Integrated optical fiber key structure parameter determination method

Abstract

The method for determining the key structure parameters of the integrated optical fiber comprises the following steps of: normalizing the beat length, the length and the rotation rate of the integrated optical fiber; according to an integrated optical fiber polarization state conversion effectiveness requirement, analyzing an integrated optical fiber variable speed section coupling mode equation and solving an eigenmode; determining a polarization performance evaluation index, and determining a rotation rate and an optical fiber length critical value according to an equivalent phase delay deviation precision requirement; and determining values of optical fiber key parameter beat length, length and rotation rate, and taking the minimum equivalent phase delay deviation as a target to obtain an optimal beat length value. According to the method, the optimization of the integrated optical fiber design can be realized on the premise of ensuring the performance of the integrated optical fiber.

Description

Technical Field

[0001] The present invention relates to the technical field of Fiber Optic Current Transformer (FOCT) measurement, and specifically relates to a method for determining key structural parameters of an integrated optical fiber. Background Art

[0002] Due to advantages such as light weight, good insulation performance, strong anti-interference ability, high reliability, and high degree of digitization, the Fiber Optic Current Transformer (FOCT) has better safety, a wider measurement range, and stronger stability compared with traditional electromagnetic current transformers, meeting the needs of the development of the new generation of smart grid informatization, digitization, and automation.

[0003] In a reflection interference FOCT, the λ / 4 wave plate is an essential component of the sensing ring. In the past, the sensing ring of a pure fiber optic current transformer usually consisted of a λ / 4 wave plate and a single-mode fiber with low birefringence, while the sensing ring of a current reflection interference FOCT usually consists of a mirror, a λ / 4 wave plate, and a highly birefringent helical fiber. There are many factors affecting the process of the λ / 4 wave plate to interchange linearly polarized light and circularly polarized light. When manufacturing the λ / 4 wave plate, errors caused by improper operation or instrument precision may result in inconsistent lengths of each λ / 4 wave plate, which will affect the measurement and damage replacement during actual application. At the same time, the λ / 4 wave plate is easily interfered by external factors. Vibration and environmental temperature changes will affect the structural stability of the λ / 4 wave plate, and the phase delay of the λ / 4 wave plate will also change when the temperature changes. Under the action of various factors, the stability and accuracy of the λ / 4 wave plate will decrease, leading to a decline in the measurement performance of the FOCT. At the same time, under the influence of external vibration and the thermal expansion and contraction of the packaging structure, the sensing optical fiber is easily affected by uncontrollable random stress, resulting in changes in polarization characteristics, thereby affecting the consistency of the performance of the manufactured wave plate. This increases the difficulty of obtaining an accurate and reliable λ / 4 wave plate. For this reason, there is currently an integrated optical fiber current sensing ring technology that uses the optical fiber drawing process to make the optical fiber into a gradient spiral structure to replace the λ / 4 wave plate. This integrated optical fiber converts the direct polarization transformation of the traditional cut-type λ / 4 optical fiber wave plate into a gradient polarization transformation, and realizes the functions of linearly polarized light transmission, polarization state conversion, and current induction on a single optical fiber.

[0004] The most important function of the integrated optical fiber variable speed section is to replace the λ / 4 wave plate to achieve the mutual conversion between linearly polarized light and circularly polarized light. Whether the equivalent phase delay angle of the integrated optical fiber variable speed section is exactly π / 2 is a key index for evaluating its polarization performance. The beat length, overall length, and pitch (i.e., the rotation rate during the optical fiber manufacturing process) of the integrated optical fiber jointly determine the polarization conversion function of the integrated optical fiber.

[0005] At present, there is little research on FOCT using integrated optical fibers in China, and related research, practice and other work need to be further carried out. During the actual operation process, the measurement performance of the Fiber Optic Current Transformer (FOCT) is susceptible to various factors such as temperature, vibration, interference, and optical path loss. There are mainly two aspects to improve the measurement current accuracy and sensitivity of FOCT: external and internal. The external method usually reduces the influence of noise, vibration, electromagnetic interference, etc. generated by the environment and other equipment in the operating conditions on FOCT through means such as isolation and temperature control; the internal method is to optimize the system structure of FOCT itself, and improve the performance and anti-interference ability of FOCT itself by adding feedback links to offset the influence of external interference, changing the optical path structure, and optimizing structure parameters, etc.

[0006] The determination of the structural parameters of the integrated optical fiber is one of the important directions to improve the measurement performance of FOCT. However, the existing research lacks a method for determining the key structural parameters of the integrated optical fiber, and there is no specific parameter guidance for the structural design of the variable-speed section of the integrated optical fiber, which is not conducive to the stability and accuracy of FOCT for measuring current. Summary of the Invention

[0007] To solve the above technical problems, the present invention provides a method for determining the key structural parameters of an integrated optical fiber. By studying the evolution law of light wave polarization inside the integrated optical fiber, using the coupled-mode theory to analyze the coupled-mode equation of polarization state evolution, the relationship expressions between the equivalent phase delay angle of the integrated optical fiber and the key parameters of fiber length, beat length, and rotation rate are analyzed, and the key structural parameters of the integrated optical fiber are determined by optimizing the polarization delay performance index. This method can realize the optimization of the integrated optical fiber design on the premise of ensuring the performance of the integrated optical fiber.

[0008] The technical solution adopted by the present invention is as follows:

[0009] A method for determining the key structural parameters of an integrated optical fiber, comprising the following steps:

[0010] Step 1: Normalize the beat length, length, and rotation rate of the integrated optical fiber;

[0011] Step 2: Analyze the coupled-mode equation of the variable-speed section of the integrated optical fiber and solve the eigenmode according to the requirement of the effectiveness of polarization state conversion of the integrated optical fiber;

[0012] Step 3: Determine the polarization performance evaluation index, and determine the critical values of the rotation rate and fiber length according to the accuracy requirement of the equivalent phase delay deviation;

[0013] Step 4: Determine the values of the key fiber optic parameters beat length, length, and rotation rate. Taking the minimum equivalent phase delay deviation as the goal, obtain the optimal beat length value.

[0014] In the said Step 1, the beat length of the integrated fiber optic variable speed section is L B , L B = 2π / Δβ, where Δβ is the difference in propagation constants of two orthogonal polarization eigenmodes in the integrated fiber optic variable speed section;

[0015] Δβ = β x -β y , β x represents the propagation constant associated with the mode polarized along the X direction (usually the mode with the transverse electric field component in the X-axis direction); β y represents the propagation constant associated with the mode polarized along the Y direction (usually the mode with the transverse electric field component in the Y-axis direction). These two parameters are the propagation constants of orthogonal polarization eigenmodes.

[0016] The overall length l of the integrated fiber optic variable speed section is normalized by the beat length L B to L = l / L B , where L represents the normalized length of the integrated fiber optic variable speed section;

[0017] At any point z along the fiber, it is normalized to Z = z / L B . The total length is l, and z is the coordinate of a point on this length, as Figure 5 shown.

[0018] The end rotation rate of the integrated fiber optic variable speed section is normalized to: Q max represents the normalized maximum rotation rate; τ max represents the actual maximum rotation rate.

[0019] The helical rotation rate distribution along the integrated fiber optic variable speed section is normalized to: Q(Z) represents the normalized rotation rate at point z; τ(z) represents the actual rotation rate at point z.

[0020] In the said Step 2, the coupling situation between two orthogonal linearly polarized modes in the integrated fiber optic is described by the coupled mode equations shown in the following formula (1):

[0021]

[0022] In formula (1): A x , A y respectively represent the two eigenmodes of the light wave along the x and y axes of the integrated fiber optic; j represents the imaginary unit in complex number calculations; τ represents the rotation rate;

[0023] Matrixing formula (1) yields:

[0024]

[0025] In formula (2): K and A represent the matrix form used for calculation;

[0026] After normal transformation and correction, the analytical solution of the coupled mode equation is obtained:

[0027]

[0028] In formula (3), A(z) represents the symbol representing the analytical solution of the coupled mode equation; P = e jφ(z)csc(2θ) ; Q = e -jφ(z)csc(2θ) ;

[0029] φ(z)=0.5arctan[2Q(z)]; θ represents an undetermined constant that is independent of the position of the integrated optical fiber;

[0030] At the beginning of the optical fiber, that is, when z = 0, the eigenmode corresponding to the incident light is A(0) = [cosθ-sinθ] T , T represents the matrix transpose, which is equivalent to linearly polarized light with a small angle θ on the birefringence axis;

[0031] The eigenmode of polarization of light waves at any position along the optical fiber is:

[0032]

[0033] In formula (4): A x (z), A y (z) represents the two eigenmodes of the light wave along the x and y axes of the integrated optical fiber at point z; e jφ(z)csc(2θ) Yes A x (z), A y (z); therefore, the propagation of the eigenmode of the integrated optical fiber in the entire optical fiber is described as:

[0034]

[0035] In step 3, after obtaining the eigenmode of the integrated fiber coupling mode equation, the polarization rate P of the light wave output through the optical fiber is obtained:

[0036]

[0037] In formula (6): α and δ represent parameters related to the specific characteristics of the incident polarized light;

[0038] α=arctan(A x / A y ),δ=arg(A x / Ay );

[0039] After the incident polarized light passes through the integrated optical fiber variable speed section, the polarization rate of the outgoing light is analytically expressed by the following formula (7):

[0040]

[0041] When the incident polarized light is linearly polarized light, its polarization rate P is 1; for elliptically polarized light, the value of its polarization rate P is between 0 and 1; and for circularly polarized light, its polarization rate P is 0.

[0042] It can be obtained from formula (7) that when Q max or L continuously increases, the polarization rate P gradually decreases, that is, the incident polarized light gradually approaches circularly polarized light.

[0043] In step 3, when the normalized length L of the integrated optical fiber variable speed section is fixed, when the maximum rotation rate Q max increases, the polarization rate P decreases, but when the maximum rotation rate Q max is large enough, the decreasing amplitude of the polarization rate P becomes smaller;

[0044] When the maximum rotation rate Q max is fixed, the polarization rate P also decreases. At the same time, when the normalized length L of the integrated optical fiber variable speed section increases to a certain value, the polarization rate P is almost no longer affected by L.

[0045] As the maximum rotation rate Q max or the normalized length L of the integrated optical fiber variable speed section continuously increases, the polarization rate P of the polarized light gradually decreases and approaches 0.

[0046] However, due to the manufacturing process of the integrated optical fiber, the overall length and the maximum rotation rate of the integrated optical fiber are not unlimited. It is necessary to consider the critical values of the above two parameters, the maximum rotation rate Q max and the normalized length L of the integrated optical fiber variable speed section, to determine the structural parameters of the integrated optical fiber variable speed section.

[0047] Assume that the outgoing polarized light is composed of two orthogonal components with equal amplitudes and a phase difference of δ e ; δ e is the equivalent phase delay of the integrated optical fiber variable speed section, then there is:

[0048] δ e = arccosP (8);

[0049] The phase delay of the standard quarter fiber wave plate is δ e0 = 90. The phase delay deviation of the fiber wave plate is used to evaluate its polarization conversion performance. The tolerance Δδ of the delay deviation e = |δe -δ e0 should not be greater than 3.6°, and the relative deviation Δδ e / δ e0 should be within 4%. The smaller the deviation, the better the performance.

[0050] The requirements for the integrated fiber polarization conversion performance are: L > 2.54 and Q L > 8.03.

[0051] In step 4, in order to analyze the relationship between the equivalent phase delay deviation of the fiber wave plate and the beat length of the fiber, L and Q L are converted to the actual values of l and τ l From equation (8), we get:

[0052]

[0053] In equation (9): Δδ e represents the equivalent phase delay deviation;

[0054] In order to analyze the influence of the beat length of the fiber on the equivalent phase delay deviation, the rotation rate τ l and the length l are respectively given by the method of controlling variables. For different maximum rotation rates τ l and fiber lengths l, as the beat length L B of the fiber increases, the relative phase delay deviation Δδ e / δ e0 of the fiber output light first decreases and then increases. Therefore, for a certain maximum rotation rate τ l and fiber length l, there is an optimal beat length that minimizes the phase delay deviation.

[0055] From equation (9), for a given τ l and l, if the beat length is taken as:

[0056]

[0057] Equation (10): L B0 represents the optimal beat length;

[0058] the delay deviation of the fiber wave plate reaches the minimum value.

[0059]

[0060] The optimal beat length L B0 is proportional to (l / τ l ) 1 / 2 and the minimum delay deviation △δ emin is negatively correlated with (l·τ l ) 1 / 2 ​

[0061] Thus, it is obtained that: the larger the product of l and τ l , the smaller △δ emin / △δ e0 , and the better the performance of the fiber optic wave plate.

[0062] When L B is the optimal beat length, when the product of l and τ l is fixed to a constant value, it corresponds to the minimum delay deviation △δ emin which can be transformed into:

[0063]

[0064] In this case, when △δ emin is a constant, the optimal beat length L B0 is directly proportional to the fiber length l or inversely proportional to the maximum rotation rate τ l .

[0065] The method for determining the key structural parameters of the integrated optical fiber of the present invention has the following technical effects:

[0066] 1) In step 1 of the present invention, in order to eliminate the influence of different units and magnitudes and make the parameters comparable, the structural parameters such as the fiber beat length, overall length, and rotation rate are normalized to dimensionless quantities for easy analysis and comparison.

[0067] 2) In step 2 of the present invention, to understand and analyze the propagation characteristics and mode coupling phenomenon of light waves in the integrated optical fiber and establish the coupled mode equation to describe the energy transfer between different modes in the optical fiber. It involves solving linear differential equations and analyzing the influence of modal coupling on the output light wave.

[0068] 3) In step 2 of the present invention, to obtain the eigenmodes of the system and analyze their influence on the propagation of optical signals. The eigen - solutions of the coupled mode equation are obtained through numerical solution or analytical solution to understand the distribution and interaction of each mode in the optical fiber.

[0069] 4) In step 3 of the present invention, evaluate the performance level of the variable - speed section of the integrated optical fiber to achieve the polarization conversion function. Set indicators such as the polarization rate and equivalent phase delay deviation, and use the evaluation indicators to determine the critical values and accuracy requirement values of the structural parameters of the integrated optical fiber. By setting indicators such as the polarization rate and equivalent phase delay deviation, the performance of the variable - speed section of the optical fiber can be clearly quantified. For example, the polarization rate can be used to measure the degree of polarization state retention, and the equivalent phase delay deviation can measure the error in the polarization conversion process. These indicators can provide clear performance goals for the design, ensuring that the variable - speed section of the optical fiber can achieve the required functional effects in actual use and avoiding over - design or under - design.

[0070] 5) In step 3 of the present invention, taking the polarization performance evaluation index as the standard, ensure that the accuracy level meets the requirements. Analyze the relationship between the equivalent phase delay deviation and the rotation rate and the fiber length to determine the critical value. Setting the critical values and accuracy requirements for these indicators can provide clear design constraints for the structural parameters of the integrated fiber, such as the core diameter, refractive index distribution, fiber material, etc. This helps to optimize the fiber size, material selection and processing technology, thereby reducing unnecessary waste and improving the design efficiency.

[0071] 6) In step 4 of the present invention, determine the values of the key structural parameters of the integrated fiber: beat length, length, and rotation rate. Combining the results of the previous steps, with the minimum equivalent phase delay deviation as the goal, obtain the optimal beat length value. Achieve the optimization of the design while ensuring the performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The present invention will be further described below with reference to the drawings and examples;

[0073] Figure 1 is a flowchart of the method of the present invention.

[0074] Figure 2 is a schematic diagram of the integrated fiber structure.

[0075] Figure 3 is τ l When taking different values of Δδ e / δ e0 versus the beat length L B variation curve graph.

[0076] Figure 4 is when l takes different values of Δδ e / δ e0 versus the beat length L B variation curve graph.

[0077] Figure 5 is a schematic diagram of z at the length l. DETAILED DESCRIPTION OF THE INVENTION

[0078] As Figure 1 shown, a method for determining the key structural parameters of an integrated fiber includes the following steps:

[0079] Step 1: Normalize the key structural parameters of the integrated fiber:

[0080] The schematic diagram of the integrated fiber structure is as Figure 2 shown. Normalize the beat length, length, and rotation rate of the fiber to more effectively compare these structural parameters with different scales and dimensions.

[0081] L B represents the beat length of the variable-speed section of the integrated fiber, LB = 2π / Δβ, where Δβ is the difference in propagation constants of two orthogonally polarized eigenmodes in the integrated optical fiber variable-speed section, Δβ = β x - β y 。

[0082] l represents the overall length of the integrated optical fiber variable-speed section, which can be normalized by the fiber beat length L B to L = l / L B Similarly, at any point z along the optical fiber, it is normalized to Z = z / L B 。

[0083] τ max represents the rotation rate at the end of the integrated optical fiber variable-speed section, which can be normalized to:

[0084] The helical rotation rate distribution along the integrated optical fiber variable-speed section can also be normalized to:

[0085] Step 2: Analyze the coupled mode equations of the variable-speed section and solve for the eigenmodes:

[0086] The coupling situation between two orthogonally linearly polarized modes in the integrated optical fiber can be described by the coupled mode equations shown in the following formula (1):

[0087]

[0088] In formula (1): A x 、A y respectively represent two eigenmodes of the light wave along the x and y axes of the integrated optical fiber.

[0089] Matrixize formula (1) to get:

[0090]

[0091] Through normal mode transformation and correction, the analytical solution of the coupled mode equation can be obtained as:

[0092]

[0093] where: P = e jφ(z)csc(2θ) 、Q = e -jφ(z)csc(2θ) In the local coordinate system, the eigenmode corresponding to the incident light at the fiber head (z = 0) is A(0) = [cosθ -sinθ] T , which is a linearly polarized light with a small angle θ relative to the birefringence axis. The light wave polarization eigenmode at any position along the optical fiber is:

[0094]

[0095] where: e jφ(z)csc(2θ) is Ax (z), A y (z) is the common phase factor, so the propagation of the eigenmode of the integrated optical fiber in the whole optical fiber can also be written as:

[0096]

[0097] After obtaining the rotation function of the integrated optical fiber variable-speed section and its eigenmodes of the coupled-mode equation, the coupling relationship between different modes in the optical fiber can be used to further design the structural parameters of the integrated optical fiber variable-speed section to meet the requirements of polarization performance indicators.

[0098] Step 3: Polarization performance evaluation indicators and critical values of rotation rate and optical fiber length:

[0099] After obtaining the eigenmodes of the integrated optical fiber coupled-mode equation and combining with the general formula, the polarization rate of the light wave output through the optical fiber can be obtained as:

[0100]

[0101] After the incident polarized light passes through the integrated optical fiber variable-speed section, the polarization rate of the outgoing light can also be analytically expressed by the following formula:

[0102]

[0103] When the incident light is linearly polarized light, its polarization rate P is 1; for elliptically polarized light, the value of its polarization rate P is between 0 and 1; and for circularly polarized light, its polarization rate P is 0.

[0104] It can be obtained from Equation (7) that when Q max or L continuously increases, the polarization rate P gradually decreases, that is, the polarized light gradually approaches circular polarization.

[0105] Separate analysis: When the optical fiber length L is fixed, when Q max increases, the polarization rate P decreases, but when Q max is large enough, the decreasing amplitude of P becomes smaller; when the rotation Q max is fixed, P also decreases. At the same time, when L increases to a certain value, P is almost not affected by L. As the maximum rotation rate Q max or the optical fiber length L continuously increases, the polarization rate P of the polarized light gradually decreases and approaches 0. However, due to the manufacturing process of the integrated optical fiber, the overall length and maximum rotation rate of the optical fiber are not unlimited. It is necessary to consider the critical values of the above two parameters of the maximum rotation rate Q max and the overall length L of the optical fiber to determine the structural parameters of the integrated optical fiber variable-speed section.

[0106] Assume that the outgoing polarized light is composed of two orthogonal components with equal amplitudes and a phase difference of δ e δe For the equivalent phase delay of the integrated optical fiber variable speed section, there is

[0107] δ e = arccosP (8);

[0108] The phase delay of the standard quarter-wave plate is δ e0 = 90°. Generally, the phase delay deviation of the optical fiber wave plate is used to evaluate its polarization conversion performance. According to the typical specifications of optical products, the tolerance Δδ of the delay deviation e = |δ e -δ e0 | should not be greater than 3.6°, and the relative deviation Δδ e / δ e0 is within 4%. The smaller the deviation, the better the performance.

[0109] According to Equation (8), when the relative phase delay deviation Δδ e / δ e0 is 1%, 2%, 3%, 4%, and 5% respectively, the critical constraint values of L and Q max are shown in Table 1 below.

[0110] Table 1 Critical values of Q max and L

[0111]

[0112] The phase delay deviation angle Δδ e shall not exceed 3.6°. However, when the values of Q and L are located in the upper right of the curve, Δδ e will exceed the deviation value of the curve. Therefore, for the requirement that the relative deviation Δδ e / δ e0 should be within 4%, that is, Δδ e / δ e0 ≤ 4%. It can be seen from Table 1 that the necessary requirements for the integrated optical fiber polarization conversion performance are: L > 2.54 and Q L > 8.03. If L ≤ 2.54 or Q L ≤ 8.03, the performance of the integrated optical fiber will not meet the minimum requirements.

[0113] In addition, the normalized length L and the normalized maximum rotation rate Q max of the integrated optical fiber variable speed section not only depend on the actual length L and the actual maximum rotation rate τ max of the optical fiber, but also are related to the beat length L B of the integrated optical fiber variable speed section.

[0114] Step 4: Control variables and determine the values of structural parameters:

[0115] To study the relationship between the equivalent phase delay deviation of the fiber optic waveplate and the prototype fiber optic beat length, L and Q L are converted to the actual values of l and τ l From Equation (8), we get:

[0116]

[0117] To study the influence of the beat length on the delay deviation, the rotation rate τ l and the length l are given respectively by the method of controlling variables.

[0118] ① Fix the fiber optic length l:

[0119] In the actual process, the fiber optic length l can be taken as 60 mm, and when the maximum rotation rates τ l take π, 2π, 3π, and 4π rad / mm respectively, the relative phase delay deviation Δδ e / δ e0 as a function of L B is shown in Figure 3 as follows.

[0120] ② Fix the maximum rotation rate τ l :

[0121] When the maximum rotation rate τ l takes 3π rad / mm and the fiber optic lengths l are 50, 60, 70, and 80 mm respectively, the curves of Δδ e / δ e0 as a function of L B are shown in Figure 4 as follows.

[0122] From the above Figure 3 , Figure 4 it can be seen that for different maximum rotation rates τ l and fiber optic lengths l, as the prototype fiber optic beat length L B increases, the relative phase delay deviation Δδ e / δ e0 first decreases and then increases. Therefore, for a certain maximum rotation rate τ l and fiber optic length l, there exists an optimal beat length that minimizes the delay deviation.

[0123] From Equation (9), for a given τ l and l, if the beat length is taken as:

[0124]

[0125] the delay deviation of the fiber optic waveplate reaches the minimum value.

[0126]

[0127] Optimal beat length L B0 is proportional to (l / τ l ) 1 / 2 , and the minimum delay deviation △δ emin is negatively correlated with (l·τ l ). 1 / 2 It can be found that the larger the product of l and τ l , the smaller △δ emin / △δ e0 , and the better the performance of the fiber wave plate. For example, if the product increases from 449.66 rad to 1012.53 rad, the relative deviation △δ emin / △δ e0 can be increased from 3% to 2%. However, if the product of l and τ l is less than certain critical values, even if L B is taken as the optimal value, the wave plate cannot achieve the expected performance. Usually, for the requirement of △δ emin / △δ e0 ≤4%, the product of l and τ l should be greater than 252.64 rad. When selecting the structural parameters for fabricating the fiber wave plate, this critical value should be considered.

[0128] Specifically, when L B is the optimal beat length and the product of l and τ l is fixed at a constant value, corresponding to some minimum delay deviations △δ emin can be transformed into:

[0129]

[0130] In this case, when △δ emin is a constant, the optimal beat length L B0 is proportional to the fiber length l or inversely proportional to the maximum rotation rate τ l .

[0131] Step Five: Comprehensive Optimization:

[0132] As can be seen from the previous two steps, the determination of the final key structural parameters includes two aspects:

[0133] ①: Determine the magnitudes of both under the critical requirements of the fiber length and the rotation rate.

[0134] ②: On the premise of ①, with the goal of minimizing the relative delay deviation, obtain the optimal value of the beat length at this time to minimize the deviation and achieve the best fiber performance.

[0135] In ①, the parameters can be determined in combination with the actual process and production situation to improve economic efficiency.

[0136] As known from ②, when only considering the minimum equivalent phase delay deviation, the optimal value of the beat length is determined by the fiber length and the rotation rate. Therefore, when optimizing the length and the rotation rate, the corresponding optimal value of the beat length can be solved. After obtaining the corresponding value of the beat length at this time, similarly, the actual production and manufacturing situation can be considered, taking into account the economic benefits and the impact on other aspects such as stability due to the change of structural parameters in the entire FOCT system.

[0137] In summary, when determining the key structural parameters of the integrated fiber, in addition to considering the necessary requirement of the minimum phase delay deviation, the impacts of economic benefits, overall system effects, etc. can also be comprehensively considered. Through the feedback method, the parameter values of the fiber length, beat length, and rotation rate can be obtained.

[0138] In the FOCT system, the phase delay angle of the ideal quarter-wave plate is 90°, and the ideal equivalent phase delay angle of the corresponding integrated fiber is also 90°. However, due to the inevitable errors in the production process, the equivalent phase delay deviation can only be controlled within the allowable range in the actual process. The relative deviation of general optical products should be within 4%. Therefore, the three conditions of the beat length, length, and rotation rate can be determined by combining this limit condition with the output light polarization rate expression.

[0139] According to the requirement of the maximum equivalent phase delay deviation, the critical values of the integrated fiber length and rotation rate can be determined, and in the design, the sizes of both should be avoided being lower than this critical value. For the beat length of the integrated fiber, it can be seen from the analysis that there is an optimal value of the beat length that minimizes the phase delay deviation. At this time, the product of the length and the rotation rate should be greater than the value corresponding to the accuracy requirement. For example, when the relative deviation is less than or equal to 4%, the product of the length and the rotation rate should be greater than 252.64 rad.

[0140] After determining the accuracy requirement, the values of the fiber length and rotation rate can be adjusted and modified according to the actual process level and the critical value limit. Finally, the corresponding optimal beat length value is solved and fed back for testing to determine whether this beat length value is reasonable and meets the requirements of other aspects.

Claims

1. A method for determining key structural parameters of an integrated optical fiber, characterized in that The following steps are involved: Step 1: normalize the beat length, length and rotation rate of the integrated optical fiber; Step 2: According to the effectiveness requirements of the integrated optical fiber polarization state conversion, the coupled mode equation of the integrated optical fiber speed change section is analyzed and the eigenmode is solved; Step 3: Determine the polarization performance evaluation index, and determine the critical value of the rotation rate and the optical fiber length according to the equivalent phase delay deviation accuracy requirement; Step 4: Determine the values ​​of the key parameters of the optical fiber, such as beat length, length, and rotation rate, and obtain the optimal beat length value with the minimum equivalent phase delay deviation as the goal.

2. The method for determining key structural parameters of an integrated optical fiber according to claim 1, characterized in that: In step 1, the beat length of the integrated optical fiber speed change section is L B , L B =2π / △β, where △β is the difference between the propagation constants of the two orthogonal polarization eigenmodes in the integrated optical fiber speed change section; △β = β x -β y , β x represents the propagation constant associated with the mode polarized along the X direction; β y represents the propagation constant associated with the mode polarized along the Y direction; The overall length l of the integrated optical fiber speed change section is calculated by the beat length L B Normalized to L = l / L B , L represents the normalized length of the integrated optical fiber speed-changing section; Any point z along the fiber is normalized to Z = z / L B ; The end rotation rate of the integrated optical fiber speed change section is normalized to: Q max represents the normalized maximum rotation rate; τ max Indicates the actual maximum rotation rate; The helical rotation rate distribution along the integrated optical fiber speed change section is normalized to: Q(Z) represents the normalized rotation rate of point z; τ(z) represents the actual rotation rate of point z.

3. The method for determining key structural parameters of an integrated optical fiber according to claim 1, characterized in that: In step 2, the coupling between the two orthogonal linear polarization modes in the integrated optical fiber is described by the coupled mode equation shown in the following formula (1): In formula (1): A x , A y Respectively represent the two eigenmodes of light waves along the x and y axes of the integrated optical fiber; j represents the imaginary unit in complex number calculation; τ represents the rotation rate; Matrixing formula (1) yields: In formula (2): K and A represent the matrix form used for calculation; After normal transformation and correction, the analytical solution of the coupled mode equation is obtained: In formula (3), A(z) represents the symbol representing the analytical solution of the coupled mode equation; P = e jφ(z)csc(2θ) ; Q = e -jφ(z)csc(2θ) ; φ(z)=0.5arctan[2Q(z)]; θ represents an undetermined constant that is independent of the position of the integrated optical fiber.

4. The method for determining key structural parameters of an integrated optical fiber according to claim 3, characterized in that: At the beginning of the optical fiber, that is, when z = 0, the eigenmode corresponding to the incident light is A(0) = [cosθ -sinθ] T , T represents the matrix transpose, which is equivalent to linearly polarized light with a small angle θ on the birefringence axis.

5. The method for determining key structural parameters of an integrated optical fiber according to claim 3, characterized in that: The eigenmode of polarization of light waves at any position along the optical fiber is: In formula (4): A x (z), A y (z) represents the two eigenmodes of the light wave along the x and y axes of the integrated optical fiber at point z; e j φ(z)csc(2θ) Yes A x (z), A y (z) common phase factor; Therefore, the propagation of the eigenmode of the integrated optical fiber in the entire optical fiber can be described as:

6. The method for determining key structural parameters of an integrated optical fiber according to claim 5, characterized in that: In step 3, after obtaining the eigenmode of the integrated fiber coupling mode equation, the polarization rate P of the light wave output through the optical fiber is obtained: In formula (6): α and δ represent parameters related to the specific characteristics of the incident polarized light; α=arctan(|A x | / |A y |), δ=arg(A x / A y ); After the incident polarized light passes through the integrated optical fiber speed change section, the polarization rate of the outgoing light is analytically expressed by the following formula (7): When the incident polarized light is linearly polarized light, its polarization rate P is 1; for elliptically polarized light, its polarization rate P is between 0 and 1; and for circularly polarized light, its polarization rate P is 0; From formula (7), we can get that when Q max Or when L continues to increase, the polarization rate P gradually decreases, that is, the incident polarized light gradually approaches circularly polarized light.

7. The method for determining key structural parameters of an integrated optical fiber according to claim 6, characterized in that: In step 3, When the normalized length L of the integrated optical fiber speed change section is fixed, the maximum rotation rate Q max As the polarization rate P increases, the polarization rate P decreases, but when the maximum rotation rate Q max When it is large enough, the decrease in polarization rate P becomes smaller; At a fixed maximum rotation rate Q max In the case of , the polarization rate P also decreases. At the same time, when the normalized length L of the integrated optical fiber speed-changing section increases to a certain value, the polarization rate P is almost no longer affected by L. With the maximum rotation rate Q max Or as the normalized length L of the integrated optical fiber speed-changing section continues to increase, the polarization rate P of the polarized light gradually decreases and approaches 0.

8. The method for determining key structural parameters of an integrated optical fiber according to claim 7, characterized in that: Consider the maximum rotation rate Q max and the critical values ​​of the normalized length L of the integrated optical fiber speed-changing section to determine the structural parameters of the integrated optical fiber speed-changing section; Assume that the outgoing polarized light consists of two waves with equal amplitude and phase difference of δ e The orthogonal components of δ e is the equivalent phase delay of the integrated optical fiber speed change section, then: d e = arccosP (8).

9. The method for determining key structural parameters of an integrated optical fiber according to claim 1, characterized in that: In step 4, in order to analyze the relationship between the equivalent phase delay deviation of the optical fiber wave plate and the beat length of the optical fiber, L and Q L Convert to l and τ l The actual value of is obtained by formula (8): In formula (9): Δδ e represents the equivalent phase delay deviation; In order to analyze the influence of the beat length of the optical fiber on the equivalent phase delay deviation, the rotation rate τ is given by the control variable method. l With length l: For different maximum rotation rates τ l and the fiber length l, as the fiber beat length L B The relative phase delay deviation of the optical fiber output light increases. e / δ e0 Both decrease first and then increase, so for a certain maximum rotation rate τ l and fiber length l, there is an optimal beat length that minimizes the phase delay deviation; From formula (9), for a given τ l and l, if the beat length is: Formula (10): L B0 represents the optimal beat length; The delay deviation of the fiber wave plate reaches a minimum: Optimal shot length L B0 and (l / τ l ) 1 / 2 Proportional to the minimum delay deviation △δ emin and (l·τ l ) 1 / 2 Negatively correlated; thus, l and τ l The larger the product of emin / △δ e0 The smaller it is, the better the performance of the fiber optic wave plate.

10. The method for determining key structural parameters of an integrated optical fiber according to claim 9, characterized in that: When L B When l is the optimal beat length, l When the product of is fixed to a constant value, it corresponds to the minimum delay deviation △δ emin can be transformed into: In this case, when △δ emin When L is a constant, the optimal beat length L B0 Proportional to the fiber length l, or to the maximum rotation rate τ l Inversely proportional.

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