JPS61104238A - Method and apparatus for measuring mode field diameter - Google Patents

Abstract

PURPOSE:To measure a mode field diameter accurately and always uniformly, by performing the correction of an integration range, a quantization error and other errors on the basis of sensitivity distribution preliminarily measured and stored. CONSTITUTION:The sensitivity distribution of each picture element of a vidicon camera 6 is preliminarily measured and the measured result is stored in the memory of a calculator 9. When the terminal image of an optical fiber 4 to be measured was picked up by this apparatus, the scanning data at the light receiving surface of said optical fiber is inputted to the calculator 9 and corrected according to the value preliminarily stored in the memory to operate a mode field diameter.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to the measurement of mode field diameter, which is one of the basic parameters of single mode optical fibers for communications.

[Conventional technology]

A method is known in which measurement light is incident on one end of an optical fiber to be measured, a pattern appearing on the other end is captured by a vidicon camera, and the pattern is observed.

This method is an extremely simple method for observing the mode field diameter, but since the characteristics of the light-receiving surface of the vidicon camera are generally not quantitatively clarified, this method allows accurate measurement of the mode field diameter. I can't.

In other words, by exciting an appropriate optical power from one end of the optical fiber to be measured, observing the pattern that appears on the other end, and measuring the diameter of the high-brightness part of the optical fiber as the mode field, it is possible to The diameter of the high-brightness portion varies depending on the intensity of the optical power. When we experimented with this, we found that when the optical power was relatively changed by 1:20, the measurement result of the moat field diameter of one optical fiber under test was equal to that of one vidicon.
.

Even when a camera was used, it was found that the thickness changed from 9.3 to 1O12 μm.

Furthermore, there is no guarantee that the characteristics of each pixel on the light-receiving surface of a vidicon camera are uniform, and the characteristics may change over time or due to temperature.

Therefore, simply measuring the mode field diameter using a vidicon camera does not always result in uniform measurements.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method and apparatus for accurately and always uniformly measuring the mode field diameter by taking advantage of the simplicity of this method.

[Means for solving problems]

In the method of the present invention, the sensitivity distribution at each position on the light-receiving surface of the vidicon camera is measured in advance and stored in the memory of a computer, and the measured values by the vidicon camera are read by scanning and given to the computer, and then the sensitivity distribution is stored in the memory of the computer. The method is characterized in that the mode field diameter is calculated by correcting the integration range, quantization error, and other factors using the sensitivity distribution stored in the sensitivity distribution.

A second invention of the present invention is an invention of a measuring device, which includes a light source, means for connecting one end of the optical fiber to be measured 11 to the light source, a vidicon camera, and an optical fiber that appears on the other end surface of the optical fiber to be measured. A mode field diameter measuring device comprising an optical means for coupling an image to the light receiving surface of the vidicon camera, and a camera control device for scanning the light receiving surface of the vidicon camera and reading out electrical signals at each position on the light receiving surface. is equipped with a computer for receiving and recording the electric signal read out by the camera control device, and the computer includes means for recording a sensitivity distribution corresponding to each position on the light receiving surface of the vidicon camera, and a It is characterized by comprising means for correcting the electric signal according to the distribution, means for calculating the mode field diameter according to the corrected value, and means for displaying or recording the calculation result of this means.

[Effect]

General (There are approximately 103XIO3 pixels (addresses) on the light-receiving surface of a bidicon camera.The sensitivity characteristics of each pixel are measured in advance and stored in the computer's memory.The mode field of the bidicon camera is When measuring the diameter, the measured value is taken into this computer in accordance with scanning, and the measured value is corrected according to the sensitivity characteristics of each address stored in advance to calculate an accurate value.

Furthermore, when calculating the moat field diameter, uniform measurement can always be performed by optimally setting the integral range according to the light beam height.

[Embodiment] FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention. Light #l is a laser diode device, and its output light is passed through an optical attenuator 2 to V? A% is input from the connector 3 to one end of the optical fiber 4 to be measured. This optical fiber to be measured 4
A lens 5 is disposed on the other end face, and is adjusted so that an image of this end face is formed on the light receiving surface of the vidicon camera 6. The vidicon camera 6 is scan-controlled by a camera control device 8. A computer 9 and an image monitor 10 are connected to this camera control device 8 .

2. Here, the number of pixels (addresses) on the light receiving surface of the vidicon camera 6 is 1024 x 1024, and the sensitivity distribution of each of these pixels is measured in advance, and the measurement results are stored in the memory of the computer 9. When this device captures an image of the end face of the optical fiber 4 to be measured, the scanning data of the light receiving surface is input to the calculation a9 and corrected to the value stored in the memory in advance, and the mode field diameter is calculated. be done.

The results are printed, displayed on a CRT screen of a computer, or recorded in memory.

An example of a method for measuring the sensitivity characteristics of a vidicon camera and a method for processing the data will be described.

Here, the mode field diameter W0 is defined as -.m=.

(1) Determination error of power distribution
'(2) (Error in determining the integral range of Equation 11) Among these, problems in determining the power distribution in (1) are that the light receiving sensitivity characteristics of the vidicon camera have nonlinear characteristics and that the light receiving surface There is a difference in sensitivity depending on the location.This characteristic must be taken into consideration when determining the integration range in (2).Next, the characteristics of the vidicon camera will be described.

Figure 2 shows a measurement system for the sensitivity characteristics of a vidicon camera. In this measurement system, an optical power meter 13 measures the amount of incident light while switching an optical switch 12. By changing the amount of incident light,
The photoelectric conversion coefficient (T coefficient) of the vidicon camera was measured.

The measurement results are shown in Figure 3.

Coordinates (X) on the light receiving surface of the camera when there is light input.

Let V (
From this figure, the relationship between the input light amount P (X, Y) and the electrical output V (X, Y) can be expressed by the following equation.

P(X,Y)=K(X,Y)V(X,Y)”r-・-・
----(21) Here, K (X, Y) is a surface sensitivity correction coefficient, and T is a photoelectric conversion coefficient (T coefficient). From this figure, the coefficient T
is found to be 0.7.

Next, Table 1 shows the measurement results of the γ value depending on the location of the light receiving surface of the vidicon camera. The light receiving element of the vidicon camera is lO
The size is μm×10 μm. Here, the position of the light receiving surface is indicated by the intersection of the X address (0 to 1023) and the Y address (θ to 1023) as shown in FIG.

Table 1 (γ value at each point) From this table, it can be seen that the γ value differs depending on the position. The variation in γ value depending on the position of several vidicon cameras was about 0.03. In this way, the T coefficient of the vidicon camera is not l, but varies depending on its position. Therefore, in order to accurately determine the light intensity distribution, it is necessary to measure the γ coefficient of the vidicon camera.

Using the measurement system shown in FIG. 2, the green sensitivity distribution in a certain direction was measured by keeping the light intensity at the center of the spot constant and moving the position of the spot incident on the light receiving part of the vidicon camera. An example of measurement at this time is shown in FIG. The white circles are the measured values of the green sensitivity distribution, and the black circles are the green sensitivity correction values. Furthermore, the broken line represents the green sensitivity correction value approximated by a quadratic function. This figure shows that this camera has low sensitivity in the center. Further, the green sensitivity distribution was measured by changing the address in the X direction using different cameras, and the green sensitivity correction distribution at that time is shown in FIG. From a to i in this figure, the X address is 472
This is the green sensitivity correction distribution when increasing by 10 addresses from 552 to 552. From this figure, it can be seen that even if the X address is slightly different, the green sensitivity distribution is different. Therefore, it is necessary to measure the green sensitivity distribution in one direction, which is necessary for spot size measurement.

Next, the integral range will be explained. From equation (1), it can be seen that the spot size changes depending on the integral range. FIG. 6 shows the relationship between the mode field diameter and the integral range of a fiber with a core diameter of 10 μm and a relative refractive index difference of 0.3%. Increasing the integration range increases the mode field diameter, and 3
It can be seen that the integral of equation (1) almost converges at a(3Wo). Next, FIG. 7 shows the measurement results of the mode field diameter when the integration range is changed. The effective cutoff wavelength and relative refractive index difference of fibers A and B used in the measurement were 1.2. czm, 0.31% and 1.17μ
m is 0.30%, and FIG. 8 shows its refractive index distribution. From FIG. 8, it can be seen that as the integration range is increased, the mode field diameter becomes larger and tends to converge. This tendency is consistent with the calculation results shown in FIG. The theoretical values of the mode field diameters of fibers A and B, calculated using the refractive index distribution in Figure 8, are t4.8, respectively.
0μI and 5.05μI. When the integral range is 3Wo, the actual measured value is 0.15 μm to 0 compared to the theoretical value from Figure 7.
．． It can be seen that it is 2 μm larger.

Next, camera sensitivity correction (γ correction) will be explained. FIG. 9 shows the relationship between the T coefficient and the measured value when the T coefficient is changed. Here, the core diameter is 10 μm, and the relative refractive index difference is 0.3%.
We considered the single mode fin of , and assumed a model that has the correct value when γ = 0.7.

From this figure, it can be seen that when the value of the coefficient T changes by ±0.1 μm, the measurement result changes by ±0.5 μm.

Therefore, in order to keep the measurement error within 0.1 μI, it is necessary to keep the measurement accuracy of the coefficient T within 0.05.

AD conversion of the received light intensity is performed using 8 bits. In other words, the measured NFP is quantized to 225 levels. Figure 10 shows the relationship between quantization level and mode field diameter. Here, the value of the quantization level represents the center value of S, 45, and y, and the center value is indicated by Pmax. The optical fiber meter used in the calculation has a core diameter of 10 μm and a relative refractive index difference of 0.3%. From this figure, it can be seen that as the integration range increases, the mode field diameter increases and eventually converges.

It can also be seen that if the quantization level is increased (the value of the converged mode field diameter becomes larger).

Further, the integral range at that time becomes larger as the quantization level increases. FIG. 11 shows the relationship between the convergence value of the spot size and the error due to quantization with respect to the quantization level. From this figure, it can be seen that even if the quantization level is set to 1000, the error is about 1%. Therefore, it can be seen that in order to keep the quantization error within 5% at present, it is necessary to set the quantization level P wax to 200 or more.

Table 2 shows an example in which the optimum measurement conditions were determined from the relationship between the measurement error and the mode field diameter discussed above. Regarding the determination of the baseline, if the integration range is set to 3Wa (3a) or higher, the integration will converge, so the baseline level should be set to 3W0 to 4W.
It is given as the average value of the level of o. In this case, it is believed that measurement errors can be kept to a minimum. The integration range was determined as follows. That is, since the measured data is 8 bits, a quantization error occurs. Therefore, in order to keep the quantization error within 5%, it is necessary to set the power level control to 200 or more as shown in FIG. 11. Furthermore, the integral range was determined to be 1.7 wo from FIG. 1O.

Table 2 Optimal measurement conditions On the other hand, it was found that linear sensitivity correction and T correction are necessary for the nonlinearity of the vidicon camera. Furthermore, in the measurement of the mode field diameter, it was found that it changes significantly with changes in the γ value. Therefore, from FIG. 9, in order to measure the spot size with an accuracy of within 0.1 μm, it is necessary to measure the value of γ with a measurement accuracy of within 0.05.

The mode field diameter of the optical loss optical fiber was measured using four types of vidicon cameras under the measurement conditions shown in Table 2.

Table 3 shows the results of measurements on nine sample fibers. The refractive index distribution of the fibers used at this time (sample symbols F to N) was measured, and the theoretical value of the mode field diameter was calculated using the refractive index distribution. The results are also shown in Table 3.

Table 3 Measurement Results The results show that the theoretical values and experimental values agree well within a range of about 0.1 μm. Therefore, the third
The measurement accuracy when measuring the mode field diameter using various vidicon cameras under the measurement conditions shown in the table is approximately 0.
．． This shows that the results are obtained with an accuracy of 1 μm.

Although the correction method and an example of the error have been described in detail above, the present invention is not limited to correction using this method, and can be implemented by performing optimal correction according to the required accuracy.

〔Effect of the invention〕

As explained above, according to the method and device of the present invention, the mode field diameter is measured using a vidicon camera, so the measuring device is simple, and it is possible to prevent variations in the characteristics of the vidicon camera or the light reception of the vidicon camera. This has the effect of being able to remove the influence of the characteristic distribution on the surface, and making it possible to always perform uniform measurements under constant conditions.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention. FIG. 2 is a diagram showing the measurement diameter of the light receiving sensitivity characteristics of the vidicon camera. FIG. 3 is a diagram showing an example of characteristics of a vidicon camera. FIG. 4 is a diagram showing an example of a line sensitivity distribution and a green sensitivity correction distribution. FIG. 5 is a diagram showing the green sensitivity correction distribution by different vidicon cameras. FIG. 6 is a diagram showing the relationship between the integral range and the mode field diameter measurement results.゛ FIG. 7 is a diagram showing the relationship between mode field diameter measurement results for different optical fibers to be measured. FIG. 8 is a diagram showing the refractive index distribution of the optical fiber under test. FIG. 9 is a diagram showing the relationship between the T coefficient and the mode field diameter measurement results. FIG. 10 is a diagram showing the relationship between mode field diameter measurement results depending on the quantization level. FIG. 11 is a diagram showing measurement errors due to quantization levels. 1... Light source, 2... Optical attenuator, 3... Connector, 4
... Optical fiber under test, 5... Lens, 6... Vidicon camera, 8... Camera control device, 9... Computer, 10... Monitor, 12... Optical switch, 13...
...Optical power meter.

Claims (5)

[Claims] (1) Measurement light is input from one end of the optical fiber to be measured, and the image appearing on the other end surface of the optical fiber to be measured is magnified by a lens and captured by a vidicon camera, thereby creating a mode field of the measurement light. In the method of measuring the diameter, the sensitivity distribution at each position on the light-receiving surface of the vidicon camera is measured in advance and stored in the memory of a computer, the measured values by the vidicon camera are read by scanning, and the values are fed to the computer, A method for measuring a mode field diameter, characterized in that the mode field diameter is calculated by correcting errors using the sensitivity distribution stored in the memory. (2) The mode field diameter measuring method according to claim (1), wherein the correction is correction of an integral range and a quantization error. (3) a light source, means for connecting one end of the optical fiber to be measured to the light source, a vidicon camera, and an optical means for coupling an image appearing on the other end surface of the optical fiber to be measured to the light receiving surface of the vidicon camera; , a camera control device that scans the light-receiving surface of the vidicon camera and reads out electrical signals at each position on the light-receiving surface; A computer for receiving and recording is provided, and the computer includes means for recording a sensitivity distribution corresponding to each position on the light receiving surface of the vidicon camera, means for correcting the electric signal according to this sensitivity distribution, and a means for correcting the electric signal according to this sensitivity distribution. 1. A mode field diameter measuring device comprising: means for calculating a mode field diameter according to the calculated value; and means for displaying or recording the calculation result of the means. (4) The mode field diameter measuring device according to claim (2), wherein the means for connecting one end of the optical fiber to be measured to the light source includes a variable attenuator for controlling the optical power passing therethrough. (5) The vidicon camera has a light-receiving surface made of a photoelectric conversion material mainly composed of PbS.
2) The mode field diameter measuring device described in item 2).