JPH0293610A - Structure of mode converter - Google Patents

Abstract

PURPOSE:To improve the deterioration of a 6dB band in a graded index fiber (GI fiber) by misaligning the axes of a single mode fiber (SM fiber) and the GI fiber by using ferrules of different diameters which insert these fibers into an adapter and a sleeve which holds the ferrules. CONSTITUTION:The sleeve 7 of a cylindrical shape or polygonal shape which presses the two ferrules 5, 5' of different diameters in a radial direction is provided into the adapter 8 to be fastened to an optical connector. Namely, the SM fiber 2 is inserted and fixed to the ferrule 5 and the GI fiber 4 to the ferrule 5'. The ferrule 5 is so formed as to be smaller in diameter than the ferrule 5'. The ferrules 5, 5' formed in such a manner are respectively inserted and fixed into the sleeve 7 in correspondence to the diameter thereof, by which the axes of the fibers inserted and fixed into the ferrules are deviated by as much as the difference in the outside diameters of the ferrules 5, 5'. The deterioration of the 6dB band is improved in this way.

Description

[Detailed Description of the Invention] [Summary] A mode converter that converts the propagation mode from single mode to multimode in an optical communication system using an optical fiber as a transmission path, and a mode that has a 6 dB band improvement effect with a simple configuration. For the purpose of providing a converter, the diameters of the ferrules of the fibers to be connected are different, and the ferrules are aligned in an axially offset state and held in place by a sleeve inside the adapter.

[Industrial application field]

The present invention relates to a conversion adapter for converting propagation modes in an optical communication system using an optical fiber as a transmission path.

Depending on the transmission capacity, there are two types of optical fiber: single mode fiber (hereinafter referred to as 3M fiber) and multimode fiber, and this multimode fiber is step index fiber (hereinafter referred to as SL fiber).
It is classified as graded index fiber (hereinafter referred to as CI fiber).

In this specification, the multimode fiber will be explained using a Gl fiber as a representative.

There are two types of light source devices, one for SM and one for GI, corresponding to 3M fiber and CI fiber, but recently there has been a trend to unify the light sources to SM light sources and apply SM light sources to existing CR fibers. is increasing.

However, when injecting light from an SM light source into a CI fiber, the transmission characteristics [6d
B Baseband characteristics expressed in the 4% F range] may appear to deteriorate, and it is desired to improve this by a simple method.

Here, the 6 dB band will be explained using FIGS. 6 and 7.

FIG. 6 is a diagram showing the attenuation of the amplitude of an optical signal due to the optical fiber transmission line, and the incident light having an amplitude of 0 that enters the optical fiber 10 is attenuated to A at the output end of the optical fiber and output.

In optical fiber transmission, due to the three causes of mode dispersion, material dispersion, and waveguide dispersion (structural dispersion), as shown in Figure 7, the higher the modulation frequency, the smaller the amplitude A of the modulation waveform appearing on the output side. Become.

The change in the amplitude ratio of input and output signals (frequency characteristics) with respect to the modulation frequency starting from this direct current is called the baseband characteristics.In particular, in the transmission band of optical fibers, the output amplitude reaches its maximum in the baseband characteristics. The modulation frequency (f in FIG. 7) at the point where it becomes 6 dB smaller than the 6 dB band is called the 6 dB band.

[Conventional technology]

The main control of the baseband characteristics of a CI fiber is the group delay time (mode dispersion) between modes propagating within the fiber. Conventionally, when the light enters a CI fiber from a 3M light source, the light inside the CI fiber has a low Since the CI fiber is in a state of next mode excitation, it was thought that the baseband characteristics would appear to be better than the original 6 dB band of CI fiber due to the reduced number of modes. Actually, the refractive index distribution of the fiber (
When the profile (hereinafter referred to as profile) is ideal as shown in FIG. 8(A), the 6 dB band appears to be improved. However, in the case of a fiber whose profile deviates from the ideal as shown in FIG. 8(B), the appearance of the 6 dB band deteriorates as described above.

The difference in refractive index distribution between FIGS. 8(A) and 8(B) is mainly due to the manufacturing method.

When manufacturing an optical fiber, it is manufactured through a two-step process of first making a preform (base material), then heating the preform to melt and soften it, and then drawing it.

As a typical method of making this preform, internal C
There are VD method (MCVD method), external CVD method, VAD method (vapor phase shaft method), etc.

According to the VAD method, a fiber having a profile as shown in FIG. 8(A) can be manufactured, but according to the internal CVD method and the external CVD method, the final
During the "collapse" process, doping materials such as GeO□ molecules escape, and a dip region 11 with a low refractive index is generated in the central portion of the core.

In this way, the profile deviates from the ideal in Figure 8 (B
) In the case of fibers shown in Figure 1, conventionally, to prevent deterioration in the 6 dB band, an S with a large core diameter was used after the 3M light source.
1 fiber is inserted several meters to several tens of meters to enlarge the emission pattern of the 3M light source, or
As described in JP-A-62-78506 and JP-A-62-78506, a method has been proposed in which a lens system is inserted after the 3M light source to enlarge the output beam size.

[Problem to be solved by the invention] The 9th evaluation of the improvement effect in the 6 dBl region by the conventional example
It is shown in FIG.

FIG. 9 shows the measurement method, in which the electrical signal from the oscillator 21 is first converted to E10 by the 3M light source 22, and then input to the conventional lens system or St fiber 24, and then further Gl
It enters the fiber 4.

Next, the light emitted from the GI fiber 4 is subjected to O/E conversion, and this signal is measured by the spectrum analyzer 27.

On the other hand, for standard measurement in the 6 dB band using the CI light source, the output of the oscillator 21 described above is converted to E10 by the Gl light source 23, and then input to the steady mode exciter 25 known as "rscs exciter", and this output is converted to E10 by the Gl light source 23. into the Gl fiber of

After the output of the Gl fiber, the E10 converter 26
and the signal is measured by the spectrum analyzer 27.

Figure 10 shows the 6 dB band measured in Figure 9 using the standard measurement method consisting of the rSM light source + conventional example and the standard measurement method consisting of the rGI light source + stationary mode exciter. The figure shows the case where the intensity is changed.

In the graph of FIG. 10, (7) is the result of measuring the original 6 dB band of the CI fiber using a standard measurement method using a combination of a CI light source and a steady mode exciter.

(() (')) shows the characteristics when an Sl fiber is connected to a 3M light source and its output is input into a Gl fiber.

(1) shows the characteristics when light from a 3M light source is incident on a GI fiber via a lens system, the numerical aperture or spot size of the incident light is arbitrarily selected, and the fiber length is used as a parameter.

(Year) shows the characteristics when the 3M light source is directly coupled to the CI fiber and excited.

The characteristics shown in (() and (tsu) in Figure 10 have a 6 dB band wider than the characteristics shown in (sai), and have a certain 6 dB
Although there is an effect of improving the B band, when compared with the characteristics that should originally be obtained in (A), it can be seen that the 6 dB band is not improved much.

Furthermore, as shown in the characteristic (1), it has been found that there is almost no improvement effect when a lens system is inserted in place of the Sl fiber.

Furthermore, JP-A-57-158604 and JP-A-62-7
As described in US Pat. No. 8,506, there is prior art that attempts to improve the 6 dB bandwidth by enlarging the spot size of the light incident on the GI fiber.

FIG. 11 shows an apparatus in which the characteristics of the known example for enlarging the spot size were tested, and the same parts as in FIG. 9 are designated by the same numbers.

In FIG. 11, the connector 28 is separated (1, 1). Using a device that changes the bonto size incident on the fiber 1, the relationship between the spot size of the light incident on the CI fiber 4 and the 6 dB band is measured. This is shown in the graph of Figure 12.

The characteristic (a) of the graph in Figure 12 shows 6 dB due to the 3M light source.
The 6dB bandwidth is approximately 200MHz,
Comparing with the original 6dB band characteristics (b) of CI fiber,
Its bandwidth was narrow and ineffective.

The present invention has been made in view of these points, and its purpose is to convert the mode of the light emitted from the 3M fiber with a simple configuration (6 in the 1,1 fiber).
An object of the present invention is to provide a mode converter that improves deterioration in the dB band with low loss.

[Means to solve the problem]

The present invention shows that when light from a 3M light source is incident on a GI fiber, the low-order mode excitation is maintained even after propagating more than 10 km in the GL fiber. This is thought to be due to the fact that the group delay time difference between low-order modes (between two or a very small number of modes) increases, resulting in band limitation.

That is, in the case of a G fiber having a profile as shown in FIG. 8(B), a dip region 11 with a low refractive index exists at the center of the core due to the optical fiber manufacturing process, so that the group between low-order modes is It is thought that the delay time difference becomes large.

Therefore, in order to improve the 6 dB band, when the light is input from the 3M fiber to the GL fiber, it must be concentrated at the center of the core of the GL fiber to prevent the light from the 3M fiber from entering, and at the same time, it is necessary to It is necessary to convert the mode to steady mode excitation.

Therefore, as shown in FIG.
In the adapter 8 that connects the and CI fiber 4, 3M
Ferrule 5 holding fiber 2 and CHI fiber 4
The outer diameters of the ferrules 5 and 5 are set to different diameters, and the outer diameters of the ferrules 5 and 5 are set to different diameters. By joining the end faces of .5゛,
This converts it into steady mode excitation that includes higher-order mode components.

[Effect]

The connection of the sleeve of the present invention will be explained using FIG.

As shown in FIG. 2(a), the diameter of the ferrule 5 into which the 3M fiber 2 is inserted is dl, and the diameter of the ferrule 5° into which the Gl fiber is inserted is d2.

The relationship between d and d2 is set such that d, > d, +.

The diameter of the sleeve 7 is changed at the notch 9 so that the ferrules 5 and ferrules 5' can be inserted and fixed.

A configuration in which ferrules 5 and 5' are inserted into this sleeve 7 is shown in FIG. 2(b).

When the ferrules 5 and 5' having different diameters are inserted into the sleeve 7, the sleeve 7 opens at the slit 6 and the ferrules 5 and 5' are pressed in the radial direction.

As a result, the ferrule 5.5° is attached to the axially continuous portion of the holding portion of the ferrule 5.5° with different diameters of the sleeve 7.
By being pressed against the ferrules, the center axes of the ferrules 5 and 5 degrees become eccentric.

This eccentricity causes the light from the 3M fiber 2 to enter the core end of the GI fiber 4.

FIG. 3(a) shows the 6 dB band when the amount of axis deviation d is changed in the mode converter according to the present invention. As the amount of axis deviation increases, the value of the 6 dB band becomes thicker (d= 1
At about 5 μm, the original 6 dB band value (27
5MHz).

Similarly, when d is set to 15 μm or more in Figure 3 (a), 6 dB
Although the band value improves, the connection loss in the converter increases accordingly, so it is not a good idea to unnecessarily increase the axis offset.

In terms of design, the most effective usable range is an axis misalignment between about d=10 μm and about 20 μm.

FIG. 3(b) shows a far-field image of the emitted light at the end of the Gl fiber, the solid line in (a) shows the emission pattern when there is no axis deviation, and the solid line in (b) shows the emission pattern when the axis deviation of the present invention is determined. The (two) broken lines indicate the steady mode pattern of the Gl fiber (injection by the 01 light source steady mode exciter).

In FIG. 3(b), it can be seen that the characteristic (A) of the present invention is almost the same as the steady mode pattern (T) of the Gl fiber, and steady mode excitation can be performed by the axis shift.

That is, by making an off-axis connection between the 3M fiber and the GI fiber as in the present invention, it is possible to mode convert the light emitted from the 3M fiber into the steady mode excitation of the GI fiber.
This makes it possible to improve deterioration in the 3 dB band. In addition, since the light propagating in the GI fiber is in a steady mode excitation state, there is little loss fluctuation even when the fiber is bent, etc., and the loss is low. Optical transmission can be performed.

〔Example〕

A sectional view of an adapter according to an embodiment of the present invention is shown in FIG.

In FIG. 4, parts that are the same as those in FIG. 1 are designated by the same numbers.

A 3M fiber 2 is inserted and fixed into the ferrule 5, and a Gl fiber 4 is inserted and fixed into the ferrule 5'.

At this time, the diameter of the ferrule 5 is made smaller than that of the ferrule 5.

By making it possible to insert and fix the ferrules 5 and 5'' configured as above respectively in accordance with the diameter of the sleeve 7, the fiber inserted and fixed in the ferrule is adjusted by the difference in the outer diameter of the ferrules 5 and 5°. The axis of is eccentric.

This sleeve 7 is inserted into the adapter 8.

The adapter 8 is threaded at both ends, and is configured to be connectable to an optical connector using the threads.

By inserting the optical connector holding the 3M fiber 2 into the ferrule 5 side of the sleeve 7 of the adapter 8, and inserting the optical connector with the Gl fiber 4 inserted and fixed into the ferrule 5° side of the adapter 8, the 3M fiber The connecting portion of the Gl fiber and the Gl fiber can be eccentrically connected to each other.

Although axis misalignment occurs at the joint surface when the optical connector is inserted, connection loss does not become a problem by making the cores of the 3M fiber 2 and the 01 fiber 4 overlap.

FIG. 5 shows the specific structure of the sleeve 7.

FIG. 5(a) shows the sleeve 7 having a cylindrical shape.

The sleeve 7 is configured such that the diameter of the cylinder of the sleeve 7 differs with the notch 3 as a boundary.

FIG. 5(b) shows a sleeve 7 having a triangular shape, and the base angle of the triangle is changed at the notch 3 as a boundary.

In this way, the diameter of the cylinder is determined by the notch 3 of the sleeve 7.
Alternatively, by changing the angle of the base angle of the triangle and using the springiness generated by the slit 6 provided in the axial direction, it is possible to insert and fix ferrules 5 and 5 degrees of different diameters, respectively.

〔Effect of the invention〕

The present invention converts light in a low-order mode excitation state into steady mode excitation by shifting the axis of the fibers using ferrules with different diameters for inserting the 3M fiber and GI fiber into the adapter, and a sleeve that holds the ferrules. By converting, it is possible to improve the band by 6 dB.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the principle of the present invention. Fig. 2 is a diagram for explaining the connection of the sleeve of the present invention. Fig. 3 shows the 6 dB band of emitted light at the end of the CI fiber and its far-field image. Graph, FIG. 4 is a cross-sectional view of an embodiment of the mode converter of the present invention, FIG. 5 is a diagram showing the structure of the sleeve of the present invention, and FIG. 6 is a diagram showing attenuation of the amplitude of an optical signal due to optical fiber transmission. , Figure 7 is a graph showing the transmission band of an optical fiber, Figure 8 is a diagram showing an example of the refractive index distribution of a Gl fiber, Figure 9 is a diagram showing a 6 dB band measurement device, and Figure 10 is a conventional example. A graph showing the characteristics of an example of 6 dB band improvement. FIG. 11 is a diagram showing an apparatus for measuring the relationship between spot size and 6 dB band. FIG. 12 is a graph showing the relationship between spot size and 6 dB band. In the figure, 5.5' indicates a ferrule, 7 indicates a sleeve, and 8 indicates an adapter. (cA)

Claims (1)

[Claims] The single mode fiber (2) on the light source side and the multimode fiber (4) on the light receiving side are butt-connected with their core axes offset, and the light emitted from the single mode fiber (2) is connected to the light receiving side. In a mode converter that changes low-order mode excitation light to steady mode excitation by inputting light into a multimode fiber (4), the single mode fiber (2) and the multimode fiber (4) are connected to each other. One of the connector ferrules (5') has a small diameter, a part in the axial direction is a continuous body, and has a slit (6) in the axial direction and a notch (3) in the center. An adapter (8) that connects a cylindrical or polygonal sleeve (7) that has different diameters with the notch (3) as a boundary and holds two ferrules (5, 5') with different diameters in the radial direction to an optical connector. ), the structure of the mode converter is characterized in that when the rib is inserted and aligned from both ends of the rib, the axially continuous portion is used as a reference plane, and the axis of the ferrule (5, 5') is eccentric. .