JP2018173604A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator capable of effectively removing stray light propagating within a substrate.SOLUTION: An optical modulator includes: a substrate 1 having an electro-optic effect; and an optical modulation unit including an optical waveguide 2 formed on the substrate and a control electrode for applying an electric field to the optical waveguide. The control electrode includes a modulation electrode for optical modulation. The optical waveguide includes an optical waveguide for signal light to let light waves to be modulated by the modulation electrode propagate. Double refractive index layers (BR1-BR3) are disposed on a front surface of the substrate having the optical waveguide for signal light formed or at least part of a rear surface of the substrate.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an optical modulator, and more particularly, to an optical modulator provided with means for removing stray light generated from an optical modulator and stray light incident on the optical modulator.

  Many optical modulators are used in the fields of optical communication and optical measurement. In order to transmit high-speed and large-capacity information, a plurality of light modulation portions are arranged on one substrate, and the entire light modulator is downsized. For example, a polarization multiplexing modulator, a quadrature amplitude modulator (QAM), and an optical modulator using multiple wavelengths have been proposed.

FIG. 1 is a schematic diagram illustrating an example of an optical modulator having a polarization combining function. An optical waveguide 2 is formed on a substrate 1 having an electrooptic effect, such as lithium niobate, and two light modulation portions (MZ1, MZ2) are provided. The light wave modulated by each light modulation portion is led out of the substrate 1 by each output optical waveguide (OP1, OP2). The output light (λ ₀₁ , λ ₀₂ ) is subjected to polarization synthesis by the wave plate WP, the reflection plate RF, and the polarization beam splitter PS to become output light (λ ′ ₀ ). Note that λ ₀ is input light to the optical modulator.

  Further, in the optical modulator of FIG. 1, the optical waveguide for radiated light (R11 and R12, R21 and R22) for deriving the radiation mode light is provided at the combined portion of the Mach-Zehnder type optical waveguides (MZ1, MZ2). Is formed.

FIG. 2 is a schematic diagram showing an example of an optical modulator using multiple wavelengths. Optical waveguides (21, 22) are provided for input light (λ ₁ , λ ₂ ) of different wavelengths incident on the substrate 1, and each optical waveguide is provided with a Mach-Zehnder optical waveguide (MZ11-22). A plurality of light modulation portions are formed. On the output side of each Mach-Zehnder type optical waveguide, output optical waveguides (OP11 to 22) and radiated light optical waveguides (R111 to 222) are arranged.

  Although not shown in FIGS. 1 and 2, a control electrode for applying an electric field to the Mach-Zehnder type optical waveguide is provided in the vicinity of each Mach-Zehnder type optical waveguide, as shown in FIG. Yes. As control electrodes, a modulation electrode (RF1 to RF3) for generating signal light and a DC bias electrode (DC1) for applying a DC bias to the optical waveguide for adjusting the bias point of the modulation curve are used as necessary. To DC3).

  In the vicinity of the multiplexing portion of the Mach-Zehnder type optical waveguide (MZ), radiated light optical waveguides (R1, R2) for deriving the radiated light are provided so as to sandwich the output optical waveguide (OP). As shown in FIG. 4, the light receiving elements (PD1, PD2) for detecting the emitted light are disposed immediately above the emitted light optical waveguide. As shown in FIG. 4B, high refractive index films (HR1, HR2) are disposed between the optical waveguide for radiated light (R1, R2) and the light receiving elements (PD1, PD2), and the optical waveguide is It plays the role of attracting propagating radiation to the light receiving element side. The light receiving elements (PD1, PD2) are fixed on the substrate 1 with an adhesive (AD). FIG. 4B shows a cross-sectional view taken along one-dot chain line A-A ′ in FIG.

  In FIG. 4, the light receiving elements (PD1, PD2) are provided separately corresponding to the respective optical waveguides for radiated light (R1, R2). However, as shown in FIG. There is also a method of receiving light waves propagating through a plurality of optical waveguides with a single light receiving element.

  Leakage light is generated from the optical waveguide at the part where the light wave is incident on the optical waveguide, the branching and multiplexing part of the Mach-Zehnder type optical waveguide constituting the optical modulation part, and the branching part to the optical waveguide for radiated light Easy to do. Such leaked light may be mixed into signal light that is output as stray light in the substrate or detected by the light receiving element together with the emitted light. As a result, the modulation characteristics of the optical modulator deteriorate, and it becomes difficult to optimally control the characteristics of the optical modulator. In particular, when a plurality of light modulation portions are arranged side by side on the same substrate and the entire substrate is downsized, the problem of stray light becomes a serious problem.

  As a method for removing stray light propagating in the substrate, as shown in Patent Document 1, a high refractive index film or a metal film is disposed on the surface of the substrate, and stray light propagating in the substrate is led out of the substrate, or metal A method of absorbing with a membrane has been proposed.

  However, in a high refractive index film, a slab waveguide is formed, stray light is only diffused, and the effect of removing stray light is extremely limited. Further, the metal film may cause a short circuit between the electrodes in a miniaturized optical modulator.

JP 2006-276518 A

  As described above, the problem to be solved by the present invention is to effectively remove stray light, arrange a plurality of light modulation portions, and further reduce the size of the light modulator. It is an object of the present invention to provide an optical modulator capable of suppressing mixing with radiated light.

In order to solve the above problems, the optical modulator of the present invention has the following technical features.
(1) In an optical modulator comprising a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and an optical modulator having a control electrode for applying an electric field to the optical waveguide, the control electrode comprises: A modulation electrode for performing optical modulation, the optical waveguide having an optical waveguide for signal light through which an optical wave modulated by the modulation electrode propagates, and the substrate on which the optical waveguide for signal light is formed A birefringent layer is disposed on at least a part of the front surface or the back surface of the substrate.

(2) In the optical modulator according to (1), when the birefringence layer is disposed on the surface of the substrate, the birefringence layer is disposed on the substrate excluding the signal light optical waveguide. It is characterized by that.

(3) In the optical modulator described in (1) above, when a birefringent layer is disposed on the surface of the substrate, the refractive index of the birefringent layer is less than the refractive index of the optical waveguide for signal light. Low and higher than the refractive index of the substrate.

(4) In the optical modulator according to any one of (1) to (3), when the birefringence layer is disposed on the back surface of the substrate, the thickness of the substrate is 20 μm or less. To do.

(5) The optical modulator according to any one of (1) to (4), wherein a metal layer is disposed on a part of the birefringence layer opposite to the substrate. To do.

(6) In the optical modulator according to any one of (1) to (5), the control electrode includes a bias electrode that applies a DC bias voltage to the optical waveguide, and the bias electrode includes the optical wave The birefringence layer is disposed on the downstream side of the modulation electrode with respect to the propagation direction, and the birefringence layer is disposed on the lower side of the bias electrode.

(7) In the optical modulator according to any one of (1) to (6), the optical modulation unit includes at least a first optical modulation part and a second optical modulation part arranged side by side. The birefringence layer is formed on the substrate between the first light modulation portion and the second light modulation portion.

  According to the present invention, the birefringence layer is disposed on at least a part of the front surface of the substrate on which the optical waveguide for signal light is formed or the back surface of the substrate. The layer can lead to the substrate outward direction and a plurality of directions. By using this action, the stray light in the substrate can be efficiently removed, and the stray light can be effectively suppressed from being mixed into the signal light and the emitted light.

It is a schematic diagram showing an example of an optical modulator having a polarization combining function. It is the schematic which shows an example of the optical modulator using multiple wavelengths. It is a figure which shows the example which uses a modulation electrode and a DC bias electrode as a control electrode which applies an electric field to a Mach-Zehnder type optical waveguide. It is a top view explaining the mode of arrangement | positioning of the optical waveguide for radiated light, and a light receiving element in order to receive radiated light. It is sectional drawing in the dashed-dotted line A-A 'of FIG. 4A. It is a top view explaining the structure which light-receives two emitted light using one light receiving element. It is sectional drawing in the dashed-dotted line B-B 'of FIG. 5A. It is a top view explaining the 1st Example concerning the optical modulator of the present invention. It is sectional drawing in the dashed-dotted line D1-D1 'of FIG. It is sectional drawing in the dashed-dotted line D2-D2 'of FIG. It is a figure explaining the 2nd Example which concerns on the optical modulator of this invention. It is a figure explaining the 3rd Example concerning the optical modulator of this invention. It is a top view explaining the 4th example concerning an optical modulator of the present invention. It is sectional drawing in the dashed-dotted line D-D 'of FIG. 10A. It is a top view explaining the 5th Example concerning the optical modulator of the present invention. It is sectional drawing in the dashed-dotted line E-E 'of FIG. 11A.

Hereinafter, the optical modulator according to the present invention will be described in detail.
As shown in FIGS. 6 to 11B, the optical modulator to which the present invention is applied includes a substrate 1 having an electro-optic effect, an optical waveguide 2 formed on the substrate, and a control for applying an electric field to the optical waveguide. In an optical modulator having an optical modulator having an electrode (not shown), the control electrode has a modulation electrode for performing optical modulation, and the optical wave propagates through the optical waveguide modulated by the modulation electrode. A birefringence layer (BR1 to BR4, BR) is provided on at least a part of the front surface of the substrate on which the signal light optical waveguide is formed or the back surface of the substrate. It is arranged.

As a substrate having an electro-optic effect, a LiNbO ₃ substrate, SrTiO ₃ , SrBi ₂ Ta ₂ O ₉ , BaTiO ₃ , KTiOPO ₄ , PLZT, or a substrate of a semiconductor material such as InP, Si, or GaAs can be used. The optical waveguide is formed by thermally diffusing Ti or the like. If necessary, it is also possible to form a ridge locally along the optical waveguide. The control electrode can be formed on the base metal layer of Au or Ti using gold plating or the like. Needless to say, various methods known in the field of optical modulators can be applied to the formation of the optical modulator of the present invention.

A feature of the present invention is that a birefringence layer is provided on at least a part of the front surface or the back surface of the substrate. As the birefringent layer, rutile (α-titanium dioxide), YVO ₄ (yttrium vanadate), or the like can be suitably used.

  As shown in FIG. 6, the end face C1 on which the input light of the substrate 1 is incident, the start position C2 of the region where the high frequency signal (RF signal) is applied to the Mach-Zehnder type optical waveguide (MZ1, MZ2), and the DC bias voltage are applied. The start position C3 of the region, the start position C4 of the output optical waveguide, and the end face C5 from which the output light of the substrate is emitted. The range of C1 to C2 is mainly an input region (IPR) where an optical waveguide for input light is arranged, the range of C2 to C3 is mainly a modulation region (RFR) where a modulation electrode is arranged, and the range of C3 to C4 is mainly The bias region (DCR) in which the DC bias electrode is disposed in the range, and the range of C4 to C5 mainly mean the output region (OPR) in which the output optical waveguide is disposed.

  The birefringence layer is disposed in the input region (IPR), modulation region (RFR), bias region (DCR), or output region (OPR) in FIG. 6 as necessary. For example, by arranging the birefringence layer BR1 in the input region, leaked light that has not entered the optical waveguide 2 at the light input portion (the coupling portion (substrate end face C1) between the substrate 1 and the optical fiber (not shown)). In addition, the leaked light that has leaked from the branch portion (20) of the optical waveguide can be emitted from the substrate 1 to the outside.

Further, by disposing the birefringent layer BR2 in the bias region (DCR), it becomes possible to emit stray light from the substrate 1 to the outside toward the multiplexing portion of the Mach-Zehnder type optical waveguide (MZ1, MZ2). .
Further, by arranging the birefringence layer BR3 in the output region (OPR), stray light entering the output optical waveguides (OP1, OP2) and the emitted light optical waveguides (R11 to R22) is emitted from the substrate 1 to the outside. It becomes possible. In particular, it is possible to suppress stray light from being mixed into the light receiving element for detecting the radiated light propagating through the radiated light optical waveguide.

  7A is a cross-sectional view taken along one-dot chain line D1-D1 'in FIG. As shown in FIG. 7A, for the birefringent layer BR1 disposed in direct contact with the surface of the substrate 1, the refractive index of the birefringent layer is lower than the refractive index of the signal light optical waveguide 2, and the substrate 1 Higher than the refractive index. This suppresses the absorption of the light wave propagating through the signal light optical waveguide 2 to the birefringent layer side, and actively stray light propagating through the substrate 1 (other than the optical waveguide 2) to the birefringent layer side. It is for sucking up.

  In some cases, the birefringence layer BR4 is disposed on the back surface of the substrate 1. This is because stray light propagating in the substrate is emitted from the back side of the substrate 1 to the outside when the thickness of the substrate is 20 μm or less. Reference sign AD in FIG. 7A indicates an adhesive layer for bonding the support substrate SP to the substrate 1 side, and the mechanical strength of the optical modulator is increased by bonding the support substrate.

  7B is a cross-sectional view taken along one-dot chain line D2-D2 'of FIG. As shown in FIG. 7B, when there are signal light optical waveguides (OP1, OP2) on the surface of the substrate 1, the birefringence layer BR3 prevents the signal light from being emitted to the outside. Therefore, the concave portions (LR1, LR2) are provided in the birefringence layer BR3 to suppress the birefringence layer (BR3) from coming into direct contact with the optical waveguide for signal light, and the signal light is emitted to the outside of the substrate 1. Is suppressed. In the recesses (LR1, LR2), a low refractive index material having a refractive index lower than that of the optical waveguide can be disposed as necessary.

  Further, as will be described later, a metal layer is arranged on a part of the birefringence layer opposite to the substrate 1 so that stray light taken into the birefringence layer does not return to the substrate 1 side again. It is also possible to do. This metal layer can also absorb stray light.

Also, as shown in FIG. 8, when a plurality of light modulation portions (MZ1, MZ2) are arranged in parallel, stray light generated in each light modulation portion is prevented from propagating to other light modulation portions. Therefore, it is preferable to form the birefringence layer BR between adjacent light modulation portions.
Further, when a plurality of light modulation portions (MZ11 to MZ22) are arranged in parallel to the embodiment of the light modulator using multiple wavelengths as shown in FIG. It is desirable to form the birefringent layer BR (for example, between MZ12 and MZ21).

  As shown in FIG. 9, the birefringence layer BR is formed except for an optical waveguide (signal light optical waveguide) through which light modulation (optical light) in which light modulation or light modulation is performed propagates. Also good. Further, as shown in FIG. 9, on the radiation optical waveguide, the birefringence is suppressed up to the region (dotted line frame) where the light receiving elements (PD1, PD2) are arranged in order to suppress the degradation of the radiation. The layer is not arranged. However, since the emitted light also becomes stray light downstream of the region where the light receiving element is disposed, it is preferable to positively remove it using a birefringence layer.

  By disposing the metal layer on the birefringent layer, it becomes possible to efficiently absorb the stray light captured by the birefringent layer. For such a metal layer, it is advantageous in manufacturing to use a part of the metal film used in the process of forming the control electrode. Further, when a part of the control electrode is used, it is necessary to prevent the electric field applied to the optical waveguide from changing (deteriorating) even when the shape of the electrode arranged on the birefringence layer is changed. For this reason, it is preferable to use a part of the DC bias electrode rather than the modulation electrode constituting the control electrode. Further, it is preferable that the birefringence layer and the metal layer be arranged in the vicinity of the optical waveguide for signal light output as much as possible. Specifically, stray light is emitted to a region where the DC bias electrode is disposed (DCR) and a region where the output optical waveguide is disposed (OPR) rather than the region where the modulation electrode is disposed (RFR) shown in FIG. A metal layer for absorption is arranged. If necessary, a metal layer can be provided on the birefringence layer BR disposed between the light modulation portions shown in FIG.

  10A and 10B show an example of the arrangement of the birefringence layers (BR, a1 to a3) and the DC bias electrode. 10B is a cross-sectional view taken along one-dot chain line D-D ′ in FIG. 10A. There is no need to dispose a metal layer in the entire region where the birefringent layer is formed. Stray light derived from the substrate by the birefringence layer (a part of the stray light propagates through the birefringence layer) is disposed on the downstream side in the propagation direction of the stray light (in particular, an optical waveguide for signal light or It is preferable to dispose a metal layer so as to prevent recombination with the radiation waveguide.

  In FIG. 10A, the right ends of the DC bias electrodes DC11 and DC12 are separated from each other. However, when both are at the same potential, the DC bias electrodes are continuously arranged along the outer peripheral shape of the birefringence layer a1. It is also possible to do. In addition, a birefringent layer is not disposed between the optical waveguide for emitted light and the optical waveguide for output light, but the birefringent layer and the metal layer are within a range not absorbing the light wave propagating through each optical waveguide. Can be placed between the two.

  FIGS. 11A and 11B show a configuration for removing radiated light that has passed through the light receiving elements (PD1, PD2) and stray light that propagates in the vicinity of the output optical waveguide. Metal layers (M1, M2) are disposed on the birefringence layer (BR). In FIG. 11B, the shape of the birefringence layer and the shape of the metal layer are the same, but the metal layer can be disposed only in a partial region of the birefringence layer.

  In addition, since the stray light is confined in the substrate and the possibility of recombination with the optical waveguide becomes higher as the thickness of the substrate having the electro-optic effect is thinner, for example, the substrate having a thickness of 20 μm or less, more preferably 10 μm or less. The present invention can be effectively applied to an optical modulator using the.

  As described above, according to the present invention, stray light propagating in the substrate is effectively removed, a plurality of light modulation portions are arranged, and even in a downsized optical modulator, stray light in the substrate is It is possible to provide an optical modulator capable of suppressing mixing in signal light and radiated light.

DESCRIPTION OF SYMBOLS 1 Substrate 2-220 Optical waveguide MZ, MZ1-MZ22 Mach-Zehnder type optical waveguide R1-R222 Radiation optical waveguide OP, OP1-OP22 Output optical waveguide RF1-RF3 Modulation electrode DC1-DC3 DC bias electrode (metal layer)
PD, PD1 to PD2 Light receiving element BR, BR1 to BR4 Birefringence layer M1 to M2 Metal layer SP Support substrate
LR1, LR2 Recessed part IPR Input area RFR Modulation area DCR Bias area OPR Output area
HR, HR1, HR2 High refractive index film AD Adhesive layer WP Wave plate RF Reflector PS Polarizing beam splitter λ _{0 to} λ ₂ input light λ ′ _0, λ o1 to λ o ₂ output light

Claims (7)

In an optical modulator comprising a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and an optical modulator having a control electrode for applying an electric field to the optical waveguide,
The control electrode has a modulation electrode for optical modulation;
The optical waveguide has a signal light optical waveguide through which a light wave modulated by the modulation electrode propagates;
A birefringence layer is disposed on at least a part of the front surface of the substrate on which the optical waveguide for signal light is formed or the back surface of the substrate.   2. The optical modulator according to claim 1, wherein when the birefringence layer is disposed on the surface of the substrate, the birefringence layer is disposed on the substrate excluding the signal light optical waveguide. Light modulator.   2. The optical modulator according to claim 1, wherein when the birefringent layer is disposed on the surface of the substrate, the refractive index of the birefringent layer is lower than the refractive index of the optical waveguide for signal light, An optical modulator having a refractive index higher than that of the optical modulator.   4. The optical modulator according to claim 1, wherein when the birefringence layer is disposed on the back surface of the substrate, the thickness of the substrate is 20 [mu] m or less.   5. The optical modulator according to claim 1, wherein a metal layer is disposed on a part of the birefringent layer opposite to the substrate.   6. The optical modulator according to claim 1, wherein the control electrode includes a bias electrode that applies a DC bias voltage to the optical waveguide, and the bias electrode is in a direction of propagation of the light wave. An optical modulator characterized in that the birefringence layer is disposed on the downstream side of the modulation electrode, and on the lower side of the bias electrode.   7. The optical modulator according to claim 1, wherein at least the first optical modulation portion and the second optical modulation portion are arranged side by side in the optical modulation portion, and the first optical modulation portion is arranged. The optical modulator, wherein the birefringence layer is formed on the substrate between the modulation portion and the second light modulation portion.

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