JPH0326776B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses an optical directional coupling device, in particular a polarization preserving fiber, to circularly polarize linearly polarized light as much as possible, and to input the circularly polarized light into an optical fiber to be measured. This invention relates to an optical directional coupling device.

Optical measuring instruments, such as optical pulse testers that measure abnormalities in optical cables by injecting optical pulses into the optical cable and measuring the backscattered light that is reflected and returned within the optical cable, have conventionally been used as An optical directional coupler as shown in the figure was used.

In FIG. 1, linearly polarized light from a laser 1 passes through a polarizing element, such as a polarizing prism 2, enters a polarization preserving fiber 4 via a self-occurring lens 3, and the linearly polarized light is directed into an optical fiber 5 to be measured. It was incident. In the same figure, reference numeral 6 denotes an optical directional coupler, and the optical directional coupler 6 is composed of the laser 1, a polarizing prism 2, a self-occurring lens 3, a polarization preserving fiber 4, and a receiver fiber 7. The polarization preserving fiber 4 is simply an assembly and manufacturing means for transmitting the light emitted from the self-occurring lens 3 as linearly polarized light to the optical fiber 5 to be measured. Note that 8 represents the optical connector 8.

The linearly polarized light emitted from the laser 1 is incident on the optical fiber 5 to be measured via the optical directional coupler 6. When the optical fiber 5 to be measured is a multimode optical fiber, The linearly polarized light incident on the optical fiber 5 is disrupted in random directions, and the reflected light that returns as backscattered light is also randomly polarized, and is branched by the optical directional coupler 6 and introduced into the receiver fiber 7. The vertically polarized component of the scattered light remains approximately constant. On the other hand, the optical fiber to be measured 5
When is a single mode optical fiber, unlike the multimode optical fiber, the optical fiber has a property that the linearly polarized light is not easily destroyed, and the light is reflected within the optical fiber 5 to be measured and returned. The linear polarization of backscattered light does not easily collapse. However, in reality, the linearly polarized light changes to elliptically polarized light based on the length of the optical fiber 5 to be measured, the stress applied to the optical fiber 5, the elliptical deformation of the core, anisotropy, heat, etc. The vertically polarized component X of the elliptically polarized light shown in is split by the polarizing prism 2 of the optical directional coupler 6 and introduced into the receiver fiber 7. By the way, when a single mode optical fiber is used as the optical fiber 5 to be measured, the direction in which the elliptical axis of the elliptically polarized light returns as described above is not constant, so the backscattered light received by the optical receiver is As shown in FIG. 3, a jagged fluctuation phenomenon occurred in the light, which caused an undesirable problem in measurement.

The purpose of the present invention is to solve the above-mentioned problems by connecting a polarization-preserving optical fiber between the conventional optical directional coupler and the optical fiber to be measured. Rotating circularly polarized light or elliptically polarized light close to circularly polarized light is incident, and the backscattered light introduced into the light receiver is received in a state where the polarization can be considered to be randomly collapsed from an electrical standpoint, It is an object of the present invention to provide an optical directional coupling device that can suppress fluctuation phenomena caused by optical rotation (instability) of the elliptical axis of elliptically polarized light.
This will be explained below with reference to the drawings from FIG. 4 onwards.

FIG. 4 shows the configuration of an embodiment of the optical directional coupling device according to the present invention, and FIG. 5 shows the measurement of backscattered light when a single mode optical fiber is measured using the optical directional coupling device according to the present invention. An example of a curve is shown.

In FIG. 4, numerals 1 to 8 correspond to those in FIG. A polarization maintaining fiber 10 is connected between an optical connector 11 to which the optical fiber 5 to be measured is connected and an optical connector 8 attached to a conventional optical directional coupler 6. Therefore, the optical directional coupler 9 according to the present invention is different from the conventional optical directional coupler 6.
The light is made to enter the optical fiber 5 to be measured via the polarization preserving fiber 10, but upon exiting from the polarization preserving fiber 10, the light is made into circularly polarized light or elliptically polarized light close to circular polarization, and then enters the optical fiber 5 to be measured. . In other words, the linearly polarized light emitted from the laser 1 is passed through the polarization preserving fiber 4 while maintaining its linear polarization.
I'll come up to it. The light whose linearity is preserved in the polarization preserving fiber 4 is transferred to the next polarization preserving fiber 10.
When entering the polarization preserving fiber 4, the polarization plane is adjusted so that the output light from the polarization preserving fiber 10 changes from linearly polarized light to circularly polarized light or elliptically polarized light close to circularly polarized light.
and 10, and as described above, nearly circularly polarized light is incident on the optical fiber 5 to be measured.

Now assume that the optical axis of the polarization preserving fiber 10 (referring to the principal axis of the ellipse in which the refractive index distribution of the polarization preserving fiber is drawn) is connected to the optical axis of the polarization preserving fiber 4 at an angle of θ. That is, since the polarization direction of the linearly polarized light emitted from the polarization preserving fiber 4 is inclined by θ with respect to the optical axis of the polarization preserving fiber 10, the amplitude within the polarization preserving fiber 10 is
It is thought that the light that is perpendicular to acosθ and asinθ is conserved. Then, when the light is emitted from the polarization preserving fiber 10, these are combined, x ² / (acos θ) ² + y ² / (asin θ) ² −2xycos δ/a ² sin
θcosθ = sin ² δ (1) (where δ is the phase difference between the two components caused by the length of the polarization preserving fiber 10) The light becomes elliptically polarized light and is emitted. To make this circularly polarized light x ² + y ² = a ² /2 ...(2), in equation (1), θ = π/4, δ = π/
2 is required. However, it is nearly impossible to cut the polarization preserving fiber 10 so that δ, that is, the phase difference between the two components occurring over the length of the polarization preserving fiber 10, becomes π/2. By cutting the polarization preserving fiber 10 on the order of the wavelength of light, δ=
This is because it must be set to π/2.

Regardless of the length of the polarization preserving fiber 10, the present invention can convert the polarization preserving fiber 10 into a polarization preserving fiber 4.
By rotating both optical axes at appropriate angles, circularly polarized light or elliptically polarized light closest to circularly polarized light is obtained.

In this way, circularly polarized light or elliptically polarized light close to circularly polarized light is emitted to the optical fiber 5 to be measured via the optical connector 11. The backscattered light reflected within the optical fiber 5 to be measured and returned is also circularly polarized light or elliptically polarized light close to circularly polarized light, and rotates at the frequency of the light. This backscattered light of circularly polarized light or elliptically polarized light close to circularly polarized light is transmitted to the optical connector 11 and the polarization preserving fiber 1.
0, passes through the optical connector 8, the polarization preserving fiber 4, and the self-occurring lens 3, and enters the polarizing prism 2. In the backscattered light incident on the polarizing prism 2, a vertically polarized component of the horizontally polarized wave of the laser 1 is split by the polarizing prism 2, passes through a photoreceiver fiber 7, and is received by a photoreceiver (not shown). For example, an avalanche photodiode is used as the light receiver, but the electrical response speed of this light receiver is much slower than the frequency of light. The backscattered light incident on the polarizing prism 2 rotates in a circle or an ellipse at a period much faster than the electrical response speed of the photoreceiver, so from an electrical standpoint, the polarization is disrupted to a random state. It can be considered that there is. Therefore, it is considered that the vertically polarized wave component branched to the light receiver side by the polarizing prism 2 and the horizontally polarized wave component that goes straight to the laser 1 side are distributed as approximately one pair. Even if the light is rotated (unstable) in the optical fiber 5 to be measured, the vertical polarization component X shown in FIG.
becomes constant. Therefore, as shown in FIG. 5, the measurement curve of the backscattered light becomes a smooth curve, and the jagged fluctuation phenomenon shown in FIG. 3 is suppressed.

In order to align the polarization planes of the polarization preserving fibers 4 and 10, a light pulse is sent to the optical fiber 5 to be measured, and the optical axis of the polarization preserving fiber 10 is rotated while observing the measurement curve of the backscattered light received by the optical receiver. Then, the polarization preserving fiber 10 is fixed at a position where the measurement curve of backscattered light is the smoothest. At this time, the linearly polarized light emitted from the polarization preserving fiber 4 to the optical fiber 10 becomes circularly polarized light or elliptically polarized light close to circularly polarized light when emitted to the optical fiber 5 to be measured.

As explained above, according to the present invention, a polarization preserving fiber is connected between a conventional optical directional coupler and an optical fiber to be measured, and the polarization plane is aligned while observing the measurement curve of backscattered light. An optical directional coupling device that can suppress the effects of optical rotation of a single mode optical fiber, that is, the effects of fluctuations in backscattered light caused by the unstable direction of the ellipse axis of elliptically polarized light, with simple operations. is realized.

[Brief explanation of the drawing]

Figure 1 shows an example of the configuration of a conventional optical directional coupler.
The figure is an explanatory diagram to explain that a jagged fluctuation phenomenon occurs in the measurement curve of backscattered light. Figure 3 shows backscatter when measuring a single mode optical fiber using a conventional optical directional coupler. An example of a light measurement curve, FIG. 4 shows the configuration of an embodiment of the optical directional coupling device according to the present invention, and FIG. 5 shows a measurement of a single mode optical fiber using the optical directional coupling device according to the present invention. An example of a measurement curve of backscattered light is shown in FIG. In the figure, 1 is a laser, 2 is a polarizing prism, 3 is a self-occurring lens, 4 is a polarization preserving fiber, 6 is an optical directional coupler, 7 is a receiver fiber, 8 is an optical connector, and 9 is an optical directional coupler , 10 represents a polarization preserving fiber, and 11 represents an optical connector.

Claims (1)

[Claims] 1. An optical directional coupler that emits linearly polarized light;
The optical directional coupler is connected to the output end of the optical directional coupler in order to convert the incident linearly polarized light into circularly polarized light or elliptically polarized light close to circularly polarized light and output it. An optical directional coupling device comprising a polarization preserving fiber positioned at a predetermined angle with respect to a polarization direction, and a holding means for holding the polarization preserving fiber at the predetermined angle.