JPS60221706A - Optical fiber - Google Patents

Abstract

PURPOSE:To decrease a coefficient of mode coupling and to prevent electric power from being diffused by providing an annular multimode core part to the outer peripheral part of a single-mode core as a center part. CONSTITUTION:The single mode transmission line consists of the single-mode core part 1 and an intermediate clad part 5 and a multimode transmission line consists of an annular multimode clad part 5 and an external clad part 3. This structure makes the electromagnetic field distribution of light of the single-mode transmission line greatly different from that of the annular multimode transmission line, so mode switching between the both is hard to occur and the diffusion of photoelectric power from a single mode area to a multimode area is suppressed low.

Description

DETAILED DESCRIPTION OF THE INVENTION When providing various services including images to subscribers of public communication lines, it is considered promising to introduce optical fibers from the viewpoint of cost and the like. In this case, the central office
The information transmitted from the terminal to the subscriber terminal (subscriber) may include VHF multiplexed signals such as audio, digitized multiplexed signals, moving images, and still images, but the combined signal band of these is 400MHz or more, and all signals When digitized, the band is several hundred MHz.

On the other hand, the signal band for transmission information from the subscriber to the center, such as voice, video, data, and fax, is at most S5M.
Hz, and even if it is digitized, it will only reach 80MHz.
It is a band of about z.゛From this point of view, the downstream transmission from the center to the subscribers uses a semiconductor laser capable of high-speed modulation as the light source, and the transmission path is a single mode fiber with broadband characteristics, and the upstream transmission from the subscribers to the center Since high-speed modulation characteristics are not required, light-emitting diodes (LFtD) are inexpensive as light sources.
It is conceivable to use a multimode fiber with good coupling efficiency with the LED as the transmission line. For this reason,
- Although it is possible to install two optical fibers per subscriber, from the viewpoint of simplifying subscriber line wiring, it is desirable to perform the above bidirectional transmission with one optical fiber. The first optical fiber used in applications such as
The structure shown in the figure was considered.

Here, l is a single mode core part, 2 is a multimode core part, and 3 is a cladding part common to both. Due to the 0-wire structure, single and multimode bidirectional transmission is possible.
It is extremely difficult to manufacture an optical fiber having such an asymmetric structure using the MCVD method or VAD method, which are methods for manufacturing a low-loss optical fiber.

Not practical. For this reason, a concentric structure shown in FIG. 2(a) has been proposed as a structure that can be easily manufactured by the above method. The refractive index distribution of each part in this structure is the second
In this case, the single mode transmission line consists of a single mode core part l and a multimode core part 2, which has a lower refractive index and acts as a cladding for the single mode core part. The multimode transmission line is composed of a multimode core section 2 and a cladding section 3. Since this structure is a concentric structure, it has the advantage of being easier to manufacture than the structure shown in FIG. 1, but it has the following problems. That is, in this structure, the difference between the propagation constant of light propagating through the single mode core section and the propagation constant of light propagating through the multimode core section is small, and the optical power incident on the single mode core section is small.

Mode conversion due to slight bending of the optical fiber, etc. As the transmission distance increases, the mode is gradually diffused into the multimode core, and after long distance transmission, it becomes a multimode distribution, resulting in multimode dispersion, making broadband transmission impossible. .

The present invention provides a single mode/multimode composite optical fiber which eliminates the above-mentioned drawbacks.The multimode core portion is formed into a ring shape, and the difference between the single mode propagation constant and the multimode propagation constant is reduced. By making it larger, the mode coupling coefficient becomes smaller, which causes power dispersion and makes it more difficult.

The present invention will be explained in detail below with reference to the drawings.
) is a sectional view showing the structure of an embodiment of an optical fiber according to the present invention. FIG. 3(b) shows the refractive index distribution of this fiber, where the single mode transmission line consists of a single mode core part 1 and an intermediate cladding part 5, and the multimode transmission line consists of a ring-shaped multimode core part 2. and the intermediate cladding part 5 and the outer cladding part 5.
It is composed of a hand section 3. In such a structure, the electromagnetic field distribution of light in a single mode transmission line is significantly different from the electromagnetic field distribution of light in a ring-shaped multimode transmission line.
Mode exchange between the two modes is difficult to occur, and the diffusion of optical power from the single mode region to the multimode region can be kept low.

FIG. 4(a) is a sectional view of another embodiment of the optical fiber according to the present invention, and FIG. 4(b) is a refractive index distribution seen from the AA' cross section in FIG. 4(a). In this structure, 6 is an air layer, and the single mode core part and the multimode cosa part are connected by a glass thin film. The difference from the case of FIG. 3(a) is that there is no intermediate cladding layer, but the basic operation is the same.

FIG. 5 is a conceptual diagram of bidirectional transmission using an optical fiber according to the present invention, and has a configuration in which the optical wavelengths are different for uplink and downlink. Light from the semiconductor laser on the center side is inputted into the single mode core portion 1ζ of the single mode/multimode composite optical fiber 23 by the lens 8. The light propagated through the fiber is
The light of this wavelength is reflected by the wavelength-selective reflecting mirror 10, passes through the lens 2, is guided to the light-receiving element 13 on the subscriber side, and is converted into an electrical signal. On the other hand, the wavelength of the light from the LED 14 on the subscriber side is different from the wavelength of the light from the laser 7, and this light is input into the multimode core section 2 of the fiber 23 by the lens 15. In this case, it is assumed that the wavelength-selective reflecting mirror IO has sufficient transmittance for light of this wavelength. The propagating light is reflected by another wavelength-selective reflecting mirror 9 that reflects the light of this wavelength and transmits the light of the wavelength from the laser 7, and is guided by the lens 20 to the center-side light-receiving element and received. . In addition, when optical power was to be supplied from the center to the subscribers, it was placed on the center side. Light from a power feeding light source 18 having a different wavelength from that of the laser 7 and LED 14 is input into the multimode core portion of the optical fiber 23 using a lens 17 and wavelength-selective reflective mirrors 16 and 9, and the emitted light is emitted at the subscriber side. The wavelength selective reflector 10.11 and the lens allow the light receiving element 2 to
1 and convert it into electrical energy.

Above, in order to simplify the explanation of the embodiments of the present invention, it is assumed that the refractive index distribution of the multimode core part is step-shaped, but the present invention can also be applied to the case of a graded refractive index distribution having broadband characteristics. It goes without saying that it can be done.

[Brief explanation of the drawing]

Figures 1 and 2 (, ) are single mode and multimode composite fibers according to the prior art, Figure 2 (b) is a refractive index distribution diagram of the optical fiber shown in Figure (a), and Figure 3 ( a) is an embodiment of the present invention, FIG. 3(b) is a refractive index distribution diagram of the optical fiber shown in FIG. 4(a), and FIG. 4(a) is another embodiment of the present invention, FIG. Figure (b) is a refraction distribution diagram of the optical fiber shown in figure (a), and Figure 5 is a conceptual diagram of single mode and multimode bidirectional transmission using the optical fiber according to the present invention. l... Single mode core part, 2... Multimode core part,
3... Cladding part S 5... Intermediate cladding part, 6.
... Hollow part, 7... Semiconductor laser, 14... Light emitting diode, 23... Single mode, multimode composite fiber% 8゜12.15, 17, 20.21... Condensing lens , 9.10, 11.16... Wavelength selective reflecting mirror, 18... Laser light source for power transmission, 13, 19, 22...
··Light receiving element. Figure 1 ￥: Jz diagram (Figure 2 (b) Figure 3 (η) Figure 3 (6) Figure 4 (phantom A' Figure 4 Cb)

Claims (1)

[Claims] A single mode/multimode composite optical fiber having a circular cross-section, having a single mode core section at the center and a ring-shaped multimode core section at the outer periphery thereof.