Optical fiber numerical aperture measurement method

Abstract

The invention relates to an optical fiber numerical aperture measurement method, which belongs to the technical field of optical fiber parameter measurement, and specifically comprises the following steps of: performing curved surface sampling on emergent light spots of a measured optical fiber, and determining sampling points and corresponding three-dimensional coordinates and angular coordinates on the basis of a scanning range of a detector and the diameter of a through hole of a variable diaphragm of the detector; traversing each sampling point by a detector to obtain the actually measured light intensity, optimizing the fitting coefficient based on the fitting light intensity of each sampling point and the actually measured light intensity to obtain the maximum light intensity amplitude meeting the precision requirement, selecting the sampling points meeting the conditions to perform circle fitting to obtain the optimal circle radius, and calculating to obtain the numerical aperture; the problems of measurement errors caused by measurement of curved wavefront through a planar array detector and complex and accurate alignment of an optical fiber and the detector during measurement in the prior art are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber parameter measurement, and particularly to a method for measuring the numerical aperture of an optical fiber. BACKGROUND

[0002] An optical fiber is a fiber made of glass or plastic, also known as a light guide fiber, which is a special medium for light beam transmission by using the principle of total reflection of light. The optical fiber has the advantages of high bandwidth, low loss, high transmission safety, no ground loop interference, small space occupation, low cost, long service life, etc., and has important applications in the fields of optical fiber communication, precision sensing, medical endoscope, radar and microwave system, security monitoring, etc. The numerical aperture (NA) is an important parameter of the optical fiber, which represents the light collecting ability of the optical fiber. The NA of the optical fiber plays a key role in the coupling of the light source and the optical fiber, and the coupling of the optical fiber and the optical fiber, and is an important factor affecting the coupling efficiency, connection loss and attenuation characteristics of the optical fiber.

[0003] When the optical fiber is shipped, the NA provided by the manufacturer is usually a nominal value, which deviates from the actual value, and the NA deviation of different batches of products is also different. In addition, different manufacturers have different definitions of the NA of the optical fiber, which may not meet the actual use requirements. In addition, the NA of the same optical fiber at different working wavelengths is also different.

[0004] Currently, the methods for measuring the NA of the optical fiber mainly include the far-field light intensity method, the far-field spot method and the refractive near-field method. Among them, the far-field light intensity method measures the light intensity distribution of the output light of the optical fiber, and calculates the NA of the optical fiber by measuring the far-field angle corresponding to the percentage of the peak intensity (different manufacturers have different definitions, such as 1%, 5% or 13.5%, or other values). The far-field spot method measures the spot radius at a distance away from the optical fiber, and the sine value of the relative angle of the spot to the light emitting point of the optical fiber is the NA of the optical fiber. The refractive near-field method measures the refractive index distribution of the core and cladding of the optical fiber, and directly calculates the NA of the optical fiber according to the theoretical formula. The above far-field measurement methods usually use a planar area array detector to directly measure the size of the entire spot, or scan the light intensity values at different points on a certain track, which is difficult to capture the peak intensity point, and has a large measurement error. SUMMARY

[0005] In view of the above analysis, the embodiments of the present application aim to provide a method for measuring the numerical aperture of an optical fiber, which solves the measurement error caused by the use of a planar area array detector to measure the curved wavefront and the complex and accurate alignment problem between the optical fiber and the detector during measurement.

[0006] In one aspect, the embodiments of the present application provide a method for measuring the numerical aperture of an optical fiber, which specifically comprises:

[0007] The exit point of the measured optical fiber is arranged opposite to the detector, and the distance between them is a preset measurement distance.

[0008] Determine each sampling point based on the scanning range of the detector and the variable aperture diameter of the detector, and obtain the three-dimensional coordinates and angular coordinates of each sampling point, wherein the three-dimensional coordinates of each sampling point are located on a spherical surface with the exit point of the measured optical fiber as the center and a preset measurement distance as the radius.

[0009] Make the detector traverse each sampling point to collect light intensity and obtain the measured light intensity of each sampling point.

[0010] Fit the light intensity of each sampling point to obtain the fitted light intensity of each sampling point, optimize the fitting coefficient based on the measured light intensity of each sampling point to obtain the fitting coefficient when the fitting accuracy meets the preset requirement, and the fitting coefficient includes the light intensity amplitude, and the light intensity amplitude when the fitting accuracy meets the preset requirement is taken as the maximum light intensity amplitude.

[0011] Select the sampling points that meet the conditions based on the maximum light intensity amplitude for circle fitting, and obtain the circle radius when the fitting effect is optimal as the optimal circle radius.

[0012] Calculate the numerical aperture based on the optimal circle radius.

[0013] The beneficial effects of the above technical solution are as follows: by using the variable aperture diameter of the detector and the scanning range to determine each sampling point and obtain the three-dimensional coordinates and angular coordinates of each sampling point, the detector can more accurately measure the light intensity of the sampling point based on the three-dimensional coordinates and angular coordinates; then the light intensity of each sampling point is fitted to obtain the fitted light intensity, the maximum light intensity amplitude that meets the fitting accuracy requirement is obtained by optimizing the fitting coefficient based on the fitted light intensity and the measured light intensity, and the circle radius when the fitting effect is optimal is obtained by fitting the sampling points that meet the conditions based on the maximum light intensity amplitude, and the numerical aperture calculated based on the optimal circle radius greatly improves the numerical aperture accuracy and reduces the measurement error compared with the prior art.

[0014] Based on the further improvement of the above method, the determination of each sampling point based on the scanning range of the detector and the variable aperture diameter of the detector, and the obtaining of the three-dimensional coordinates of each sampling point specifically include:

[0015] Obtain the horizontal scanning range and the vertical scanning range of the detector, divide the horizontal scanning range and the vertical scanning range into horizontal and vertical coordinates of each sampling point at equal intervals with the variable aperture diameter of the detector as the interval, and obtain the radial coordinate based on the horizontal and vertical coordinates of each sampling point and the preset measurement distance.

[0016] The three-dimensional coordinate calculation formula of the sampling point is as follows:

[0017]

[0018] In the formula, x i , yj , z i，j are the transverse, longitudinal, radial coordinates of the corresponding sampling points, M is the number of transverse sampling points, N is the number of longitudinal sampling points, X is the transverse scanning range, Y is the longitudinal scanning range, and R is the preset measurement distance.

[0019] The beneficial effect of the above further improvement scheme is that by adopting the detector variable aperture hole diameter and scanning range to determine each sampling point and obtain the three-dimensional coordinates of each sampling point, the detector can more accurately measure the light intensity of the sampling point based on each three-dimensional coordinate.

[0020] Based on the further improvement of the above method, the angular coordinates of each sampling point are obtained by the following formula, which is specifically represented as:

[0021]

[0022] In the formula, is the x i corresponding transverse angular coordinate, θ j is the y j corresponding longitudinal angular coordinate.

[0023] The beneficial effect of the above further improvement scheme is that based on the corresponding transverse coordinates, longitudinal coordinates and measurement distances of each sampling point, the corresponding transverse and longitudinal angular coordinates of each sampling point can be obtained, which facilitates the detector to more accurately measure the light intensity of the sampling point based on each angular coordinate.

[0024] Based on the further improvement of the above method, the fitting light intensity of each sampling point is obtained by fitting the light intensity of each sampling point, and the fitting coefficient is optimized based on the measured light intensity of each sampling point to obtain the fitting coefficient when the fitting precision meets the preset requirement, comprising:

[0025] S1: selecting the maximum light intensity sampling point coordinates;

[0026] S2: setting the initial value of the fitting coefficient;

[0027] S3: fitting the light intensity of each sampling point based on the current fitting coefficient and the maximum light intensity coordinates to obtain the fitting light intensity of each sampling point;

[0028] S4: calculating the sum of squares of the difference between the fitting light intensity and the measured light intensity value of each sampling point;

[0029] When the sum of squares is greater than or equal to a first preset threshold, a first iteration is started to optimize the fitting coefficient, and the first iteration specifically includes:

[0030] SA1: synchronously reducing the current fitting coefficient by a preset step size, and then

[0031] performing S3-S4,

[0032] SA2: judging whether the sum of squares obtained in the current iteration round is greater than or equal to the first preset threshold value, if yes, returning to step SA1 until the iteration round reaches the first preset iteration number;

[0033] Otherwise, terminating the first iteration, and taking the light intensity amplitude in the fitting coefficient in the current round as the maximum light intensity amplitude;

[0034] When the first iteration round reaches the first preset iteration number, and the sum of squares less than or equal to the first preset threshold value is still not found, resetting the current fitting coefficient as the fitting coefficient initial value, and starting a second iteration, the second iteration specifically includes:

[0035] SAA1: increasing the current fitting coefficient by a preset step length,

[0036] performing S3-S4,

[0037] SAA2: judging whether the sum of squares obtained in the current iteration round is greater than or equal to the first preset threshold value, if yes, returning to step SAA1 until the iteration round reaches the second preset iteration number;

[0038] Otherwise, terminating the second iteration, and taking the light intensity amplitude in the fitting coefficient in the current round as the maximum light intensity amplitude;

[0039] When the second iteration round reaches the second preset iteration number, and the sum of squares less than or equal to the first preset threshold value is still not found, taking the light intensity amplitude in the fitting coefficient corresponding to the minimum value in all the sums of squares as the maximum light intensity amplitude.

[0040] The beneficial effect of the above further improved scheme is that the light intensity amplitude with fitting precision meeting the preset requirement can be obtained, and more accurate and smaller error results can be obtained in subsequent data processing.

[0041] Based on the further improvement of the above method, the fitting coefficient further includes fitting coefficients a and b; and the fitting light intensity of each sampling point is represented as:

[0042]

[0043] In the formula, I'(x i , y j ) is the fitting light intensity, a and b are the fitting coefficients, x Imax , y Imax are the horizontal and vertical coordinates corresponding to the maximum light intensity, and A is the light intensity amplitude.

[0044] The beneficial effect of the above further improved scheme is that more accurate fitting light intensity can be obtained based on the horizontal and vertical coordinates of each sampling point and the horizontal and vertical coordinates corresponding to the maximum light intensity.

[0045] Based on the further improvement of the above method, the sum of squares of the difference between the fitting light intensity and the measured light intensity of each sampling point is calculated by the following formula, which is specifically represented as:

[0046] In the formula, I(x i , y j ) is the measured light intensity, and Δ is the sum of squares of the difference between the fitting light intensity and the measured light intensity of each sampling point.

[0047] The beneficial effect of the above further improvement scheme is that the sum of squares of the difference between the fitting light intensity and the measured light intensity can be obtained to find the light intensity amplitude that meets the preset accuracy requirement.

[0048] Based on the further improvement of the above method, the fitting circle is performed on the sampling points that meet the condition based on the maximum light intensity amplitude, and the circle radius at the optimal fitting effect is obtained as the optimal circle radius, including:

[0049] SS1: set the initial value of the current circle radius and the initial coordinates of the center of the fitting circle, and the center coordinates of the fitting circle include the horizontal coordinates and the vertical coordinates;

[0050] SS2: find all sampling points from the sampling points whose measured light intensity meets the preset proportion with the maximum light intensity amplitude;

[0051] SS3: perform circle fitting on each sampling point to obtain the radius of each fitting circle;

[0052] SS4: calculate the sum of squares of the difference between the square sum of each fitting circle radius and the square of the initial value of the circle radius;

[0053] When the sum of squares is greater than or equal to a second preset threshold, a first iteration is started, and the first iteration specifically includes:

[0054] SSA1: simultaneously reduce the horizontal coordinates and the vertical coordinates of the center of the current fitting circle and the current circle radius by a preset step size,

[0055] perform SS3-SS4,

[0056] SSA2: determine whether the sum of squares obtained in the current iteration round is greater than or equal to the second preset threshold, if yes, return to step SSA1 until the iteration round reaches a first preset iteration number,

[0057] otherwise, terminate the first iteration, and take the current circle radius corresponding to the sum of squares obtained in the current round as the optimal circle radius;

[0058] When the first iteration round reaches the second preset iteration number, and the sum of squares less than or equal to the second preset threshold is still not found, the current circle radius is reset to the current circle radius initial value, the current fitting circle center horizontal coordinate and the vertical coordinate are reset to the fitting circle center initial coordinate, and the second iteration is started, and the second iteration specifically includes:

[0059] SSAA1: synchronously increasing the current fitting circle center horizontal coordinate, the vertical coordinate, and the current circle radius by a preset step,

[0060] performing SS3-SS4,

[0061] SSAA2: determining whether the sum of squares obtained in the current iteration round is greater than or equal to the second preset threshold, if yes, returning to step SSAA1 until the iteration round reaches the second preset iteration number,

[0062] otherwise, terminating the second iteration, and taking the sum of squares corresponding to the current circle radius obtained in the current iteration round as the optimal circle radius;

[0063] When the second iteration round reaches the first preset iteration number, and the sum of squares less than or equal to the second preset threshold is still not found, the current circle radius corresponding to the minimum value in all sums of squares is taken as the optimal circle radius.

[0064] The beneficial effect of the above further improved scheme is that the optimal current circle radius can be obtained, and subsequent data processing can obtain more accurate and smaller error results.

[0065] Based on the further improvement of the above method, the sum of squares of the square difference of each fitting circle radius found by the following formula is calculated, and the formula is specifically represented as:

[0066] (x s -c x ) 2 +(y t -c y ) 2 =r 2 , in the formula, x s , y t are the horizontal and vertical coordinates of each sampling point found, c x , c y are the horizontal and vertical coordinates of the fitting circle center, and r is the fitting circle radius.

[0067] The beneficial effect of the above further improved scheme is that the fitting circle radius of each sampling point found by the fitting circle is obtained.

[0068] Based on the further improvement of the above method, the sum of squares of the square difference of each fitting circle radius square and the current circle radius square is calculated by the following formula, and the formula is specifically represented as:

[0069] In the formula, r' is the current circle radius, and Γ is the sum of squares of the difference between the square of each fitting circle radius and the square of the current circle radius.

[0070] The beneficial effect of the further improved scheme is that the sum of squares of the difference between the square of each fitting circle radius and the square of the current circle radius can be obtained to find the optimal current circle radius.

[0071] Based on the further improvement of the above method, the numerical aperture is calculated based on the optimal circle radius by the following formula, which is specifically represented as:

[0072] In the formula, NA is the numerical aperture.

[0073] The beneficial effect of the further improved scheme is that the accurate numerical aperture can be calculated based on the optimal circle radius.

[0074] In the present application, each of the technical schemes described above can be combined with each other to realize more preferred combination schemes. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification, or will be understood by implementing the present application. The purposes and other advantages of the present application can be realized and obtained from the contents specifically indicated in the specification and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0075] The accompanying drawings are included to provide a further understanding of the present application, and are incorporated in and constitute a part of this specification, illustrate embodiments of the present application, and together with the description serve to explain the principles of the present application, and should not be necessarily construed as limiting the present application.

[0076] Figure 1 The present application is an embodiment method flowchart.

[0077] Figure 2 The present application is an embodiment measured optical fiber exit light sampling point three-dimensional coordinate and angular coordinate schematic diagram.

[0078] Figure 3 The present application is an embodiment measured optical fiber exit light sampling point distribution plane projection schematic diagram.

[0079] Figure 4 The present application is an embodiment measured optical fiber exit light fitting light intensity 3D distribution schematic diagram.

[0080] Figure 5 The present application is an embodiment measured optical fiber exit light fitting light intensity 2D distribution and circle fitting schematic diagram. DETAILED DESCRIPTION

[0081] The preferred embodiments of the present application will be described in detail below with reference to the drawings, wherein the drawings constitute a part of this application, and are used to explain the principles of the embodiments of the present application, but not to limit the scope of the present application.

[0082] One specific embodiment of the present application discloses a method for measuring the numerical aperture of an optical fiber, as shown in the figure. Figure 1

[0083] The method specifically comprises:

[0084] The exit point of the measured optical fiber is arranged opposite to the detector, and the distance between them is a preset measurement distance;

[0085] Based on the scanning range of the detector and the diameter of the through hole of the variable diaphragm of the detector, each sampling point is determined, and the three-dimensional coordinates and the angular coordinates of each sampling point are obtained, wherein the three-dimensional coordinates of each sampling point are located on a spherical surface with the exit point of the measured optical fiber as the center and the preset measurement distance as the radius;

[0086] The detector traverses each sampling point to collect light intensity, and the measured light intensity of each sampling point is obtained;

[0087] The light intensity fitting of each sampling point is performed to obtain the fitting light intensity of each sampling point, the fitting coefficient is optimized based on the measured light intensity of each sampling point, the fitting coefficient is obtained when the fitting precision meets the preset requirement, and the fitting coefficient includes the light intensity amplitude, the light intensity amplitude when the fitting precision meets the preset requirement is taken as the maximum light intensity amplitude;

[0088] Based on the maximum light intensity amplitude, the sampling points meeting the conditions are selected for circle fitting, and the circle radius when the fitting effect is optimal is taken as the optimal circle radius;

[0089] The numerical aperture is calculated based on the optimal circle radius.

[0090] Generally, for the measurement of the numerical aperture of an optical fiber, the exit point of the measured optical fiber is arranged opposite to the detector, the light emitted from the exit point of the measured optical fiber presents as a gradually enlarged light column, and after passing through the measurement distance, the light column is projected onto the measured detector to form a light spot. The detector detects the light spot, and the scanning range of the detector is determined by the size of the area that can be scanned by the detector. In the prior art, the measurement error will be caused by using a planar array detector to measure the intensity distribution of the divergent wavefront. In addition, the optical fiber and the detector need to be accurately aligned during the measurement, the measurement error of the light intensity peak value is large, and the measurement process is complex. In the present embodiment, as shown in the figure, the three-dimensional coordinates and the angular coordinates of each sampling point are output by the curved surface detection of the light spot curved surface, which is more in line with the actual characteristics of light, and collimation is not required, thereby simplifying the measurement setting process. Figure 2

[0091] ​​Specifically, sampling points are determined based on the detector scanning range and the diameter of the variable aperture. The detector scanning range refers to the light spot area (X-axis) projected onto the detector by the light emitted from the emitting point of the fiber under test. Multiple uniformly distributed sampling points are obtained by equally spacing the horizontal and vertical scanning ranges of the detector, using the diameter of the variable aperture as the interval. In existing technologies, detectors use planar arrays for detection; however, in this embodiment, the sampling points are distributed on the curved surface of the light spot, such as... Figure 3 As shown, compared with uniformly distributed sampling points on a plane, the light intensity values ​​obtained from sampling points on a curved surface are more accurate. Therefore, the three-dimensional coordinates and angular coordinates of each sampling point need to be calculated using the following formula, which specifically includes:

[0092]

[0093] In the formula, x i y j , z i，j These are the horizontal, vertical, and radial coordinates of the corresponding sampling points. M is the number of horizontal sampling points, N is the number of vertical sampling points, and R is the preset measurement distance.

[0094]

[0095] In the formula, It is x i Corresponding to the horizontal angular coordinate, θ j It is y j Corresponding vertical angular coordinates.

[0096] In practice, the calculation of the three-dimensional coordinates and angular coordinates of each sampling point and the measurement of the light intensity of the sampling point can be carried out simultaneously, or the three-dimensional coordinates and angular coordinates of each sampling point can be calculated first, and then the detector can be adjusted to collect the light intensity of each sampling point one by one based on the three-dimensional coordinates and angular coordinates of each sampling point.

[0097] In a preferred example, such as Figure 3 As shown, the detector measures the light intensity at each sampling point on the curved surface of the emitted light spot from the fiber under test in an "S" shaped trajectory, as detailed below:

[0098] When i = 1, x1 = X / MX / 2, j = 1, 2, ..., N.

[0099] y1=Y / NY / 2, y2=2Y / NY / 2,...,y N =YY / 2=Y / 2;

[0100]

[0101] When i = 2, x2 = 2X / M - X / 2, j = N, N-1,..., 2, 1,

[0102] y N = Y - Y / 2 = Y / 2, y N-1 = Y*(N-1) / N - Y / 2 = Y / 2 - Y / N,..., y1 = Y / N - Y / 2;

[0103]

[0104] When i = 3, x3 = 3X / M - X / 2, j = 1, 2,..., N;

[0105] y1 = Y / N - Y / 2, y2 = 2Y / N - Y / 2,..., y N = Y - Y / 2 = Y / 2;

[0106]

[0107] By analogy, until the detector traverses the three-dimensional coordinates of each sampling point, the angular coordinates, and completes the measurement of the light intensity of each sampling point to obtain the measured light intensity, wherein the angular coordinates are used to adjust the pose of the detector when measuring the corresponding sampling point. The preferred example is only one preferred form of the scanning trajectory of the detector for each sampling point, which has shorter scanning time compared to other forms, but does not limit the form of the scanning trajectory of the detector for each sampling point. Optionally, the scanning trajectory of the detector for each sampling point can adopt the forms of "S" type, inverse "S" type, row-by-row re-scanning, column-by-column re-scanning, random point selection, random line selection, random block selection, etc.

[0108] Next, it is necessary to fit the light intensity of each sampling point and find the optimal fitting coefficient based on the measured light intensity to find the maximum light intensity amplitude, which is used for subsequent data processing and numerical aperture calculation.

[0109] Further, the light intensity of each sampling point is fitted to obtain the fitting light intensity of each sampling point, and the fitting coefficient is optimized based on the measured light intensity of each sampling point to obtain the fitting coefficient when the fitting precision meets the preset requirement, including:

[0110] S1: selecting the maximum light intensity sampling point coordinates;

[0111] Optionally, the sampling point coordinates corresponding to the maximum measured light intensity value, or the transverse and longitudinal coordinates are both 0, are selected as the maximum light intensity sampling point coordinates;

[0112] S2: setting the initial value of the fitting coefficient;

[0113] The fitting coefficient includes a, b, and A, wherein A is the light intensity amplitude. Illustratively, the initial values of a, b, and A are all set to 1;

[0114] S3: fitting light intensity of each sampling point based on current fitting coefficient, maximum light intensity coordinate to obtain each sampling point fitting light intensity, fitting light intensity of each sampling point is expressed as:

[0115]

[0116] In the formula, I'(x i , y j ) is fitting light intensity, x s , y x is transverse, longitudinal coordinate corresponding to maximum light intensity;

[0117] As shown in Figure 4 , the measured optical fiber exit light intensity distribution is approximately Gaussian distribution, and the fitting light intensity of each sampling point is obtained by fitting each sampling point through Gaussian function;

[0118] S4: calculating the sum of squares of the difference between each sampling point fitting light intensity and measured light intensity value, further, calculating the sum of squares of the difference between each sampling point fitting light intensity and measured light intensity through the following formula, the formula is specifically expressed as:

[0119] In the formula, I(x 2 , y t ) is measured light intensity, Δ is the sum of squares of the difference between each sampling point fitting light intensity and measured light intensity;

[0120] Specifically, the sum of squares of the difference between each sampling point fitting light intensity and measured light intensity is calculated by least square method, that is, the deviation between fitting light intensity and measured light intensity.

[0121] Further, the optimal solution is obtained by judging whether the sum of squares is less than the first preset threshold value, wherein the first preset threshold value is set according to the definition mode of the measured fiber numerical aperture, in principle, the smaller the deviation between each sampling point fitting light intensity and corresponding actual light intensity, the more ideal the maximum light intensity amplitude obtained; if the sum of squares is not optimal solution, the optimal solution is obtained by changing the current fitting coefficient by a preset step length to iteratively calculate a plurality of sums of squares, specifically including:

[0122] When the sum of squares is greater than or equal to the first preset threshold value, the first iteration is started, and the fitting coefficient is optimized, the first iteration specifically includes:

[0123] SA1: reducing the current fitting coefficient by a preset step length, wherein, for example, the preset step length is 0.01, then

[0124] performing S3-S4,

[0125] SA2: judging whether the sum of squares obtained in the current iteration round is greater than or equal to the first preset threshold value, if yes, returning to step SA1 until the iteration round reaches the first preset iteration number;

[0126] Otherwise, terminating the first iteration, and taking the light intensity amplitude in the fitting coefficient in the current round as the maximum light intensity amplitude;

[0127] When the first iteration round reaches the first preset iteration number, and the sum of squares less than or equal to the first preset threshold value is still not found, resetting the current fitting coefficient as the fitting coefficient initial value, and starting the second iteration, the second iteration specifically includes:

[0128] SAA1: synchronously increasing the current fitting coefficient by a preset step size,

[0129] performing S3-S4,

[0130] SAA2: judging whether the sum of squares obtained in the current iteration round is greater than or equal to the first preset threshold value, if yes, returning to step SAA1 until the iteration round reaches the second preset iteration number;

[0131] Otherwise, terminating the second iteration, and taking the light intensity amplitude in the fitting coefficient in the current round as the maximum light intensity amplitude;

[0132] When the second iteration round reaches the second preset iteration number, and the sum of squares less than or equal to the first preset threshold value is still not found, taking the light intensity amplitude in the fitting coefficient corresponding to the minimum value in all sums of squares as the maximum light intensity amplitude.

[0133] Further, according to the definition of the numerical aperture of the measured optical fiber, the sampling points meeting the conditions are selected based on the maximum light intensity amplitude for circle fitting, and the circle radius when the fitting effect is optimal is taken as the optimal circle radius, including:

[0134] SS1: setting a current circle radius initial value and initial coordinates of a fitting circle center, the initial coordinates of the fitting circle center including horizontal coordinates and vertical coordinates;

[0135] Specifically, the corresponding sampling point coordinates are found from the fitting light intensity of each sampling point based on the maximum light intensity amplitude, as the initial coordinates of the fitting circle center, and the current circle radius initial value is set to 1 for example;

[0136] SS2: find all sampling points from each sampling point, the measured light intensity and the maximum light intensity amplitude meet the preset proportion, wherein the preset proportion refers to the maximum light intensity amplitude proportion defined by different measured fiber numerical aperture values, common ones are 1%, 5%, 13.5%; select the sampling points with light intensity value close to the product of the maximum light intensity amplitude and the corresponding proportion from each sampling point, preferably, select the sampling points with light intensity value within the error range of ±1% of the product of the maximum light intensity amplitude and the corresponding proportion,

[0137] SS3: calculate the radius of each fitting circle for each sampling point to obtain the radius of each fitting circle;

[0138] Specifically, the radius of each fitting circle is calculated by the following formula, which is specifically represented as:

[0139] (x s -c x ) 2 +(y t -c y ) 2 =r 2 , wherein x s , y t is the horizontal and vertical coordinates of each sampling point, c x , c y is the horizontal and vertical coordinates of the center of the fitting circle, and r is the radius of the fitting circle;

[0140] SS4: calculate the sum of squares of the difference between the square of each fitting circle radius and the square of the current circle radius;

[0141] Specifically, the sum of squares of the difference between the square of each fitting circle radius and the square of the initial circle radius is calculated by the following formula, which is specifically represented as:

[0142] wherein r' is the initial circle radius, and Γ is the sum of squares of the difference between the square of each fitting circle radius and the square of the initial circle radius;

[0143] When the sum of squares is greater than or equal to a second preset threshold, a first iteration is started, which specifically includes:

[0144] SSA1: reduce the horizontal and vertical coordinates of the center of the current fitting circle, the current circle radius by a preset step size,

[0145] perform SS3-SS4,

[0146] SSA2: determine whether the sum of squares obtained in the current iteration round is greater than or equal to the second preset threshold, if yes, return to step SSA1 until the iteration round reaches the second preset iteration number,

[0147] Otherwise, terminate the first iteration, and take the current circle radius obtained in the current iteration as the optimal circle radius;

[0148] When the first iteration reaches the second preset iteration number, and the sum of squares is still greater than the second preset threshold, reset the current circle radius to the initial value of the current circle radius, and reset the horizontal and vertical coordinates of the center of the current fitting circle to the initial coordinates of the center of the fitting circle, and start the second iteration, which specifically includes:

[0149] SSAA1: increase the horizontal and vertical coordinates of the center of the current fitting circle and the current circle radius by a preset step size,

[0150] perform SS3-SS4,

[0151] SSAA2: determine whether the sum of squares obtained in the current iteration is greater than or equal to the second preset threshold, if yes, return to step SSAA1 until the iteration reaches the second preset iteration number,

[0152] Otherwise, terminate the second iteration, and take the sum of squares corresponding to the current circle radius obtained in the current iteration as the optimal circle radius;

[0153] When the second iteration reaches the first preset iteration number, and the sum of squares is still greater than the second preset threshold, take the current circle radius corresponding to the minimum sum of squares as the optimal circle radius.

[0154] Further, based on the optimal circle radius, the numerical aperture is calculated by the following formula, which is specifically represented as:

[0155] In the formula, NA is the numerical aperture.

[0156] Compared with existing technologies, the fiber numerical aperture measurement method provided in this embodiment performs curved surface sampling on the emitted light spot of the fiber under test. Specifically, it determines each sampling point and its corresponding three-dimensional coordinates and angular coordinates based on the detector scanning range and the diameter of the detector's variable aperture. Then, the detector traverses each sampling point to collect light intensity, obtaining the measured light intensity of each sampling point. Next, it performs light intensity fitting on each sampling point to obtain the fitted light intensity of each sampling point. Based on the measured light intensity of each sampling point, it optimizes the fitting coefficient to obtain the fitting coefficient when the fitting accuracy meets the preset requirements. The fitting coefficient includes the light intensity amplitude. The light intensity amplitude when the fitting accuracy meets the preset requirements is taken as the maximum light intensity amplitude. Based on the maximum light intensity amplitude, sampling points that meet the conditions are selected for circle fitting, and the radius of the circle with the best fitting effect is obtained as the optimal circle radius. Finally, the numerical aperture is calculated based on the optimal circle radius and the distance between the emitted point of the fiber under test and the detector. By performing curved surface sampling on the emitted light spot of the fiber under test, accurate and convenient light intensity measurement can be achieved without collimating the emitted light from the fiber under test with the detector. Furthermore, the numerical aperture obtained based on the optimized search calculation process greatly improves the measurement accuracy and reduces the measurement error.

[0157] For example, the measurement distance from the emitting end of the fiber under test to the detector is R = 25cm, the detector's horizontal and vertical scanning ranges are X = 7.5cm and Y = 7.5cm respectively, and the number of sampling points for the emitted light from the fiber under test in the horizontal and vertical directions are M = 150 and N = 150 respectively. The resulting 3D distribution map of the emitted light intensity is shown below. Figure 4 As shown, after fitting the light intensity using a Gaussian function and optimizing the fitting coefficients, the maximum light intensity amplitude A = 0.6 was obtained, and the coordinates of the sampling point corresponding to the maximum light intensity (x... Imax y Imax The distances are (0.1cm, 0.05cm), and the optimal fitting coefficients are a = 3 and b = 3.1. The planar distribution of the intensity of the light emitted from the fiber under test is shown below. Figure 5 As shown, the three intensity profiles when the light intensity drops to 1%, 5%, and 13.5% of the peak intensity are respectively as follows: Figure 5 As shown in a, b, and c, the optimal fitting circle radii obtained by performing a circle fitting on this intensity profile are r, respectively. 1％ =3.6918cm, r 5％ =2.9811cm and r 13.5％ =2.4368cm, the corresponding numerical aperture of the optical fiber is calculated to be NA. 1％ =0.1477, NA 5％ =0.1192 and NA 13.5％ =0.0975.

[0158] Those skilled in the art can understand that all or part of the processes of the above-mentioned embodiment methods can be completed by instructing the relevant hardware by a computer program, and the program can be stored in a computer readable storage medium. The computer readable storage medium is a disk, an optical disk, a read-only memory, a random access memory, etc.

[0159] The above description is merely preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method of measuring the numerical aperture of an optical fiber, comprising: The method specifically includes: The fiber optic output point under test is positioned directly opposite the detector, and the two are at a preset measurement distance apart. Each sampling point is determined based on the detector scanning range and the diameter of the detector's variable aperture, and the three-dimensional coordinates and angular coordinates of each sampling point are obtained. The three-dimensional coordinates of each sampling point are all located on a sphere with the emission point of the fiber under test as the center and the preset measurement distance as the radius. The detector is made to traverse each sampling point to collect light intensity, and the measured light intensity at each sampling point is obtained. The light intensity of each sampling point is obtained by fitting the light intensity of each sampling point. The fitting coefficient is optimized based on the measured light intensity of each sampling point to obtain the fitting coefficient when the fitting accuracy meets the preset requirements. The fitting coefficient includes the light intensity amplitude. The light intensity amplitude when the fitting accuracy meets the preset requirements is taken as the maximum light intensity amplitude. Based on the maximum light intensity amplitude, sampling points that meet the conditions are selected for circle fitting, and the circle radius with the best fitting effect is obtained as the optimal circle radius. The numerical aperture is calculated based on the optimal circle radius.

2. The method of claim 1, wherein, The process of determining each sampling point based on the detector scanning range and the diameter of the variable aperture, and obtaining the three-dimensional coordinates of each sampling point, specifically includes: The detector's lateral and longitudinal scanning ranges are obtained. The lateral and longitudinal scanning ranges of the detector are divided at equal intervals with the diameter of the detector's variable aperture as the spacing to obtain the horizontal and vertical coordinates of each sampling point. The surface coordinates are obtained based on the horizontal and vertical coordinates of each sampling point and the preset measurement distance. The formula for calculating the three-dimensional coordinates of the sampling points is as follows: In the formula, x i , y j , and z i，j are the transverse, longitudinal, and radial coordinates of the corresponding sampling points, M is the number of transverse sampling points, N is the number of longitudinal sampling points, X is the transverse scanning range, Y is the longitudinal scanning range, and R is a preset measurement distance.

3. The method of claim 2, wherein, The angular coordinates of each sampling point are obtained using the following formula, which is specifically expressed as follows: wherein is x i corresponds to a lateral angular coordinate, θ j is y j corresponds to a longitudinal angular coordinate.

4. The method for measuring the numerical aperture of an optical fiber according to claim 3, characterized in that, The process involves fitting the light intensity at each sampling point to obtain the fitted light intensity, and then optimizing the fitting coefficients based on the measured light intensity at each sampling point to obtain the fitting coefficients that meet the preset requirements for fitting accuracy. This includes: S1: Select the coordinates of the sampling point with the maximum light intensity; S2: Set initial values ​​for the fitting coefficients; S3: Based on the current fitting coefficients and the coordinates of the maximum light intensity, the light intensity of each sampling point is fitted to obtain the fitted light intensity of each sampling point; S4: Calculate the sum of the squares of the differences between the fitted light intensity and the measured light intensity at each sampling point; When the sum of the squares is greater than or equal to a first preset threshold, the first iteration is initiated to optimize the fitting coefficients. The first iteration specifically includes: SA1: Simultaneously decrease the current fitting coefficient with a preset step size, then Execute S3-S4, SA2: Determine whether the sum of squares obtained in the current iteration is greater than or equal to the first preset threshold. If so, return to step SA1 until the iteration reaches the first preset number of iterations. Otherwise, terminate the first iteration and take the light intensity amplitude in the fitting coefficients of the current round as the maximum light intensity amplitude; If, after the first iteration reaches the first preset iteration count, no sum of squares less than or equal to the first preset threshold is found, the current fitting coefficients are reset to their initial values, and the second iteration is initiated. The second iteration specifically includes: SAA1: Simultaneously increase the current fitting coefficient with a preset step size. Execute S3-S4, SAA2: Determine whether the sum of squares obtained in the current iteration is greater than or equal to the first preset threshold. If so, return to step SAA1 until the iteration reaches the second preset number of iterations. Otherwise, terminate the second iteration and take the light intensity amplitude in the fitting coefficients of the current round as the maximum light intensity amplitude; If, after the second iteration reaches the second preset iteration number, no sum of squares less than or equal to the first preset threshold is found, then the light intensity amplitude among the fitting coefficients corresponding to the minimum sum of all squares is taken as the maximum light intensity amplitude.

5. The method for measuring the numerical aperture of an optical fiber according to claim 4, characterized in that, The fitting coefficients also include fitting coefficients a and b; the fitted light intensity at each sampling point is expressed as follows: In the formula, I'(x i , y j ) is the fitted light intensity, a and b are the fitted coefficients, x Imax , y Imax are the transverse and longitudinal coordinates corresponding to the maximum light intensity, and A is the light intensity amplitude.

6. The method of claim 5, wherein, The sum of the squares of the differences between the fitted light intensity and the measured light intensity at each sampling point is calculated using the following formula, which is specifically expressed as follows: In the formula, I(x) i y j ) is the measured light intensity, and Δ is the sum of the squares of the differences between the fitted light intensity and the measured light intensity at each sampling point.

7. The method of claim 6, wherein, The step of selecting sampling points that meet the conditions based on the maximum light intensity amplitude for circle fitting, and obtaining the circle radius with the best fitting effect as the optimal circle radius, includes: SS1: Set the initial value of the current circle radius and the initial coordinates of the fitted circle center, wherein the fitted circle center coordinates include horizontal coordinates and vertical coordinates; SS2: Identify all sampling points from which the measured light intensity and the maximum light intensity amplitude satisfy a preset ratio; SS3: Perform circle fitting calculations on each identified sampling point to obtain the radius of each fitted circle; SS4: Then calculate the sum of the squares of the differences between the squares of the radii of each fitted circle and the square of the current circle's radius; When the sum of the squares is greater than or equal to a second preset threshold, the first iteration is initiated, and the first iteration specifically includes: SSA1: Simultaneously decreases the horizontal coordinates of the center of the currently fitted circle, the vertical coordinates, and the radius of the currently fitted circle by a preset step size. Execute SS3-SS4, SSA2: Determine if the sum of squares obtained in the current iteration is greater than or equal to the second preset threshold. If so, return to step SSA1 until the first preset number of iterations is reached. Otherwise, terminate the first iteration and take the sum of the squares obtained in the current round as the current circle radius as the optimal circle radius; If, after the first iteration reaches the second preset iteration count, no sum of squares less than or equal to the second preset threshold is found, the current circle radius is reset to its initial value, the horizontal coordinate of the current fitted circle center is reset to its initial coordinate, and the vertical coordinate is reset to the initial coordinate of the fitted circle center. Then, the second iteration is initiated. The second iteration specifically includes: SSAA1: Simultaneously increase the horizontal and vertical coordinates of the center of the currently fitted circle and the radius of the current circle by a preset step size. Execute SS3-SS4, SSAA2: Determine if the sum of squares obtained in the current iteration is greater than or equal to the second preset threshold. If so, return to step SSAA1 until the second preset number of iterations is reached. Otherwise, terminate the second iteration and take the sum of the squares obtained in the current iteration as the current circle radius as the optimal circle radius; If, after the second iteration reaches the first preset iteration number, no sum of squares less than or equal to the second preset threshold is found, then the current circle radius corresponding to the minimum value among all sums of squares is taken as the optimal circle radius.

8. The method of claim 7, wherein, The radius of each fitted circle is obtained by performing circle fitting calculation on each of the identified sampling points using the following formula, which is specifically expressed as follows: (x s -c x ) 2 +(y t -c y ) 2 =r 2 , where x s , y t are the horizontal and vertical coordinates of each sample point, c x , c y are the horizontal and vertical coordinates of the center of the fitted circle, and r is the radius of the fitted circle.

9. The method of claim 8, wherein, The sum of the squares of the differences between the squares of the radii of each fitted circle and the square of the current circle radius is calculated using the following formula, which is specifically expressed as follows: In the formula, r′ is the current circle radius, and Γ is the sum of the squares of the differences between the squares of the fitted circle radii and the squares of the current circle radius.

10. The method for measuring the numerical aperture of an optical fiber according to claim 8, characterized in that, The numerical aperture is calculated based on the optimal circle radius using the following formula, which is specifically expressed as follows: In the formula, NA is the numerical aperture.

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