A tunable optical waveguide based on the electro-optic effect of ferroelectric domains

Abstract

The present invention discloses a tunable optical waveguide based on the electro-optic effect of ferroelectric domains. The waveguide comprises a domain structure, a first domain wall, a second domain wall, a positive electrode, a negative electrode, and an insulating layer. The domain structure is divided into positive and negative domains, with the negative domain sandwiched in the middle and a positive domain disposed on each side. A first domain wall and a second domain wall are formed at the junction of the positive and negative domains, respectively. The first and second domain walls are covered with metal electrodes, which are electrically connected to the first and second domain walls. By applying a voltage to the positive and negative electrodes, an electric field is applied to the negative domain, generating total reflection in the negative domain to confine the laser. When the voltage is very low, the waveguide disappears, thereby achieving modulation of laser transmission in the waveguide. The present invention utilizes the conductive properties of the domain walls of the ferroelectric crystal to perform electric field modulation, generate a photoelectric effect, and increase the crystal refractive index of the inversion between the domain walls, thereby forming a planar or strip waveguide. The preparation process is simple and the cost is low, and the waveguide achieves modulation of laser transmission.

Description

Technical Field

[0001] The present invention relates to an adjustable optical waveguide, in particular to an adjustable optical waveguide based on ferroelectric domain electro-optic effect. Background Art

[0002] The electro-optic effect can be manipulated by an electric field, producing a refractive index change through the Pockels effect or the Pockels cell effect. Although the refractive index change induced by the electric field is small, it can be precisely displayed and measured using methods such as interferometry, leading to numerous important applications in optical communications, distance measurement, displays, information processing, and sensors. Most commonly used electro-optic crystals are ferroelectric crystals, such as KDP, KDA, KTP, LiNbO3, and LiTaO3. To produce a sufficiently large electro-optic effect, conventional bulk crystals require a high modulation voltage, typically ranging from several thousand volts to tens of thousands of volts. To reduce the modulation voltage and increase the modulation speed, waveguides can be fabricated on the crystal, such as buried proton exchange waveguides, titanium diffused waveguides, or ridge waveguides. By shortening the distance between the positive and negative electrodes, the electric field strength is increased, reducing the voltage, and the modulation voltage can be lowered to below a few volts. However, the optical transmission area and power of strip waveguides are limited by their cross-sectional area. Applying voltage to planar waveguides requires embedding large conductive electrodes within the material, making the fabrication process complex and costly.

[0003] In nonlinear optics, one form of phase matching in ferroelectric crystals is quasi-phase matching (QPM). This phase matching utilizes the inversion characteristics of ferroelectric domains within the ferroelectric to perform periodic ferroelectric domain reversals, thereby compensating for the phase mismatch generated by the nonlinear coupling process. Creating planar or strip waveguides with simple fabrication processes, low costs, and adjustable laser transmission is an urgent challenge. Summary of the Invention

[0004] Objective of the invention: The objective of the present invention is to provide a tunable optical waveguide based on the electro-optic effect of ferroelectric domains.

[0005] Technical solution: The present invention includes a domain structure, a first domain wall, a second domain wall, a positive electrode, a negative electrode and an insulating layer. The domain structure is divided into positive domains and negative domains. The negative domain is sandwiched in the middle, and a positive domain is set on each side. The first domain wall and the second domain wall are formed at the junction of the positive domain and the negative domain, respectively. The first domain wall and the second domain wall are covered with metal electrodes, and the metal electrodes are conductively connected to the first domain wall and the second domain wall. By applying voltage to the positive electrode and the negative electrode, an electric field is applied to the negative domain, and an electro-optical effect is generated under the action of the electric field. The positive and negative voltages are set in the crystal axis to increase the refractive index of the electro-optical effect, which is greater than the refractive index of the positive domains on both sides. Total reflection is generated in the negative domain to confine the laser. When the voltage is zero or the voltage is very small, the waveguide disappears, realizing the modulation of the laser transmission in the waveguide.

[0006] Furthermore, the positive domain and the negative domain are both ferroelectric crystals, and the ferroelectric crystals include any one of lithium niobate, lithium tantalate and potassium titanyl phosphate.

[0007] Furthermore, the domain structure is domain-inverted through a domain inversion process, the inverted portion is called a negative domain, and the uninverted portion is called a positive domain.

[0008] Furthermore, the covered area under the positive electrode is a positive domain, and the uncovered area is a negative domain.

[0009] Furthermore, the domain inversion process adopts any one of a room temperature electric field polarization method, a growth stripe method and a poly-domain method.

[0010] Furthermore, the structural size range of the domain inversion is: domain inversion thickness 0.1μm≤D≤100μm; waveguide width 1μm≤H≤10mm; positive and negative electrode thickness 1nm≤T≤100μm.

[0011] Furthermore, the positive and negative voltages range from 1V to 1000V.

[0012] Furthermore, the crystal axis is a direction selected based on the electro-optic coefficient of the material so as to increase the refractive index of the domain inversion portion.

[0013] Furthermore, the waveguide includes but is not limited to curved, circular and cross structures.

[0014] Furthermore, the positive electrode and the negative electrode are insulated and filled with insulating material.

[0015] Beneficial effects: Compared with the existing technology, the present invention has the following significant advantages: it utilizes the conductive properties of the domain walls of ferroelectric crystals to perform electric field modulation, produce a photoelectric effect, increase the inverted crystal refractive index between domain walls, thereby forming a planar or strip waveguide, with a simple preparation process and low cost, and realizes the modulation of laser in waveguide transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram of the present invention;

[0017] Figure 2 is a schematic cross-sectional view of the present invention;

[0018] Figure 3 Schematic diagram of the 2×2 waveguide beam splitter structure. DETAILED DESCRIPTION

[0019] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0020] like Figure 1As shown, the present invention includes a domain structure, a first domain wall 1, a second domain wall 2, a positive electrode 5, a negative electrode 6 and an insulating layer 7. The domain structure is divided into a positive domain 3 and a negative domain 4, wherein the negative domain 4 is sandwiched in the middle, with a positive domain 3 set on each side, and a first domain wall 1 and a second domain wall 2 are formed at the junction of the positive domain 3 and the negative domain 4, respectively. The first domain wall 1 and the second domain wall 2 are covered with metal electrodes, and the metal electrodes are conductively connected to the first domain wall 1 and the second domain wall 2. By applying voltage to the positive electrode 5 and the negative electrode 6, an electric field is applied to the negative domain 4, and an electro-optical effect is generated under the action of the electric field. The positive and negative voltages are set in the crystal axis so that the refractive index of the electro-optical effect increases and is greater than the refractive index of the positive domains 3 on both sides, and total reflection is generated in the negative domain 4 to confine the laser. When the voltage is zero or the voltage is very small, the waveguide disappears, thereby realizing modulation of the laser transmission in the waveguide.

[0021] The optical material used in the present invention is a ferroelectric crystal, such as lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or potassium titanyl phosphate (KTiOPO4). Domain inversion can be achieved through room-temperature electric field polarization. As shown in the figure below, the inverted portion is called the negative domain, and the uninverted portion is called the positive domain. A domain wall with a certain conductivity forms between the positive and negative domains.

[0022] The present invention covers the two domain walls of the negative domain with metal electrodes. Voltage is applied through the metal electrodes and the domain walls. Under the action of the voltage, the domain walls conduct, applying an electric field to the negative domain, generating an electro-optical effect. By selecting appropriate positive and negative voltages and the appropriate orientation of the crystal axis, the refractive index of the electro-optical effect increases to a value greater than that of the positive domains on either side, thereby generating total internal reflection in the negative domain, confining the laser light.

[0023] In addition, the waveguide is formed when voltage is applied and disappears when the voltage is zero or very small. Therefore, this characteristic can be used to modulate the transmission of laser in the waveguide. When the waveguide is formed, the tail end outputs laser, and when the waveguide disappears, the light intensity at the tail end is very weak, forming an intensity modulation effect of the emitted laser.

[0024] like Figure 2 Figure 2 shows a schematic cross-sectional view of the present invention, where the thickness of the positive and negative electrodes is T, the width of the waveguide is H, and the thickness of the waveguide is D. Lithium niobate is a uniaxial crystal, with its optical axis c-axis along the Z direction and refractive index ellipsoidal symmetry in the X and Y directions. To maintain insulation between the positive and negative electrodes, an insulating material such as photoresist is used.

[0025] The ferroelectric materials include lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium titanyl phosphate (KTiOPO4), etc., and the preferred material is lithium niobate.

[0026] The above-mentioned ferroelectric domain inversion process includes a room temperature electric field polarization method, a growth stripe method, a poly-sheet multi-domain method, etc., and the room temperature electric field polarization method is preferred.

[0027] The structural dimensions of the domain inversion structure are as follows: domain inversion thickness 0.1 μm ≤ D ≤ 100 μm; waveguide width 1 μm ≤ H ≤ 10 mm; and positive and negative electrode thickness 1 nm ≤ T ≤ 100 μm. The preferred dimensions are D ≈ 1 μm, H ≈ 1 mm, and T ≈ 100 nm.

[0028] The voltage applied to the positive and negative electrodes is in the range of 1V to 1000V.

[0029] According to the electro-optic effect, under the action of an external electric field E, the refractive index principal axis of the lithium niobate crystal changes, causing the refractive index to change in all directions. According to the linear electro-optic effect matrix of lithium niobate, an electric field E is applied along the X-axis direction, then E y =E x =E, the refractive index ellipsoid equation of lithium niobate crystal becomes:

[0030]

[0031] After two coordinate transformations and ignoring the second-order small quantity, the new refractive index can be obtained as follows:

[0032]

[0033] n′ z =n e (4)

[0034] like Figure 1 As shown, the polarization direction of the input light is in the Y direction, so here we only focus on equation (3). In equation (3), the electric field in the X direction is generated by the charges on the positive and negative domain walls. After applying voltage to the positive and negative electrodes, the positive and negative charges are distributed on the adjacent domain walls, thus forming an electric field E in the X direction. The change in refractive index generated by equation (3) produces electro-optical modulation.

[0035] The axial direction of the crystal is such that, according to the sign of the electro-optic coefficient of the material, the voltage is applied in the direction that increases the refractive index of the domain inversion part. For example, the thickness of the domain inversion of lithium niobate crystal is D = 1 μm, and the electro-optic coefficient is γ 22 =3.4pm / v, so when the voltage difference between the positive and negative electrodes is V=1000V, according to formula (5)

[0036]

[0037] It can be concluded that this increase in refractive index is similar to that of a proton exchange waveguide, and a waveguide similar to that of a proton exchange or titanium diffusion waveguide can be formed.

[0038] In addition, the above waveguide can form a curved, ring-shaped, cross-shaped structure, such as the curved cross waveguide shown in the figure below, which forms a 2×2 waveguide beam splitter after voltage is applied. The negative domain part in the figure is prepared by making a beam splitter mask, photolithography, and room temperature electric field polarization method. The covered area under the prepared positive electrode is the positive domain, and the uncovered area is the negative domain area. The arrangement of the above electrodes is as follows Figure 3 As shown, a curved waveguide with an increased refractive index is formed in a waveguide beam splitter.

[0039] Example 1:

[0040] Using lithium niobate as the substrate, the room temperature electric field polarization process is carried out. Figure 1 The domains shown are reversed and retained Figure 1 The dimensions of the above-mentioned processing structure are: waveguide width D = 1μm, waveguide width H = 1mm, electrode material is copper, and the coating thickness is 100nm. Figure 1 In the example shown, a 1000V voltage is applied to the positive electrode and grounded to the negative electrode, creating a high-intensity electric field in the domain-inverted region. The input laser has a wavelength of 1.5μm and is polarized along the X-axis. The electro-optic effect produces a refractive index increase of approximately 0.018. Waveguide coupling is achieved through optical fiber, and the light intensity at the output port is approximately equal to the input light intensity. When the voltage is removed, the electro-optic effect disappears, the waveguide disappears, and laser transmission in the waveguide ceases. The light intensity at the output port is approximately zero, thus achieving voltage-controlled light intensity modulation.

[0041] Example 2:

[0042] like Figure 3 As shown, the 2×2 waveguide is prepared according to the above steps. Figure 3 Apply positive and negative voltages to the positive and negative electrodes to form Figure 3 The 2×2 waveguide coupler shown. After removing all voltages, Figure 3 The waveguide structure shown here disappears, along with the waveguide coupler. Applying corresponding voltages only to positive electrode 1, negative electrode 5, and negative electrode 2 results in a structure with only the left-side curved waveguide. Signals are output only from the left input port to the left output port. Applying corresponding voltages only to positive electrode 1, negative electrode 5, positive electrode 3, and negative electrode 4 results in a curved strip waveguide extending from the lower left to the upper right. Signals are output only from the left input port to the right output port.

[0043] By combining the above-mentioned straight waveguide, 2×2 coupler and other similar waveguide devices, electrical signals can be converted into optical signals, realizing a hybrid electro-optical chip that performs electrical and optical signal operations.

Claims

1. A tunable optical waveguide based on the electro-optic effect of ferroelectric domains, characterized by: The invention comprises a domain structure, a first domain wall (1), a second domain wall (2), a positive electrode (5), a negative electrode (6) and an insulating layer (7). The domain structure is divided into a positive domain (3) and a negative domain (4). The negative domain (4) is sandwiched in the middle, and a positive domain (3) is set on each side. The first domain wall (1) and the second domain wall (2) are formed at the junction of the positive domain (3) and the negative domain (4). The first domain wall (1) and the second domain wall (2) are covered with metal electrodes. The metal electrodes are conductive with the first domain wall (1) and the second domain wall (2). By applying voltage to the positive electrode (5) and the negative electrode (6), an electric field is applied to the negative domain (4), and an electro-optical effect is generated under the action of the electric field. The positive and negative voltages are set in the crystal axis so that the refractive index of the electro-optical effect increases and is greater than the refractive index of the positive domains (3) on both sides. Total reflection is generated in the negative domain (4) to confine the laser. When the voltage is zero or very small, the waveguide disappears, thereby realizing the modulation of the laser transmission in the waveguide.

2. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 1, characterized in that: The positive domain (3) and the negative domain (4) are both ferroelectric crystals, and the ferroelectric crystals include any one of lithium niobate, lithium tantalate and potassium titanyl phosphate.

3. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 1, characterized in that: The domain structure is domain-inverted by a domain inversion process, and the inverted portion is called a negative domain (4), and the uninverted portion is called a positive domain (3).

4. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 1, characterized in that: The covered area below the positive electrode (5) is a positive domain (3), and the uncovered area is a negative domain (4).

5. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 3, characterized in that: The domain inversion process adopts any one of a room temperature electric field polarization method, a growth stripe method and a poly-sheet multi-domain method.

6. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 3, characterized in that: The structural size range of the domain inversion is: domain inversion thickness 0.1 μm ≤ D ≤ 100 μm; Waveguide width 1μm≤H≤10mm; positive and negative electrode thickness 1nm≤T≤100μm.

7. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 1, characterized in that: The positive and negative voltages range from 1V to 1000V.

8. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 1, characterized in that: The crystal axis is a direction selected based on the electro-optic coefficient of the material so as to increase the refractive index of the domain inversion portion.

9. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 1, characterized in that: The waveguides include, but are not limited to, curved, circular, and crossed structures.

10. The tunable optical waveguide based on the ferroelectric domain electro-optic effect according to claim 1, characterized in that: The positive electrode (5) and the negative electrode (6) are kept insulated and filled with insulating material.

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