Electro-optic modulator and method of making the same

Abstract

This application provides an electro-optic modulator and its fabrication method. The electro-optic modulator includes: a device layer; an optical waveguide located on the device layer, the optical waveguide including at least a first waveguide interference arm and a second waveguide interference arm; a coplanar traveling-wave electrode located on the device layer; and a control layer covering the first waveguide interference arm, used to change the effective refractive index of the optical waveguide. The length of the control layer enables the electro-optic modulator to operate in the linear bias region. This invention controls the zero-point bias of the electro-optic modulator by introducing a control layer, completely eliminating the introduction of an external electric field and avoiding interference with the original electrical signal.

Description

Technical Field

[0001] This application relates to the field of modulator technology, specifically to an electro-optic modulator and its fabrication method. Background Technology

[0002] Currently, clinical electroencephalography (EEG) measurement techniques primarily rely on electrode guidance, circuit amplification, and signal recording to acquire brain signals from the subject's scalp. This electrical method directly acquires signals with a low signal-to-noise ratio, making it impossible to observe subthreshold potential changes. Analysis has identified three main reasons for this reduced signal-to-noise ratio: The weak (1μV-200μV) values ​​of EEG signals, the unstable signals and the susceptibility to interference cause a decrease in the signal-to-noise ratio of the signals acquired by the EEG detection method. The complex and ever-changing testing environment reduces the signal-to-noise ratio of EEG signals; Electromagnetic signals in combined detection techniques interfere with EEG signals. The third factor mentioned above is particularly prominent in EEG detection methods and is an inherent problem of electrical detection techniques that cannot be eliminated. To address the numerous problems in electrical EEG acquisition technology, new EEG signal acquisition methods need to be explored. Optical detection methods are passive and resistant to electromagnetic interference, and most optical sensors offer high measurement sensitivity and a wide range of adjustable parameters, exhibiting advantages such as corrosion resistance, light weight, flexibility, and good biocompatibility in the field of physiological signal detection. Therefore, research on optical detection methods for physiological signals has become a hot topic in recent years. The most common method for converting electrical signals into optical signals is to use electro-optic modulators. Currently, lithium niobate (LiNbO3) crystals possess a large electro-optic coefficient, excellent optical transmittance (up to 98% in the 400-5000nm wavelength range), optical uniformity, low hygroscopicity, and mature processing technology. Therefore, LiNbO3 modulators made from this material have been widely used in high-frequency optical communication and fiber optic sensing. In 2017 and 2020, electro-optic modulators packaged with LiNbO3 Mach-Zehnder (MZ) modulation chips were successfully used to acquire electrocardiogram (ECG) and alpha brainwave (AEG) signals.

[0003] However, EEG signals are extremely weak compared to traditional electrical signals, much weaker than the half-wave voltage of a lithium niobate electro-optic modulator. If the amplitude of light intensity variation during measurement is small, a high-magnification amplifier is required, which introduces noise. To minimize noise while ensuring a significant output signal amplitude within a small range, its linear operating region must be at a 0V bias position. This eliminates the need for an electric field line at the sensing end that captures the EEG signal without applying a DC bias, ensuring the modulation depth of the lithium niobate electro-optic modulator and the passive operation of the sensing element. The theoretical sensitivity can reach 0.04mW / V. Traditional high-speed lithium niobate electro-optic modulators often use an external DC bias voltage to dynamically adjust the zero-point potential. However, because the EEG signal value is small, using an external bias introduces external interference. Summary of the Invention

[0004] The problem this application aims to solve is to control the zero-point bias of the electro-optic modulator by introducing a control layer, without introducing an external electric field, thus avoiding interference with the original electrical signal.

[0005] To solve the above-mentioned technical problems, the technical solution adopted in this application is: an electro-optic modulator, comprising:

[0006] Device layer;

[0007] An optical waveguide is located on the device layer, and the optical waveguide includes at least a first waveguide interference arm and a second waveguide interference arm;

[0008] Coplanar traveling wave electrodes are located on the device layer; and

[0009] A control layer, covering the first waveguide interferometer arm, is used to change the effective refractive index of the optical waveguide. The length of the control layer enables the MZ electro-optic modulator to operate in the linear bias region.

[0010] Preferably, the control layer is a silicon oxide layer.

[0011] Preferably, the coplanar traveling wave electrode comprises:

[0012] The center electrode is located between the first waveguide interferometer arm and the second waveguide interferometer arm;

[0013] The ground electrode is located outside the first and second waveguide interferometer arms.

[0014] Preferably, it includes:

[0015] A first electro-optic modulator is formed on the device layer. The first electro-optic modulator includes an optical waveguide and a coplanar traveling wave electrode. The optical waveguide includes a first waveguide interference arm and a second waveguide interference arm.

[0016] The first electro-optic modulator is calibrated;

[0017] An intermediate control layer is formed on the first waveguide interferometer arm of the first electro-optic modulator to form the second electro-optic modulator;

[0018] The length of the intermediate control layer is adjusted, and the interference intensity curve of the second electro-optic modulator is measured to form an electro-optic modulator with a control layer.

[0019] The control layer is used to change the effective refractive index of the optical waveguide, and the length of the control layer enables the electro-optic modulator to operate in the linear bias region.

[0020] Preferably, the method for calibrating the first electro-optic modulator includes:

[0021] The first electro-optic modulator receives an optical signal, which is split into two coherent beams with the same amplitude and enters the first waveguide interferometer arm and the second waveguide interferometer arm respectively.

[0022] When an external electric field is applied to the coplanar traveling wave electrode, a phase difference is generated between the first waveguide interferometer arm and the second waveguide interferometer arm, and interference occurs.

[0023] The output light amplitude of the first electro-optic modulator is measured to obtain its maximum and minimum amplitude, and the amplitude difference between the maximum and minimum amplitude is obtained.

[0024] Preferably, the control layer is a silicon oxide layer.

[0025] Preferably, the length of the intermediate control layer is adjusted by etching the intermediate control layer.

[0026] Preferably, the coplanar traveling wave electrode comprises:

[0027] The center electrode is located between the first waveguide interferometer arm and the second waveguide interferometer arm;

[0028] The ground electrode is located outside the first and second waveguide interferometer arms.

[0029] The advantages and positive effects of this application are as follows: This application introduces a control layer to control the zero-point bias of the electro-optic modulator, completely eliminating the introduction of an external electric field and avoiding interference with the original electrical signal. Furthermore, unlike typical communication environments, the temperature of the human body surface is constant, constituting a static working environment. Zero-point drift caused by temperature changes is negligible. Therefore, a fixed zero-point bias is set to compensate for zero-point drift caused by process defects or material inhomogeneity. Attached Figure Description

[0030] Figure 1 A three-dimensional structural schematic diagram of an electro-optic modulator according to an embodiment of the present invention is shown;

[0031] Figure 2A schematic diagram of the structure of a first electro-optic modulator according to an embodiment of the present invention is shown;

[0032] Figure 3 The interference intensity curve of the output light from the optical waveguide is shown;

[0033] Figure 4 A schematic diagram of the structure of a second electro-optic modulator according to an embodiment of the present invention is shown;

[0034] Figure 5 A schematic diagram of the structure for etching the intermediate control layer 103 is shown. Detailed Implementation

[0035] To better understand this application, the following description, in conjunction with specific embodiments and accompanying drawings, will further illustrate this application.

[0036] Figure 1 A three-dimensional structural schematic diagram of an electro-optic modulator according to an embodiment of the present invention is shown, as follows: Figure 1 As shown, the electro-optic modulator 10 includes: a substrate 105, a device layer 101 located on the substrate 105, wherein an optical waveguide 102, a coplanar traveling wave electrode 104 and a control layer 103 are formed on the device layer 101.

[0037] In this embodiment, the materials of the substrate 105 and the device layer 101 are, for example, lithium niobate, but this is not a limitation. In other embodiments, any semiconductor material can be selected as needed.

[0038] Optical waveguide 102 is located on the surface of device layer 101 and includes a first branch structure 1023, a first waveguide interferometer arm 1021, a second waveguide interferometer arm 1022, and a second branch structure 1024. The first branch structure 1023 is a Y-shaped branch, including one input terminal and two output terminals. The input terminal of the first branch structure 1023 serves as the input terminal of the optical waveguide 102 for receiving optical signals. The two output terminals of the first branch structure 1023 are coupled to the input terminals of the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022, respectively. The second branch structure 1024 is a Y-shaped branch, including two input terminals and one output terminal. The two input terminals of the second branch structure 1024 are coupled to the output terminals of the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022, respectively. The output terminal of the second branch structure 1024 serves as the output terminal of the optical waveguide 102, outputting optical signals.

[0039] The coplanar traveling wave electrode 104 includes a center electrode 1041 and a ground electrode 1042. The center electrode 1041 is located between the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022, and the ground electrode 1042 is located outside both the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022. The coplanar traveling wave electrode 104 is used to apply an external electric field to the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022.

[0040] The control layer 103 covers the first waveguide interferometer arm 1021. The control layer 103 is used to change the effective refractive index of the optical waveguide 102, and the length of the control layer 103 causes the electro-optic modulator 10 to operate in the linear bias region.

[0041] In this embodiment, the coplanar traveling wave electrode 104 applies an external electric field to the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022. Due to the electro-optic effect, the refractive index of the light in the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 changes. Specifically, the input end of the first branch structure 1023 receives the optical signal and splits it into two coherent beams with the same amplitude via the first branch structure 1021. These beams then enter the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022, respectively. Due to the effect of the applied electric field, a phase difference is generated in the light in the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022. Furthermore, since the first waveguide interferometer arm 1021 is covered with a control layer 103, its refractive index is reduced. Compared to the second waveguide interferometer arm 1022, the optical path length of the first waveguide interferometer arm 1021 is reduced, resulting in a phase delay. The light output from the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 interferes at the second branch structure 1024 and is output via the second output terminal 1024b of the second branch structure 1024. In this embodiment, by adjusting the length of the control layer 103, the electro-optic modulator operates in the linear bias region.

[0042] The second embodiment of the present invention provides a method for fabricating an electro-optic modulator, the method comprising:

[0043] S10: Form the first electro-optic modulator on the device layer.

[0044] Figure 2 A schematic diagram of the structure of the first electro-optic modulator according to an embodiment of the present invention is shown, as follows: Figure 2 As shown, the first electro-optic modulator includes: a substrate 105, a device layer 101 located on the substrate 105, wherein an optical waveguide 102 and a coplanar traveling wave electrode 104 are formed on the device layer 101.

[0045] In this embodiment, the materials of the substrate 105 and the device layer 101 are, for example, lithium niobate, but this is not a limitation. In other embodiments, any semiconductor material can be selected as needed.

[0046] Optical waveguide 102 is located on the surface of device layer 101 and includes a first branch structure 1023, a first waveguide interferometer arm 1021, a second waveguide interferometer arm 1022, and a second branch structure 1024. The first branch structure 1023 is a Y-shaped branch, including one input terminal and two output terminals. The input terminal of the first branch structure 1023 serves as the input terminal of the optical waveguide 102 for receiving optical signals. The two output terminals of the first branch structure 1023 are coupled to the input terminals of the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022, respectively. The second branch structure 1024 is a Y-shaped branch, including two input terminals and one output terminal. The two input terminals of the second branch structure 1024 are coupled to the output terminals of the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022, respectively. The output terminal of the second branch structure 1024 serves as the output terminal of the optical waveguide 102, outputting optical signals.

[0047] The coplanar traveling wave electrode 104 includes a center electrode 1041 and a ground electrode 1042. The center electrode 1041 is located between the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022, and the ground electrode 1042 is located outside both the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022. The coplanar traveling wave electrode 104 is used to apply an external electric field to the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022.

[0048] S20: Calibrate the first electro-optic modulator.

[0049] In this step, the coplanar traveling-wave electrode 104 applies an external electric field to the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022. The optical waveguide 102 receives the optical signal, which is split into two coherent beams with the same amplitude via the first branch structure 1023 and enters the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 respectively. Due to the applied electric field, the refractive indices of the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 change, thereby generating a phase difference. The output light from the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 enters the second branch structure 1024 and interferes at the second branch structure 1024, and is output via the second branch structure 1024. The amplitude of the output light from the optical waveguide is measured to obtain its maximum amplitude. and minimum amplitude And obtain the amplitude difference between the maximum and minimum amplitude values. ,in, .

[0050] In a specific embodiment, the optical electric field received at the input end of optical waveguide 1-2 is:

[0051]

[0052] The electric field of the light wave entering the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 is as follows:

[0053]

[0054] The electric field of the optical wave output from the output terminal of the optical waveguide 102 is:

[0055]

[0056] In the formula, Phase shift of the first optical wave interferometer arm This is due to the phase shift in the second interferometer arm, caused by the imperfect symmetry between the two arms. If the phase shift is introduced by the control layer, then the output optical power is:

[0057]

[0058] In the formula, It is the total phase shift introduced by the electro-optic modulator, by , and Composition, considering the losses of the lithium niobate electro-optic modulator under practical conditions, .

[0059] Figure 3 The interference intensity curve of the output light from optical waveguide 102 is shown. Figure 3 As can be seen from this, the output optical power is at 0.5 There exists an approximately linear working region, with a total phase shift of [missing information]. At this point, the first electro-optic modulator operates in the near-linear operating region, achieving maximum modulation efficiency.

[0060] S30: An intermediate control layer 103a is formed on the first waveguide interferometer arm 1021 of the first electro-optic modulator to form a second electro-optic modulator. Figure 4 As shown. The intermediate control layer 103a is used to change the refractive index of the optical waveguide. The intermediate control layer 103a is, for example, a silicon dioxide layer, but is not limited to this. The material of the intermediate control layer 103a can be specifically selected as needed.

[0061] S40: Adjust the length of the intermediate control layer and measure the interference intensity curve of the second electro-optic modulator to form an electro-optic modulator with a control layer.

[0062] In this step, the coplanar traveling wave electrode 104 applies an external electric field to the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022. The optical waveguide 102 receives the optical signal, which is split into two coherent beams with the same amplitude via the first branch structure 1023 and enters the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 respectively. Due to the effect of the applied electric field, the refractive index of the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 changes, thereby generating a phase difference. Since the first waveguide interferometer arm 1021 is covered with an intermediate control layer 103a, its refractive index is reduced, and the optical path of the first waveguide interferometer arm 1021 is reduced compared to the second waveguide interferometer arm 1022, resulting in a phase delay. The output light from the first waveguide interferometer arm 1021 and the second waveguide interferometer arm 1022 enters the second branch structure 1024 and interferes at the second branch structure 1024, and is output via the second branch structure 1024.

[0063] Furthermore, Figure 5 A schematic diagram of the structure for etching the intermediate control layer 103a is shown, as follows. Figure 5 As shown, the intermediate control layer 103a is etched to adjust its length.

[0064] Specifically, the second electro-optic modulator is slowly immersed in an etching solution, with a portion of the intermediate control layer 103a submerged and a portion partially exposed. The submerged portion of the intermediate control layer 103a is then removed. During the etching process of the intermediate control layer 103a, the interference intensity is simultaneously measured. As the length of the intermediate control layer 103a immersed in the etching solution increases, its length decreases, causing the optical path length of the signal light transmitted in the first waveguide interferometer arm 1021 beneath the intermediate control layer 103a to continuously change. This results in a continuous change in the phase at the output end of the optical waveguide, thereby altering the interference intensity of the output light. The interference intensity of the output light from the optical waveguide 102 reaches the linear region, i.e., 0.5. ( When the radius of the neighborhood of the approximate linear interval is reached, the second electro-optic modulator is removed and the etching is stopped. At this time, the intermediate control layer 103a is etched to form the final control layer 103, and the final electro-optic modulator 10 is obtained. The electro-optic modulator 10 operates in the linear bias region.

[0065] In this invention, no external electric field is introduced, thus avoiding interference with the original electrical signal; preliminary simulations show that, with the refractive index of lithium niobate... waveguide refractive index The control layer material is silicon dioxide, which has a mature process and controllable properties. Silicon dioxide has a refractive index... Taking the transmission TE mode as an example, the effective refractive index of the upper waveguide without a silicon dioxide control layer (the upper layer of the waveguide is an air layer) is 2.1507; the effective refractive index of the lower waveguide covered with a silicon dioxide control layer (the upper layer of the waveguide is a silicon dioxide layer with a thickness of 1μm) is 2.1503, and the theoretical phase delay is 1.63 rad / mm.

[0066] In specific implementations of this invention, the structure of the electro-optic modulator may be appropriately adjusted depending on the circumstances. This embodiment does not impose any restrictions on this.

[0067] In this application, a control layer is introduced to control the zero-point bias of the electro-optic modulator, without introducing any external electric field, thus avoiding interference with the original electrical signal. Furthermore, unlike typical communication environments, the temperature of the human body surface is constant, constituting a static working environment. Zero-point drift caused by temperature changes is negligible. Therefore, a fixed zero-point bias is set to compensate for zero-point drift caused by process defects or material inhomogeneity.

[0068] The embodiments of this application have been described in detail above, but the content described is only a preferred embodiment of this application and should not be considered as limiting the scope of this application. All equivalent changes and improvements made within the scope of this application should still fall within the scope of this patent.

Claims

1. A method for fabricating an electro-optic modulator, wherein, include: A first electro-optic modulator is formed on the device layer. The first electro-optic modulator includes an optical waveguide and a coplanar traveling wave electrode. The optical waveguide includes a first waveguide interference arm and a second waveguide interference arm. The first electro-optic modulator is calibrated; An intermediate control layer is formed on the first waveguide interferometer arm of the first electro-optic modulator to form the second electro-optic modulator; The length of the intermediate control layer is adjusted, and the interference intensity curve of the second electro-optic modulator is measured to form an electro-optic modulator with a control layer. The control layer is used to change the effective refractive index of the optical waveguide, and the length of the control layer enables the electro-optic modulator to operate in the linear bias region. The method for calibrating the first electro-optic modulator includes: The first electro-optic modulator receives an optical signal, which is split into two coherent beams with the same amplitude and enters the first waveguide interferometer arm and the second waveguide interferometer arm respectively. When an external electric field is applied to the coplanar traveling wave electrode, a phase difference is generated between the first waveguide interferometer arm and the second waveguide interferometer arm, and interference occurs. The output light amplitude of the first electro-optic modulator is measured to obtain its maximum and minimum amplitude, and the amplitude difference between the maximum and minimum amplitude is obtained.

2. The method for fabricating the electro-optic modulator according to claim 1, wherein, The intermediate control layer is a silicon oxide layer.

3. The method for fabricating the electro-optic modulator according to claim 1, wherein, The length of the intermediate control layer is adjusted by eroding the intermediate control layer.

4. The method for fabricating the electro-optic modulator according to claim 1, wherein, Coplanar traveling wave electrodes include: The center electrode is located between the first waveguide interferometer arm and the second waveguide interferometer arm; The ground electrode is located outside the first and second waveguide interferometer arms.

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