JP2016071256A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator with which it is possible to suppress the coupling loss of an optical waveguide and reduce the device size.SOLUTION: An optical modulator includes a modulation substrate 1 having a substrate 1 formed with lithium niobate and being 20 μm or less in thickness, a plurality of optical waveguides 10 formed on the substrate, and a modulation electrode (not shown) for modulating at least some of optical waves propagating the plurality of optical waveguides. The optical modulator has a polymer waveguide 21 constituting the core part of an optical waveguide outside of the modulation substrate, and the polymer waveguide is provided with a wave-splitting or a wave-combining part for connecting the plurality of optical waveguides 10 formed on the modulation substrate with each other. In a portion A connecting the polymer waveguide and the modulation substrate, the height of the polymer waveguide 21 is set to the same thickness as the modulation substrate 1.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical modulator, and in particular, a modulation substrate including an optical waveguide and a modulation electrode on a substrate formed of lithium niobate (LN), and a polymer waveguide including an optical waveguide formed of a polymer. The present invention relates to an optical modulator combining the above.

  An optical modulator having an optical waveguide on a substrate formed of lithium niobate (LN) (waveguide-type LN modulator) has a small wavelength chirp and is capable of phase / intensity modulation. (Refer to Patent Document 1). As characteristics representing the performance of the LN modulator, there are a drive voltage (Vπ) and a modulation band. The smaller the drive voltage and the larger the modulation band, the better the characteristics of the LN modulator. However, since the drive voltage and the modulation band are normally in a trade-off relationship, it is difficult to extend the modulation band by fixing the drive voltage (see Patent Document 2).

  In recent years, multilevel modulation schemes in which the phase and intensity are modulated have become mainstream (see Non-Patent Documents 1 and 2). In order to manipulate the phase, it is necessary to arrange a plurality of Mach-Zehnder type waveguides (MZ structures). For example, in the case of a quaternary PSK modulator, three MZ structures are required, and in the case of a polarization multiplexed quaternary PSK modulator, six MZ structures are required.

  Further, in the quaternary PSK modulator, the input light needs to be divided into four branches. Each of the separated waveguides is subjected to phase modulation at the action part of the MZ structure. In this phase modulation electrode, a coplanar structure is applied to each MZ structure. The width of the ground electrode (GND electrode) between the MZ structures of the quaternary PSK modulator needs to be at least 200 μm in consideration of the crosstalk of the electrical signals of each MZ structure and the ground function of the GND electrode.

  For this reason, the separation interval of the first Y branch of the light 4 branching portion is 100 μm or more. In order to reduce the size of the optical modulator, it is necessary to shorten the length of the Y branch. On the other hand, it is possible to shorten the length of the working region where the electric field formed by the modulation electrode acts on the optical waveguide, but if it is shortened, it is necessary to increase the driving voltage and an inexpensive driver cannot be used. For this reason, it has been studied to shorten the length of the Y branch rather than shortening the length of the action region (see Non-Patent Documents 1 and 2 and Patent Document 3).

In miniaturization of a multilevel modulator using LN, it is necessary to shorten the Y branch section. However, when a Ti diffusion optical waveguide is provided on a substrate formed of conventional LiNbO ₃ , the difference in refractive index between the core and the clad cannot be increased (see Non-Patent Document 3). The bending waveguide cannot be shortened. In order to solve this problem, it has been proposed to apply a PLC (planar lightwave circuit) made of quartz glass to the substrate portion on which the Y branch portion is formed. However, in the connection between the PLC and the LN, it is difficult to make the coupling loss of all the optical waveguides the same at the same time in the alignment between a plurality of waveguides (arrays), and there are many optical connection points (optical fiber and PLC Between the PLC and the LN chip), a problem in reliability arises.

  In addition, for alignment (optical axis adjustment) between the PLC and the LN substrate, reference light is incident from the input side of the optical waveguide, and the both are aligned while monitoring the reference light output from the output side. So-called active alignment is indispensable, and there is a problem that the burden of adjustment work increases.

JP 2010-185977 A Japanese Patent Publication No. 7-50265 Japanese Patent No. 3800594 Japanese Patent No. 4691481

Koichi Murata and 1 other, “Current Status and Future Prospects of Optical Integrated Circuit Technology for Digital Coherent Systems”, Optronics, No.7, pp113-120, 2012 Masaki Sugiyama, “LN modulators that are the key to phase modulation”, Optronics, No.3, pp107-111, 2011 Makoto Minakata, “LiNbO3 optical waveguide device”, IEICE Transactions C-I, Vol. J77-C-I, No. 5, pp 194-205, May 1994 Yosuke Furuta, 3 others, "Ti diffusion into LiNbO3 crystal", Laser Research, Vol. 21, No. 9, pp981-986, September 1993 Kazuo Eda, 7 others, "Direct bonding of piezoelectric materials", IEICE Technical Report, US95-24, EMD95-20, CPM95-32, July 1995 Hideki Takagi, et al., "Room-temperature bonding of lithium niobate and silicon wafers by argon-beam surface activation", Applied Physics Letters, Vol. 74, No. 16, pp2387-2389, 19 April 1999

  The problem to be solved by the present invention is to solve the above-mentioned problems, to provide an optical modulator capable of suppressing the coupling loss of the optical waveguide and miniaturizing.

In order to solve the above problems, the present invention has the following technical features.
(1) A substrate formed of lithium niobate and having a thickness of 20 μm or less, a plurality of optical waveguides formed on the substrate, and at least part of light waves propagating through the plurality of optical waveguides An optical modulator comprising a modulation substrate having a modulation electrode, and having a polymer waveguide that is formed of a polymer and forms a core portion of an optical waveguide outside the modulation substrate, the polymer waveguide being A branching section or a combining section for connecting the plurality of optical waveguides formed on the modulation substrate to each other, and a height of the polymer waveguide at a portion connecting the polymer waveguide and the modulation substrate. Is set to be equal to the thickness of the modulation substrate.

(2) In the optical modulator according to (1), the modulation substrate and the polymer waveguide are bonded to a reinforcing substrate via a resin layer, and the refractive index of the resin layer is the polymer waveguide. It is characterized by being set lower than the refractive index.

(3) The optical modulator according to (1) or (2) above, wherein the connection portion between the modulation substrate and the polymer waveguide is directly connected without any other material. To do.

(4) The optical modulator according to any one of (1) to (3), further including a protective film that covers the polymer waveguide, wherein the refractive index of the protective film is higher than the refractive index of the polymer waveguide. It is characterized by being set low.

(5) In the optical modulator according to (2), a groove from which a part of the polymer waveguide or the resin layer is removed is formed at a bonding position between the end of the polymer waveguide and the optical fiber. An end of the optical fiber is disposed in the groove.

  As in the present invention, a substrate made of lithium niobate and having a thickness of 20 μm or less, a plurality of optical waveguides formed on the substrate, and at least a part of light waves propagating through the plurality of optical waveguides are modulated. An optical modulator comprising a modulation substrate having a modulation electrode for forming a polymer waveguide having a polymer waveguide that is formed of a polymer and forms a core portion of an optical waveguide outside the modulation substrate. The waveguide includes a branching section or a combining section that connects the plurality of optical waveguides formed on the modulation substrate to each other, and the polymer waveguide is provided at a portion that connects the polymer waveguide and the modulation substrate. Is set to be the same as the thickness of the modulation substrate, so that the optical waveguide formed by the polymer waveguide can be bent larger. Place to get But, it is possible to shorten the length of the Y-branch (length in the optical axis direction), while implementing downsizing of the optical modulator.

  In addition, by setting the height of the polymer waveguide to be the same as the thickness of the modulation substrate, it is possible to suppress the coupling loss between the polymer waveguide and the optical waveguide formed on the modulation substrate. In particular, since the lithium niobate substrate has a thickness of 20 μm or less, there is little change in size due to thermal expansion of the substrate, and the influence of changes in size and position due to the difference in linear expansion coefficient from the polymer waveguide is suppressed.

  Further, the modulation substrate and the polymer waveguide are joined to the reinforcing substrate via a resin layer, and the refractive index of the resin layer is set lower than the refractive index of the polymer waveguide. In addition to functioning as a clad of the polymer waveguide, light waves leaking from the polymer waveguide escape to the resin layer and the reinforcing substrate rather than the thin LN substrate. Therefore, compared with the conventional optical modulator using a thin LN substrate including a Y branch. However, it is possible to reduce the deterioration of characteristics due to light leakage.

  Furthermore, since the connecting part between the modulation substrate and the polymer waveguide is directly connected without any other material, the coupling loss between the optical waveguide formed on the modulation substrate and the polymer waveguide is further suppressed. It becomes possible to do. In particular, since an adhesive layer that connects the two becomes unnecessary, internal stress due to environmental temperature changes is not applied to the contact surfaces of both, and refractive index changes due to shape changes and internal stresses are suppressed, and coupling loss is further suppressed. .

  In addition, since the protective film has a protective film covering the polymer waveguide, and the refractive index of the protective film is set to be lower than the refractive index of the polymer waveguide, the protective film is physically attached to the polymer waveguide from the external environment. In addition to protecting from chemical stimulation, the protective film functions as a cladding of the polymer waveguide, and it is also possible to suppress light waves propagating through the polymer waveguide from leaking out of the polymer waveguide.

  A groove from which a part of the polymer waveguide or the resin layer is removed is formed at the joining position between the end of the polymer waveguide and the optical fiber, and the end of the optical fiber is disposed in the groove. Optical axis adjustment between the modulator and the optical fiber can be easily and accurately performed. Positioning using a mark called a groove, so-called passive alignment can be performed, and conventional active alignment becomes unnecessary.

It is a figure explaining the outline of the optical modulator which concerns on this invention, and is the figure which showed a mode that the height of the core part of a LN board | substrate and a polymer waveguide becomes especially the same. It is a figure explaining process (a)-(c) of the manufacturing process of the optical modulator which concerns on this invention. It is a figure explaining process (d)-(e) of the manufacturing process of the optical modulator which concerns on this invention. It is a figure explaining process (f)-(g) of the manufacturing process of the optical modulator which concerns on this invention. It is a figure explaining the prior art example (a) using PD-QPSK modulator, and this invention (b). It is a figure explaining the application example of a PD-QPSK modulator. It is a figure explaining the connection of the optical modulator which concerns on this invention, and an optical fiber.

Hereinafter, the optical modulator of the present invention will be described in detail using preferred examples.
As shown in FIG. 1, the present invention is formed of lithium niobate and has a thickness of 20 μm or less, a plurality of optical waveguides 10 formed on the substrate, and the plurality of optical waveguides. In an optical modulator including a modulation substrate 1 having a modulation electrode (not shown) for modulating at least a part of a light wave, the optical modulator is formed of a polymer, and a core portion of an optical waveguide outside the modulation substrate The polymer waveguide 21 includes a branching portion or a multiplexing portion that connects the plurality of optical waveguides 10 formed on the modulation substrate to each other, and the polymer waveguide and the waveguide In the portion A where the modulation substrate is connected, the height of the polymer waveguide 21 is set to be the same as the thickness of the modulation substrate 1.

  A feature of the optical modulator according to the present invention is that a part of the optical waveguide is formed of a polymer waveguide. The polymer waveguide can increase the difference in refractive index between the core and the clad as compared with the optical waveguide formed in lithium niobate, in particular, the Ti thermal diffusion waveguide. For this reason, since the bending of the optical waveguide can be increased, the length of the Y branch (length in the optical axis direction) can be shortened even when the same separation interval is obtained in the Y branch as compared with the LN substrate or the like. Miniaturization of the optical modulator can be realized.

  Furthermore, in the optical modulator according to the present invention, the height of the polymer waveguide 21 is set to be the same as the thickness of the modulation substrate 1 as indicated by the dotted line portion indicated by the symbol A in FIG. With this configuration, it is possible to further suppress the coupling loss between the polymer waveguide 21 and the optical waveguide 10 formed on the modulation substrate 1. Particularly, since the modulation substrate 1 formed of lithium niobate has a thickness of 20 μm or less, the size change due to the thermal expansion of the substrate 1 is small, and the size change and position due to the difference in the linear expansion coefficient with the polymer waveguide 21 are small. The impact of change is small.

  In the optical modulator of FIG. 1, the modulation substrate 1 and the polymer waveguide 21 are joined to the reinforcing substrate 3 via the resin layer 4. This makes it possible to compensate for the insufficient mechanical strength of the modulation substrate 1 having a thickness of 20 μm or less and to support the polymer waveguide 21. In particular, the refractive index of the resin layer 4 is preferably set lower than the refractive index of the polymer waveguide 21. In this case, the resin layer 4 can function as a part of the clad of the polymer waveguide 21. Further, it is possible to suppress the light wave propagating through the polymer waveguide 21 from leaking to the reinforcing substrate 3 side.

  The connection portion (the portion indicated by reference numeral A) between the modulation substrate 1 and the polymer waveguide 21 is directly connected without any other material. This means that an adhesive layer that joins both a conventional PLC and an LN substrate is not necessary in the optical modulator of the present invention. Since a different material such as an adhesive layer is not interposed, the coupling loss between the optical waveguide formed on the modulation substrate and the polymer waveguide can be further suppressed. In addition, since both are bonded without using an adhesive, it is not affected by internal stress accompanying changes in the environmental temperature of the adhesive. The refractive index change is also suppressed.

  Further, as will be described later, a protective film covering the polymer waveguide 21 can be provided. In this case, by setting the refractive index of the protective film lower than the refractive index of the polymer waveguide 21, the protective film not only protects the polymer waveguide from physical and chemical stimulation from the external environment, but also protects it. In addition to the film functioning as a cladding of the polymer waveguide, it is possible to prevent light waves propagating through the polymer waveguide from leaking out of the polymer waveguide.

  As the material of the polymer waveguide used in the optical modulator of the present invention, polymethyl methacrylate (PMMA), polysilane, CYTOP (trade name cytop, manufactured by Asahi Glass Co., Ltd., transparent fluororesin), etc. can be suitably used. is there.

  As a material used for the protective film, polymethyl methacrylate (PMMA), polysilane, CYTOP (trade name cytop, manufactured by Asahi Glass Co., Ltd., transparent fluororesin) and the like can be suitably used. However, it is necessary to adjust the refractive index so that the refractive index is lower than that of the core portion of the polymer waveguide.

  As the reinforcing substrate, it is preferable to use a material having a linear expansion coefficient close to that of the modulating substrate. For example, LN, quartz glass, BK7, or the like can be used. In addition, as a material for the resin layer that joins the modulation substrate and the reinforcing substrate, a photo-curing adhesive such as acrylic can be suitably used in consideration of workability and the like.

Next, an example of the manufacturing method of the optical modulator of the present invention will be described with reference to FIGS.
A Ti diffusion waveguide is formed on the LN substrate by the method described in Patent Document 3. Since the formed Ti diffusion waveguide has irregularities on the surface (see Non-Patent Document 4), the surface of the LN wafer (about 0.1 to 0.2 μm) is planarized by polishing or the like. As shown in FIG. 2A, the LN substrate 1 provided with the optical waveguide 10 is directly bonded to the surface of a holding substrate 100 such as a Si wafer (see Non-Patent Documents 5 and 6).

  The space between the Si wafer 100 of FIG. 2A and the waveguide end face of the LN substrate 1 is filled with a resin 20 (material forming the core of the polymer waveguide) (see FIG. 2B). Next, as shown in FIG. 2C, the LN substrate 1 and the resin 20 are ground and polished to a thickness of 20 μm or less, for example, about 10 μm.

  In order to hold the LN substrate 1 and the resin 2 having a thickness of about 10 μm, the reinforcing substrate 3 is bonded via a low refractive index adhesive (resin layer) 4 as shown in FIG. Thereafter, by removing the Si wafer 100 as the holding substrate with potassium hydroxide or the like, it is possible to realize a state in which the surfaces of the LN substrate 1 and the resin 2 are aligned as shown in FIG. it can.

  Using the Ti diffusion waveguide 10 of the LN substrate 1 as a reference, the resin 2 (the layer that becomes the core portion of the polymer waveguide) is patterned in the shape of the optical waveguide 21 using RIE (reactive ion etting) or the like ( (Refer FIG.4 (f)).

  Thereafter, as shown in FIG. 4G, a protective film 5 (cladding layer) can be added in consideration of subsequent processes (electrode formation, mounting, etc.). The modulation electrode can be formed at any time after the surface of the LN substrate 1 shown in FIG. 3E is exposed, but after the protective film 5 is formed in order to protect the formed polymer waveguide. Preferably, a modulation electrode is formed.

  The optical waveguides formed in the optical modulator of the present invention are all optically connected (positioned) with reference to the wafer constituting the LN substrate 1 (passive alignment is possible), and mass production is possible. . In addition, since the optical waveguides can be connected without using an adhesive, it is possible to ensure reliability. For example, since there is no inclusion such as an adhesive between the joints, the characteristic deterioration due to the softening of the adhesive due to a temperature change does not occur.

  The downsizing that is a feature of the optical modulator of the present invention will be described. As described above, a DQPSK (Differentiaol Quadrature Phase Shift Keying) modulator and a PD-QPSK (Dual Polarization Quadrature Phase Shift Keying) modulator are provided with 2 2 It is necessary to provide two or three branch circuits. In addition, it is necessary to ensure that the spacing between the waveguides after branching is 100 μm or more, and in the vertical direction of the LN substrate 11 shown in FIG. 5A in proportion to the number of Mach-Zehnder waveguides in parallel. Height increases. Along with this, the length required for the optical branching portion (length in the horizontal direction in the figure) also increases, and the length L in the horizontal direction (light propagation direction) of the LN substrate 11 of the optical modulator becomes even longer. . 5 and 6 show examples of the PD-QPSK modulator. The optical waveguide 12 formed on the LN substrate 11 is connected to four Mach-Zehnder type optical waveguides through two branches. One of the two light waves emitted from the LN substrate 11 is output as X-polarized light, and the other becomes Y-polarized light through the polarization rotation element 6. The two light waves are combined by the polarization combining means 7 and output.

  On the other hand, in order to cope with high-speed modulation and the like, in order to keep the driving voltage low, it is necessary to ensure the electrode length (interaction length between light (optical waveguide) and electric field) at a certain value or more. For this reason, in order to reduce the size of the optical modulator and shorten the length L in the propagation direction of the light wave, the optical branch circuit must be shortened. However, the refractive index difference of the Ti diffusion waveguide has a limit to increase due to the solid solution limit. As a result, the refractive index difference between the core and the cladding cannot be increased, and the bending of the optical waveguide becomes steep. It was difficult to do.

  In the present invention, as shown in FIG. 5 (b), the optical branch circuit is constituted by a polymer waveguide 24. A shaded area 23 is a formation area of the polymer waveguide 24. Since the polymer waveguide can increase the refractive index difference between the core and the clad as compared with the Ti thermal diffusion waveguide formed on the LN substrate, the optical waveguide can be bent more steeply. As a result, the length L ′ connecting the LN substrate 13 and the polymer waveguide forming region 23 in FIG. 5B is much shorter than the conventional one in FIG. Can be realized. Further, as shown in FIG. 5B, by replacing the bent portion or the branched portion of the optical waveguide with a polymer waveguide, it is possible to reduce the locations where light can leak. Temporarily, since the light leaked from the polymer waveguide has a bulk structure, the influence on the optical characteristics of the optical modulator is negligible.

  FIG. 6 shows an example in which the optical branch circuit on the emission side as well as the incident side of the PD-QPSK modulator is replaced with a polymer waveguide 28. As a result, the size can be further reduced as compared with the example of FIG. Reference numeral 15 denotes an LN substrate, 16 denotes a Ti thermal diffusion waveguide, 26 and 28 denote polymer waveguides, and 25 and 27 denote regions where polymer waveguides are formed. As shown in FIG. 6, when most of the optical branch circuit is formed of a polymer waveguide, a thermo-optic effect (TO effect) or the like is used to control the phase of the light wave propagating through the optical waveguide. When voltage control is desired, a phase shifter may be installed on the LN substrate side.

  In FIG. 1 (FIG. 4 (f)), the polymer waveguide removes the resin that forms the polymer waveguide, leaving only the optical waveguide portion, but only the optical waveguide portion is formed in a convex shape. In addition to the optical waveguide portion 21 on the adhesive layer (resin layer 4), the resin used for the polymer waveguide may be left. The optical waveguide may be a ridge-shaped waveguide that forms a groove along the optical waveguide.

  Next, a method for aligning the optical modulator and the optical fiber of the present invention by passive alignment will be described. As shown in FIG. 7A, when the material 2 of the light incident / exit portion of the optical modulator is a polymer, the groove 8 can be formed in the fiber connecting portion. The grooves can be formed simultaneously with the formation of the polymer waveguide 21 as shown in FIG. Since the fiber diameter of a normal optical fiber is 125 μm, it is difficult to process a V-groove as much as 63 μm on a difficult-to-process LN substrate, but the polymer can be easily obtained by using RIE using oxygen. A V-groove can be formed. Moreover, since not only the resin 2 that forms the polymer waveguide but also the resin that forms the adhesive layer 4 with the reinforcing substrate can be processed together, the grooves 8 in FIGS. 7A and 7B can be easily formed. Is possible.

  After the groove 8 is formed, as shown in FIG. 7C, the optical fiber 9 and the optical modulator substrate (chip) are optically connected by passive alignment by disposing the tip of the optical fiber in the groove. Can do.

  As described above, according to the present invention, it is possible to provide an optical modulator that can suppress the coupling loss of the optical waveguide and can be miniaturized.

1,11,13,15 LN substrate 2,20 Resin layer forming polymer waveguide 3 Reinforcing substrate 4 Adhesive layer (resin layer)
5 Protective layer 6 Polarization rotating means 7 Polarization combining means 8 Groove (V groove)
9 Optical fiber 10, 12, 14, 16 Optical waveguide formed on LN substrate 21, 24, 26, 28 Polymer waveguide (core part)
23, 25, 27 Polymer waveguide formation region 100 Holding substrate

Claims (5)

A substrate formed of lithium niobate and having a thickness of 20 μm or less; a plurality of optical waveguides formed on the substrate; and a modulation electrode for modulating at least a part of light waves propagating through the plurality of optical waveguides; In an optical modulator comprising a modulation substrate having
A polymer waveguide formed of a polymer and constituting a core portion of an optical waveguide outside the modulation substrate;
The polymer waveguide includes a branching section or a combining section that connects the plurality of optical waveguides formed on the modulation substrate,
An optical modulator characterized in that the height of the polymer waveguide is set to be equal to the thickness of the modulation substrate at a portion connecting the polymer waveguide and the modulation substrate.   2. The optical modulator according to claim 1, wherein the modulation substrate and the polymer waveguide are joined to a reinforcing substrate via a resin layer, and the refractive index of the resin layer is greater than the refractive index of the polymer waveguide. An optical modulator characterized by being set low.   3. The optical modulator according to claim 1, wherein a connection portion between the modulation substrate and the polymer waveguide is directly connected without any other material.   4. The optical modulator according to claim 1, further comprising a protective film that covers the polymer waveguide, wherein the refractive index of the protective film is set lower than the refractive index of the polymer waveguide. Features an optical modulator.   3. The optical modulator according to claim 2, wherein a groove from which a part of the polymer waveguide or the resin layer is removed is formed at a joint position between the end portion of the polymer waveguide and the optical fiber. An optical modulator comprising an end portion of an optical fiber.

Images