JPS6236203B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an optical fiber for single mode transmission, and is particularly suitable for application to a communication fiber. This is the basic mode of optical fiber for communications.
In order to realize so-called single mode transmission of only the HE ₁₁ mode, the refractive index of the core is _n1 , its radius is a, the refractive index of the cladding is _n2 , and the wavelength of the transmitted light is When λ and pi are π, it is necessary to satisfy the following condition: 2.405>2a/λ·π√ ² ₁ − ² ₂ . In this way, optical fibers whose core diameter is designed to be thin to satisfy the above inequality are called single-mode fibers, and because there is no multimode dispersion, pulse delays do not occur, making them ideal for future high-capacity fibers. It is highly anticipated as a communication channel. However, in what is actually called a single mode fiber, there are HE ₁₁ (v) mode in which the electric field is along the vertical direction and HE ₁₁ (H) mode in which the magnetic field is in the horizontal direction. It is common for the rate distribution to have a more or less non-axisymmetric shape such as an ellipse, rather than a perfectly concentric axisymmetric shape. Therefore, the difference in group velocity between the HE ₁₁ (v) mode and the HE ₁₁ (H) mode causes a pulse broadening at the receiving end similar to multimode dispersion, which reduces the transmission capacity of a single mode fiber. This is considered to be one of the main reasons for the limitations on Therefore, if only the HE ₁₁ (v) mode is excited at the transmitting end, and this HE ₁₁ (v) mode propagates as it is to the receiving end, and only the HE ₁₁ (v) mode is detected at the receiving end. , the above-mentioned difference in group velocity is no longer a problem, but in a very common step-type single mode fiber,
It is extremely rare for the HE ₁₁ mode to reach the receiving end without its plane of polarization being distorted due to inhomogeneity or deformation of the internal structure. For this reason, a single mode fiber has a structure in which the planes of polarization of the two optical fibers remain unchanged even under deformation such as bending and twisting that optical fibers normally undergo. Various publications have been published so far. For example, by applying internal stress to an optical fiber so that its refractive index distribution is non-axisymmetric, the tensor-like refractive index in two mutually perpendicular planes along the longitudinal direction of the optical fiber becomes small. A method that reduces the coupling of the orthogonal polarization components by giving a phase velocity difference between the two orthogonal polarization components, or a method that further advances this idea and makes the core part extremely polarized. There is a method that reduces the coupling of these orthogonally polarized components by shaping it into an axially symmetric shape, for example an ellipse, to give a difference in the phase velocity of the polarized components in the major axis direction and the minor axis direction. Are known. in general,
The coupling coefficient between two orthogonal polarization components per unit transmission length of the optical fiber (determined by the degree of bending or twisting due to deformation of the optical fiber, or unevenness of the internal structure, etc.) is c, and the phase velocity between them ( If the difference in phase constant) is Δβ, the strength of the coupling between both polarization components is approximately proportional to c/Δβ. These two conventional examples attempt to obtain a state as close to single polarization transmission as possible by giving a phase velocity difference to two polarization components that are orthogonal to each other. has a mode in which it can be transmitted. Therefore, when the coupling coefficient c is large, that is, when the deformation of the optical fiber is large, or when the length of the optical fiber is very long, the phase velocity difference Δ
Even if β is relatively large, the strength of coupling between both polarization components increases, and there is a risk that the conditions for single mode transmission will eventually be lost. The present invention eliminates the drawbacks of what was conventionally called a single-mode fiber, and provides an optical fiber that, based on a novel idea, can in principle transmit only a single mode of single polarization. The purpose is to The configuration of the first single mode transmission optical fiber of the present invention that achieves this object has a major axis and a minor axis in a cross section perpendicular to the light transmission direction, and has a uniform refractive index distribution. a core portion having a symmetrical shape, a second clad portion that faces the core portion in the minor axis direction of the core portion, and has a refractive index lower than that of the core portion; and a cladding part formed in an annular shape surrounding the core part and having a refractive index higher than the refractive index of the second cladding part and lower than the refractive index of the core part. Further, the configuration of the optical fiber for single mode transmission according to the second aspect of the present invention has a major axis and a minor axis in a cross section perpendicular to the light transmission direction, and is refracted from the center toward the peripheral edge. a symmetrical core portion whose refractive index gradually decreases; a second clad portion that faces the core portion in the minor axis direction thereof and has a refractive index lower than the refractive index of the periphery of the core portion; a cladding portion formed in an annular shape to surround a second cladding portion and the core portion, and having a refractive index higher than that of the second cladding portion and lower than a refractive index at the center of the core portion; It is equipped with the following. Hereinafter, an embodiment of the optical fiber according to the present invention will be described in detail with reference to FIG. 1 and the subsequent drawings. The first aspect of the present invention is shown in FIG. 1A and FIG.
As shown in Fig. 1b, which shows the refractive index distribution along the B arrow direction, a non-containing material having a major axis (vertical dimension in Fig. 1a) and a minor axis (horizontal dimension in Fig. 1a) is shown. A refractive index lower than the refractive index n ₂ of the cladding part 2 that faces the core part 1 in its minor axis direction and covers the core part 1 is provided at both ends of the axially symmetrical core part 1 in the radial direction. A second cladding portion 3 of n _d is formed, and the refractive index n ₁ of the core portion 1 is higher than the refractive index n ₂ of the cladding portion 2 . In addition, the second aspect of the present invention is as shown in FIG. 2a showing the cross-sectional structure of an optical fiber according to an embodiment thereof, and FIG. A core portion 1 having an elliptical cross-sectional shape and having a refractive index with a square distribution, and a refractive index that covers this core portion 1 and has a refractive index lower than the refractive index n ₁ of the core portion 1.
At a part of the boundary with the cladding part 2 of n ₂ , this core part 1
A second cladding part 3 is formed opposite to the second cladding part ₃ with a refractive index n _d lower than the refractive index n 2 of the cladding part 2 . As can be easily deduced from these examples,
In the present invention, in the refractive index distribution formed by the core portion 1 and the cladding portion 2, a second cladding portion 3 having a refractive index n
Although this is the technique of forming the smallest groove (valley) 4 of _d , it is not necessary that the cross-sectional shape of the core part 1 according to the second invention among the above-mentioned embodiments is an ellipse; It may also have a symmetrical shape, for example the shape shown in FIG. 1a. Next, it will be shown that an optical fiber having such a structure is a fiber capable of transmitting a single-polarized single-mode signal. If the width (diameter) of the core part 1 is 2a, the radial coordinate with the center of this core part 1 as the origin is r, and the refractive index distribution is expressed as a function of r by n(r), when r<a If r>a becomes. Here, ρ is a coefficient related to the depth of the groove 4 in the second cladding part 3 between the core part 1 and the cladding part 2, and when ρ=1, it is the case of a conventional optical fiber without the groove 4. , ρ is larger than 1, it means that there is a groove 4 as shown in the drawing. Further, α is a parameter representing the state of the refractive index distribution of the core portion 1, and α
=∞, the refractive index change between the core part 1 and the cladding part 2 takes a step shape as shown in FIG. 1b, and α
When =2, it becomes a square distribution as shown in FIG. 2b, but it is generally expressed as an α-th power distribution. Note that Δ is called the normalized refractive index difference,

The value is equal to [Formula]. In an optical fiber with such a refractive index distribution,
The condition for the cutoff frequency to appear in the HE ₁₁ mode, which is the basic mode, is that if the groove 4 has a width as shown in Figures 1b and 2b, and the width of the groove 4 is t, then ρ>α+2/2・G(t) is given by g(t), but g(t) is g when t=0.
When (0)=1 and t=∞, g(∞)=∞, which is an almost monotonically increasing function. Therefore, even if the value of ρ is a constant value ρ ₀ , the cutoff frequency varies depending on the width t of the groove 4, so the condition for the cutoff frequency to appear in the HE ₁₁ mode is g(t)=2ρ ₀ / It can be rewritten as α+2. From the above, as shown in Figs. 1 and 2, the grooves 4 of the second cladding part 3 are not formed in the vertical direction, but the grooves 4 of the second cladding part 3 are formed in the left-right direction at right angles thereto. By doing this, a single polarization single mode region appears on the frequency axis. FIG. 3 is a graph for explaining this, showing the normalized delay time as a function of the normalized frequency v for the HE ₁₁ , TE ₀₁ , and TM ₀₁ modes, respectively. FIG. 3 corresponds to the embodiment shown in FIGS. 1 and 2, but
In Figures 1a and 2a, the HE ₁₁ mode which has an electric field in the horizontal direction corresponds to the curve p in Figure 3, and in Figures 1a and 2a, it has an electric field in the vertical direction.
Since the HE ₁₁ mode corresponds to the curve q in FIG. 3, a single polarization single mode region s is realized. In other words, by designing the optical fiber so that the operating normalized frequency falls within the single-polarization single-mode region s obtained in this way, a single-polarization single-mode optical fiber can be obtained. I can do it. As described above in detail with examples,
In principle, the optical fiber of the present invention has a single polarized HE ₁₁
Because only one mode is transmitted, extremely stable single-polarization single-mode transmission is possible. In the conventionally known quasi-single-polarized single-mode optical fiber that transmits orthogonal dual-polarization modes, the plane of polarization collapses and orthogonal mode components occur when subjected to large deformation, whereas the optical fiber according to the present invention Even if the fiber undergoes such deformation, it can only transmit the HE ₁₁ mode, which is a polarized wave component in one direction, so the orthogonal mode component generated by the deformation immediately dissipates, achieving stable waveguide characteristics. can.

[Brief explanation of the drawing]

FIG. 1a is a sectional view of an embodiment of the first optical fiber for single mode transmission according to the present invention, and FIG. 1b
2 is a graph showing the refractive index distribution along the BB arrow direction, FIG.
Figure b is a graph showing the refractive index distribution along the BB arrow direction, and Figure 3 is a graph showing the normalized delay time HE as a function of the normalized frequency in the embodiments shown in Figures 1 and 2. ₁₁ mode, TE ₀₁ mode, TM ₀₁
This is a graph showing modes. In the drawings, 1 is a core portion, 2 is a cladding portion, and 3 is a second cladding portion.

Claims (1)

[Claims] 1. A symmetrical core portion having a major axis and a minor axis in a cross section perpendicular to the light transmission direction and having a uniform refractive index distribution; a second cladding part that faces the core part and has a refractive index lower than that of the core part; An optical fiber for single mode transmission, comprising a cladding portion having a refractive index higher than the refractive index of the second cladding portion and lower than the refractive index of the core portion. 2. A symmetrical core portion having a major axis and a minor axis in a cross section perpendicular to the light transmission direction, and a refractive index gradually decreasing from the center toward the peripheral edge, and a core portion having a symmetrical shape in the minor axis direction of the core portion. a second cladding part that faces the core part and has a refractive index less than or equal to the refractive index of the periphery of the core part; an optical fiber for single mode transmission, comprising a cladding portion having a refractive index higher than that of the second cladding portion and lower than a refractive index at the center of the core portion.