Optical waveguide element, optical modulation device using the same, and optical transmission device

Abstract

To provide an optical waveguide element that suppresses the effects on the optical waveguide caused by peeling of the resistive element in the terminating resistor, oxidation of the resistive element itself, and heat generation in the terminating resistor. [Solution] An optical waveguide element comprising a substrate 1 on which an optical waveguide is formed, and a modulation electrode for modulating a light wave propagating through the optical waveguide and a resistor RG disposed at the end of the modulation electrode, wherein the modulation electrode comprises a signal electrode ES and a ground electrode EG, the resistor is covered with an insulating film PL, the signal electrode and the ground electrode are arranged on the insulating film at a position that overlaps with the resistor when the optical waveguide element is viewed in plan view (a), and the insulating film is characterized in that openings (OPS, OPG) are formed for electrically connecting the signal electrode and the ground electrode and the resistor.

Description

Technical Field

[0001] The present invention relates to an optical waveguide device, an optical modulation device using the same, and an optical transmission device. In particular, the present invention relates to a substrate on which an optical waveguide is formed, and an optical waveguide device provided on the substrate with a modulation electrode for modulating a light wave propagating through the optical waveguide and a resistor disposed at an end portion of the modulation electrode.

Background Art

[0002] In the fields of optical communication and optical measurement, optical waveguide devices in which optical waveguides are formed on substrates such as lithium niobate (LN) are widely used. To modulate a light wave propagating through an optical waveguide, a modulation electrode composed of a signal electrode and a ground electrode is used. When a microwave, which is a modulation signal, is input to the modulation electrode, the end portion thereof is connected to a termination resistor so that the microwave is not reflected at the end portion of the modulation electrode.

[0003] As also disclosed in Patent Document 1, in FIG. 1, a signal electrode ES and a ground electrode EG are formed on a substrate 1 on which an optical waveguide (not shown) is formed. A termination substrate 2 is disposed adjacent to the substrate 1, and a resistor RG constituting a termination resistor and a ground electrode GN are formed on the termination substrate. Reference numeral WG denotes a wiring (wire) that electrically connects the substrate 1 and the termination substrate 2. With this configuration, the modulation signal propagated through the signal electrode ES is converted into heat by the resistor of the termination substrate and dissipated as heat.

[0004] In recent years, miniaturization of optical waveguide devices such as optical modulators has been demanded, and the need for optical waveguide devices that do not use a termination substrate on which a termination resistor is mounted has been increasing. In this case, as shown in FIG. 2, it is conceivable to dispose a resistor RG between the end portion of the signal electrode ES on the substrate 1 and the ground electrode EG. Reference numeral BF denotes a buffer layer that suppresses absorption of a light wave propagating through an optical waveguide (not shown) formed on the substrate 1 by the signal electrode ES and the ground electrode EG. FIG. 2(b) shows a cross-sectional view taken along the dashed-dotted line A-A' in FIG. 2(a).

[0005] As shown in Figure 2, in a structure where a terminating resistor is formed on the substrate 1 (chip) of an optical waveguide element, when electricity flows through the terminating resistor and the resistor RG heats up and expands, a delamination phenomenon of the resistor occurs if the coefficient of linear expansion of the resistor RG and the surrounding material are different. The surrounding material includes the substrate 1, the signal electrode ES, the ground electrode EG, and the buffer layer BF.

[0006] Furthermore, during the manufacturing process of optical waveguide elements and when using them, the presence of oxygen in the surrounding atmosphere of the resistor RG can cause oxidation of the resistor itself. In particular, oxidation reactions are more likely to occur when the resistor is heated. Moreover, because the thickness of the resistor on the chip is extremely thin, less than 1 μm, oxidation of the resistor causes a large change in the resistance value of the termination resistor. When the resistance value changes, an impedance mismatch occurs between the signal electrode and the termination resistor. This results in reflection of the microwave, which is the modulating signal, and affects the operation of the optical modulator.

[0007] Furthermore, the heat generated is transferred to the connected signal electrodes, changing the refractive index of the optical waveguide through thermo-optic effects. This can affect the branching ratio in the optical multiplexing and branching sections, and cause noise in the modulated signal of the optical modulation section. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2011-209456 [Overview of the project] [Problems that the invention aims to solve]

[0009] The problem that this invention aims to solve is to provide an optical waveguide element that solves the above-mentioned problems and suppresses the effects on the optical waveguide due to peeling of the resistor at the termination resistor, oxidation of the resistor itself, and heat generation at the termination resistor. Furthermore, it aims to provide an optical modulation device and an optical transmission device using such an optical waveguide element. [Means for solving the problem]

[0010] To solve the above problems, the optical waveguide element, optical modulation device, and optical transmission device of the present invention have the following technical features. (1) An optical waveguide element comprising a substrate on which an optical waveguide is formed, and on the substrate a modulation electrode for modulating a light wave propagating through the optical waveguide and a resistor disposed at the end of the modulation electrode, wherein the modulation electrode comprises a signal electrode and a ground electrode, the resistor is covered with an insulating film, the signal electrode and the ground electrode are arranged on the insulating film at a position that overlaps with the resistor when the optical waveguide element is viewed from above, and the insulating film has openings formed therein for electrically connecting the signal electrode and the ground electrode and the resistor.

[0011] (2) The optical waveguide element described in (1) above is characterized in that a conductive pad portion having an area larger than the area of ​​the opening is arranged on the resistor, and the insulating film is arranged to cover the conductive pad portion.

[0012] (3) The optical waveguide element described in (1) above is characterized in that the area of ​​the opening for connecting the ground electrode and the resistor is larger than the area of ​​the opening for connecting the signal electrode and the resistor.

[0013] (4) In the optical waveguide element described in (1) above, the resistor is composed of a plurality of resistive parts, and the resistive parts are arranged to be symmetric with respect to a straight line in the direction in which the signal electrode extends.

[0014] (5) The optical waveguide element described in (1) above is characterized in that the insulating film is composed of an inorganic material, an organic material, or a combination thereof.

[0015] (6) The optical waveguide element described in any of (1) to (5) above is an optical modulation device characterized by comprising an optical fiber housed in a housing that inputs or outputs an optical wave to the optical waveguide.

[0016] (7) The optical modulation device described in (6) above is characterized in that it has an electronic circuit inside the housing that amplifies the modulation signal input to the modulation electrode.

[0017] (8) An optical transmission device characterized by having an optical modulation device as described in (6) above and an electronic circuit that outputs a modulation signal for causing the optical modulation device to perform a modulation operation. [Effects of the Invention]

[0018] The present invention provides an optical waveguide element comprising a substrate on which an optical waveguide is formed, a modulation electrode for modulating light waves propagating through the optical waveguide, and a resistor disposed at the termination of the modulation electrode, wherein the modulation electrode comprises a signal electrode and a ground electrode, the resistor is coated with an insulating film, the signal electrode and the ground electrode are arranged on the insulating film at a position that overlaps with the resistor when the optical waveguide element is viewed from above, and the insulating film has openings for electrically connecting the signal electrode and the ground electrode and the resistor, thereby providing an optical waveguide element that suppresses peeling of the resistor at the termination resistor, oxidation of the resistor itself, and the impact on the optical waveguide due to heat generation at the termination resistor. Furthermore, by using this optical waveguide element, it is possible to provide optical modulation devices and optical transmission devices with similarly excellent characteristics. [Brief explanation of the drawing]

[0019] [Figure 1] This figure shows an example of a conventional optical waveguide element (when using a termination substrate). [Figure 2] This figure shows an example of a conventional optical waveguide element (where a termination resistor is placed on the chip). [Figure 3] This figure illustrates embodiment (1) of the optical waveguide element of the present invention. [Figure 4] This figure illustrates embodiment (2) of the optical waveguide element of the present invention. [Figure 5] This figure illustrates embodiment (3) of the optical waveguide element of the present invention. [Figure 6]This is a diagram for explaining Example (4) of the optical waveguide device of the present invention. [Figure 7] This is a diagram for explaining Example (5) of the optical waveguide device of the present invention. [Figure 8] This is a diagram for explaining Example (6) of the optical waveguide device of the present invention. [Figure 9] This is a diagram for explaining Example (7) of the optical waveguide device of the present invention. [Figure 10] This is a diagram showing an optical transmission device according to the present invention.

Mode for Carrying Out the Invention

[0020] Hereinafter, the present invention will be described in detail using preferred examples. The present invention is, for example, as shown in FIGS. 3 to 8, a substrate 1 on which an optical waveguide is formed, and on the substrate, a modulation electrode for modulating light waves propagating through the optical waveguide and a resistor RG disposed at the end of the modulation electrode. In the optical waveguide device, the modulation electrode includes a signal electrode ES and a ground electrode EG, the resistor is covered with an insulating film PL, and the signal electrode and the ground electrode are disposed on the insulating film at a position overlapping the resistor when the optical waveguide device is viewed in plan (a). The insulating film is formed with openings (OPS, OPG) for electrically connecting the signal electrode, the ground electrode, and the resistor.

[0021] As the substrate 1 used in the optical waveguide element of the present invention, a substrate having an electro-optic effect can be used. Specifically, single-crystal materials such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate), which are ferroelectric materials, or materials obtained by doping these substrate materials with MgO or the like, can be used. Furthermore, these materials can be formed into films using vapor phase growth methods such as sputtering, evaporation, or CVD. In addition, a substrate obtained by bonding an electro-optic effect substrate to another substrate and then thinning the electro-optic effect substrate can be used. Moreover, semiconductor substrates such as Si, substrates composed of compound semiconductors (GaAs, GaN, InP, ZnSe, CdTe) or silicon-based semiconductors (SiC, etc.), substrates used in SiPh (silicon photonics), and organic material substrates such as EO polymers can also be used. The optical waveguide element of the present invention is suitable for materials affected by heat generated at the termination resistor. For example, it is preferable to apply the present invention to substrates 1 made of materials that have a different coefficient of thermal expansion than the resistor and are prone to delamination, or materials in which the refractive index of the optical waveguide changes with temperature changes.

[0022] The substrate 1 on which the optical waveguide is formed can also be one on which the holding substrate is directly bonded or bonded and fixed via an adhesive layer such as resin. For the holding substrate to be directly bonded, it is preferable that it has a lower refractive index than the optical waveguide or the substrate on which the optical waveguide is formed, but this is not limited to this. In the case of direct bonding, an intermediate layer such as a metal oxide or metal may be included in the bonding portion. Furthermore, the holding substrate is preferably made of a material with a similar coefficient of thermal expansion to the optical waveguide substrate, such as a substrate containing an oxide layer, such as a low dielectric constant substrate of SiO2 or Al2O3, including glass, quartz, fused silica, synthetic silica, Eagle Glass, alkali glass, alkali-free glass, lead glass, Pyrex® glass, soda glass, sapphire, alumina, etc. In addition, it is also possible to use the same LN substrate as the optical waveguide substrate, or a composite substrate in which a silicon film or LN crystal film is formed on the surface of an oxide film formed on a wafer substrate abbreviated as SOI or LNOI. If the refractive index of the holding substrate is higher than that of the optical waveguide substrate, a layer with a lower refractive index than that of the optical waveguide substrate (an intermediate layer), such as SiO2, is provided between the optical waveguide substrate and the holding substrate.

[0023] As optical waveguides, it is possible to use optical waveguides formed by thermal diffusion of a high refractive index material such as Ti into an optical waveguide substrate, optical waveguides formed by the proton exchange method, and rib-type optical waveguides in which the substrate has convex parts corresponding to the optical waveguide by etching the substrate other than the optical waveguide or forming grooves on both sides of the optical waveguide. Furthermore, in order to match the rib-type optical waveguide, it is possible to increase the refractive index further by thermal diffusion methods such as Ti or the proton exchange method. In terms of size, the rib-type optical waveguide is a fine structure with a width and height of about 1 μm or less in order to enhance light confinement.

[0024] A buffer layer (cladding layer) BF is placed on the upper side of the substrate 1 containing the optical waveguide. This is not only to suppress the scattering of light waves due to surface roughness of the optical waveguide itself, but also to prevent the light waves propagating through the optical waveguide from being absorbed by the wiring when part of a control electrode (modulation electrode, etc.) or its wiring is placed on the upper side of the optical waveguide.

[0025] As the buffer layer, a dielectric material with a lower refractive index and higher transparency than the optical waveguide substrate is used. Specifically, materials such as SiO2, Al2O3, MgF2, La2O3, ZnO, HfO2, MgO, CaF2, and Y2O3 are used, including oxides and fluorides of metal elements from groups 1 to 17 of the periodic table. It goes without saying that while the buffer layer BF is necessary in areas where electrodes are placed to overlap with the optical waveguide, resistors and electrodes can also be directly placed on the substrate 1 other than the optical waveguide to form a termination resistor.

[0026] For the resistor RG, materials that are resistant to heat (materials with a high melting point) or materials that can secure sufficient resistance even in small sizes due to the narrow spacing between the electrodes (materials with high electrical resistivity) are preferred. Specifically, Ti, Nb, Ta, Cr, Zr and their compounds TiN, NbN, TaN, Cr2N, ZrN, and RuO2 used in chip resistors can be used.

[0027] The insulating layer (PL) should preferably have high electrical insulation properties and low thermal conductivity. Specifically, inorganic materials such as SiO2, SiN, Ta2O5, and Nb2O5, and organic materials such as permanent resists (resins) can be used, and combinations of these are also acceptable.

[0028] The first feature of the optical waveguide element of the present invention is that, as shown in Figure 3, in order to suppress oxidation of the resistor RG, it is covered with an insulating film PL, and in order to suppress peeling of the resistor RG, the signal electrode ES and the ground electrode EG are arranged so that they overlap with the resistor RG when the optical waveguide element is viewed in plan (see Figure 3(a)). Due to this configuration, as shown in Figure 3(b), openings (OPS, OPG) for electrically connecting the signal electrode ES and the ground electrode EG with the resistor RG are formed in the insulating film PL. Figure 3(b) is a cross-sectional view along the dashed line B-B' in Figure 3(a). The same applies to the other drawings below.

[0029] In the case of Figure 3, the resistance value generated in the resistor RG changes depending on the shape and arrangement of the apertures (OPS, OPG). Therefore, the second feature of the optical waveguide element of the present invention is that, as shown in Figure 4, a conductive pad portion PA having a larger area than the area of ​​the apertures (OPS, OPG) is placed on the resistor RG, and the signal electrode ES and ground electrode EG are connected to the conductive pad portion PA. As a result, the resistance value of the resistor RG is determined by the position and shape of the conductive pad portion PA on the resistor, and is largely independent of the precision when forming the apertures, making it possible to achieve a stable resistance value.

[0030] In the embodiments shown in Figures 5 and 6, the area of ​​the opening OPG for connecting the ground electrode EG and the resistor RG is set to be larger than the area of ​​the opening OPS for connecting the signal electrode ES and the resistor RG. With this configuration, the heat generated in the resistor is actively dissipated from the larger area of ​​the opening, resulting in a smaller amount of heat being transmitted to the signal electrode ES, and thus suppressing the temperature rise of the optical waveguide (optical waveguide section and optical branch section) via the signal electrode.

[0031] In Figure 5, each opening is formed with only one opening, whereas in Figure 6, the opening on the ground electrode side is formed with multiple openings. Either shape can be used.

[0032] In the embodiment shown in Figure 7, the resistor is composed of multiple resistive portions RGP, and the resistive portions are arranged symmetrically with respect to a straight line (dotted line C) in the direction in which the signal electrode ES extends. By composing the resistor with multiple resistive portions in this way, the heat source is dispersed, and the temperature rise of the resistor can be suppressed. Furthermore, even if the resistive portions RGP are selectively cut with a laser or the like to adjust the overall resistance of the resistor, the resistive portions that are not cut are always covered with an insulating film, which suppresses oxidation. Naturally, the more resistive portions RGP are cut, the higher the resistance value will be. The cross-sectional view of each resistive portion will be one of the shapes shown in Figures 3(b), 4(b), 5(b), and 6(b).

[0033] When constructing a circuit with multiple resistors, the intention behind arranging them symmetrically with respect to the dashed line C is that if there is a difference in resistance values ​​on either side of the dashed line C of the termination resistor, or if there are irregularities in the distribution of resistance values, the microwaves propagating through the signal electrode ES may be partially reflected by the termination resistor. Furthermore, when cutting multiple resistive sections to adjust the resistance value, it is desirable to cut the resistive sections in positions that are as symmetrical as possible.

[0034] The embodiment shown in Figure 8 illustrates the termination resistance when the signal electrode is a differential electrode. In a differential electrode configuration, out-of-phase modulated signals (S+ and S-) are applied to the signal electrode. The arrangement of the signal electrode and the ground electrode is "G, S+, S-, G" (where G is ground), and a resistor (RG1 or RG3) is placed between the signal electrode ES (S+ or S-) and the ground electrode EG. Furthermore, a resistor RG2, whose resistance is twice that of the resistors (RG1 or RG3), is placed between the signal electrode ES (S+) and the signal electrode ES (S-). The electrical connection between the signal electrode ES or ground electrode EG and the resistors (RG1-RG3) is configured as shown in Figures 3 to 6.

[0035] Figure 9 shows an embodiment in which the resistor RG, the termination resistor of the signal electrode ES, is positioned away from the branching and multiplexing sections (WG1, WG2) of the optical waveguide WG. This makes it possible to suppress the effects of heat from the resistor, such as changes in the branching ratio due to heat transfer to the branching section or inability to properly perform multiplexing due to heat transfer to the multiplexing section. Furthermore, if heat from the resistor is transferred to the optical waveguide section where an electric field due to a DC voltage is applied and the optical waveguide section (operating section) where a modulated signal is applied, noise components generated by the thermal change may be superimposed on the signal or the bias point of the optical modulator may shift. Therefore, it is preferable to keep the resistor away from these optical waveguide sections.

[0036] Furthermore, as shown in Figure 9, by arranging the terminating resistor (resistor RG) at equidistant distances from the multiplexing section (and similarly from the action section), heat can be applied uniformly to the multiplexing section, thereby suppressing the occurrence of a shift in the bias point of the modulation signal between each Mach-Zehnder type optical waveguide.

[0037] Figure 10 shows an example of an optical transmission device. In recent years, optical modulation devices that integrate driver ICs and optical waveguide elements in the same housing, such as high-bandwidth-coherent driver modulators (HB-CDMs), have attracted attention, and there is a growing need for configurations suitable for miniaturization, such as the optical waveguide element of the present invention.

[0038] In the optical modulation device of the present invention, an optical waveguide element is arranged in a housing CA made of metal or the like. Input light L1, such as from a semiconductor laser LD, is input to the optical waveguide WG formed in the optical waveguide element within the housing via an optical fiber or other optical component such as a lens. On the other hand, the light wave emitted from the optical waveguide element is input to another optical fiber F and becomes output light L2. When outputting light, polarization combining means and optical components OB such as lenses are used as needed. Modulation electrodes (not shown) are formed on the substrate 1 of the optical waveguide element. In addition, reinforcing members are arranged on the substrate of the input / output section of the optical waveguide element as needed to increase mechanical strength.

[0039] In an optical modulation device, a driver circuit element DRV that generates an electrical signal S applied to the modulation electrode of the optical waveguide element is arranged adjacent to the optical waveguide element, and the optical waveguide element and the driver circuit element DRV are housed in the same housing CA.

[0040] Furthermore, it is possible to configure the device as an optical transmitter by adding a signal generator DSP (Digital Signal Processing Unit) that generates the modulation signal So input to the driver circuit element DRV. It is also possible to integrate the enclosure CA and the signal generator DSP into a single chassis.

[0041] When the optical modulation device of the present invention houses optical waveguide elements within a housing, it is usually hermetically sealed with nitrogen gas or the like. However, since the oxidation of the resistor is suppressed in the present invention, it may be shipped in its pre-sealed state (in the atmosphere) or used as an optical modulation device in a sub-assembly that is not hermetically sealed. [Industrial applicability]

[0042] As described above, the present invention makes it possible to provide an optical waveguide element that suppresses the effects on the optical waveguide due to peeling of the resistor in the terminating resistor, oxidation of the resistor itself, and heat generation in the terminating resistor. Furthermore, it is also possible to provide an optical modulation device and an optical transmission device using such an optical waveguide element. [Explanation of Symbols]

[0043] 1 circuit board RG resistor PL insulating film BF Buffer Layer WG optical waveguide ES signal electrode EG ground electrode OPS, OPG opening

Claims

1. An optical waveguide element comprising a substrate on which an optical waveguide is formed, and on the substrate, a modulation electrode for modulating the light wave propagating through the optical waveguide and a resistor disposed at the end of the modulation electrode, The modulation electrode comprises a signal electrode and a ground electrode, The resistor is coated with an insulating film, The signal electrode and the ground electrode are positioned on the insulating film at a location that overlaps with the resistor when the optical waveguide element is viewed in plan view. An optical waveguide element characterized in that the insulating film has openings formed therein for electrically connecting the signal electrode and the ground electrode with the resistor.

2. In the optical waveguide element according to claim 1, A conductive pad portion having an area larger than the area of ​​the opening is arranged on the resistor. The optical waveguide element is characterized in that the insulating film is arranged to cover the conductive pad portion.

3. In the optical waveguide element according to claim 1, An optical waveguide element characterized in that the area of ​​the opening for connecting the ground electrode and the resistor is larger than the area of ​​the opening for connecting the signal electrode and the resistor.

4. In the optical waveguide element according to claim 1, The resistor is composed of multiple resistive parts, The optical waveguide element is characterized in that the resistive portion is arranged symmetrically with respect to a straight line in the direction in which the signal electrode extends.

5. In the optical waveguide element according to claim 1, The optical waveguide element is characterized in that the insulating film is composed of an inorganic material, an organic material, or a combination thereof.

6. An optical modulation device characterized in that the optical waveguide element according to any one of claims 1 to 5 is housed in a housing and comprises an optical fiber that inputs or outputs an optical wave to the optical waveguide.

7. An optical modulation device according to claim 6, characterized in that it has an electronic circuit inside the housing that amplifies the modulation signal input to the modulation electrode.

8. An optical transmitting device characterized by comprising an optical modulation device according to claim 6, and an electronic circuit that outputs a modulation signal for causing the optical modulation device to perform a modulation operation.

Images