CN112987346A

CN112987346A - Thin-film electro-optic modulator easy to realize electro-optic wave velocity matching and preparation method

Info

Publication number: CN112987346A
Application number: CN202110330085.XA
Authority: CN
Inventors: 帅垚; 杨小妮; 乔石珺; 吴传贵; 罗文博
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2021-03-29
Filing date: 2021-03-29
Publication date: 2021-06-18
Anticipated expiration: 2041-03-29
Also published as: CN112987346B

Abstract

The invention belongs to the field of integrated optical devices, and particularly relates to a thin-film electro-optic modulator capable of easily realizing electro-optic wave velocity matching and a preparation method thereof. The cavity is arranged on the buried layer, so that most of microwave signals concentrated on the buried layer enter the cavity structure to be transmitted after being introduced into the cavity under the condition of no cavity originally, and the effective refractive index of the microwaves is greatly reduced; so that the electro-optical wave velocity matching is easy to realize. Even when the bandwidth is higher than 20GHz, good electro-optical wave speed matching can be realized, so that the optimal modulation efficiency is kept. Compared with the prior art, the communication speed is faster, and the distance between the electrode and the optical waveguide is not increased, so that the electro-optical superposition integral is not influenced, and the half-wave voltage is not increased.

Description

Thin-film electro-optic modulator easy to realize electro-optic wave velocity matching and preparation method

Technical Field

The invention belongs to the field of integrated optical devices, and particularly relates to a thin-film electro-optic modulator capable of easily realizing electro-optic wave velocity matching and a preparation method thereof.

Background

The optical communication technology has the advantages of high transmission rate, large transmission bandwidth, strong electromagnetic interference resistance, low power consumption, strong multiplexing capability, no RC delay and the like, and is gradually a hotspot of research in academic circles. Typical integrated photonic devices in optical communication networks include optical waveguide couplers, wavelength division multiplexers, micro-ring modulators, electro-optic modulators, micro-ring filters, and the like. The electro-optical modulator is one of key devices of an optical interconnection system, an optical calculation system and an optical communication system, and is used for loading an electric signal to be transmitted into an optical wave, converting monochromatic light emitted by laser into an optical signal carrying information, and converting the electric signal into the optical signal.

Traditional electro-optical modulator is mostly the bulk material structure, and the device volume is great and the integrated level is lower, because when the device size is great relatively, electro-optical modulator's half-wave voltage can be very high, consequently can bring great power loss. In recent years, with the development of thin film technology, thin film devices are promoted to have a hot tide, ridge-shaped optical waveguides etched on optical thin film crystals show excellent characteristics, large refractive index difference can be realized, optical power is more concentrated on the optical waveguides, and light is better limited to be transmitted in the thin film crystals, so that the half-wave voltage of an electro-optical modulator is greatly reduced, the power consumption of the modulator is reduced, and the miniaturization of the devices is realized. For example, C.Wang1, et al, "Integrated lithium niobate electro-optical modulators operating at CMOS-compatible zeolites", Nature,2018,562(7725): 101-. The traveling wave electrode of the integrated lithium niobate electro-optical modulator belongs to a distribution parameter structure, and the electrode is a coplanar microstrip transmission line; only when the microwave is consistent with the light wave along the transmission direction of the waveguide along the transmission direction of the electrode, the electric signal and the optical signal of the electro-optical modulator can be transmitted forwards at the same phase speed, so that the electric field is positively acted on the light wave, and the modulation effect is achieved.

In the prior art, there are many methods for realizing the speed matching of the electro-optical wave, such as increasing the thickness of the buffer layer directly below the electrode or increasing the thickness of the electrode. Although electro-optical matching can be achieved by increasing the thickness of the buffer layer below the electrodes, the extra buffer layer reduces electro-optical overlap integral, which increases half-wave voltage and affects modulation efficiency. Since the aspect ratio of the electrode is large, it is difficult to control the uniformity of the electrode growth in the process by increasing the thickness of the electrode. More importantly, the two ways of realizing the electro-optical wave speed matching both have the problem of difficulty in realizing the electro-optical wave matching under a higher (higher than 20GHz) electrical bandwidth, so that the improvement of the electrical bandwidth is limited, and the information transmission rate of the communication system is reduced.

Disclosure of Invention

The purpose of the invention is: aiming at the problems in the prior art, the thin-film electro-optic modulator easy to realize electro-optic wave speed matching and the preparation method thereof are provided, the effective refractive index of microwave is reduced through the cavity structure below the electrode, the electro-optic wave speed matching is favorably realized, and the thin-film electro-optic modulator has the advantages of simple process, high modulation efficiency and the like.

In order to achieve the purpose, the invention adopts the following technical scheme:

a thin film electro-optic modulator easy to realize electro-optic wave velocity matching comprises a substrate layer, a thin film layer and a traveling wave electrode;

a buried layer is arranged between the substrate layer and the thin film layer, and 3 cavities are arranged on the buried layer;

the traveling wave electrode is positioned on the thin film layer and comprises two ground electrodes and a central source electrode, and the central source electrode is positioned between the two ground electrodes; the 3 electrodes respectively correspond to one cavity, and each cavity is overlapped with the central connecting line of the corresponding electrode; and an optical waveguide is arranged between the ground electrode and the central source electrode, and is made in the thin film layer and made of the same material as the thin film layer.

Further, the width of each cavity is smaller than or equal to the width of the corresponding electrode. So as to avoid the cavity from reducing the refractive index of the light wave by overlarge and reduce the matching effect of the speed of the electro-optic wave.

Further, the substrate layer is made of monocrystalline silicon, lithium niobate, sapphire or quartz glass; the buried layer 002 is made of silicon dioxide, BCB (benzocyclobutene), PMMA (polymethyl methacrylate), or PDMS (polydimethylsiloxane) having a dielectric constant of less than 3; the material of the thin film wave layer is lithium niobate, silicon nitride, silicon carbide, gallium arsenide or gallium nitride; the two side ground electrodes 007 and the central source electrode 008 are made of gold, platinum, titanium, aluminum or copper.

A method for preparing a thin film electro-optic modulator easy to realize electro-optic wave speed matching comprises the following steps:

step 1, obtaining a lithium niobate thin film layer with the thickness of less than 1 mu m by ion implantation;

step 2, growing a silicon dioxide bonding layer on the silicon substrate layer;

step 3, patterning the silicon dioxide bonding layer by adopting an ultraviolet lithography technology and a reactive ion etching technology to obtain a buried layer and three cavities on the buried layer;

step 4, bonding the lithium niobate thin film layer obtained in the step 1 to a substrate layer by adopting a hydrophilic group or wafer bonding process;

step 5, adopting a reactive ion etching process to obtain an optical waveguide in the lithium niobate thin film layer obtained in the step 1;

and 6, growing and patterning the ground electrode layers and the central source electrode layer on the two sides of the lithium niobate waveguide flat plate layer by adopting photoetching and magnetron sputtering processes.

The invention has the beneficial effects that: the cavity is arranged on the buried layer, so that most of microwave signals concentrated on the buried layer enter the cavity structure to be transmitted after being introduced into the cavity under the condition of no cavity originally, and the effective refractive index of the microwaves is greatly reduced; so that the electro-optical wave velocity matching is easy to realize. Even when the bandwidth is higher than 20GHz, good electro-optical wave speed matching can be realized, so that the optimal modulation efficiency is kept. Compared with the mode of adding a buffer layer in the prior art, the distance between the electrode and the optical waveguide cannot be increased, so that the electro-optic overlap integral cannot be influenced, and the half-wave voltage cannot be increased. Compared with the method of increasing the thickness of the electrode in the prior art, the method does not need to consider the requirements of the electrode on the process. The whole device has simple structure and is easy to manufacture.

Drawings

FIG. 1 is a schematic diagram of an electro-optic modulator according to the present invention;

FIG. 2(a) is a schematic cross-sectional view of an electric field mode of a modulator without a cavity structure;

FIG. 2(b) is a schematic diagram illustrating a cross-sectional simulation of an electric field mode according to an embodiment of the present invention;

FIG. 2(c) is a graph of the effective refractive index of microwaves as a function of microwave frequency for an electro-optic modulator and a modulator without a cavity structure in accordance with the present invention;

FIG. 3 is a graph showing the relationship between the cavity height corresponding to the electro-optic modulator and the source electrode and the effective refractive index N of the microwave;

FIG. 4 is a graph showing the relationship between the cavity width corresponding to the electro-optic modulator and the source electrode and the effective refractive index N of the microwave in accordance with the present invention;

fig. 5(a) - (g) are process flow diagrams of the thin-film electro-optic modulator easy to implement electro-optic wave velocity matching according to the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings.

As shown in fig. 1, the thin film electro-optical modulator provided by the present invention, which is easy to implement electro-optical wave velocity matching, includes a substrate layer 001, a thin film layer 005, and a traveling wave electrode. A buried layer 002 is arranged between the substrate layer 001 and the

thin film layer

005, and 3 cavities are arranged on the buried layer 002. The traveling wave electrode is positioned on the film layer 005 and comprises two ground electrodes 007 and a central source electrode 008, and the central source electrode 008 is positioned between the two ground electrodes 007; the 3 electrodes respectively correspond to one cavity, and each cavity is overlapped with the central connecting line of the corresponding electrode. An optical waveguide is arranged between the ground electrode 007 and the central source electrode 008, the optical waveguide is manufactured in the thin film layer and is made of the same material as the thin film layer in the electro-optical modulator, and the effective refractive index of the microwave is larger than that of the optical wave due to the fact that the dielectric constant of the metal electrode is large; in this case, a medium having a low dielectric constant may be added around the metal electrode, thereby lowering the microwave effective refractive index. Air has a dielectric constant of about 1, and is a particularly ideal dielectric material; therefore, in the present embodiment, three cavities are provided in the buried layer 002, each of the 3 electrodes corresponds to one cavity, and each cavity overlaps with its corresponding electrode center line. The effective refractive index of the microwave is reduced through the cavity arranged on the buried layer, so that the wave velocity matching between the electro-lights is easier to realize, and the matching degree is higher.

In specific implementation, the thickness of the substrate layer 001 is 100 μm, and the selected material is silicon. The film layer material is an x-cut monocrystal lithium niobate film, and the preferable thickness is 450 nm. The whole modulator is an M-Z type modulator, and the traveling wave electrode is of a coplanar waveguide structure and is made of gold; during manufacturing, the thicknesses of the two ground electrodes 007 and the central source electrode 008 are both 4 μm, the width 112 of the central source electrode is 10 μm, and the distance 111 between the ground electrode and the source electrode is 8 μm. Because the ridge waveguide can effectively improve the modulation efficiency, the optical waveguide of the embodiment is preferably the ridge waveguide, and a space is arranged between the ridge of the ridge waveguide and the traveling wave electrode. The waveguide ridge layer 006 has a thickness of 350nm and a width of 1 μm. The three cavities all have a height 113 of 5 μm, the two cavities 003 corresponding to the ground electrode have a width of 12 μm, and the cavity 004 corresponding to the central source electrode has a width of 10 μm.

To further illustrate the advantages of the present invention, experimental comparisons of thin film electro-optic modulators provided by the present invention with conventional electro-optic modulators were made. The experimental verification adopts the following condition parameters: the wavelength of the light wave is 1550nm, the width of the ridge optical waveguide is 1 μm, the height is 350nm, the effective refractive index of the TE mode light wave is 1.7994, and the microwave signal is 20 GHz.

FIG. 2(a) is a schematic cross-sectional view of an electric field mode of a modulator without a cavity structure; FIG. 2(b) is a schematic diagram illustrating a comparison of simulated cross sections of the electric field mode according to the embodiment of the present invention. Under the same parameter conditions, comparing fig. 2(a) and fig. 2(b), it can be known that after the introduction of the cavity structure, the microwave signal is mostly transmitted into the cavity structure.

FIG. 2(c) is a graph of the effective refractive index of microwaves as a function of microwave frequency for an electro-optic modulator and a modulator without a cavity structure in accordance with the present invention; as can be seen from fig. 2(c), the effective microwave refraction of the electro-optical modulator of the present invention is 0.2 lower than that of the non-cavity structure, and thus, the electro-optical modulator of the present invention achieves the reduction of the effective microwave refraction and is easier for the electro-optical wave velocity matching.

The size of the cavity affects the refractive index of the light wave, and thus the speed matching of the electro-optic wave. When the width of each cavity is smaller than that of the corresponding electrode, the larger the cavity is, the better the electro-optic matching performance is; therefore, the present embodiment limits the size of the 3 cavities, i.e., the width of each cavity is less than or equal to the width of the corresponding electrode.

Fig. 3 is a graph showing a relationship between a cavity width corresponding to the electro-optical modulator and the source electrode and an effective refractive index N of the microwave, and fig. 4 is a graph showing a relationship between a cavity width corresponding to the electro-optical modulator and the source electrode and an effective refractive index N of the microwave. When the cavity width used for the cavity corresponding to the central source electrode in fig. 3 is 10 μm, the abscissa of the electro-optical wave velocity matching point is 1.15 μm. When the cavity height used for the cavity corresponding to the central source electrode in fig. 4 is 3 μm, the abscissa of the electro-optical wave velocity matching point is 4.25 μm. As can be seen from fig. 3 and 4, when the cavity height of the cavity corresponding to the central source electrode is higher and the cavity width is wider, the effective refractive index of the microwave is lower, the matching degree of the electro-optical wave speed is optimal, and the modulation effect is the best. It should be noted, however, that the cavity width of each cavity should not exceed the width of the corresponding electrode, so as to avoid the cavity from excessively lowering the refractive index of the light wave.

In summary, the electro-optical modulator of the present invention is easy to implement electro-optical wave velocity matching, and can maintain the optimal matching degree even under high bandwidth. Compared with the prior art, the communication speed is faster, and the distance between the electrode and the optical waveguide is not increased, so that the electro-optical superposition integral is not influenced, and the half-wave voltage is not increased.

Fig. 5(a) to (g) are process flows of the thin-film electro-optic modulator of the present embodiment, which is easy to implement the electro-optic wave velocity matching, and include the following steps:

step 1, as shown in fig. 5(a), a lithium niobate thin film layer 103 having a thickness of 450nm is obtained by ion implantation.

And 2, oxidizing and growing a silicon dioxide bonding layer 104 with the thickness of 1.5 microns on the silicon substrate layer 001 with the thickness of 4.7 microns as shown in figure 5 (b).

And step 3, as shown in fig. 5(c), patterning the silicon dioxide bonding layer 104 by combining an ultraviolet lithography process and a reactive ion etching process to obtain a buried layer 002, a cavity 003 at two sides of the buried layer and a central cavity-004 of the buried layer.

And 4, as shown in fig. 5(d), bonding the lithium niobate thin film layer 103 obtained in the step 1 to the silicon substrate layer 001 by adopting a hydrophilic group bonding process.

And 6, as shown in fig. 5(f), etching the lithium niobate thin film layer obtained in the step 5 by adopting a reactive ion etching process to obtain a waveguide flat layer 005 with the thickness of 100nm and a waveguide ridge layer 006 with the thickness of 350 nm.

Step 7, as shown in fig. 5(g), growing and patterning the ground electrode layers 007 on the two sides and the central source electrode layer 008 on the lithium niobate waveguide flat plate layer 005 by adopting photoetching and magnetron sputtering processes; the electrode material is gold, and the thickness of the gold electrode is 4 μm.

The above-mentioned embodiments are further illustrative for the purpose, content and advantages of the present invention, and it should be understood that the above-mentioned embodiments are only illustrative for the purpose of the present invention and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A thin film electro-optic modulator easy to realize electro-optic wave velocity matching comprises a substrate layer, a thin film layer and a traveling wave electrode; the method is characterized in that:

2. A thin film electro-optic modulator for facilitating the matching of the velocity of an electro-optic wave as claimed in claim 1 wherein: the width of each cavity is less than or equal to the width of the corresponding electrode.

3. A thin film electro-optic modulator for facilitating the matching of the velocity of an electro-optic wave as claimed in claim 1 wherein: the substrate layer is made of monocrystalline silicon, lithium niobate, sapphire or quartz glass; the buried layer 002 is made of silicon dioxide, BCB (benzocyclobutene), PMMA (polymethyl methacrylate), or PDMS (polydimethylsiloxane) having a dielectric constant of less than 3; the material of the thin film wave layer is lithium niobate, silicon nitride, silicon carbide, gallium arsenide or gallium nitride; the two side ground electrodes 007 and the central source electrode 008 are made of gold, platinum, titanium, aluminum or copper.

4. A method for preparing a thin film electro-optic modulator easy to realize electro-optic wave velocity matching is characterized by comprising the following steps: the method comprises the following steps:

step 2, growing a silicon dioxide bonding layer on the silicon substrate layer;