Fiber polarization state measurement device and method based on rotating quarter-wave plate

Abstract

This invention provides a fiber polarization state measurement device based on the rotating quarter-wave plate method, including a transmission device and a detection device. The transmission device includes a fiber coupler, a single-mode polarization-maintaining fiber, and a fiber collimator. The detection device includes a quarter-wave plate, a rotating stage, a polarizer, and a power meter. The beam to be measured is coupled into the single-mode polarization-maintaining fiber through the fiber coupler, transmitted through the single-mode polarization-maintaining fiber to the fiber collimator, collimated, and then incident on the quarter-wave plate. The rotating stage drives the quarter-wave plate to rotate within an angle range of 0° to 180°. After passing through the polarizer, the beam is incident on the power meter, and the optical power values ​​measured by the power meter at at least eight angles are recorded. The output of the power meter is connected to a computer.

Description

Technical Field

[0001] This invention relates to the field of measurement technology, and in particular to a fiber optic polarization state measurement device and method based on a rotating quarter-wave plate. Background Technology

[0002] In fields such as fiber optic sensing, fiber optic communication, and coherent detection, the polarization state stability of light waves propagating within optical fibers directly affects the measurement accuracy and signal quality of the system. Especially in interferometer systems constructed using polarization-maintaining fibers or ordinary single-mode fibers, the polarization state of light waves undergoes uncontrollable changes during transmission due to the inherent birefringence effect of the fiber and external environmental disturbances. This leads to decreased interference signal contrast and increased phase noise, severely limiting system performance. Therefore, achieving accurate measurement of the polarization state of light waves in optical fibers is a crucial prerequisite for conducting polarization-dependent modulation and improving the stability of interferometric systems.

[0003] Currently, most common polarization state measurement methods rely on commercial polarization analyzers or polarization measurement systems based on a four-detector structure. These devices typically employ discrete optical elements and reconstruct the polarization state by measuring the light intensity in four specific polarization directions in a laboratory environment. However, existing measurement methods have the following drawbacks: First, the equipment is expensive, especially polarization analyzers suitable for online fiber testing, which integrate precise waveplates and detector arrays, making them costly. Second, the measurement system structure is complex, often employing spatial optical paths or fiber optic pigtail structures, which have high requirements for the operating environment and are difficult to apply directly in integrated and portable engineering scenarios. Third, existing methods often require multiple adjustments or calibrations during the measurement process, which is cumbersome and not conducive to quickly obtaining polarization state information. Summary of the Invention

[0004] This invention provides a fiber polarization state measurement device based on the rotating quarter-wave plate method, including a transmission device and a detection device. The transmission device includes a fiber coupler, a single-mode polarization-maintaining fiber, and a fiber collimator. The detection device includes a quarter-wave plate, a rotating stage, a polarizer, and a power meter. The beam to be measured is coupled into the single-mode polarization-maintaining fiber through the fiber coupler, transmitted through the single-mode polarization-maintaining fiber to the fiber collimator, collimated, and then incident on the quarter-wave plate. The rotating stage drives the quarter-wave plate to rotate within an angle range of 0° to 180°. After passing through the polarizer, the beam is incident on the power meter, and the optical power values ​​measured by the power meter at at least eight angles are recorded. The output of the power meter is connected to a computer.

[0005] The present invention also provides a method for measuring the polarization state of an optical fiber based on a rotating quarter-wave plate, comprising the following steps: Step S100: System initialization. Install a quarter-wave plate on a rotary table, determine the relative angular relationship between the fast axis of the quarter-wave plate and the polarizer, and set the transmission direction of the polarizer to the reference zero-degree direction. In step S200, after the light to be tested is coupled to the fiber optic coupler, it is transmitted through a single-mode polarization-maintaining fiber and collimated by the fiber optic collimator, and then incident sequentially on a quarter-wave plate and a polarizer with the transmission axis fixed. In step S300, a quarter-wave plate is driven to rotate at equal angular intervals within the range of 0° to 180° by a rotary table. After each step is completed, a power meter is used to record the power value of the light emitted through the polarizer. Step S400: Record each rotation angle using a computer. Corresponding power value Fourier analysis was performed on the power value sequence to extract the constant term coefficients. Second harmonic sine coefficient , fourth harmonic cosine term coefficient and the coefficient of the fourth harmonic sine term Fourier coefficients are used to calculate the four Stokes parameters of the beam under test. , , , ; Step S500: Determine the degree of polarization of the polarized light based on the Stokes parameters. Ellipticity 1. Length and width of the circumscribed rectangle polarization direction Solve the problem.

[0006] Furthermore, the method is characterized in that step S400 specifically comprises: Step S410: Record the power values ​​corresponding to at least eight rotation angles using a computer. ; Step S420: Obtain the Fourier coefficients using the following formula. , in, Represents the number of experimental groups. This represents the corresponding angle; 0° represents the direction of the polarizer's transmission axis. To record the total number of power values, The rotation angle of the rotary table is The corresponding reading on the power meter; Step S430: Obtain the Stokes parameters using the following formula. .

[0007] Furthermore, in step S500, the degree of polarization of the polarized light is obtained using the following formula. Ellipticity 1. Length and width of the circumscribed rectangle and and polarization direction : .

[0008] Furthermore, the calculated degree of polarization Ellipticity, expressed in terms of a Poincaré sphere Circumscribed rectangle and Represented by a polarization ellipse.

[0009] Compared with the prior art, the present invention has the following significant advantages: (1) It does not require replacing optical components to adjust the optical path and optical power. During the measurement process, all optical components are placed in the measurement optical path throughout the process, which can realize stable optical non-contact measurement of the full range of fiber polarization state; (2) The measurement accuracy can be improved by increasing the number of measurements and reducing the quarter-wave plate rotation angle interval. It can realize the detection of fiber polarization parameters of all types of polarization state, is less affected by environmental interference, and has a low cost; (3) It has a simple structure and small size, can be portable and integrated, and is convenient for on-site testing of fiber optic communication lines and local polarization state detection in large optical equipment. Moreover, it is easy to operate and can be operated by non-professionals. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the structure of the present invention.

[0011] Figure 2 This is a schematic diagram illustrating the change in light intensity of the beam under test as the quarter-wave plate rotates.

[0012] Figure 3 This is a schematic diagram of the Poincaré sphere representation of the beam to be measured in an embodiment of the present invention.

[0013] Figure 4 This is a schematic diagram representing the polarization ellipse of the beam to be tested in an embodiment of the present invention.

[0014] Among them, 10 is a transmission device, 101 is an optical fiber coupler, 102 is a single-mode polarization-maintaining fiber, and 103 is an optical fiber collimator; 20 is a detection device, 201 is a quarter-wave plate, 202 is a rotating stage, 203 is a polarizer, and 204 is a power meter. Detailed Implementation

[0015] This embodiment provides a fiber optic polarization state measurement device based on a rotating quarter-wave plate. When measuring the polarization state of light in an optical fiber, a quarter-wave plate is driven to rotate within an angle range of 0° to 180° using a rotating stage. Optical power values ​​measured by at least eight angle power meters are recorded. Based on the functional relationship between the measured optical power values ​​and the Stokes parameters of the incident light, the Stokes parameters of the beam under test are inverted, thereby determining its polarization state. The specific implementation device is as follows: Combination Figure 1 A fiber optic polarization state measurement device based on the rotating quarter-wave plate method includes an optical fiber coupler 101, a single-mode polarization-maintaining fiber 102, an optical fiber collimator 103, a quarter-wave plate 201, a rotating stage 202, a polarizer 203, and a power meter 204. The power meter 204 is connected to a computer to calculate the polarization state. The beam to be measured is coupled into the optical fiber through the optical fiber coupler 101, transmitted through the single-mode polarization-maintaining fiber 102 to the optical fiber collimator 103, collimated, and then incident on the quarter-wave plate 201. The rotating stage 202 drives the quarter-wave plate 201 to rotate within an angle range of 0° to 180°. After passing through the polarizer 203, it is incident on the power meter 204, and the optical power values ​​measured by at least eight angles are recorded. The output of the power meter 204 is connected to a computer, and the polarization state is calculated based on the functional relationship between the measured optical power value and the Stokes parameter of the incident light.

[0016] Furthermore, the rotation of the quarter-wave plate is controlled via a rotary table.

[0017] A fiber polarization state measurement method based on the rotating quarter-wave plate method includes the following steps: Step 1: System initialization. Install the quarter-wave plate 201 on the rotary table 202, determine the relative angular relationship between the fast axis of the quarter-wave plate 201 and the polarizer 203, and set the light transmission direction of the polarizer 203 to the reference zero-degree direction. Step 2: After the light to be tested is coupled to the fiber coupler 101, it is transmitted through the single-mode polarization-maintaining fiber 102 and collimated by the fiber collimator 103, and then incident sequentially on the quarter-wave plate 201 and the polarizer 203 whose transmission axis is fixed. Step 3: Drive the quarter-wave plate 201 to rotate at equal angular intervals within the range of 0° to 180° using the rotary stage 202. After each step is completed, use the power meter 204 to record the power value corresponding to the light emitted by the polarizer 203. Step 4: Record each rotation angle using a computer. Corresponding power value Fourier analysis was performed on the power value sequence to extract the constant term coefficients. Second harmonic sine coefficient , fourth harmonic cosine term coefficient and the coefficient of the fourth harmonic sine term The four Stokes parameters of the beam under test are calculated using the Fourier coefficients. , , , ; Step 5: Determine the degree of polarization of the polarized light based on the Stokes parameters. Ellipticity 1. Length and width of the circumscribed rectangle polarization direction Solve the problem.

[0018] Furthermore, step 4 specifically involves: Step 41: Use a computer to record the power values ​​corresponding to at least eight rotation angles. ; Step 42, obtain the Fourier coefficients using the following formula: in, Represents the number of experimental groups. This represents the corresponding angle; 0° represents the transmission axis direction of polarizer 203. To record the total number of power values, The rotation angle of the rotary table 202 is The corresponding reading of the power meter 204; Step 43, obtain the Stokes parameters using the following formula: .

[0019] Furthermore, step 5 specifically includes: The degree of polarization of polarized light is obtained using the following formula. Ellipticity 1. Length and width of the circumscribed rectangle polarization direction : The calculated degree of polarization is represented by a Poincaré sphere, and the ellipticity and circumscribed rectangle are represented by polarization ellipses.

[0020] Example 1: This example provides a specific application of a fiber optic polarization state measurement method based on a rotating quarter-wave plate, including the following process: Step S1: Initialize the detection system: Mount the quarter-wave plate 201 on the rotary stage 202 and determine the relative angular relationship between the fast axis of the quarter-wave plate 201 and the transmission axis of the polarizer 203. The specific calibration method is as follows: Remove the light source to be tested, and incident a known linearly polarized light (such as linearly polarized light with a polarization direction consistent with the reference zero degree) onto the system. Rotate the quarter-wave plate 201 until the power meter 204 reading is at its minimum. At this point, the fast axis of the quarter-wave plate 201 is parallel to the transmission axis of the polarizer 203, and this position is recorded as the zero point of the fast axis. Afterward, set the transmission axis direction of the polarizer 203 to the reference zero degree direction.

[0021] In step S2, the light to be tested is coupled to the fiber optic coupler 101, and then transmitted through the single-mode polarization-maintaining fiber 102 and collimated into a parallel beam by the fiber optic collimator 103. The parallel beam is then incident sequentially on the quarter-wave plate 201 and the polarizer 203 whose transmission axis is fixed. In this embodiment, the wavelength of the light to be tested is 1550nm.

[0022] In step S3, the quarter-wave plate 201 is driven to rotate at equal angular intervals within the range of 0° to 180° by the rotary stage 202. The power value corresponding to the emitted light from the polarizer 203 at at least eight angles is recorded using the power meter 204. In this embodiment, the measurement accuracy is improved by increasing the number of measurements and reducing the rotation angle interval of the quarter-wave plate 201, and 805 power values ​​are collected.

[0023] Step S4: Record each rotation angle using a computer. Corresponding power value Fourier analysis was performed on the power value sequence to extract the constant term coefficients. Second harmonic sine coefficient , fourth harmonic cosine term coefficient and the coefficient of the fourth harmonic sine term Thus, the curve of the measured optical power changing with the rotation angle of the quarter-wave plate 201 is obtained, as shown below. Figure 2 The figure shows the calculation of four Stokes parameters of the beam under test using Fourier coefficients. , , , Thus, the Poincaré sphere of the light to be measured can be obtained, such as Figure 3 As shown.

[0024] Step S5: Based on the Stokes parameters mentioned above, calculate the degree of polarization of the polarized light. Ellipticity: 1.18 1.0°, length of the circumscribed rectangle 0.8515, width of the bounding rectangle The polarization direction angle is 0.5244. The polarization angle is -33.8°, from which the polarization ellipse of the light to be measured can be obtained, as follows: Figure 4 As shown.

[0025] In summary, this invention eliminates the need to replace optical components or adjust the optical path and power. During the measurement process, all optical components are placed within the measurement optical path throughout, enabling stable non-contact optical measurement of the full range of fiber polarization states.

Claims

1. A fiber optic polarization state measurement device based on the rotating quarter-wave plate method, characterized in that, It includes a transmission device (10) and a detection device (20). The transmission device (10) includes an optical fiber coupler (101), a single-mode polarization-maintaining fiber (102), and an optical fiber collimator (103). The detection device (20) includes a quarter-wave plate (201), a rotating stage (202), a polarizer (203), and a power meter (204). The beam to be tested is coupled into a single-mode polarization-maintaining fiber (102) through an optical fiber coupler (101), and transmitted to an optical fiber collimator (103) through the single-mode polarization-maintaining fiber (102). After collimation, it is incident on a quarter-wave plate (201). The quarter-wave plate (201) is driven to rotate within an angle range of 0° to 180° using a rotary stage (202). After passing through a polarizer (203), it is incident on a power meter (204), and the optical power values ​​measured by at least eight angle power meters are recorded. The output end of the power meter (204) is connected to a computer.

2. A method for measuring the polarization state of an optical fiber based on the device described in claim 1, characterized in that, Includes the following steps: Step S100: System initialization. Install the quarter-wave plate (201) on the rotary table (202), determine the relative angle relationship between the fast axis of the quarter-wave plate (201) and the polarizer (203), and set the light transmission direction of the polarizer (203) to the reference zero-degree direction. In step S200, the light to be tested is coupled to the fiber optic coupler (101), transmitted through the single-mode polarization-maintaining fiber (102), and collimated by the fiber optic collimator (103), and then incident sequentially onto the quarter-wave plate (201) and the polarizer (203) whose transmission axis is fixed. In step S300, the quarter-wave plate is driven to rotate at equal angular intervals within the range of 0° to 180° by a rotating stage (202). After each step is completed, the power value corresponding to the light emitted by the polarizer (203) is recorded by a power meter (204). Step S400: Record each rotation angle using a computer. Corresponding power value Fourier analysis was performed on the power value sequence to extract the constant term coefficients. Second harmonic sine coefficient , fourth harmonic cosine term coefficient and the coefficient of the fourth harmonic sine term Fourier coefficients are used to calculate the four Stokes parameters of the beam under test. , , , ; Step S500: Determine the degree of polarization of the polarized light based on the Stokes parameters. Ellipticity 1. Length and width of the circumscribed rectangle polarization direction Solve the problem.

3. The method according to claim 2, characterized in that... Step S400 in this method specifically includes: Step S410: Record the power values ​​corresponding to at least eight rotation angles using a computer. ; Step S420: Obtain the Fourier coefficients using the following formula. ， in, Represents the number of experimental groups. This represents the corresponding angle; 0° represents the direction of the polarizer's transmission axis. To record the total number of power values, The rotation angle of the rotary table is The corresponding reading on the power meter; Step S430: Obtain the Stokes parameters using the following formula. 。 4. The method according to claim 3, characterized in that, In step S500, the degree of polarization of the polarized light is obtained using the following formula. Ellipticity 1. Length and width of the circumscribed rectangle and and polarization direction : 。 5. The method according to claim 4, characterized in that, Calculated polarization degree Ellipticity, expressed in terms of a Poincaré sphere Circumscribed rectangle and Represented by a polarization ellipse.

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