A lithium niobate modulator

Abstract

The application discloses a lithium niobate modulator, which realizes impedance matching components by using coaxial transmission lines, can allow the impedance matching components to be integrated with radio frequency connectors and modulator housings while improving the S11 bandwidth of the modulator, compared with existing impedance matching components based on the coplanar waveguide mode, improves the integration of the modulator, reduces the size of the modulator and the number of components, and improves the overall reliability of the modulator. In addition, the design requirements of the lithium niobate modulator chip are simplified, the redundancy of key parameters in chip design (including fire line width, electrode spacing, electrode thickness, etc.) is allowed to be increased, and the bandwidth of the device is obviously improved. And by reducing the width of the chip, the yield of the chip of the same size wafer can be improved, which is extremely beneficial to the industrial application of high-speed lithium niobate modulators.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical communication devices, and in particular to a lithium niobate modulator. BACKGROUND

[0002] Lithium niobate crystal has high electro-optic coefficient, fast response and other characteristics, and the electro-optic modulator prepared therefrom has large bandwidth, low half-wave voltage and other characteristics, and is a key core device in the field of optical communication. S11 and S21 are two important performance parameters of the lithium niobate modulator, and -10dB is usually taken as the determination standard of the S11 bandwidth (electrical bandwidth), and -3dB is usually taken as the determination standard of the S21 bandwidth (modulation bandwidth). In order to improve the modulation bandwidth S21 of the lithium niobate modulator, a traveling wave electrode design is usually adopted, and the characteristic impedance thereof is generally between 30Ω and 40Ω (the typical value is 34Ω), which is different from the 50Ω output impedance of the signal source. When the traveling wave electrode is directly connected with the signal source, the impedance discontinuity or impedance mismatch is the main reason for causing the S11 and S21 ripple phenomenon, and is also an important reason for causing the low S11 bandwidth.

[0003] Figures 1-3 The characteristic impedance of a typical high-speed lithium niobate modulator traveling wave electrode, and the S11 and S21 bandwidths when the traveling wave electrode is directly connected with the signal source are respectively shown. As shown in FIG. 1, Figure 2 when the traveling wave electrode is directly connected with the signal source, the S11 bandwidth is -10.5dB@2GHz, which is close to the determination standard -10dB of the S11 bandwidth. Considering the consistency of the product manufacturing process, the impedance mismatch will obviously seriously affect the qualification rate of the S11 bandwidth.

[0004] In order to improve the S11 bandwidth, one of the solutions is to use an impedance matching circuit. Figures 4-5 The structure and cross-sectional schematic diagram of a lithium niobate modulator in the prior art (US5138480) are respectively shown, which includes a lithium niobate substrate 110, MZ optical waveguides 230, 231 and 232, a chip electrode unit composed of an input pad 130 (including 112, 114, 116), traveling wave electrodes 120, 122, 124 and an output pad 132 (including 112, 114, 116), a ceramic circuit board 150 for providing an impedance matching circuit, and a radio frequency connector 176. In order to improve the S11 bandwidth, a section of coplanar waveguide transmission line is added between the traveling wave electrode and the radio frequency connector 176, the transmission line is made on the ceramic circuit board, and the characteristic impedance thereof continuously changes from 50Ω of the signal source to 34Ω of the traveling wave electrode. With the continuous change of the impedance, the impedance mismatch or sudden change is avoided, and thus the S11 can be reduced to a certain extent. The disadvantage is that the ceramic circuit board 150 and the lithium niobate chip need to be jointly packaged in the shell of the modulator, which will increase the size and complexity of the modulator and reduce the reliability thereof.

[0005] Since the line width (W h ) to the electrode spacing (G) (see for example Figure 5 ) is negatively correlated with its characteristic impedance, i.e., increasing the live wire width (W h ) to the electrode spacing (G) can reduce the characteristic impedance. Therefore, another impedance matching circuit implementation scheme has been proposed in the prior art, which adds an impedance transition section at the rear end of the chip input pad, wherein the ratio of the live wire width to the electrode spacing is reduced to continuously change the characteristic impedance from 34Ω to 50Ω, and then the input pad is connected, for example Figure 6 As shown in the figure. However, the impedance transition section used in this solution is also a coplanar waveguide transmission line, which can improve the S11 bandwidth to ≤-13dB in the 40GHz range. However, the addition of this impedance transition section to the chip reduces the chip width to a typical value of 2mm, which is approximately 60% wider than a chip without the transition section.

[0006] It can be seen that the two existing solutions for the impedance matching circuit in the lithium niobate modulator both adopt the implementation form of coplanar waveguide, which requires the addition of additional components and increases the size of the chip and the device. Summary of the Invention

[0007] In response to the above-mentioned problems existing in the prior art, the present invention discloses a lithium niobate modulator, which realizes an impedance matching component by utilizing a coaxial transmission line structure. While effectively improving the S11 bandwidth of the modulator, it allows the impedance matching component to be integrated with the radio frequency connector and the housing of the modulator. Compared with the existing impedance matching component realized based on the coplanar waveguide method, it can improve the integration of the modulator, reduce the size of the modulator and the number of components, and improve the overall reliability of the modulator. In addition, by realizing the impedance matching component outside the chip, the design requirements of the lithium niobate modulator chip are simplified, and the redundancy of key parameters in the chip design (including the width of the hot wire, the electrode spacing, the electrode thickness, etc.) can be increased, which has a significant improvement effect on improving the bandwidth of the device and reducing the half-wave voltage. Moreover, by reducing the chip width, the chip yield of wafers of the same size can be improved, which is extremely beneficial for the industrial application of high-speed lithium niobate modulators.

[0008] Specifically, the lithium niobate modulator of the present invention may include an electrode unit and a radio frequency signal access structure;

[0009] The radio frequency signal access structure is used to connect the radio frequency signal to the electrode unit, and includes a radio frequency connector and an impedance matching component;

[0010] The radio frequency connector is used to connect to a radio frequency signal source via a coaxial transmission line;

[0011] The impedance matching component is arranged between the RF connector and the electrode unit for matching the impedance between the RF connector and the electrode unit in the form of a coaxial transmission line.

[0012] Further, the impedance matching component comprises an outer conductor, an inner conductor and a filling medium filled between the outer conductor and the inner conductor, the inner conductor being electrically connected to the electrode unit.

[0013] Preferably, the electrode unit comprises a traveling wave electrode, the outer conductor is integrally formed with a housing of the lithium niobate modulator, and the inner conductor is electrically connected to a hot wire of the traveling wave electrode through an electrical connector.

[0014] Preferably, the electrical connector is in the form of a ribbon bond.

[0015] Further, the impedance matching component has a portion with a continuously changing ratio D / d of an inner diameter D of the outer conductor to an outer diameter d of the inner conductor for realizing a smooth transition from a first characteristic impedance to a second characteristic impedance, the first characteristic impedance being a characteristic impedance of the RF connector, and the second characteristic impedance being a characteristic impedance of the electrode unit.

[0016] Optionally, in the portion with the continuously changing ratio D / d, the outer diameter d of the inner conductor and / or the inner diameter D of the outer conductor varies linearly, cosinely or tangentially along an axial direction.

[0017] Further, the impedance matching component comprises a first medium matching section and a second medium matching section.

[0018] The first medium matching section is filled with a first filling medium, which is air.

[0019] The second medium matching section is filled with a second filling medium, which is a solid filling medium.

[0020] One of the first and second medium matching sections is for connecting the RF connector, and the other is for connecting the electrode unit.

[0021] Preferably, the outer conductors of the first and second medium matching sections are integrally formed with a housing of the lithium niobate modulator, the first and second medium matching sections share the inner conductor, and the inner conductor is fixed to the housing of the lithium niobate modulator through the second filling medium.

[0022] Optionally, the second filling medium is glass.

[0023] Preferably, one of the first and second medium matching sections is cylindrical, and the other comprises a portion with a continuously changing ratio D / d of an inner diameter D of the outer conductor to an outer diameter d of the inner conductor.

[0024] Optionally, the lithium niobate modulator of the present application can be an intensity modulator, a phase modulator, a polarization modulator or a lithium niobate thin film modulator. BRIEF DESCRIPTION OF DRAWINGS

[0025] The specific embodiments of the present application will be further described in details below with reference to the accompanying drawings.

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the accompanying drawings required to be used in the embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description are only some embodiments of the present application, and other accompanying drawings can also be obtained by those skilled in the art without any creative effort based on these accompanying drawings.

[0027] Figures 1-3 The characteristic impedance of the traveling wave electrode in a typical high-speed lithium niobate modulator, and the parameters S11 and S21 when the traveling wave electrode is directly connected to the signal source are shown respectively;

[0028] Figures 4-5 The structural schematic diagram and the cross-sectional view of a lithium niobate modulator in the prior art are shown respectively;

[0029] Figure 6 Another implementation scheme of the impedance matching circuit for the lithium niobate modulator in the prior art is shown;

[0030] Figure 7 A perspective view of the lithium niobate modulator according to the present application is schematically shown;

[0031] Figure 8 A cross-sectional view of the impedance matching component according to the present application is schematically shown;

[0032] Figure 9 A partial enlarged view of the lithium niobate modulator according to the present application is schematically shown;

[0033] Figure 10 A preferred example of the impedance matching component according to the present application is schematically shown;

[0034] Figure 11 Another preferred example of the impedance matching component according to the present application is schematically shown;

[0035] Figure 12 and 13 The S11 parameter test results of the lithium niobate modulator of the present application and the S11 parameter test results of the lithium niobate modulator without using the impedance matching component are shown respectively. DETAILED DESCRIPTION

[0036] In the following, exemplary embodiments of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example in order to convey the spirit of the present application to those skilled in the art to which the present application pertains. Therefore, the present application is not limited to the embodiments disclosed herein.

[0037] Similar to typical high-speed lithium niobate modulators, the lithium niobate modulator of the present application can include input / output tail fibers, an electrode unit (e.g., a traveling wave electrode), a lithium niobate chip, a radio frequency signal access structure, and a housing, as shown in Figure 7

[0038] In the lithium niobate modulator, the radio frequency signal access structure can include an impedance matching component and a radio frequency connector, the radio frequency connector connects a signal source through a coaxial transmission line to receive a radio frequency signal, and the impedance matching component has two ends connected to the radio frequency connector and an input pad of the electrode unit, respectively, to introduce the radio frequency signal into the electrode unit in an impedance matching manner; an output pad of the electrode unit is connected to a terminal load. Therefore, when modulating an optical signal, the radio frequency signal emitted by the signal source enters the lithium niobate modulator through the coaxial transmission line, and then passes through the radio frequency connector, the impedance matching component, the input pad, the electrode unit, the output pad, and the terminal load in sequence.

[0039] Unlike the existing implementation of the impedance matching component, the impedance matching component of the present application will be implemented with the help of a coaxial transmission line, so that the characteristic impedance of the signal source (e.g., 50Ω) can be continuously and smoothly transitioned to the characteristic impedance of the electrode unit (e.g., 34Ω), which improves the S11 bandwidth of the modulator while allowing the impedance matching component to be integrated with the radio frequency connector and possibly a (glass) insulator, reducing the width of the chip and the size of the modulator device.

[0040] Figure 8 A cross-sectional view of the impedance matching component of the present application is shown. As shown in the figure, the impedance matching component can include an outer conductor, an inner conductor, and a filling medium filled between the two. When the outer diameter of the outer conductor of the impedance matching component is D, the outer diameter of the inner conductor is d, and the dielectric constant of the filling medium between the two is ε r , the characteristic impedance Z0 of the impedance matching component is:

[0041]

[0042] ​Therefore, the ratio D / d of the outer diameter D of the outer conductor to the inner diameter d of the inner conductor in the impedance matching component can be continuously changed, so as to realize a continuous and smooth transition from the first characteristic impedance (i.e. the characteristic impedance of the signal source, for example 50 Ω) of the RF connector to the second characteristic impedance (for example the characteristic impedance of the traveling wave electrode, 34 Ω) of the electrode unit, eliminate the phenomenon of impedance mutation or impedance mismatch from the RF connector to the electrode unit, and realize the continuous impedance between the signal source and the electrode unit (for example the traveling wave electrode), thereby effectively improving the S11 bandwidth of the modulator. For example, when air is used as the filling medium, the D / d is continuously changed from 2.5 to 1.5, so as to continuously change the characteristic impedance of the impedance matching component from 56 Ω to 25 Ω; when glass is used as the filling medium, the D / d is continuously changed from 6.8 to 1.8, so as to continuously change the characteristic impedance of the impedance matching component from 58 Ω to 18 Ω.

[0043] According to the present application, in the portion of the impedance matching component where the D / d is continuously changed, the outer diameter d of the inner conductor and / or the inner diameter D of the outer conductor can be changed along the axial direction in any appropriate continuous and smooth curve, such as a linear, cosine or tangent curve, so as to provide the required continuous and smooth transition from the first characteristic impedance to the second characteristic impedance.

[0044] Further, in the impedance matching component realized based on the coaxial transmission line, the outer conductor can be integrally formed with the shell of the lithium niobate modulator, and the inner conductor can be fixed to the shell of the modulator through the filling medium and electrically connected to the input pad of the electrode unit through the electrical connection, as shown in Figure 9 Thus, the impedance matching component, the RF connector and the insulator can be integrally integrated on the shell of the modulator, which is extremely advantageous for reducing the chip width and the size of the modulator device.

[0045] In the example shown in Figure 9 It can be noted that the electrical connection between the inner conductor and the input pad can be realized by means of a bonding ribbon as the electrical connection, and further realize the electrical connection with the hot wire of the traveling wave electrode, for example.

[0046] In the preferred example, for the convenience of assembly and improvement of the overall structural strength, the impedance matching component can include a first medium matching section and a second medium matching section. The first medium matching section can be filled with air as the first filling medium, and the second medium matching section can be filled with a solid medium (for example glass) as the second filling medium. The outer conductors of the first and second medium matching sections can be integrally formed with the shell of the lithium niobate modulator, and the first and second medium matching sections share one inner conductor, and the inner conductor is fixedly connected to the shell of the lithium niobate modulator through the second filling medium, for example as shown in Figure 10 .

[0047] In this preferred example, one of the first and second dielectric matching sections (e.g. the second dielectric matching section) can be used to connect the radio frequency connector, and the other (e.g. the first dielectric matching section) can be used to connect the electrode unit.

[0048] Further, a portion (e.g. an approximately conical portion) in which the ratio D / d of the outer conductor inner diameter D to the inner conductor outer diameter d is continuously varied can be provided only in one of the first and second dielectric matching sections (e.g. the second dielectric matching section) for achieving a continuous smooth transition of the required characteristic impedance, and a standard coaxial transmission line structure in which the outer conductor inner diameter D is constant in the axial direction is employed in the other. Figure 10 and 11 Further, a portion (e.g. an approximately conical portion) in which the ratio D / d of the outer conductor inner diameter D to the inner conductor outer diameter d is continuously varied can be provided only in one of the first and second dielectric matching sections (e.g. the second dielectric matching section) for achieving a continuous smooth transition of the required characteristic impedance, and a standard coaxial transmission line structure in which the outer conductor inner diameter D is constant in the axial direction is employed in the other.

[0049] For example, in the example shown in Figure 10 , the first dielectric matching section (i.e. the air section) using air as the filling medium is used to connect the live wire of the travelling wave electrode, and the second dielectric matching section (i.e. the glass section) using glass as the filling medium is used to connect the radio frequency connector, wherein a D / d continuously varied portion in which the outer conductor inner diameter D is continuously varied in the axial direction is provided only in the first dielectric matching section, and the outer conductor inner diameter D of the second dielectric matching section is constant in the axial direction and is in the form of a cylinder as a whole. In this example, when the inner conductor outer diameter d = 0.38 mm, the outer conductor inner diameter D of the first dielectric matching section can gradually decrease from the outside to the inside from 0.88 mm to 0.67 mm, thereby causing the characteristic impedance to smoothly decrease from 50 Ω to 34 Ω, so as to achieve impedance matching between the signal source and the travelling wave electrode.

[0050] For example, in the example shown in Figure 11 , the first dielectric matching section (i.e. the air section) using air as the filling medium is used to connect the live wire of the travelling wave electrode, and the second dielectric matching section (i.e. the glass section) using glass as the filling medium is used to connect the radio frequency connector, wherein a D / d continuously varied portion in which the outer conductor inner diameter D is continuously varied in the axial direction is provided only in the first dielectric matching section, and the outer conductor inner diameter D of the second dielectric matching section is constant in the axial direction and is in the form of a cylinder as a whole. In this example, when the inner conductor outer diameter d = 0.38 mm, the outer conductor inner diameter D of the first dielectric matching section can gradually decrease from the outside to the inside from 0.88 mm to 0.67 mm, thereby causing the characteristic impedance to smoothly decrease from 50 Ω to 34 Ω, so as to achieve impedance matching between the signal source and the travelling wave electrode.

[0051] Figure 12 and 13The S11 parameter test results of the lithium niobate modulator of the present application and the S11 parameter test results of the lithium niobate modulator without using the impedance matching component are shown respectively, wherein the S11 bandwidth (<-10dB) is increased from 6GHz to 22GHz in the lithium niobate modulator of the present application, and the S11 parameter is also reduced by 4-5dB in the low frequency band within 10GHz, which proves that the lithium niobate modulator of the present application can effectively improve the S11 bandwidth.

[0052] Meanwhile, the chip width of the lithium niobate modulator of the present application can be reduced to 1.25-1.4mm, compared with the prior art using the impedance transition section on the chip (for example Figure 6 , the typical chip width of which is 2mm), due to the reduction of the chip width, the number of chips produced by a 3-inch wafer is increased by 50%, and the chip yield is greatly improved.

[0053] Those skilled in the art can understand that the present application can be applied to high-speed lithium niobate intensity modulators, high-speed lithium niobate phase modulators, high-speed lithium niobate polarization modulators, high-speed lithium niobate thin film modulators and other optoelectronic devices.

[0054] As can be seen from the above, in the lithium niobate modulator of the present application, the impedance matching component is realized by using the coaxial transmission line structure, which can effectively improve the S11 bandwidth of the modulator (actually measured: the S11 bandwidth can be increased by more than 10GHz, and the S11 parameter can be reduced by 4-5dB in the low frequency band): at the same time, the impedance matching component of the present application can be integrated with the radio frequency connector and the shell of the modulator, compared with the existing impedance matching component based on the coplanar waveguide mode, the integration degree of the modulator can be improved, the size of the modulator and the number of components can be reduced, and the overall reliability of the modulator is improved. By reducing the chip width, the chip yield of the same size wafer can be improved, for example, by more than 50%, which is extremely beneficial to the industrial application of high-speed lithium niobate modulators. In addition, by realizing the impedance matching component outside the chip, the design requirements of the lithium niobate modulator chip are simplified, allowing the redundancy of key parameters in chip design (including the fire width, electrode spacing, electrode thickness, etc.) to be increased, which has obvious improvement effect on improving the bandwidth of the device and reducing the half-wave voltage.

[0055] Although the present application has been described in detail with reference to specific embodiments in the accompanying drawings, those skilled in the art will readily recognize that the above embodiments are merely exemplary, used to illustrate the principles of the present application, and do not limit the scope of the present application, and those skilled in the art can make various combinations, modifications and equivalent replacements to the above embodiments without departing from the spirit and scope of the present application.

Claims

1. A lithium niobate modulator comprising an electrode unit and a radio frequency signal access structure; The radio frequency signal access structure is used to connect the radio frequency signal to the electrode unit, and includes a radio frequency connector and an impedance matching component; The radio frequency connector is used to connect to a radio frequency signal source via a coaxial transmission line; The impedance matching component is provided between the RF connector and the electrode unit, and is used to achieve impedance matching between the RF connector and the electrode unit in the form of a coaxial transmission line; The impedance matching component includes an outer conductor, an inner conductor and a filling medium filled between the outer conductor and the inner conductor, and the inner conductor is electrically connected to the electrode unit; The impedance matching component has a portion where the ratio D / d of the inner diameter D of the outer conductor to the outer diameter d of the inner conductor continuously changes, and is used to achieve a smooth transition from a first characteristic impedance to a second characteristic impedance, where the first characteristic impedance is the characteristic impedance of the RF connector and the second characteristic impedance is the characteristic impedance of the electrode unit; In the portion where D / d changes continuously, the outer diameter d of the inner conductor and / or the inner diameter D of the outer conductor changes in a linear, cosine or tangent curve along the axial direction.

2. The lithium niobate modulator according to claim 1, wherein: The electrode unit includes a traveling wave electrode, the outer conductor is formed integrally with the shell of the lithium niobate modulator, and the inner conductor is electrically connected to the live wire of the traveling wave electrode through an electrical connector.

3. The lithium niobate modulator according to claim 2, wherein: The electrical connector is in the form of a bonded gold ribbon.

4. The lithium niobate modulator according to claim 1, wherein: The impedance matching component includes a first dielectric matching section and a second dielectric matching section; The first medium matching section is filled with a first filling medium, which is air; The second medium matching section is filled with a second filling medium, which is a solid filling medium; One of the first and second dielectric matching sections is used to connect to a radio frequency connector, and the other is used to connect to an electrode unit.

5. The lithium niobate modulator according to claim 4, wherein: The outer conductors of the first and second dielectric matching sections are integrally formed with the housing of the lithium niobate modulator. The first and second dielectric matching sections share an inner conductor, and the inner conductor is fixed to the housing of the lithium niobate modulator via a second filling medium.

6. The lithium niobate modulator according to claim 4, wherein: The second filling medium is glass.

7. The lithium niobate modulator according to claim 4, wherein: One of the first and second dielectric matching sections is cylindrical, and the other includes a portion where the ratio D / d of the inner diameter D of the outer conductor to the outer diameter d of the inner conductor continuously changes.

8. The lithium niobate modulator according to any one of claims 1 to 7, which is an intensity modulator, a phase modulator, a polarization modulator or a lithium niobate thin film modulator.

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