JPH0293606A - Structure of mode converter - Google Patents

Abstract

PURPOSE:To improve the deterioration of a 6dB band in a graded index fiber (GI fiber) at a low loss by subjecting a fiber which is reduced in the diameter of the front end part to misalignment by the mode converter fixed by a binding material in the state while the fiber is held in contact with the inside wall of a ferrule. CONSTITUTION:The fiber 1 which is formed smaller in the diameter of the front end than the inside diameter of the ferrule 2 is inserted into the ferrule 2 in an adapter 6 which connects a single mode fiber (SM fiber) 14 and the GI fiber 1. The fiber 1 is fixed by the binding material while the front end thereof is held in contact with the inside wall of the ferrule 2. The ferrule 2 is fixed to a housing to constitute the adapter 6. An optical connector holding the SM fiber 14 and the GI fiber 1 is connected by this adapter 6, by which the misalignment is generated between the SM fiber 14 and the GI fiber 1. The exit light from the SM fiber 14 is mode-converted to the stationary mode excitation of the GI fiber 1 by connecting the SM fiber and the GI fiber with misalignment in such a manner that the deterioration of the 6dB band is improved.

Description

[Detailed Description of the Invention] [Summary] This invention relates to 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 has a band improvement effect of 6 dB with a simple configuration. For the purpose of providing a mode converter, the diameter of the tip of the fiber inserted into the ferrule is reduced, and the ferrule is fixed to the housing with adhesive, with the tip of the fiber in contact with the inner wall of the ferrule. The configuration is such that a single mode fiber and a multimode fiber are connected using a mode converter.

[Industrial Field of Application] 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 these multimode fibers include step index fiber (hereinafter referred to as SL fiber) and graded index fiber. Fiber C (hereinafter referred to as Gl 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 GI fiber, but recently there has been a trend to unify the light sources to SM light sources and apply SM light sources to existing GI fibers. is increasing.

However, when injecting light from an SM light source into a CI fiber, the transmission characteristics (6d
The baseband characteristics expressed in the B band may appear outdated, and it is desired to improve this by a simple method.

Here, the 6 dB band will be explained using FIGS. 7 and 8.

FIG. 7 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 the amplitude A0 entering 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 8, 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 (frequency characteristic) of the input/output signal with respect to the modulation frequency starting from this direct current is called the baseband characteristic, and in particular, the transmission band of optical fiber has a maximum output amplitude of A in the baseband characteristic. The modulation frequency (f in FIG. 8) at the point where it becomes 6 dB smaller than the 6 dB band is called the 6 dB band.

[Conventional technology]

The main thing that controls the baseband characteristics of a GI fiber is the group delay time (mode dispersion) between modes propagating within the fiber. Conventionally, when the light enters a GI fiber from a 3M light source, the light inside the GL fiber has a low Since the next mode is excited, it was thought that the baseband characteristics would appear to be better than the original 6 dB band of Gl fiber due to the reduced number of modes. Actually, the refractive index distribution of the fiber (
, hereinafter referred to as profile) is ideal as shown in FIG. 9(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. 9(B), the 6 dB band looks outdated as described above.

The difference in refractive index distribution between FIGS. 9(A) and 9(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. 9(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 9 (B
) In the case of a fiber like the one shown in (), to prevent degradation in the 6 dB band, an SI fiber with a large core diameter is inserted several meters to several tens of meters after the 3M light source to enlarge the output pattern of the 3M light source. or as described in JP-A-57-158604 and JP-A-6278506,
Methods such as inserting a lens system after the M light source to enlarge the output beam size have been used.

[Problem to be solved by the invention]

The first evaluation of the 6 dB band improvement effect by the conventional example
This is shown in Figure O and Figure 11.

FIG. 10 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 after entering the conventional lens system or the SL fiber 24, it is further
l fiber 20.

Next, the output light of the Gl fiber 20 is O/E converted, and this signal is measured by the spectrum analyzer 27.

On the other hand, for standard measurement in the 6 dB band using the Gl light source, the output of the oscillator 21 described above is converted to E10 by the Gl light source 23, and further inputted 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 CI fiber.

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

Figure 11 shows the 6 dB band measured in Figure 10 using the standard measurement method consisting of the rSM light source + conventional example and the standard measurement method consisting of the rGI light source + steady mode exciter. The figure shows the case where the intensity is changed.

In the graph of FIG. 11, (a) is the result of measuring the original 6 dB band of the Gl fiber using a standard measurement method using a combination of a CI light source and a steady mode exciter.

(A) and (T) show 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 CI 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 of a 3M light source directly connected to the Gl fiber and excited.

The characteristics shown in Cb) ('>) in Figure 11 are (o)
The 6 dB band is wider than the characteristics shown in (a), and there is a certain degree of 6 dB band improvement effect, but when compared with the characteristics that should originally be obtained in (a), it can be seen that the 6 dB band is not much improved.

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. 12 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. 11 are designated by the same numbers.

In Figure 12, pull apart the connector 28 and press G! The graph in FIG. 13 shows the relationship between the spot size of light incident on the Gl fiber 20 and the 6 dB band, which was measured using a device that changes the spot size incident on the fiber.

The characteristic (a) of the graph in Figure 13 shows 6 dB due to the 3M light source.
The 6dB bandwidth is approximately 200MHz,
Comparing with the original 6 dB band characteristic (b) of Gl 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 using a simple configuration, and to achieve a 6 dB reduction in the CI fiber.
An object of the present invention is to provide a mode converter that improves band deterioration with low loss.

[Means to solve the problem]

According to the present invention, when light from a 3M light source is incident on a Gl fiber, the low-order mode excitation is maintained even after propagating for more than 10 km in the G 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) becomes large due to this, and the band is limited.

That is, in the case of a Gl fiber having a profile as shown in FIG. 9(B), there is a dip region 11 with a low refractive index in the center of the core due to the optical fiber manufacturing process, so the group delay between lower-order modes is It is thought that the time difference will be 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.
A single mode fiber or a multimode fiber having a tip end smaller than the inner diameter of the ferrule 2 is inserted into the fiber 1, and the tip end of the fiber 1 is fixed with an adhesive in contact with the inner wall of the ferrule 2.

This ferrule 2 is fixed to a housing 4 to form an adapter.

This adapter allows you to connect 3M fiber and GI fiber by connecting optical connectors that hold 3M fiber and GI fiber.
Axis misalignment occurs between the I fibers, and the light of the low-order mode component from the 3M fiber is converted into steady mode excitation containing the high-order mode component.

[Effect]

The configuration of the axis deviation of the present invention will be explained using FIG. 2.

As shown in FIG. 2, if the tip of the GI fiber 13 is made thin and the tip of the GI fiber is in contact with the inner wall of the ferrule 2, the center of the core 32'' of the 3M fiber 5 and the GI fiber 13 The center of the core 32 is eccentric.

Due to this eccentricity, the light of the 3M fiber 14 is incident on the end of the core of the Gl fiber 13.

FIG. 3(a) shows that in the mode converter according to the present invention,
The 6 dB band is shown when the amount of axis deviation d is changed, and as the amount of axis deviation increases, the value of the 6 dB band increases,
At about d=15 μm, the original 6 dB band value (275 MHz) of CI fiber is reached.

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 CI fiber, the solid line in (a) shows the emission pattern without axis deviation, and the solid line in (b) shows the emission pattern in the case of axis deviation in the present invention. The output pattern when set to a predetermined value is shown, and the (two) broken lines indicate the steady mode pattern of the CI fiber (G
Injection by steady mode exciter) is shown.

In FIG. 3(b), it can be seen that the characteristic pattern (a) of the present invention is substantially the same as the steady mode characteristic pattern (2) of the Gl fiber, and the steady mode excitation can be performed by the axis shift.

That is, by making an off-axis connection between the 3M fiber and the Gl 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 G fiber.
This makes it possible to improve deterioration in the 6 dB band.

Furthermore, since the light propagating within the Gl fiber is in a steady mode excitation state, there is little loss variation even when the fiber is bent, and low-loss optical transmission can be performed.

〔Example〕

FIG. 4 shows the manufacturing process of the ferrule of the present invention.

The outer diameter of the fiber 1 to be inserted into the ferrule 2 is reduced by etching the cladding at the tip in the axial direction of the fiber or by drawing it in a molten state.

As shown in FIG. 4(a), the fiber 1 is inserted into the hole of the ferrule 2, made to protrude from the hole of the ferrule 2, and bent so that the fiber 1 comes into contact with the hole of the ferrule 2.

Then, as shown in FIG. 4(b), a space is created between the ferrule 2 and the fiber 1, so this space is filled with adhesive 3 and the fiber 1 is fixed to the ferrule 2.

Furthermore, as shown in FIG. 4(C), by cutting off the fiber 1 protruding from the ferrule 2 and polishing the end face of the ferrule 2, it is possible to construct a ferrule 2 in which the optical fiber is eccentric.

FIG. 5 shows a first embodiment in which a connector connecting a 3M fiber and a Gl fiber are connected using the mode converter of the present invention.

In FIG. 5, parts that are the same as those in FIG. 1 are designated by the same numbers.

5 is a mode converter, 6 is an adapter, 7 is a 3M connector plug, and 8 is a GI connector plug.

The mode converter 5 shows an example in which a Gl fiber is used for the ferrule 2, and the eccentric end of the ferrule 2 is inserted into one end of an adapter 6 for connecting an optical connector.

A 3M fiber is inserted and fixed into the ground pair of the adapter 6, and a 3M connector plug 7 having a ferrule with the same diameter as the ferrule of the mode converter 5 is inserted.

In the adapter 6, the 3M connector ferrule 9 and the ferrule 2 are connected, and the eccentricity of the ferrule 2 allows light to enter the core end of the GI fiber. It can be converted into steady mode excited light including the next mode component.

Furthermore, CI connector plug 8 with GI fiber inserted and fixed
can be inserted into the non-eccentric side of the mode converter 5, and the mode-converted light can be input to the transmission path of the CI fiber.

FIG. 6 shows a second embodiment of the invention.

FIG. 6 shows that threaded portions for connecting the 3M connector plug 7 and the CI connector plug 8 are provided at both ends of the housing 4.
A mode conversion adapter that enables direct connection between a 3M connector plug 7 and a GI connector plug 8 using a mode converter 5 is shown.

In the first and second embodiments, the fiber inserted and fixed into the ferrule 2 in the eccentric mode converter 5 is a Gl fiber, but the same effect can be obtained even if a 3M fiber is used.

In this case, it is preferable to connect the eccentric end of the ferrule 2 to the connector plug 8 because connection loss can be reduced.

[Effects of the Invention] The present invention provides a mode in which the ferrule is fixed to the housing by reducing the diameter of the tip of the fiber inserted into the ferrule and fixing the tip of the fiber with adhesive in a state in which it is in contact with the inner wall of the ferrule. By shifting the axis between the single mode fiber and the multimode fiber using a converter, it is possible to convert the light in the low-order mode excitation state to steady mode excitation and improve the band by 6 dB.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the principle of the present invention, Fig. 2 is a diagram showing the configuration of axis deviation of the present invention, and Fig. 3 is a diagram showing the principle of the present invention.
Graph showing the 6 dB band of the emitted light at the end of the fiber and its far-field pattern, FIG. 4 is a diagram showing the manufacturing process of the ferrule, FIG. 5 is a diagram showing the first embodiment of the present invention, and FIG. The figure shows a second embodiment of the present invention, FIG. 7 is a graph showing the attenuation of the amplitude of an optical signal in an optical fiber transmission line, FIG. 8 is a graph showing the transmission band of the optical fiber, and FIG. 9 is a graph showing the optical fiber transmission band. A diagram showing an example of the refractive index distribution of a Gl fiber, Figure 10 is 6
FIG. 11 is a graph showing the characteristics of a conventional example of improving the 6 dB band; FIG. 12 is a diagram showing a device for measuring the relationship between the spot size and the 6 dB band. FIG. 13 is a graph showing the relationship between spot size and 6 dB band. In the figure, 1 is the fiber, 2 is the ferrule, 3 is the adhesive, 4 is the housing, and 5 is the mode converter. 1. Demonstration No. 6
Old 'g 9 mouths

Claims (1)

[Claims] When the light emitted from the single-mode fiber on the light source side enters the multi-mode fiber on the receiving side, the light in the low-order mode is excited in a steady mode at the coupling portion between the single-mode fiber and the multi-mode fiber. In the mode converter for changing the mode into
The fiber (1) is processed to be thinner than the inner diameter of the ferrule (2), the fiber (1) is inserted into the ferrule 2, and the tip of the fiber (1) is placed in contact with the inner wall of the ferrule (2). 3), and the ferrule (2) is fixed to a housing (4) connectable to an optical connector.