JPH0766092B2 - Mode conversion code - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A code for converting a propagation mode from a single-mode fiber to a multimode in an optical communication system using an optical fiber as a transmission line. For the purpose of providing a conversion code, optical connectors are provided at both ends of the optical fiber, and the phenol of one optical connector is eccentric.

[Industrial field of application]

The present invention relates to a conversion adapter that converts a propagation mode in an optical communication system using an optical fiber as a transmission line.

Depending on the transmission capacity, there are single mode fibers (hereinafter referred to as SM fibers) and multimode fibers depending on the transmission capacity. These multimode fibers include step index fibers (hereinafter referred to as SI fibers) and graded index fibers (hereinafter referred to as SI fibers). Hereinafter referred to as GI fiber).

In the present specification, the GI fiber will be described as a representative of the multimode fiber.

Supports SM fiber and GI fiber, and SM
There are two types, one for light source and one for GI. Recently, there is an increasing tendency to unify the light source for SM as a light source for SM and apply the light source for SM to existing GI fiber.

However, when light is incident on the GI fiber from the SM light source, the transmission characteristics [baseband characteristics expressed in the 6 dB band] originally possessed by the GI fiber may be apparently deteriorated. It is requested to improve.

Here, the 6 dB band will be described with reference to FIGS. 8 and 9.

FIG. 8 is a diagram showing the attenuation of the amplitude of the optical signal by the optical fiber transmission line, and the incident light having the amplitude A ₀ incident on the optical fiber 10 is attenuated to A at the output end of the optical fiber and output.

In optical fiber transmission, due to three causes of mode dispersion, material dispersion, and waveguide dispersion (structural dispersion), as shown in FIG. 9, the higher the modulation frequency, the smaller the amplitude A of the modulation waveform that appears on the output side. Become.

This change in the input / output signal amplitude ratio (frequency characteristic) with respect to the modulation frequency starting from direct current is called the baseband characteristic. Especially, in the optical fiber transmission band, the maximum output amplitude in the baseband characteristic is A ₀ to 6 dB. The modulation frequency (f _{1 in} FIG. 9) at the point where it becomes smaller is called the 6 dB band.

[Conventional technology]

The group delay time (mode dispersion) between the modes propagating in the fiber mainly controls the baseband characteristics of the GI fiber. Conventionally, when the SM light source enters the GI fiber,
Since the light in the GI fiber is in the state of low-order mode excitation,
It was thought that the baseband characteristics would seem better than the original 6 dB band of GI fiber due to the smaller number of modes.

When the refractive index distribution (hereinafter referred to as profile) of the fiber is actually ideal as shown in FIG. 10 (A), 6d
The B band apparently improves. But the profile is number 10
In the case of a fiber that is not ideal as shown in FIG. 6B, the 6 dB band apparently deteriorates as described above.

The difference in the refractive index distribution between FIGS. 10 (A) and 10 (B) is mainly due to the manufacturing method.

When manufacturing an optical fiber, a preform (base material) is first prepared, and then the preform is heated, melted and softened, and drawn to form a two-step process.

As a typical method of making this preform, internal CV
D method (MCVD method), external CVD method, VAD method (vapor phase axis method)
And so on.

According to the VAD method, a fiber having a profile as shown in FIG. 10 (A) can be produced, but according to the internal CVD method and the external CVD method, GeO ₂ molecules can be produced in the final “collapse” process unless special care is taken. Doping materials such as the above will escape, and a dip region 11 having a low refractive index will be generated in the central portion of the core.

Thus, in the case of a fiber whose profile is not ideal as shown in Fig. 10 (B), SI fiber having a large core diameter is placed several meters after the SM light source for deterioration of 6 dB band. From a few tens of meters to expand the emission pattern of the SM light source, or in Japanese Patent Laid-Open Nos. 57-158604 and 62-78.
As described in No. 506, a method has been proposed in which a lens system is inserted after the SM light source to expand the output beam size.

[Problems to be Solved by the Invention]

An evaluation of the 6 dB band improvement effect by the conventional example is shown in FIGS. 11 and 12.

FIG. 11 shows a measuring method, in which the electric signal from the oscillator 21 is first E / O converted by the SM light source 22, and then incident on the lens system or SI fiber 24 according to the conventional example, and then on to the GI fiber 20.

Next, the light emitted from the GI fiber 20 is O / E converted, and this signal is measured by the spectrum analyzer 27.

On the other hand, the standard measurement in the 6 dB band with the GI light source is the oscillator 21 above.
E / O conversion of the output of the GI light source 23, further incident on the steady mode exciter 25 known by the name of "SGS exciter",
This output is incident on the GI fiber.

After the output of the GI fiber, it similarly enters the E / O converter 26, and the signal is measured by the spectrum analyzer 27.

Fig. 12 shows "SM light source + conventional example" measured in Fig. 11.
The 6 dB band according to the standard measurement method and the 6 dB band according to the standard measurement method including the "GI light source + steady mode exciter" are shown for the case where the length of the GI fiber (measurement object) is changed.

In the graph of Fig. 12, (a) shows the original 6 dB of GI fiber by the standard measurement method of the combination of GI light source and steady mode exciter.
It is the result of measuring the band. (A) and (c) are SI for SM light source
The characteristics are shown in which the fiber is connected and the output is incident on the GI fiber. (D) shows the characteristics when the light from the SM light source is incident on the GI fiber through the lens system, the numerical aperture or spot size of the incident light is arbitrarily selected, and the fiber length is used as a parameter.

(E) shows the characteristics when the SM light source is excited by direct coupling to the GI fiber.

The characteristics shown in (a) and (c) of FIG. 12 have a wider 6 dB band than the characteristics shown in (e), and have a certain 6 dB band improvement effect, but the characteristics originally expected from (a). It can be seen that the 6 dB band is not so much improved when compared with.

Also, as shown in the characteristic (d), it was found that there is almost no improvement effect when the lens system is inserted instead of the SI fiber.

Further, as described in JP-A-57-158604 and JP-A-62-78506, there is a prior art that attempts to improve the 6 dB band by enlarging the spot size of the incident light on the GI fiber. .

FIG. 13 shows an apparatus that has been subjected to a characteristic test of a known example for enlarging the spot size, and the same parts as those in FIG. 11 are designated by the same numbers.

In Fig. 13, the GI measured using a device that separates the connector 28 and changes the spot size incident on the GI fiber.
The relationship between the spot size of the incident light on the fiber 20 and the 6 dB band is shown in the graph of FIG.

The 6 dB band due to the SM light source is shown in the characteristic (a) of the graph of FIG. 14, and the 6 dB bandwidth is about 200 MHz, which is narrower than the characteristic of the GI fiber's original 6 dB bandwidth (b).
It wasn't effective.

The present invention has been made in view of such a point, and an object thereof is to reduce the loss of the 6 dB band in the GI fiber by performing mode conversion of the light emitted from the SM fiber with a simple configuration. It is to provide a mode conversion code to improve in.

[Means for solving problems]

The present invention, when the light from the SM light source is incident on the GI fiber,
Due to the fact that low-order mode excitation is maintained even after propagating in the GI fiber for more than 10 km, the cause of the 6 dB band deterioration is
It was considered that this is because the group delay time difference between the low-order modes (between two or a very small number of modes) becomes large due to the disorder of the profile, and the band is limited.

That is, it has a profile as shown in FIG.
In the case of GI fiber, since the dip region 11 having a low refractive index exists in the center of the core in the optical fiber manufacturing process,
It is considered that the group delay time difference between the low-order modes becomes large.

Therefore, in order to improve the 6 dB band, when the light enters the SM fiber to the GI fiber, it is concentrated in the core center of the GI fiber so that the light from the SM fiber does not enter, and the higher-order mode component is included. It is necessary to perform mode conversion into steady mode excitation.

Therefore, according to the present invention, as shown in FIGS. 1 (a) and 1 (b), the SM fiber or the GI fiber used as the mode conversion code with the optical connectors 6 and 7 attached to both ends is used as a light source side SM fiber and a light receiving side. It is inserted between GI fibers, and the optical axis center of the core of one optical connector 6 on the side connecting different types of fibers is displaced from the center of the outer periphery of the ferrule 61 by a predetermined amount. Mode conversion can be realized by causing an axis shift between the fiber and the GI fiber.

Further, as shown in FIG. 2, as the mode conversion code, the one in which the optical connectors 6 and 7 are attached to both ends of the steady mode exciter 8 formed by alternately splicing the SM fiber and the GI fiber is used. With the configuration, the characteristics can be improved as compared with the conversion code using only the eccentricity of the optical connectors 6 and 7.

Also, as shown in Fig. 3, SM with optical connector 7 attached to one end
By providing a fiber and a GI fiber to which an optical connector 7'is attached at the other end, the SM fiber and the end surface of the GI fiber on which the optical connector is not attached are connected by axially offsetting each other. , SM fiber and GI
Mode conversion between fibers can be realized.

[Action]

As shown in FIG. 1 (b), by shifting the axis of the ferrule of the optical connector by a predetermined amount d, the light from the SM fiber 2
Since the light is incident at a position off the core center of the GI fiber 3, the light of low-order mode excitation passing near the center of the core in the GI fiber 3 is reduced, and the light of higher-order mode component is generated, resulting in axis misalignment. The GI fiber 3
Will be mode-converted to the steady mode excitation of.

Further, instead of the optical fiber inserted between the optical connectors as shown in FIG. 2, it becomes more effective to use a steady mode exciter composed of the optical fiber.

As shown in Fig. 3, use an ordinary optical connector as the optical connector, and use SM fiber and GI fiber as the fiber between the optical connectors, and join the SM fiber and GI fiber by offsetting the joint surface by d. Then, since the light from the SM fiber 2 is incident on the position deviated from the core center of the GI fiber 4,
In the GI fiber 3, light of low-order mode excitation passing near the center of the core is reduced and light of higher-order mode component is generated.

FIG. 4 (a) shows the 6 dB band when the axial deviation amount d is changed by the mode conversion code according to the present invention, and the value of the 6 dB band increases as the axial deviation amount increases, and d = 15 μm.
At about m, the GI fiber's original 6 dB bandwidth value (275 MHz) is reached.

When d is set to 15 μm or more in FIG. 4 (a), the 6 dB band value is similarly improved, but since the connection loss in the conversion code is increased accordingly, it is not a good idea to perform misalignment.

From a design point of view, the axial deviation between d = about 10 μm and about 20 μm is the most effective range of use.

FIG. 4 (b) shows a far-field image of the outgoing light at the end of the GI fiber, the solid line in (a) shows the outgoing pattern when there is no axis deviation, and the solid line in (a) shows the axial deviation of the present invention. The emission pattern when set to a predetermined value is shown, and the broken line in (c) shows the steady mode pattern of the GI fiber (GI light source + incidence by the steady mode exciter).

In FIG. 4 (b), it can be seen that the characteristic (a) of the present invention is substantially the same as the steady mode pattern (c) of the GI fiber, and steady mode excitation can be performed due to the axis deviation.

That is, by performing axis misalignment between the SM fiber and the GI fiber as in the present invention, the output light from the SM fiber is mode-converted into the steady mode excitation of the GI fiber, which results in 6 dB.
Bandwidth degradation can be improved.

Further, since the light propagating in the GI fiber is in the steady mode excitation state, it is possible to perform low-loss optical transmission with little loss fluctuation even when the fiber is bent.

〔Example〕

Referring to FIG. 1 for explaining the first embodiment of the present application with reference to FIG. 1, 2 is an SM fiber, 3 is a GI fiber, 4, 6 and 7 are optical connectors, 61 is an eccentric side connector ferrule, 71 Is a non-eccentric side connector ferrule, 5 is a mode conversion code, and 9 is a connector connection adapter.

The mode conversion cord 5 connects the optical connectors 6 and 7 to both ends of the SM fiber or the GI fiber.

At this time, the optical fiber insertion hole of the ferrule 61 of one optical connector 6 is configured such that the optical connector of the optical connector is displaced from the center of the optical axis of the core by a predetermined amount d with respect to the center of the outer periphery of the ferrule. When the optical connector is connected to the optical connector, the core positions of the optical connectors are eccentric to each other, resulting in axial misalignment.

This is shown in FIG. 1 (b).

The connection loss can be reduced by using the GI fiber 3 for the mode conversion code, as compared with the case of using the SM fiber 2.

The specific structure of the ferrule 61 is shown in FIGS.

FIG. 5 shows the structure of the first ferrule.

Insert the optical fiber insertion hole of the ferrule 61 into the optical fiber 2 or 3
Is larger than the outer diameter of the ferrule 61, the optical fiber 2 or 3 is fixed to the inner wall of the hole of the ferrule 61 by contacting a part thereof, and the gap 57 is filled with the adhesive 57 so that the ferrule 61 has an axial deviation d. The following is a configuration that can be applied.

FIG. 6 shows the structure of the second ferrule.

Insert the optical fiber insertion hole of the ferrule 61 into the optical fiber 2 or 3
The diameter is substantially the same as that of the ferrule 61, and the ferrule 61 is opened at a position displaced by d from the central axis.

FIG. 2 shows a second embodiment of the present invention.

Similar to the first embodiment, the ferrule 61 of the optical connector 6 is configured such that the center of the optical axis of the core is offset from the center of the outer periphery of the ferrule by a predetermined amount d, and between the optical connector 6 and the optical connector 7. For example, core diameter and crat diameter are 50μm and 12 respectively.
5μm SI fiber 8a, GI fiber 8b, SI fiber 8c 3
A steady mode exciter 8 in which different types of fibers are connected in order is provided to make mode conversion more effective.

In addition, 81 is a fusion splicing part.

A third embodiment will be described with reference to FIG. In FIG. 3, ordinary optical connectors 7 and 7'are used as optical connectors, and SM fibers and GI fibers are connected to the optical connectors 7 and 7 ', respectively.
Connect the M fiber and GI fiber, which are not connected to the optical connector, with the end faces of the optical fibers offset.

An enlarged view of this connecting portion is shown in FIG.

Reference numeral 51 denotes a portion that is fusion-spliced by discharge heating or the like in a state where the end-treated optical fiber is off-axis, and the outer circumference thereof is surrounded by a reinforcing steel wire 55 and a Holmet adhesive 59, and is covered with a heat treatment tube 55. The optical fiber connection portion is reinforced.

Further, the outer side of the cord is covered with a box-shaped or tubular member 58 made of metal, plastic, or the like, and at the end of the member 58, the cord jacket 54 and the tensile strength fiber 53 are fixed and fixed.

Although not shown in the figure, a cover is provided on the member 58, and the connecting portion 50 is fixed by a mechanical fastening means such as a screw or an adhesive.

Therefore, due to this axis shift, the light emitted from the SM fiber 2 is not incident on the center of the core of the GI fiber 3 in a concentrated manner, so that it is converted into steady mode excitation including higher order mode components.

The SM fiber core 2A clad is 2B, and the GI fiber core 3A is 3B.

〔The invention's effect〕

The present invention relates to a cord for connecting an SM fiber and a GI fiber, in which the optical connector at one end is axially displaced, or the connecting portion between the single mode fiber and the multi-mode fiber connected between the optical connectors of the mode conversion cord is axially displaced. By doing so, it is possible to convert the light in the low-order mode excitation state from the SM fiber into the steady-mode excitation.

[Brief description of drawings]

FIG. 1 is the first principle diagram and embodiment, FIG. 2 is the second principle diagram and embodiment, FIG. 3 is the third principle diagram and embodiment, and FIG. 4 is the output at the GI fiber end. A graph showing the 6 dB band of the emitted light and its far-field pattern, FIG. 5 is a diagram showing the configuration of the first eccentric ferrule, FIG. 6 is a diagram showing the configuration of the second eccentric ferrule, and FIG. 7 is an optical fiber connection. Fig. 8 is an enlarged view of the part, Fig. 8 is a diagram showing the attenuation of the optical signal amplitude in the optical fiber transmission line, Fig. 9 is a graph showing the transmission band of the optical fiber, and Fig. 10 is an example of the refractive index distribution of the GI fiber. FIG. 11 is a diagram showing a 6 dB band measuring device, FIG. 12 is a graph showing characteristics of a conventional 6 dB band improvement example, and FIG. 13 is a graph for measuring the relationship between the spot size and the 6 dB band. FIG. FIG. 14 is a graph showing the relationship between spot size and 6 dB band. In the figure, 2 is an SM fiber, 3 is a GI fiber, 6 and 7 are optical connectors, 61 is an eccentric side connector ferrule, 71 is a non-eccentric side connector, and 5 is a mode conversion code.

Claims (3)

[Claims] 1. A mode conversion code which is inserted between a light source side single mode fiber and a light receiving side multimode fiber and which performs mode conversion of light emitted from the light source side single mode fiber. Optical connectors (6, 7) are attached to both ends of the optical fiber used for the cord, and the optical axis center of the core of one optical connector on the side connecting different types of fibers is located with respect to the center of the outer periphery of the ferrule (61). A mode conversion code characterized by a certain amount of axis misalignment. 2. A mode conversion code which is inserted between a light source side single mode fiber and a light receiving side multimode fiber, and which performs mode conversion of light emitted from the light source side single mode fiber. Optical connectors (6, 7) are attached to both ends of a steady mode exciter (8) in which fibers are alternately fused and connected,
A mode conversion code, characterized in that the center of the optical axis of the core of the ferrule (61) of one of the optical connectors is offset from the outer circumference of the ferrule by a predetermined amount. 3. A mode conversion code, which is inserted between a light source side single mode fiber and a light receiving side multimode fiber, and which performs mode conversion of light emitted from the light source side single mode fiber, and which has an optical signal at one end. A single mode fiber with a connector (7) attached and a multimode fiber with an optical connector (7 ') attached at one end are provided, and the single mode fiber and the optical connector (7,7') of the multimode fiber are attached. A mode conversion code, characterized in that the end faces of the fibers which have not been formed are connected with their axes displaced by a predetermined amount.