JP2000266956A - Optical fiber type wave-length filter element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify constitution, reduce a cost, and reduce an insertion loss of propagation light. SOLUTION: This filter element is an optical fiber type wave-length filter element wherein plural optical coupling parts 5 for generating optical coupling between plural cores (the first core 3, the second core 4) are formed intermittently (with intervals) in an optical fiber 1 having the plural cores(the first core 3, the second core 4) to allow single mode propagation for plural lights different in their wave-lengths, and a light propagation coefficient and/or a wave-guide length between the adjacent optical coupling parts 5, 5 is/are different in the plural cores (the first core 3, the second core 4).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber type wavelength filter element, which has a particularly simple structure, is low in cost, and has a small insertion loss of propagating light.

[0002]

2. Description of the Related Art In recent years, a wavelength division multiplex transmission system has been used in the field of optical communication. Accordingly, optical components for various uses have been developed. Wavelength filter elements, which are one of such optical components, are used for blocking light in unnecessary wavelength bands and for flattening the wavelength of amplification characteristics of optical fiber amplifiers, and their applications are expanding.

Conventionally, a Fabry-Perot interference filter using an etalon has been known as a wavelength filter element. In this method, a plurality of reflectors are opposed to each other to repeatedly reflect light to be split between the reflectors, thereby generating interference fringes. The characteristics as a filter can be obtained by adjusting the rate and the like.

[0004]

However, this Fabry-Perot interference filter requires a plurality of reflectors to be manufactured and the spacing and reflectance of these reflectors must be adjusted, so that the manufacturing process is complicated and the price is high. There was a problem. Further, when the propagation light is once taken out of the optical fiber, transmitted through a wavelength filter element such as a Fabry-Perot interference filter, and then incident on the optical fiber again, there is a problem that the insertion loss tends to increase. The present invention has been made in view of the above circumstances, and has as its object to provide a low-cost wavelength filter element having a simple configuration. Furthermore, another object is to provide a wavelength filter element that can reduce the insertion loss of propagation light.

[0005]

According to the present invention, there is provided an optical fiber having a plurality of cores capable of transmitting a plurality of lights having different wavelengths in a single mode.
An optical fiber type wavelength filter element in which a plurality of optical coupling portions for generating light coupling between the plurality of cores are intermittently formed, wherein a light propagation constant and / or an adjacent light coupling portion are provided between the plurality of cores. An optical fiber type wavelength filter element characterized by different waveguide lengths between optical coupling parts is proposed. Further, it is preferable that the optical coupling portion is formed by heating and stretching a part of the optical fiber in the length direction. Further, it is preferable to use an optical fiber in which one of the plurality of cores is located near the center thereof.

[0006]

FIG. 1 is a perspective view showing an example of a wavelength filter element according to the present invention. In FIG. 1, reference numeral 1 denotes a twin-core fiber (optical fiber). FIG. 2 is a sectional view of the twin-core fiber 1. This twin core fiber 1
A first core 3 and a second core 4 having a higher refractive index than the clad 2 in the clad 2 having a flat refractive index distribution.
Are provided. The first core 3 is located near the center of the clad 2 (twin-core fiber 1).

The number of these cores can be changed as appropriate depending on the number of wavelengths to be demultiplexed, and is not particularly limited as long as it is two or more. Here, an example in which two cores are provided for convenience will be described. These first core 3 and second core 3
The core 4 is capable of transmitting a plurality of lights having different wavelengths in a single mode. The first core 3 and the second core 4 have different propagation constants of the fundamental modes of the plurality of lights. Here, the plurality of lights having different wavelengths are lights to be propagated and demultiplexed by the optical fiber type wavelength filter element, and light of at least two or more wavelengths is set.

The difference in propagation constant between the first core 3 and the second core 4
And the relative refractive index difference between the core and the clad. In this example, the refractive index distribution shape has a flat step-type refractive index, but may have a complicated shape such as a core of a dispersion-shifted optical fiber.

For example, when the refractive index distribution shapes of the first core 3 and the second core 4 are made equal, the propagation constant difference can be provided by setting the relative refractive index difference to a different value. When the first core 3 and the second core 4 have different refractive index distribution shapes, the relative refractive index difference can be set to the same value or different values. The refractive index distribution shape, relative refractive index difference, and the like of the first core 3 and the clad 2 or the second core 4 and the clad 2 can be appropriately changed depending on required characteristics.

Although the distance between the first core 3 and the second core 4 is not particularly limited, one of the lengths (several meters to several tens of meters) used as an optical fiber component is one. Preferably, light propagating through the core is not affected by light propagating through the other core. If the distance between the cores is too short, light coupling between the cores occurs at portions other than the optical coupling portion 5 described below, which is inconvenient. However, when the difference between the propagation constants of the fundamental modes of the respective cores is large, the coupling hardly occurs, so that the distance between the cores can be relatively reduced.

Table 1 shows the characteristics of the twin-core fiber 1 of this example. The outer diameter D of the clad 2 is 125.
5 μm, the outer diameters and relative refractive index differences of the first core 3 and the second core 4 were 9.7 μm, 0.33%, and 9.6, respectively.
μm and 0.35%. The distance L1 from the center of the first core 3 to the outer edge of the clad 2 is 62.6 μm. The center distance L2 between the first core 3 and the second core 4 is 34.1 μm.

[0012]

[Table 1]

The cladding 2 is usually made of fluorine-added quartz glass or pure quartz glass. The first core 3 and the second core 4 are each made of germanium-added quartz glass, pure quartz glass, or the like.

In the twin-core fiber 1, a plurality of extending portions (optical coupling portions) 5 are formed intermittently at predetermined intervals in the length direction. The number of the extending portions (optical coupling portions) 5 is not particularly limited as long as it is plural. When three or more extending portions are provided, the intervals between the extending portions may be set to be equal depending on the desired filtering characteristics, or may be set to be different from each other. These extending portions (optical coupling portions) 5 heat a part of the length of the twin-core fiber 1 in the longitudinal direction to a temperature equal to or higher than the softening point of the material of the twin-core fiber 1 (about one hundred and several hundred degrees) and extend in the longitudinal direction. It is obtained by doing. When this heating stretching is performed, the diameter of the clad 2 is gradually reduced toward the center of the heating portion to be tapered, and the diameters of the first core 3 and the second core 4 are also reduced. Is reduced, and the effect of optical coupling in which light propagating in one core is coupled to the other core is obtained.

This optical fiber type wavelength filter element is usually used by being fixed to a reinforcing member made of a material having a linear expansion coefficient close to that of the material of the twin core fiber 1 such as quartz glass from the viewpoint of environmental temperature resistance. At this time, if the length of the optical fiber type wavelength filter element is long, two or more reinforcing devices are arranged and fixed at the respective positions of the plurality of optical coupling portions 5.

Here, a normal single mode optical fiber is connected to both ends of the twin core fiber 1, respectively.
It is assumed that light is incident from one single mode optical fiber. Since the core of the single-mode optical fiber is located near the center of the cladding, light propagating through this core enters the first core 3 of the twin-core fiber 1. Then, the first core 3 and the second core 4 pass through the first optical coupling section 5.
Distributed to the first core 3 and the second core 4
Then, the light is recombined at the adjacent optical coupling section 5 and an interference action occurs. That is, the same operation as that of a so-called Mach-Zehnder interference system is obtained.

FIG. 3 is an explanatory view showing an example of a Mach-Zehnder interference system. In this interference system, two parallel single-mode optical fibers (hereinafter abbreviated as optical fibers) 11 and 11 are 3 dB. By the couplers 12, 12,
It is configured to be connected at two locations with an interval. 3dB
The lengths of the two optical fibers 11, 11 between the couplers 12, 12 are different.

When light of two different wavelengths λ 1 and λ 2 is incident on one optical fiber 11, these lights are branched via the first 3 dB coupler 12 and split into two optical fibers 1.
Lights of both λ1 and λ2 travel through each of 1 and 11. Then, these lights are transmitted to the second 3 dB coupler 12.
, And as shown in FIG.
The light of λ1 is distributed to the other optical fiber 11, and the light of λ2 is distributed to the other optical fiber 11. At this time, the wavelength of the light distributed to one optical fiber 11 and the other optical fiber 11 differs depending on the phase difference of the light that interferes with the second 3 dB coupler 12.

FIG. 4 shows that the first 3 dB coupler 12 branches into the optical fibers 11 and 11 so that the electric field strengths of λ 1 and λ 2 are halved, and then recombines at the second coupling section 3 dB coupler 12. 7 is a graph showing the phase difference between two interference lights propagating through the two optical fibers 11 and the amount of light of the incident-side optical fiber 11 after recombination. The same result is obtained for both λ1 and λ2.

From this graph, when the phase difference is an even multiple of π, the light propagating through the two optical fibers 11, 11 is assumed to be 1 when the power of the light propagating through the optical fiber 11 on the incident side is set to 1. Is obtained. That is, the power is twice as high as that before the recombination. In the case of an odd number multiple, these lights cancel each other out and the power becomes zero. Therefore, if the phase difference is set so that the power is doubled for λ1, and the phase difference is set so that the power is zero for λ2, the light of λ1 is
1 and the light of λ2 can be distributed to the other optical fiber 11. As a result, characteristics as shown in FIG. 3 are obtained, and characteristics as a filter for selectively removing λ2 light from incident λ1 and λ2 light are obtained. The phase difference is
It is proportional to the difference in light propagation constant between the two optical fibers 11. Further, the phase difference can be adjusted by the difference in the waveguide length between the two optical fibers 11. Therefore, the period of the phase change can be adjusted by adjusting one or both of the propagation constant difference and the waveguide length difference between the optical fibers 11 between the 3 dB couplers 12.

That is, in the example of the optical fiber type wavelength filter element of the present invention shown in FIGS. 1 and 2, each of the first core 3 and the second
By adjusting one or both of the propagation constant difference between the first and second cores 4 and 5 and the waveguide length difference between the first core 3 and the second core 4 between the optical coupling units 5 and 5, Λ incident on the first core 3 from the single mode optical fiber connected to the
Of the light of 1 and λ2, the light of λ2 is finally coupled to the second core 4, and the light of λ1 can be selectively extracted from the single mode optical fiber connected to the other end.
The characteristics as an optical filter can be obtained. In the optical fiber type wavelength filter element of the present invention, since a plurality of cores are in the same clad, even if there is an external influence, the difference of the external influence between the cores is very small, and stable optical filter characteristics are obtained. can get.

In the optical fiber type wavelength filter element of this example, if the one having a large propagation constant difference between the first core 3 and the second core 4 is used, the length between the optical coupling parts 5 and 5 becomes longer. There is an advantage that can be shortened. For this reason, the size of the element is reduced, and the space efficiency can be improved.
Further, by fixing two or more optical coupling portions 5 to one reinforcing device, the number of reinforcing devices for fixing the optical fiber type wavelength filter element can be reduced. Since the influence of the expansion and contraction of the reinforcing device due to a change in the environmental temperature and the like becomes uniform for the plurality of optical coupling parts 5, the environmental temperature resistance can be further improved.

As described above, the optical fiber type wavelength filter element shown in FIG. 1 and FIG.
In addition, two or more optical coupling portions 5 are formed intermittently by heating and stretching in the longitudinal direction. Therefore, the number of parts is small, and it can be manufactured by a simple operation. In addition, since it is an optical fiber type element, it can be directly connected to an optical fiber for transmission,
The insertion loss of propagating light can be reduced.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. A twin-core fiber having the same configuration as that shown in FIGS. 1 and 2 was manufactured. A coating layer made of an acrylic resin for surface protection was provided on the outer periphery of the twin core fiber. The two cores of this twin-core fiber
As shown in Table 1, the difference between the outer diameter and the refractive index was relatively close. Next, the twin-core fiber was cut to a length of 2 m, and an acrylic coating resin near the center was cut to a length of about 2 m.
It was removed over 0 mm. Then, the center of the exposed clad was heated using a gas burner until it was melted, and the fiber was gently stretched in the longitudinal direction of the fiber.

At this time, a general single-mode optical fiber is placed on both ends of the twin-core fiber in advance for about 2 minutes.
Each m was fused and connected. Then, during this operation of heating and drawing, a superluminescent laser emitting at a wavelength of about 1.55 μm is connected from one end of one single-mode optical fiber, and the other single-mode optical fiber is connected to an end of the other single-mode optical fiber. An optical spectrum analyzer was connected, and at the time of the above-mentioned heating and stretching operation, a laser beam was incident on the twin-core fiber, and the spectrum of the amount of transmitted light was monitored by the optical spectrum analyzer. The stretching was terminated when the transmitted light amount decreased by 3 dB from the initial value in the wavelength band of 1.55 μm.

Next, the stretched portion was fixed to a general quartz glass reinforcing device with an adhesive. Then, the coating resin was similarly removed at a position 50 cm away from the stretched position to the emission side, and stretching was performed. Along with this stretching, the spectrum of the amount of transmitted light changed, and the stretching was stopped when the amount of transmitted light at a wavelength of 1.55 μm became about 4 dB as shown in FIG. Then, the stretched portion was similarly fixed to a quartz glass reinforcing device with an adhesive. It was confirmed that this optical fiber type wavelength filter element had characteristics as a wavelength filter.

[0027]

As described above, since the optical fiber type wavelength filter element of the present invention can be constituted by one member, the number of parts is small, and a part of the optical fiber in the longitudinal direction is heated. It can be manufactured by a simple operation of stretching. Therefore, the cost is lower than in the past. Further, since it is an optical fiber type element, it can be directly connected to a transmission optical fiber, and the insertion loss of propagating light can be reduced. Further, by using an optical fiber in which one of the plurality of cores is located near the center of the optical fiber, it can be connected to a normal single mode optical fiber with low loss.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an example of a wavelength filter element of the present invention.

FIG. 2 is a cross-sectional view of the twin-core fiber shown in FIG.

FIG. 3 is an explanatory diagram showing an example of a Mach-Zehnder interference system.

FIG. 4 is a graph showing the phase difference of the interference light and the amount of light coupled from the optical fiber on the incident side to the other optical fiber in the Mach-Zehnder interference system shown in FIG.

FIG. 5 is a graph showing a spectrum of a transmitted light amount obtained at the time of manufacturing the optical fiber type wavelength filter element of the example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Twin core fiber (optical fiber), 2 ... Cladding, 3 ... 1st core, 4 ... 2nd core, 5 ... Optical coupling part (extended part).

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Kenji Nishiide 1440, Mukurosaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant Co., Ltd. (72) Inventor Ryozo Yamauchi 1440, Musaki, Sakura City, Chiba Prefecture Fujikura Sakura Plant Co., Ltd. F term (reference) 2H038 AA22 AA34 BA25

Claims (3)

[Claims] 1. An optical fiber having a plurality of cores capable of propagating a plurality of lights having different wavelengths in a single mode, and a plurality of optical coupling portions for generating light coupling between the plurality of cores are formed intermittently. An optical fiber type wavelength filter element,
An optical fiber type wavelength filter element, wherein the plurality of cores have different light propagation constants and / or different waveguide lengths between adjacent optical coupling portions. 2. An optical fiber type wavelength filter element according to claim 1, wherein an optical coupling portion is formed by heating and stretching a part of the optical fiber in the longitudinal direction. . 3. The optical fiber type wavelength filter device according to claim 1, wherein one of the plurality of cores is located near a center of the optical fiber.