A lithium niobate electro-optical modulator and its preparation method

Abstract

The present invention provides a lithium niobate electro-optic modulator, comprising a substrate, a ridge waveguide made of a lithium niobate film on an insulator (LNOI), an electrode, a dielectric (Dielectrics) and a covering layer. The lithium niobate film is placed on a micron-thick insulator, the insulating layer is silicon dioxide, and the thickness of the lithium niobate film is 300-900 nanometers. First, a ridge waveguide is processed on the lithium niobate film, electrodes are arranged on both sides of the ridge waveguide, and the area between the ridge waveguide and the electrode is filled with a dielectric. A covering layer is arranged above the ridge waveguide, the electrode and the dielectric. The dielectric constant of the dielectric is greater than the dielectric constant of the covering layer. The present invention also provides a method for preparing a lithium niobate electro-optic modulator. The lithium niobate electro-optic modulator provided by the present invention can reduce the half-wave voltage to about 1 / 4 of the original without changing the waveguide and electrode structure, and does not introduce additional production costs.

Description

Technical Field

[0001] The invention belongs to the field of optical information processing, and in particular relates to a lithium niobate electro-optic modulator and a preparation method thereof. Background Art

[0002] Laser external modulation technology has always been a research hotspot in the field of optical communications. Phase and intensity modulators based on the electro-optic effect of lithium niobate have large bandwidth and low loss, and have great application prospects in the field of high-speed optical communications. Electro-optic modulators based on titanium diffusion or proton exchange waveguides on bulk lithium niobate have been commercialized and widely used. The titanium diffusion or proton exchange waveguides based on bulk lithium niobate in the prior art have a waveguide width of 5-8 microns, and the electrodes and waveguides are covered with silicon dioxide. Due to the large waveguide width, in order to obtain a lower half-wave voltage, the device design size is 0.8-4 cm. The large waveguide width also limits its optical localization capability, resulting in a limited modulation bandwidth (about 10 GHz). In addition, the design size of the device is too large, which makes the device inconvenient for large-scale integration and difficult to meet the demand for rapid increase in modulation bandwidth in modern communications.

[0003] In order to solve the above problems, electro-optic modulators based on lithium niobate thin films on insulators (LNOI) have appeared in the prior art. Compared with titanium diffusion or proton exchange waveguides in traditional bulk lithium niobate, the ridge waveguide based on lithium niobate thin films has a smaller width and stronger light localization capability. For example, in the prior art, there is a Mach-Zehnder interferometer intensity modulator with a modulation bandwidth of 100 GHz prepared on lithium niobate thin films using a CMOS-compatible process. Its electrode length is 1 cm, and the half-wave voltage is reduced to 2.3 volts.

[0004] As an indicator directly related to device power consumption, half-wave voltage has always been one of the indicators that technicians are most concerned about. Reducing the half-wave voltage can directly reduce device power consumption. Under the premise of large-scale integration, the reduction of power consumption of a single device means a significant reduction in overall power consumption. Therefore, on the basis of existing technologies, it is beneficial to further invent a technical means to reduce the half-wave voltage of a lithium niobate electro-optical modulator without affecting the device size and modulation capability. Since this technical means of reducing the half-wave voltage saves device costs by reducing power consumption, it is preferred that additional production costs should not be introduced in the large-scale integrated production process to avoid offsetting the cost reduction effect provided by the reduced power consumption. Summary of the invention

[0005] The present invention provides a lithium niobate electro-optic modulator, comprising a waveguide, an electrode and a covering layer, characterized in that the covering layer is arranged on the surface of the waveguide and the electrode and covers the surface of the waveguide and the electrode, and a dielectric is arranged between the waveguide and the electrode.

[0006] Furthermore, the dielectric constant of the material used for the dielectric is greater than the dielectric constant of the material of the covering layer.

[0007] Furthermore, the waveguide includes a single-ridge waveguide, the electrodes are multiple, the multiple electrodes include at least a first electrode and a second electrode, and the first electrode and the second electrode are respectively arranged on both sides of the single-ridge waveguide.

[0008] Further, the dielectric includes a first portion and a second portion, the first portion is disposed between the first electrode and the single-ridge waveguide, and the second portion is disposed between the second electrode and the single-ridge waveguide.

[0009] Furthermore, the waveguide includes a double-ridge waveguide, the double-ridge waveguide includes a second ridge waveguide and a third ridge waveguide, the electrodes are multiple, the multiple electrodes include at least a third electrode, a fourth electrode and a fifth electrode, the third electrode is arranged on the outside of the second ridge waveguide, the fourth electrode is arranged between the second ridge waveguide and the third ridge waveguide, and the fifth electrode is arranged on the outside of the third ridge waveguide.

[0010] Further, the dielectric includes third, fourth, fifth and sixth parts, the third part is arranged between the third electrode and the second ridge waveguide, the fourth part is arranged between the second ridge waveguide and the fourth electrode, the fifth part is arranged between the fourth electrode and the third ridge waveguide, and the sixth part is arranged between the third ridge waveguide and the fifth electrode.

[0011] Furthermore, the material of the double-ridge waveguide includes an X- or Y-cut lithium niobate film on an insulator.

[0012] Furthermore, the insulator includes a silicon dioxide insulating layer.

[0013] Furthermore, the double-ridge waveguide insulating layer is disposed on a semiconductor or dielectric substrate.

[0014] Furthermore, a covering layer is provided on the double-ridge waveguide, the plurality of electrodes, and the plurality of portions of the dielectric.

[0015] Furthermore, the material of the covering layer includes silicon dioxide.

[0016] Furthermore, the material of the dielectric includes aluminum nitride, the material of the electrode includes gold, and the material of the waveguide includes lithium niobate.

[0017] The present invention also provides a method for preparing a lithium niobate electro-optic modulator, which specifically comprises the following steps:

[0018] Step 1: Disposing a lithium niobate film on an insulator on a semiconductor or dielectric substrate;

[0019] Step 2, etching the lithium niobate film to obtain a ridge waveguide;

[0020] Step 3, plating a plurality of electrodes on both sides of the ridge of the ridge waveguide;

[0021] Step 4: depositing a dielectric between the plurality of electrodes and the ridge of the ridge waveguide;

[0022] Step five: covering the ridge waveguide, the plurality of electrodes and the dielectric with a silicon dioxide layer.

[0023] Furthermore, in step one, the material of the semiconductor substrate includes silicon, the material of the dielectric substrate includes lithium niobate or quartz, and the insulator includes a silicon dioxide insulating layer.

[0024] Furthermore, in step 1, the lithium niobate film is X-cut or Y-cut.

[0025] Furthermore, in step 2, a ridge waveguide is fabricated on the lithium niobate film using photolithography and etching processes.

[0026] Furthermore, in step 2, the ridge waveguide includes a single-ridge waveguide or a double-ridge waveguide.

[0027] Furthermore, in step three, the plurality of electrodes are processed by using alignment photolithography and electron beam evaporation processes.

[0028] Furthermore, in step four, the material of the dielectric includes aluminum nitride.

[0029] Furthermore, in step five, a chemical vapor deposition process is used to cover the silicon dioxide layer.

[0030] Compared with the prior art, the lithium niobate electro-optic modulator provided by the present application has at least the following beneficial technical effects:

[0031] 1. The lithium niobate ridge waveguide provided in the present application is processed based on a lithium niobate film on an insulator, so the waveguide width is small and the light localization ability is strong.

[0032] 2. The lithium niobate electro-optic modulator provided in the present application can significantly reduce the half-wave voltage without changing the waveguide structure, size and electrode length of the device, thereby reducing the power consumption of the entire device.

[0033] 3. The preparation method of the lithium niobate electro-optic modulator provided by the present application, wherein the processes adopted are all CMOS compatible processes, and large-scale integration can be achieved by using some existing production equipment. It is not necessary to introduce additional production costs due to the technical means of reducing the half-wave voltage provided by the present application, so as to prevent the cost reduction effect provided by reducing the half-wave voltage from being offset by the increase in production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a structural schematic diagram of an embodiment of the present application;

[0035] Figure 2 It is an overall schematic diagram of an embodiment of the present application;

[0036] Figure 3 is a schematic diagram of the external electric field distribution between electrodes on both sides of a waveguide according to an embodiment of the present application;

[0037] Figure 4 It is a schematic diagram of the light field distribution in the waveguide region of an embodiment of the present application. DETAILED DESCRIPTION

[0038] The following describes embodiments of the present invention with reference to the drawings in the specification to make its technical content clearer and easier to understand. The present invention can be embodied through many different forms of embodiments, and the scope of protection of the present invention is not limited to the embodiments mentioned in the text. In the present application, if there are two waveguides, the space between the two waveguides is called the inner side, and the side of each waveguide opposite to the inner side is called the outer side. The single-ridge waveguide mentioned in the present application refers to only one ridge waveguide formed on the substrate material, and the double-ridge waveguide refers to two ridge waveguides formed on the substrate material. The half-wave voltage mentioned in the present application specifically refers to the magnitude of the external voltage applied when the phase difference of the two-arm optical signal is π for a Mach-Zehnder modulator.

[0039] like Figure 1 , Figure 2 As shown, this embodiment uses silicon as the substrate material. Specifically, the thickness of the silicon substrate 1 is 500 microns. On the silicon substrate 1, an insulator and a lithium niobate film 3 on the insulator are provided. In this embodiment, the insulator is specifically a silicon dioxide insulating layer 2. Preferably, the thickness of the silicon dioxide insulating layer 2 is set to 2 microns. The lithium niobate film 3 is X-cut, that is, the lithium niobate crystal is placed in a Figure 2In the coordinate system shown, the section of the lithium niobate crystal is perpendicular to the x-axis. In other similar embodiments, the lithium niobate film 3 can also be cut in a Y-cut manner. The material constituting the lithium niobate film 3 is preferably 5% magnesium-doped lithium niobate, that is, the molar number of magnesium ions accounts for 5% of the sum of the molar numbers of lithium niobate and magnesium ions. The size of the lithium niobate film 3 is 1 cm × 1.5 cm, and the thickness of the lithium niobate film 3 is 600 nanometers before processing. A ridge waveguide is constructed based on the lithium niobate film 3, and the height of the constructed lithium niobate ridge waveguide is 350 nanometers and the width is 1 micron. In this embodiment, two ridge waveguides 31 and 32 are constructed based on the lithium niobate film. In other similar embodiments, only one ridge waveguide can be constructed according to actual needs to produce similar technical effects. Electrodes are arranged on both sides of each ridge waveguide 31 and 32. Taking the two ridge waveguides arranged in this embodiment as an example, in order to ensure that each ridge waveguide has electrodes on both sides, at least three electrodes 41, 42, and 43 need to be arranged. Among them, the electrode 41 is arranged on the left side of the ridge waveguide 31, the electrode 42 is arranged between the ridge waveguide 31 and the ridge waveguide 32, and the electrode 43 is arranged on the right side of the ridge waveguide 32. It should be noted that the left side and the right side mentioned in this specification are only used to distinguish the orientations of the two waveguides and the three electrodes, and cannot be used to limit the specific positions. In other similar embodiments, if only one ridge waveguide is constructed, at least two electrodes need to be set, and the positions of these two electrodes are respectively located on both sides of the ridge waveguide. The material of the electrodes 41, 42, and 43 is preferably gold, and other conductors can also be used according to actual needs. The height of the electrodes 41, 42, and 43 is 1 micron. After the above settings, the overall structure of this embodiment is as follows. Figure 2 shown.

[0040] exist Figure 2 Based on the above, an aluminum nitride dielectric is provided in the region between the ridge waveguide and the electrode. In this embodiment, since there are two ridge waveguides 31, 32 and three electrodes 41, 42, 43, the aluminum nitride dielectric needs to be divided into four parts 51, 52, 53, 54. Figure 1As shown, the first part 51 is arranged in the area between the electrode 41 and the ridge waveguide 31, the second part 52 is arranged in the area between the ridge waveguide 31 and the electrode 42, the third part 53 is arranged in the area between the electrode 42 and the ridge waveguide 32, and the fourth part 54 is arranged in the area between the ridge waveguide 32 and the electrode 43. The aluminum nitride dielectric fills the area between the ridge waveguide and the electrode, and the thickness of the aluminum nitride dielectric is approximately equal to the height of the ridge waveguides 31 and 32. In other similar embodiments, if only one waveguide and two electrodes are provided, the aluminum nitride dielectric needs to be divided into two parts. If more waveguides and electrodes are provided, the aluminum nitride dielectric needs to be filled into the area between each electrode and the waveguide. On the top layer of the device, a silicon dioxide covering layer 6 is provided, and the silicon dioxide covering layer 6 simultaneously covers the electrode, the ridge waveguide and the aluminum nitride dielectric.

[0041] This embodiment can reduce the half-wave voltage of the lithium niobate electro-optic modulator based on the following principles:

[0042] Since the X-cut lithium niobate modulator supports the TE mode, applying an external electric field in the Z direction can utilize its maximum electro-optic coefficient γ 33 Since the external electric field acting on the waveguide is non-uniform, the superposition factor Γ is introduced to represent the strength of the interaction. The superposition factor Γ satisfies the following formula:

[0043]

[0044] Among them, G is the electrode distance, V is the applied voltage, E is the external electric field distribution function, and E' is the normalized optical electric field distribution function.

[0045] The influence of the external electric field on the waveguide refractive index can be expressed as follows:

[0046]

[0047] Among them, n e is the extraordinary refractive index, n z is the principal refractive index in the Z direction of the refractive index ellipsoid, E z is the electric field outside the center of the waveguide.

[0048] It can be seen from the above formula that when the device length and the external voltage remain unchanged, increasing the electric field E in the center of the waveguide z , or increasing the superposition factor Γ can have a greater impact on the waveguide refractive index and obtain a larger refractive index change Δn e , so that the light wave obtains a larger phase change. If the phase change required by the light wave is certain, then only a smaller half-wave voltage is needed to meet the requirement.

[0049] Among the factors that affect the superposition factor Γ, the electrode spacing G and the applied voltage V remain unchanged. When the applied voltage V is constant, the partial pressure U in the ridge waveguide region between the electrodes is w The partial pressure U of the aluminum nitride dielectric region m Satisfies the following formula:

[0050]

[0051] Among them, U w is the partial pressure in the waveguide area, U m is the partial pressure of the dielectric area, d w is the waveguide area width, d m is the width of the dielectric region, ε rm is the relative dielectric constant of the dielectric, ε rw is the waveguide relative dielectric constant.

[0052] From the above formula, it can be obtained that the proportion of the waveguide area to the total applied voltage satisfies the following formula:

[0053]

[0054] In this embodiment, aluminum nitride is used as the dielectric material. The relative dielectric constant of aluminum nitride is 8. In the prior art, in the prior art, no dielectric is set between the waveguide and the electrode, but a silicon dioxide covering layer is directly set. In this case, silicon dioxide will be filled into the area between the waveguide and the electrode. The relative dielectric constant of silicon dioxide is 3.9. According to the above formula, when the waveguide width and the electrode size and position remain unchanged, in this embodiment, a dielectric with a higher relative dielectric constant is additionally filled between the lithium niobate ridge waveguide and the electrode, so that the external electric field strength in the waveguide area is enhanced, thereby reducing the half-wave voltage.

[0055] The COMSOL Multiphysics software is used to simulate the external electric field distribution between the electrodes of the lithium niobate electro-optic modulator and the optical field distribution of the waveguide. Figure 3 and Figure 4 As shown. Among them, Figure 3 The gray value in represents the magnitude of the electric field component in the x direction. The whiter the color, the greater the electric field intensity. Figure 4 The gray value in represents the intensity of the light field in the waveguide in the x direction. The whiter the color, the greater the light field intensity. Figure 3 and Figure 4 The applied voltage V=5V is used in the examples. The superposition factor Γ of this embodiment is calculated to be 4.4771×10 -9 kg 2 m 4 / (s 6 A 2), external electric field E z =1.0364×10 6 V / m. At the same time, the superposition factor Γ of the technical solution with the same waveguide and electrode dimensions as in this embodiment and only silicon dioxide is filled between the waveguide and the electrode is calculated to be 1.4275×10 -9 kg 2 m 4 / (s 6 A 2 ), external electric field E z =8.5926×10 5 V / m. It can be seen that, by adopting the technical solution provided by the present application, the superposition factor is increased to 3 times compared with the prior art, and the external electric field intensity at the center of the waveguide is also significantly improved. It can be calculated that the half-wave voltage of the electro-optic modulator is reduced to about 1 / 4 of the original, which is a significant effect.

[0056] The preparation method of the lithium niobate electro-optic modulator in this embodiment is as follows:

[0057] 1. Prepare a silicon substrate with a thickness of 500 microns;

[0058] 2. A lithium niobate film on an insulator with a size of 1×1.5 cm and a thickness of 600 nanometers is selected and placed on a silicon substrate, wherein the insulator is an insulating layer of silicon dioxide material with a thickness of 2 micrometers, and the lithium niobate film is X-cut and the material is 5 mol% magnesium-doped lithium niobate;

[0059] 3. Using photolithography and etching technology, a lithium niobate ridge waveguide is processed on the lithium niobate film. The lithium niobate ridge waveguide has a height of 350 nanometers and a width of 1 micron;

[0060] 4. Electrodes are deposited on both sides of the waveguide using alignment photolithography and electron beam evaporation processes. The material of the electrodes is preferably gold, and the electrode height is 1 micron.

[0061] 5. Deposit aluminum nitride dielectric between the electrode and the waveguide using aligned photolithography and chemical vapor deposition processes;

[0062] 6. Use chemical vapor deposition to deposit silicon dioxide on the surface, covering the surface of the waveguide, electrode and dielectric.

[0063] The processing technology used in this embodiment is a CMOS compatible process. During the processing, no additional production cost will be generated due to the addition of new technical means. Due to the need for large-scale integration, the increase in the production cost of a single device will lead to a significant increase in the overall cost. This will offset the cost reduction effect brought about by reducing power consumption. The technical means provided in this application reduce the half-wave voltage and thus reduce the power consumption of the device without causing additional production costs, truly ensuring the effect of reducing the cost of using the device.

[0064] The lithium niobate thin film electro-optic modulator designed by the present invention is a Mach-Zehnder type light intensity modulator, which includes structures such as waveguide, electrode, dielectric, and insulating layer. Among them: the waveguide is a ridge waveguide, which plays a light guiding role, and the light is confined and modulated therein; the insulating layer is located under the lithium niobate thin film, providing a large refractive index difference, so that the light field is better localized in the ridge waveguide on the film; the electrode is a conductive metal material deposited on both sides of the ridge waveguide, and an external voltage is applied to form an electric field between the electrodes; the covering layer is a silicon dioxide material deposited after the waveguide, electrode, dielectric and other structures are processed, which plays the role of packaging and adjusting the modulation bandwidth of the device; the dielectric is aluminum nitride deposited between the ridge waveguide and the metal electrode. This structure is the innovative design part of the present invention, and uses a material with a higher relative dielectric constant than the covering layer to achieve the function of increasing the electric field at the waveguide.

[0065] The working principle of the electro-optic modulator is: by applying an external voltage to the electrode, the optical signal transmitted in the ridge waveguide is modulated due to the electro-optic effect of the lithium niobate material, and carries the information of the modulated external voltage. In particular, in the Mach-Zehnder type optical intensity modulator, the light in the two ridge waveguides is modulated to produce a phase difference, and after interference, an intensity modulated optical signal is obtained.

[0066] The preferred specific embodiments of the present invention are described in detail above. It should be understood that ordinary technicians in the field can make many modifications and changes based on the concept of the present invention without creative work. Therefore, all technical solutions that can be obtained by technicians in the technical field based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A lithium niobate electro-optic modulator, comprising a waveguide, a plurality of electrodes, a dielectric and a cover layer, characterized in that: The covering layer is arranged on the surface of the waveguide and the multiple electrodes, and covers the surface of the waveguide and the multiple electrodes; the waveguide includes a ridge waveguide, which is constructed based on a lithium niobate film on an insulator, and the lithium niobate film on an insulator is arranged on a semiconductor or dielectric substrate; the ridge waveguide is processed on the lithium niobate film by using photolithography and etching processes; the multiple electrodes are respectively arranged on both sides of the ridge waveguide; the lithium niobate film is X-cut or Y-cut; the dielectric is arranged in the area between the ridge part of the waveguide and the electrode; the material of the dielectric includes aluminum nitride.

2. The lithium niobate electro-optic modulator according to claim 1, characterized in that: The dielectric constant of the material used for the dielectric is greater than the dielectric constant of the material of the cover layer.

3. The lithium niobate electro-optic modulator according to claim 2, characterized in that: The waveguide includes a single-ridge waveguide, the single-ridge waveguide includes a first ridge waveguide, the electrode includes a first electrode and a second electrode, and the first electrode and the second electrode are respectively arranged on both sides of the first ridge waveguide.

4. The lithium niobate electro-optic modulator according to claim 3, characterized in that: The dielectric includes a first portion and a second portion, the first portion being disposed between the first electrode and the first ridge waveguide, and the second portion being disposed between the second electrode and the first ridge waveguide.

5. The lithium niobate electro-optic modulator according to claim 2, characterized in that: The waveguide includes a double-ridge waveguide, the double-ridge waveguide includes a second ridge waveguide and a third ridge waveguide, the multiple electrodes include a third electrode, a fourth electrode and a fifth electrode, the third electrode is arranged on the outside of the second ridge waveguide, the fourth electrode is arranged between the second ridge waveguide and the third ridge waveguide, and the fifth electrode is arranged on the outside of the third ridge waveguide.

6. The lithium niobate electro-optic modulator according to claim 5, characterized in that: The dielectric includes a third part, a fourth part, a fifth part, and a sixth part. The third part is arranged between the third electrode and the second ridge waveguide, the fourth part is arranged between the second ridge waveguide and the fourth electrode, the fifth part is arranged between the fourth electrode and the third ridge waveguide, and the sixth part is arranged between the third ridge waveguide and the fifth electrode.

7. The lithium niobate electro-optic modulator according to claim 6, characterized in that: The insulator includes a silicon dioxide insulating layer.

8. The lithium niobate electro-optic modulator according to claim 7, characterized in that: The insulating layer is disposed on a semiconductor or dielectric substrate.

9. The lithium niobate electro-optic modulator according to claim 8, characterized in that: A cover layer is provided on the double-ridge waveguide, the plurality of electrodes, and a plurality of portions of the dielectric.

10. The lithium niobate electro-optic modulator according to claim 9, characterized in that: The material of the cover layer includes silicon dioxide.

11. The lithium niobate electro-optic modulator according to claim 1, characterized in that: The material of the plurality of electrodes includes gold, and the material of the waveguide includes lithium niobate.

12. A method for preparing a lithium niobate electro-optic modulator, characterized in that: The following steps are involved: Step 1: a lithium niobate film on an insulator is provided on a semiconductor or dielectric substrate; the lithium niobate film is X-cut or Y-cut; Step 2: etching the lithium niobate film by photolithography and etching process, and obtaining a ridge waveguide on the lithium niobate film; Step 3, plating a plurality of electrodes on both sides of the ridge waveguide; Step 4: depositing a dielectric between the plurality of electrodes and the ridge of the ridge waveguide; the material of the dielectric comprises aluminum nitride; Step five: providing a covering layer on the ridge waveguide, the plurality of electrodes and the dielectric.

13. The method for preparing a lithium niobate electro-optic modulator according to claim 12, characterized in that: In step 1, the material of the semiconductor substrate includes silicon, the material of the dielectric substrate includes lithium niobate or quartz, and the insulator includes a silicon dioxide insulating layer.

14. The method for preparing a lithium niobate electro-optic modulator according to claim 12, characterized in that: In step 2, the ridge waveguide includes a single-ridge waveguide or a double-ridge waveguide.

15. The method for preparing a lithium niobate electro-optic modulator according to claim 12, characterized in that: In step three, the plurality of electrodes are processed by using alignment photolithography and electron beam evaporation processes.

16. The method for preparing a lithium niobate electro-optic modulator according to claim 12, characterized in that: In step five, the cover layer is formed by a chemical vapor deposition process.

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