JPS60244833A - Measuring method of structure of single-mode optical fiber - Google Patents

Abstract

PURPOSE:To take an accurate measurement by using an image pickup tube for near-infrared light with short wavelength or visible light by detecting the center position of a ground mode by using light with wavelength shorter than the in-use wavelength of the single-mode optical fiber. CONSTITUTION:The single-mode optical fiber which has 1.3mum in-use wavelength and is about 2m long is used as the optical fiber 1 to be measured and excited by a semiconductor laser 8 with 0.85mum oscillation wavelength. The projection end surface of the optical fiber 1 is observed through a TV camera 10 which uses the image pickup tube for near-infrared light with short wavelength and a video processor 5 calculates its light intensity distribution to detect the center position of the ground mode. Then, an LED11 which emits light in the nearly the same 0.85mum wavelength band with incident light is used as a light source for lighting the end surface, and an end surface image of the optical fiber 1 is observed through the TV camera 10 and video processor 5 to detect the external diameter of the objective optical fiber and its center position.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the structure of a single mode optical fiber with high precision.

Generally, the quantities measured as structural parameters of a single mode optical fiber are the outer diameter, core diameter, core non-circularity, and eccentricity between the outer diameter and the core. Among these, particularly important quantities that govern the splice loss of a single mode optical fiber are the outer diameter and the amount of eccentricity between the outer diameter and the core. However, since a single mode optical fiber is in a state where only the fundamental mode 8 propagates at the wavelength used, what is truly important is not the eccentricity of the outer diameter and the core, but the eccentricity of the outer diameter and the fundamental mode. (difference between the center of the outer diameter and the center of the fundamental mode).

When the core is deformed from a perfect circle, the center of the core and the center of the fundamental mode generally do not coincide. Therefore, it is necessary to measure the outer diameter and the amount of eccentricity of the fundamental mode, but it has been extremely difficult to perform this measurement using conventional techniques.

In other words, the conventionally widely used method for measuring the structure of optical fibers involves injecting white light into a short optical fiber sample, about a few tens of inches long, and observing the output end face with a microscope. In this case, what is observed on the output end face is the shape of the core, and it is impossible to detect the center of the fundamental mode. On the other hand, as a method for measuring the dimensions of the fundamental mode, as shown in FIG. T via system 6
A method of measuring using a V-right camera and an image processing device 5 is known. Therefore, as shown in Fig. 2, an optical system for illuminating the output end face consisting of an end face illumination light source 6 and a nof mirror 7 is added, and the outer diameter and the center of the outer diameter of the optical fiber to be measured are determined. If it can be measured, it is thought that the amount of eccentricity of the fundamental mode can be measured. However, the wavelength used for single mode optical fiber is usually 1.6
It is near-infrared light with a large wavelength such as μm or 1.55 μm, and for this reason, the TV right camera has no choice but to use an image pickup tube for near-infrared light with a large wavelength. Infrared light imaging tubes have disadvantages in that they have significantly lower sensitivity and resolution than visible light imaging tubes or near-infrared light imaging tubes with small wavelengths. Further, the light source 6 for illuminating the output end face must be a near-infrared light having a wavelength exactly the same as the wavelength used, considering the chromatic aberration of the lens system. For this reason, it is conceivable to use a laser beam such as a semiconductor laser to obtain a sufficient amount of light to match the image pickup tube, but since the laser light source has good coherence, there is a problem in that it generates speckle noise. As described above, it is extremely difficult to measure the outer diameter and the eccentricity of the fundamental mode using the conventional techniques.

The present invention aims to eliminate the above-mentioned conventional drawbacks and provide a method for easily and accurately measuring the outer diameter and the eccentricity of the fundamental mode of a single-mode optical fiber, Therefore, in the method for measuring the structure of a single mode optical fiber according to the present invention, light having a wavelength shorter than the working wavelength of the single mode optical fiber is incident from one end of the single mode optical fiber to be measured. The fiber is bent so that only the fundamental mode propagates, and the center position of the fundamental mode is detected on the output end face of the single mode optical fiber, and the output end face is set to y, which is different from the incident light.
The method is characterized in that the outer diameter and outer diameter of the single moat 8-optical fin and the eccentricity of the fundamental mode are measured by illuminating with a light source of the same wavelength and detecting the outer diameter and center of the output end. shall be. In the present invention, since the center position of the fundamental mode can be detected using light with a wavelength shorter than the wavelength used by the single moat 8-optical ino 2, the incident light with a small wavelength can be used as the incident light for detecting the center position of the fundamental mode. Near-infrared light or visible light can be used, and therefore accurate measurements can be performed using a near-infrared light or visible light imaging tube with a small wavelength and good sensitivity and resolution. Furthermore, since a light source other than a laser beam such as an LED having an emission wavelength that is the same as that of the incident light can be used as a light source for illuminating the output end face, the problem of speckle noise does not occur.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 6 shows an example of an apparatus configuration for carrying out the method of the present invention. In this figure, the same reference numerals as in FIG. 1.2 indicate the same constituent means. In this example, a single mode optical fiber with a wavelength of 1.6 μm and a length of approximately 277 L is used as the optical fiber 1 to be measured.
It is excited by a μm semiconductor laser 8. Optical fiber to be measured 1
Although it does not become a single mode optical fiber for light with a wavelength of 0.85 μm when stretched straight, it is possible to bend the optical fiber 1 appropriately by wrapping the middle part of the optical fiber 1 around a round bar 9 as shown in the figure. By adding this, it is possible to realize a state in which only the fundamental mode can propagate. Therefore, the output end face of the optical fiber 1 to be measured is observed with the TV right camera 0 using an image pickup tube for near-infrared light with a small wavelength, and the light intensity distribution is determined by the image processing device 5, thereby determining the base mode. The center position can be detected. Next, the above incident light is y
Using an LEDil that emits light in the 0.85 μm band of the same wavelength as a light source for end face illumination, the outer diameter of the optical fiber to be measured is determined by observing the end face image of the optical fiber to be measured 1 using the TV camera 10 and the image processing device 5. and detect its center position. From the above measurements, the outer diameter of the optical fiber to be measured and the eccentricity of the fundamental mode can be determined.

In the above measurements, near-infrared light with a small wavelength is used as the incident light for detecting the fundamental mode center position, and an imaging tube for near-infrared light with a small wavelength with good sensitivity and resolution is used as the observation means. Therefore, highly accurate measurements can be made. Further, since an LED light source having the same wavelength as the incident light can be used as a light source for illuminating the output end face, the problem of scratch noise does not occur. Furthermore, the measuring device is simple in construction because it is obtained by adding an optical system for end face illumination and a fiber bending means to the fundamental mode dimension measuring system.

In the above embodiment, near-infrared light with a small wavelength is used as the incident light, and an imaging tube for near-infrared light with a small wavelength is used as the observation means, but in the present invention, visible light and near-infrared light are used as the incident light. A visible light imaging tube can also be used as the observation means.

As described above, the present invention provides a method for easily and accurately measuring the outer diameter of a single mode optical fiber and the eccentricity of the outer diameter and the fundamental mode.

[Brief explanation of the drawing]

Figure 1 shows a conventional measurement system for measuring the dimensions of the fundamental mode of a single-mode optical fiber, and Figure 2 shows the outer diameter of a single-mode optical fiber and the eccentricity of the outer diameter and fundamental mode. FIG. 6 is a diagram showing an example of the measurement system used in the method of the present invention. 1... Optical fiber to be measured, 5... Image processing device,
7... Half mirror 18... Semiconductor laser, 9...
・Round bar, 10...TV camera for near-infrared light, 11...
LED patent applicant Sumitomo Electric Industries, Ltd. (4 others)

Claims (4)

[Claims] (1) Light with a wavelength shorter than the working wavelength of the single-mode optical fiber is introduced from one end of the single-mode optical fiber to be measured, and the single-mode optical fiber is bent so that it is substantially in the fundamental mode. When the central position of the fundamental mode is detected on the end face at the output of the single-mode optical fiber, the output end face is illuminated with light having the same wavelength as the incident light. , a structure of a single-mode optical fiber characterized in that the outer diameter and outer diameter of the single-mode optical fiber and the eccentricity of the fundamental mode are measured by polarizing the outer diameter of the output end and the center. Measurement method. (2) A method according to claim 1, in which a semiconductor laser is used as a light source for the incident light. (3) The method according to claim 1 or 2, in which an LED is used as the light source for illuminating the output end. (4) Claim 1 or 2 uses a TV camera for visible light or near-infrared light with a small wavelength as means for detecting the central position of the base mode, the outer diameter of the output end, and the central position. Section method.