JP2020166050A - Light control element - Google Patents

Abstract

To provide a light control element capable of preventing unnecessary light from being mixed into an optical modulation part or output light and improving properties, such as the extinction ratio of an optical modulation signal.SOLUTION: A light control element comprises: first optical waveguides (21 and 23) formed on one surface of a first substrate (11); an irradiation part (21) radiating the first optical waveguides with light and an emission part (23) emitting the light from the first optical waveguides provided in the end of the first substrate; a second optical waveguide (22) formed on one surface of a second substrate (12); and a light modulation part modulating the light transmitting the second optical waveguide and provided in the second substrate. Translation parts (S1 and S2) of the first and second optical waveguides are optically coupled.SELECTED DRAWING: Figure 3

Description

The present invention relates to an optical control element, and more particularly to an optical control element in which a substrate having a light incident portion and a light emitting portion and a substrate having a light modulation portion are separated.

Since the optical modulators used in next-generation optical fiber communication are used for multi-value modulation, they are compact and have low optical loss, and also have an extinction ratio (ON / OFF extinction ratio) of the output optical signal. It needs to be expensive. In order to realize miniaturization of optical modulators and the like, an optical modulator having a thin plate structure has been proposed in which the overlapping efficiency of light and electric field is improved in order to reduce the driving voltage.

In the conventional light modulator having a thin plate structure, as shown in FIGS. 1 and 2, a substrate 1 such as lithium niobate (LN) is thinned to a thickness of 20 μm or less, and an optical waveguide 2 is formed on the substrate 1. ing. FIG. 1 is a plan view of an optical modulator which is an optical control element, and electrodes such as modulation electrodes are omitted for simplification. FIG. 2 is a side view of FIG. The substrate 1 to be a thin plate is joined to the holding substrate 10 via the adhesive layer 3. Lin indicates incident light, and Lout indicates emitted light.

The incident portion where the light wave is incident on the optical waveguide 2 formed on the substrate 1 and the exit portion where the light wave is emitted from the optical waveguide 2 are formed on the same substrate as the substrate 1 on which the optical modulation portion is formed. For example, stray light due to a mismatch between the mode field diameter of the optical fiber and the incident part, and synchrotron radiation generated from the optical modulator (the junction of the Mach-Zehnder type optical waveguide) when the light modulator is turned off are generated by the optical waveguide. It remains inside the thin plate-structured substrate 1 formed. Since these unnecessary lights such as stray light and synchrotron radiation are incident on the output optical fiber together with the output light emitted from the exit portion, there is a problem that the signal level in the OFF state cannot be sufficiently lowered. In addition, unnecessary light enters the optical waveguide that constitutes the optical modulation unit, causing problems such as deterioration of the modulation signal.

In order to remove stray light and synchrotron radiation from the thin substrate 1, in Patent Document 1, a high refractive index member is partially arranged on the surface of the substrate, and in Patent Document 2 and Patent Document 3, an unnecessary optical waveguide is provided. Is disclosed on the substrate and the like.

However, in an optical modulator that integrates many Mach-Zehnder-type optical waveguides, such as a nested optical waveguide that incorporates multiple Mach-Zehnder-type optical waveguides in a nested manner, unnecessary light such as stray light and synchrotron radiation is completely eliminated. It is difficult to remove. Moreover, in an optical modulator or the like that performs high-frequency modulation, there is a problem that the characteristics of the modulated signal are greatly deteriorated by mixing a small amount of unnecessary light with the signal light or output light of the optical modulation section.

Japanese Patent No. 4658658 Japanese Patent No. 6299170 Japanese Unexamined Patent Publication No. 2016-191820

The problem to be solved by the present invention is optical control that solves the above-mentioned problems, suppresses unnecessary light from being mixed into the optical modulation section and output light, and improves characteristics such as the extinction ratio of the optical modulation signal. It is to provide an element.

In order to solve the above problems, the optical control element of the present invention has the following technical features.
(1) A first optical waveguide is formed on one surface of the first substrate, and at the end of the first substrate, an incident portion for incident light on the first optical waveguide and the first optical waveguide. A second optical waveguide is formed on one surface of the second substrate, and the second substrate is photomodulated to modulate the light propagating through the second optical waveguide. It is characterized in that the first optical waveguide and the second optical waveguide are photocoupled in a translational portion.

(2) The optical control element according to (1) above is characterized in that the thickness of the second substrate is 20 μm or less.

(3) In the optical control element according to (1) or (2) above, the first substrate is a quartz-based material or lithium niobate, and the second substrate is lithium niobate. To do.

(4) The optical control element according to any one of (1) to (3) is characterized in that the first substrate and the second substrate are directly bonded to each other.

(5) In the optical control element according to any one of (1) to (4) above, in the translational portion, at least one of the end portion of the first optical waveguide or the end portion of the second optical waveguide. Is characterized in that a tapered structure is formed in which the width of the optical waveguide gradually narrows toward the tip of the optical waveguide.

(6) In the optical control element according to any one of (1) to (5) above, in the translational section, at least one of the first substrate or the second substrate has a periodic refractive index. It is characterized in that a structure is formed.

(7) In the optical control element according to any one of (1) to (6) above, an orthogonal bias electrode for optical control is provided on either the first substrate or the second substrate. And.

(8) The optical control element according to any one of (1) to (7) above, characterized in that an electrode for optical coupling is provided at the translational portion of the second substrate.

In the present invention, the first optical waveguide is formed on one surface of the first substrate, and at the end of the first substrate, an incident portion that incidents light on the first optical waveguide, and the first light An exit portion that emits light from the waveguide is provided, a second optical waveguide is formed on one surface of the second substrate, and the second substrate is a light that modulates the light propagating through the second optical waveguide. A modulation unit is provided, and in the translational portion of the first optical waveguide and the second optical waveguide, both are photocoupled, so that unnecessary light is prevented from being mixed into the optical modulation section and the output light. It is possible to provide an optical control element having improved characteristics such as an extinction ratio of an optical modulation signal.

It is a top view which shows the example of the conventional optical control element. It is a side view of the optical control element of FIG. It is a side view which shows the 1st Example which concerns on the optical control element of this invention. It is a top view of the optical control element shown in FIG. It is a top view which shows the 2nd Example which concerns on the optical control element of this invention. It is a top view which shows the 3rd Example which concerns on the optical control element of this invention. It is a top view which shows the 4th Example which concerns on the optical control element of this invention. It is a side view which shows the 5th Example which concerns on the optical control element of this invention. It is a top view which shows the 6th Example which concerns on the optical control element of this invention. It is a top view explaining the connection structure (the 1) between optical waveguides used for the optical control element of this invention. It is a top view explaining the connection structure (the 2) between optical waveguides used for the optical control element of this invention. It is a side view explaining the connection structure (the 3) between optical waveguides used for the optical control element of this invention. It is a top view explaining the connection structure (the 4) between optical waveguides used for the optical control element of this invention.

Hereinafter, the optical control element of the present invention will be described in detail with reference to suitable examples.
In the optical control element of the present invention, as shown in FIGS. 3 and 4, a first optical waveguide (21, 23) is formed on one surface of a first substrate (11), and a first optical waveguide (21, 23) is formed at an end of the first substrate. An incident portion (21) for injecting light into the first optical waveguide and an emitting portion (23) for emitting light from the first optical waveguide are provided, and a second substrate (12) is provided with a first surface. The second optical waveguide (22) is formed, and the second substrate is provided with an optical modulation unit that modulates the light propagating through the second optical waveguide, and the first optical waveguide and the second optical waveguide are provided. In the translational portions (S1 and S2) with the waveguide, both are optically coupled.

As shown in FIG. 3, the optical control element of the present invention includes a first substrate (11) having an incident portion (21) and an emitted portion (23) of light, and an optical modulation unit (Mach-Zehnder type optical light in FIG. 4). The second substrate (12) having the two branched waveguide portions of the waveguide is separated. Then, the first substrate (11) and the second substrate (12) are directly bonded to each other without passing through the adhesive layer 3 such as resin as shown in FIG. 2, and the optical bond between the substrates (black in FIG. 3). (See the part where the fill arrow is displayed) is used.

Unnecessary light due to the mismatch between the optical fiber and the mode field diameter of the incident portion generated in the incident portion (21) of the first substrate (11) propagates in the first substrate, so that the second substrate has the optical modulation portion. It does not propagate to (12), and it is suppressed that unnecessary light is mixed into the optical modulation section.

Further, the unnecessary light generated at the incident portion of the first substrate (11) spreads inside the substrate (11) (in the thickness direction of the substrate) and is more than the unnecessary light propagating in the substrate 1 (thin plate) of FIG. , It is hard to be mixed in the output light.

Further, the unnecessary light remaining inside the substrate in the modulation OFF state generated in the optical modulation portion of the second substrate (12) is not coupled to the emission portion (23) of the first substrate (11). ..

A top view of FIG. 3 is shown in FIG. 4 for comparison with FIG. Optical waveguides (21, 23) formed on the first substrate (11) and optical waveguides (22) formed on the second substrate (12) in the vicinity of the incident portion (21) and the exit portion (23). Is overlapped with each other in the translational portions (S1 and S2), and is photocoupled between the substrates.

As the first substrate (11), a plane lightwave circuit (PLC; Planar Lightwave Circuit) using a quartz-based material such as a glass substrate and an LN substrate made of a lithium niobate material can be used.

Examples of the second substrate (12) include an LN substrate made of a lithium niobate (LN) material having an electro-optical effect. Examples of the LN substrate include magnesium oxide-doped (MgO-doped) LN having high photodamage resistance, in addition to LN having a congluent composition. It is also possible to adjust the refractive index by doping the LN substrate with metal ions by thermal diffusion or ion implantation.

The thickness of the second substrate (12) is set to 20 μm or less, more preferably 10 μm or less in order to match the speed of the microwave and the light wave of the modulated signal. Further, it is more preferable to dispose a material having a lower dielectric constant than that of the second substrate on the portion of the first substrate located on the lower surface side of the optical modulation portion of the second substrate (12).

In the translation section (S1, S2), when the propagation distance between the waveguides arranged in close proximity satisfies the perfect bond length, the evanescent coupling of the photoelectric magnetic field causes the power to be completely transferred from one waveguide to the other waveguide. Transition to. Therefore, from the optical coupling from the optical waveguide (21) formed on the first substrate to the optical waveguide (22) formed on the second substrate, and from the optical waveguide (22) formed on the second substrate. Optical coupling to the optical waveguide (23) formed on the first substrate becomes possible.

When a PLC made of a glass material is used for the first substrate (11), the refractive index of the PLC is 1.5 with respect to the refractive index of 2.2 of the LN, and the difference in the refractive indexes between the two is large. Propagation constants are also very different. Therefore, even if the optical waveguides are simply arranged in close proximity, no optical coupling occurs. Therefore, by forming a layer made of a material having a refractive index higher than that of the optical waveguide of the first substrate, the propagation constant of the optical waveguide formed on the second substrate is propagated by the optical waveguide formed on the second substrate. By approaching a constant, optical coupling between optical waveguides becomes possible.

On the other hand, when LN is used for the first substrate (11), the propagation constant of the optical waveguide formed on the first substrate is close to the propagation constant of the optical waveguide formed on the second substrate, so that the material has a high refractive index. Optical coupling between optical waveguides is possible by arranging them in close proximity without forming a layer made of the same.

FIG. 5 shows a second embodiment of the optical control element of the present invention, in which a modulation electrode (CE) for optical control, an orthogonal bias electrode (BE), and an optical waveguide (22) are mounted on a second substrate (12). (Branch part and combine part of Mach-Zehnder type optical waveguide) are formed. Since the optical coupling portions (S1 and S2) are limited to the light incident portion (21) and the light emitting portion (23), the coupling efficiency in the optical coupling portion is between both arms (branched waveguide) of the Mach-Zehnder type optical waveguide. It has the advantage of not affecting the difference in light intensity. On the other hand, in order to form the optical branching portion and the optical modulation portion on the second substrate (12) having a thin plate structure, a highly accurate process technique is required.

FIG. 6 shows a third embodiment of the optical control element of the present invention, in which the first substrate and the second substrate (12) are formed by dividing the branch of the optical waveguide. A modulation electrode (CE) for optical control, an orthogonal bias electrode (BE), and a part of a branch portion of the optical waveguide are formed on the second substrate (12). There is an advantage that a part of the optical branching portion can be formed on a first substrate (PLC or the like) capable of producing an optical waveguide with high accuracy.

FIG. 7 shows a fourth embodiment according to the optical control element of the present invention, in which all the branch portions of the optical waveguide and the orthogonal bias electrode (BE) are formed on the first substrate, and the second substrate (12) is formed. This is a configuration in which a modulation electrode (CE) for optical control is formed. It is an advantage that all the optical branching portions are formed on a first substrate (PLC or the like) that can be manufactured with high accuracy. However, the orthogonal bias electrode (BE) needs to be driven by a thermo-optical effect. Since the second substrate having a thin plate structure has only a linear optical waveguide (22) and a modulation electrode (CE) for optical control, the difficulty of the required process technique is higher than that of the configuration shown in FIG. It also has the advantage of lowering.

FIG. 8 is a side view illustrating a fifth embodiment of the optical control element of the present invention. The first substrate (11) and the second substrate (12) do not have to be the same size, and the second substrate can be made into a small substrate for a more accurate process such as reduced projection exposure and electron beam lithography. The technology can be applied. Further, the number of the second substrate does not have to be one, and it is possible to arrange a plurality of second substrates side by side on the first substrate.

FIG. 9 shows a sixth embodiment according to the optical control element of the present invention, in which optical coupling is performed in order to perform optical coupling in the translational portion of the optical waveguide of the first substrate and the second substrate (12) with high efficiency. An example of forming an electrode (OE) for light is shown. By utilizing the electro-optical effect, it is possible to adjust the propagation constant of the optical waveguide, so that optical coupling can be performed with high efficiency.

FIG. 10 is a diagram for explaining the connection structure (No. 1) between the optical waveguides used in the optical control element of the present invention, and in order to reduce the drive voltage, the second substrate (12) has micron-level fineness. It is a top view when a simple optical waveguide (WG2) is formed. For low loss optical coupling, the optical waveguide (WG2) formed on the second substrate has a tapered structure (T2) at the optical coupling portion.

FIG. 11 is a diagram illustrating another connection structure (No. 2) between optical waveguides. Due to the low loss optical coupling from the first optical waveguide (WG1) formed on the first substrate (11) to the second optical waveguide (WG2) formed on the second substrate (12). The configuration in which the first optical waveguide (WG1) formed on the first substrate is formed into a tapered structure T1 at the optical coupling portion is shown. The tapered structure facilitates mode transition of the optical waveguide formed on each substrate.

FIG. 12 is a diagram illustrating another connection structure (No. 3) between optical waveguides. Low-loss optical coupling from the first optical waveguide (21) formed on the first substrate (11) to the second optical waveguide (22) formed on the second substrate (12), or a second. Due to low loss optical coupling from the second optical waveguide (22) formed on the substrate (12) to the first optical waveguide (23) formed on the first substrate (11). A configuration is shown in which a high refractive index layer (4) made of a material having a higher refractive index than that of the optical waveguides (21, 23) of the first substrate is formed. By forming a high refractive index layer between the first optical waveguide and the second optical waveguide, the difference in propagation constant between the two optical waveguides is relaxed, so that optical coupling between the optical waveguides becomes possible.

FIG. 13 is a diagram for explaining another connection structure (No. 4) of the optical waveguide feeling. For low loss and low wavelength characteristic optical coupling from the first optical waveguide (WG1) to the second optical waveguide (WG2) and from the second optical waveguide (WG2) to the first optical waveguide (WG1). , A structure having a photonic crystal structure having a periodic refractive index structure in the optical coupling portion is shown. By using the photonic bandgap, which is a forbidden band of light, optical coupling with low loss and excellent wavelength characteristics becomes possible. The photonic crystal structure can be formed on either the first substrate or the second substrate in the dotted line region A of FIG.

As described above, according to the present invention, it is possible to provide an optical control element that suppresses unnecessary light from being mixed into an optical modulation unit or output light and has improved characteristics such as an extinction ratio of an optical modulation signal. It becomes.

11 1st substrate 12 2nd substrate 21 and 23 1st optical waveguide 22 2nd optical waveguide S1 to S6 Translation part (optical coupling part)
CE Modulation Electrode BE Orthogonal Bias Electrode OE Optical Coupling Electrode

Claims (8)

A first optical waveguide is formed on one surface of the first substrate,
At the end of the first substrate, an incident portion for incident light on the first optical waveguide and an exit portion for emitting light from the first optical waveguide are provided.
A second optical waveguide is formed on one surface of the second substrate,
The second substrate is provided with an optical modulation section that modulates the light propagating in the second optical waveguide.
An optical control element characterized in that, in a translational portion between the first optical waveguide and the second optical waveguide, both are optically coupled. The optical control element according to claim 1, wherein the thickness of the second substrate is 20 μm or less. The optical control element according to claim 1 or 2, wherein the first substrate is a quartz-based material or lithium niobate, and the second substrate is lithium niobate. The optical control element according to any one of claims 1 to 3, wherein the first substrate and the second substrate are directly bonded to each other. In the optical control element according to any one of claims 1 to 4, in the translational portion, the tip of the optical waveguide is attached to at least one of the end portion of the first optical waveguide or the end portion of the second optical waveguide. An optical control element characterized in that a tapered structure is formed in which the width of the optical waveguide gradually narrows toward. In the optical control element according to any one of claims 1 to 5, in the translational portion, a structure having a periodic refractive index is formed on at least one of the first substrate and the second substrate. An optical control element characterized by this. The optical control element according to any one of claims 1 to 6, wherein an orthogonal bias electrode for optical control is provided on either the first substrate or the second substrate. The optical control element according to any one of claims 1 to 7, wherein an electrode for optical coupling is provided in the translational portion of the second substrate.

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