JPS5831565B2 - optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber in which the polarization direction of the lowest order mode, HE1, is kept constant among transmission lines used in optical communications.

In conventional optical fibers, it has been difficult to maintain the polarization direction of the propagation mode constant for the reasons described below.

A normal optical fiber has a perfectly circular core, and the refractive index distribution is as shown in FIG. 1A, where the refractive index of core 1 is n.

On the other hand, there is a so-called step type in which the refractive index nd of the cladding 2 constituting the outer periphery of the core decreases discontinuously, or a gray type in which the refractive index changes continuously from the core 1 to the reflood 21 as shown in Figure B. It is a dead shape, and the refractive index distribution is symmetrical at any angular position with respect to the core center.

The refractive index of each recoa in the circumferential direction was uniform.

Generally, the propagation mode of such an optical fiber includes a pair of two modes having equal propagation constants (referred to as degeneracy) and polarization directions orthogonal to each other.

In an actually produced optical fiber, due to fluctuations in the core diameter in the longitudinal direction of the optical fiber, or residual stress existing in the core, there is a possibility that the difference between the pair of degenerate modes whose polarization directions are orthogonal to each other is has the disadvantage that mode coupling occurs, and as a result, as the mode propagates through the optical fiber in its longitudinal direction, its polarization direction rotates.

If the polarization direction is rotated in this way, it cannot be used as an optical fiber for polarization multiplex transmission or for a fiber gyro.

One way to solve this problem is to make the cross-sectional shape of the core rectangular or elliptical as shown in Fig. 2 A and B, respectively, and to generate two mutually orthogonal modes, HE1□ even, which is the lowest mode, HE1□ mode. A method has been proposed in which the polarization direction of the HE11 mode is kept constant by solving the degeneracy between the HE11 mode and the HE11 odd mode, that is, by creating a certain amount of difference in the propagation constant, and the effectiveness of this method has been confirmed through experiments. It is being

However, it is difficult to accurately control the dimensions of such rectangular or elliptical cores using conventional manufacturing techniques for optical fibers with perfectly circular cores. It has the disadvantage that conventional optical fiber manufacturing techniques cannot be applied to the manufacturing of optical fibers with elliptical cores.

This invention was made in consideration of these points, and it is an optical fiber that can maintain the polarization direction of the lowest mode, HEl, and the mode constant, and can also have a perfectly circular core shape. This is what I am trying to propose.

FIG. 3 is a sectional view of an embodiment of the optical fiber according to the present invention, and in this embodiment, the refractive index distribution in the circumferential direction of the core is changed.

For this reason, the refractive index was made different between the angle range portion 1a indicated by diagonal lines in the core 1 and the other portion 1b.

However, the core 1 composed of these image parts 1 a and 1 b
The cross section of is a perfect circle.

A cladding 2 is formed around the outer periphery of the core 1 .

θ is the angle at which the diagonal line portion 1a of the core extends with respect to the coordinate axis 3 whose origin is the center of the core 1.

shall be. Refractive index distribution n(r
, θ) are assumed as follows.

r is the distance from the core center, and θ is the angle from the axis 3.

Tax θ.

~π/2, a is the radius of the core 10, and Δ is the relative refractive index difference between the core 1 and the cladding 2, which takes a positive value.

The propagation constant of the propagation mode of an optical fiber having such an asymmetric refractive index distribution in the circumferential direction (C) can be obtained by numerical calculation from analysis results based on known wave theory.

Figure 4 shows θ as an example.

= π/4, n, = 1.46, θ when g = 1/2
= 0 button and shows the propagation constant difference △β between HE1□ even mode and HEl, odd mode with polarization direction in π/2, and the vertical axis is normalized by dividing △β by the wave number in vacuum. propagation constant difference,
The horizontal axis is the normalized frequency defined by V = k n 1a v' 2 ys, and △ is indicated in the figure.

From FIG. 4, it can be seen that by changing Δ and selecting V, it is possible to obtain any Δβ.

Next, we will discuss the relationship between the propagation constant difference Δβ and mode coupling, and show that if Δβ can be set appropriately, mode coupling can be avoided and the polarization direction can be kept constant.

According to the well-known mode coupling theory, the necessary condition for two modes with a propagation constant difference of △β to couple is the existence of a fluctuation component with a period A that satisfies △βA = 2π in longitudinal fluctuations such as the core diameter. It is to be.

FIG. 5 shows an example of a power spectrum P(Δβ) representing the intensity of the above-mentioned fluctuation of the optical fiber.

The vertical axis is P(Δβ), and the horizontal axis is Δβ.

Therefore, when fluctuations having such a power spectrum shape exist, mode coupling can be prevented by making the propagation constant difference Δβ between the modes larger than Δβ0.

For this reason, in the embodiment shown in FIG. 3, it is possible to prevent coupling between the HE1 even mode and the HE11 odd mode, as shown in FIG. HE11 incident with the polarization direction matched to /2
The mode propagates without rotating its polarization direction.

Normal optical fiber fluctuation power spectrum P(△
β) is not necessarily in the same form as shown in Figure 5, but in general, the component △β-0 is the strongest and its intensity gradually decreases as △β increases, and above a certain △β0, the component is almost absent. The distribution shape is such that it does not last long.

The value of △βo/ is estimated to be about 10 to 10,
Compared to the calculated value of △β/ in Fig. 4, △β〉△β.

It is possible to fully satisfy the following conditions.

Furthermore, when producing an optical fiber having the structure as shown in the example of FIG. It has become clear from other experimental results that birefringence occurs in the directions of θ=0 and π/2.

Since this birefringence has the effect of further increasing the propagation constant difference Δβ, an optical fiber fabricated based on the example shown in FIG. 3 can obtain a Δβ greater than the calculated value in FIG. 4. .

Therefore △β. Even for an optical fiber with /k greater than 10, the relative refractive index difference Δ is kept at about 0.05, and the housing is relatively easy to manufacture, such as Δβ>Δβ.

In other words, the desired effect can be sufficiently obtained.

In addition to this embodiment, the same effect can be obtained by varying the refractive index distribution of the core or cladding portion in the circumferential direction.

Note that θ as shown in FIG. It is effective when the angle is −45° and the shape is symmetrical with respect to the axes 3 and 4.

As explained above, in the optical fiber according to the present invention, the polarization direction of the propagation mode can be kept constant by controlling only the refractive index distribution while the core shape is a perfect circle. It is easy to control, and the core parts 1a and 1b made by the conventional optical fiber manufacturing method.
For example, in the above embodiment, two optical fiber preforms having respective refractive indexes are divided into four equal opening angles with respect to the center of the core, and two of them are combined to create a new preform. It has the advantage that it can be drawn into an optical fiber.

In the fiber of this invention, a mode incident in a certain polarization direction propagates while maintaining the polarization direction, so it can be used as an optical fiber for polarization multiplex transmission, an optical fiber for fiber gyros, and a waveguide for optical integrated circuits. Its range of applications is extremely wide-ranging.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the refractive index distribution of the core and cladding of a conventional optical fiber, Fig. 2 is a diagram showing the cross-sectional shape of the core of a conventionally proposed optical fiber, and Fig. 3 is a diagram showing an embodiment of the present invention. 4 is a diagram showing the relationship between the normalized frequency Δβ/ and the normalized frequency V in the example of FIG. 3, and FIG. 5 is a diagram showing the power of fluctuation in the optical fiber. FIG. 3 is a diagram showing the relationship between spectrum and Δβ. 1: core, 2: cladding, 1a, 1b: core portions with mutually different refractive indexes.

Claims (1)

[Claims] 1. In an optical fiber in which the boundary between the core and cladding of the optical fiber and the outer periphery of the cladding are formed in a perfect circle on a plane perpendicular to the axis, and only the HE11 mode, which is the lowest mode, is propagated, the perfect circle The refractive index distribution of the optical fiber is line-symmetrical with respect to two axes that are orthogonal to each other passing through the center of the optical fiber, and one of the two axes is set as a reference axis around the center; The other of the two axes is rotated by 90° so as to overlap the reference axis so that the refractive index distribution with respect to the other axis after rotation is different from the refractive index distribution with respect to the reference axis before rotation. The refractive index distribution of the optical fiber is set such that the axis can be selected, and the HE
1. All 1. 1. An optical fiber characterized in that the optical fiber is configured to create a difference in propagation constant between HE11 even mode and HE11 odd monide, which have mutually orthogonal polarization directions.