JPH03144337A - Measuring method for characteristic of optical fiber - Google Patents

Abstract

PURPOSE:To make measurement of a transmission loss accurately by measuring an incident light by putting a light source light in a stable mode distribution by winding an optical fiber in the shape of a character 8 and by putting it further in a steady mode distribution by making it pass through a mode filter, when characteristics are measured by comparing light outputs at an incidence end and an emission end of the optical fiber with each other. CONSTITUTION:An optical fiber measuring device is constructed of a light source 1, an optical fiber exciting unit composed of a mode scrambler 2 and a mode filter 3, and a power meter 5. As to an optical fiber constituting the exciting unit, an optical fiber similar to an optical fiber 4 of which a transmission loss and others are to be measured is used, and it is constituted of a multicomponent optical fiber of high numerical aperture and large aperture, for instance. The incidence end 4a of the optical fiber 4 is connected to the mode filter 3 of the exciting unit and the emission end 4b thereof is connected to the power meter 5, while LED and a spectroscope are used for the light source 1. In this constitution, the optical fiber 4 is wound in the shape of a character 8 in a number of times on the mode scrambler 2. As for the filter 3, a fiber obtained by stretching the optical fiber in the shape of a taper is employed therefor.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for measuring characteristics such as transmission loss measurement of an optical fiber, and particularly relates to a method for measuring characteristics of an optical fiber with a large refractive index such as a multi-component optical fiber. .

[Problems to be solved by conventional techniques and inventions] For example, No. 9
As shown in the figure, when transmitting light from a light source 10 through an optical fiber 40 and measuring transmission loss with a power meter 50 connected to the output end P, ! Cut P s and position P
Compare the output at , and the output at P. In this case, it is important to specify the mode of the incident light because the transmission loss of a multimode optical fiber generally depends greatly on the incident excitation conditions, and the input excitation conditions are standardized for silica-based multimode optical fibers.

However, in large-diameter, high-NA fibers such as step-index multicomponent optical fibers, the large difference in refractive index causes strong light confinement, producing various modes of light, making it difficult to create standard excitation conditions. Met. As a countermeasure for this, for example, 500 m to 1 k between the light source and the incident end.
There is a method of creating a steady mode using a long dummy fiber of g+.

When using steady mode light, the mode distributions at the input end and output end match, making accurate measurements possible, but in this case, the power of the light decreases while transmitting through the dummy fiber, and the The problem is that the measurable range is limited because the output is low.

[Purpose of the Invention] The present invention solves these conventional problems and can create incident light in a mode extremely close to the steady mode, thereby making it possible to create an incident light beam with a mode extremely close to the steady mode. The purpose is to provide a method that can accurately measure characteristics.

[Means for Solving the Problems] The method for measuring characteristics of an optical fiber according to the present invention achieves the above-mentioned object. 8 fibers
After the light source light is made into a stable mode distribution using a mode scrambler wound in the shape of an arrow, the light with the stable mode distribution is passed through a mode filter to enter the input end of the optical fiber as a substantially steady mode distribution. It was designed to do so.

[Example] FIG. 1 is a diagram showing an example of an optical fiber measuring device for applying the optical fiber characteristic measuring method of the present invention.
It consists of an optical fiber exciter consisting of a light source 1, a mode scrambler 2 and a mode filter 3, and a power meter 5. The optical fiber constituting the optical exciter is similar to the optical fiber to be measured 4 whose transmission loss etc. are to be measured, and has, for example, a high numerical aperture (for example, NA=0.5) and a large diameter (for example, a core diameter of 20 mm).
0μm.

It consists of a multi-component optical fiber with a cladding diameter of 250 μm.

The input end 4a of the optical fiber to be measured 4 is connected to the mode filter 3 of such an optical fiber exciter, and the output end 4b is connected to the power meter 5. Light source l is LE
Any light source can be used, such as a D-light source and a spectrometer.

As shown in Figure 2, the mode scrambler 2 consists of an optical fiber wound many times in a figure-eight pattern, and effectively scrambles the modes by causing mutual transmission of optical power between multiple modes within the multi-component optical fiber. and the numerical aperture (N
A) A stable mode distribution that does not depend on A) can be obtained.

FIG. 3 shows the results of measuring the far field pattern (F F P) at the output end when the NA of the incident light source is changed. As is clear from the figure, when the number of windings is increased, the 50% value of the FFP peak converges to a constant value regardless of the NA of the incident light source, and a stable mode distribution is obtained by the mode scrambler. I understand that.

The number of windings of mode scrambler 2 is NA=0.5,
In the case of a multi-component optical fiber with a diameter of 200/250, 15
0 tan or more is required. However, if the number of windings is too large, the transmission loss will increase, so about 200 turns is preferable. In addition, when the above optical fiber has a bending diameter of φ10 and a winding of 200 turns, the increase in transmission loss of the mode scrambler is 1.6 dB when the incident NA is 0.65.
0.25 is 0.8dB, long length (2000m)
This is much smaller than when using a dummy fiber.

Mode filter 3 removes a specific mode,
The light with a stable mode distribution obtained by the mode scrambler is converted into a steady mode. That is, when the light with a stable mode distribution obtained by the mode scrambler 2 is propagated through the optical fiber 4, the mode changes at the output end 4b, but by changing to a steady mode, the mode distribution at the input end 4a changes. The mode distribution at the output end 4b can be made substantially equal.

As such a filter 3, an optical fiber stretched into a tapered shape as shown in FIG. 4 is suitable. The diameter, length, etc. of the small diameter portion 3a of the mode filter 3 are designed according to the standard distribution of the steady mode.
In the case of 250 optical fiber, the diameter of the small diameter part is 160 to 17
The thickness should be approximately 0 μm. In the case of multi-component optical fibers, the mode distribution for this mode filter design cannot be determined by propagating a sufficient distance to obtain a steady mode distribution, so it is usually difficult to determine the mode distribution for the fiber length 1 or 2. It can be determined from the average FFP at the output end when propagating through the optical fiber.

The loss increase due to such a mode filter is 0.5d
Even when combined with the increase in loss of the mode scrambler 2, the increase in loss of the present optical fiber exciter can be suppressed to about 2 dB. In this way, the light from the light source 1 becomes a stable mode independent of the incident conditions by propagating through the figure-eight mode scrambler 2, and further becomes a steady mode distribution by the mode filter 3. Therefore, the moto distributions at the input end 4a and the output end 4b of the optical fiber 4 to be measured are substantially equal, so that characteristics such as transmission loss can be accurately measured.

Furthermore, since the incident excitation light has a steady mode distribution, the loss addition law holds true for transmission loss, and transmission loss can be measured with good reproducibility.

Example 1 In the measuring device shown in Fig. 1, the optical fiber 4 was 2P diameter 2
A multicomponent type S1 optical fiber with a diameter of 00 μm, a cladding diameter of 250 μm, a numerical aperture of 0650, and a length of 2000 m is used.The mode scrambler 2 has a bending diameter of φ10 and the number of windings is 200 turns.The mode filter 3 has a diameter of the small diameter part. 165
A micrometer was used. Such mode scrambler 2
The FFP of each of the input end 4a and output end 4b of the optical fiber 4 was measured when the optical fiber 4 was excited in combination with the mode filter 3 and the mode filter 3. Note that as the light source 1, an 850 nm LED was used.

In addition, the mode filter in this embodiment has a reference NA of FF.
The 15% value of the P peak is 0642, and the 50% value is 0.32.

The results are shown in Figure 5.

As a comparative example, FIG. 8 shows each FFP at the input end and the output end when only the mode scrambler 2 similar to Example 1 was used without using the mode filter 3. As is clear from FIGS. 5 and 8, according to the measurement method of the present invention, the patterns at the incident end and the exit end almost match, and a nearly steady mode distribution can be obtained.

In this example, five types of optical fibers 4 with different transmission losses were used.
When the loss value was measured, it almost matched the value measured using a 2000 m long dummy fiber, and there was no difference in the excitation effect between the two.

Example 2 Using the same light source 1, mode scrambler 2, and mode filter 3 as in Example 1, five types of optical fibers 41 to 45 with different transmission losses and lengths of 200 m were fused and spliced to calculate the loss during splicing. was measured (Figure 6).

FIG. 7 shows the relationship between the sum of losses and the loss upon connection when each optical fiber is measured separately without being connected. As is clear from the figure, it was confirmed that according to the measurement method of the present invention, the law of addition holds true when measuring transmission loss.

Example 3 Using the same mode scrambler and mode filter as in Example 1, the transmission loss of an optical fiber having a length L = 1000 m was measured 10 times.

As is clear from the results shown in the table, the reproducibility of the measurements was very good.

[Effects of the Invention] As is clear from the above embodiments, according to the optical fiber characteristic measuring method of the present invention, the conventional excitation conditions can be created by using a specially shaped mode scrambler and mode filter. It is possible to establish standard excitation conditions for large-diameter, high-NA fibers, which have been difficult to achieve, and it is possible to excite the optical fiber in a substantially steady mode, making it possible to accurately measure transmission loss. Furthermore, since it is possible to excite with a small increase in loss without using a long dummy fiber, it is possible to measure a long optical fiber.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an optical fiber measuring device to which the optical fiber characteristic measuring method of the present invention is applied; FIG. 2 is a diagram showing an embodiment of a mode scrambler according to the present invention; Figure 3 is a graph showing the relationship between mode stability and the number of windings by the mode scrambler, Figure 4 is a diagram showing an embodiment of the mode filter according to the present invention, and Figure 5 is a graph showing the relationship between the stability of the mode by the mode scrambler and the number of wraps. A diagram showing the far-field pattern at the input end and output end in the characteristic measurement method, Figure 6 is a diagram showing the configuration of loss measurement in Example 2, and Figure 7 is the sum of each optical fiber loss and connection loss in the same example. FIG. 8 is a graph showing the far-field pattern at the input end and output end when only a mode scrambler is used, and FIG. 9 is a diagram showing a conventional optical fiber loss measurement method. 1... Light source 2... Mode scrambler 3... Mode filter 4... Optical fiber to be measured 5... Power meter

Claims (1)

[Claims] When comparing the optical output at the input end and the output end of an optical fiber to measure its characteristics, the light source light is made into a stable mode distribution using a mode scrambler made by winding the optical fiber in a figure 8 pattern, and then a mode filter is applied. A method for measuring characteristics of an optical fiber, characterized in that the light having the stable mode distribution is made to pass through and enter the input end of the optical fiber as a substantially steady mode distribution.