JPH0616125B2 - Mode conversion adapter - Google Patents

Description

The present invention relates to an adapter for converting a propagation mode from a single mode to a multimode in an optical communication system using an optical fiber as a transmission line, and a mode conversion having a 6 dB band improvement effect with a simple configuration. For the purpose of providing an adapter, it is divided into a single mode fiber side flange and a multimode fiber side flange, and by shifting the flanges and joining them, an axis misalignment occurs when the connector is inserted. The configuration is a mode conversion adapter that obtains the excitation mode.

[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 multi-mode fibers as types of optical fibers, and these multi-mode fibers are step index fibers (hereinafter referred to as SI fibers).
And graded index fiber (hereinafter referred to as GI fiber).

In the present specification, the GI fiber will be representatively described as the multimode fiber.

There are two types of light source devices, one for SM and one for GI, corresponding to SM fiber and GI fiber. Recently, there is a tendency to unify the light source for SM and to apply the SM light source to the existing GI fiber. Is increasing.

However, when the light is incident on the GI fiber from the SM light source, the transmission characteristics [6d originally possessed by the GI fiber
The base band characteristic expressed in the B band] may be apparently deteriorated, and it is desired to improve this by a simple method.

Here, the 6 dB band will be described with reference to FIGS. 5 and 6.

FIG. 5 is a diagram showing the attenuation of the amplitude of the optical signal by the optical fiber transmission line, and the incident light with 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. 6, the higher the modulation frequency, the smaller the amplitude A of the modulation waveform that appears on the output side. Become.

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

[Conventional technology]

The base band characteristic of the GI fiber is mainly controlled by the group delay time (mode dispersion) between the modes propagating in the fiber. Conventionally, when the SM light source enters the GI fiber, the light in the GI fiber becomes low. Since the next mode is excited, it has been considered that the baseband characteristics are apparently better than the original 6 dB band of the GI fiber due to the smaller number of modes. In fact, when the refractive index distribution (hereinafter referred to as profile) of the fiber is ideal as shown in FIG. 7 (A), the 6 dB band is apparently improved. However, in the case of a fiber whose profile is not ideal as shown in FIG. 7 (B), the 6 dB band apparently deteriorates as described above.

The difference in the refractive index distribution between FIGS. 7A and 7B is mainly due to the manufacturing method.

When manufacturing an optical fiber, a preform (base material) is first produced, 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 C
There are a VD method (MCVD method), an external CVD method, a VAD method (vapor phase axis attaching method) and the like.

According to the VAD method, a fiber having a profile as shown in FIG. 7 (A) can be manufactured. However, 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.

In this way, in contrast to the degradation of the 6 dB band in the case of a fiber having a profile that is not ideal as shown in FIG. 7 (B), conventionally, an SI fiber having a large core diameter is placed several meters after the SM light source. From a few tens of meters to enlarge the emission pattern of the SM light source, or in Japanese Patent Laid-Open No. 57-15860.
As described in No. 4 and Japanese Patent Laid-Open No. 62-78506, a method has been proposed in which a lens system is inserted in the subsequent stage of the SM light source to expand the size of the emitted beam.

[Problems to be Solved by the Invention]

The evaluation of the 6 dB band improvement effect according to the conventional example is the eighth.
Shown in Figures and 9.

FIG. 8 shows a measuring method. First, the electric signal from the oscillator 21 is E / O converted by the SM light source 22 and is incident on the lens system according to the conventional example or the SI fiber 24, and then GI is further converted.
It is incident on the 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, in the standard measurement of the 6 dB band by the GI light source, the output of the oscillator 21 is E / O converted by the GI light source 23, and further enters the steady mode exciter 25 known by the name such as "SGS exciter". The output is incident on the GI fiber described above.

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

FIG. 9 shows the 6 dB band measured by the “SM light source + conventional example” and the 6 dB band measured by the standard measurement method consisting of the “GI light source + steady mode exciter” measured in FIG. It shows the case where the length of is changed.

In the graph of FIG. 9, (a) shows the original 6 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 dB band.

(A) and (c) show characteristics in which the SI fiber is connected to the SM light source 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 directly coupled to the GI fiber and excited.

The characteristics shown in (a) and (c) of FIG. 9 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 improved so much when compared with.

Further, 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, JP-A-57-158604 and JP-A-62-8
There is prior art that seeks to improve the 6 dB band by enlarging the spot size of the light incident on the GI fiber, as described in No. 506.

FIG. 10 shows an apparatus which has been subjected to a characteristic test of a known example for enlarging the spot size, and the same parts as those in FIG. 8 are designated by the same reference numerals.

The relationship between the spot size of the incident light on the GI fiber 20 and the 6 dB band measured by using the device for separating the connector 28 in FIG. 10 and changing the spot size incident on the GI fiber is shown in the graph of FIG. Show.

The characteristic (a) of the graph of FIG. 11 shows the 6 dB band by the SM light source, and the 6 dB bandwidth is about 200 MHz, which is narrow and effective compared with the characteristic (b) of the original 6 dB band of the GI fiber. Was not.

The present invention has been made in view of the above points, and an object thereof is to achieve 6 dB in a GI fiber by mode-converting the light emitted from the SM fiber with a simple structure.
It is an object of the present invention to provide a mode conversion adapter that improves band deterioration with low loss.

[Means for solving problems]

According to the present invention, when light from the SM light source is incident on the GI fiber, low-order mode excitation is maintained even after propagating in the GI fiber for 10 km or more. It is 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 and the band is limited.

That is, in the case of a GI fiber having a profile as shown in FIG. 7 (B), since the dip region 11 having a low refractive index exists in the center of the core in the optical fiber manufacturing process, the group delay between low order modes is large. It is considered that the time difference becomes large.

Therefore, in order to improve the 6 dB band, when the light enters from the SM fiber to the GI fiber, the light from the SM fiber is prevented from entering by concentrating on the central part of the core of the GI fiber, 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 FIG. 1 (a), in a mode conversion adapter for changing light of low-order mode excitation to steady mode excitation, a single-mode side flange 51 to be coupled with a single-mode side connector 6 is provided. , Multimode side connector 7
Provided with a multi-mode side flange 52 to be coupled with, the single-mode side flange 51 and the multi-mode side flange 52,
By arranging the optical fibers so that the optical axis centers of the cores of the optical fibers are displaced from each other by a predetermined amount d, an axial displacement is caused between the SM fiber and the GI fiber, and mode conversion is realized.

Further, as shown in FIG. 1 (b), a single mode side flange 51 for connecting the single mode side connector 6 and a multimode side flange 52 for connecting the multimode side connector 7 are provided, and the single mode side connector 6 is provided. Flange to join
51 or a flange that connects to the connector on the multimode side
The connector insertion hole of 52 is configured to be larger than the diameter of the ferrule of the connector, and the screw hole 55 for fixing the ferrule of the connector in the flange is provided, and the optical axis centers of the cores of the optical fibers are displaced from each other by a predetermined amount. By the screw 56,
The connector ferrule is fixed, an axis shift is caused between the SM fiber and the GI fiber, and mode conversion is realized.

[Action]

As shown in FIG. 1 (a), the optical connector flange is divided into two, and the axes of the ferrule insertion portions of the two flanges are displaced by a predetermined amount d, so that the light from the SM fiber 2 is G
It is incident on a position deviated from the center of the core of the I fiber 3.

Therefore, 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, and by properly selecting the axis deviation amount d, the steady state of the GI fiber 3 is reduced. The mode will be converted to mode excitation.

At this time, by making the core portion of the SM fiber 2 included in the area of the core portion of the GI fiber 3, the connection loss can be reduced.

Similarly, as shown in FIG. 1 (b), by dividing the flange into two and enlarging the ferrule insertion hole of one of the flanges, play can be created between the ferrule insertion holes.

This play is fixed by shifting the ferrule of the connector by a predetermined amount with respect to the central axis of the insertion hole by the screw 56, so that the light from the SM fiber 2 is incident on the position deviated from the core center of the GI fiber 3. It

For this reason, the light of low-order mode excitation passing near the center of the core in the GI fiber 4 is reduced, and the light of higher-order mode component is generated. By appropriately selecting the axis deviation amount d, the GI
It will be mode-converted into the steady-state excitation of the fiber 4.

FIG. 2 (a) shows a 6 dB band when the axis deviation amount d is changed in the mode conversion adapter according to the present invention,
The value in the 6 dB band increases as the amount of axis deviation increases, and reaches the original 6 dB band value (275 MHz) of the GI fiber at about d = 15 μm.

Similarly, when d is set to 15 μm or more in FIG. 2 (a), the 6 dB band value is similarly improved, but since the connection loss in the conversion adapter is increased accordingly, it is not a good idea to increase the axis deviation unnecessarily.

From a design standpoint, an axis deviation between d = about 10 μm and about 20 μm is the most effective range of use.

FIG. 2 (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 output pattern of the field pair set to a predetermined value is shown. The broken line in (c) is the GI
The steady mode pattern of the fiber (light source for GI + incident by steady mode exciter) is shown.

In FIG. 2 (b), the characteristic pattern (a) of the present invention is GI.
This is almost the same as the steady mode characteristic pattern (c) of the fiber, and it can be seen that steady mode excitation can be performed due to axis misalignment.

That is, by performing axis misalignment between the SM fiber and the GI fiber as in the present invention, the light emitted from the SM fiber can be mode-converted into the steady mode excitation of the GI fiber, thereby improving the degradation of the 6 dB band.

Furthermore, 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 variation even when the fiber is bent.

〔Example〕

 The first embodiment of the present application is shown in FIG.

In FIG. 1 (a), 2 is SM fiber, 3 is GI fiber, 6 is SM fiber side connector, 61 is SM fiber side connector ferrule, 7 is GI fiber side connector,
71 is a GI fiber side connector ferrule, 5 is a mode conversion adapter, 51 is a SM fiber side flange, 52 is a GI fiber side flange, 54 and 54 'are ferrule insertion holes, and 53 is a flange fixing screw.

The mode conversion adapter 5 is an S that holds the SM fiber 2.
G holding M fiber side connector 6 and GI fiber 3
Flange for fixing the I-fiber side connector 7 with a screw
51 and 52 are provided respectively.

The flanges 51 and 52 have ferrule insertion holes into which ferrules 61 and 71 of the corresponding connectors are inserted and fixed.
54 and 54 'are provided.

The flanges 52 and 52 are displaced so that the central axes of the ferrules 61 and 71 which are inserted and fixed in the insertion holes are displaced by a predetermined amount, and the screws are screwed.
The flange 51 and the flange 52 are fixed by 53.

An enlarged view of this joint is shown in FIG.

In the figure, 2A is an SM fiber core, 2B is an SM fiber clad, 3A is a GI fiber core, 3B is a GI fiber clad, and the same parts as those in FIG.

Ferrule insertion holes 54 and 5 of flange 51 and flange 52 in the figure
Since 4'is fixed in a shifted state, the central axes of the ferrules 61 and 71 of the SM fiber side connector 6 and the GI fiber side connector 7 are eccentric to the mode conversion adapter 5.

Due to this eccentricity, the central axes of the SM fiber 2 and the GI fiber 3 are displaced by d.

Due to this axis shift d, 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 a higher-order mode component.

FIG. 1 (b) shows a second embodiment of the present invention.

Similar to the first embodiment, a flange 51 for inserting the SM fiber side connector 6 and a flange 52 for inserting the GI fiber side connector 7 are provided.

The ferrule insertion hole 54 'of the flange 52 into which the GI fiber side connector 7 is inserted is configured to be larger than the ferrule 71 of the GI fiber side connector 7, and the ferrule insertion hole 54' is provided with a screw that is open in the radial direction of the ferrule 71. Hole
55 will be provided.

After inserting the GI fiber side connector 7 into the flange 52, the screw 56 is inserted into the screw hole 55, and the ferrule 71 of the GI fiber side connector 7 is pushed down by the screw 56.

As a result, the ferrule 71 of the GI fiber side connector 7
Has a configuration deviated from the central axis of the core of the connector 61 on the SM fiber side facing each other.

Due to the axis shift obtained by the above configuration, the light emitted from the SM fiber 2 is not concentrated and incident on the center of the core of the GI fiber 3, so that it is converted into steady mode excitation including higher order mode components.

Although the ferrule insertion hole 54 'of the flange 52 into which the GI fiber 3 is inserted is made larger than the ferrule 71 in this embodiment, the diameter of the hole 54 of the flange 51 into which the SM fiber 2 is inserted may be increased.

FIG. 4 shows a third embodiment of the present invention.

The third embodiment is a diagram showing a modification of the second embodiment, in which the flange 52 has a plurality of screw holes and the ferrule 71 is adjusted.
It makes it possible to perform precisely.

The flange 52 is provided with a screw hole 57 in addition to the screw hole 55, and the screw 58
Adjust ferrule 71 with and 56.

Also in this embodiment, the diameter of the hole 54 of the flange 51 into which the SM fiber 2 is inserted may be increased as in the second embodiment.

〔The invention's effect〕

The present invention relates to an adapter for connecting a single-mode fiber and a multi-mode fiber, in which the flange connecting the single-mode fiber side connector and the multi-mode fiber side connector is displaced, or by a screw provided on the flange, a shaft is attached to the ferrule of the connector to be inserted. With the configuration that causes the shift, the light in the low-order mode excitation state from the SM fiber is converted into the steady-mode excitation, and 6 dB is obtained.
A mode conversion adapter that improves the bandwidth can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing the principle and embodiment of the present invention. FIG.
A graph showing the 6 dB band of the output light and the far-field pattern at the end of the I fiber, FIG. 3 is an enlarged view of the ferrule insertion hole in FIG. 1, FIG. 4 is a view showing a third embodiment, and FIG. FIG. 6 is a diagram showing the attenuation of the amplitude of the optical signal in the optical fiber transmission line. FIG. 6 is a graph showing the transmission band of the optical fiber. FIG. 7 is a diagram showing an example of the refractive index distribution of the GI fiber. FIG. 8 is 6 dB. FIG. 9 is a diagram showing a band measuring device, FIG. 9 is a graph showing characteristics of a conventional 6 dB band improvement example, and FIG. 10 is a diagram showing a device for measuring a relationship between a spot size and a 6 dB band. FIG. 11 is a graph showing the relationship between the spot size and the 6 dB band. In the figure, 2 is SM fiber, 3 is GI fiber, 6 is SM fiber side connector, 61 is SM fiber side connector ferrule, 7 is GI fiber side connector, 71 is GI fiber side connector, 5 is mode conversion adapter, 51 is SM Fiber side flange, 52 is GI fiber side flange, 54, 54 '
Is a ferrule insertion hole, and 53 is a flange fixing screw.

Front page continuation (72) Inventor Kazuya Sasaki 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Within Fujitsu Limited (56) References JP 63-71811 (JP, A) JP 62-78506 (JP, JP, A)

Claims (2)

[Claims] 1. When the light emitted from the light source side single mode fiber (2) is incident on the light receiving side multi mode fiber (3), the single mode fiber (2) and the multi mode fiber (3) are coupled at the joint portion. A mode conversion adapter for changing incident light to the multimode fiber from light of low order mode excitation to steady mode excitation by axially shifting the core centers of the single mode fiber and the multimode fiber, The single mode side flange (51) coupled to the mode fiber side connector (6) and the multimode side flange (52) coupled to the multimode fiber side connector are provided, and the single mode side flange (51) is provided. The multi-mode side flange (52) is fixed such that the optical axis centers of the cores of the optical fibers are displaced from each other by a predetermined amount. Dapta. 2. When the light emitted from the light source side single mode fiber (2) is incident on the light receiving side multimode fiber (3), the single mode fiber and the single mode fiber are coupled at the coupling portion of the single mode fiber and the multimode fiber. The single mode fiber side connector 6 is coupled in a mode conversion adapter for changing the incident light to the multimode fiber from the light of low order mode excitation to the steady mode excitation by decentering the core center of the multimode fiber. A single mode side flange 51 and a multimode side flange 52 for connecting to the multimode fiber side connector 7 are provided, and the flange 51 for connecting to the single mode fiber side connector 6 or the multimode fiber side connector is connected. The connector insertion hole of the flange 52 is provided with a ferrule of the connector. The diameter is larger than the diameter, and a screw hole (55) for fixing the ferrule of the connector in the flange is provided, and the screw (56) is used so that the optical axis centers of the mutual optical fiber cores are displaced by a predetermined amount. A mode conversion adapter that fixes the connector ferrule.