
the relay substrate due to an increase in transmission rate is
suppressed, without violating the demand for downsizing of the
optical modulator, at the same level as or lower than the cost in
the related art, and also considering manufacturability.
Specifically, two radiation microwaves of a radiation mode
generated by the signal conductor pattern as the high-frequency
waves propagate through the signal conductor patterns on the relay
substrate (hereinafter referred to as propagation radiation
microwaves) and a radiation mode generated along with the different
mode conversion at the connection point between the signal input
terminal and the signal conductor pattern (hereinafter referred to
as connection point radiation microwaves) are simultaneously
suppressed from propagating through the relay substrate, and the
generation of high-frequency energy transfer between adjacent
signal conductor patterns is suppressed. Hereinafter, embodiments
of the present invention will be described with reference to the
drawings.
First Embodiment
[0038] First, a first embodiment of the present invention will be
described. FIGS. 1 and 2 are diagrams showing a configuration of an
optical modulator 100 according to the first embodiment of the
present invention. Here, FIGS. 1 and 2 are a plan view and a side
view of the optical modulator 100, respectively.
[0039] The optical modulator 100 includes an optical modulation
element 102, a housing 104 that houses the optical modulation
element 102, an input optical fiber 108 for inputting light into
the optical modulation element 102, and an output optical fiber 110
that guides the light output from the optical modulation element
102 to the outside of the housing 104.
[0040] The optical modulation element 102 is a DP-QPSK modulator
that performs optical modulation of 400 Gb/s, for example, and
includes, for example, four Mach-Zehnder type optical waveguides
provided on an LN substrate. The four Mach-Zehnder type optical
waveguides are provided with four signal electrodes 112a, 112b,
112c, and 112d that respectively modulate light waves propagating
through the Mach-Zehnder type optical waveguide. As known in the
related art, on the surface of the LN substrate of the optical
modulation element 102, for example, ground electrodes 122a, 122b,
122c, 122d, and 122e (not shown in FIG. 1, and shown in FIG. 3)
constituting a coplanar waveguide (CPW) are provided for the four
signal electrodes 112a, 112b, 112c, and 112d.
[0041] Specifically, the ground electrodes 122a, 122b, 122c, 122d,
and 122e are disposed so as to sandwich the signal electrodes 112a,
112b, 112c, and 112d therebetween in the surface of the LN
substrate surface, and constitute a coplanar waveguide having a
predetermined characteristic impedance in a predetermined operating
frequency together with the four signal electrodes 112a, 112b,
112c, and 112d.
[0042] Four high-frequency electrical signals (modulation signals)
are input to the four signal electrodes 112a, 112b, 112c, and 112d,
respectively. These high-frequency electrical signals cooperate to
control the propagation of the light wave in the four Mach-Zehnder
type optical waveguides, and perform the operation of DP-QPSK
modulation of 400 Gb/s as a whole.
[0043] The two light rays output from the optical modulation
element 102 are polarized and combined by a lens optical system
(not shown), for example, and guided to the outside of the housing
104 through the output optical fiber 110.
[0044] The housing 104 includes a case 114a to which the optical
modulation element 102 is fixed, and a cover 114b. In order to
facilitate understanding of the configuration inside the housing
104, only a part of the cover 114b is shown on the left side in
FIG. 1, but actually, the cover 114b is disposed to cover the
entire box-shaped case 114a and hermetically seals the inside of
the housing 104. The case 114a is made of a metal or a ceramic
plated with gold, for example, and functions electrically as an
electric conductor. The housing 104 is usually provided with a
plurality of pins for DC control or the like, which are omitted in
FIG. 1.
[0045] The case 114a includes electrical connectors 116a, 116b,
116c, and 116d having signal input terminals 124a, 124b, 124c, and
124d for inputting high-frequency electrical signals respectively
applied to the signal electrodes 112a, 112b, 112c, 112d of the
optical modulation element 102. In addition, a relay substrate 118
is accommodated in the housing 104. As will be described later, the
relay substrate 118 has signal conductor patterns 330a, 330b, 330c,
and 330d that electrically connect the signal input terminals 124a,
124b, 124c, and 124d and one ends of the signal electrodes 112a,
112b, 112c, and 112d of the optical modulation element 102
respectively, and ground conductor patterns 340a, 340b, 340c, 340d,
and 340e.
[0046] The other ends of the signal electrodes 112a, 112b, 112c,
and 112d of the optical modulation element 102 are terminated by a
terminator 120 having a predetermined impedance. Thus, the
electrical signals input to the one ends of the signal electrodes
112a, 112b, 112c, and 112d propagate in the signal electrodes 112a,
112b, 112c, and 112d as traveling waves.
[0047] Each of the electrical connectors 116a, 116b, 116c, and 116d
is, for example, a socket of a push-on type coaxial connector. The
cylindrical ground conductors of these electrical connectors 116a,
116b, 116c, and 116d are electrically connected and fixed to the
case 114a. Therefore, the case 114a corresponds to a structure
connected to the ground potential. The signal input terminals 124a,
124b, 124c, and 124d are, for example, center conductors (core
wires) extending along the cylindrical center lines of the ground
conductors in the connector sockets that are the electrical
connectors 116a, 116b, 116c, and 116d.
[0048] FIG. 3 is a partial detail view of a part A in FIG. 1, and
shows a configuration of the relay substrate 118 and its
surroundings. FIG. 4 is a perspective view of the front surface
418e (the surface shown in FIGS. 1 and 3) of a single relay
substrate 118 as viewed from the side where the signal input
terminal 124a and the like are disposed.
[0049] The signal conductor patterns 330a, 330b, 330c, and 330d and
the ground conductor patterns 340a, 340b, 340c, 340d, and 340e are
provided on the front surface 418e of the relay substrate 118.
[0050] The ground conductor patterns 340a, 340b, 340c, 340d, and
340e are provided so as to sandwich the signal conductor patterns
330a, 330b, 330c, and 330d in the surface of the front surface 418e
of the relay substrate 118. Thus, the signal conductor patterns
330a, 330b, 330c, and 330d constitute coplanar lines together with
the ground conductor patterns 340a, 340b, 340c, 340d, and 340e,
respectively.
[0051] As shown in FIG. 3, the signal electrodes 112a, 112b, 112c,
and 112d of the optical modulation element 102 are electrically
connected to one ends of the signal conductor patterns 330a, 330b,
330c, and 330d of the relay substrate 118, respectively, by wire
bonding using the conductor wires 126, for example. Here, the
conductor wire 126 can be a gold wire, for example.
[0052] Further, in the optical modulation element 102, the ground
electrodes 122a, 122b, 122c, 122d, and 122e that constitute the
coplanar line together with the signal electrodes 112a, 112b, 112c,
and 112d are electrically connected to one ends of the ground
conductor patterns 340a, 340b, 340c, 340d, and 340e of the relay
substrate 118, respectively, by wire bonding using the conductor
wires 126, for example, in the same manner as described above. The
wire bonding using the conductor wire 126 described above is an
example, and the present invention is not limited to this. Instead
of wire bonding of the conductor wire 126, ribbon bonding using a
conductor ribbon such as a gold ribbon can be used.
[0053] As shown in FIGS. 3 and 4, the signal input terminals 124a,
124b, 124c, and 124d of the electrical connectors 116a, 116b, 116c,
and 116d disposed on the case 114a of the housing 104 are fixed and
electrically connected to the other ends of the signal conductor
patterns 330a, 330b, 330c, and 330d of the relay substrate 118,
respectively. These fixing and electrical connection can be
performed by using solder, brazing material, or conductive
adhesive, for example.
[0054] Here, the side of the relay substrate 118 on which the
signal conductor patterns 330a, 330b, 330c, and 330d and the signal
input terminals 124a, 124b, 124c, and 124d are respectively
connected is referred to as a signal input side 418a, and a side
surface of the relay substrate 118 having the signal input side
418a as one side is referred to as an input side surface 418b.
Further, the side of the relay substrate 118 that faces the signal
input side 418a, that is, the side where the signal conductor
patterns 330a, 330b, 330c, and 330d and the signal electrodes 112a,
112b, 112c, and 112d of the optical modulation element 102 are
connected is referred to as a signal output side 418c, and the side
of the relay substrate 118 having the signal output side 418c as
one side is referred to as an output side surface 418d. In FIG. 3,
of the relay substrate 118, the side surface on the right side in
FIG. 3 orthogonal to the input side surface 418b is referred to as
a right side surface 418g, and the side surface on the left side in
FIG. 3 is referred to as a left side surface 418h. Further, the
surface of the relay substrate 118 that faces the front surface
418e is referred to as a rear surface 418f.
[0055] As shown in FIG. 3, the signal input terminals 124a, 124b,
124c, and 124d of the electrical connectors 116a, 116b, 116c, and
116d are disposed on the signal conductor patterns 330a, 330b,
330c, and 330d of the relay substrate 118 so as to extend from the
signal input side 418a with a predetermined length L1. Here, the
portions where the signal conductor patterns 330a, 330b, 330c, and
330d are connected to the signal input terminals 124a, 124b, 124c,
and 124d on the signal input side 418a and the vicinity thereof are
referred to as input connection points.
[0056] In particular, in the relay substrate 118 of the optical
modulator 100 according to the present embodiment, on the front
surface 418e where the signal conductor patterns 330a, 330b, 330c,
and 330d are formed, grooves 350a, 350b, 350c, 350d, and 350e
extending from the signal input side 418a (or the input side
surface 418b) are provided in the ground conductor patterns 340a,
340b, 340c, 340d, and 340e. Further, the grooves 350a, 350b, 350c,
350d, and 350e have a predetermined depth from the front surface
418e of a single relay substrate 118 (not penetrating the relay
substrate 118). Here, in the present embodiment, as an example, the
grooves 350a, 350b, 350c, 350d, and 350e are configured to have the
same size as each other.
[0057] Hereinafter, the signal electrodes 112a, 112b, 112c, and
112d of the optical modulation element 102 are collectively
referred to as the signal electrode 112, and the ground electrodes
122a, 122b, 122c, 122d, and 122e are collectively referred to as
the ground electrode 122. Further, the electrical connectors 116a,
116b, 116c, and 116d are collectively referred to as the electrical
connector 116, and the signal input terminals 124a, 124b, 124c, and
124d are collectively referred to as the signal input terminal 124.
Further, the signal conductor patterns 330a, 330b, 330c, and 330d
of the relay substrate 118 are collectively referred to as the
signal conductor pattern 330, and the ground conductor patterns
340a, 340b, 340c, 340d, and 340e are collectively referred to as
the ground conductor pattern 340. Further, the grooves 350a, 350b,
350c, 350d, and 350e are collectively referred to as a groove
350.
[0058] Here, since the four signal input terminals 124 are the
central conductors of the four electrical connectors 116 that are
coaxial connectors as described above, the propagation mode of the
high-frequency electrical signal propagating through the signal
input terminals 124 is the coaxial mode. In addition, as described
above, the signal conductor patterns 330 on the relay substrate 118
constitute a coplanar line together with the six ground conductor
patterns 340, and propagate high-frequency electrical signals in
the coplanar mode (CPW mode).
[0059] Therefore, at the input connection point where the signal
conductor pattern 330 is connected to the signal input terminal
124, the propagation mode conversion (different mode conversion)
from the coaxial mode to the CPW mode occurs. Therefore, at each of
the input connection points, connection point radiation microwaves
are generated along with the different mode conversion and can
propagate inside the relay substrate 118.
[0060] The signal conductor pattern 330 of the relay substrate 118
generates propagation radiation microwaves when a high-frequency
electrical signal propagates in the longitudinal direction starting
from the input connection point, and the propagation radiation
microwaves can propagate in the relay substrate 118. The
propagation radiation microwave can be generated from each part in
the longitudinal direction of the signal conductor pattern 330, and
the generation intensity thereof generally has a maximum intensity
at an input connection point, for example, and decreases as it goes
away from the signal input side 418a.
[0061] In the optical modulator 100 having the above configuration,
since the ground conductor pattern 340 is provided with the groove
350 extending from the signal input side 418a, the connection point
radiation microwaves that can be generated at the input connection
point and propagation radiation microwaves generated in the
vicinity of the input connection point of the signal conductor
pattern 330 are prevented from propagating inside the relay
substrate 118 by the air walls formed by the grooves 350,
respectively.
[0062] In other words, in the optical modulator 100, the transfer
of high-frequency energy between the signal conductor patterns 330
through each of the connection radiation microwave and the
propagation radiation microwave is suppressed by the presence of
the groove 350.
[0063] As a result, even in a case where the optical modulator 100
is operated at a transmission rate of 400 Gb/s or higher, the
distance between the signal conductor patterns 330 is not increased
(thus, the relay substrate 118 and the optical modulator 100 are
not increased in size), and the crosstalk between the signal
conductor patterns 330 can be effectively reduced at a cost equal
to or lower than that in the related art, so good optical
modulation characteristics can be realized.
[0064] Here, according to the knowledge of the inventors of the
present invention, it is preferable that the size of the groove 350
has any of the following relationships, when the length of the
signal input terminal 124 extending from the signal input side 418a
on the signal conductor pattern 330 is L1, the length of the groove
350 extending from the signal input side 418a is L2, and the width
measured in the direction orthogonal to the longitudinal direction
is W (see FIG. 3). Here, the length L1 is a length of the signal
input terminal 124 extending on the signal conductor pattern 330
from the signal input side 418a as described above. Hereinafter,
the length L1 is referred to as a terminal extension length L1.
L2>L1 (1)
L2>W (2)
[0065] The length L2 is a length in which the groove 350 extends
from the signal input side 418a along the groove 350, and is not a
distance between the signal input side 418a and the end of the
groove 350 in a direction orthogonal to the signal input side 418a.
For example, in a case where the groove 350 is formed in a curved
line instead of a straight line, the length is measured along the
curved line (for example, the length in which the center line in
the width direction of the groove 350 extends). In addition, in a
case where the width W varies along the extending direction of the
groove 350, the width W can be set to the average value or the
maximum value of the width in each portion.
[0066] Expression (1) means that the signal input terminal 124 is
configured such that the length L2 of the groove 350 extending from
the signal input side 418a is longer than the terminal extension
length L1 extending from the signal input side 418a. Here, it is
desirable that L2>L1 because the connection point radiation
microwave can be generated over the range of the terminal extension
length L1 at the input connection point, so when the length L2 of
the groove is shorter than the terminal extension length L1, the
effect of suppressing propagation of the connection point radiation
microwaves is reduced.
[0067] Further, Expression (2) means that the groove 350 is formed
such that the length L2 extending is longer than the width W
measured along the direction orthogonal to the direction extending
from the signal input side 418a. Here, it is desirable that L2>W
because the propagation radiation microwave that can be generated
from the signal conductor pattern 330 in the vicinity of the signal
input side 418a may not be suppressed when the length L2 of the
groove 350 is shorter than the width W and because the width of the
groove 350 may be unnecessarily increased, and the mechanical
strength of the relay substrate 118 may be reduced when
W>L2.
[0068] In FIG. 3, the signal input terminal 124a and the groove
350a are taken as an example, and the terminal extension length L1
and the length L2 and width W of the groove 350 are shown, but
these can be defined similarly for the other grooves 350b, 350c,
350d, and 350e. In the present embodiment, the five grooves 350 are
formed to have the same size as each other, but the present
invention is not limited to this. As long as the relationship of
Expression (1) or Expression (2) is satisfied in each of the
grooves 350, the grooves 350 may be configured to have different
sizes. The above-described terminal extension length L1, the length
L2 and width W of the groove 350, and the preferable conditions
shown in Expression (1) or Expression (2) are the same even in the
first to fifth modification examples to be described later and can
be applied thereto.
[0069] Further, if a metal film is formed on the inner surface of
the groove 350 (if metallization is performed), the effect of
suppressing the propagation of the connection point radiation
microwaves and the propagation radiation microwaves into the relay
substrate 118 can be further enhanced. The metal films are
desirably provided on at least two inner surfaces of the inner
surfaces of the grooves 350 facing each other in the direction of
the signal input side 418a. In addition to this, it is more
desirable that a metal film is also formed on the bottom surface (a
surface parallel to the front surface 418e) and/or the end surface
(a surface parallel to the input side surface 418b) of the groove
350.
[0070] In the present embodiment, one groove 350 is provided in
each of the five ground conductor patterns 340, but the present
invention is not limited to this. From the viewpoint of reducing
crosstalk between the adjacent signal input terminals 124 and/or
between the adjacent signal conductor patterns 330, the ground
conductor patterns 340 (that is, in the present embodiment, the
ground conductor patterns 340b, 340c, and 340d) formed between at
least the adjacent signal conductor patterns 330 may have at least
one groove 350.
[0071] However, if the groove 350 is also formed in the ground
conductor patterns 340a, 340e that are not sandwiched between the
adjacent signal conductor patterns 330 as in the present
embodiment, the propagation of connection point radiation
microwaves and propagation radiation microwaves, which are
generated from, for example, the input connection point of the
outermost signal conductor patterns 330a, 330d and propagate to and
reflected on the right side surface 418g and the left side surface
418h orthogonal to the input side surface 418b of the relay
substrate 118 can be reduced. Thereby, for example, the connection
point radiation microwaves and the propagation radiation microwaves
generated from the signal conductor patterns 330a, 330d and
returning to themselves are reduced, and noise generated in the
signal conductor patterns 330a, 330d caused by the reflected
microwaves returning can be reduced.
[0072] In the present embodiment, one ground conductor pattern 340
is provided between the adjacent signal conductor patterns 330, but
the present invention is not limited to this. There may be a
plurality of ground conductor patterns 340 provided between
adjacent signal conductor patterns 330. In this case, a groove
similar to the groove 350 can be formed in at least one ground
conductor pattern 340 provided between adjacent signal conductor
patterns 330. Alternatively, a groove similar to the groove 350 can
be provided in the ground conductor pattern 340 which is adjacent
to each of the signal conductor patterns 330 (closest to each of
the signal conductor patterns 330), among the plurality of ground
conductor patterns 340 provided between the adjacent signal
conductor patterns 330.
[0073] In the present embodiment, each of the signal conductor
patterns 330 is formed as a linear shape extending in a direction
orthogonal to the signal input side 418a as an example, but the
present invention is not limited thereto. The signal conductor
pattern 330 is formed in a straight line or a curved line that is
not orthogonal to the signal input side 418a, according to the
interval between the signal input terminals 124, the interval
between the signal electrodes 112 in the optical modulation element
102, or other electrical requirements.
[0074] In FIG. 3, the pattern interval between the signal conductor
pattern 330 and the ground conductor pattern 340 is illustrated as
being substantially constant, but the present invention is not
limited thereto. The patterns may be formed at different intervals,
for example, in the part in the range of the length L2 from the
signal input side 418a in which the groove 350 is formed and the
other parts such that the distributed impedance of the coplanar
line formed by the signal conductor pattern 330 and the ground
conductor pattern 340 is in a predetermined value range in the
operating frequency range of the optical modulation element 102,
according to the technique in the related art.
[0075] Next, a modification example of the relay substrate 118 that
can be used in the optical modulator 100 according to the first
embodiment will be described.
First Modification Example
[0076] FIG. 5 is a diagram illustrating a configuration of a relay
substrate 518 according to a first modification example. The relay
substrate 518 can be used instead of the relay substrate 118 in the
optical modulator 100 shown in FIG. 1. In FIG. 5, the same
reference numerals as those in FIG. 4 are used for the same
components as those of the relay substrate 118 shown in FIG. 4, and
the above description of FIG. 4 is adopted.
[0077] The relay substrate 518 shown in FIG. 5 has the same
configuration as that of the relay substrate 118 shown in FIG. 4,
except that grooves 550a, 550b, 550c, 550d, and 550e are provided
instead of the grooves 350a, 350b, 350c, 350d, and 350e. The
grooves 550a, 550b, 550c, 550d, and 550e have the same
configuration as the grooves 350a, 350b, 350c, 350d, and 350e, but
are different in that the grooves do not extend up to a signal
output side 418c of the relay substrate 518, where an electrical
signal is output from the signal conductor pattern 330 to the
signal electrode 112 of the optical modulation element 102, and the
entire of each of the grooves 550a, 550b, 550c, 550d, and 550e is
formed so as to penetrate to the rear surface 418f facing the front
surface 418e, from the front surface 418e of the relay substrate
518.
[0078] Further, the grooves 550a, 550b, 550c, 550d, and 550e are
different from the grooves 350a, 350b, 350c, 350d, and 350e in that
metal films are formed on three inner surfaces of the grooves,
respectively (metallized). Hereinafter, the grooves 550a, 550b,
550c, 550d, and 550e are collectively referred to as a groove
550.
[0079] The relay substrate 518 having the above-described
configuration is formed such that the entire of each of the grooves
550 extending from the signal input side 418a on the front surface
418e of the relay substrate 518 penetrate to the rear surface 418f
of the relay substrate 518. Therefore, propagation of the two
connection point radiation microwaves and propagation radiation
microwaves generated at the input connection point and the vicinity
thereof into the relay substrate 518 is further suppressed as
compared with the relay substrate 118 shown in FIG. 4. Further, in
the present modification example, since the metal film is formed on
all three inner surfaces of the groove 550, the effect of
suppressing the propagation of the connection point radiation
microwave and the propagation radiation microwave is strengthened
than that of the relay substrate 118. Thus, the effect of reducing
the transfer of high-frequency energy between the signal conductor
patterns 330 through the connection radiation microwave and the
propagation radiation microwave is further enhanced as compared
with the relay substrate 118.
Second Modification Example
[0080] FIG. 6 is a diagram illustrating a configuration of a relay
substrate 618 according to a second modification example. The relay
substrate 618 can be used instead of the relay substrate 118 in the
optical modulator 100 shown in FIG. 1. In FIG. 6, the same
reference numerals as those in FIG. 4 are used for the same
components as those of the relay substrate 118 shown in FIG. 4, and
the above description of FIG. 4 is adopted.
[0081] The relay substrate 618 shown in FIG. 6 has the same
configuration as that of the relay substrate 118 shown in FIG. 4,
except that grooves 650a, 650b, 650c, 650d, and 650e are provided
instead of the grooves 350a, 350b, 350c, 350d, and 350e. The
grooves 650a, 650b, 650c, 650d, and 650e have a partially similar
configuration to the grooves 350a, 350b, 350c, 350d, and 350e, but
are different in that the grooves extend from the signal input side
418a of the relay substrate 618 up to the signal output side 418c
of the relay substrate 618, where an electrical signal is output
from the signal conductor pattern 330 to the signal electrode 112
of the optical modulation element 102.
[0082] In other words, the present modification example corresponds
to the case where the length L2 of the groove 350 is set to the
same value as the width Ws (see FIG. 3) of the relay substrate 118
in the relay substrate 118 shown in FIG. 3 (L2=Ws). Hereinafter,
the grooves 650a, 650b, 650c, 650d, and 650e are collectively
referred to as a groove 650. In addition, this example is an
example, and in a case where the groove 350 is not a straight line,
it is not established that (L2=Ws).
[0083] In the relay substrate 618 having the above-described
configuration, the propagation of the propagation radiation
microwaves that can be generated from each part in the longitudinal
direction of the signal conductor pattern 330 into the relay
substrate 618 is continuously suppressed by the groove 650 from the
signal input side 418a to the signal output side 418c, so the
transfer of high-frequency energy between the signal conductor
patterns 330 is further reduced as compared with the relay
substrate 118 of FIG. 4. Further, since the groove 650 is formed so
as to reach the signal output side 418c from the signal input side
418a in this way, the structure of the relay substrate 618 is
simplified, and thus the relay substrate 618 is manufactured more
easily compared to the relay substrate 118 of FIG. 4.
[0084] Note that the above-described effect of reducing the
transfer of high-frequency energy is strengthened by forming a
metal film on the two inner surfaces of each groove 650 (that is,
two inner surfaces facing each other in the direction of the signal
input side 418a of the relay substrate 618). Further, if a metal
film is also formed on the bottom surface (surface parallel to the
front surface 418e) of each groove 650, the energy of the microwave
that propagates through the relay substrate 618 and is emitted from
the bottom surface to the air is also reduced, so crosstalk between
the signal conductor patterns 330 can be further reduced.
[0085] Here, in a case where a metal film is formed on the bottom
surface of the groove 650, when the relay substrate 618 is fixed to
the housing 104 using solder or brazing material, the solder or
brazing material that protrudes from the rear surface 418f of the
relay substrate 618 may reach the metal film on the bottom surface
of the groove 650 and reach the ground conductor pattern 340
through the metal film on the inner surface of the groove 650.
Solder or brazing material that reaches the ground conductor
pattern 340 makes it difficult to weld the wire in the ground
conductor pattern 340, for example, during wire bonding between the
ground electrode 122 of the optical modulation element 102 and the
ground conductor pattern 340. Therefore, in a case where the solder
or brazing material reaches the ground conductor pattern 340
through the metal film of the groove 650, for example, the metal
film may be formed on the bottom surface of the groove 650 in the
vicinity of the signal output side 418c.
[0086] In the present modification example, since the groove 650 is
provided up to the signal output side 418c, in a case where the
confinement strength of the high-frequency signal in the signal
conductor pattern 330 is weak, or there is a difference in the
consistency of the propagation mode at the signal output point of
the signal output side 418c (the connection point of the signal
conductor pattern 330 and the signal electrode 112), the
disturbance (reflection or radiation of a high-frequency electrical
signal) of the propagation mode may occur due to the presence of
the groove 650 at the signal output point of the signal output side
418c. Therefore, it can be said that the present modification
example is a suitable configuration in a case where it is desired
to simplify the structure of the relay substrate 618 while
facilitating its manufacture, and further reduce the crosstalk
between the signal conductor patterns 330, particularly in the
design in which the confinement strength of the high-frequency
signal in the signal conductor pattern 330 is sufficiently
secured.
Third Modification Example
[0087] FIG. 7 is a diagram illustrating a configuration of a relay
substrate 718 according to a third modification example. The relay
substrate 718 can be used instead of the relay substrate 118 in the
optical modulator 100 shown in FIG. 1. FIG. 7 is different from
FIG. 4 in that the configuration of the relay substrate 718 is
shown using a three-view drawing instead of a perspective view. In
FIG. 7, the same reference numerals as those in FIG. 4 are used for
the same components as those of the relay substrate 118 shown in
FIG. 4, and the above description of FIG. 4 is adopted.
[0088] The relay substrate 718 shown in FIG. 7 has the same
configuration as that of the relay substrate 118 shown in FIG. 4,
except that groove 750a, 750b, 750c, 750d, and 750e are provided
instead of the grooves 350a, 350b, 350c, 350d, and 350e.
[0089] Different from the grooves 350, the grooves 750a, 750b,
750c, 750d, and 750e are formed such that the depths of the ends of
these grooves 750a, 750b, 750c, 750d, and 750e measured from the
front surface 418e at the signal input side 418a are deeper than
the depths of the grooves 750a, 750b, 750c, 750d, and 750e measured
from the front surface 418e at the other ends of these grooves
750a, 750b, 750c, 750d, and 750e. Specifically, the grooves 750a,
750b, 750c, 750d, and 750e have the bottom surfaces formed
stepwise, and are formed so as to penetrate through to the rear
surface 418f of the relay substrate 518, in a range of a
predetermined distance L3 (e<b) from the signal input side
418a.
[0090] That is, the grooves 750a, 750b, 750c, 750d, and 750e are
formed such that the depths thereof increase stepwise (in the
present modification example, in two steps) and the grooves reach
the rear surface 418f at a predetermined distance L3 from the
signal input side 418a. In other words, the grooves 750a, 750b,
750c, 750d, and 750e are formed such that the depths thereof
decrease stepwise toward the signal output side 418c. Hereinafter,
the grooves 750a, 750b, 750c, 750d, and 750e are collectively
referred to as a groove 750.
[0091] In the relay substrate 718 having the above-described
configuration, the portion of the groove 750 in a range of a
predetermined distance L3 from the signal input side 418a is
provided to penetrate to the rear surface 418f, so propagation of
the connection point radiation microwave and the propagation
radiation microwave at the input connection point and in the
vicinity thereof into the relay substrate 718 is effectively
suppressed, as in the Modification Example 1.
[0092] Further, in the relay substrate 718, the groove 750 is
formed such that it extends longer than the groove 350 of the relay
substrate 118 toward the signal output side 418c and the depth
thereof decreases stepwise. Therefore, the disturbance of the
propagation mode in the signal conductor pattern 330 that may occur
due to the presence of the groove 750 decreases stepwise toward the
signal output side 418c and does not occur in the signal output
side 418c. On the other hand, since the propagation radiation
microwaves generated from each part in the length direction of the
signal conductor pattern 330 gradually decrease toward the signal
output side 418c, even if the depth of the groove 750 decreases
stepwise toward the signal output side 418c, the amount of the
propagation radiation microwaves generated from each part of the
signal conductor pattern 330 and propagating through the relay
substrate 718 can be suppressed to a substantially constant value
along the longitudinal direction of the signal conductor pattern
330.
[0093] Therefore, in the relay substrate 718, while the disturbance
of the propagation mode of the signal conductor pattern 330 due to
the presence of the groove 750 is smoothly eliminated toward the
signal output side 418c, the high-frequency energy transfer between
the signal conductor patterns 330 through the connection point
radiation microwaves and the propagation radiation microwaves can
be effectively reduced.
[0094] Here, in the present modification example, it is desirable
that the predetermined distance L3 of the portion in which the
groove 750 extends up to the rear surface 418f of the relay
substrate 718 satisfies either of the followings from the same
reason as the length L2 in the relay substrate 118 (suppression of
propagation of connection point radiation microwaves and
propagation radiation microwaves described above).
L3>L1 (3)
L3>W (4)
[0095] In Expressions (3) and (4), L1 and W are the terminal
extension length and the width of the groove 750, respectively,
defined in the same manner as the example of the relay substrate
118 described above.
[0096] In the present modification example, the groove 750 is
deeply formed in two steps up to the rear surface 418f, but the
number of steps is not limited to two. For example, the number of
steps may be one. That is, the depth of the groove 750 may be
constant from the position of the predetermined distance L3 to the
end of the groove 750. Alternatively, the number of steps may be
three or more.
[0097] Alternatively, the depth of the groove 750 may be configured
to be continuously deep from the end of the groove 750. In this
case, the bottom surface of the groove 750 can reach the rear
surface 418f at the signal input side 418a (that is, L3=0 can be
set).
[0098] In the present modification example, the groove 750 reaches
the rear surface 418f at least at the signal input side 418a, but
the present invention is not limited to this. The grooves 750 may
be formed such that the depth measured from the front surface 418e
at the signal input side 418a is deeper than the depth measured
from the front surface 418e at the ends of the grooves 750. For
example, even if the groove 750 does not necessarily reach the rear
surface 418f at the signal input side 418a, the same effect as
described above can be achieved.
[0099] Even in the modification example, as in the case of the
relay substrates 118, 518, and 618, the above-described effect of
reducing the transfer of high-frequency energy can be strengthened
by forming a metal film on the two inner surfaces of the groove 750
or the two inner surfaces and the bottom surface of the groove
750.
Fourth Modification Example
[0100] FIG. 8 is a diagram illustrating a configuration of a relay
substrate 818 according to a fourth modification example. The relay
substrate 818 can be used instead of the relay substrate 118 in the
optical modulator 100 shown in FIG. 1. In FIG. 8, the same
reference numerals as those in FIG. 4 are used for the same
components as those of the relay substrate 118 shown in FIG. 4, and
the above description of FIG. 4 is adopted.
[0101] The relay substrate 818 shown in FIG. 8 has the same
configuration as that of the relay substrate 118 shown in FIG. 4,
except that a ground conductor 840 that is in contact with the
housing 104 and has a ground potential is provided on the rear
surface 418f. The relay substrate 818 is different from the relay
substrate 118 in that grooves 850a, 850b, 850c, 850d, and 850e are
provided instead of the grooves 350a, 350b, 350c, 350d, and 350e.
Hereinafter, the grooves 850a, 850b, 850c, 850d, and 850e are
collectively referred to as a groove 850.
[0102] The groove 850 has the same configuration as that of the
groove 350, but is different from the groove 350 in that a metal
film is formed on the bottom surface and two inner surfaces
thereof, and six vias 860 are provided on each of the bottom
surfaces. In FIG. 8, for ease of understanding, only the rightmost
via in the drawing of the groove 850 is denoted by reference
numeral 860, but it is to be understood that five vias drawn in
circles with the same diameter as the vias on the left side of the
via with the reference numeral 860 attached thereto are also the
vias 860.
[0103] These vias 860 electrically connect the metal film on the
bottom surface of the groove 850 and the ground conductor 840
formed on the rear surface 418 f of the relay substrate 818.
Accordingly, the ground conductor 840 on the rear surface 418f is
electrically connected to the ground conductor pattern 340 on the
front surface 418e through the via 860 and the metal film provided
on the bottom surface and the two inner surfaces of the groove
850.
[0104] Since the relay substrate 818 having the above-described
configuration is formed with the via 860 that connects the metal
film on the bottom surface of the groove 850 and the ground
conductor 840 on the rear surface 418f of the relay substrate 818,
it is possible to block the radiation microwaves that are to
propagate inside the relay substrate 818 through the groove 850 of
the above-described two radiation microwaves (connection point
radiation microwaves and propagation radiation microwaves).
Therefore, in the relay substrate 818, high-frequency energy
transfer between the signal conductor patterns 330 through the two
radiation microwaves can be suppressed.
[0105] Further, since the ground conductor pattern 340 on the front
surface 418e and the ground conductor 840 on the rear surface 418f
are connected by the via 860 having a length shorter than the
thickness of the relay substrate 818, a higher ground effect than
that of the relay substrate 118 (uniform ground potential
distribution) can be achieved. In the present embodiment, the
number of vias is six, but the present invention is not limited to
this. The vias have the same outer diameter, but may have different
outer diameters as long as the above effects are achieved. In
particular, in a case where the outer diameter of the via on the
connection point side between the signal input terminal and the
signal conductor pattern is made larger than the outer diameters of
the other vias, it is possible to obtain a higher ground effect
(such as uniform ground potential distribution) at the connection
point between the signal input terminal and the signal conductor
pattern where the above two radiation microwaves are likely to
occur, while maintaining the mechanical strength of the
substrate.
Fifth Modification Example
[0106] FIG. 9 is a diagram illustrating a configuration of a relay
substrate 918 according to a fifth modification example. The relay
substrate 918 can be used instead of the relay substrate 118 in the
optical modulator 100 shown in FIG. 1. In FIG. 9, the same
reference numerals as those in FIG. 4 are used for the same
components as those of the relay substrate 118 shown in FIG. 4, and
the above description of FIG. 4 is adopted.
[0107] The relay substrate 918 shown in FIG. 9 has the same
configuration as that of the relay substrate 118 shown in FIG. 4,
except that signal conductor patterns 930a, 930b, 930c, and 930d
are provided instead of the signal conductor patterns 330a, 330b,
330c, and 330d. Hereinafter, the signal conductor patterns 930a,
930b, 930c, and 930d are collectively referred to as a signal
conductor pattern 930.
[0108] The signal conductor patterns 930a, 930b, 930c, and 930d
have the same configuration as the signal conductor patterns 330a,
330b, 330c, and 330d, but have the planar shape on the relay
substrate 918 different from the signal conductor patterns 330a,
330b, 330c, and 330d.
[0109] That is, each of the signal conductor patterns 330a, 330b,
330c, and 330d is formed as a linear shape extending in a direction
orthogonal to the signal input side 418a, whereas each of the
signal conductor patterns 930a, 930d is configured to include a
straight line extending at an angle in the direction different from
the direction orthogonal to the signal input side 418a. Each of the
signal conductor patterns 930b and 930c includes a curved
portion.
[0110] Thus, in the relay substrate 918, on the signal input side
418a, the ends of the signal conductor pattern 930a and 930b are
disposed adjacent to the right side in FIG. 9 to form one group,
and the ends of the signal conductor patterns 930c and 930d are
disposed adjacent to the left side in FIG. 9 to form another group.
For example, in a case where the optical modulation element 102 is
a small-sized and integrated modulator such as a DP-QPSK modulator,
such grouping of the ends of the signal conductor patterns 930 on
the signal input side 418a can be employed in a case where two
groups of high-frequency electrical signals are respectively input
to two nested Mach-Zehnder modulators which modulate two orthogonal
polarized light beams, respectively.
[0111] The relay substrate 918 is different from the relay
substrate 118 in that ground conductor patterns 940a, 940b, 940c,
and 940d are provided instead of the ground conductor patterns
340a, 340b, 340c, and 340d. Hereinafter, the ground conductor
patterns 940a, 940b, 940c, and 940d are collectively referred to as
a ground conductor pattern 940.
[0112] The ground conductor pattern 940 has the same configuration
as that of the ground conductor pattern 340, but is different in
that an edge adjacent to the signal conductor pattern 930 is formed
in a straight line and/or a curved line so as to form a coplanar
line in accordance with the shape of the signal conductor pattern
930.
[0113] The relay substrate 918 is different from the relay
substrate 118 in that groove 950a, 950b, 950c, 950d, and 950e are
provided instead of the grooves 350a, 350b, 350c, 350d, and 350e.
Hereinafter, the grooves 950a, 950b, 950c, 950d, and 950e are
collectively referred to as a groove 950.
[0114] Here, the grooves 950a, 950b, 950c, 950d, and 950e have the
same configuration as the grooves 350a, 350b, 350c, 350d, and 350e,
but are different from the grooves 350a, 350b, 350c, 350d, and 350e
in the following points.
[0115] First, in the relay substrate 118 in FIG. 4, one groove 350
is provided in each of the ground conductor patterns 340, whereas
in the relay substrate 918 in FIG. 9, no groove is provided in the
ground conductor patterns 940a, 940e that are not sandwiched
between the adjacent signal conductor patterns 930. This is because
in the relay substrate 918, propagation radiation microwaves
propagating from the signal conductor patterns 930a and 930d toward
the right side surface 418g and the left side surface 418h of the
relay substrate 918, respectively do not reach the other signal
conductor patterns 930 directly, which does not contribute much to
the crosstalk between the signal conductor patterns 930.
[0116] In the relay substrate 918, the ends of the signal conductor
patterns 930b and 930c on the signal input side 418a are formed so
as to be separated from each other such that no significant
crosstalk occurs. Therefore, in the relay substrate 918, no groove
is provided in the ground conductor pattern 940c sandwiched between
the signal conductor patterns 930b, 930c.
[0117] In the relay substrate 918, two grooves 950a, 950b each
having a straight line and a curved line are provided on the ground
conductor pattern 940b sandwiched between the signal conductor
patterns 930a, 930b so as to follow the respective shapes of the
signal conductor patterns 930a, 930b. In addition, in the relay
substrate 918, two grooves 950c, 950d each having a curved line and
a straight line are provided on the ground conductor patterns 940d
sandwiched between the signal conductor patterns 930c, 930d so as
to follow the respective shapes of the signal conductor patterns
930c, 930d.
[0118] In the relay substrate 918 having the above configuration,
the number and shape of grooves provided in the ground conductor
pattern 940 are determined in accordance with the shape of the
signal conductor pattern 930. That is, a groove is not provided in
the propagation path of the radiation microwave that does not
substantially contribute to the occurrence of crosstalk between the
adjacent signal conductor patterns 330 (in the present modification
example, the substrate portion of the relay substrate 918 on which
the ground conductor patterns 940a, 940c, and 940d are formed).
Therefore, the relay substrate 918 can be manufactured easily and
inexpensively by simplifying the processing steps.
[0119] In the relay substrate 918, in the propagation path of two
radiation microwaves (connection point radiation microwaves and
propagation radiation microwaves) that can be sandwiched between
two signal conductor patterns 930 and cause crosstalk between the
signal conductor patterns 930 (in the present modification example,
the substrate portion on which the ground conductor patterns 940b,
940d are formed), a groove 950 having a linear portion or a curved
portion along the shape of the signal conductor pattern 930 at a
position adjacent to each signal conductor pattern 930 having a
curved portion is provided.
[0120] Thus, on the relay substrate 918, two radiation microwaves
can be prevented from propagating through the relay substrate 918
between the adjacent signal conductor patterns 930, and the
propagation of unnecessary microwaves radiated from the curved
portion of the signal conductor pattern 930 can also be
suppressed.
[0121] In the present modification example, two respective grooves
950a, 950b, and 950c, 950d are respectively provided in the ground
conductor patterns 940b, 940d configured as one ground conductor
pattern, but the present invention is not limited thereto. For
example, the ground conductor pattern 940b may be divided into
right and left in the drawing, a groove 950a may be formed in the
divided right portion of the drawing, and a groove 950b may be
provided in the divided left portion of the drawing. Similarly, the
ground conductor pattern 940d may be divided into right and left in
the drawing, a groove 950c may be formed in the divided right
portion of the drawing, and a groove 950d may be provided in the
divided left portion of the drawing.
Second Embodiment
[0122] Next, a second embodiment of the present invention will be
described. The present embodiment is an optical transmission
apparatus mounted with an optical modulator related to either of
the optical modulator 100 according to the first embodiment or the
optical modulators 100 including the relay substrates 518, 618,
718, 818, and 918 according to the first to fifth modification
examples of the first embodiment.
[0123] FIG. 10 is a diagram illustrating a configuration of an
optical transmission apparatus according to the present embodiment.
The present optical transmission apparatus 1000 includes an optical
modulator 1002, a light source 1004 that inputs light to the
optical modulator 1002, a modulation signal generation part 1006,
and a modulation data generation part 1008.
[0124] The optical modulator 1002 may be an optical modulator
related to either of the optical modulator 100 according to the
first embodiment, or the optical modulators 100 including the relay
substrates 518, 618, 718, 818, and 918 according to the first to
fifth modification examples of the first embodiment. Here, in order
to avoid redundant descriptions and facilitate understanding, it is
assumed that the optical modulator 1002 is the optical modulator
100 including the relay substrate 118 below.
[0125] The modulation data generation part 1008 receives
transmission data given from the outside, generates modulation data
for transmitting the transmission data (for example, data obtained
by converting or processing transmission data into a predetermined
data format), and outputs the generated modulation data to the
modulation signal generation part 1006.
[0126] The modulation signal generation part 1006 is an electronic
circuit (drive circuit) that outputs an electrical signal for
causing the optical modulator 1002 to perform a modulation
operation, generates a modulation signal which is a high-frequency
signal for making the optical modulator 1002 perform an optical
modulation operation according to the modulation data, based on the
modulation data which is output by the modulation data generation
part 1008, and inputs the generated modulation signal to the
optical modulator 1002. The modulation signal includes four
high-frequency electrical signals corresponding to the four signal
electrodes 112a, 112b, 112c, and 112d of the optical modulation
element 102 provided in the optical modulator 1002.
[0127] The four high-frequency electrical signals are input from
the signal input terminals 124a, 124b, 124c, and 124d of the
electrical connectors 116a, 116b, 116c, and 116d of the optical
modulator 1002 to the signal conductor patterns 330a, 330b, 330c,
and 330d on the relay substrate 118, and are input to the signal
electrodes 112a, 112b, 112c, and 112d of the optical modulation
element 102 through the signal conductor pattern 330a.
[0128] Thus, the light output from the light source 1004 is, for
example, DP-QPSK modulated by the optical modulator 1002 and output
as modulated light from the optical transmission apparatus
1000.
[0129] In particular, in the optical transmission apparatus 1000,
as the optical modulator 1002, an optical modulator related to
either of the optical modulator 100 according to the first
embodiment, or the optical modulators 100 including the relay
substrates 518, 618, 718, 818, and 918 according to the first to
fifth modification examples of the first embodiment is used.
Therefore, in the optical transmission apparatus 1000, it is
possible to secure stable and good optical modulation
characteristics by effectively reducing crosstalk between a
plurality of high-frequency electrical signals that drive the
optical modulation element 102 accompanying an increase in
transmission rate, and thus stable and good transmission
characteristics can be realized.
[0130] The present invention is not limited to the configurations
of the above-described embodiment and modification examples, and
can be realized in various aspects without departing from the
spirit thereof.
[0131] For example, in the above-described embodiments and
modification examples, the preferable size conditions indicated by
Expressions (1) and (2) for the groove 350 shown in the description
of the relay substrate 118 can be applied similarly to grooves 550,
650, 750, 850, and 950 on the relay substrates 518, 618, 718, 818,
and 918. In this case, since the length L2 of the groove is defined
as the length that the groove extends, in the example of the groove
950, it is the length measured along the shape of the groove 950
formed in a straight line or a curved line.
[0132] In addition, for example, even if a single relay substrate
is configured by combining the characteristic portions of the relay
substrates 118, 518, 618, 718, 818, and 918 shown in the
above-described embodiments and modification examples, the same
effects as those shown in the above-described modification examples
can be achieved. For example, a plurality of grooves each having
the same configuration as the grooves 350, 550, 650, 750, 850, and
950 are mixed in a plurality of ground conductor patterns, and at
least one groove can be provided in at least one ground conductor
pattern formed on the front surface 418e.
[0133] Alternatively, in the relay substrates 618, 718, and 918, a
metal film may be provided on the entire inner surfaces of the
grooves 650, 750, and 950, a metal film may be provided on the rear
surface 418f, and a via similar to that of the relay substrate 818
may be provided. Alternatively, in the relay substrate 618, the
depth of the groove 650 may change stepwise or continuously as in
the relay substrate 718. In this case, for example, the groove 650
may be configured such that the depth thereof changes stepwise or
continuously within a range not reaching the rear surface 418f.
[0134] As described above, the optical modulator 100 described
above includes the optical modulation element 102 including the
plurality of signal electrodes 112, the plurality of signal input
terminals 124 for inputting the electrical signals applied to the
signal electrodes 112, and a relay substrate 118 on which a
plurality of signal conductor patterns 330 and a plurality of
ground conductor patterns 340 electrically connecting the signal
input terminals 124 and the signal electrodes 112 are formed. In
addition, the optical modulator 100 includes a housing 104 that
houses the optical modulation element 102 and the relay substrate
118. On the signal input side 418a of the relay substrate 118 where
the electrical signal from the signal input terminal 124 is input
to the signal conductor pattern 330, the signal input terminal 124
is disposed to extend from the signal input side 418a onto the
signal conductor pattern 330. The relay substrate 118 has at least
one groove 350 extending from the signal input side 418a, in at
least one ground conductor pattern 340 formed between adjacent
signal conductor patterns 330, on the front surface 418e on which
the signal conductor pattern 330 is formed. The groove 350 is
formed such that the length extending from the signal input side
418a is longer than the length of the signal input terminal 124
extending from the signal input side 418a.
[0135] According to this configuration, two radiation microwaves of
a radiation mode generated from the signal conductor pattern 330 in
the vicinity of the input connection point due to the propagation
of the high-frequency signal by the groove 350 provided in the
relay substrate 118 (propagation radiation microwaves) and a
radiation mode generated along with different mode conversion at
the connection point between the signal input terminal 124 and the
signal conductor pattern 330 (connection point radiation
microwaves) are suppressed from propagating in the relay substrate
118, and the generation of high-frequency energy transfer between
adjacent signal conductor patterns 330 can be suppressed. That is,
in the optical modulator 100, since the transfer of the
high-frequency energy is suppressed by the groove 350 having a
simple configuration provided in the relay substrate 118, it is
possible to realize good optical modulation characteristics, by
effectively suppressing an increase in a crosstalk between signal
conductor patterns 330 due to an increase in a transmission rate,
without increasing the cost and ensuring ease of manufacture,
without violating the demand for downsizing of the optical
modulator 100.
[0136] Further, a relay substrate 618 provided with a groove 650
different from the groove 350 can be used for the optical modulator
100. The groove 650 is formed so as to extend up to a signal output
side 418c of the relay substrate 118, where an electrical signal is
output from the signal conductor pattern 330 to the signal
electrode 112 of the optical modulation element 102.
[0137] According to this configuration, the propagation of the
propagation radiation microwaves that can be generated from each
part in the longitudinal direction of the signal conductor pattern
330 into the relay substrate 618 is continuously suppressed by the
groove 650 from the signal input side 418a to the signal output
side 418c. Therefore, the transfer of high-frequency energy between
the signal conductor patterns 330 is further reduced as compared
with the relay substrate 118. Further, since the groove 650 is
formed so as to reach the signal output side 418c from the signal
input side 418a in this way, the structure of the relay substrate
618 is simplified, so the manufacture becomes easy.
[0138] Further, a relay substrate 718 provided with a groove 750
different from the groove 350 can be used for the optical modulator
100. The groove 750 is formed such that the depth of the end of the
groove 750 measured from the front surface 418e of the relay
substrate 718 at the signal input side 418a is deeper than the
depth of the groove 750 measured from the front surface 418e at the
other end of the groove 750.
[0139] According to this configuration, the groove 750 is formed
such that the depth becomes shallower as it goes away from the
signal input side 418a, so the disturbance of the propagation mode
in the signal conductor pattern 330 that may occur due to the
presence of the groove 750 decreases toward the signal output side
418c. Since the large portion of the propagation mode disturbance
corresponds to the portion where the groove 750 is formed more
deeply, the propagation in a part where the disturbance is large
and the propagation microwaves are frequently generated is more
effectively suppressed than a deeper groove part.
[0140] Therefore, in the relay substrate 718, while the disturbance
of the propagation mode of the signal conductor pattern 330 due to
the presence of the groove 750 is smoothly eliminated toward the
signal output side 418c, the high-frequency energy transfer between
the signal conductor patterns 330 through the connection point
radiation microwaves and the propagation radiation microwaves can
be suppressed to a certain level or less along the longitudinal
direction of the signal conductor pattern 330.
[0141] Further, in the relay substrate 718, the groove 750 is
formed such that the depth measured from the front surface 418e of
the relay substrate 718 becomes stepwise or continuous in depth
from the other end of the groove 750 toward the signal input side
418a. According to this configuration, the groove whose depth
changes can be formed with a simple configuration.
[0142] Further, in the relay substrate 718, the groove 750 is
formed up to the rear surface 418f facing the front surface 418e of
the relay substrate 718 at the signal input side 418a, or is formed
up to the rear surface 418f facing the front surface 418e of the
relay substrate 718 within a range of a predetermined distance L3
from the signal input side 418a.
[0143] According to this configuration, at the signal input side
418a where connection point radiation microwaves and propagation
radiation microwaves are most likely to be generated or in the
vicinity thereof, the effect of suppressing the propagation of the
connection point radiation microwaves and propagation radiation
microwaves into the relay substrate 718 can be strengthened.
[0144] In addition, a metal film may be formed on the inner surface
of the grooves 350, 650, 750, and 950 of the relay substrates 118,
618, 718, and 918. According to this configuration, the effect of
suppressing the propagation of the connection point radiation
microwaves and propagation radiation microwaves into these relay
substrates by these grooves can be strengthened.
[0145] Further, the grooves 350, 650, 750, and 950 of the relay
substrates 118, 618, 718, and 918 may further have a metal film
formed on their bottom surfaces. According to this configuration,
it is possible to enhance the effect of suppressing the crosstalk
between the signal conductor patterns 330 by suppressing the
connection point radiation microwave and the propagation radiation
microwave propagating in the relay substrate from being radiated
into the air from the bottom surfaces of these grooves.
[0146] Further, a relay substrate 818 provided with a groove 850
different from the groove 350 can be used for the optical modulator
100. A ground conductor 840 is formed on the rear surface 418f
facing the front surface 418e of the relay substrate 818, and a via
860 that connects a metal film on the bottom surface and a ground
conductor 840 of the rear surface 418f of the relay substrate 818
is formed on the bottom surface of the groove 850.
[0147] According to this configuration, since the via 860 that
connects the metal film on the bottom surface of the groove 850 and
the ground conductor 840 on the rear surface 418f of the relay
substrate 818 is formed, it is possible to block the radiation
microwaves that are to propagate inside the relay substrate 818
through the groove 850, of the above-described connection point
radiation microwaves and propagation radiation microwaves.
Therefore, in the relay substrate 818, high-frequency energy
transfer between the signal conductor patterns 330 through the
connection point radiation microwaves and the propagation radiation
microwaves can be further suppressed.
[0148] Further, a relay substrate 518 provided with a groove 550
different from the groove 350 can be used for the optical modulator
100. The groove 550 does not extend up to a signal output side 418c
of the relay substrate 518, where an electrical signal is output
from the signal conductor pattern 330 to the signal electrode 112
of the optical modulation element 102, and the entire of each of
the grooves 550 is formed so as to penetrate to a rear surface 418f
of the relay substrate 518 facing the front surface 418e.
[0149] According to this configuration, since the entire of each of
the grooves 550 penetrates up to the rear surface 418f facing the
front surface 418e of the relay substrate 518, the propagation of
the connection point radiation microwave and the propagation
radiation microwave generated at the input connection point and the
vicinity thereof into the relay substrate 518 is further
suppressed.
[0150] Further, in the relay substrate 518, a metal film is further
formed on the inner surface of the groove 550 formed so as to
penetrate from the front surface 418e to the rear surface 418f.
According to this configuration, since the metal film is formed on
the inner surface of the groove 550, the effect of suppressing the
propagation of the connection point radiation microwave and the
propagation radiation microwave, and therefore the effect of
reducing the transfer of high-frequency energy between the signal
conductor patterns 330 through these radiation microwaves can be
further strengthened.
[0151] In the relay substrates 118, 518, 618, 718, 818, and 918 of
the optical modulator 100, the grooves 350, 550, 650, 750, 850, and
950 are formed such that the length L2 extending from the signal
input side 418a is longer than the width W measured along the
direction orthogonal to the extending direction. According to this
configuration, it is possible to effectively suppress the
propagation of connection point radiation microwaves and
propagation radiation microwaves, which can be generated from each
part in the longitudinal direction of the signal conductor pattern
330 along with the propagation of the high frequency signal, into
these relay substrates.
[0152] The optical modulator 100 including any of the
above-described relay substrates 118, 518, 618, 718, 818, and 918
constitutes the optical transmission apparatus 1000 together with
the modulation signal generation part 1006 which is an electronic
circuit that outputs an electrical signal for causing the optical
modulator 100 to perform a modulation operation. According to this
configuration, it is possible to realize stable and good
transmission characteristics, by effectively reducing crosstalk
between a plurality of high-frequency electrical signals that drive
the optical modulation element 102 due to an increase in
transmission rate.
* * * * *