JPS63125906A - Inspecting method for optical fiber coupler - Google Patents

Abstract

PURPOSE:To measure the wavelength characteristics of the degree of coupling of an optical fiber coupler in a short time by using a heat ray generated by heating an incidence-side optical fiber as a light source, branching it at a welding and elongation part, and measuring the intensity of projection light appearing on a projection side and inspecting the coupling rate. CONSTITUTION:The optical fiber is heated from its side face and then the optical fiber reaches certain constant temperature and considered to be a kind of black body to become a light source which has a spectrum based on the Planck's rule of thermal radiation. When the temperature of the optical fiber is sufficiently high, it becomes a light source which generates light in a wavelength range (0.6-1.6mum) used for optical fiber transmission. This light is propagated in the core part of the optical fiber coupler, branched at the welding and elongation part, and projected from core parts 3 and 4 in the end surfaces of the fibers 1 and 2. This method allows the heating length of the optical fiber to be set optionally long and the energy generated by the heating is found by using the Planck's expression and Wien's displacement law and proportional to the value obtained by raising the heating temperature to the 5th power, so light with high energy is made incident on the optical fiber. Consequently, the light can easily be made incident on the optical fiber as compared with a conventional inspecting method, the measurement time can be shortened, and the measurement accuracy is excellent.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for inexpensively testing the wavelength dependence of the coupling ratio of an optical fiber coupler.

[Prior Art] Optical fiber couplers, which are constructed by fusing multiple optical fibers and then stretching the fused portion into a tapered shape, are used for optical distribution and optical coupling in transmission lines for optical communications, and will continue to be used in the future. It is thought that demand will continue to increase. Furthermore, for optical distribution and optical coupling for wavelength multiplexed transmission, an optical fiber coupler that matches the wavelength used over a wide wavelength range is required.

FIG. 4 shows an example of a conventional method for inspecting the coupling ratio of a 2×2 optical fiber coupler manufactured by fusing and drawing two identical optical fibers. In FIG. 4, 1. la, 2, 2a
are optical fibers, 3, 3a, 4.4a are core portions of each optical fiber, and 5.5a, 6, 6a are cladding portions, respectively. Optical fibers 1 and la and 2 and 2a are fused and
Before drawing, each optical fiber was one piece of optical fiber, and these are integrated at the fusion/drawing section 7. A lens 12 is attached to the core portion 3a at one end of this optical fiber coupler.
When light from the light source 11 is incident through the fiber 1.2, the incident light is branched at the fusion/stretching section 7, and the branched light is emitted from the core section 3.4 of the end face of the fiber 1.2. Now, assuming that the intensity of the light emitted from the core part 3 is Il and the intensity of the light emitted from the core part 4 is I2, the coupling ratio R when the light enters the fiber 1a of this optical fiber coupler and is coupled to the fiber 1
1 is determined by . Depending on the degree of stretching of the fusion/stretching section 7,
The coupling ratio R1 can be controlled within a range of 0 to 1.0.

Reference numerals 8 and 9 shown in FIG. 4 are light receiving elements that detect the intensity of light emitted from the core portion 3.4, and 10 is an arithmetic circuit that calculates the coupling ratio R1.

The conventional coupling ratio inspection system has a light source 1 as shown in FIG.
1 is made to enter the core portion 3a of the optical fiber la through the lens 12.

As the light source 11, a semiconductor laser or a light emitting diode is used, and the wavelength is 0, which is oscillated by these light sources.
, 85 μm band, 1.3 μm V, 1.55 μm
It can only be measured at specific wavelengths of H. On the other hand, with the development of wavelength division multiplexing transmission systems, optical fiber couplers that can be used in a wide range of wavelength bands have been developed (D, B, Mort
imore, Electronics Letters
, Vol, 21°No., 17. P, 742.1985)
. To measure the coupling ratio of such a wide wavelength band operating optical fiber coupler, use a white light source such as a halogen lamp and a spectrometer instead of the light source 11 to obtain light of an arbitrary wavelength, and then use the lens 12 to measure the coupling ratio. It is necessary to condense the light and input it into the core 3a of the optical fiber 1a. However, since the core diameter of the car-moat optical fiber is small and the numerical aperture is small,
It is difficult to increase the amount of incident light, and since low levels are detected using methods such as lock-in detection, there is a problem in that measurement takes a long time, 30 to 60 minutes.

[Problems to be Solved by the Invention] The present invention solves the above-mentioned conventional drawbacks, that is, the fact that it can only be tested at a specific wavelength or that it takes a long time to test a wide wavelength band, The purpose is to provide a method for measuring characteristics in a short time.

[Means for Solving the Problems] In order to achieve such an object, the optical fiber coupler inspection method of the present invention involves fusing and stretching a plurality of optical fibers each having a core portion and a cladding portion. In the inspection method for optical fiber couplers, the light source is a hot ray generated by heating the optical fiber on the input side, and the intensity of the emitted light that is branched at the fusion/stretching part and appears on the output side is measured to determine the coupling ratio. It is characterized by inspecting.

[Operation] When an optical fiber is heated from the side, the optical fiber reaches a certain temperature and can be regarded as a kind of black body, becoming a light source with a spectrum according to Blank's radiation law. This light source is
If the temperature of the optical fiber is high enough, it becomes a light source that generates light in the wavelength range (0.6 μm to 1.6 μm) used for optical fiber transmission. This light propagates through the core part of the optical fiber coupler, is split at the fusion/stretching part, and becomes the fiber 1.2.
The light is emitted from the core portion of the end face.

This method allows the heating length of the optical fiber to be set arbitrarily long,
In addition, the energy generated by heating can be calculated from Blank's equation and Wien's displacement side, and is proportional to the fifth power of the heating temperature. Light can be input into an optical fiber. Therefore, compared to the conventional method, the signal processing system can be simplified and the measurement time can be significantly shortened.

[Example] Hereinafter, the present invention will be explained in detail by referring to an example shown in the drawings.

Example 1 FIG. 1 is a diagram explaining the present invention in detail.

1, la; 2.2a are single mote optical fibers of the same size, e.g. 2 m long, outer diameter 125
μm.

The diameters of the cores 3, 3a, 4, and 4a are 8.0 μm, the refractive index difference is 0.3%, the cutoff wavelength is 1.1 μm, the cladding has a 5in2 composition, and the core has a 5i02-GeO□ composition.

At the center, two optical fibers are integrated by a fusion/drawing method, and the outer diameter of the fusion/drawing section 7 is about 30 μm. The coupling ratio of this optical fiber coupler is wavelength 1
．． The result of measurement using a conventional method using a 3 μm semiconductor laser was 50.2% in the case of la-+1.

In the case of la→2, it was 49.8%. The coating material on the portion 1a of this optical fiber coupler was removed over a length of 10 cm, and the optical fiber coupler was inserted into an electric furnace 21 containing a cylindrical carbon heating element 20 having an inner diameter of 1 cm and a length of 5c+n. The temperature of the heating element was 1000°C, and was controlled to have a temperature fluctuation range of ±0.5°C.

The light from the output end core portions 3 and 4 of the optical fiber coupler is received by light receiving elements 8 and 9 such as photoconductive elements, phototransistors, and photodiodes, and is then analyzed by an optical spectrum analyzer 22.
The spectra were each measured in a measurement wavelength range of 1.18 to 1.6 μm using the following, and the coupling ratio was calculated using equation (1). The total time required for the measurement was about 5 minutes. The measurement results are shown in Figure 2. As shown in FIG. 2, the coupling ratio at the output end core portion 3 changes from 74% to 0% in the region from near the cutoff wavelength of 1.1 μm to 1.6 μm, and
．． At 3 μm, it shows 50±0.1%. As shown in this embodiment, compared to conventional inspection methods, light can be easily introduced into an optical fiber, measurement time can be shortened, and measurement accuracy is also good.

In the above embodiments, the case where the cutoff wavelength is 1.1 .mu.m in a 0-mode fiber is shown, but the present invention can also be applied to an optical fiber coupler used at a shorter wavelength (for example, 0.6 .mu.m). Furthermore, in the case of a multimode optical fiber, it is easier to input light than in the case of a single mode optical fiber, and it goes without saying that more accurate measurement is possible. Furthermore, it is possible to measure not only 2×2 optical fiber couplers but also star couplers such as IXIo and IX100.

Example 2 As a second example, an example will be described in which the coupling ratio was measured during manufacturing of an optical fiber coupler by a fusion bonding/drawing method.

An oxyhydrogen flame 23 was used in place of the electric furnace 21 in the example shown in FIG. 1, and the heating temperature of the fiber was set to 1000°C as in the first example, and was controlled within the range of ±1°C.

Furthermore, as shown in FIG. 3, a bandpass type wavelength filter 24.25 is inserted between the output end core parts 3, 4 of the optical fiber coupler and the light receiving element 8.9, and the light source generated by the heating of the oxyhydrogen flame is removed. From the light inside, 0.6 μm, 0.
7 μm, 0.8 μm, 0.9 μm, 1
．． 0 μm, 1.1 μm, 1.2 μm, 1
．． 3 μm.

They were detected by the light receiving element 8.9 with center passing wavelengths of 1.4 μm, 1.5 μm, and 1.6 μm, respectively. At the time of these detections, as a matter of course, heating for stretching is stopped. Eleven band-pass diameter wavelength filters were placed on the circumference of the rotating disk at equal intervals from the center and rotated at a speed of 1 rpm by a rotary motor. Output signal of each light receiving element 1. ,I2
The voltage is then amplified via the current-voltage converter 26, and the arithmetic circuit 10 requires a coupling ratio time corresponding to each bandpass wavelength filter. For example, when the wavelength used by the optical fiber coupler is 1.3 μm, the center wavelength is 1.3 μm.
Using a 3 μm band-pass wavelength filter, the coupling ratio was measured during the non-heating period between one heating drawing step and the next heating drawing step in the optical fiber coupler manufacturing process.

An optical fiber coupler with a coupling ratio of 50±0.1% was realized with a 3 dB coupler.

[Effects of the Invention] As explained above, the present invention is capable of measuring the wavelength dependence of the degree of coupling of an optical fiber coupler in a short time and with high accuracy. It is also effective as an inspection method during the manufacturing process.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an embodiment of the present invention, Fig. 2 is a coupling ratio measurement obtained by the method of the present invention 1, la...single mode optical fiber, 2, 2a... single mode optical fiber,
3.3a, 4, 4a... Core part, 5.5a, 6, 6a... Clad part, 7... Fusion bonding.
Extension part, 8.9... Light receiving element, lO... Arithmetic circuit, 11... Light source, 12... Lens, 20... Carbon heating element, 21... Electric furnace, 22... Optical spectrum analyzer, 24.25...
- Filter, 26... Current-voltage converter. Figure 2

Claims (1)

[Claims] 1) In a method for inspecting an optical fiber coupler formed by fusing and drawing a plurality of optical fibers each having a core portion and a cladding portion, a heat ray generated by heating an input side optical fiber is used as a light source. A method for inspecting an optical fiber coupler, characterized in that the coupling ratio is inspected by measuring the intensity of the emitted light that is split at the fusion/stretching part and appears on the output side. 2) inserting a bandpass wavelength filter between the output side end face of the optical fiber and a light receiving element that receives the output light;
2. The method of inspecting an optical fiber coupler according to claim 1, wherein the coupling ratio is inspected by guiding only light of a desired wavelength from the light source to the light receiving element.