Optical modulator and optical transmission apparatus using the same

Abstract

Provided is an optical modulator and an optical transmission device that can reliably perform electrical connection between a signal electrode and a signal wiring of a wiring substrate even when the width of the signal electrode in an active portion of an optical control substrate is narrow. An optical modulator includes an optical control substrate having at least an optical waveguide including a branch waveguide that branches one light wave into two, and a control electrode for applying an electric field to the branch waveguide, and a wiring substrate provided with a wiring that relays an electric signal applied to the control electrode or a wiring that terminates the electric signal, characterized in that the control electrode includes a signal electrode, the wiring includes a signal wiring, and in a portion in which the signal electrode and the signal wiring are electrically connected, the interval at which the branch waveguide sandwiches the signal electrode is wider than the interval at which the branch waveguide sandwiches the signal electrode in an active portion of the control electrode in which the control electrode applies an electric field to the branch waveguide.

Description

TECHNICAL FIELD

[0001] The present application relates to an optical modulator and an optical transmission apparatus using the same, and particularly to an optical modulator including: an optical control substrate having an optical waveguide including at least a branch waveguide branching one light wave into two, and a control electrode for applying an electric field to the branch waveguide; and a wiring substrate provided with a wiring relaying an electric signal applied to the control electrode. BACKGROUND

[0002] In the field of optical communication or optical measurement, an optical modulator using an optical control substrate having an optical waveguide and a control electrode for controlling a light wave propagating in the optical waveguide is frequently used. In recent years, the optical modulator is required to be broadband or small, and for example, a plurality of different high-frequency signals are simultaneously applied to one optical modulator.

[0003] Further, in applying an electric signal to each control electrode including a modulation electrode, a relay substrate is used in order to simplify the process of electric wiring. As shown in Figure 1 Patent Document 1, in electric connection from the relay substrate 2 to the optical control substrate 1, wire bonding WB is exclusively used in terms of easiness of wiring. However, in an optical modulator of a frequency of several tens of GHz or more, particularly exceeding 100 GHz, such as a 96GBaud-800G modulator, the propagation loss of the electric signal of the wire bonding increases.

[0004] In addition, as shown in Figure 1 , in a case where a plurality of control electrodes (S1 to S4) are used, the length of the wiring from each input port disposed near the side edge of the optical control substrate in which the relay substrate is disposed to the active part of each control electrode (the range shown by the dotted line on the left side of the arrow A) differs, and a difference in electric characteristics occurs between the control electrodes (each port). Figure 1

[0005] In Figure 2 , in order to realize the smallness of the optical modulator, the light input and output (Lin, Lout1, Lout2) part is disposed at the same side of the optical control substrate 1. In this optical modulator, regarding the input electric signal Sin, the range from each input port to the active part of each control electrode (S1 to S4) (the part shown by the arrow A) differs between the control electrodes, and a difference in electric characteristics occurs as in Figure 1 Figure 2 In , only the signal electrode S is shown by a dotted line. The symbol PD is a light detecting member for detecting a part of the emission light emitted from the combining part of the optical waveguide OW.

[0006] ​When the relay substrate is arranged along the side of the light control substrate, space for arranging the relay substrate needs to be additionally ensured, and thus it is difficult to downsize the frame body that houses the light control substrate and the like. Further, in the relay substrate, space for path design for impedance adjustment is also needed, and thus it is more difficult to achieve downsizing.

[0007] As a method for solving these problems, a structure in which the wiring substrate is arranged so as to overlap the light control substrate is also proposed in Patent Literature 2. However, in the electrical connection between the wiring substrate and the light control substrate, it is difficult to sufficiently ensure the electrical connection between the wiring substrate and the signal electrode, particularly the signal wiring involved in the modulation signal, and the signal electrode. That is, as shown in Figure 2 in order to enhance the constraint of the optical waveguide on the optical wave, a thin plate rib structure in which the width or thickness of the optical waveguide is set to about 1 μm or less is proposed. Therefore, the interval between the signal electrode and the ground electrode arranged so as to sandwich the optical waveguide becomes narrower and narrower, and the width of the signal electrode itself also becomes extremely narrow, and thus it is difficult to sufficiently ensure the area of the connection portion of the signal wiring of the wiring substrate and the signal electrode.

[0008] Prior Art Documents

[0009] Patent Literature

[0010] Patent Literature 1: Japanese Patent No. 5494704

[0011] Patent Literature 2: Japanese Patent Application Laid-Open No. 2014-191250 SUMMARY

[0012] Problems to be Solved by the Invention

[0013] The present application has been made to solve the problems described above, and to provide an optical modulator that can reliably perform electrical connection between a signal electrode and a signal wiring of a wiring substrate even when the width of the signal electrode in an active portion of a light control substrate is narrowed. In addition, to provide an optical transmission device using the optical modulator.

[0014] Technical Means for Solving the Problems

[0015] To solve the problems described above, the optical modulator and the optical transmission device of the present application have the following technical features.

[0016] (1) An optical modulator comprising: an optical control substrate having an optical waveguide including at least a branch waveguide branching one light wave into two, and a control electrode for applying an electric field to the branch waveguide; and a wiring substrate provided with a wiring relaying an electric signal applied to the control electrode or a wiring terminating the electric signal, characterized in that the control electrode includes a signal electrode, the wiring includes a signal wiring, and in a portion electrically connecting the signal electrode and the signal wiring, a gap of the branch waveguide sandwiching the signal electrode is wider than a gap of the branch waveguide sandwiching the signal electrode in a portion where the control electrode applies an electric field to the branch waveguide.

[0017] (2) The optical modulator according to the (1), characterized in that the wiring relaying the electric signal is formed together with the wiring terminating the electric signal in the wiring substrate.

[0018] (3) The optical modulator according to the (1) or (2), characterized in that, in the electrically connected portion, electrical connection is made using flip-chip connection.

[0019] (4) The optical modulator according to any one of the (1) to (3), characterized in that the optical waveguide is formed of a rib structure of a thin plate having a height of 1 μm or less.

[0020] (5) The optical modulator according to any one of the (1) to (4), characterized in that a driver circuit element generating an electric signal applied to the control electrode is disposed adjacent to the optical control substrate, and an output terminal of the driver circuit element is connected to the wiring of the wiring substrate.

[0021] (6) An optical transmission apparatus characterized by comprising: the optical modulator according to any one of the (1) to (5); and a signal generator generating a modulation signal input to the driver circuit element.

[0022] Effects of the Invention

[0023] By the present application, an optical modulator includes an optical control substrate having an optical waveguide including at least a branch waveguide branching one light wave into two, and a control electrode for applying an electric field to the branch waveguide, and a wiring substrate provided with a wiring relaying an electric signal applied to the control electrode, in which the control electrode includes a signal electrode, the wiring includes a signal wiring, and in a portion in which the signal electrode and the signal wiring are electrically connected, a gap of the branch waveguide sandwiching the signal electrode is wider than a gap of the branch waveguide sandwiching the signal electrode in a portion in which the control electrode applies an electric field to the branch waveguide, so that a width of the signal electrode in the portion in which the signal electrode and the signal wiring are electrically connected can be sufficiently ensured, thereby enabling reliable electrical connection of both. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a view showing an example of a conventional optical modulator.

[0025] Figure 2 is a view showing another example of a conventional optical modulator.

[0026] Figure 3 is a view showing an example of an optical modulator of the present application.

[0027] Figure 4 is an enlarged view of a portion Sc of an input electric signal in the control electrode of Figure 3

[0028] Figure 5 is a view showing a state in which a wiring substrate (relay substrate) 20 is overlaid on Figure 3 the optical control substrate 1 of the optical modulator of

[0029] Figure 6 is a cross-sectional view showing a state of electrical connection of the signal electrode of Figure 5 and the wiring substrate.

[0030] Figure 7 is a view showing another example of an optical modulator of the present application.

[0031] Figure 8 is a view showing a state in which a wiring substrate (relay substrate) 20 and a wiring substrate (terminal substrate) 21 are overlaid on Figure 7 the optical control substrate 1 of the optical modulator of

[0032] Figure 9 is a view showing an optical transmission device of the present application.

[0033] EXPLANATION OF SYMBOLS

[0034] 1: optical control substrate

[0035] 2: relay substrate​

[0036] 20: wiring substrate (relay substrate)

[0037] 20L: wiring

[0038] 20P: pad portion for input

[0039] 21: wiring substrate (terminal substrate)

[0040] 21PG, 21PS: pad portion for output

[0041] 100: substrate

[0042] 110: reinforcing substrate

[0043] A: arrow

[0044] C: symbol

[0045] CS: frame

[0046] DC1, DC2: region

[0047] DRV: driver circuit element

[0048] DSP: signal generator

[0049] F: optical fiber

[0050] FC: flip chip connection portion

[0051] Gc: connection portion of ground electrode

[0052] Glp, Slp: lower electrode portion

[0053] GND: ground electrode

[0054] Gup, Sup: upper electrode portion

[0055] L1, L2: light

[0056] Lin: input light

[0057] Lout, Lout1, Lout2: output light

[0058] OW, OW1, OW2: optical waveguide

[0059] PC: polarization combining member

[0060] PD: light-receiving element

[0061] S: signal electrode

[0062] S1, S2, S3, S4: control electrode

[0063] Sc: connection portion of signal electrode

[0064] Sc2: connection portion

[0065] Sin: electric signal

[0066] So: modulated signal

[0067] Sout: electric signal (modulated signal)

[0068] TE: terminal element

[0069] W1, W2: interval

[0070] WB: wire bonding DETAILED DESCRIPTION

[0071] Hereinafter, the present application will be described in detail using preferred examples.

[0072] As Figures 3 to 8 shown, in the present application, an optical modulator includes an optical control substrate 1 having at least an optical waveguide OW including a branch waveguide branching one light wave into two, and a control electrode for applying an electric field to the branch waveguide, and a wiring substrate (20, 21) provided with a wiring relaying an electric signal applied to the control electrode or a wiring terminating the electric signal, the optical modulator being characterized in that the control electrode includes a signal electrode S, and the wiring includes a signal wiring, as Figure 4 shown, in a portion (Sc on the signal electrode side) electrically connecting the signal electrode and the signal wiring, an interval Wl of the branch waveguide sandwiching the signal electrode is wider than an interval W2 of the branch waveguide sandwiching the signal electrode in an active portion (symbol A) of the control electrode applying an electric field to the branch waveguide.

[0073] Figure 3 is a view showing the optical control substrate 1 constituting the optical modulator of the present application, in which the optical waveguide OW and the control electrode (signal electrode S, ground electrode not shown) are formed in the optical control substrate 1. Figure 4 is an enlarged view of a portion of the signal electrode S of Figure 3 which an input electric signal is inputted. Further, Figure 5 is a view showing a case where a wiring substrate (relay substrate 20) including a line relaying an electric signal is overlaid on the optical control substrate 1 of Figure 4 . In addition, Figure 6 is a view showing a portion of a cross-sectional view at a dotted line B-B' in Figure 5

[0074] ​As the optical control substrate 1 containing the optical waveguide OW, strong dielectric substrates with electro-optic effects, such as lithium niobate (LN), lithium tantalate (LT), and lead lanthanum zirconate titanate (PLZT), can also be used. Alternatively, substrates formed by vapor-phase growth of these materials on a reinforcing substrate can also be used. Furthermore, substrates using various materials such as semiconductor materials like InP or organic materials can also be used. The reinforcing substrate can be made of materials such as quartz or crystal.

[0075] As a method for forming optical waveguides, such as Figure 6 As shown, ribbed optical waveguides can be used, which are formed by etching the substrate surface other than the optical waveguides (OW1, OW2) or forming grooves on both sides of the optical waveguides, etc., to make the portion of the substrate corresponding to the optical waveguides convex. Furthermore, while forming ribbed optical waveguides, composite optical waveguides can also be used, such as those formed on the substrate surface of Ti through thermal diffusion or proton exchange methods to form high refractive index portions.

[0076] To achieve velocity matching between the microwave and light waves in the modulation signal, the substrate 100 on which the optical waveguide is formed is a thin plate with a thickness of 5 μm or less, more preferably 3 μm or less. When forming the thin plate, there are methods such as grinding to achieve a thinness while the substrate 100 is attached to the reinforcing substrate 110 using direct bonding, or forming the substrate 100 as a vapor-phase grown film on the reinforcing substrate 110. The height of the ribbed optical waveguides (OW1, OW2) is set to 2 μm or less, more preferably 1 μm or less.

[0077] The control electrodes that apply an electric field to the optical waveguides (OW1, OW2) can be a combination of a signal electrode and a ground electrode for propagating electrical signals such as modulation signals, or a bias electrode for applying a DC bias voltage. As a method for forming control electrodes along the optical waveguides, they can be constructed by laminating Au onto a substrate metal such as Au or Ti using a plating method.

[0078] like Figure 6As shown, in the optical waveguide (OW1, OW2), in the case of a rib structure of a thin plate having a height of 1 μm or less with a convex portion, regarding the control electrode, particularly the modulation electrode formed by the signal electrode S and the ground electrode GND, a structure for efficiently applying an electric field to the optical waveguide and a structure for achieving matching of the propagation speed of the electric signal and the light and reducing the propagation loss of the electric signal need to be considered. Therefore, the lower electrode portions (Slp and Glp) are provided for the signal electrode S and the ground electrode GND, respectively, and these lower electrode portions are arranged in close proximity to the optical waveguide (OW1, OW2). In addition, the upper electrode portions (Sup and Gup) having a width narrower than the lower electrode portions are arranged on the upper side of the lower electrode portions (Slp and Glp) and separate the signal electrode S and the ground electrode GND, and a sufficient sectional area in the signal electrode and the like is ensured, for example, by setting the height of the upper electrode portions to several tens of μm or more.

[0079] Figure 3 is a plan view showing an example of the optical control substrate 1 used in the optical modulator of the present application. The optical waveguide OW is configured to form eight branch waveguides by causing the input light Lin to pass through three stages of branch portions, and thereafter, to finally emit two output lights (Lout1, Lout2) by passing through two stages of combining portions.

[0080] The dashed-line boxes DC1 and DC2 show regions in which DC bias electrodes for performing bias control are arranged. The symbol C is a region in which the optical waveguide is bent, and by setting the height or width of the optical waveguide to 1 μm or less, the confinement of light can be enhanced and the radius of curvature can be reduced. The region of the dashed-line box A shows an active portion in which an electric field is applied to the optical waveguide (branch waveguide) by the control electrode (modulation electrode). The PD is a light-receiving element that receives a portion of the output light (Lout1, Lout2) before emission. Instead of such a light-receiving element, the light-receiving element can be arranged so as to receive the radiation mode light in the combining portion as a technique known to those skilled in the art.

[0081] In Figure 3 only the signal electrode S of the control electrode is shown. As shown in the enlarged view in Figure 4 , the input portion Sc of the signal electrode S is composed of a portion thicker than the width of the signal electrode in the active portion. This functions as a connection portion on the signal electrode side when flip-chip connection is performed, as described later. As shown in Figure 4 , the width of the electrical connection portion Sc of the signal electrode S is widened, and in correspondence thereto, the interval of the branch waveguides (OW) is also widened. Therefore, in the connection portion Sc of the signal electrode, the interval W1 of the branch waveguides (OW) sandwiching the signal electrode S is wider than the interval W2 of the branch waveguides sandwiching the signal electrode S in the active portion A. In this way, the reason why the optical waveguide can be sharply bent is that the light of the optical waveguide is strongly confined as described above.

[0082] Figure 4 The ground electrode GND is also clearly shown in the form of a clamped signal electrode S. Similar to the signal electrode, the ground electrode also needs to be electrically connected to the wiring (ground wiring) of the wiring board. Figure 4 In the diagram, the connection portion of the ground electrode is explicitly shown in the dashed line Gc. Generally, connecting the ground electrode first, and then connecting the signal electrode, relative to the direction of travel of the modulation signal, allows for more stable movement of the electrical signal. More preferably, multiple (e.g., four) connection portions of the ground electrode can be provided, with the connection portion of the signal electrode sandwiched between the connection portions of the signal electrode in the direction of travel of the modulation signal. L1 is the light input to the branch waveguide, and L2 is the light propagating in the branch waveguide.

[0083] The input portion Sc of the signal electrode S is composed of a portion that is wider than the signal electrode of the active portion, but its thickness can also be greater (e.g., 5 μm to 30 μm). By increasing the thickness, breakage or peeling of the signal electrode during flip-chip bonding can be prevented, thereby improving the bonding strength.

[0084] Flip-chip interconnects improve signal propagation loss compared to wire bonding. Furthermore, flip-chip interconnects allow for more consistent wiring lengths across multiple signal electrodes, thus reducing differences in electrical characteristics between control electrodes.

[0085] When an electrical signal is input to the signal electrode S, it is used Figure 5 The wiring board (relay board) 20 shown is configured to overlap with the optical control board 1, thereby miniaturizing the size of the frame housing the optical control board. On the wiring board 20, as an example, an input pad portion 20P is formed along one side edge of the wiring board 20. This input pad portion 20P is electrically connected to a wiring 20L disposed on the lower surface (the surface on the side of the optical control board) of the wiring board 20. The wiring 20L includes signal wiring and ground wiring, and the other end of the wiring 20L is electrically connected to a flip-chip connection portion FC. Figure 3 or Figure 4 The connection portion Sc of the signal electrode and the connection portion Gc of the ground electrode are shown. Figure 6 This illustrates the connection between the flip-chip connector FC in the signal electrode S and the wiring board 20 side (signal wiring not shown). The dimensions of the flip-chip connector are, for example, a width of approximately 30 μm to 60 μm, and the width of the connector portion of the signal electrode is correspondingly set to approximately 30 μm to 80 μm.

[0086] about Figure 3 The structure of the signal electrode S is configured such that, after passing through the active portion A, it interacts with... Figure 2The existing example is connected to the output pad portion arranged along the side of the light control substrate 1, and outputs an electrical signal (modulated signal) to the outside (Sout). In contrast, in Figure 7 and Figure 8 , it is also possible to configure such that a portion (connection portion Sc2) thicker than the active portion is formed at the end portion on the output side of the signal electrode, and is directly electrically connected to the wiring substrate (terminal substrate) 21. Figure 7 is a modification example of Figure 3 , Figure 8 is a modification example of Figure 5 .

[0087] Figure 8 The symbol 21 in is a terminal substrate, and an output pad portion 21PS connected to the signal electrode is formed on the terminal substrate. In addition, an output pad portion 21PG connected to the ground electrode can also be provided. On the output side of the signal electrode S, in general, the signal electrode is connected first, and then the ground electrode is connected with respect to the traveling direction of the modulated signal, and the emission of the modulated signal from the signal electrode can be suppressed. Furthermore, it is preferable that the connection portions of the ground electrodes can also be provided in a manner of sandwiching the connection portion of the signal electrode in front and back in the traveling direction of the modulated signal. A terminal element TE such as a terminal resistor is provided on the terminal substrate 21, and is electrically connected to the output pad portion (21PS, 21PG).

[0088] Figure 8 In Figure 8 , the terminal substrate 21 is provided separately from the relay substrate 20, but it is also possible to form a wiring substrate in which the two are integrated. Of course, in the optical modulator of the present application, the relay substrate can also be configured as before, such that only the terminal substrate is overlapped and arranged on the light control substrate 1 as in . In addition, in the case where the light receiving element PD is arranged in close proximity to the wiring substrate, the shape of the wiring substrate and the arrangement of the light receiving element can also be set in a manner in which the wiring substrate (20, 21) does not overlap the light receiving element PD. The light receiving element PD can also be configured to not only detect the emitted light from the combining portion of the optical waveguide OW, but also branch and detect a portion of the optical wave propagating in the optical waveguide OW.

[0089] Figure 9 is a diagram showing an example of an optical transmission device.

[0090] In the optical modulator, a driver circuit element DRV that generates an electrical signal applied to the control electrode is arranged adjacent to the light control substrate 1, and the output terminal of the driver circuit element is connected to the wiring of the wiring substrate 20. The input light Lin is input to the light control substrate 1 using an optical fiber F via an optical member such as a lens. Figure 9The light control substrate 1. On the other hand, the light wave emitted from the light control substrate 1 is synthesized, for example, via a polarization combining member PC and input to another optical fiber via an optical member such as a lens, becoming output light Lout.

[0091] Further, a signal generator DSP that generates a modulation signal So input to the driver circuit element DRV can be provided to configure an optical transmission device. The optical modulator and the driver circuit element can be assembled to a frame CS, and further the frame CS and the signal generator DSP can be assembled to one base plate.

[0092] Industrial applicability

[0093] As described above, according to the present application, an optical modulator that can reliably perform electrical connection between a signal electrode and a signal wiring of a wiring substrate even when the width of the signal electrode in the active portion of the light control substrate is narrowed can be provided. Further, an optical transmission device using the optical modulator can be provided.

Claims

1. An optical modulator comprising: An optical control substrate having an optical waveguide including at least a branch waveguide branching one optical wave into two, and a control electrode for applying an electric field to the branch waveguide; and A wiring substrate provided with a wiring relaying a modulation signal applied to the control electrode, the optical modulator characterized in that the control electrode includes a signal electrode and a ground electrode sandwiching the signal electrode, the wiring includes a signal wiring and a ground wiring, in a portion electrically connecting the signal electrode and the signal wiring, the signal electrode is sandwiched by the branch waveguides branching into two, the interval of the branch waveguides sandwiching the signal electrode being wider than the interval of the branch waveguides sandwiching the signal electrode in an active portion of the control electrode applying an electric field to the branch waveguides, the signal electrode is electrically connected to the signal wiring on the front side of the modulation signal with respect to the direction of travel of the modulation signal.

2. The optical modulator of claim 1, wherein, A driver circuit element generating an electric signal applied to the control electrode is disposed adjacent to the optical control substrate, and an output terminal of the driver circuit element is connected to the wiring of the wiring substrate.

3. An optical modulator comprising: An optical control substrate having an optical waveguide including at least a branch waveguide branching one optical wave into two, and a control electrode for applying an electric field to the branch waveguide; and A wiring substrate provided with a wiring terminating a modulation signal applied to the control electrode, the optical modulator characterized in that the control electrode includes a signal electrode and a ground electrode sandwiching the signal electrode, the wiring includes a signal wiring and a ground wiring, in a portion electrically connecting the signal electrode and the signal wiring, the signal electrode is sandwiched by the branch waveguides branching into two, the interval of the branch waveguides sandwiching the signal electrode being wider than the interval of the branch waveguides sandwiching the signal electrode in an active portion of the control electrode applying an electric field to the branch waveguides, the signal electrode is electrically connected to the signal wiring on the front side of the modulation signal with respect to the direction of travel of the modulation signal.

4. The optical modulator of claim 1 or 3, wherein, The wiring relaying the modulation signal and the wiring terminating the modulation signal are formed in the wiring substrate.

5. The optical modulator of claim 1 or 3, wherein, In the portion of electrical connection, flip chip connection is used for electrical connection.

6. The optical modulator of claim 1 or 3, wherein, The optical waveguide is formed of a rib structure of a thin plate having a height of 1 μm or less.

7. An optical transmitting device, characterized in that, Comprise: the optical modulator according to claim 2; and a signal generator generating a modulation signal input into the driver circuit element.

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