JPH10142568A - Waveguide type optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to decrease driving voltage by partly removing a buffer layer from the places where waveguides do not exist and moving the ends of the buffer layer and electrodes to the positions deviating from the ends of the waveguides. SOLUTION: The surface of a crystal substrate 1 is provided with the two Y-shaped branch type waveguides functioning as the incident side waveguide 2 and the exist side waveguide 3 and phase shift parts 4. The buffer layer 5 is formed exclusive of the places where waveguide passages or the electrodes do not exist and the ends of the buffer layer 5 and the electrodes are deviated in the central direction of the waveguides. A plane waveguide type electrode structure of the shape similar to the shape of the buffer layer 5 is formed of signal electrodes 6 and two grounding electrodes 7 on the buffer layer 5. Further, connectors and packages 8 for the connectors are mounted at the device. Microwaves are applied via the connectors to the signal electrodes 6. Fibers and fiber packages 9 are mounted on both sides of the device 6 to enable the incidence and exit of the light on and from the device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical modulator and a waveguide type optical switch used in various optical systems such as high-speed optical communication, optical switching network, optical information processing, optical image processing and the like.

[0002]

2. Description of the Related Art Waveguide optical modulators and optical switches are very important elements in realizing various optical systems such as high-speed optical communication, optical switching networks, optical information processing and optical image processing. It is. Waveguide optical modulators are formed on several types of substrates by various manufacturing methods. However, most research on waveguide optical devices focuses on LiNbO ₃ and semiconductors (eg, GaAs).
System). The method of in-diffusion of titanium into LiNbO ₃ provides a convenient and relatively simple method for forming a low-loss strip waveguide with good electro-optical properties on a substrate. Important parameters of the waveguide type optical modulator are drive power (or drive voltage), modulation bandwidth, characteristic impedance, and insertion loss.
Among these parameters, the modulation bandwidth and the drive voltage had a trade-off relationship. That is, it has been difficult to simultaneously satisfy the requirements for a wide modulation bandwidth and a low driving voltage. Therefore, research on waveguide type optical modulators
We focus on optimizing this trade-off relationship.

[0003] The bandwidth of a waveguide modulator is limited by microwave attenuation and speed mismatch between light waves and microwaves. In addition to the above, mainly the type, material and arrangement of electrodes and the dielectric constant of a substrate Depends on. Traveling wave electrodes are widely used for broadband applications. The concept is to configure the electrode as an extension of the drive transmission line. for that reason,
The characteristic impedance of the electrodes must be the same as the characteristic impedance of the power supply and the cable. The modulation speed in this case is limited by the difference in transit time (or phase speed or effective refractive index) of the light wave and the microwave.
The traveling-wave electrode structures that can be used include: 1) Asymmetric Strip Line
(Hereinafter abbreviated as ASL)) or an asymmetric planar strip (Asymmetric Coplanar S)
and a 2) planar waveguide (hereinafter, abbreviated as ACPS) type electrode structure.
There are two types of es (hereinafter abbreviated as CPW) type electrode structures.

[0004] Velocity mismatch and microwave attenuation can be reduced by optimizing buffer layer parameters and electrode parameters. Similarly, the driving voltage in a trade-off relationship with the bandwidth is the overlap integral of radio waves (ie, microwaves) and light waves, that is, the overlap integral, and further, the thickness of the buffer layer, the signal electrode and It is related to the distance between the ground electrodes and the length of the signal electrode / ground electrode. In order to increase the bandwidth while maintaining the speed matching and the characteristic impedance close to 50Ω, it is necessary to increase the thickness of the buffer layer and to provide a thick electrode having a small electrode width and a large gap between the electrodes. Further, the waveguide parameters are fixed with respect to the required bandwidth / characteristic impedance condition. However, this is undesirable because it increases the drive voltage of the device.

The present inventors have also realized an optical modulator which uses a thick buffer layer and a thick electrode structure, and has a wide band and a relatively low driving voltage. This is "A wide
-Band Ti: LiNbO ₃ optical m
odulator with the convention
al coplanar waveguide type
e Electrode (Broadband TI: LiNbO ₃ Optical Modulator with Conventional Planar Waveguide Electrodes) ”(IEEE Photonics Technology)
gy Letters, Vol. 4, No. 9 (1992),
1020 to 1022). The present inventors have achieved a bandwidth of 20 GHz and a driving voltage of 5 V for an electrode length of 2.5 cm.

FIG. 5A shows an optical waveguide structure (Mach-Zehnder interferometer) and a Mach-Zehnder structure having a CPW electrode structure.
2 shows a basic configuration of a Zehnder interferometer optical modulator (of the above embodiment). FIG. 5B is a cross-sectional view along the line AA ′. Waveguides 2, 3, and 4 are formed on a crystal substrate (such as a lithium niobate crystal) 1 having an electro-optic effect by forming a titanium metal film strip and performing thermal diffusion into the crystal. I have. The optical waveguide structure includes two Y-branch waveguides (both sides on the incident side and the exit side) 2 and 3
And a phase shift unit 4. It is covered with a buffer layer (SiO ₂ dielectric layer) 5 having a thickness of 1 μm. The buffer layer is uniformly coated on the optical waveguide structure. A planar waveguide (CPW) type electrode structure including a signal electrode and two ground electrodes is formed thereon. This electrode structure has a size of 2.5 cm in length, 7 μm in width, and 12 μm in thickness. The distance between the signal electrode and the ground electrode is 15 μm. According to this structure, the thickness of the buffer layer, the electrode width (waveguide width), the distance between the electrodes, and the like are fixed, so that the overlap integral between the light wave and the radio wave is fixed, and the driving voltage is limited.

One method for reducing the driving voltage is to use a ridge-type waveguide structure, which is disclosed in Japanese Patent Application Laid-Open No. Hei 6-300995. In this patent (FIGS. 1 and 2), an optical waveguide structure including a Y-branch waveguide on an incident side and an exit side and a phase shift unit includes:
It has a ridge structure. Except for the region of the phase shift unit on the interferometer arm, the other region (that is, the region between the interferometer arms) is covered with the buffer layer, and the CPW electrode structure is formed on the buffer layer. Although this structure is required to have a low driving voltage, it uses a horizontal electric field (instead of a normal vertical electric field), and the overall driving voltage is increased about three times as compared with using a vertical electric field. I do. This has been described in several papers on optical modulators, for example, "Waveguide Electro."
Optical Modulators (Waveguide Electro-Optic Modulator) "(IEEE Transactions o
nMicrowave Theory and Tec
hniques, Vol. MTT-30, No. 8 (1
982), pages 1121 to 1137, and especially FIG. 3) on page 1123. Further, manufacturing a ridge-type waveguide involves a complicated manufacturing process. Therefore, to obtain an optical modulator with a wide bandwidth and a low driving voltage, the configuration for a vertical electric field commonly used in almost all known documents is almost useless for the above-described configuration using a horizontal electric field. . Therefore, there is a strong demand for a new structure capable of reducing the driving voltage.

[0008]

According to the present invention, by allowing the overlap integral to be increased, the buffer layer parameters and electrode parameters, particularly the buffer layer thickness (ie, in practice, the overlap integral) are increased. ) Solves the above problem that the driving voltage is limited.

In a waveguide type optical modulator, electrodes are arranged on a waveguide, and an electric field (either a vertical electric field or a horizontal electric field) is applied through the electrodes. Due to the linear photoelectric effect (Pockels effect), the refractive index profile of the waveguide changes in proportion to the applied voltage. As a result of this change in refractive index,
A phase shift is induced electro-optically, and modulation is effected as a result of this phase shift.

In general, the change Δn in the refractive index induced electro-optically is expressed by equation (as a function of the applied voltage V).

[0011]

(Equation 1) Here, _ne is the extraordinary refractive index of the substrate crystal, r ₃₃ is the electro-optic coefficient of the substrate crystal (in the case of using a vertical electric field, the substrate crystal by z-cut), E (x, y) is the applied electric field, V
Is the applied voltage, G is the distance between the electrodes, Γ is the applied electric field and the optical mode field (hereinafter simply referred to as the optical field)
This is the overlap integral with. The value of the overlap integral depends on the distance between the electrodes, the waveguide optical mode profile, the electric field profile, and the thickness of the buffer layer. The value of Γ is between 0 and 1, and is given by Expression 2.

[0012]

(Equation 2) Where Φ (x, y) is a two-dimensional optical field,
E (x, y) is a two-dimensional electric field. Since the applied electric field profile and the optical field profile (ie, the mode profile) are different, the overlap integral Γ represents the amount of overlap between these two mode profiles. In order to decrease the voltage, the value of Γ needs to be increased to be as close as possible to the theoretical limit of 1, but the achievable values are the thickness of the buffer layer, the width of the electrode, The range was 0.3 to 0.6 depending on parameters such as the interval.

The induced total phase shift amount Δβ over the interaction length L (ie, the length of the electrode) is given by equation (3).

[0014]

(Equation 3) Here, λ is the operating wavelength. Switching operation (that is, on-off operation) is possible by setting the induced phase shift amount to zero (on state) or π (off state).

Here, when the voltage is 0 volt,
No phase shift occurs and the modulator / switch is turned on. On the other hand, when voltage V is applied, a phase shift of π radians occurs, and the modulator / switch is turned off.

Instead of ΔβL, replace π, and (3)
When the equation is modified, the product VL of the voltage and the length is expressed by Equation 4.

[0017]

(Equation 4) V _π (= V) is an applied voltage corresponding to the phase shift π, and is also called a switching voltage.

It is an object of the present invention to improve the overlap integral Γ. The optical field is near the center of the waveguide (the actual position, ie the deviation from the center, is obtained by the parameters of the waveguide design and manufacturing).
And the electric field is concentrated at the end of the electrode. In the embodiment shown in FIG. 5, the value of Δ is small. This is because the maximum point of the electric field intensity is different from the maximum point of the optical field intensity.

[0019]

According to the present invention, a buffer layer is partially removed from a place where no waveguide exists (that is, between waveguides) and a position shifted from an end of the waveguide. By moving the buffer layer and the electrode structure to the above, a waveguide type optical device in which the maximum point of the optical field intensity is matched with the maximum point of the electric field intensity can be obtained.
In this case, the buffer layer is always used on the waveguide from the viewpoint of reducing the TM mode loss. When electrodes are directly arranged on the waveguide without providing a buffer layer between the waveguides,
The TM mode of the light wave is lost. Thus, removing the buffer layer where there is no waveguide or electrode structure helps to increase the overlap integral without adversely affecting the TM mode loss. At the same time, shifting the ends of the buffer layer and the electrode to a position offset from the end of the waveguide (ie, toward the center of the waveguide) further increases the overlap integral. Waveguide manufacturing parameters, buffer layer parameters, and electrode parameters should not change the velocity matching state and microwave attenuation state.
Optimized by proper design.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIG. 1 shows the structure of a basic waveguide type optical device according to the present invention. FIG. 1A is a plan view of a waveguide type optical device according to the first embodiment of the present invention, and FIG.
It is sectional drawing which follows the AA 'plane of (a).

A crystal substrate having an electro-optic effect (for example,
Waveguides 2, 3, 4 on lithium niobate crystal 1)
Formed a titanium metal film strip having a width of 5 to 20 μm and a thickness of 500 to 1200 angstroms,
It is formed by in-diffusion in the crystal at 100 ° C. for 5 to 12 hours. Two Y-branch waveguides functioning as the input side waveguide 2 and the output side waveguide 3 and a phase shift unit 4 are provided. It is covered with a buffer layer 5 (dielectric layer having a dielectric constant of 1.2 to 40) having a thickness (T) of 1 to 10 μm. The buffer layer 5 is formed except for a portion where the waveguide or the electrode does not exist (that is, between the electrodes), and the ends of the buffer layer and the electrode are shifted toward the center of the waveguide. On the buffer layer 5, a planar waveguide (CPW) type electrode structure having the same shape as the buffer layer is formed. This electrode structure is composed of a signal electrode 6 and two ground electrodes 7, and the signal electrode has a width (W) of 5 to 20 μm and a length (L).
It has a size of 10 to 70 mm and a thickness of 3 to 40 μm, while the ground electrode has a width of 100 to 9000 μm and a length of 10 to 7 μm.
It has a size of 0 mm and a thickness of 3 to 40 μm. The distance G between the signal electrode 6 and the ground electrode 7 is 1 / W / G.
The distance is selected to be 0.1 (that is, G = 5 to 200 μm).

Further, a connector and a connector package 8 are attached to the device. Microwaves (radio waves) are applied to the signal electrodes 6 via the connector. Fibers and the fiber package 9 are attached to both sides of the device to enable light to enter and exit the device.

Second Embodiment FIG. 2 shows the structure of a basic waveguide type optical device according to the present invention. FIG. 2A is a plan view of a waveguide type optical device according to a second embodiment of the present invention, and FIG.
It is sectional drawing which follows the AA 'plane of (a).

A crystal substrate having an electro-optic effect (for example,
Waveguides 2, 3, 4 on lithium niobate crystal 1)
Formed a titanium metal film strip having a width of 5 to 20 μm and a thickness of 500 to 1200 angstroms,
It is formed by in-diffusion in the crystal at 100 ° C. for 5 to 12 hours. Two Y-branch waveguides functioning as the input side waveguide 2 and the output side waveguide 3 and a phase shift unit 4 are provided. It is covered with a buffer layer 5 (dielectric layer having a dielectric constant of 1.2 to 40) having a thickness (T) of 1 to 10 μm. The buffer layer 5 is formed except for a portion where the waveguide or the electrode does not exist (that is, between the electrodes), and the ends of the buffer layer and the electrode are shifted toward the center of the waveguide. Further, the thickness of the buffer layer changes stepwise so that a portion thereof is thinner than the rest.
On the buffer layer 5, a planar waveguide (CPW) type electrode structure having the same shape as the lower buffer layer is formed. This electrode structure is composed of a signal electrode 6 and two ground electrodes 7, and the signal electrode has a width (W) of 5 to 20 μm and a length (L) of 10 to 70 m.
m, and a thickness of 3 to 40 μm, while the ground electrode has a size of 100 to 9000 μm in width, 10 to 70 mm in length, and 3 to 40 μm in thickness. The distance G between the signal electrode 6 and the ground electrode 7 is selected such that the value of W / G is 1 to 0.1 (that is, G = 5 to 200 μm).

Further, a connector and a connector package 8 are attached to the device. Microwaves (radio waves) are applied to the signal electrodes 6 via the connector. Fibers and the fiber package 9 are attached to both sides of the device to enable light to enter and exit the device.

Third Embodiment FIG. 3 shows the structure of a basic waveguide type optical device according to the present invention. FIG. 3A is a plan view of a waveguide type optical device according to a third embodiment of the present invention, and FIG.
It is sectional drawing which follows the AA 'plane of (a).

A crystal substrate having an electro-optic effect (for example,
Waveguides 2, 3, 4 on lithium niobate crystal 1)
Formed a titanium metal film strip having a width of 5 to 20 μm and a thickness of 500 to 1200 angstroms,
It is formed by in-diffusion in the crystal at 100 ° C. for 5 to 12 hours. Two Y-branch waveguides functioning as the input side waveguide 2 and the output side waveguide 3 and a phase shift unit 4 are provided. It is covered with a buffer layer 5 (dielectric layer having a dielectric constant of 1.2 to 40) having a thickness (T) of 1 to 10 μm. The buffer layer 5 is formed except for a portion where the waveguide or the electrode does not exist (that is, between the electrodes), and the ends of the buffer layer and the electrode are shifted toward the center of the waveguide. On the buffer layer 5, an asymmetric planar strip (ACPS) type (or asymmetric stripline (ASL)) type electrode structure having the same shape as the buffer layer is formed. It has a width (W) of 5 to 20 μm and a length (L) of 10
Signal electrode 6 having a thickness of 3 to 40 μm and a width of 10
0-9000 μm, length 10-70 mm, thickness 3-40
It consists of a single ground electrode 7 of μm. The distance G between the signal electrode 6 and the ground electrode 7 is such that the value of W / G is 1 to 0.1 (that is, G = 5 to 200 μm). Next, the connector and the connector package 8 are attached to the device. Microwaves (radio waves) are applied to the signal electrodes 6 via the connector. On each side of the device is attached a fiber and its fiber package 9,
Light is allowed to enter and exit the device.

Fourth Embodiment FIG. 4 shows the structure of a basic waveguide type optical device according to the present invention. FIG. 4A is a plan view of a waveguide type optical device according to a fourth embodiment of the present invention, and FIG.
It is sectional drawing which follows the AA 'plane of (a).

A crystal substrate having an electro-optic effect (for example,
Waveguides 2, 3, 4 on lithium niobate crystal 1)
Formed a titanium metal film strip having a width of 5 to 20 μm and a thickness of 500 to 1200 angstroms,
It is formed by in-diffusion in the crystal at 100 ° C. for 5 to 12 hours. Two Y-branch waveguides functioning as the input side waveguide 2 and the output side waveguide 3 and a phase shift unit 4 are provided. It is covered with a buffer layer 5 (dielectric layer having a dielectric constant of 1.2 to 40) having a thickness (T) of 1 to 10 μm. The buffer layer 5 is formed except for a portion where the waveguide or the electrode does not exist (that is, between the electrodes), and the ends of the buffer layer and the electrode are shifted toward the center of the waveguide. Further, the thickness of the buffer layer changes stepwise so that a portion thereof is thinner than the rest.
On the buffer layer 5, an asymmetric planar strip (ACPS) type (or asymmetric stripline (ASL)) type electrode structure having the same shape as the buffer layer below is formed. This is a width (W) of 5 to 20 μm and a length (L) of 10 to 70 m
m, a signal electrode 6 having a thickness of 3 to 40 μm and a width of 100 to 90
It is composed of a single ground electrode 7 having a thickness of 10 μm, a length of 10 to 70 mm, and a thickness of 3 to 40 μm. The distance G between the signal electrode 6 and the ground electrode 7 is such that the value of W / G is 1 to 0.1 (that is, G = 5).
200 μm). Next, the connector and the connector package 8 are attached to the device. Microwaves (radio waves) are applied to the signal electrodes 6 via the connector. Fibers and their fiber packages 9 are attached to both sides of the device to allow light to enter and exit the device.

[0030]

According to the present invention, the buffer layer is partially removed from a place where no waveguide exists, and the buffer layer and the end of the electrode are moved to a position shifted from the end of the waveguide. The drive voltage can be reduced.

[Brief description of the drawings]

FIG. 1A is a plan view showing a basic configuration of a waveguide type optical device according to a first embodiment of the present invention. FIG. 2B is a sectional view taken along line AA ′ of FIG.

FIG. 2A is a plan view showing a basic configuration of a waveguide type optical device according to a second embodiment of the present invention. FIG. 2B is a sectional view taken along the line AA ′ in FIG.

FIG. 3A is a plan view showing a basic configuration of a waveguide type optical device according to a third embodiment of the present invention. FIG. 3B is a sectional view taken along line AA ′ in FIG.

FIG. 4A is a plan view showing a basic configuration of a waveguide type optical device according to a fourth embodiment of the present invention. FIG. 4B is a sectional view taken along line AA ′ of FIG.

FIG. 5A is a plan view showing a basic configuration of a conventional waveguide type optical device. FIG. 5B is a sectional view taken along the line AA ′ in FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 electro-optic crystal 2 incidence-side Y-shaped waveguide 3 emission-side Y-shaped waveguide 4 phase shifter 5 buffer layer 6 signal electrode 7 ground electrode 8 connector and connector package / mount 9 fiber / fiber package

Claims (5)

[Claims] 1. A crystal substrate having an electro-optic effect, at least one waveguide provided on the crystal substrate, a buffer layer formed on the crystal substrate, and an electrode formed on the buffer layer. Wherein the end of the buffer layer is shifted from the end of the waveguide. 2. A crystal substrate having an electro-optic effect, at least one waveguide provided on the crystal substrate, a buffer layer formed on the crystal substrate, one signal electrode, and surrounding the signal electrode. An electrode group consisting of two ground electrodes, a connector and a package / mount for the connector,
A fiber and a fiber package attached to both ends of the optical waveguide, wherein the buffer layer is selectively formed except at a place where the waveguide or the electrode does not exist (that is, between the electrodes); And an end of the electrode is shifted toward a center of the waveguide. 3. A crystal substrate having an electro-optic effect, at least one waveguide provided on the crystal substrate, a buffer layer formed on the crystal substrate, one signal electrode, and surrounding the signal electrode. An electrode group consisting of two ground electrodes, a connector and a package / mount for the connector,
A fiber and a fiber package attached to both ends of the optical waveguide, wherein the buffer layer is selectively formed except at a place where the waveguide or the electrode is not present (that is, between the electrodes), and The buffer layer and the end of the electrode are offset from each other in the direction of the center of the waveguide, and the thickness of the buffer layer changes stepwise so that a part thereof is thinner than the remaining part. A waveguide type optical device, comprising: 4. A crystal substrate having an electro-optic effect, at least one waveguide provided on the crystal substrate, a buffer layer formed on the crystal substrate, one signal electrode and one ground electrode. An electrode group, a connector and a package / mount for the connector, and a fiber and a fiber package attached to both ends of the optical waveguide, wherein the buffer layer is provided in a place where the waveguide or the electrode is not present (that is, the electrode). (B), and the end portions of the buffer layer and the electrode are offset from each other in the direction of the center of the waveguide. 5. A crystal substrate having an electro-optic effect, at least one waveguide provided on the crystal substrate, a buffer layer formed on the crystal substrate, one signal electrode and one ground electrode. An electrode group, a connector and a package / mount for the connector, and a fiber and a fiber package attached to both ends of the optical waveguide, wherein the buffer layer is provided in a place where the waveguide or the electrode is not present (that is, the electrode). And the end portions of the buffer layer and the electrode are shifted in the center direction of the waveguide, and the thickness of the buffer layer is partially reduced. A waveguide type optical device characterized by being changed stepwise so as to be thinner than the remaining part.