JPS6034088B2 - Optical fiber padded - Google Patents

Description

Detailed Description of the Invention '1} Technical Field of the Invention The present invention relates to an optical attenuator used as an optical component in an optical fiber communication system, and particularly used in high-speed (large-capacity) analog communication or high-speed digital communication. It concerns fiber optic pads (fixed attenuators).

[2] Background of the technology Optical attenuators reduce the power level of light propagating in optical fibers, and are used to set levels in various measurements and to adjust levels to ensure the normal performance of optical components. It is absolutely necessary for such purposes.

There are fixed types and variable types of optical attenuators, and fixed types are often used in optical fiber communication systems. This type of optical attenuator has features such as less reflected light from the optical attenuator, good processing of unnecessary light, and especially in the case of a fixed attenuator, accurate attenuation amount. Furthermore, it is desirable to have a simple structure, a 4-inch shape, and a lightweight one.

When the reflected light from the optical attenuator returns to the optical path, the circuit components on the light emitting side are affected in various ways by this reflected light. In order to reduce this reflected light as much as possible, attempts have been made to use an attenuation film or to arrange the attenuation film at an angle with respect to the optical axis, but a fully satisfactory optical attenuator has not been proposed. Prior Art and Problems FIGS. 1 to 3 are schematic diagrams for explaining conventional fixed optical attenuators, respectively.

The conventional optical attenuator shown in Fig. 1 uses an optical fiber (
(single mode) 10 and a receiving side optical fiber (single mode) 11 are arranged so that their respective optical axes 10c and 11c are aligned in a straight line, and each end face 1
0d and 11d are arranged apart from each other at a distance of D.

The end faces 10d and 11d are formed on planes perpendicular to the optical walls 10c and 11c, respectively. In addition, the code 10a, Ila
indicates the cores of the optical fibers 10 and 11, and 10
b and 11b each indicate a cladding. In this optical attenuator, the amount of attenuation increases or decreases in proportion to the increase or decrease in the separation distance D. Therefore, there is an advantage that in order to obtain a desired amount of attenuation, it is sufficient to set a separation distance ○ corresponding to the amount of attenuation. Therefore, this optical attenuator has no problem with optical coupling, but since the end surfaces 10d and 11d are flat, a large amount of the emitted light 12 is reflected on the end surface 11d, and the core 10a is the light path on the light emitting side.
The disadvantage is that it returns to To prevent this reflection,
Even if the end face 11d is coated with a transmission film, there is a problem that the ratio of return power to input power is on the order of 2B to 3B. The conventional optical attenuator shown in FIG.
Single-mode optical fibers 20 and 21 are connected to their respective optical axes 20c and 21c as in the conventional example shown in FIG.
are arranged so that they are aligned in a straight line.

The respective end surfaces 20d and 21d are optical axes 20c,
An attenuating film 21e (for example, an ND filter) is vapor-deposited on the light-receiving side end face 21d. Note that 20a and 21a represent the core, and 200 and 21b represent the cladding, respectively. In the case of this optical attenuator, a light absorber (not shown) having a slightly larger refractive index than those of the claddings 20b and 21b is wound around the outer periphery of the claddings 20b and 21b. 21
The unnecessary light incident on b is well processed (absorbed). However, in the case of this optical attenuator, it is very difficult to process the inclined surfaces of the end surfaces 20d and 21d, and in practice, the inclinations of the oblique end surfaces 20d and 21d are different from the optical axes 20c and 21d, respectively.
1c, the amount of reflected light from the attenuating film 21e returns to the light-emitting side optical path (core 20a), and in order to reduce this return reflected light, the inclination angle (light In order to increase the angle formed with the vertical plane of the shaft, it becomes more difficult to process the inclined surface and the end surfaces 20d and 21d
It is difficult to arrange the alignment with the damping film 2, and the amount of attenuation is
Since it depends on 0e, the amount of attenuation varies due to processing errors, and there is a problem that the degree of attenuation is good. The conventional optical attenuator shown in FIG.
1d are formed in a plane perpendicular to the respective optical axes 30c and 31c, and are closely coupled to each other, and the optical fibers 30 and 31 are arranged with the cores 30a and 31a offset from each other. ing.

This optical attenuator is constructed only for cores 30a and 31a, and is set within a range where end faces 30d and 31d of cores 30a and 31a are at least bonded (overlapping) to each other. Therefore, the amount of attenuation is between the end faces 30d and 3
It is determined by the size of the connecting portion with 1d, that is, the diagonally shaded portion 32 shown in the opening of FIG. In this case, it is formed based on a known calculation formula showing the relationship between the amount of axial misalignment S (see figure A) between the cores 30a and 31a and the coupling loss (considered to be substantially the same as the amount of attenuation). It is. Since the light that enters the optical fiber 31 is a parallel ray, it is assumed that the light that enters the cladding 31b is completely absorbed by the cladding 31b, and the behavior of the light that enters the cladding 31b is not considered. It has not been. However, subsequent research has shown that in reality, even though the light that has entered the cladding 31b is parallel, the light rays cross the ship's side to some extent as it travels. It was found that the light was incident on the cladding 31a on the light receiving side. In the case of optical fibers, the refractive index of the cladding portion is smaller than the refractive index of the core portion, so this can be easily explained theoretically. Therefore, since this conventional optical attenuator is formed only for the cores 30a and 31a as described above, there is a problem in that a large amount of attenuation cannot be obtained. As explained above, conventional optical attenuators do not sufficiently consider reflected light and scattered light, and are used in optical systems that handle coherent light, such as measurement systems, communication systems, medical optical video discs, etc. This includes the problem that it is unsuitable for use.

{41 Purpose of the Invention In view of the above-mentioned conventional problems, the present invention focuses on the behavioral phenomenon of light incident on the cladding, and provides a simple structure with very little reflected light, a wide range of attenuation, and a method for setting the attenuation. is easy,
The object of the present invention is to provide an optical fiber pad that can also be used in an optical fiber system using coherent light.

■Structure of the invention In order to achieve this object, according to the present invention, optical fibers each having an end face formed on a plane perpendicular to the optical axis are arranged in a state in which the end faces are in close contact with each other. At the same time, the optical fibers are arranged in an off-axis state such that at least a part of the core end face of the other optical fiber is coupled to the clad end face of one of the optical fibers, and the light receiving side of the optical fiber is An optical fiber pad is provided in which the length of the fiber is set to a predetermined length corresponding to a desired amount of optical attenuation.

[61 Embodiments of the Invention Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIGS. 4 to 8 are diagrams for explaining embodiments of the present invention.

FIG. 4 is a longitudinal cross-sectional view of a first embodiment of the optical fiber pad according to the present invention, FIG. 5 is a cross-sectional view taken along the line F--F in FIG. 4, and FIG. Pad axis deviation amount S (
A graph showing the relationship between 〃m) and attenuation L (dB) based on measured data, FIG. 7 is a diagram showing the second embodiment, and FIG. 8 is a modification of the optical fiber pad shown in FIG. 4. FIG. As shown in FIG. 4, the optical fiber pad of the present invention includes a single-mode optical fiber 40 on the light-emitting side, a single-mode optical fiber 41 on the light-receiving side having a predetermined length 1, and an axis misalignment S of these two optical fibers. Adapter 42 that holds and fixes the respective end surfaces 40d and 41d in close contact with each other.
It consists of

In addition, the code|symbol 43 shows a light receiving element (for example, APD). One end face 40d of the optical fiber 40 on the light emitting side is the optical axis 4.
It is formed in a plane perpendicular to 0c and is inserted into the adapter 42 with iron fitting, and the other end surface 40e side protrudes from the adapter 42. This protruding end surface 40e is coupled to an optical fiber from a light emitting element (for example, a semiconductor laser LD) by an optical connector or the like (not shown). The optical fiber 41 on the light receiving side has a predetermined length 1, and similarly to the optical fiber 40, one end surface 41d is formed in a plane perpendicular to the optical axis 41c, and the end surface 41d is the end surface of the optical fiber 40. 40d is inserted into the adapter 42 with iron fittings. The other end surface 41e has a light receiving element (for example, an AP
D) is coupled to 43. The adapter 42 connects the optical fiber 4
0 and 41 are formed so that the respective optical wires 40c and 41c can be fitted together with an axis offset of S, and the end surface 40
The optical fibers 40 and 41 are held and fixed in a state where d and 41d are in close contact with each other. As shown in FIG. 6, the length 1 of the optical fiber 41 is one element related to the amount of attenuation L (dB), and the value of L changes in proportion to a change in this length 1. Therefore, the optical fiber pad's vertical axis deviation amount S(r
m) and the length 1 of the optical fiber 41, a desired attenuation amount L (clothing) can be easily obtained. Now, as shown in FIG. 4, the end faces of the cores 40a, 41a of the optical fibers 40, 41 are completely separated from each other, and there is no joint between them, and the end faces of the claddings 41b, 40b facing each other are connected to each other. They are placed closely together. Therefore, the light emitted from the core 40a (coherent light)
is almost entirely incident on the cladding 41b, and the cladding 4
There is almost no reflected light at the end face of portion 1b. As explained in FIG. 3 above, even though this incident light is a coherent light, it expands to some extent while propagating inside the cladding 41b, and some of it, for example, the light ray indicated by arrow G, enters the core 41b. incident. Other light rays that do not enter the core 41a, that is, unnecessary light, are efficiently absorbed by an absorber (not shown) that is wrapped around the outer periphery of the cladding 41b and has a refractive index slightly larger than that of the cladding 41b. Further, as described above, since there is almost no reflected light from the end face of the cladding 41b, the optical components on the light emitting side are not adversely affected by the reflected light. In this way, cores 40a and 41
Even when the core 40a is not directly coupled to the core 40a, the light emitted from the core 40a enters the core 41a and is transmitted while being greatly attenuated by the cladding 41b. The present invention was devised taking into consideration the phenomenon that the light beam incident on the cladding 41b enters the cladding 41a, and is essentially different from conventional optical attenuators. . Therefore, the amount of axial deviation S between the cores 40a and 41a is, for example, the fifth
Like the core 41a shown by the dashed line in the figure, from S to S.
It can be set in a wide range up to 2. This means that the optical fiber pad according to the present invention allows the attenuation amount to be set over a wide range from small to large, which was not possible with the prior art. In FIG. 6, the broken line 45 is the curve when the length 1 of the receiving side optical fiber/<41 is set to 2, and the dashed line 47 is the curve when 1 is set to 0.5 degrees.

In this figure, the same parts as in FIG. 4 are given the same reference numerals. Therefore, 40 and 41 are single mode optical fibers, 40a and 41a are cores, 40b and 41b are claddings, 43 is a light receiving element (avalanche photodiode, APD), and 44 is a light emitting element (semiconductor laser, LD
) are shown respectively. In addition, each symbol is set as follows. P,: Optical power output from the optical fiber 40 on the light emitting side, P2: Optical power input to the optical fiber 41 on the light receiving side, S: Optical power 4 of each of the cores 40a and 41a
Axis deviation amount between 0c and 41c (French m), L: Attenuation amount (ship)
, L=101og(P,/P2), a: core 40a, 4
Radius of 1a (French m), 2A: Outer diameter of cladding 40b, 41b (French m), n.: Refractive index of core 40a, 41a, Mountain: Refractive index of cladding 40b, 41b, △: Core 40a, 41a and Relative refractive index difference (%) between claddings 40b and 41b, Δ=(n, one peak)/nl, input: wavelength of light (〃m).

As is clear from FIG. 6, the optical fiber pad of the present invention has a large attenuation amount L (for example, 30 ~4M old) can be easily obtained with good accuracy.

Further, as shown in FIG. 7 (second embodiment), the optical fiber pad of the present invention or the optical fiber 41 on the light receiving side is formed into a ring-shaped loop 48 in the middle, and from this loop 48 part. It is also possible to reduce the size by utilizing the radiation loss of light, or to further increase the attenuation amount for the straight optical fiber 41 of the same length.

Furthermore, a photodetector, etc. (
It is also possible to arrange a sensor (not shown) and use it for monitoring the amount of attenuation. Further, an optical fiber 40 shown in FIG.
The connection state between and 41 can be formed in various modifications. For example, as shown in FIG.
It is also possible to removably form an optical fiber 41' having a desired length by cutting the intermediate part of the iron-filled part of the optical fiber 41 that is inserted into the optical fiber 2, and other modifications are possible. It is also possible. '71 Effects of the Invention As explained in detail above, the optical fiber pad of the present invention has a simple structure, almost no reflected light, and good processing of unnecessary light. It has great effects, such as good accuracy, wide attenuation setting range, and easy setting of particularly large attenuation amounts.

[Brief explanation of the drawing]

1 to 3 are diagrams for explaining respective conventional optical attenuators, FIG. 4 is a vertical cross-sectional view of a first embodiment of an optical fiber pad according to the present invention, and FIG. 6 is a graph showing an example of the relationship between the amount of axis deviation S (mountain m) and the amount of attenuation L (dB) of the optical fiber pad of the present invention, and FIG. The figure shows the second embodiment, and FIG. 8 shows a modification of the first embodiment shown in FIG. 4. 40...Single mode optical fiber on the light emitting side,
41...Single mode optical fiber on the light receiving side,
40a, 41a... Core, 40b, 41b... Clad, 40c, 41c... Optical axis, 40d, 40e, 41d,
41e... End face, 42... Adapter, 43...
...Photodetector (APD), 44...Light emitting device (
LD), 48...Loop, S...Amount of axis misalignment (rm) between optical fibers 40 and 41, L...Amount of attenuation (clothing), 1...
...The length (skin) of the optical fiber 41 on the light receiving side. Figure 1 Figure 2 Figure 3 Figure 5 Key Figure 6 Figure 7 Figure 8

Claims (1)

[Claims] 1 Optical fibers each having an end face formed on a plane perpendicular to the optical axis are arranged in such a manner that the end faces are in close contact with each other, and the optical fiber clad end face of one of the optical fibers is connected to the end face of the other optical fiber.・The optical fibers are arranged in an off-axis state so that at least a portion of the core end face portions are connected, and the length of the optical fiber on the light receiving side of the optical fiber is set to a predetermined length corresponding to the desired amount of optical attenuation. Optical fiber padded featuring: