Dual-polarization quadrature phase shift keying modulator

Abstract

The utility model discloses a dual-polarization quadrature phase shift keying modulator chip based on a lithium niobate film, which adopts an active polarization conversion scheme, so that compared with a traditional passive polarization conversion scheme based on an asymmetric dielectric waveguide, the dual-polarization quadrature phase shift keying modulator chip has the advantages that the polarization extinction ratio can be improved by more than 10dB; therefore, the eye pattern and the bit error rate of the receiving end are effectively improved. Particularly, in a DP-QPSK modulator chip scheme based on an X-cut lithium niobate film, an optical waveguide is bent by 90 degrees (namely, Y conduction is deflected to Z conduction) at the output end side of a second quadrature phase shift keying modulation QPSK module QPSK-X, and then active polarization conversion is implemented, so that two-section active polarization conversion control can be conveniently provided, polarization conversion is more accurately controlled, and the polarization conversion efficiency is improved. And the polarization extinction ratio is improved.

Description

Technical Field

[0001] The utility model relates to the field of optical fiber communication, in particular to a dual-polarization quadrature phase shift keying modulator chip based on lithium niobate thin film. Background Technique

[0002] In the field of coherent communication, a dual-polarization quadrature phase shift keying (DP-QPSK) modulator is a widely used high-speed modulator.

[0003] Figure 1 Fig. shows a basic structure of a DP-QPSK modulator, which includes two QPSK modulators with orthogonal polarizations, such as a QPSK-X modulator and a QPSK-Y modulator. As Figure 1 shown, each QPSK modulator is a dual-parallel Mach-Zehnder modulator, and each QPSK modulator inputs two signals, namely an in-phase signal I and a quadrature-phase signal Q. Since the optical polarization directions of the two QPSK modulators are the same (for example, both are TE modes for an X-cut lithium niobate thin film modulator), it is necessary to rotate the optical path polarization direction of the QPSK-X modulator by 90° before beam combination to convert the TE mode to the TM mode, and then combine its optical path with that of the QPSK-Y modulator, so as to achieve dual-polarization (TE / TM) operation. Among them, the device for realizing the polarization rotation (PR) and beam combination (PBC) functions is a PRC (Polarization Rotator Combiner) module.

[0004] In a DP-QPSK modulator, the quality of the polarization rotation (PR) performance directly affects the eye diagram and bit error rate at the receiving end. The index characterizing the PR performance is the polarization extinction ratio. Taking the incident TE light as an example, after polarization rotation, the ideal output optical polarization state is the TM mode, and the actual output is a mixed polarization state, including both TE and TM polarization modes. The ratio of the optical power of the TM mode to that of the TE mode is the polarization extinction ratio. The higher the polarization extinction ratio, the better the polarization rotation performance. For example, in Figure 2 the QPSK-X channel, the TE mode is rotated into the TM mode after PR, and the residual TE-mode optical power is combined with the QPSK-Y channel and then transmitted to the receiving end in the form of the signal of the Y channel, forming channel crosstalk noise.

[0005] In the prior art, for a DP-QPSK modulator based on lithium niobate thin film, the PRC module usually uses an asymmetric dielectric waveguide structure to realize, that is, the PR and PBC functions are provided simultaneously by means of the asymmetric dielectric waveguide structure, as Figure 2-3As shown. For the PRC module based on this asymmetric medium structure, the polarization extinction ratio after polarization conversion is relatively low (10 - 20 dB), which has a greater impact on the eye diagram and bit error rate, resulting in phenomena such as the inability to open the eye diagram and a high bit error rate. Therefore, it is necessary to improve the polarization extinction ratio in the polarization rotation process of the DP-QPSK modulator and reduce the polarization crosstalk between the two channels (QPSK-X and QPSK-Y). Summary of the Invention

[0006] To address this problem, the present invention proposes a dual-polarization quadrature phase shift keying modulator chip structure based on lithium niobate thin film. Among them, by adopting an active polarization conversion scheme, compared with the passive polarization conversion scheme based on an asymmetric dielectric waveguide in the prior art, the polarization extinction ratio can be increased by more than 10 dB, thereby effectively improving the eye diagram and bit error rate at the receiving end. Especially in the DP-QPSK modulator chip scheme based on X-cut lithium niobate thin film, by bending the optical waveguide 90 degrees (i.e., deflecting from Y-conduction to Z-conduction) at the output side of the second quadrature phase shift keying modulation QPSK module QPSK-X and then implementing active polarization conversion, it is possible to conveniently provide two-stage active polarization conversion control, more precisely control the polarization conversion, and improve the polarization extinction ratio.

[0007] Specifically, the dual-polarization quadrature phase shift keying modulator may include a first quadrature phase shift keying modulation module, a second quadrature phase shift keying modulation module, a polarization rotation module, and a polarization beam combining module;

[0008] The polarization rotation module is configured to rotate the polarization direction of the second optical signal output by the second quadrature phase shift keying modulation module so as to be orthogonal to the polarization direction of the first optical signal output by the first quadrature phase shift keying modulation module;

[0009] The polarization beam combining module is configured to combine the first optical signal and the second optical signal whose polarization directions are orthogonal to each other;

[0010] It is characterized in that the dual-polarization quadrature phase shift keying modulation module is formed based on lithium niobate material, and the polarization rotation module is an active module.

[0011] Preferably, the lithium niobate material is lithium niobate thin film.

[0012] More preferably, the lithium niobate thin film is X-cut lithium niobate thin film.

[0013] Furthermore, the first optical signal is input into the polarization beam combining module through an X-cut Y-conduction lithium niobate thin film optical waveguide;

[0014] The second optical signal is input into the polarization beam combining module through an X-cut Z-conduction lithium niobate thin film optical waveguide;

[0015] The polarization rotation module is configured to rotate the polarization direction of a second optical signal transmitted in the X-cut Z-propagation lithium niobate thin film optical waveguide.

[0016] Furthermore, the polarization rotation module includes a tuning electrode and a control electrode;

[0017] The tuning electrode is configured to apply an electric field in the Y direction to the X-cut Z-propagation lithium niobate thin film optical waveguide;

[0018] The control electrode is configured to apply an electric field in the X direction to the X-cut Z-propagation lithium niobate thin film optical waveguide.

[0019] Furthermore, the polarization rotation module includes a first polarization rotation unit and a second polarization rotation unit;

[0020] The first polarization rotation unit includes a first tuning electrode and a first control electrode;

[0021] The second polarization rotation unit includes a second tuning electrode and a second control electrode.

[0022] Furthermore, the X-cut Z-propagation lithium niobate thin film optical waveguide is a ridge waveguide formed by etching a lithium niobate thin film;

[0023] The tuning electrode is formed on both sides of the ridge waveguide;

[0024] The control electrode is formed above the ridge waveguide.

[0025] Furthermore, a silicon oxide isolation layer is formed between the tuning electrode and the ridge waveguide, and a silicon oxide isolation layer is formed between the control electrode and the ridge waveguide.

[0026] Furthermore, a silicon oxide passivation layer is also covered on the tuning electrode and the control electrode.

[0027] Optionally, the first quadrature phase shift keying modulation module and the second quadrature phase shift keying modulation module are dual parallel Mach-Zehnder modulators. Description of the Drawings

[0028] The following further elaborates on the specific embodiments of the present invention in conjunction with the drawings.

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following briefly introduces the drawings required for use in the description of the embodiments or the prior art. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0030] Figure 1 shows a basic structure of a DP-QPSK modulator;

[0031] Figure 2 shows a PRC module implemented based on an asymmetric dielectric waveguide structure in the prior art;

[0032] Figure 3 shows another PRC module implemented based on an asymmetric dielectric waveguide structure in the prior art;

[0033] Figure 4 shows an example of a dual-polarization quadrature phase shift keying modulator according to the present invention, which is implemented based on a lithium niobate thin film;

[0034] Figure 5 shows an example of an active polarization rotation module according to the present invention, which is implemented based on a lithium niobate thin film. Detailed implementation manners

[0035] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example to fully convey the spirit of the present invention to those skilled in the art to which the present invention pertains. Therefore, the present invention is not limited to the embodiments disclosed herein.

[0036] Figure 4 shows an example of a dual-polarization quadrature phase shift keying modulator according to the present invention.

[0037] As Figure 4 shown, the dual-polarization quadrature phase shift keying modulator may include a first quadrature phase shift keying modulation module QPSK-Y, a second quadrature phase shift keying modulation module QPSK-X, a polarization rotation module, and a polarization beam combining module PBC.

[0038] The first quadrature phase shift keying modulation module QPSK-Y may modulate an input optical signal (which is, for example, in the TE mode) to output a first optical signal.

[0039] The second quadrature phase shift keying modulation module QPSK-X may modulate an input optical signal (which is, for example, in the TE mode) to output a second optical signal.

[0040] As an example, the first / second quadrature phase shift keying modulation module may be in the form of a dual-parallel Mach-Zehnder modulator.

[0041] The polarization rotation module PR can be arranged on the output side of the second quadrature phase shift keying modulation module, and is used to rotate the polarization direction of the second optical signal, so as to be orthogonal to the polarization direction of the first optical signal. For example, the second optical signal in the TE mode is deflected into the TM mode, so as to realize the polarization mode orthogonal to the TE mode of the first optical signal.

[0042] The polarization beam combining module PBC can be arranged behind the polarization rotation module PR, and is used to combine the first optical signal (TE mode) and the second optical signal (TM mode) whose polarization directions are orthogonal to each other.

[0043] Different from the implementation method of adopting an asymmetric dielectric waveguide structure in the dual-polarization quadrature phase shift keying modulator based on lithium niobate (thin film) materials in the prior art, in the present invention, the polarization rotation module can be implemented in an active manner, so as to allow precise control of the rotation of the polarization direction of the optical signal. For example, the polarization extinction ratio of the conversion from the TE mode to the TM mode is improved, which is beneficial to improving the eye diagram and bit error rate at the receiving end.

[0044] The following will be combined with Figure 4-5 Specifically describe an example of the dual-polarization quadrature phase shift keying modulator based on lithium niobate thin film materials according to the present invention, so as to better understand the working principle of the present invention.

[0045] In Figure 4 In the example, the dual-polarization quadrature phase shift keying modulator is formed based on X-cut lithium niobate thin film materials. Therefore, the input optical signal will be transmitted to the first / second quadrature phase shift keying modulation QPSK module in the TE mode through the X-cut Y-conduction lithium niobate thin film optical waveguide to receive the corresponding modulation processing.

[0046] The first quadrature phase shift keying modulation QPSK module QPSK-Y outputs the first optical signal, which is input into the polarization beam combining module PBC through the X-cut Y-conduction lithium niobate thin film optical waveguide in the TE mode.

[0047] The second quadrature phase shift keying modulation QPSK module QPSK-X also outputs a second optical signal in the TE mode. Among them, since the lithium niobate crystal belongs to the 3m point group and its electro-optic tensor is a 6×3 matrix, the r61 component can be used to achieve polarization mode conversion. Therefore, in the present invention, in order to achieve active polarization rotation in the X-cut lithium niobate thin film modulator, at the output side of the second quadrature phase shift keying modulation QPSK module QPSK-X, by deflecting the lithium niobate thin film optical waveguide direction by 90 degrees, that is, the optical waveguide transmission direction is deflected from the Y direction to the Z direction, the second optical signal is input into the polarization beam combining module through the X-cut Z-conduction lithium niobate thin film optical waveguide, so that polarization rotation can be achieved by applying a control electric field in the X direction. At the same time, due to the existence of birefringence, the refractive indices of the TE and TM modes are different. In order to achieve the highest conversion efficiency, a tuning electric field in the Y direction can also be applied for phase modulation.

[0048] As Figure 4 shown, at the output side of the second quadrature phase shift keying modulation QPSK module QPSK-X, the second optical signal in the TE mode enters the X-cut Z-conduction lithium niobate thin film optical waveguide and is transmitted toward the polarization beam combining module.

[0049] At the same time, the active polarization rotation module includes a tuning electrode and a control electrode. Among them, the tuning electrode is configured to apply a tuning electric field in the Y direction to the X-cut Z-conduction lithium niobate thin film optical waveguide, while the control electrode is configured to apply an electric field in the X direction to the X-cut Z-conduction lithium niobate thin film optical waveguide. Therefore, by adjusting the voltage magnitudes on the tuning electrode and the control electrode, the polarization rotation acting on the second optical signal can be accurately controlled, and the polarization extinction ratio of the second optical signal converted from the TE mode to the TM mode can be improved.

[0050] The inventor further found through research that the above-mentioned X-direction electric field and Y-direction electric field for polarization rotation both act on the optical waveguide through metal electrodes. Limited by the processing technology, the electric field generated by the control electrode applying the X-direction electric field is not a perfect X direction, and a partial Y-direction electric field will also be generated. Moreover, the electric field generated by the tuning electrode applying the Y-direction electric field is not a perfect Y direction, and a partial X-direction electric field will also be generated. This will inevitably cause interference between the tuning electric field and the control electric field and affect the effectiveness of polarization rotation.

[0051] Therefore, as a preferred example, the polarization rotation module may include a first polarization rotation unit and a second polarization rotation unit.

[0052] The first polarization rotation unit includes a first tuning electrode and a first control electrode, which are used for primary polarization rotation of the second optical signal. Similarly, a second tuning electrode and a second control electrode are provided in the second polarization rotation unit for correction on the basis of primary polarization conversion to further improve the polarization extinction ratio.

[0053] As an example, through a two-stage active polarization rotation control scheme, the polarization extinction ratio can ultimately be increased to 25 - 30 dB, which is more than 10 dB higher than that of the asymmetric medium scheme in the prior art.

[0054] Figure 5 Fig. shows an example of an active polarization rotation module implemented based on a lithium niobate thin film material according to the present invention.

[0055] As Figure 5 shown, a lithium niobate thin film ridge optical waveguide can be formed on a lithium niobate thin film wafer through, for example, an etching process.

[0056] Then, deposit a first layer of silicon oxide thin film and lithograph a pattern to form a silicon oxide isolation layer for subsequent use as an isolation layer between the electrode and the ridge optical waveguide.

[0057] Next, deposit a control electrode and a tuning electrode respectively and lithograph patterns respectively, so as to form tuning electrodes on both sides of the ridge waveguide and a control electrode above the ridge waveguide. Among them, there is a silicon oxide isolation layer between each of the tuning electrodes on both sides and the ridge waveguide, and there is a silicon oxide isolation layer between the control electrode above and the ridge waveguide.

[0058] Finally, deposit another layer of silicon oxide as a passivation layer.

[0059] In summary, in the DP-QPSK modulator chip structure of the present invention, by adopting an active polarization conversion scheme, compared with the passive polarization conversion scheme implemented based on an asymmetric dielectric waveguide in the prior art, the polarization extinction ratio can be increased by more than 10 dB, thereby effectively improving the eye diagram and bit error rate at the receiving end. Among them, especially in the DP-QPSK modulator chip scheme based on an X-cut lithium niobate thin film, by bending the optical waveguide 90 degrees (i.e., deflecting from Y conduction to Z conduction) at the output side of the second quadrature phase shift keying modulation QPSK module QPSK-X and then implementing active polarization conversion, a two-stage active polarization conversion control can be conveniently provided, the polarization conversion can be more precisely controlled, and the polarization extinction ratio can be increased.

[0060] Although the present invention has been described above with reference to specific embodiments in conjunction with the accompanying drawings, it is easy for those skilled in the art to recognize that the above embodiments are merely exemplary and are used to illustrate the principle of the present invention, which will not limit the scope of the present invention. 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 invention.

Claims

1. A dual-polarization quadrature phase shift keying modulator, which comprises a first quadrature phase shift keying modulation module, a second quadrature phase shift keying modulation module, a polarization rotation module, and a polarization beam combining module; The polarization rotation module is configured to rotate the polarization direction of a second optical signal output by the second quadrature phase shift keying modulation module, so as to be orthogonal to the polarization direction of a first optical signal output by the first quadrature phase shift keying modulation module; The polarization beam combining module is configured to combine the first optical signal and the second optical signal whose polarization directions are orthogonal to each other; It is characterized in that The dual-polarization quadrature phase shift keying modulation module is formed based on a lithium niobate material, and the polarization rotation module is an active module.

2. The dual-polarization quadrature phase shift keying modulator according to claim 1, characterized in that, The lithium niobate material is a lithium niobate thin film.

3. The dual-polarization quadrature phase shift keying modulator according to claim 2, characterized in that, The lithium niobate thin film is an X-cut lithium niobate thin film.

4. The dual-polarization quadrature phase shift keying modulator according to claim 3, wherein The first optical signal is input into the polarization beam combining module through an X-cut Y-propagating lithium niobate thin film optical waveguide; The second optical signal is input into the polarization beam combining module through an X-cut Z-propagating lithium niobate thin film optical waveguide; The polarization rotation module is configured to rotate the polarization direction of the second optical signal transmitted in the X-cut Z-propagating lithium niobate thin film optical waveguide.

5. The dual-polarization quadrature phase shift keying modulator according to claim 4, wherein The polarization rotation module includes a tuning electrode and a control electrode; The tuning electrode is configured to apply an electric field in the Y direction to the X-cut Z-propagating lithium niobate thin film optical waveguide; The control electrode is configured to apply an electric field in the X direction to the X-cut Z-propagating lithium niobate thin film optical waveguide.

6. The dual-polarization quadrature phase shift keying modulator according to claim 5, wherein The polarization rotation module includes a first polarization rotation unit and a second polarization rotation unit; The first polarization rotation unit includes a first tuning electrode and a first control electrode; The second polarization rotation unit includes a second tuning electrode and a second control electrode.

7. The dual-polarization quadrature phase shift keying modulator according to claim 5, characterized in that, The X-cut Z-propagating lithium niobate thin film optical waveguide is a ridge waveguide formed by etching a lithium niobate thin film; The tuning electrode is formed on both sides of the ridge waveguide; The control electrode is formed above the ridge waveguide.

8. The dual-polarization quadrature phase shift keying modulator according to claim 7, wherein A silicon oxide isolation layer is formed between the tuning electrode and the ridge waveguide, and a silicon oxide isolation layer is formed between the control electrode and the ridge waveguide.

9. The dual-polarization quadrature phase shift keying modulator according to claim 8, characterized in that, A silicon oxide passivation layer also covers the tuning electrode and the control electrode.

10. The dual-polarization quadrature phase shift keying modulator according to any one of claims 1-9, characterized in that The first quadrature phase shift keying modulation module and the second quadrature phase shift keying modulation module are dual-parallel Mach-Zehnder modulators.

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