Optical modulator and optical transmission apparatus using same

An optical modulator includes a relay substrate having signal conductor patterns that connect signal input terminals and signal electrodes of an optical modulation element and ground conductor patterns, and a housing, in which the signal conductor pattern includes a component mounting portion including an electrical circuit element. Two ground conductor patterns sandwiching the signal conductor pattern are formed in an asymmetrical shape in a plan view within a component mounting area having a square shape in the plan view centered on the component mounting portion. A direction of a side of the component mounting area having the square shape is same as the extending direction of the signal conductor pattern and a length of a side of the component mounting area is equal to a distance from a center of the component mounting portion to a portion on an adjacent signal conductor pattern closest to the center.

TECHNICAL FIELD

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 a signal electrode of an optical modulation element, and an optical transmission apparatus using the optical modulator.

BACKGROUND ART

In high-speed/large-capacity optical fiber communication systems, optical modulators incorporating waveguide type optical modulation elements are used in many cases. Particularly, optical modulation elements using LiNbO3(hereinafter, also referred to as LN) having electro-optic effects for substrates can realize high-bandwidth optical modulation characteristics with small optical losses, so the optical modulation elements are widely used in high-speed/large-capacity optical fiber communication systems.

The optical modulation element using the LN substrate includes Mach-Zehnder type optical waveguides and signal electrodes for applying a high-frequency electrical signal as a modulation signal to the optical waveguides. Then, the signal electrodes provided in the optical modulation element are connected to lead pins and connectors that are signal input terminals provided on a housing of the optical modulator, via a relay substrate provided in the housing in which the optical modulation element is accommodated. Thus, since the lead pins and connectors that are signal input terminals are connected to a circuit substrate on which an electronic circuit for causing the optical modulator to perform a modulation operation is mounted, an electrical signal output from the electronic circuit is applied to the signal electrodes of the optical modulation element via the relay substrate.

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 metro networks.

An optical modulator that performs QPSK modulation (QPSK optical modulator) and an optical modulator that performs DP-QPSK modulation (DP-QPSK optical modulator) include a plurality of Mach-Zehnder type optical waveguides having a so-called nested structure called a nested type, each of which includes at least one signal electrode. Therefore, the optical modulators include a plurality of signal electrodes, and the above-described DP-QPSK modulation operation is performed in cooperation with high-frequency electrical signals applied to the signal electrodes.

In such an optical modulator, an electrical circuit element such as an electric filter for improving high-frequency characteristics or the like may be mounted on a signal line formed on a relay substrate (Patent Literature Nos. 1 and 2).

FIG.15is a plan view illustrating an example of a configuration of an optical modulator including such a relay substrate on which an electrical circuit element is mounted in the related art. An optical modulator2200includes, for example, an optical modulation element2202which is a DP-QPSK modulator formed on an LN substrate, and a housing2204in which the optical modulation element2202is accommodated. Here, the housing2204includes a case2214aand a cover2214b. The optical modulator2200also includes an input optical fiber2208and an output optical fiber2210which are fixed to the case2214aand perform an input and output of light to the optical modulation element2202.

Four signal input terminals2224a,2224b,2224c, and2224d(hereinafter, collectively also referred to as a signal input terminal2224) for inputting a high-frequency electrical signal for driving the optical modulation element2202from an external electronic circuit are further provided, in the case2214aof the housing2204. Specifically, the signal input terminal2224is, for example, a center electrode of electrical connectors2216a,2216b,2216c, and2216d(hereinafter, collectively also referred to as an electrical connector2216) which are high-frequency coaxial connectors. The high-frequency electrical signals input from the respective signal input terminals2224are input to one ends of the four signal electrodes2212a,2212b,2212c, and2212d(hereinafter, collectively also referred to as a signal electrode2212) provided in the optical modulation element2202via a relay substrate2218accommodated in the housing2204, and terminated by a terminator2220with a predetermined impedance provided at the other end of the signal electrode2212.

The optical modulation element2202outputs two modulated light beams from two output optical waveguides2226aand2226b, and the two output light beams are combined into one beam by a polarization-combining part2228including a polarization beam combining prism or the like. The combined light is output to the outside of the housing2204via the output optical fiber2210.

FIG.16is a diagram illustrating the relay substrate2218and its periphery in the optical modulator2200illustrated inFIG.15. Ground electrodes2222a,2222b,2222c,2222d, and2222eare provided in the optical modulation element2202so that each of the signal electrodes2212constitutes a coplanar waveguide (CPW).

Further, on the relay substrate2218, the signal conductor patterns2230a,2230b,2230c, and2230d(hereinafter, collectively also referred to as a signal conductor pattern2230) respectively connecting the four signal input terminals2224and the four signal electrodes2212of the optical modulation element2202are formed. These signal conductor patterns2230form a high-frequency signal line together with ground conductor patterns2240a,2240b,2240c,2240d, and2240earranged on the relay substrate2218so as to sandwich the signal conductor pattern2230in a plane direction of the substrate.

Each of the four signal conductor patterns2230of the relay substrate2218is provided with component mounting portions2250a,2250b,2250c, and2250d(hereinafter, collectively also referred to as a component mounting portion2250) on which an electric filter for improving high-frequency characteristics is mounted, for example.FIGS.17and18are a partial detailed view of a part J of the relay substrate2218illustrated inFIG.16and a cross-sectional view taken along arrow line XVIII-XVIII inFIG.16. These drawings illustrate the configuration of the component mounting portion2250bas an example of the component mounting portion2250, and other component mounting portions2250a,2250c, and2250dmay have the same configuration.

The component mounting portion2250bhas, for example, the same configuration as the electric filter described in Patent Literature 1. That is, the component mounting portion2250bincludes a thin film resistor2252b(a cross-hatched portion illustrated inFIGS.17and18) formed as an electrical circuit element in a part of the signal conductor pattern2230b, and a capacitor2254bmounted on the signal conductor pattern2230b. Further, the signal conductor pattern2230bof the component mounting portion2250bis formed wider than, for example, other portions.

The thin film resistor2252bis formed with a portion of the signal conductor pattern2230bwith a desired thickness so that the portion has a desired resistance value, and is formed to be thinner than the thickness of the other portion, for example. Further, the capacitor2254bis, for example, a single plate capacitor, and a lower surface electrode portion of the capacitor is fixed on a wide portion of the signal conductor pattern2230bconnected to one end of the thin film resistor2252b, for example, by soldering. On the other hand, an upper surface electrode of the capacitor2254bis connected over the wide portion of the signal conductor pattern2230bconnected to the other end of the thin film resistor2252b, for example, by wire bonding using a conductor wire2270. Thus, the component mounting portion2250bconstitutes an electric filter in which the thin film resistor2252band the capacitor2254bare connected in parallel.

Incidentally, the DP-QPSK optical modulator as described above is often used at a transmission rate of 100 Gb/s at present, and development to expand this transmission rate to 400 Gb/s is also in progress. With the increase in the frequency of the modulator operation, components with excellent high-frequency characteristics can be selected as the electrical circuit elements (capacitor2254bor the like) mounted on the component mounting portion2250as described above, or the impedance of the component mounting portion2250is matched with the line impedance of the signal conductor pattern2230.

However, the component mounting portion2250described above can be a discontinuity point for a high-frequency (microwave) electrical signal propagating in the signal conductor pattern2230due to, for example, a difference in physical shape between the electrical circuit element and the signal conductor pattern2230, displacement of a mounting position of the electrical circuit element, and the like. As a result, a part of the microwave electrical signal leaks from the component mounting portion2250and becomes a leaked microwave2290(FIG.16), which can act as noise on the adjacent signal conductor pattern2230and the signal electrode2212on the optical modulation element2202.

In particular, in an optical modulator such as the optical modulator2200in which high-frequency electrical signals given to a plurality of signal electrodes2212cooperate to perform DP-QPSK modulation, it is desirable that all the high-frequency electrical signals are input to the signal electrode2212of the optical modulation element2202without being affected by noise or the like, and the generation of noise as described above may adversely affect the operation of the optical modulation element2202.

Further, the demand for miniaturization of the optical modulator2200remains unchanged, and the relay substrate2218is being miniaturized along with the miniaturization of the housing2204of the optical modulator2200. As a result, a plurality of different high-frequency signals are propagated in close proximity to each other in the narrow relay substrate2218, and electrical crosstalk between high-frequency signal lines such as the signal conductor pattern2230due to leaked microwaves as described above cannot be ignored.

That is, in an optical modulator in the related art, it is required to reduce crosstalk between high-frequency electrical signals caused by leaked microwaves that may occur from component mounting portion provided on the relay substrate and to realize appropriate modulation characteristics.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

From the above background, in an optical modulator, it is required to effectively suppress the influence of leaked microwaves which may be generated from a portion of a relay substrate that electrically connects each of signal electrodes of an optical modulation element and each of signal input terminals, and that is provided with an electrical circuit element constituting an electric filter or the like, and to realize appropriate optical modulation characteristics.

Solution to Problem

According to one aspect of the present invention, there is provided an optical modulator including: an optical modulation element that includes 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 the signal input terminals to the signal electrodes, and a plurality of ground conductor patterns are formed; and a housing in which the optical modulation element and the relay substrate are accommodated, in which at least one signal conductor pattern includes at least one component mounting portion including an electrical circuit element, two ground conductor patterns sandwiching the at least one signal conductor pattern on the relay substrate are formed in an asymmetrical shape in a plan view within a component mounting area having a square shape in the plan view centered on the component mounting portion, with respect to a straight line extending in an extending direction of the at least one signal conductor pattern in the component mounting portion, and a direction of a side of the component mounting area having the square shape is same as the extending direction of the at least one signal conductor pattern in the component mounting portion and a length of a side of the component mounting area is equal to a distance from a center of the component mounting portion to a portion on an adjacent signal conductor pattern closest to the center.

According to another aspect of the present invention, there is provided an optical modulator including: an optical modulation element that includes 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 the signal input terminals to the signal electrodes, and a plurality of ground conductor patterns are formed; and a housing in which the optical modulation element and the relay substrate are accommodated, in which at least one signal conductor pattern includes at least one component mounting portion including an electrical circuit element, two ground conductor patterns sandwiching the at least one signal conductor pattern on the relay substrate are formed asymmetrically within a component mounting area having a square shape in the plan view centered on the component mounting portion, in terms of a presence or absence of vias, diameters of vias, or the numbers of vias, and a direction of a side of the component mounting area having the square shape is same as the extending direction of the at least one signal conductor pattern in the component mounting portion and a length of a side of the component mounting area is equal to a distance from the at least one signal conductor pattern to a closest adjacent signal conductor pattern.

According to still another aspect of the present invention, the two ground conductor patterns are formed to have portions in which distances from respective edges of the two ground conductor patterns to opposite edges of the at least one signal conductor pattern are different from each other in the component mounting area.

According to still another aspect of the present invention, one ground conductor pattern of the two ground conductor patterns includes a portion in which a width measured in a direction orthogonal to the extending direction of the at least one signal conductor pattern is narrower than a width of an other ground conductor pattern of the two ground conductor patterns, in the component mounting area.

According to still another aspect of the present invention, one ground conductor pattern of the two ground conductor patterns has a discontinuous section in the component mounting area, and an other ground conductor pattern does not have a discontinuous section in the component mounting area.

According to still another aspect of the present invention, the other ground conductor pattern of the two ground conductor patterns has a portion in which a separation distance to a portion of the signal conductor pattern constituting the component mounting portion is formed longer than a separation distance to other portions of the signal conductor pattern within a corresponding component mounting area.

According to still another aspect of the present invention, the optical modulation element is configured to generate two modulated light beams, each of which is modulated by a pair of the electrical signals, and the relay substrate is configured to propagate the pair of electrical signals by a pair of the signal conductor patterns adjacent to each other.

According to still another aspect of the present invention, there is provided 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.

Advantageous Effects of Invention

According to the present invention, it is possible to effectively suppress the influence of leaked microwaves that may be generated from the component mounting portion provided with the electrical circuit element in the relay substrate, and to realize appropriate optical modulation characteristics.

This application claims the benefit of Japanese Patent Application No. 2019-146242 filed on Aug. 8, 2019, the disclosure of which is herein incorporated by reference in its entirety.

DESCRIPTION OF EMBODIMENTS

The present invention changes a direction of microwaves that can leak at a component mounting portion of a relay substrate by making an electrode configuration around the component mounting portion asymmetric, and reduces the influence on its own signal electrode, adjacent signal electrodes, and the like. Specifically, optical modulators illustrated in the following embodiments and modification examples thereof include an optical modulation element and a relay substrate provided with a component mounting portion including an electrical circuit element. Then, in these optical modulators, in order to solve the above-described problems, in a predetermined component mounting area of at least one signal conductor pattern provided with a component mounting portion, two ground conductor patterns sandwiching the at least one signal conductor pattern are formed asymmetrically with respect to an extending direction of the signal conductor pattern. Specifically, in the component mounting area, separation distances between the signal conductor pattern and the two adjacent ground conductor patterns are asymmetrical to each other, the widths of the two ground conductor patterns are asymmetrical to each other, or the presence or absence of vias, the diameters of vias, and the numbers of vias in the two ground conductor patterns are different from each other. Thus, in the optical modulator, the intensity of leaked microwave generated from the component mounting portion toward the adjacent signal conductor pattern is reduced, and crosstalk between the adjacent signal conductor patterns is reduced.

First Embodiment

First, a first embodiment of the present invention will be described.FIG.1is a plan view illustrating a configuration of an optical modulator100according to the first embodiment of the present invention,FIG.2is a side view of the optical modulator100, andFIG.3is a partial detailed view of a part A inFIG.1.

The optical modulator100includes an optical modulation element102, a housing104in which the optical modulation element102is accommodated, an input optical fiber108for inputting light into the optical modulation element102, and an output optical fiber110that guides the light output from the optical modulation element102to the outside of the housing104.

The optical modulation element102is, for example, a DP-QPSK modulator that performs optical modulation of 400 Gb/s, 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 electrodes112a,112b,112c, and112d(hereinafter, collectively also referred to as a signal electrode112) that respectively modulate light waves propagating through the Mach-Zehnder type optical waveguide. In addition, as known in the related art, on a surface of the LN substrate of the optical modulation element102, for example, ground electrodes122a,122b,122c,122d, and122e(seeFIG.3and not illustrated inFIG.1) are provided so that each of the four signal electrodes112a,112b,112c, and112dincludes a coplanar waveguide (CPW).

Specifically, the ground electrodes122a,122b,122c,122d, and122e(hereinafter, collectively also referred to as a ground electrode122) are disposed to respectively sandwich the signal electrodes112a,112b,112c, and112dtherebetween in a surface of the LN substrate, and constitute a coplanar waveguide having a predetermined characteristic impedance in a predetermined operating frequency together with the four signal electrodes112a,112b,112c, and112d.

Four high-frequency electrical signals (modulation signals) are respectively input to the four signal electrodes112. 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.

Specifically, two pairs of high-frequency electrical signals, one pair of which includes two high-frequency electrical signals, are applied to the four respective signal electrodes112. The optical modulation element102is configured to generate two modulated light beams each of which is modulated by one pair of electrical signals. The two generated modulated light beams are respectively output from two output optical waveguides126aand126bthat form a part of the optical modulation element102. In the present embodiment, two high-frequency electrical signals forming one pair are applied to the signal electrodes112aand112bto generate modulated light output from the output optical waveguide126a, and other two high-frequency electrical signals forming another pair are applied to the signal electrodes112cand112dto generate modulated light output from the output optical waveguide126b. These two modulated light beams are combined into one beam by a polarization-combining part128including a polarization beam combining prism or the like, and then output to the outside of the housing104via the output optical fiber110.

The housing104includes a case114ato which the optical modulation element102is fixed and a cover114b. In order to facilitate understanding of the configuration inside the housing104, only a part of the cover114bis illustrated on the left side inFIG.1, but actually, the cover114bis disposed to cover the entire box-shaped case114aand hermetically seals the inside of the housing104. The case114ais made of a metal or a ceramic plated with gold, for example, and functions electrically as an electric conductor. The housing104can be usually provided with a plurality of pins for DC control or the like, which are omitted inFIG.1.

In the case114a, electrical connectors116a,116b,116c, and116d(hereinafter, collectively also referred to as electrical connectors116), which are coaxial connectors including signal input terminals124a,124b,124c, and124d(hereinafter, collectively also referred to as a signal input terminal124) that input the high-frequency electrical signal to be applied to each of the signal electrodes112a,112b,112c, and112dof the optical modulation element102are provided.

Each of the electrical connectors116is, for example, a socket for a push-on coaxial connector, including a cylindrical ground conductor, and the signal input terminal124includes a center conductor (core wire) extending along a center line of the cylindrical ground conductor. Each of the cylindrical ground conductors is electrically connected and fixed to the case114a. Therefore, the case114aconstitutes a part of a ground line that supplies a ground potential. Further, each of the signal input terminals124is electrically connected to one end of each of the signal electrodes112of the optical modulation element102, via a relay substrate118.

The other end of the signal electrode112of the optical modulation element102is terminated by a terminator120having a predetermined impedance. Thus, the electrical signals input to the one ends of the signal electrodes112respectively propagate in the signal electrodes112as traveling waves.

FIG.3illustrates a configuration of the relay substrate118and its surroundings. On the relay substrate118, signal conductor patterns330a,330b,330c, and330d(hereinafter, collectively also referred to as a signal conductor pattern330) and ground conductor patterns340a,340b,340c,340d,340e, and340f(hereinafter, collectively also referred to as a ground conductor pattern340) are formed.

On the relay substrate118, a rear surface ground conductor (not illustrated) is formed on, for example, the entire surface of a rear surface facing a front surface (a surface illustrated inFIG.3in which the signal conductor pattern330and the ground conductor pattern340are formed). The rear surface ground conductor is fixed to the case114aof the housing104with, for example, solder, a brazing material, a conductive adhesive, or the like. Thus, the rear surface ground conductor becomes a ground line component. Each of the ground conductor patterns340is connected to the rear surface ground conductor and connected to the ground line through an appropriate via (not illustrated).

The ground conductor patterns340a,340b,340c,340d,340e, and340fare respectively provided to sandwich the signal conductor patterns330a,330b,330c, and330din a front surface of the relay substrate118. Thus, each of the signal conductor patterns330and the ground conductor pattern340form a coplanar waveguide.

In the present embodiment, the signal conductor pattern330extends in an upward-downward direction illustrated inFIG.3, and among sides of the relay substrate118, one end of a side on a lower side illustrated inFIG.3is connected to the signal input terminal124. Here, among the sides of the relay substrate118, a side on which the signal conductor pattern330and the signal input terminal124are connected is referred to as a signal input side318a.

Each of the signal electrodes112of the optical modulation element102is electrically connected to another end of the signal conductor pattern330of the relay substrate118, on an upper side illustrated inFIG.3among sides of the relay substrate118, by wire bonding using a conductor wire326, for example. The conductor wire326can be a gold wire, for example. Here, among the sides of the relay substrate118, a side on which the signal conductor pattern330and the signal electrode112of the optical modulation element102are connected is referred to as a signal output side318b. In the present embodiment, the signal input side318aand the signal output side318bform two sides facing each other in the relay substrate118in a plan view. Among the sides of the relay substrate118inFIG.3, the other two sides facing each other, other than the signal input side318aand the signal output side318bare referred to as side edges (lateral sides)318cand318d.

In the optical modulation element102, the respective ground electrodes122that constitute the coplanar waveguide together with the signal electrodes112are electrically connected to one ends of the respective ground conductor patterns340at the signal output side318bof the relay substrate118, by wire bonding using the conductor wires326, for example, in the same manner as described above. The wire bonding using the conductor wire326described above is an example, and the present invention is not limited thereto. Instead of wire bonding of the conductor wire326, for example, ribbon bonding using a conductor ribbon such as a gold ribbon can be used.

The signal conductor patterns330a,330b,330c, and330dinclude, for example, component mounting portions350a,350b,350c, and350d(hereinafter, collectively also referred to as a component mounting portion350) which are portions (dark shaded portions illustrated inFIG.3) provided with an electrical circuit element constituting an electric filter, respectively. Here, the electrical circuit element refers to an active element and/or a passive element serving as a functional element constituting the circuit, and does not include a wire pattern or a land (solder) provided exclusively for electrical connection.

For example, similar to the component mounting portion2250illustrated inFIGS.17and19, the component mounting portion350may be configured by mounting an electrical circuit element such as a capacitor and/or forming an electrical circuit element such as a thin film resistor on a portion of the signal conductor pattern330provided wider than the others. That is, for example, similar to the signal conductor pattern2230billustrated inFIGS.17and19, a wide portion is formed in the signal conductor pattern330, a capacitor similar to the capacitor2254bis mounted, and a thin film resistor similar to the thin film resistor2252bis formed in a part of the wide portion of the signal conductor pattern330, thereby configuring the component mounting portion350.

In the present embodiment, in particular, the two ground conductor patterns340aand340bsandwiching the signal conductor pattern330aon the relay substrate118are formed in an asymmetrical shape in a plan view within a component mounting area360ahaving a square shape in the plan view centered on the component mounting portion350a, with respect to a straight line extending in an extending direction of the signal conductor pattern330ain the component mounting portion350a.

Further, the component mounting area360ais defined as an area having a square shape in the plan view in which a direction of a side of the component mounting area having the square shape is same as the extending direction of the at least one signal conductor pattern330ain the component mounting portion350aand a length of a side of the component mounting area is equal to a distance (pitch) W11from the center of the component mounting portion350ato a portion on the adjacent signal conductor pattern330bclosest to the center thereof (for example, the center in a width direction of the signal conductor pattern330bin the portion).

Here, the center of the component mounting portion350may be the center of any of the electrical circuit elements included in the component mounting portion350, or the center of the circumscribed rectangle (described later) including the electrical circuit elements constituting the component mounting portion350. InFIG.3, for the sake of description, it is assumed that a boundary line in the width direction (a boundary line between the left and right illustrated inFIG.3) of the component mounting portion350indicates an edge position in the width direction of the corresponding signal conductor pattern330in the component mounting portion350. Further, inFIG.3, the width of the portion of the signal conductor pattern330in the component mounting portion350is wider than that of the other portion, but the width is not limited thereto. The width of the portion of the signal conductor pattern330in the component mounting portion350may be the same as that of the other portion or may have a narrower width than the other portion.

In the present embodiment, since the ground conductor pattern340ahas a notched portion342a(hatched portion illustrated inFIG.3), the ground conductor patterns340aand340bare formed such that separation distances (gaps) from respective edges of the ground conductor patterns340aand340bto opposite edges of the signal conductor pattern330ahave different portions (that is, asymmetrical portions) in the component mounting area360a. Further, in the present embodiment, since the signal conductor pattern330is formed parallel to the side edge318dand linearly, a length w11of one side of the component mounting area360ais equal to a distance (pitch) w21between a center line in the width direction of the signal conductor pattern330aand a center line in the width direction of the signal conductor pattern330b. That is, w11=w21.

The signal conductor pattern330aand the ground conductor patterns340aand340bcan be formed such that the characteristic impedance of the signal conductor pattern330abecomes a predetermined value, for example, inside and outside the component mounting portion350aby adjusting a clearance (separation distance) between the conductor patterns, a formation width of the signal conductor pattern330a, and the like.

In general, in a high-frequency signal line including a signal conductor and a ground conductor, the smaller the distance between the signal conductor and the ground conductor, the stronger the confinement of the high-frequency signal in the signal conductor.

In the relay substrate118, there are portions where separation distances from the signal conductor pattern330ato the ground conductor patterns340aand340bare different from each other in the component mounting area360a. Therefore, the leaked microwave generated from the component mounting portion350ais biased in a direction in which the separation distance from the adjacent ground conductor pattern is large, that is, in the direction of the ground conductor pattern340a(for example, inFIG.3, in a direction area sandwiched by arrows of two alternate long and short dash lines extending from the component mounting portion350a) and emitted to have a higher intensity than the other directions.

On the other hand, since the signal conductor pattern330acan be configured to have, for example, the same characteristic impedance inside and outside the component mounting area360a, the total amount of leaked microwaves generated from the component mounting portion350ais almost the same as a case where the ground conductor patterns340aand340bare formed symmetrically.

As a result, the intensity of the leaked microwave in a direction other than the direction area sandwiched by the arrows of the alternate long and short dash lines is relatively reduced, and crosstalk from the signal conductor pattern330ato the signal conductor pattern330bdue to the leaked microwave generated from the component mounting portion350ais reduced.

Similarly, in the present embodiment, since the ground conductor pattern340fhas a notched portion342d(hatched portion illustrated inFIG.3), the ground conductor patterns340eand340fare configured such that separation distances (gaps) from respective edges of the ground conductor patterns340eand340fto opposite edges of the signal conductor pattern330dhave different portions in the component mounting area360d.

Here, the component mounting area360dis defined as an area having a square shape in the plan view in which a direction of a side of the component mounting area having the square shape is same as the extending direction of the at least one signal conductor pattern330din the component mounting portion350dand a length of a side of the component mounting area is equal to a distance (pitch) W12from the center of the component mounting portion350dto a portion on the adjacent signal conductor pattern330cclosest to the center thereof (for example, the center in a width direction of the signal conductor pattern330cin the portion). In the present embodiment, since the signal conductor pattern330is formed parallel to the side edge318dand linearly, a length w12of one side of the component mounting area360dis equal to a distance (pitch) w22between a center line in the width direction of the signal conductor pattern330dand a center line in the width direction of the signal conductor pattern330c. That is, w12=w22.

Thus, the ground conductor patterns340eand340fare formed in an asymmetrical shape in a plan view with respect to the straight line extending in the extending direction of the signal conductor pattern330din the component mounting portion350d, in the component mounting area360d. Therefore, the leaked microwave generated from the component mounting portion350dhas a higher intensity than the other directions in the direction area sandwiched by the arrows of the two alternate long and short dash lines extending from the component mounting portion350dinFIG.3, for example. As a result, crosstalk from the signal conductor pattern330dto the signal conductor pattern330cdue to the leaked microwave generated from the component mounting portion350dis reduced.

In particular, in an optical modulator that generates two modulated light beams, each of which is modulated by a pair of high-frequency electrical signals, such as the optical modulator100that performs DP-QPSK modulation, the two paired high-frequency electrical signals often carry information for a phase difference between them. Therefore, crosstalk between the two paired high-frequency electrical signals also generates a phase noise in addition to an intensity noise, and can have a greater influence on the modulation characteristics of the optical modulator100than crosstalk between unpaired high-frequency electrical signals.

Since the paired high-frequency electrical signals are generally relayed by using two adjacent signal conductor patterns on the relay substrate, it is extremely important to suppress such crosstalk between two adjacent signal conductor patterns.

The optical modulator100of the present embodiment is configured to respectively propagate two high-frequency electrical signals forming one pair of the two pairs of high-frequency electrical signals by the adjacent signal conductor patterns330aand330b, and to propagate an other two paired high-frequency electrical signals, by the other adjacent signal conductor patterns330cand330d. Then, with the above configuration, the crosstalk from the signal conductor pattern330ato the signal conductor pattern330b, that is, crosstalk from one of the two high-frequency electrical signals forming one pair to the other is reduced. Further, with the above configuration, the crosstalk from the signal conductor pattern330dto the signal conductor pattern330c, that is, crosstalk from one of the two high-frequency electrical signals forming the other pair to the other is also reduced. As a result, the optical modulator100can effectively reduce the influence of leaked microwaves and realize appropriate optical modulation characteristics.

In the present embodiment, it is assumed that the component mounting portion350has the same configuration as the component mounting portion2250billustrated inFIGS.17and18, but the present invention is not limited thereto. As illustrated inFIG.4, the component mounting portion350may include, for example, electrical circuit elements352aand352b, which are two chip components connected in parallel to each other in the plane direction of the relay substrate, which are inserted by dividing the signal conductor pattern330a. In this case, the area of the component mounting portion350can be the area of the circumscribed rectangle (rectangle of the alternate long and short dash line illustrated inFIG.4) including the electrical circuit elements352aand352bconstituting the component mounting portion350.

In this case, the center of the component mounting portion350acan be the center of the rectangle of the alternate long and short dash line indicating the area of the component mounting portion350a. Alternatively, the center of any of the shapes of the plurality of electrical circuit elements352aand352bconstituting the component mounting portion350a(for example, the shape center362aof the electrical circuit element352aindicated by the plus sign inFIG.4) may be the center of the component mounting portion350a.

Further, as described above, the extending direction of the signal conductor pattern330in the component mounting portion350when the signal conductor pattern330has a divided portion in the component mounting portion350can be a propagation direction of the high-frequency signal flowing from the signal conductor pattern330or a propagation direction of the high-frequency signal flowing out to the signal conductor pattern330for any of the electrical circuit elements included in the component mounting portion350. InFIG.4, as an example, a propagation direction362bof the high-frequency signal flowing from the signal conductor pattern for the electrical circuit element352ais drawn as the extending direction of the signal conductor pattern330ain the component mounting portion350a.

In the present embodiment, the separation distances between the signal conductor pattern330and the two ground conductor patterns340are formed asymmetrically in the two component mounting areas360aand360ddefined for the two signal conductor patterns330aand330d. However, the present invention is not limited thereto. Further, one signal conductor pattern330may be provided with a plurality of component mounting portions350and a component mounting area360, and as illustrated inFIG.3, each of the four signal conductor patterns may be provided with the component mounting portions350and the component mounting areas360. Therefore, the separation distances can be configured to be asymmetrical to each other in the at least one component mounting area360of the at least one signal conductor pattern330according to the generation state of the leaked microwave on the relay substrate118.

As described above, the component mounting area360is defined as “an area having a square shape in the plan view in which a direction of a side of the component mounting area having the square shape is same as the extending direction of the at least one signal conductor pattern330in the component mounting portion350and a length of a side of the component mounting area is equal to a distance from the center of the component mounting portion350to a portion on the adjacent signal conductor pattern330closest to the center thereof”. This is because an effective electric field of microwaves is distributed in this area and it is possible to effectively affect an effective electric field profile of microwaves by making the electrode configuration asymmetric in this portion, and as a result, the direction of the leaked microwave can be effectively adjusted.

Next, a modification example of the relay substrate used in the optical modulator100will be described.

First Modification Example

FIG.5is a diagram illustrating a configuration of a relay substrate518according to a first modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate518can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1. InFIG.5, the same reference numerals as those inFIG.3are used for the same components as those of the components of the relay substrate118illustrated inFIG.3, and the above description ofFIG.3will be incorporated herein.

The relay substrate518has the same configuration as the relay substrate118, but also in the component mounting areas360band360cfor the component mounting portions350band350cof the signal conductor patterns330band330c, two ground conductor patterns sandwiching the signal conductor patterns330band330c, respectively, are configured asymmetrically with respect to a straight line extending in the extending direction of the signal conductor patterns330band330cin the component mounting portions350band350c.

Here, similar to the component mounting area360a, the component mounting area360bis defined as an area having a square shape in the plan view in which a direction of a side of the component mounting area having the square shape is same as the extending direction of the at least one signal conductor pattern330bin the component mounting portion350band a length of a side of the component mounting area is equal to a distance (pitch) W13from the center of the component mounting portion350bto a portion on the adjacent signal conductor pattern330aclosest to the center thereof (for example, the center in a width direction of the signal conductor pattern330ain the portion). In the present embodiment, since the signal conductor pattern330is formed parallel to the side edge318dand linearly, a length w13of one side of the component mounting area360bis equal to the distance (pitch) w21between the center line in the width direction of the signal conductor pattern330band the center line in the width direction of the signal conductor pattern330a. That is, w13=w21.

Further, similar to the component mounting area360d, the component mounting area360cis defined as an area having a square shape in the plan view in which a direction of a side of the component mounting area having the square shape is same as the extending direction of the at least one signal conductor pattern330cin the component mounting portion350cand a length of a side of the component mounting area is equal to a distance (pitch) W14from the center of the component mounting portion350cto a portion on the adjacent signal conductor pattern330dclosest to the center thereof (for example, the center in a width direction of the signal conductor pattern330din the portion). In the present embodiment, since the signal conductor pattern330is formed parallel to the side edge318dand linearly, a length w14of one side of the component mounting area360cis equal to the distance (pitch) w22between a center line in the width direction of the signal conductor pattern330cand a center line in the width direction of the signal conductor pattern330d. That is, w14=w22.

More specifically, the relay substrate518has the same configuration as the relay substrate118, but is different from the relay substrate118in that it includes ground conductor patterns540cand540dinstead of the ground conductor patterns340cand340d. The ground conductor pattern540chas the same configuration as the ground conductor pattern340c, but is different from the ground conductor pattern340cin that it has a notched portion342b(hatched portion illustrated inFIG.5) within the component mounting area360b. Thus, the ground conductor patterns340band540care formed to have portions in which the separation distances (gaps) from the respective edges of the ground conductor patterns340band540cto the opposite edges of the signal conductor pattern330bare different from each other in the component mounting area360b.

Therefore, similar to the component mounting portion350a, the leaked microwave generated from the component mounting portion350bhas a higher intensity than the other directions, toward the ground conductor pattern540cin which the confinement of high-frequency signals is weakened by the presence of the notched portion342b, for example, inFIG.5, in a direction area sandwiched by arrows of two alternate long and short dash lines extending from the component mounting portion350b. As a result, crosstalk from the signal conductor pattern330bto the signal conductor pattern330adue to the leaked microwave generated from the component mounting portion350bis reduced.

Therefore, in the relay substrate518, crosstalk between the signal conductor patterns330aand330brespectively propagating the two high-frequency electrical signals forming one pair is effectively suppressed.

Similarly, the ground conductor pattern540dhas the same configuration as the ground conductor pattern340d, but is different from the ground conductor pattern340din that it has a notched portion342c(hatched portion illustrated inFIG.5) within the component mounting area360c. Thus, the ground conductor patterns540dand340eare formed to have portions in which the separation distances (gaps) from the respective edges of the ground conductor patterns540dand340eto the opposite edges of the signal conductor pattern330care different from each other in the component mounting area360c.

As a result, the leaked microwave generated from the component mounting portion350chas a higher intensity than the other directions, in the direction of the ground conductor pattern540din which the confinement of high-frequency signals is weakened by the presence of the notched portion342c, for example, inFIG.5, in a direction area sandwiched by arrows of two alternate long and short dash lines extending from the component mounting portion350c. As a result, crosstalk from the signal conductor pattern330cto the signal conductor pattern330ddue to the leaked microwave generated from the component mounting portion350cis reduced.

Therefore, in the relay substrate518, crosstalk between the signal conductor patterns330cand330d, which respectively propagate two high-frequency electrical signals forming the other pair, is effectively suppressed.

As a result, the relay substrate518can realize further appropriate optical modulation characteristics than the case where the relay substrate118is used.

In the first embodiment and the first modification example, it is assumed that the signal conductor pattern330is formed in a linear shape, but the present invention is not limited thereto. Similarly, in the other modification examples and embodiments illustrated below, the signal conductor pattern330can be configured in any shape including curves and bends depending on the required electrical characteristics, the arrangement of the signal input terminals124, and/or the arrangement of the signal electrodes112of the optical modulation element102, and the like.

Here, in an optical modulator having an optical modulation element using an LN substrate, the external dimensions of the relay substrate are typically, for example, several mm×ten and several mm. In a relay substrate of this size, the length of one side of the component mounting area360having a square shape in a plan view, is, for example, about several hundred μm, and the signal conductor pattern330can generally be regarded as a straight line within the component mounting area.

Second Modification Example

FIG.6is a diagram illustrating a configuration of a relay substrate718according to a second modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate718can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1. InFIG.6, the same reference numerals as those inFIG.3are used for the same components as those of the components of the relay substrate118illustrated inFIG.3, and the above description ofFIG.3will be incorporated herein.

The relay substrate718has the signal conductor pattern330formed at the same position as the relay substrate118, and each of these signal conductor patterns330includes the component mounting portion350, in the same manner as the relay substrate118. Therefore, in the relay substrate718, the component mounting areas360aand360dare defined for each of the component mounting portions350of the signal conductor patterns330, in the same manner as the relay substrate118.

The relay substrate718has the same configuration as the relay substrate118, but is different from the relay substrate118in that it includes ground conductor patterns740aand740finstead of the ground conductor patterns340aand340f. The ground conductor pattern740ahas the same configuration as the ground conductor pattern340aof the relay substrate118illustrated inFIG.3, but is different from the ground conductor pattern340ain that it has a discontinuous section742a(that is, no conductor is provided) in the component mounting area360a.

Therefore, in the relay substrate718, the effect of confining the high-frequency signal on the side of the ground conductor pattern740aof the signal conductor pattern330ain the component mounting area360ais weaker than that in the case of using the ground conductor pattern340a. Therefore, the leaked microwaves generated in the component mounting area360aare more likely to be unevenly distributed on the side of the ground conductor pattern740aas compared with the relay substrate118illustrated inFIG.3. As a result, in the relay substrate718, the crosstalk from the signal conductor pattern330ato the signal conductor pattern330bdue to the leaked microwave is further reduced as compared with the relay substrate118.

Similarly, the ground conductor pattern740fhas the same configuration as the ground conductor pattern340fof the relay substrate118illustrated inFIG.3, but is different from the ground conductor pattern340fin that it has a discontinuous section742din the component mounting area360d. Therefore, the leaked microwaves generated in the component mounting area360dare more likely to be unevenly distributed on the side of the ground conductor pattern740fas compared with the relay substrate118illustrated inFIG.3. As a result, in the relay substrate718, the crosstalk from the signal conductor pattern330dto the signal conductor pattern330cdue to the leaked microwave is further reduced as compared with the relay substrate118.

In particular, the configuration in which the discontinuous sections742aand742dare provided in the ground conductor patterns740aand740fas illustrated inFIG.6can greatly weaken the ground intensities of the ground conductor patterns740aand740fwith respect to the ground conductor patterns340band340eas compared with the configuration in which the notched portions342aand342dare provided as illustrated inFIG.3(that is, the impedance of the ground conductor patterns740aand740fwith respect to the ground line can be made higher than the impedance of the ground conductor patterns340band340ewith respect to the ground line). Therefore, the relay substrate718of the present modification example has an effective configuration when it is desired to bias the propagation direction of the leaked microwave generated from the component mounting portion350while keeping the length of the asymmetrical portion of the two ground conductor patterns340in the component mounting area360as short as possible.

Third Modification Example

FIG.7is a diagram illustrating a configuration of a relay substrate818according to a third modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate818can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1. InFIG.7, the same reference numerals as those inFIGS.3,5, and6are used for the same components as those of the components of the relay substrates118,518, and718illustrated inFIGS.3,5, and6, and the above descriptions ofFIGS.3,5, and6will be incorporated herein.

The relay substrate818has the same configuration as the relay substrate518of the first modification example illustrated inFIG.5, but includes ground conductor patterns740aand740fsimilar to the relay substrate718of the second modification example illustrated inFIG.6instead of the ground conductor patterns340aand340f, and includes ground conductor patterns840cand840dinstead of the ground conductor patterns540cand540d.

The ground conductor patterns840cand840dhave the same configurations as the ground conductor patterns540cand540dof the relay substrate518illustrated inFIG.5, but are different from the ground conductor patterns540cand540din that they have discontinuous sections842band842cin the component mounting areas360band360c.

Since the relay substrate818having the above configuration includes the ground conductor patterns740aand740f, similar to the relay substrate718of the second modification example illustrated inFIG.6, the crosstalk from the signal conductor pattern330ato the signal conductor pattern330band the crosstalk from the signal conductor pattern330dto the signal conductor pattern330care further reduced as compared with the relay substrate118illustrated inFIG.3.

Further, since the relay substrate818has the sections842band842cin which conductors are not formed in the ground conductor patterns840cand840d, respectively, the leaked microwaves generated in the component mounting areas360band360care more likely to be unevenly distributed on the sides of the ground conductor patterns840cand840das compared with the relay substrate518illustrated inFIG.5. As a result, in the relay substrate818, the crosstalk from the signal conductor pattern330bto the signal conductor pattern330aand from the signal conductor pattern330cto the signal conductor pattern330ddue to the leaked microwaves is further reduced as compared with the relay substrate518.

Fourth Modification Example

FIG.8is a diagram illustrating a configuration of a relay substrate918according to a fourth modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate918can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1, similar to the relay substrate818of the third modification example illustrated inFIG.7. InFIG.8, the same reference numerals as those inFIGS.3and7are used for the same components as those of the components of the relay substrates118and818illustrated inFIGS.3and7, and the above descriptions ofFIGS.3and7will be incorporated herein.

The relay substrate918has the same configuration as the relay substrate818of the third modification example illustrated inFIG.7, and similar to the relay substrate818, crosstalk between the signal conductor patterns330aand330band between the signal conductor patterns330cand330ddue to leaked microwaves is further reduced as compared with the relay substrate518of the first modification example illustrated inFIG.5.

As described above, the relay substrate918has the same configuration as the relay substrate818of the third modification example illustrated inFIG.7, but is different from the relay substrate818of the third modification example illustrated inFIG.7in that it includes ground conductor patterns940band940einstead of the ground conductor patterns340band340e. The ground conductor pattern940bhas the same configuration as the ground conductor pattern340bof the relay substrate818of the third modification example illustrated inFIG.7, but is different from the ground conductor pattern340bin that recess portions942aand942bare provided on the edges facing the component mounting portions350aand350b, respectively, in the component mounting areas360aand360b.

That is, in the relay substrate918, the other ground conductor pattern940bfacing the one ground conductor pattern740ahaving the section742ain which the conductor is not provided has a recess portion942ain which a separation distance to a portion of the signal conductor pattern330aconstituting the component mounting portion350ais formed longer than a separation distance to other portions of the signal conductor pattern330awithin the corresponding component mounting area360a.

Normally, when the electrical circuit element is mounted on the signal conductor pattern330ain the component mounting portion350a, the mounting position of the electrical circuit element may change due to manufacturing variation or the like. For example, in the relay substrate818illustrated inFIG.7, when such a displacement in the mounting position occurs in the direction of the ground conductor pattern340b, which has a narrower separation distance from the signal conductor pattern330athan the ground conductor pattern740a, the characteristic impedance of the signal conductor pattern330acan be changed.

Since the relay substrate918includes the ground conductor pattern940bhaving the recess portion942ainstead of the ground conductor pattern340b, even if the mounting position of the electrical circuit element in the component mounting portion350ashifts to the ground conductor pattern940bside, the ratio of the displacement amount of the mounting position to the separation distance between the component mounting portion350aand the ground conductor pattern940bis smaller than that of the relay substrate818having no recess portion942a.

Therefore, in the relay substrate918, the fluctuation of the characteristic impedance of the signal conductor pattern330adue to the displacement of the mounting position can be alleviated (made smaller) as compared with the relay substrate818in the component mounting area360a.

Similarly, in the relay substrate918, the other ground conductor pattern940bfacing the one ground conductor pattern840chaving the section842bin which the conductor is not provided has a recess portion942bin which a separation distance to a portion of the signal conductor pattern330bconstituting the component mounting portion350bis formed longer than a separation distance to other portions of the signal conductor pattern330bwithin the corresponding component mounting area360b. Therefore, in the relay substrate918, the fluctuation of the characteristic impedance of the signal conductor pattern330bdue to the displacement of the mounting position of the electrical circuit element in the component mounting portion350bcan be alleviated (made smaller) as compared with the relay substrate818in the component mounting area360b.

Similarly, the ground conductor pattern940eof the relay substrate918has the same configuration as the ground conductor pattern340eof the relay substrate818of the third modification example illustrated inFIG.7, but is different from the ground conductor pattern340ein that recess portions942cand942dare provided on the edges facing the signal conductor patterns330cand330d, respectively, in the component mounting areas360cand360d.

That is, in the relay substrate918, the other ground conductor pattern940efacing the one ground conductor pattern840dhaving the section842cin which the conductor is not provided has a recess portion942cin which a separation distance to a portion of the signal conductor pattern330cconstituting the component mounting portion350cis formed longer than a separation distance to other portions of the signal conductor pattern330cwithin the corresponding component mounting area360c. Further, in the relay substrate918, the other ground conductor pattern940efacing the one ground conductor pattern740fhaving the section742din which the conductor is not provided has a recess portion942din which a separation distance to a portion of the signal conductor pattern330dconstituting the component mounting portion350dis formed longer than a separation distance to other portions of the signal conductor pattern330dwithin the corresponding component mounting area360d.

Thus, in the relay substrate918, the fluctuation of the characteristic impedance of the signal conductor patterns330cand330ddue to the displacement of the mounting positions of the electrical circuit elements in the component mounting portions350cand350dcan be alleviated (made smaller) as compared with the relay substrate818, respectively, in the component mounting areas360cand360d.

InFIG.8, it is assumed that the recess portions942a,942b,942c, and942d(hereinafter, collectively also referred to as a recess portion942) are configured to be longer than the length of the component mounting portion350along the extending direction of the corresponding signal conductor pattern330, but the present invention is not limited thereto. The recess portion942may be configured to be shorter than the length of the component mounting portion350along the extending direction of the corresponding signal conductor pattern330.

Fifth Modification Example

FIG.9is a diagram illustrating a configuration of a relay substrate1018according to a fifth modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate1018can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.3. InFIG.9, the same reference numerals as those inFIG.3are used for the same components as those of the components of the relay substrate118illustrated inFIG.3, and the above description ofFIG.3will be incorporated herein.

The relay substrate1018has the signal conductor pattern330formed at the same position as the relay substrate118of the first embodiment illustrated inFIG.3, and similar to the relay substrate118, each signal conductor pattern330is provided with the component mounting portion350. Therefore, in the relay substrate1018, the component mounting area360is defined for each of the component mounting portions350, in the same manner as the relay substrate118.

In the relay substrates118,518,818, and918illustrated inFIGS.3,5,6, and7according to the first embodiment and the first to fourth modification examples described above, in the component mounting area360, two ground conductor patterns1040sandwiching the signal conductor pattern330are configured to have contour shapes that are asymmetrical to each other. On the other hand, in the relay substrate1018according to the present modification example, in the component mounting area360, the two ground conductor patterns sandwiching the signal conductor pattern330are configured to be asymmetrical with respect to the mode of forming vias by setting diameters and the numbers of the vias provided in the two ground conductor patterns to be different from each other.

Specifically, the relay substrate1018of the present modification example has the same configuration as the relay substrate118illustrated inFIG.3, but is different from the relay substrate118in that it includes ground conductor patterns1040a,1040b,1040c,1040d,1040e, and1040f(collectively also referred to as a ground conductor pattern1040) instead of the ground conductor patterns340a,340b,340c,340d,340e, and340f.

The ground conductor patterns1040a,1040b,1040c,1040d,1040e, and1040fhave the same configurations as the ground conductor patterns340a,340b,340c,340d,340e, and340f, respectively.

However, unlike the ground conductor patterns340aand340fof the relay substrate118illustrated inFIG.3, the ground conductor pattern1040aand1040fdo not have notched portions342aand342d. Further, the ground conductor patterns1040a,1040b,1040c,1040d,1040e, and1040feach have a plurality of vias1070having the same diameter, for example. InFIG.9, although the reference numeral1070is attached to only one via for each of the ground conductor patterns1040in order to avoid redundant description, it should be understood that the other circles having the same diameter as the circle to which the reference numeral1070is attached are also the vias1070.

In particular, in the relay substrate1018, the two ground conductor patterns1040sandwiching the signal conductor pattern330are configured such that the diameters and numbers of vias provided in the ground conductor pattern differ from each other in the component mounting area360. Specifically, in the component mounting area360a, the ground conductor pattern1040bis provided with six vias1070, whereas the ground conductor pattern1040ais provided with four vias1070and one via1072having a diameter smaller than that of the via1070.

In general, in a high-frequency signal line including a signal conductor and a ground conductor, the smaller the impedance from the ground conductor to a ground line (that is, the more so-called ground strengthening is sufficient), the stronger confinement of the high-frequency signal in the signal conductor.

In the component mounting area360aof the relay substrate1018, since the vias are provided in the ground conductor patterns1040aand1040bwith the above configuration, the impedance of the ground conductor pattern1040awith respect to the rear surface ground conductor (not illustrated) constituting the ground line provided on the back surface of the relay substrate1018is higher than the impedance of the ground conductor pattern1040bwith respect to the rear surface ground conductor. That is, in the component mounting area360a, the high-frequency confinement effect of the signal conductor pattern330ais lower on the side of the ground conductor pattern1040athan on the side of the ground conductor pattern1040b.

Therefore, the leaked microwave generated in the component mounting portion350ais biased in the direction of the ground conductor pattern1040a(for example, inFIG.9, in a direction area sandwiched by arrows of two alternate long and short dash lines extending from the component mounting portion350a) and propagate to have a higher intensity than the other directions.

As a result, the intensity of the leaked microwave in a direction area other than the direction area sandwiched by the arrows of the alternate long and short dash lines is relatively reduced, and crosstalk from the signal conductor pattern330ato the signal conductor pattern330bdue to the leaked microwave generated from the component mounting portion350ais reduced.

Similarly, in the component mounting area360bof the relay substrate1018, the ground conductor pattern1040bis provided with six vias1070, whereas the ground conductor pattern1040cis provided with four vias1070and one via1072having a diameter smaller than that of the via1070. Therefore, similar to the above, the impedance of the ground conductor pattern1040cwith respect to the rear surface ground conductor is higher than the impedance of the ground conductor pattern1040bwith respect to the rear surface ground conductor, and crosstalk from the signal conductor pattern330bto the signal conductor pattern330adue to the leaked microwave generated from the component mounting portion350bis reduced.

Further, the same applies to the component mounting areas360cand360d. That is, in the component mounting areas360cand360d, corresponding portions on the ground conductor pattern1040dare provided with six vias1070, respectively, whereas the ground conductor patterns1040dand1040fare provided with four vias1070and one via1072having a diameter smaller than that of the via1070.

Therefore, the leaked microwaves generated in the component mounting portions350cand350dare unevenly distributed in the directions of the ground conductor patterns1040dand1040f, respectively. As a result, the crosstalk from the signal conductor pattern330cto the signal conductor pattern330d, from the signal conductor pattern330dto the signal conductor pattern330c, and from the signal conductor pattern330dto the signal conductor pattern330cdue to the leaked microwaves is reduced.

In the present modification example, the two ground conductor patterns sandwiching the signal conductor pattern have been described as having different diameters and numbers of vias, but the present invention is not limited thereto. For example, the diameters of the vias are the same in the two ground conductor patterns but only the numbers may be different from each other, or the numbers of the vias are the same in the two ground conductor patterns but the diameters of at least some vias may be different from the diameters of other vias.

Sixth Modification Example

FIG.10is a diagram illustrating a configuration of a relay substrate1118according to a sixth modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate1118can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1. InFIG.10, the same reference numerals as those inFIGS.3and9are used for the same components as those of the components of the relay substrates118and1080illustrated inFIGS.3and9, and the above descriptions ofFIGS.3and9will be incorporated herein.

In the relay substrate1118, different numbers of vias are provided in the two ground conductor patterns sandwiching the signal conductor pattern330in the component mounting area360. Further, the two ground conductor patterns are formed to include portions whose widths measured in a direction orthogonal to the extending direction of the signal conductor pattern330are different from each other in the component mounting area360.

Specifically, the relay substrate1118has the same configuration as the relay substrate1018of the fifth modification example illustrated inFIG.9, but is different from the relay substrate1018in that it includes ground conductor patterns1140a,1140c,1140d, and1140finstead of the ground conductor patterns1040a,1040c,1040d, and1040f.

The ground conductor pattern1140ahas the same configuration as the ground conductor pattern1040aof the relay substrate1018of the fifth modification example illustrated inFIG.9, but includes a via1070at the position of the via1072instead of the via1072. That is, the relay substrate1118is provided with different numbers of vias1070having the same diameter on the ground conductor patterns1140aand1040bin the component mounting area360a.

Further, the ground conductor pattern1140ahas a narrow portion1174a(hatched portion illustrated inFIG.10) in the component mounting area360a. The narrow portion1174ais formed such that the width measured in the direction orthogonal to the extending direction of the signal conductor pattern330ais shorter than the width direction length of the portion of the ground conductor pattern1040bextending within the component mounting area360a.

Thus, in the relay substrate1118, the ground conductor pattern1140aincludes the narrow portion1174aas a portion having a higher impedance to the ground line (for example, the rear surface ground conductor) than the ground conductor pattern1040bin the component mounting area360a.

Therefore, in the component mounting area360a, the effect of confining the high-frequency signal in the signal conductor pattern330ais lower on the side of the ground conductor pattern1140ahaving the narrow portion1174athan on side of the ground conductor pattern1040b.

Therefore, the leaked microwave generated in the component mounting portion350ais biased in the direction of the ground conductor pattern1140a(for example, inFIG.10, in a direction area sandwiched by arrows of two alternate long and short dash lines extending from the component mounting portion350a) and emitted to have a higher intensity than the other directions.

As a result, the intensity of the leaked microwave in a direction area other than the direction area sandwiched by the arrows of the alternate long and short dash lines is relatively reduced, and crosstalk from the signal conductor pattern330ato the signal conductor pattern330bdue to the leaked microwave is reduced.

Similarly, the ground conductor pattern1140chas a narrow portion1174b(hatched portion illustrated inFIG.10) in the component mounting area360b. The narrow portion1174bis formed such that the width measured in the direction orthogonal to the extending direction of the signal conductor pattern330bis shorter than the width direction length of the portion of the ground conductor pattern1040bextending within the component mounting area360b.

Therefore, the leaked microwave generated in the component mounting portion350bis unevenly emitted in the direction of the ground conductor pattern1140c, and the crosstalk from the signal conductor pattern330bto the signal conductor pattern330adue to the leaked microwave is reduced.

Similarly, in the relay substrate1118, the ground conductor patterns1140dand1140feach have a different number of vias1070from the ground conductor pattern1040ein each of the component mounting areas360cand360d. Further, in the relay substrate1118, the ground conductor patterns1140dand1140fhave narrow portions1174cand1174d, respectively, in the component mounting areas360cand360d. The narrow portions1174cand1174deach are formed such that the widths measured in the direction orthogonal to the extending direction of the signal conductor patterns330cand330dare shorter than the width direction length of the portion of the ground conductor pattern1040cextending within the component mounting areas360cand360d.

Therefore, the leaked microwaves generated in the component mounting portions350cand350dare unevenly distributed and propagate in the directions of the ground conductor patterns1140dand1140f, respectively. As a result, the crosstalk from the signal conductor pattern330bto the signal conductor pattern330a, from the signal conductor pattern330cto the signal conductor pattern330d, and from the signal conductor pattern330dto the signal conductor pattern330cdue to the leaked microwaves is reduced.

In the relay substrate1118illustrated inFIG.10, the narrow portion1174aand the like are formed with a width wider than the diameter of the via1070, but the present invention is not limited thereto. The narrow portion1174aand the like may be formed with a width narrower than the diameter of the via1070.

Seventh Modification Example

FIG.11is a diagram illustrating a configuration of a relay substrate1218according to a seventh modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate1218can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1, similar to the relay substrate1118of the sixth modification example illustrated inFIG.10. InFIG.11, the same reference numerals as those inFIG.10are used for the same components as those of the components of the relay substrate1118illustrated inFIG.10, and the above description ofFIG.10will be incorporated herein.

The relay substrate1218has the same configuration as the relay substrate1118of the sixth modification example illustrated inFIG.10, but is different from the relay substrate1118in that it includes ground conductor patterns1240a,1240b,1240c,1240d,1240e, and1240finstead of the ground conductor patterns1140a,1040b,1040c,1140d,1140e, and1040f.

The ground conductor patterns1240a,1240c,1240d, and1240fhave the same configurations as the ground conductor patterns1140a,1140c,1140d, and1140fof the relay substrate1118of the sixth modification example illustrated inFIG.10, but are different from the relay substrate1118in that they include narrow portions1274a,1274b,1274c, and1274d, respectively, similar to the narrow portions1174a,1174b,1174c, and1174dof the sixth modification example illustrated inFIG.10and having a width narrower than the diameter of the via1070in the component mounting areas360a,360b,360c, and360d.

That is, the narrow portions1274aand1274beach are formed such that the widths measured in the direction orthogonal to the extending direction of the signal conductor patterns330aand330bare shorter than the width direction length of the portion of the ground conductor pattern1240bextending within the component mounting areas360aand360band are narrower than the diameter of the via1070. Further, the narrow portions1274cand1274deach are formed such that the widths measured in the direction orthogonal to the extending direction of the signal conductor patterns330cand330dare shorter than the width direction length of the portion of the ground conductor pattern1240eextending within the component mounting areas360cand360dand are narrower than the diameter of the via1070.

Thus, the ground conductor patterns1240a,1240c,1240d, and1240fhave narrow portions1274a,1274b,1274c, and1274dhaving higher impedances between the ground conductor patterns1240a,1240c,1240d, and1240fand the ground line than the narrow portions1174a,1174b,1174c, and1174dof the relay substrate1118of the sixth modification example illustrated inFIG.10, and the crosstalk between the signal conductor patterns330aand330band between the signal conductor patterns330cand330ddue to the leaked microwaves is reduced.

Further, the ground conductor patterns1240band1240eof the relay substrate1218have the same configurations as the ground conductor patterns1040band1040eof the relay substrate1118of the sixth modification example illustrated inFIG.10, but are different from the relay substrate1118of the sixth modification example illustrated inFIG.10in that they include recess portions1276a,1276b,1276c, and1276d(hereinafter, collectively also referred to as a recess portion1276), respectively, similar to the recess portions942a,942b,942c, and942dof the relay substrate914of the fourth modification example illustrated inFIG.8in the component mounting areas360a,360b,360c, and360d.

That is, in the relay substrate1218, the ground conductor pattern1240bhas recess portions1276aand1276bin which separation distances to portions of the signal conductor patterns330aand330bconstituting the component mounting portions350aand350bare formed longer than separation distances to other portions of the signal conductor patterns330aand330bwithin the corresponding component mounting areas360aand360b. Further, the ground conductor pattern1240ehas recess portions1276dand1276din which separation distances to portions of the signal conductor patterns330cand330dconstituting the component mounting portions350cand350dare formed longer than separation distances to other portions of the signal conductor patterns330cand330dwithin the corresponding component mounting areas360cand360d.

As a result, in the relay substrate1218, similar to the relay substrate914of the fourth modification example illustrated inFIG.8, when the electrical circuit elements constituting the component mounting portions350a,350b,350c, and350dare mounted on the signal conductor patterns330a,330b,330c, and330d, even if the mounting position of the corresponding electrical circuit element shifts in the direction of the adjacent ground conductor patterns1240band1240e, the fluctuation of the characteristic impedance of the signal conductor patterns330a,330b,330c, and330ddue to the displacement of the mounting position can be alleviated as compared with the relay substrate1118.

In addition, inFIGS.10and11, it is assumed that vias are provided in the narrow portions1174a,1274, and the like but the present invention is not limited thereto. Even in a configuration in which vias are not provided in the narrow portions1174a,1274, and the like, the same effect as described above can be obtained.

Further, in the relay substrate1218of the present modification example and the relay substrate1118of the seventh modification example illustrated inFIG.10, it is assumed that the two ground conductor patterns are formed to include portions having different numbers of vias from each other and having different widths measured in a direction orthogonal to the extending direction of the signal conductor pattern330, in the component mounting area360, but the present invention is not limited thereto. For example, the two ground conductor patterns are formed to include portions whose widths measured in a direction orthogonal to the extending direction of the signal conductor pattern330are different from each other in the component mounting area360, but may have the same number of vias or no vias at all. Even with this configuration, the impedances of the two ground conductor patterns and the ground line may be different from each other, and the same effect as described above can be obtained.

Eighth Modification Example

FIG.12is a diagram illustrating a configuration of a relay substrate1318according to an eighth modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate1318can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1. InFIG.12, the same reference numerals as those inFIG.3are used for the same components as those of the components of the relay substrate118illustrated inFIG.3, and the above description ofFIG.3will be incorporated herein.

The relay substrate1318has the same configuration as the relay substrate118illustrated inFIG.3, but is different from the relay substrate118in that it includes ground conductor patterns1340a,1340b,1340c,1340d,1340e, and1340finstead of the ground conductor patterns340a,340b,340c,340d,340e, and340f.

The ground conductor patterns1340aand1340fhave the same configurations as the ground conductor patterns340aand340fof the relay substrate118illustrated inFIG.3, but are different from the relay substrate118in that they do not have the notched portions342aand342d. Further, in the present modification example, the ground conductor patterns1340a,1340c,1340d, and1340fdo not have vias in the component mounting areas360a,360b,360c, and360d. However, these ground conductor patterns1340a,1340c,1340d, and1340fmay have vias (not illustrated) outside the component mounting areas360a,360b,360c, and360d.

The ground conductor patterns1340b,1340eeach have two vias1370in the component mounting areas360a,360b,360c,360d. Here, each of the ground conductor patterns1340band1340emay have vias (not illustrated) outside the component mounting areas360a,360b,360c, and360d.

That is, in the relay substrate1318, the vias1370are present only in the ground conductor patterns1340band1340e, respectively, and vias are not present in the ground conductor patterns1340a,1340c,1340d, and1340f, in the component mounting areas360a,360b,360c, and360d.

Thus, in the relay substrate1318, the impedance between each of the ground conductor patterns1340band1340eand the ground line in the component mounting areas360a,360b,360cand360dis lower than that of the ground conductor patterns1340a,1340c,1340d, and1340f.

Therefore, in the relay substrate1318, in the component mounting areas360a,360b,360c, and360d, the effect of confining the high-frequency signals in the signal conductor pattern330a,330b,330c, and330dis relatively lower on the side of the ground conductor patterns1340a,1340c,1340d, and1340fthan on the side of the ground conductor patterns1340band1340e.

As a result, the leaked microwaves generated in the component mounting portions350a,350b,350c, and350dare emitted unevenly in the directions of the ground conductor patterns1340a,1340c,1340d, and1340f, respectively.

Therefore, the crosstalk between the signal conductor patterns330aand the signal conductor pattern330band between the signal conductor pattern330cand the signal conductor pattern330ddue to the leaked microwaves is reduced.

Ninth Modification Example

FIG.13is a diagram illustrating a configuration of a relay substrate1418according to a ninth modification example, and is a diagram corresponding to the partial detailed view of the first embodiment illustrated inFIG.3. The relay substrate1418can be used instead of the relay substrate118in the optical modulator100illustrated inFIG.1, similar to the relay substrates518and1318of the first modification example and the eighth modification example illustrated inFIGS.5and12.

InFIG.13, the same reference numerals as those inFIGS.3,5, and12are used for the same components as those of the components of the relay substrates118,518, and1318illustrated inFIGS.3,5, and12, and the above descriptions ofFIGS.3,5, and12will be incorporated herein.

The relay substrate1318has the same configuration as the relay substrate118illustrated inFIG.3, but includes the ground conductor patterns1340band1340eof the relay substrate1318of the eighth modification example illustrated inFIG.12instead of the ground conductor patterns340band340e, and includes the ground conductor patterns540cand540dof the relay substrate518of the first modification example illustrated inFIG.5instead of the ground conductor patterns340cand340d.

Thus, in the relay substrate1418, similar to the relay substrate1318illustrated inFIG.12, since the vias1370are provided only on one ground conductor pattern in the component mounting area360, it has an effect of reducing crosstalk between the signal conductor patterns330. Further, in the relay substrate1418, since notches342a,342b,342c, and342din the relay substrate518illustrated inFIG.5are also formed, an even higher crosstalk reduction effect than that of the relay substrate1318illustrated inFIG.12is exhibited.

Second Embodiment

Next, a second embodiment of the present invention will be described. The present embodiment provides an optical transmission apparatus on which any optical modulator of the optical modulator100according to the first embodiment and the optical modulators100using the relay substrates518,718,818,918,1018,1118,1218,1318, and1418according to the modification examples is mounted.

FIG.14is a diagram illustrating a configuration of an optical transmission apparatus according to the present embodiment. An optical transmission apparatus2100includes an optical modulator2102, a light source2104that inputs light to the optical modulator2102, a modulation signal generation part2106, and a modulation data generation part2108.

The optical modulator2102can be any optical modulator of the optical modulator100according to the first embodiment and the optical modulators100using the relay substrates518,718,818,918,1018,1118,1218,1318, and1418according to the modification examples described above. Here, in order to avoid redundant description and facilitate understanding, in the following description, the optical modulator2102is assumed to be the optical modulator100according to the first embodiment.

The modulation data generation part2108receives 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 part2106.

The modulation signal generation part2106is an electronic circuit (drive circuit) that outputs an electrical signal for causing the optical modulator2102to perform a modulation operation, generates a modulation signal which is a high-frequency signal for making the optical modulator2102perform an optical modulation operation according to the modulation data, based on the modulation data which is output by the modulation data generation part2108, and inputs the generated modulation signal to the optical modulator2102. The modulation signal includes four high-frequency electrical signals corresponding to the four signal electrodes112a,112b,112c, and112dof the optical modulation element102provided in the optical modulator2102. Here, the high-frequency electrical signals input to the signal electrodes112aand112bform a pair, and output light output from one output optical waveguide126aof the optical modulation element102is modulated.

Further, the high-frequency electrical signals input to the signal electrodes112cand112dform the other pair, and output light output from the other output optical waveguide126bof the optical modulation element102is modulated.

The four high-frequency electrical signals are input from the signal input terminals124a,124b,124c, and124dof the respective electrical connectors116a,116b,116c, and116dof the optical modulator2102to the signal conductor patterns330a,330b,330c, and330don the relay substrate118, and are input to the signal electrodes112a,112b,112c, and112dof the optical modulation element102via the signal conductor pattern330aor the like.

Thus, the light output from the light source2104is, for example, DP-QPSK modulated by the optical modulator2102and output as modulated light from the optical transmission apparatus2100.

In particular, in the optical transmission apparatus2100, as the optical modulator2102, any optical modulator of the optical modulator100according to the first embodiment and the optical modulators100using the relay substrates518,718,818,918,1018,1118,1218,1318, and1418according to the modification examples is used. Therefore, in the optical transmission apparatus2100, it is possible to effectively reduce an increase in crosstalk between the high-frequency electrical signals for driving the optical modulation element102due to the above-described space leaked microwaves, particularly, crosstalk due to space leaked microwaves between the signal lines propagating the two paired high-frequency electrical signals. Therefore, in the optical transmission apparatus2100, it is possible to ensure stable and appropriate optical modulation characteristics, and to realize stable and appropriate transmission characteristics.

The present invention is not limited to the configurations of the embodiments and the modification examples described above, and can be realized in various aspects without departing from the spirit thereof.

For example, in the above-mentioned relay substrates118,518,718,818,918,1018,1118,1218,1318, and1418, it is assumed that a portion in which the two ground conductor patterns340are formed asymmetrically with each other is included in the component mounting area360of two or more signal conductor patterns330, but the present invention is not limited thereto. In any of the configurations of the relay substrates118,518,718,818,918,1018,1118,1218,1318, and1418, it is possible to include a portion in which the two ground conductor patterns340are formed asymmetrically with each other in the component mounting area360of the at least one signal conductor pattern330according to the generation state of the leaked microwave in the relay substrate118or the like.

Further, in the first embodiment and the modification examples thereof described above, it is assumed that all of the signal conductor patterns330are provided with one component mounting portion350, but the present invention is not limited thereto. The component mounting portion350may be provided in at least one signal conductor pattern330, or a plurality of component mounting portions350may be provided for one signal conductor pattern330. That is, at least one signal conductor pattern330may be provided with at least one component mounting portion350.

Further, the number of vias formed in the ground conductor pattern of the relay substrates1018,1118,1218,1318, and1418illustrated inFIGS.9,10,11,12, and13is an example, and the present invention is not limited thereto. Any number and size of vias may be provided in any manner as long as the gist of the above-mentioned characteristic configurations is not exceeded forFIGS.9,10,11,12, and13. For example, in the relay substrate1018according to the fifth modification example illustrated inFIG.9, the two ground conductor patterns may be provided with any number and size of vias in any manner as long as the numbers and diameters of the vias provided in the two ground conductor patterns are different from each other in the component mounting area360.

Further, the characteristic configurations of the relay substrate118,518, and the like according to the first embodiment and the modification examples thereof described above can be combined in any manner to form one relay substrate. For example, the characteristic configuration in which the propagation direction of the leaked microwaves is biased due to the asymmetry of the vias illustrated inFIG.9can be combined with the configuration illustrated inFIG.3to form one relay substrate. Further, the recess portion942of the relay substrate918of the fourth modification example illustrated inFIG.8or the recess portion1276of the relay substrate1218of the seventh modification example illustrated inFIG.11may be applied to the relay substrate according to another modification example to reduce the impedance fluctuation of the signal conductor pattern330due to the displacement of the mounting position of the electrical circuit element in the component mounting portion350.

Further, in the above-described embodiment, the optical modulation element102is a DP-QPSK modulator configured by using an LN substrate, but the present invention is not limited thereto. For example, the optical modulation element102may be any optical modulation element configured by using a semiconductor substrate.

Further, in the above-described embodiment, the optical modulation element102, the relay substrate118, and the like are accommodated in the housing104, but in addition to these, an electronic circuit element (driver element) for operating the optical modulation element102may also be accommodated in the housing104.

As described above, the optical modulator100according to the above-described embodiment includes the optical modulation element102including a plurality of signal electrodes112, a plurality of signal input terminals124for inputting electrical signals to be respectively applied to the signal electrode112, the relay substrate118, and the housing104in which the optical modulation element102and the relay substrate118are accommodated. The relay substrate118is formed with a plurality of signal conductor patterns330that electrically connect the signal input terminals124to the signal electrodes112, and a plurality of ground conductor patterns340. At least one signal conductor pattern330includes at least one component mounting portion350including an electrical circuit element. The two ground conductor patterns340sandwiching the at least one signal conductor pattern330on the relay substrate118are formed in an asymmetrical shape in a plan view with respect to the at least one signal conductor pattern330, in the component mounting area360having a square shape in the plan view centered on the component mounting portion350. Here, the component mounting area360is an area having the square shape in the plan view in which a direction of a side is same as the extending direction of the at least one signal conductor pattern330in the component mounting portion350and a length of a side is equal to a distance from the at least one signal conductor pattern330to the closest adjacent signal conductor pattern330.

Further, as illustrated by an example in the relay substrate1018,1318, or the like, two ground conductor patterns (for example,1040aand1040bor1340aand1340b) sandwiching the at least one signal conductor pattern330are formed asymmetrically in the component mounting area360having the square shape in the plan view centered on the component mounting portion350in terms of a presence or absence of vias, diameters of vias, or the numbers of vias to be different from each other.

According to these configurations, it is possible to bias the propagation direction of the leaked microwaves that may be generated from the component mounting portion350provided with the electrical circuit element in the relay substrate118, to suppress the crosstalk between the signal conductor patterns330, and to realize appropriate optical modulation characteristics.

Further, as illustrated by an example as the notched portion342aor the like in the relay substrate118,518, or the like, the two ground conductor patterns sandwiching at least one signal conductor pattern330are formed to have portions in which distances from respective edges of the two ground conductor patterns to opposite edges of the at least one signal conductor pattern330are different from each other in the component mounting area360.

According to the configurations, simply by changing the shape of the ground conductor pattern, it is possible to easily bias the propagation direction of the leaked microwaves generated from the component mounting portion350, to suppress the crosstalk between the signal conductor patterns330, and to realize appropriate optical modulation characteristics.

Further, as illustrated by an example as the narrow portion1174aor the like in the relay substrate1118, or the like, one of the two ground conductor patterns sandwiching at least one signal conductor pattern330includes a narrow portion in which a width measured in the direction orthogonal to the extending direction of the at least one signal conductor pattern330is narrower than that of the other of the two ground conductor patterns, in the component mounting area360.

According to the configuration, it is possible to strengthen the bias in the propagation direction of the leaked microwaves generated from the component mounting portion350, to suppress the crosstalk between the signal conductor patterns330, and to realize appropriate optical modulation characteristics.

Further, as illustrated as an example as the section742aor the like in the relay substrate818or the like, one of the two ground conductor patterns sandwiching at least one signal conductor pattern330has a discontinuous section in the component mounting area360.

According to the configuration, it is possible to further strengthen the bias in the propagation direction of the leaked microwaves generated from the component mounting portion350, to suppress the crosstalk between the signal conductor patterns330, and to realize further appropriate optical modulation characteristics.

Further, as illustrated by an example as the recess portion942a,1276a, or the like in the relay substrates918and1218, the other ground conductor pattern that does not include the narrow portion or the discontinuous section, of the two ground conductor patterns sandwiching the at least one signal conductor pattern330, is formed such that a separation distance to the signal conductor pattern330in the component mounting portion350is longer than a separation distance to the signal conductor pattern330in the portion other than the component mounting portion350.

According to this configuration, the change in the characteristic impedance of the signal conductor pattern330due to the displacement of the mounting position of the electrical circuit element in the component mounting portion350is suppressed, and it is possible to suppress crosstalk between the signal conductor patterns330while reducing manufacturing variations and realize appropriate optical modulation characteristics.

Further, the optical modulator100can use, for example, the optical modulation element102that performs DP-QPSK modulation, which is configured to generate two modulated light beams, each of which is modulated by a pair of electrical signals, and the relay substrate118and the like may be configured to propagate the pair of electrical signals by a pair of signal conductor patterns adjacent to each other, for example, the signal conductor patterns330aand330b.

With this configuration, crosstalk propagated by the adjacent signal conductor patterns330between the two paired high-frequency electrical signals can be effectively reduced, so that it is possible to realize appropriate optical modulation characteristics.

Further, the optical transmission apparatus according to the second embodiment described above includes the modulation operation on the optical modulator100using any of the relay substrates described in the first embodiment or the modification examples thereof, the modulation signal generation part2106which is an electronic circuit that outputs an electrical signal for causing the optical modulator100to perform a modulation operation, and the like. With this configuration, for example, the propagation of leaked microwaves, which becomes noticeable as the transmission rate is increased, is suppressed and crosstalk and the like between a plurality of high-frequency electrical signals that drive the optical modulation element102is effectively reduced, so it is possible to realize stable and appropriate transmission characteristics.

REFERENCE SIGNS LIST