Optical waveguide element, optical modulation device using optical waveguide element, and optical transmission apparatus

Abstract

The present application provides an optical waveguide element, an optical modulation device using the optical waveguide element, and an optical transmission apparatus. The optical waveguide element can make the length of a substrate having an electro-optic effect short and can sufficiently ensure a space required for arrangement of an optical receiving element. An optical waveguide element has a substrate (1) having an electro-optic effect, an optical waveguide (2) formed on the substrate, and an optical receiving element (PD) arranged on the substrate and monitoring an optical wave propagating in the optical waveguide or an optical wave radiated from the optical waveguide, characterized by having a monitoring optical waveguide (22) extending from the optical waveguide to the optical receiving element, the monitoring optical waveguide having a turn-back portion (23) with respect to an outgoing direction of the optical waveguide, and the optical receiving element being arranged at a portion (24) after the turn-back portion of the monitoring optical waveguide.

Description

Technical Field

[0001] The present invention relates to optical waveguide elements and optical modulation devices and optical transmitting apparatuses using optical waveguide elements, particularly to optical waveguide elements having a substrate, an optical waveguide, and an optical receiving element, wherein the substrate has an electro-optic effect, the optical waveguide is formed on the substrate, the optical receiving element is disposed on the substrate, and monitors light waves propagating in or radiated from the optical waveguide. Background Technology

[0002] In the fields of optical communication and optical measurement, optical waveguide elements such as optical modulators, in which optical waveguides are formed on substrates with electro-optic effects, such as lithium niobate (LN), are widely used. Moreover, in optical modulators, the output light and emitted light emitted from the combiner of the Mach-Zehnder optical waveguide are monitored in order to control the bias voltage in optical modulation based on the Mach-Zehnder optical waveguide.

[0003] As in Patent Document 1, a design is made to place optical receiving elements such as PDs on a substrate. Figure 1 In this way, the interference light in the combiner section of the Mach-Zehnder type optical waveguide 2 formed on the substrate 1 is output to the outside of the substrate 1 through the output waveguide 21. Moreover, the light with complementary light intensity generated by the interference at the combiner section is used as monitoring light by the monitoring optical waveguide 22 and guided to the light receiving element PD.

[0004] Such a configuration of optical receiving elements in a dual-polarized optical modulator with multiple Mach-Zehnder type optical waveguides connected in parallel for coherent communication provides sufficient monitoring sensitivity without loss relative to the output light. Furthermore, by arranging multiple optical receiving elements on the substrate, a small optical modulator can be realized, and therefore it is widely used.

[0005] By placing the light-receiving element on a substrate, the size of the optical waveguide element can be reduced to some extent. However, in recent years, the demand for further miniaturization has been increasing, particularly the requirement to further shorten the length of the optical waveguide. Figure 1 The substrate 1 shown has a length L of Mach-Zehnder type optical waveguide 2. Therefore, it is necessary to shorten the distance from the combiner portion of the Mach-Zehnder type optical waveguide to the output end of the optical waveguide, which makes it difficult to ensure the placement of the optical receiving element PD.

[0006] Furthermore, as in Patent Document 2, a reinforcing member is disposed on a portion of the substrate. Figure 1As shown, a reinforcing member 3 is disposed on the upper side of the substrate 1 along the edge of the substrate 1 at the input end where incident light (Lin) enters the optical waveguide and at the output end where outgoing light (Lout) exits. This is to prevent waveguide defects when grinding the end faces of the input or output ends of the optical waveguide, or to ensure the bonding area when optical components such as optical fibers and lenses are bonded and fixed opposite to the input and output ends.

[0007] Adhesives are used to bond the light-receiving element PD to the substrate 1 and the reinforcing member 3 to the substrate 1. During the bonding of the light-receiving element or the reinforcing member 3, adhesive may overflow from the light-receiving element or the reinforcing member, thus hindering the bonding of the other. Therefore, to prevent the adhesives used on the various components from contacting each other, the spacing between the light-receiving element PD and the reinforcing member 3 needs to be widened. This makes it more difficult to shorten the distance between them. Figure 1 The length L of the substrate 1.

[0008] Prior art literature

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2017-211504

[0011] Patent Document 2: Japanese Patent Application Publication No. 2017-187522 Summary of the Invention

[0012] The problem to be solved by the present invention is to solve the aforementioned problems and provide an optical waveguide element that can shorten the length of a substrate with an electro-optic effect and sufficiently ensure the space required for the configuration of an optical receiving element, an optical modulation device using the optical waveguide element, and an optical transmitting device.

[0013] In order to solve the above-mentioned problems, the optical waveguide element, optical modulation device and optical transmission device of the present invention have the following technical features.

[0014] (1) An optical waveguide element comprising: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; and an optical receiving element disposed on the substrate and monitoring optical waves propagating in or radiated from the optical waveguide, characterized in that the optical waveguide element includes a monitoring optical waveguide extending from the optical waveguide to the optical receiving element, the monitoring optical waveguide having a folded-back portion relative to the emission direction of the optical waveguide, and the optical receiving element being disposed at a portion after the folded-back portion of the monitoring optical waveguide.

[0015] (2) In the optical waveguide element described in (1) above, the characteristic is that, on the upper side of the substrate, a reinforcing member is joined to the substrate along one side of the substrate where the output end of the optical waveguide is disposed, and the folded-back portion is formed between the substrate and the reinforcing member.

[0016] (3) In the optical waveguide element described in (1) or (2) above, the input end and the output end of the optical waveguide are arranged along the same side of the substrate.

[0017] (4) In the optical waveguide element described in (3) above, the optical receiving element is characterized in that it is disposed outside the range of the diffusion angle of the leakage light leaking from the input end to the substrate.

[0018] (5) In the optical waveguide element described in (2) above, a light-shielding portion is formed on the side of the reinforcing member facing the optical receiving element or on the side of the optical receiving element facing the reinforcing member.

[0019] (6) In any one of (1) to (5) above, the optical waveguide element is characterized in that the monitoring optical waveguide is a ridge-type optical waveguide.

[0020] (7) An optical modulation device, characterized in that it comprises: an optical waveguide element as described in any one of (1) to (6) above; a housing for housing the optical waveguide element; and an optical fiber for inputting light waves from outside the housing into the optical waveguide, or outputting light waves from the optical waveguide to outside the housing.

[0021] (8) In the optical modulation device described in (7) above, the optical modulation device is characterized in that the optical modulation device has an electronic circuit inside the housing that amplifies the modulation signal input to the optical waveguide element.

[0022] (9) An optical transmitting device, characterized in that it comprises: an optical modulation device as described in (7) or (8) above; and an electronic circuit that outputs a modulation signal that causes the optical modulation device to perform a modulation operation.

[0023] Invention Effects

[0024] According to the present invention, since the optical waveguide element comprises: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; and an optical receiving element disposed on the substrate and monitoring light waves propagating in or radiated from the optical waveguide, the optical waveguide element having a monitoring optical waveguide extending from the optical waveguide to the optical receiving element, the monitoring optical waveguide having a folded-back portion relative to the emission direction of the optical waveguide, and the optical receiving element disposed in a portion after the folded-back portion of the monitoring optical waveguide, it is possible to provide an optical waveguide element that can shorten the length of the substrate having an electro-optic effect and sufficiently ensure the space required for the placement of the optical receiving element, an optical modulation device utilizing the optical waveguide element, and an optical transmission device. Attached Figure Description

[0025] Figure 1 This is a top view showing an existing optical waveguide component.

[0026] Figure 2 (a) is a top view, (b) is a side view, and (c) and (d) are cross-sectional views at the dashed line S in (a), representing a first embodiment of the optical waveguide element of the present invention.

[0027] Figure 3 These are (a) a top view and (b) a cross-sectional view illustrating a second embodiment of the light control element of the present invention.

[0028] Figure 4 This is a top view showing a third embodiment of the light control element of the present invention.

[0029] Figure 5 These are (a) a top view and (b) a side view illustrating a fourth embodiment of the light control element of the present invention.

[0030] Figure 6 These are (a) a top view and (b) a side view illustrating a fifth embodiment of the light control element of the present invention.

[0031] Label Explanation

[0032] 1 substrate

[0033] 2 Optical waveguide

[0034] 3. Reinforcing components

[0035] 22. Surveillance optical waveguide

[0036] 23 Turnaround section

[0037] PD optical receiver element Detailed Implementation

[0038] The present invention will now be described in detail using preferred embodiments.

[0039] The present invention is as follows Figure 2 As shown, the optical waveguide element includes: a substrate 1 having an electro-optic effect; an optical waveguide 2 formed on the substrate; and an optical receiving element (PD) disposed on the substrate and monitoring light waves propagating in or radiated from the optical waveguide. The element is characterized by having a monitoring optical waveguide 22 extending from the optical waveguide to the optical receiving element, the monitoring optical waveguide having a folded-back portion 23 relative to the emission direction of the optical waveguide, and the optical receiving element disposed in a portion (24) after the folded-back portion of the monitoring optical waveguide. Figure 2 (a) is a top view. Figure 2 (b) is a side view.

[0040] As the substrate 1 with electro-optic effect, substrates such as lithium niobate (LN), lithium tantalate (LT), and PLZT (lead lanthanum zirconate titanate) can be used, as well as composite substrates obtained by bonding vapor-phase grown films based on these materials to different types of substrates.

[0041] In addition, various materials such as semiconductor materials and organic materials can also be used as optical waveguides.

[0042] As a method for forming an optical waveguide, the portion of the substrate corresponding to the optical waveguide can be etched onto the substrate surface other than the waveguide, or grooves can be formed on both sides of the waveguide to create a convex ridge-shaped optical waveguide. Alternatively, an optical waveguide can be formed by creating a high-refractive-index portion on the substrate surface using methods such as thermal diffusion or proton exchange with Ti. Composite optical waveguides can also be formed by diffusing high-refractive-index materials into the ridge-shaped waveguide portion. In particular, since the radius of curvature is reduced in the folded-back portion of a monitoring optical waveguide, a ridge-shaped optical waveguide structure with enhanced light containment is preferred.

[0043] To achieve velocity matching between the microwave and light waves in the modulation signal, the thickness of the substrate on which the optical waveguide is formed is set to 10 μm or less, more preferably 5 μm or less. Furthermore, the ratio h / t of the height h of the ridge waveguide (from the bottom edge of the grooves on both sides of the ridge waveguide to the top edge of the ridge waveguide protrusion) to the substrate thickness t of the ridge waveguide portion (from the bottom surface of the substrate to the top edge of the ridge waveguide protrusion) is set to 0.8 or less. Particularly when the substrate thickness t is 1 μm or less, h / t is preferably set to the range of 0.6 to 0.8. Moreover, a vapor-grown film can be formed on the reinforcing substrate 1, and this film can be processed into the shape of the optical waveguide described above.

[0044] To improve mechanical strength, the substrate with the optical waveguide is directly bonded to or bonded to a reinforcing substrate via an adhesive layer such as resin. As the reinforcing substrate for direct bonding, a material with a lower refractive index than the optical waveguide and the substrate with the optical waveguide, and a thermal expansion coefficient close to that of the optical waveguide, such as quartz, is preferred. Alternatively, a reinforcing substrate with a refractive index equal to or higher than that of the substrate with the optical waveguide can also be used. In this case, a low-refractive-index layer is formed between the reinforcing substrate and the optical waveguide substrate.

[0045] Alternatively, when bonding to a reinforcing substrate via an adhesive layer, an LN substrate or the like can also be used as the reinforcing substrate.

[0046] When using optical waveguide elements as optical modulation devices, modulation electrodes are formed along the optical waveguide, especially the branch waveguides of Mach-Zehnder type optical waveguides, in order to modulate the light waves propagating in the optical waveguide. Moreover, in order to control the DC bias voltage of the optical modulation device, bias electrodes can also be configured independently of the modulation electrodes.

[0047] The optical waveguide element of the present invention is characterized as follows: Figure 2 By providing a foldback portion 23 midway through the monitoring optical waveguide 22 connecting the optical waveguide 2 to the optical receiving element PD, the length L of the substrate 1 is shortened. Furthermore, the foldback portion 23 is positioned between the reinforcing member 3, which is disposed along the end face of the substrate 1, and the substrate 1; in other words, when viewed from above... Figure 2 The reinforcing member 3 is arranged to overlap with at least a portion of the folded-back portion 23, as shown in (a). This allows for a further reduction in the length L of the substrate 1.

[0048] Furthermore, since the folded-back portion 23 can be formed in the wide portion that also includes the lower side of the reinforcing member 3, the radius of curvature can be increased to suppress the light propagation loss caused by bending at the folded-back portion.

[0049] The optical waveguide (foldback section, input / output waveguide) located on the lower side of the reinforcing member 3 can use, for example... Figure 2 The structure (groove) shown in (c) is the part other than the ridge waveguide that has been removed, as shown in [the diagram]. Figure 2 A ridge waveguide with slots of constant width formed on both sides of the waveguide, as in (d). The width of the slot is preferably set to about 1 to 10 times the width of the ridge waveguide. Figure 2 (c) and Figure 2 (d) is Figure 2 The cross-sectional view at the dashed line S in (a) shows the shape of the optical waveguide formed on the substrate 1.

[0050] Therefore, the return section and the input / output waveguides can maintain the function of a ridge waveguide with a large sealing effect for light waves while ensuring sufficient bonding area between the reinforcing member 3 and the substrate 1. Furthermore, there is no concern about the reinforcing member 3 tilting relative to the bonding surface of the substrate 1 during bonding. In addition, this ensures a stable adhesive layer thickness regardless of the waveguide's cross-section or pattern shape, reducing variations in adhesive strength and controlling adhesive overflow from the reinforcing member.

[0051] like Figure 2 As shown in (a), the distance between the light receiving element PD and the reinforcing member 3 can also be set to be greater than the distance between the combining section of the Mach-Zehnder waveguide (the section that generates the light wave detected by the light receiving element) and the reinforcing member 3. This ensures the longest possible length of the operating section for light modulation, and also prevents contact between the reinforcing member 3 and the adhesive that fixes the light receiving element PD to the substrate 1.

[0052] Figure 3 A monitoring optical waveguide (22, 22') for monitoring emitted light is provided by means of a narrowed Mach-Zehnder type optical waveguide output waveguide 20. Figure 3 (a) is a top view. Figure 3 (b) is Figure 3 The cross-sectional view at the single-dotted line A-A' of (a).

[0053] Two monitoring optical waveguides are provided with foldback sections (23, 23') and extend towards the monitoring optical waveguides (24, 24') that pass under the optical receiving elements (PD1, PD2). Alternatively, the optical receiving elements (PD1) and (PD2) can be separately constructed and disposed on each monitoring optical waveguide (24, 24'), or they can be integrally formed within a support member 4. When a three-branch structure is used in the combining section of the Mach-Zehnder type optical waveguide, the outputs of the two optical receiving elements are combined into a single monitoring output. This reduces the mixing of monitoring light into the output light, resulting in a high ON / OFF extinction ratio, and further reduces the shift in the modulation curve between the monitoring light and the output light.

[0054] When integrated via support member 4, such as Figure 3 As shown in (b), the support member 4 needs to be configured across the branch waveguide of the Mach-Zehnder type optical waveguide, so it is preferable to form a recess G in a portion of the support member 4. Moreover, a buffer layer can also be formed on the branch waveguide to prevent the light wave from being absorbed by the optical receiving element side.

[0055] In order to guide at least a portion of the light wave propagating in the monitoring optical waveguide toward the optical receiving element (PD), it can be done as follows: Figure 3 As in (b), the optical receiving element is placed close to the optical waveguide, thereby attracting the evanescent component from the optical waveguide toward the optical receiving element. Furthermore, a slanted cut or the like can be formed in a part of the optical waveguide to cause the light wave to be reflected or scattered toward the optical receiving element.

[0056] Furthermore, a reflective surface can be formed on the side, upper or lower surface, or inside of the support member 4 to reflect light waves within the support member, thereby improving the light receiving sensitivity of the light receiving element. In particular, when a reflective surface is formed inside the support member 4, it is possible to suppress the incident light waves from one of the light receiving elements (PD1) and (PD2) to the other, thus enabling the reception of more stable monitoring light.

[0057] In addition, to ensure the stability of the light received and characteristics of the monitoring light, and to improve the reliability of fixing the light receiving element, a base structure can be separately installed on the lower surface of the light receiving element (PD).

[0058] Figure 4This is an example where the light wave monitored by the optical receiving element PD is set as a part of the output light from the multiplexing section. An optical coupler (dashed line portion C) is configured in a part of the output waveguide 20 to guide a part of the output light to the monitoring optical waveguide. Various structures such as directional couplers, MMI couplers, and cross couplers can be used as the optical coupler C.

[0059] Figure 5 The diagram shows the input and output ends of an optical waveguide arranged along the same side (right side of the figure) of substrate 1. Furthermore, the input (Lin) light wave is branched into two, each modulated by a nested optical waveguide, and the two modulated lights are polarized and combined to output a single output light (Lout). Additionally, the electrical signal (Sin) used for optical modulation is input from the opposite side of the sides of substrate 1 where the input and output light waves are located. Figure 5 (b) is relative to Figure 5 (a) is a top view and a side view.

[0060] Lens 51, located on the incident side of the light wave, and lenses (52, 53), located on the exit side, are integrally held in optical block 5 and joined to the sides of substrate 1 and reinforcing member 3. Furthermore, if necessary, an optical block may also be provided that integrally holds optical components such as wavelength plate 54 and polarization combining members (55, 56). Alternatively, the end face of an optical fiber inserted and fixed to a member such as a capillary tube may be directly joined to the sides of substrate 1 and reinforcing member 3 along with the capillary tube.

[0061] As electrodes disposed on substrate 1, there are a modulation electrode (the thick dashed line part in the figure, only the signal electrode is shown, and the ground electrode is omitted) that is input to the electrical signal Sin, a bias electrode (B2) that controls the bias voltage of the main Mach-Zehnder type optical waveguide of the nested optical waveguide, and a bias electrode (B1) that controls the bias voltage of the secondary Mach-Zehnder type optical waveguide.

[0062] The dashed line LS depicted on the input side of the optical waveguide 2 represents the leakage light of the light wave incident from the end face of the substrate 1. The diffusion angle (divergence angle θ) of the leakage light is calculated using θ = λ / πw0 based on the beam radius w0 of the optical coupling section and the wavelength λ. By placing the optical receiving element outside the range of the leakage light diffusion angle, it is possible to suppress the situation where uncoupled light incident from the coupling section enters the optical receiving element and becomes noise in the optical receiving element.

[0063] exist Figure 5 In (a), the shape of the terminal of the monitoring optical waveguide is bent toward the side of the substrate 1 (the upper and lower edges of the substrate 1 in the figure). This is to prevent the light waves (useless light) emitted from the terminal of the monitoring optical waveguide from re-entering the optical waveguide 2. The light waves can also be absorbed by electrodes formed on the surface of the substrate 1, but the light waves can also be emitted to the outside from the side of the substrate 1.

[0064] Figure 6 and Figure 5 Similarly, the optical receiving element PD is configured to avoid leakage light LS from the input end of the optical waveguide. In addition, light-shielding portions can be formed on the sides of the optical receiving element PD and the reinforcing member 3 to suppress unwanted light from entering the optical receiving element or unwanted light from being reflected by the side of the reinforcing member and entering the optical waveguide 2 and the optical receiving element PD.

[0065] The aforementioned optical waveguide element can be housed within a housing, and an optical fiber can be installed within the optical waveguide of the waveguide element to allow light waves to enter and exit from the outside of the housing, thereby constructing an optical modulation device. Furthermore, electronic circuitry such as a driver IC for modulating the modulator can also be built into the housing. In particular... Figure 5 , Figure 6 In the case of waveguides with such folded-back structures, modulation devices with excellent high-frequency characteristics can be realized by directly configuring the interface, driver IC, and modulation element for high-frequency signals.

[0066] Furthermore, the optical modulation device may include electronic circuits such as a digital signal processor and a driver IC that generate electrical signals input to the modulation electrodes formed on the substrate of the optical waveguide element, as well as a laser source and control circuits, thereby constituting an optical transmission device. The electronic circuits may be housed within the same housing as the optical waveguide element, or they may be located outside the housing.

[0067] Industrial applicability

[0068] As described above, according to the present invention, it is possible to provide an optical waveguide element that can be configured to have a short length of a substrate having an electro-optic effect and can sufficiently ensure the space required for the configuration of an optical receiving element, an optical modulation device using the optical waveguide element, and an optical transmitting device.

Claims

1. An optical waveguide element comprising: a substrate having an electro-optic effect; An optical waveguide is formed on this substrate; And a light receiving element, disposed on the substrate, for monitoring light waves propagating in or radiated from the optical waveguide, characterized in that, The optical waveguide element includes a monitoring optical waveguide extending from the optical waveguide to the optical receiving element. The monitoring optical waveguide has a reflective section relative to its outgoing direction. The optical receiving element is positioned a portion after the folded-back section of the monitoring optical waveguide. On the upper side of the substrate, a reinforcing member is joined to the substrate along one side of the substrate where the output end of the optical waveguide is disposed, and the folded-back portion is formed between the substrate and the reinforcing member.

2. The optical waveguide element according to claim 1, characterized in that, The input and output ends of the optical waveguide are arranged along the same side of the substrate.

3. The optical waveguide element according to claim 1 or 2, characterized in that, The light receiving element is positioned outside the range of the diffusion angle of the leaked light leaking from the input terminal to the substrate.

4. The optical waveguide element according to claim 1, characterized in that, A light-shielding portion is formed on the side of the reinforcing member facing the light-receiving element or on the side of the light-receiving element facing the reinforcing member.

5. The optical waveguide element according to claim 1 or 2, characterized in that, The optical waveguide used for monitoring is a ridge-type optical waveguide.

6. An optical modulation device, characterized in that, have: The optical waveguide element according to any one of claims 1 to 5; The housing contains the optical waveguide element; as well as The optical fiber carries light waves from outside the housing into the optical waveguide, or carries light waves from the optical waveguide out of the housing.

7. The optical modulation device according to claim 6, characterized in that, The optical modulator has an electronic circuit inside the housing that amplifies the modulation signal input to the optical waveguide element.

8. An optical transmitting device, characterized in that, have: The optical modulation device according to claim 6 or 7; and The electronic circuit outputs a modulation signal that causes the optical modulator to perform modulation.

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