Method and system for testing characteristic indexes of lithium niobate polarization controller

Abstract

The invention discloses a lithium niobate polarization controller characteristic index test method and test system, the lithium niobate polarization controller comprises a substrate, an optical waveguide, an electrode A, an electrode B and an electrode C, and the electrode B is grounded. The method comprises the following steps: connecting a light source along the length direction of the optical waveguide, taking the propagation direction of light as a Z axis, and connecting the light source to the optical waveguide; establishing a coordinate system by taking the distribution direction of the electrode A, the electrode B and the electrode C as a Y axis and taking a direction perpendicular to a plane formed by the Z axis and the Y axis as an X axis; a modulation electric field in the X direction is applied to the lithium niobate polarization controller, the polarization state at the moment is analyzed, and conversion voltage is obtained; a modulation electric field in the Y direction is applied to the lithium niobate polarization controller, the polarization state at the moment is analyzed, and half-wave voltage is obtained. The refractive index of the lithium niobate crystal is directly regulated and controlled through an external electric field, the optical waveguide is equivalent to an adjustable wave plate, external devices are omitted, and efficient, accurate and low-cost testing of characteristic indexes of the lithium niobate polarization controller is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of electro-optic modulation communication, and particularly relates to a method and a system for testing the characteristic indexes of a lithium niobate polarization controller. Background Art

[0002] The lithium niobate polarization controller belongs to the category of electro-optic modulators. Its structure is different from that of common lithium niobate phase modulators and intensity modulators. It is mainly used for regulating the polarization characteristics of optical signals, such as measuring the polarization characteristics in couplers, amplifiers, attenuators, and wavelength division multiplexers. The half-wave voltage and the conversion voltage are important indexes for measuring the optoelectronic conversion efficiency of the lithium niobate polarization controller, and directly reflect the magnitude of the driving voltage of the device. Among them, the conversion voltage is the voltage applied when all the energy of the optical wave signal rotates from the transverse electric (TE) mode to the transverse magnetic (TM) mode, and the half-wave voltage is the voltage required for the optical wave signal to change by 180° between the TE mode and the TM mode.

[0003] At present, the conventional method for measuring the half-wave voltage of a polarization controller mainly uses the method of starting oscillation with a polarizer for measurement. Specifically as follows: First, the incident light passes through the polarization controller and is modulated by an external voltage; then the output light at the output end of the polarization controller enters the polarizer for starting oscillation; further, the polarized light passing through the polarizer is connected to a photodetector; finally, the photodetection signal and the external voltage signal are connected to an oscilloscope together, and the voltage corresponding to half a cycle of the photodetection signal is intercepted to deduce the half-wave voltage and the conversion voltage of the polarization controller. However, this method requires adding auxiliary devices and equipment such as a polarizer, a photodetector, and an oscilloscope, resulting in a high cost problem, and there is a large measurement blind area under some input polarization states. Summary of the Invention

[0004] Aiming at the deficiencies of the above-mentioned prior art, the technical problem to be solved by the present invention is: to provide a method and a system for testing the characteristic indexes of a lithium niobate polarization controller that can quickly and simply measure the half-wave voltage and the conversion voltage of the lithium niobate polarization controller while reducing the measurement blind area.

[0005] To solve the above technical problem, a technical solution adopted by the present invention is: to provide a method for testing the characteristic indexes of a lithium niobate polarization controller. The lithium niobate polarization controller includes a substrate, an optical waveguide disposed on the substrate, an electrode B disposed directly above the optical waveguide, and electrodes A and C spaced apart on both sides of the optical waveguide. The electrode B is grounded, and the method includes the following steps:

[0006] A light source is connected along the length direction of the optical waveguide. Taking the propagation direction of the light as the Z-axis, the distribution direction of the electrodes A, B, and C as the Y-axis, and the direction perpendicular to the plane formed by the Z-axis and the Y-axis as the X-axis, a coordinate system is established.

[0007] Apply a modulation electric field in the X direction on the lithium niobate polarization controller through the host computer, analyze the polarization state at this time, and then obtain the conversion voltage;

[0008] Apply a modulation electric field in the Y direction on the lithium niobate polarization controller through the host computer, analyze the polarization state at this time, and then obtain the half-wave voltage.

[0009] Further, in the step of applying a modulation electric field in the X direction on the lithium niobate polarization controller through the host computer, analyzing the polarization state at this time, and then obtaining the conversion voltage, the following sub-steps are included:

[0010] Input a modulation voltage in the Y direction in the host computer to compensate for the voltage in the Y direction inherent in the lithium niobate crystal material, and input a modulation voltage in the X direction in the host computer to apply a modulation electric field in the X direction;

[0011] Obtain the Stokes parameters based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and display the polarization state of the polarized light through the Poincaré sphere;

[0012] Adjust the modulation voltage in the X direction until several circles of circular polarization states are displayed on the Poincaré sphere;

[0013] Calculate the conversion voltage based on the modulation voltage in the X direction at this time. Among them, the modulation voltage in the X direction corresponding to one circle of circular polarization state is equal to four times the conversion voltage.

[0014] Further, in the step of applying a modulation electric field in the Y direction on the lithium niobate polarization controller through the host computer, analyzing the polarization state at this time, and then obtaining the half-wave voltage, the following sub-steps are included:

[0015] Input a modulation voltage in the X direction in the host computer to compensate for the voltage in the X direction inherent in the lithium niobate crystal material, and input a modulation voltage in the Y direction in the host computer to apply a modulation electric field in the Y direction;

[0016] Calculate the Stokes parameters based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and display the polarization state of the polarized light through the Poincaré sphere;

[0017] Adjust the modulation voltage in the Y direction until several circles of circular polarization states are displayed on the Poincaré sphere;

[0018] Calculate the half-wave voltage based on the modulation voltage in the Y direction at this time. Among them, the modulation voltage in the Y direction corresponding to one circle of circular polarization state is equal to twice the half-wave voltage.

[0019] Further, in the step of obtaining the Stokes parameters based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and displaying the polarization state of the polarized light through the Poincaré sphere, the following sub-steps are included:

[0020] Obtain the Jones matrix of the equivalent wave plate of the optical waveguide under the modulation electric field based on the modulation voltage in the X direction and the modulation voltage in the Y direction;

[0021] Obtain the Jones vector of the output polarized light based on the Jones matrix and the Jones vector of the input polarized light;

[0022] Calculate the Stokes parameters based on the Jones vector of the output polarized light;

[0023] Map the Stokes parameters onto the Poincaré sphere to display the polarization state of the output polarized light.

[0024] Further, in the step of obtaining the Jones matrix of the equivalent wave plate of the optical waveguide under the modulation electric field based on the modulation voltage in the X direction and the modulation voltage in the Y direction, the following sub-steps are included:

[0025] Calculate the angle by which the principal axis of the refractive index ellipsoid rotates relative to the corresponding coordinate axis in the original coordinate system after applying the modulation electric field, denoted as the first rotation angle;

[0026] Calculate the phase difference introduced by the equivalent wave plate based on the modulation voltage in the X direction and the modulation voltage in the Y direction, denoted as the first phase difference;

[0027] Obtain the Jones matrix of the equivalent wave plate based on the first rotation angle and the first phase difference.

[0028] Further, in the step of calculating the phase difference introduced by the equivalent wave plate based on the modulation voltage in the X direction and the modulation voltage in the Y direction, the following sub-steps are included:

[0029] After the principal axes of the refractive index ellipsoid are aligned, obtain a new coordinate system, and calculate the refractive index in the X' direction and the refractive index in the Y' direction in the new coordinate system;

[0030] Obtain the refractive index difference based on the refractive index in the X' direction and the refractive index in the Y' direction;

[0031] Obtain the light wavelength and the thickness of the equivalent wave plate, and obtain the phase difference introduced by the equivalent wave plate based on the refractive index difference, the light wavelength, and the thickness of the equivalent wave plate.

[0032] Further, the calculation formula of the first rotation angle is as follows:

[0033]

[0034] In formula (1), F x represents the modulation voltage in the X direction, F y represents the modulation voltage in the Y direction, and θ represents the first rotation angle;

[0035] The calculation formula of the first phase difference is as follows:

[0036]

[0037] In formula (II), μ represents the first phase difference, n x' -n y' represents the refractive index difference, d represents the thickness of the equivalent wave plate, and λ represents the optical wavelength;

[0038] In the step of obtaining the Jones matrix of the equivalent wave plate based on the first rotation angle and the first phase difference, the Jones matrix is expressed as:

[0039]

[0040] In formula (III), μ represents the first phase difference, and θ represents the first rotation angle.

[0041] Furthermore, in the step of calculating the refractive index in the X' direction and the refractive index in the Y' direction in the new coordinate system, the calculation formula for the refractive index in the X' direction is as follows:

[0042]

[0043] In formula (IV), F x represents the modulation voltage in the X direction, F y represents the modulation voltage in the Y direction, n x' represents the refractive index in the X' direction, n o represents the refractive index of the ordinary light, γ 22 represents the electro-optic coefficient tensor, and θ represents the first rotation angle;

[0044] The calculation formula for the refractive index in the Y' direction is as follows:

[0045]

[0046] In formula (V), n y' represents the refractive index in the Y' direction;

[0047] In the step of obtaining the refractive index difference based on the refractive index in the X' direction and the refractive index in the Y' direction, the calculation formula for the refractive index difference is as follows:

[0048] n x' -n y' =n o 3 γ 22 (cos(2θ)F y +sin(2θ)F x (VI)

[0049] In formula (VI), n x' -n y' represents the refractive index difference.

[0050] Further, in the step of obtaining the Jones vector of the output polarized light based on the Jones matrix and the Jones vector of the input polarized light, the calculation formula of the Jones vector of the output polarized light is as follows:

[0051]

[0052] In formula (VII), β represents the polarization angle of the input polarized light, represents the phase difference of the input polarized light, a represents the amplitude component of the output polarized light in the X direction, b represents the phase difference of the output polarized light, and c represents the amplitude component of the output polarized light in the Y direction;

[0053] In the step of calculating the Stokes parameters based on the Jones vector of the output polarized light, the Stokes parameters include the total light intensity S0, the light intensity difference S1 between 0 degrees and 90 degrees, the light intensity difference S2 between plus and minus 45 degrees, and the light intensity difference S3 between left-handed light and right-handed light. Among them, S0 = a 2 + c 2 , S1 = a 2 - c 2 , S2 = 2accosb, S3 = 2acsinb.

[0054] To solve the above technical problems, another technical solution adopted by the present invention is: to provide a test system for the characteristic indexes of a lithium niobate polarization controller, including:

[0055] A light source for providing input light to the lithium niobate polarization controller;

[0056] A polarization analyzer for analyzing the polarization state of the output light obtained after the input light passes through the lithium niobate polarization controller;

[0057] An upper computer for displaying the polarization state of the output light on the Poincaré sphere interface, and also for controlling the voltage signal output of the DC voltage source by inputting the modulation voltage in the X direction and the modulation voltage in the Y direction;

[0058] A DC voltage source for outputting a corresponding voltage signal under the control of the upper computer to apply a modulation electric field in the X direction or a modulation electric field in the Y direction on the lithium niobate polarization controller.

[0059] The test method and test system for the characteristic indexes of the lithium niobate polarization controller of the present invention have at least the following beneficial effects: By directly regulating the refractive index of the lithium niobate crystal through an externally applied electric field, the optical waveguide is equivalent to an adjustable wave plate, eliminating external devices such as polarizers and photodetectors, significantly reducing the hardware cost and test complexity; By the host computer controlling the application of X / Y direction modulation electric fields in real time and combining the Poincaré sphere visualization technology, the change of the output light polarization state can be dynamically captured, the Stokes parameters can be accurately calculated, and the half-wave voltage and conversion voltage are directly correlated through the circular polarization state, so that the target parameters can be quickly converted, avoiding the errors introduced by the photoelectric signal processing in the traditional method, and greatly reducing the measurement blind area; Adopting a fully electrically controlled adjustment method, the polarization state regulation and parameter calculation can be completed by inputting the modulation voltage through the host computer, without the need for manual frequent adjustment of optical devices, the operation process is highly automated, significantly reducing the risk of human error and improving the test efficiency; The traditional method is sensitive to the input polarization state and has a measurement blind area, while the present invention can cover the regulation requirements of any input polarization state through the electric field equivalent wave plate technology, and is applicable to the polarization characteristic tests of various optical devices such as couplers, wavelength division multiplexers, and amplifiers, with stronger versatility; The system has a fast response speed and a short measurement period, and can quickly obtain the accurate values of the half-wave voltage and conversion voltage, providing an efficient tool for the performance optimization and quality control of the lithium niobate polarization controller. Description of the Drawings

[0060] The drawings described herein are used to provide a further understanding of the present application, and constitute a part of the present application. The schematic embodiments and descriptions thereof of the present application are used to explain the present application, and do not constitute an improper limitation of the present application. In the drawings:

[0061] Figure 1 It is a schematic structural diagram of a lithium niobate polarization controller.

[0062] Figure 2 It is a flowchart of an embodiment of the test method for the characteristic indexes of the lithium niobate polarization controller of the present invention.

[0063] Figure 3 is Figure 2 The flowchart of step S200 in

[0064] Figure 4 is Figure 3 The flowchart of step S220 in

[0065] Figure 5 is Figure 4 The flowchart of step S221 in

[0066] Figure 6 It is a modulation schematic diagram in the Y direction of the lithium niobate polarization controller.

[0067] Figure 7Schematic diagram of modulation in the X direction of the lithium niobate polarization controller.

[0068] Figure 8 Schematic cross-sectional view of the refractive index ellipsoid after applying modulation voltages in the X and Y directions.

[0069] Figure 9 Schematic diagram of polarization state analysis on the Poincare sphere.

[0070] Figure 10 Schematic diagram of the circular polarization state of one turn of the Poincare sphere in this embodiment.

[0071] Figure 11 is Figure 2 the flowchart of step S300 in

[0072] Figure 12 Schematic diagram of an embodiment of the test system for the characteristic indexes of the lithium niobate polarization controller of the present invention. Specific embodiments

[0073] The present invention will be further described below with reference to the accompanying drawings.

[0074] Please refer to Figure 1 , the lithium niobate polarization controller includes a substrate, an optical waveguide disposed on the substrate, an electrode B disposed directly above the optical waveguide, and electrodes A and C spaced apart on both sides of the optical waveguide. Among them, electrode B is grounded, and positive and negative voltages can be applied to electrodes A and C. According to the prior art, the voltage V A applied to electrode A is = 2V0 * δ * sin(α) - V π * δ * cos(α) + V A,Bias , the voltage V C applied to electrode C is = 2V0 * δ * sin(α) + V π * δ * cos(α) + V C,Bias , and the voltage V B of electrode B is = 0. When the electrode compensation voltage V A,Bias on electrode A and the electrode compensation voltage V C,Bias on electrode C are both 0, the half-wave voltage conversion voltage Among them, δ represents the value of the wave plate (i.e., the retardation of the wave plate). It can be seen from this that both the half-wave voltage and the conversion voltage are affected by the wave plate. However, because in the actual operation process, the specific value of the wave plate cannot be calculated, and since it can be seen from the above formula that both the half-wave voltage and the conversion voltage are affected by the wave plate, therefore, in the present invention, the optical waveguide (lithium niobate crystal) is equivalent to an arbitrarily adjustable wave plate by applying an external modulation electric field, and then the characteristic indexes of the lithium niobate polarization controller are tested, and the characteristic indexes are the half-wave voltage and the conversion voltage. The half-wave voltage is the voltage required when the phase change of the optical wave signal between the TE mode and the TM mode is 180°, and the conversion voltage is the voltage applied when all the energy of the optical wave signal rotates from the TE mode to the TM mode (i.e., the phase change is 90°).

[0075] Please refer to Figure 2 , which is a flowchart of an implementation manner of the method for testing the characteristic indexes of the lithium niobate polarization controller of the present invention. The method for testing the characteristic indexes of the lithium niobate polarization controller in this implementation manner includes the following steps:

[0076] S100. Establish a coordinate system.

[0077] Specifically, please refer to Figure 1 , a light source is connected along the length direction of the optical waveguide, the propagation direction of the light is used as the Z axis, the distribution direction (TE direction) of the electrodes A, B, and C is used as the Y axis, and the direction (TM direction) perpendicular to the plane formed by the Z axis and the Y axis is used as the X axis to establish a coordinate system.

[0078] S200. Detect the conversion voltage.

[0079] Specifically, an X-direction modulation electric field is applied to the lithium niobate polarization controller through the upper computer, the polarization state at this time is analyzed, and then the conversion voltage is obtained. Please refer to Figure 3 , this step S200 includes the following sub-steps:

[0080] S210. Apply an X-direction modulation electric field.

[0081] Specifically, first input a Y-direction modulation voltage F y in the upper computer to compensate for the Y-direction voltage inherent in the lithium niobate crystal material, and then input an X-direction modulation voltage F x in the upper computer to apply an X-direction modulation electric field. The upper computer controls the voltages applied to the electrodes A and C according to the input modulation voltage, and the voltage V A on the electrode A = (F x + F y ) / 2, and the voltage V c on the electrode C = (F x - F y) / 2. It can be seen that after applying the same and co - directional electric field voltages to electrodes A and C, the voltage in the Y - direction is dynamically cancelled out, leaving only the modulation voltage in the X - direction. It should be noted that, in order to protect the device, in this embodiment, the input modulation voltage in the X - direction should be greater than or equal to - 150V and less than or equal to + 150V.

[0082] S220. Obtain the polarization state.

[0083] Specifically, obtain the Stokes parameters based on the modulation voltage in the X - direction and the modulation voltage in the Y - direction, and display the polarization state of the polarized light through the Poincaré sphere. Please refer to Figure 4 , this step S220 includes the following sub - steps:

[0084] S221. Calculate the Jones matrix.

[0085] Specifically, obtain the Jones matrix of the equivalent waveplate of the optical waveguide under the modulation electric field based on the modulation voltage in the X - direction and the modulation voltage in the Y - direction. Please refer to Figure 5 , this step S221 includes the following sub - steps:

[0086] S221a. Calculate the first rotation angle.

[0087] Specifically, calculate the angle by which the principal axis of the refractive index ellipsoid rotates relative to the corresponding coordinate axis in the original coordinate system after applying the modulation electric field, denoted as the first rotation angle. First, assume that when there is no external electric field, the refractive index ellipsoid equation of lithium niobate can be expressed as:

[0088]

[0089] where n x represents the refractive index in the X - direction, n y represents the refractive index in the Y - direction, n z represents the refractive index in the Z - direction, and satisfies n x = n y = n o , n z = n e , n o represents the refractive index of the o - light (ordinary light), n e represents the refractive index of the e - light (extraordinary light).

[0090] If only considering the linear electro - optic effect, the action of the external electric field F=(F x , F y , F z ) on the refractive index ellipsoid of the optical waveguide is expressed as:

[0091]

[0092] Because the electro-optic coefficient tensor of lithium niobate crystal is:

[0093]

[0094] When there is an external electric field F=(F x ,F y ,F z ), the refractive index ellipsoid of the lithium niobate optical waveguide is transformed into:

[0095]

[0096] In this embodiment, light propagates along the Z axis, and there is only an electric field in the plane where the X axis and the Y axis are located, satisfying the simplified formula:

[0097]

[0098] At this time, the refractive index ellipsoid is principal-axised to obtain the angle at which the principal axis of the refractive index ellipsoid rotates relative to the corresponding coordinate axis in the original coordinate system. The first rotation angle is obtained by the following formula:

[0099]

[0100] Where θ represents the first rotation angle, F x represents the modulation voltage in the X direction, F y Indicates the modulation voltage in the Y direction. Figure 6 , if only the Y direction is modulated, then F x Approaches 0, tan(2θ)=0, θ=0. Please refer to Figure 7 , if only the X direction is modulated, then F y Approaching 0, tan(2θ) = +∞ or -∞, or In the actual modulation process, F x and F y It cannot be completely zero, and voltage compensation in this direction is required. This also explains that in step S210, the modulation voltage F in the Y direction must first be input into the host computer. y This is to compensate for the voltage in the Y direction of the lithium niobate crystal material and the reason why the modulation voltage in the X direction must be input into the host computer in step S310 to compensate for the voltage in the X direction of the lithium niobate crystal material.

[0101] S221 b. Calculate the first digit phase difference.

[0102] Specifically, the phase difference introduced by the equivalent wave plate is calculated based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and is recorded as the first phase difference. Figure 8, the specific method is as follows: after aligning the principal axes of the refractive index ellipsoid, a new coordinate system is obtained, and the refractive index in the X' direction and the refractive index in the Y' direction in the new coordinate system are calculated; a refractive index difference is obtained based on the refractive index in the X' direction and the refractive index in the Y' direction; the optical wavelength and the thickness of the equivalent wave plate are acquired, and a phase difference introduced by the equivalent wave plate is obtained based on the refractive index difference, the optical wavelength, and the thickness of the equivalent wave plate.

[0103] The refractive index in the X' direction is obtained through the following formula:

[0104]

[0105] where F x represents the modulation voltage in the X direction, F y represents the modulation voltage in the Y direction, n x' represents the refractive index in the X' direction, n o represents the refractive index of the ordinary light, γ 22 represents the electro-optic coefficient tensor, and θ represents the first rotation angle.

[0106] The refractive index in the Y' direction is obtained through the following formula:

[0107]

[0108] where n y' represents the refractive index in the Y' direction.

[0109] The refractive index difference is obtained through the following formula:

[0110] n x' -n y' =n o 3 γ 22 (cos(2θ)F y +sin(2θ)F x )

[0111] where n x' -n y' represents the refractive index difference.

[0112] When light waves propagate along the Z direction in a lithium niobate crystal, if an electric field in the X and Y directions is applied to the crystal, the effect of the lithium niobate crystal on the light waves is equivalent to a wave plate whose principal axis of the crystal makes an angle θ with the X direction of the crystal. The refractive indices of the two principal axes of the wave plate are n x' and n y' respectively. By reasonably setting the modulation voltage in the X direction and the modulation voltage in the Y direction, the lithium niobate crystal can be equivalent to an arbitrary wave plate with adjustable angle and delay amount (closely related to the refractive index), thereby completing the regulation of any output light polarization state.

[0113] According to the properties of the wave plate, the first phase difference is obtained by the following formula:

[0114]

[0115] where μ represents the first phase difference, n x' -n y' represents the refractive index difference, d represents the thickness of the equivalent wave plate, λ represents the light wavelength. In this embodiment, the thickness of the equivalent wave plate can take the length value of the waveguide.

[0116] S221 c. Calculate the Jones matrix of the equivalent wave plate.

[0117] Specifically, the Jones matrix of the equivalent wave plate is obtained based on the first rotation angle and the first phase difference. According to Figure 8 the refractive index ellipsoid shown, the Jones matrix of the equivalent wave plate is expressed as:

[0118]

[0119] where i represents the imaginary number, μ represents the first phase difference, θ represents the first rotation angle, and the derivation process of obtaining the Jones matrix is a mature algorithm, which will not be elaborated here.

[0120] S222. Calculate the Jones vector of the output polarized light.

[0121] Specifically, the Jones vector of the output polarized light is obtained based on the Jones matrix and the Jones vector of the input polarized light. Multiply the Jones matrix by the Jones vector of the input polarized light to obtain the Jones vector of the output polarized light. The specific derivation process of the Jones vector of the input polarized light is a mature existing technology, which will not be elaborated here. The Jones vector of the output polarized light is obtained by the following formula:

[0122]

[0123] where β represents the polarization angle of the input polarized light, represents the phase difference of the input polarized light, a represents the amplitude component of the output polarized light in the X direction, b represents the phase difference of the output polarized light, and c represents the amplitude component of the output polarized light in the Y direction.

[0124] S223. Calculate the Stokes parameters.

[0125] Specifically, the Stokes parameters are calculated based on the Jones vector of the output polarized light. The Stokes parameters include the total light intensity S0, the light intensity difference S1 between 0 degrees and 90 degrees, the light intensity difference S2 between plus and minus 45 degrees, and the light intensity difference S3 between left-handed light and right-handed light. Among them, S0 = a 2 +c 2 , S1 = a 2 -c 2, S2 = 2accosb, S3 = 2acsinb.

[0126] S224. Display the polarization state of the output polarized light.

[0127] Specifically, refer to Figure 9 , map the Stokes parameters to the Poincaré sphere to display the polarization state of the output polarized light.

[0128] S230. Adjust the modulation voltage in the X direction.

[0129] Specifically, adjust the modulation voltage in the X direction until several circles of circular polarization states are displayed on the Poincaré sphere. As the modulation voltage increases, more and more polarization states are displayed on the Poincaré sphere. To avoid the problem of inaccurate counting due to too many circular polarization states, the number of circles of circular polarization should be as small as possible. In this embodiment, adjust the modulation voltage in the X direction until one circle of circular polarization state is displayed on the Poincaré sphere. Please refer to Figure 10 .

[0130] S240. Calculate the conversion voltage.

[0131] Specifically, calculate the conversion voltage according to the modulation voltage in the X direction at this time. According to the definition of the conversion voltage, the modulation voltage in the X direction corresponding to one circle of circular polarization state is equal to four times the conversion voltage. Divide the modulation voltage in the X direction input at this time by four to obtain the conversion voltage of the lithium niobate polarization controller in this embodiment.

[0132] S300. Detect the half-wave voltage.

[0133] Specifically, apply a modulation electric field in the Y direction on the lithium niobate polarization controller through the host computer, analyze the polarization state at this time, and then obtain the half-wave voltage. Please refer to Figure 11 , this step S300 includes the following sub-steps:

[0134] S310. Apply a modulation electric field in the Y direction.

[0135] Specifically, first input the modulation voltage F in the X direction in the host computer x to compensate for the voltage in the X direction inherent in the lithium niobate crystal material, and input the modulation voltage F in the Y direction in the host computer y to apply a modulation electric field in the Y direction. The host computer controls the voltages applied to electrode A and electrode C according to the input modulation voltages. The voltage V on electrode A A = (F x + F y ) / 2, and the voltage V on electrode C c = (F x - F y) / 2. It can be seen that after applying equal and opposite electric field voltages to electrode A and electrode C, the voltage in the X direction is dynamically cancelled out, leaving only the modulation voltage in the Y direction. It should be noted that in order to protect the device, in this embodiment, the input modulation voltage in the Y direction should be greater than or equal to -150V and less than or equal to +150V.

[0136] S320. Obtain the polarization state.

[0137] Specifically, the Stokes parameters are calculated based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and the polarization state of the output polarized light is displayed through the Poincaré sphere. For the specific steps, please refer to the content of step S220, which will not be elaborated here one by one.

[0138] S330. Adjust the modulation voltage in the Y direction.

[0139] Specifically, adjust the modulation voltage in the Y direction until several circles of circular polarization states are displayed on the Poincaré sphere. As the modulation voltage increases, more and more polarization states are displayed on the Poincaré sphere. To avoid the problem of inaccurate counting due to too many circular polarization states, the number of circles of circular polarization should be as small as possible. In this embodiment, adjust the modulation voltage in the Y direction until one circle of circular polarization state is displayed on the Poincaré sphere.

[0140] S340. Calculate the half-wave voltage.

[0141] Specifically, calculate the half-wave voltage according to the modulation voltage in the Y direction at this time. According to the definition of the half-wave voltage, the modulation voltage in the Y direction corresponding to one circle of circular polarization state is equal to twice the half-wave voltage. Divide the modulation voltage in the Y direction input at this time by two to obtain the half-wave voltage of the lithium niobate polarization controller in this embodiment.

[0142] By modulating the refractive index of the optical waveguide through an electric field, the input polarized light with any polarization state can be modulated into the output polarized light with any polarization state, and the polarization state of the output light is characterized by the Stokes parameters on the Poincaré sphere. Further analyzing the polarization state, the half-wave voltage and conversion voltage of the lithium niobate polarization controller can be obtained.

[0143] Please refer to Figure 12, which is a schematic diagram of an embodiment of the test system for the characteristic indexes of the lithium niobate polarization controller of the present invention. The test system for the characteristic indexes of the lithium niobate polarization controller in this embodiment is used to implement the test method for the characteristic indexes of the lithium niobate polarization controller as described in the above embodiment. Specifically, the test system for the characteristic indexes of the lithium niobate polarization controller in this embodiment includes a light source 1, a polarization analyzer 2, a host computer 3, and a DC voltage source 4. The light source 1 is used to provide input light for the lithium niobate polarization controller. The polarization analyzer 2 is used to analyze the polarization state of the output light obtained after the input light passes through the lithium niobate polarization controller. The host computer 3 is used to display the polarization state of the output light obtained by the polarization analyzer 2 on the Poincaré sphere interface, and is also used to control the voltage signal output of the DC voltage source 4 by inputting the modulation voltage in the X direction and the modulation voltage in the Y direction. The DC voltage source 4 is used to output a corresponding voltage signal under the control of the host computer 3 to apply a modulation electric field in the X direction or a modulation electric field in the Y direction on the lithium niobate polarization controller. The DC voltage source 4 includes a first DC voltage source 4a and a second DC voltage source 4b. The first DC voltage source 4a and the second DC voltage source 4b are respectively used to apply voltages on the electrodes A and C of the lithium niobate polarization controller to form a modulation electric field in the X direction or a modulation electric field in the Y direction.

[0144] When building the test system for the characteristic indexes of the lithium niobate polarization controller in this embodiment, connect the input optical fiber of the lithium niobate polarization controller to the light source 1, connect the output optical fiber of the lithium niobate polarization controller to the polarization analyzer 2, connect the electrodes A and B of the lithium niobate polarization controller to the first DC voltage source 4a and the second DC voltage source 4b respectively, and connect the program control buses of the first DC voltage source 4a, the second DC voltage source 4b, and the polarization analyzer 2 to the host computer 3.

[0145] The present invention directly regulates the refractive index of a lithium niobate crystal through an externally applied electric field, equivalent to an adjustable wave plate for an optical waveguide, eliminating external devices such as a polarizer and a photodetector, significantly reducing the hardware cost and test complexity; by the host computer controlling the application of X / Y direction modulation electric fields in real time to a DC voltage source, combined with the Poincaré sphere visualization technique, the change in the polarization state of the output light can be dynamically captured, the Stokes parameters can be accurately calculated, the half-wave voltage and the conversion voltage are directly correlated through the circular polarization state, and the target parameters can be quickly converted, avoiding errors introduced by photoelectric signal processing in the traditional method, and significantly reducing the measurement blind area; adopting a fully electronically controlled adjustment method, the polarization state regulation and parameter calculation can be completed by inputting a modulation voltage to the host computer, without the need for manual frequent adjustment of optical devices, the operation process is highly automated, significantly reducing the risk of human error and improving the test efficiency; the traditional method is sensitive to the input polarization state and has a measurement blind area, while the present invention can cover the regulation requirements for any input polarization state through the electric field equivalent wave plate technology, and is applicable to the polarization characteristic tests of various optical devices such as couplers, wavelength division multiplexers, and amplifiers, with stronger versatility; the system has a fast response speed and a short measurement period, and can quickly obtain accurate values of the half-wave voltage and the conversion voltage, providing an efficient tool for the performance optimization and quality control of lithium niobate polarization controllers.

[0146] The above content only expresses the preferred embodiments of the present invention, and the description is relatively specific and detailed, but it should not be construed as a limitation to the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A test method for the characteristic indexes of a lithium niobate polarization controller, the lithium niobate polarization controller comprising a substrate, an optical waveguide disposed on the substrate, an electrode B disposed directly above the optical waveguide, and electrodes A and C spaced apart on both sides of the optical waveguide, the electrode B being grounded, characterized in that, It includes the following steps: Access a light source along the length direction of the optical waveguide. Take the light propagation direction as the Z-axis, the distribution direction of the electrode A, electrode B, and electrode C as the Y-axis, and the direction perpendicular to the plane formed by the Z-axis and Y-axis as the X-axis to establish a coordinate system; Apply a modulation electric field in the X direction on the lithium niobate polarization controller through the host computer, analyze the polarization state at this time, and then obtain the conversion voltage; Apply a modulation electric field in the Y direction on the lithium niobate polarization controller through the host computer, analyze the polarization state at this time, and then obtain the half-wave voltage.

2. The test method for the characteristic indexes of the lithium niobate polarization controller according to claim 1, characterized in that, In the step of applying a modulation electric field in the X direction on the lithium niobate polarization controller through the host computer, analyzing the polarization state at this time, and then obtaining the conversion voltage, it includes the following sub-steps: Input a modulation voltage in the Y direction in the host computer to compensate for the voltage in the Y direction inherent in the lithium niobate crystal material, and input a modulation voltage in the X direction in the host computer to apply a modulation electric field in the X direction; Obtain the Stokes parameters based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and display the polarization state of the output polarized light through the Poincaré sphere; Adjust the modulation voltage in the X direction until several circles of circular polarization states are displayed on the Poincaré sphere; Calculate the conversion voltage according to the modulation voltage in the X direction at this time, where the modulation voltage in the X direction corresponding to one circle of circular polarization state is equal to four times the conversion voltage.

3. The test method for the characteristic indexes of the lithium niobate polarization controller according to claim 1, characterized in that, In the step of applying a modulation electric field in the Y direction on the lithium niobate polarization controller through the host computer, analyzing the polarization state at this time, and then obtaining the half-wave voltage, it includes the following sub-steps: Input a modulation voltage in the X direction in the host computer to compensate for the voltage in the X direction inherent in the lithium niobate crystal material, and input a modulation voltage in the Y direction in the host computer to apply a modulation electric field in the Y direction; Calculate the Stokes parameters based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and display the polarization state of the output polarized light through the Poincaré sphere; Adjust the modulation voltage in the Y direction until several circles of circular polarization states are displayed on the Poincaré sphere; Calculate the half-wave voltage according to the modulation voltage in the Y direction at this time, where the modulation voltage in the Y direction corresponding to one circle of circular polarization state is equal to twice the half-wave voltage.

4. The test method for the characteristic indexes of the lithium niobate polarization controller according to claim 2 or claim 3, characterized in that In the step of obtaining the Stokes parameters based on the modulation voltage in the X direction and the modulation voltage in the Y direction, and displaying the polarization state of the output polarized light through the Poincaré sphere, it includes the following sub-steps: Obtain the Jones matrix of the equivalent waveplate of the optical waveguide under the modulation electric field based on the modulation voltage in the X direction and the modulation voltage in the Y direction; Obtain the Jones vector of the output polarized light based on the Jones matrix and the Jones vector of the input polarized light; Calculate the Stokes parameters based on the Jones vector of the output polarized light; Map the Stokes parameters to the Poincaré sphere to display the polarization state of the output polarized light.

5. The test method for the characteristic indexes of the lithium niobate polarization controller according to claim 4, wherein In the step of obtaining the Jones matrix of the equivalent waveplate of the optical waveguide under the modulation electric field based on the modulation voltage in the X direction and the modulation voltage in the Y direction, it includes the following sub-steps: Calculate the angle by which the principal axis of the refractive index ellipsoid rotates relative to the corresponding coordinate axis in the original coordinate system after applying the modulation electric field, denoted as the first rotation angle; Calculate the phase difference introduced by the equivalent waveplate based on the modulation voltage in the X direction and the modulation voltage in the Y direction, denoted as the first phase difference; Obtain the Jones matrix of the equivalent wave plate based on the first rotation angle and the first phase difference.

6. The test method for the characteristic indexes of the lithium niobate polarization controller according to claim 5, wherein, In the step of calculating the phase difference introduced by the equivalent wave plate based on the modulation voltage in the X direction and the modulation voltage in the Y direction, the following sub-steps are included: After the principal axes of the refractive index ellipsoid are aligned, a new coordinate system is obtained, and the refractive index in the X' direction and the refractive index in the Y' direction in the new coordinate system are calculated; Obtain the refractive index difference based on the refractive index in the X' direction and the refractive index in the Y' direction; Obtain the optical wavelength and the thickness of the equivalent wave plate, and obtain the phase difference introduced by the equivalent wave plate based on the refractive index difference, the optical wavelength, and the thickness of the equivalent wave plate.

7. The test method for the characteristic indexes of the lithium niobate polarization controller according to claim 5, characterized in that, The calculation formula of the first rotation angle is as follows: In formula (1), F x represents the modulation voltage in the X direction, F y represents the modulation voltage in the Y direction, and θ represents the first rotation angle; The calculation formula of the first phase difference is as follows: In formula (II), μ represents the first-order phase difference, n x' -n y' represents the refractive index difference, d represents the thickness of the equivalent wave plate, and λ represents the optical wavelength; In the step of obtaining the Jones matrix of the equivalent wave plate based on the first rotation angle and the first phase difference, the Jones matrix is expressed as: In Equation (III), μ represents the first phase difference, and θ represents the first rotation angle.

8. The test method for the characteristic indexes of the lithium niobate polarization controller according to claim 6, characterized in that, In the step of calculating the refractive index in the X' direction and the refractive index in the Y' direction in the new coordinate system, the calculation formula of the refractive index in the X' direction is as follows: In Equation (4), F x represents the modulation voltage in the X direction, F y represents the modulation voltage in the Y direction, n x' represents the refractive index in the X' direction, n o represents the refractive index of the ordinary light, γ 22 represents the electro-optic coefficient tensor, and θ represents the first rotation angle; The calculation formula of the refractive index in the Y' direction is as follows: In formula (V), n y' represents the refractive index in the Y' direction; In the step of obtaining the refractive index difference based on the refractive index in the X' direction and the refractive index in the Y' direction, the calculation formula of the refractive index difference is as follows: n x' -n y' =n o 3 γ 22 (cos(2θ)F y +sin(2θ)F x )(Six) In formula (VI), n x' -n y' represents the refractive index difference.

9. The test method for the characteristic index of the lithium niobate polarization controller according to claim 7, characterized in that, In the step of obtaining the Jones vector of the output polarized light based on the Jones matrix and the Jones vector of the input polarized light, the calculation formula of the Jones vector of the output polarized light is as follows: In formula (VII), β represents the polarization angle of the input polarized light, represents the phase difference of the input polarized light, a represents the amplitude component of the output polarized light in the X direction, b represents the phase difference of the output polarized light, and c represents the amplitude component of the output polarized light in the Y direction; In the step of calculating the Stokes parameters based on the Jones vector of the output polarized light, the Stokes parameters include the total light intensity S0, the light intensity difference S1 between 0 degrees and 90 degrees, the light intensity difference S2 between plus and minus 45 degrees, and the light intensity difference S3 between left-handed light and right-handed light. Among them, S0 = a 2 + c 2 , S1 = a 2 - c 2 , S2 = 2ac cos b, S3 = 2ac sin b.

10. A test system for the characteristic indexes of a lithium niobate polarization controller, characterized in that, Including: A light source for providing input light to the lithium niobate polarization controller; A polarization analyzer for analyzing the polarization state of the output light obtained after the input light passes through the lithium niobate polarization controller; An upper computer for displaying the polarization state of the output light on the Poincaré sphere interface, and also for controlling the voltage signal output of the DC voltage source by inputting the modulation voltage in the X direction and the modulation voltage in the Y direction; A DC voltage source for outputting a corresponding voltage signal under the control of the upper computer to apply a modulation electric field in the X direction or a modulation electric field in the Y direction on the lithium niobate polarization controller.

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