JPH01316706A - Mode conversion type optical fiber - Google Patents

Abstract

PURPOSE:To eliminate radiation loss by varying the specific refractive index difference and core radius of the mode conversion type optical fiber from a multimode optical fiber to a single-mode optical fiber in order. CONSTITUTION:The combination (DELTA, a) of the specific refractive index difference defined by the refractive index of the core and the refractive index of the clad and the radius (a) of the core is (DELTAG1, aG1) and (DELTASM, aSM) at both ends of the optical fiber with length L and when deltaDELTA=(DELTAG1-DELTASM)/(m-1) and deltaa=(aG1-aSM)/(m-1), equations I and II hold at respective positions in the lengthwise direction Z. In the equations I and II, (n-1)/m.L<=Z<=n/m.L, where (n) is 1, 2..., and m>=3. Consequently, when light is propagated in an SM fiber, the cutoff of high order mode and the radiation loss due to electric field mismatching are eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a mode converting optical fiber that can be used to couple a multimode optical fiber and a single mode optical fiber.

[Conventional technology]

In conventional optical transmission lines, single-mode optical fibers with a wide bandwidth of several tens of GHz are used for long-distance, high-capacity optical transmission media with repeating intervals of several tens of times or more and signal transmission speeds of several hundred Mbit/sec or more. In addition, the relay interval is several tens of kilometers or less, and the signal transmission speed is several hundred Mb.
For short-distance, small-capacity optical transmission media of less than it/sec, the number 1 is used.
A multi-mode optical fiber type with a narrow band of 00 MH is adopted.

As mentioned above, the reason why single-mode optical fiber is used as a long-distance, high-capacity optical transmission medium in conventional optical transmission lines is that, for example, in long-distance, high-capacity optical transmission lines such as trunk lines, the broadband nature of the optical transmission medium is However, since the number of connection points is small and the requirements for connection characteristics are low, a single mode optical fiber with a wide band is suitable although it is difficult to connect.

On the other hand, the reason why multimode optical fibers are used for short-distance, small-capacity source media is that, for example, short-distance, small-capacity transmission lines such as subscriber systems require many connection points and extremely high connection characteristics. This is because there is a low demand for wideband characteristics of optical transmission media, and therefore multimode optical fibers, which have a narrowband but are easy to connect, are suitable.

By the way, in recent years, in the field of communications, in addition to traditional telephone services, various services that require broadband transmission lines have been introduced, such as single-picture information services using high-definition televisions and video conferencing services. As a result, the demand for broadband transmission lines is increasing even in subscriber systems.

In addition, the connection technology between single-mode optical fibers has improved significantly, and the connection characteristics between single-mode optical fibers have now been improved to the same level as the connection characteristics between multi-mode optical fibers. Therefore, it is thought that single-mode optical fibers will be introduced in subscriber systems in the future.

In this case, existing multimode optical fibers are installed in the subscriber system, and it is necessary to coexist with newly installed single-mode optical fibers, and technology for connecting these single-mode optical fibers and multimode optical fibers is required. New developments are required.

[Problem to be solved by the invention]

However, the only known connection technology for current single-mode optical fibers and multi-mode optical fibers is direct coupling, such as by fusion splicing. It has not yet been possible to eliminate direction-dependent splice loss characteristics.

For example, when a single mode optical fiber and a multimode optical fiber are directly connected using the fusion splicing method, the splice loss when an optical signal is input from the single mode optical fiber to the multimode optical fiber is several tenths (dB/splice).
However, when an optical signal is input from a multi-mode optical fiber to a single-mode optical fiber, the loss is very large at about 20 dB/connection. For this reason, when single-mode optical fibers and multi-mode optical fibers coexist in the same transmission path, the propagation direction of optical signals is limited to one direction, making bidirectional communication impossible. Ta.

Therefore, in the present invention, when used in the case of connecting a single mode optical fiber and a multimode optical fiber, the connection loss is small no matter which side the optical signal enters from. The object of the present invention is to provide a mode conversion type optical fiber that can be used as an optical transmission line connected to a mode optical fiber.

[Means to solve the problem]

The mode conversion type optical fiber according to the present invention includes a core made of a transparent material having a predetermined maximum refractive index and a predetermined radius, and a core made of a translucent material having a refractive index smaller than the maximum refractive index, covering the outer peripheral surface of the core. In an optical fiber equipped with a cladding made of a transparent material, the combination of the relative refractive index difference Δ defined by the refractive index of the core and the refractive index of the cladding and the radius a of the core (Δ
GI, aGI) are (ΔGl'a ) and (Δ a ) at both ends of the length of optical fiber, respectively, and δΔ
−(ΔGI SM' SMC
l-68M)/(m-1).

δa −(a c+−a SM) / (m 1 )
Then, at each position in the length direction Z, a (Z
) −aG, −(n−1) δaΔ(Z) ring ΔG,
-(n-1) 66m
m;n-1121...Im;m≧3) Since the mode conversion type optical fiber according to the present invention is configured as described above, it can be used as a multimode fiber. As G I
(Graded 1ndex) fiber, connect it to one end surface of Z-0 of the above-mentioned mode conversion type optical fiber, and connect the single-mode optical fiber to the other end surface of Z-L of the above-mentioned mode conversion type optical fiber. mode conversion can be performed. In other words, at Z-0, from the above equation, a(Z-0)ma, Δ(Z-0)-aGl, G! The Z-L has the same characteristics as an SM (single mode) fiber, and from the above equation, a(Z-L)ma Δ'(Z-L)-68M.

Therefore, G! When transmitting an optical signal from the fiber to the 3M fiber via the mode conversion type optical fiber according to the present invention, among the optical fibers that have entered the mode conversion type optical fiber from the Gl fiber, the higher mode signals are transmitted through the mode conversion type optical fiber. As it propagates, the relative refractive index difference Δ and core radius a
changes, so it is gradually coupled to lower-order modes,
Eventually most of the higher mode signals will be coupled into the fundamental mode. Further, the optical signal with a wide electric field distribution propagates through the mode conversion type optical fiber, and as it approaches the 3M fiber, it is focused at the core radius aS- of the 3M fiber. As a result, the spread of the electromagnetic field distribution of the optical signal input from the Gl fiber into the mode conversion type optical fiber according to the present invention becomes smaller as it approaches the connection end face with the 3M fiber, resulting in well-concentrated fundamental mode light. Since it becomes a signal, there is no radiation loss due to blocking of higher-order modes or electric field mismatch when propagating through a 5M fiber.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to FIGS. 1 to 5 of the accompanying drawings. In addition, in the description of the drawings, the same elements are given the same reference numerals, and redundant description will be omitted.

FIG. 1 is a sectional view of a mode conversion type optical fiber according to an embodiment of the present invention. The length of this mode conversion type optical fiber 5 is 400 m, and the relative refractive index difference Δ and core radius a are (1.0%, 25 μm), (0.9%, 22 μm), respectively.
m), (0,8%.

19 μm), (0.7%, 16 μm), (0.6%, 1
3μm), (0.5%, 10μm), (0.4%, 7μm)
Prepare 8 optical fibers of 50 m each (0.3%, 4 μm) and fusion splice them in descending order of relative refractive index difference Δ and core radius a (in descending order from the right in the figure). It is composed of Core 5 of mode conversion type optical fiber 5
The compositions of the cladding 52 and the cladding 52 are (Si 02
+G c O2) and (SIO2). Then, the relative refractive index Δ and core radius a of the mode conversion type optical fiber 5
When plotted as a function of the distance from the end surface to which the G summer connector is connected, it becomes as shown in FIGS. 2 and 3, and it can be seen that it follows the formula <1) below.

In other words, the relative refractive index difference Δ and core radius a of the mode conversion optical fiber 5 are the difference between the Gl fiber (connected to the left end surface in the figure) and the 3M fiber (connected to the right end surface in the figure). (Δ+a) are (ΔGl' aGl) and (Δ
a), and on the other hand, δΔ−8M' SM (ΔG1−ΔSM) / (m −1), δa■
The length of the mode conversion optical fiber 5 is (aGl-aSM) / (m-1), and the distance in the length direction is Z
Expressed as, a (Z) a c + (n-1) δaΔ(
Z ) ” F revision-→1-1 ) δΔ−m
m;n■1.2. ...
, mum≧3) ...(1).

FIG. 4 shows an experimental system prototyped using the mode conversion type optical fiber 5 described above. In this system, in order to input the optical signal into the Gl fiber 2, the L
An ED8 is provided, and the power of the emitted light is J? at the output end of the 8M fiber 4. A power meter 9 is provided to determine J. Reference numeral 10 indicates a reel around which each optical fiber is wound. Gl
The fiber 3 and the mode conversion type optical fiber 5 are coupled by a Gl connector (multimode optical fiber connector) 6,
The 5M fiber 4 and the mode conversion type optical fiber 5 are coupled by an 8M connector (single mode optical fiber connector) 7.

In this case, the mode conversion type optical fiber 5 has the same relative refractive index difference Δ as the GI fiber 2 at the joint of the GI connector 6. I and core radius aGl, this part can be connected with the same connector as the connection between Gl fibers,
At the joining part of the 5M connector 7, the mode conversion type optical fiber 5 has the same relative refractive index difference Δ8M and core radius a8 as the 5M fiber 4.
M,l! 1. Therefore, connection can be made with the same connector used to connect SM fibers. Gl fiber 2
The length of the 5M fiber 4 is 500 m, and the relative refractive index difference Δ and core radius a are ΔG, −1.0%, and aG, respectively.
l = 25 μm sΔ5Ill-0.3%, a 5n
-4 μm. In addition, the compositions of the core and blood of the GI fiber 2.5M fiber 4 are (SlO2
+ G e O2) and (3102).

Using this experimental system, we used Gl fiber 2 to 5M fiber 4.
As a result of calculating the insertion loss and coupling loss, the sum of both losses was 1.9 dB. Conventionally, when a GI fiber and a 3M fiber were directly connected, the CI fiber had multiple propagation modes, and the spread width of the electromagnetic field distribution of each mode was sufficiently large compared to the core radius of the 3M fiber. Immediately after the signal enters the 3M fiber, the higher mode light is blocked and most of the fundamental mode light is emitted, actually
It was confirmed that the power of the light that can be propagated in the M fiber is only about 1% of the power of light in the Gl fiber, which can be reduced to about one-tenth or less compared to the 20 dB splice loss.

FIG. 5 shows how the Gl fiber 2, which is a multimode fiber in the conventionally installed optical cable 1, and the 5M fiber 4 of the newly installed optical cable 3 are connected to a mode conversion type optical fiber according to an embodiment of the present invention. A configuration in which fibers 5 are used is shown. In this embodiment, the transmission line ′ which connects each fiber is
There are two systems.

According to this configuration, the GI fiber 2 is injected into the mode conversion type optical fiber 5 via the GI connector 6, and the 5M
The optical signal reaching the 5M fiber 4 via the connector 7 is turned into a fundamental mode optical signal with a small spread of electric field distribution and well focused by the mode conversion type optical fiber 5. Therefore, there is no radiation loss due to cutoff of higher-order modes or electric field mismatch in the SM fiber 4, and the optical signal is allowed to propagate from the GI fiber 3 to the 5M fiber 4 with low loss.

In this way, in the embodiment of the present invention, G! fiber 2 to 5
Since the transmission of the optical signal to the M fiber 4 has a coupling loss of about 2.0 dB, and conversely, the transmission of the optical signal from the 5M fiber 4 to the GL fiber 2 is performed without any problem, there is no restriction on the propagation direction of the optical signal. .

Therefore, two-way communications such as video conferences and computer communications can be realized using an existing system (a system in which a 5M fiber 4 is installed in a subscriber transmission line). In the above embodiment, one of the two transmission lines is connected to the Gl fiber 2.
The optical signal can be sent from the 5M fiber 4 to the 5M fiber 4, and the other can be used to send the optical signal from the 5M fiber 4 to the Gl fiber 2.

The present invention is not limited to the above embodiments, and various modifications are possible.

In this example, a mode conversion type optical fiber satisfying equation (1) was constructed by flexibly splicing optical fibers having different relative refractive index differences and core radii. It is also possible to continuously change at least one of the following.

〔Effect of the invention〕

As explained above in detail, in the present invention, the relative refractive index difference and core radius of the mode conversion type optical fiber are sequentially changed from those of a multimode optical fiber to those of a single mode optical fiber. The optical signal entering the mode optical fiber has a small spread of electromagnetic field distribution and becomes a well-focused fundamental mode light, and in the single mode optical fiber, there is no radiation loss due to cutoff of higher-order modes or electric field mismatch. Optical transmission from multimode optical fiber to single mode optical fiber becomes possible with low loss.

Therefore, bidirectional communication can be appropriately performed between the single mode optical fiber and the multimode optical fiber.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a mode conversion type optical fiber according to an embodiment of the present invention, and FIG. 2 is a diagram showing changes in the relative refractive index difference of the mode conversion type optical fiber according to an embodiment of the present invention. , 3rd
The figure shows changes in the core radius of a mode converting optical fiber according to an embodiment of the present invention, and FIG. Configuration diagram of the experimental system when combining the 5th
The figure is a configuration diagram of a system in which a G1 fiber and an 8M fiber are connected using a mode conversion type optical fiber according to an embodiment of the present invention. 1.3...Optical cable, 2...Gl fiber, 49
．．・8M fiber, 5...mode conversion optical fiber,
6...G! Connector, 7...8M connector, 8...
・LED, 9...Power meter, 51...Core, 5
2...Clad. Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] A core made of a translucent material having a predetermined maximum refractive index and a predetermined radius, and a translucent material having a refractive index smaller than the maximum refractive index and covering the outer peripheral surface of the core. In the optical fiber, a combination (Δ, a) of a relative refractive index difference Δ defined by the refractive index of the core and the refractive index of the cladding and a radius a of the core is a light beam of length L. (Δ_G_I, a_G_I) and (Δ_S_M, a_S_M) at both ends of the fiber, respectively, and δΔ=−(Δ_G_I−Δ_S_M)/(m−1), δ
When a=(a_G_I-a_S_M)/(m-1), at each position in the length direction Z, a(Z)=a_G_
I-(n-1)δa Δ(Z)=Δ_G_I-(n-1)δΔ (However, [(n-1)/m]L≦Z≦(n/m)L;n
=1, 2,..., m; m≧3).