JP2947423B2 - Optical fiber preform refractive index distribution measurement method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for measuring the refractive index distribution of an optical fiber preform.

[Prior art] (1) Regarding starting preform and finished preform: Generally, when producing an optical fiber preform, first, a fifth preform is used.
As shown in the figure, a part of the core 12 and a part of the clad 14 are simultaneously formed (hereinafter, this stage is referred to as a base material 10), and then the insufficient clad 16 is removed by an external method or a jacket method. It is completed by adding (this is hereinafter referred to as completed base material 18).

At that time, in order to estimate the insufficient cladding amount,
At the stage when the starting base material 10 was prepared, its refractive index distribution was measured, and the relative refractive index difference Δ1 was determined based on the refractive index of the cladding portion 14.
And then estimated the cladding shortage.

At the same time, various transmission characteristics were estimated from the relative refractive index difference Δ1.

(2) Measurement of Refractive Index Distribution An example of a method of measuring a refractive index distribution is as follows. As shown in FIG.
a light beam parallel to the optical axis 22 at a position A of y on the y axis
24 are incident perpendicular to the x-axis.

The light beam 24 bends at the refraction angle φ, passes through the lens 26,
It enters B (the distance y 'from the optical axis 22) of the position detector 28 provided at the defocusing point.

 From the value of y ′, φ is obtained by calculation.

When the incident position of the light beam 24 is gradually changed in the y-axis direction and the corresponding y 'is obtained, the distribution of the refraction angle φ is obtained. From the calculation, the refractive index as shown in the lower part of FIG. A distribution is obtained.

 From this refractive index distribution, a relative refractive index difference Δ1 is obtained.

[Problems to be Solved by the Invention] Pure quartz is usually used to add the missing cladding in the starting base material 10.

Therefore, the refractive index of the clad portion of the completed base material is the same value as the refractive index of quartz, and the relative refractive index difference Δ2
Is based on this value (right side in FIG. 5).

However, components (eg, chlorine-based components) used in the dehydration step remain in the clad portion 14 of the starting base material 10, and do not have the same value as the refractive index of quartz.

Therefore, the value of the relative refractive index difference differs between the starting base material and the completed base material, and the transmission characteristics estimated for the starting base material and the transmission characteristics of the completed base material do not match.

[Means for Solving the Problems] As shown in FIG. 1, a transparent glass tube 30 having the same refractive index as the cladding (usually pure quartz) to be added is coaxially covered on the outside of the starting base material 10. Then, the refractive index distribution is measured.

[Operation] When the relative refractive index difference Δ3 is determined from the refractive index values of the transparent glass tube 30 and the core 12 in the refractive index distribution obtained by the above measurement (see FIG. 4), the value is , The relative refractive index difference Δ2 of the completed base material 18.

[Description] Since a cladding to be added later is usually pure quartz, a pure quartz tube is used as the transparent glass tube 30 as well.

In order to keep the transparent glass tube 30 coaxial with the starting base material 10, for example, as shown in FIGS.
Intervene.

Starting base material 10 covered with a transparent glass tube 30 as shown in FIG.
Is immersed in a matching oil 20 as shown in FIG. 1 and a refractive beam distribution is measured by injecting a light beam 24 perpendicular to the axial direction, for example, as in the case of the conventional FIG.

At this time, the matching oil 20 is also inserted between the transparent glass tube 30 and the starting base material 10.

The profile of the refractive index distribution obtained by the above measurement is shown on the far right of FIG. 4 (the refractive index distributions of the starting base material and the finished base material are also shown for comparison).

From this refractive index distribution, the relative refractive index difference Δ1 of the starting base material 10 can be determined, and at the same time, the relative refractive index difference Δ3 of the starting base material 10 with respect to the refractive index of the transparent glass tube 30 can be determined.

Since the refractive index of the transparent glass tube 30 is the same as the refractive index of the cladding 16 to be added later, Δ3 is equal to the relative refractive index difference Δ2 of the completed base material.

[Effects of the Invention] When an optical fiber preform is manufactured by adding a missing clad to a starting preform consisting of a core and a clad, the starting preform is prepared at the stage of producing the starting preform. In order to measure a refractive index distribution by analyzing a light beam perpendicular to the axial direction and analyzing the emitted light, a transparent glass tube having the same refractive index as the clad to be added is placed outside the starting base material. Since the above measurement is performed while coaxially covering the base material, the relative refractive index difference of the finished base material can be determined at the same time when the refractive index distribution of the starting base material is measured.

Therefore, the accuracy of the estimation of the insufficient cladding amount and the estimation of the transmission characteristics is improved.

[Brief description of the drawings]

1 to 4 relate to an embodiment of the present invention, FIG. 1 is an explanatory view of a refractive index distribution measurement, FIG. 2 is an explanatory view of a state where a transparent glass tube 30 and a rubber ring 32 are combined, FIG. 3 is an explanatory view showing a state in which a transparent glass tube 30 is covered on the starting base material 10, FIG. 4 is an explanatory view showing the refractive index distribution and the refractive index distributions of the conventional starting base material 10 and completed base material 18 together, FIG. 5 is an explanatory view of the refractive index distribution of the starting base material 10 and the finished base material 18, and FIG. 6 is an explanatory view showing an example of a conventional refractive index distribution measurement of the starting base material 10. 10: Starting material, 12: Core 14: Clad 16: Added cladding 18: Finished material 20: Matching oil 22: Optical axis, 24: Light beam 26: Lens, 28: Position detector 30: Transparent glass tube, 32: Rubber ring

Claims (1)

(57) [Claims] An optical fiber preform is produced by adding a missing clad to a starting preform consisting of a core and a clad. Then, when a light beam perpendicular to the axial direction is incident, and the emitted light is analyzed to measure the refractive index distribution, a transparent glass tube having the same refractive index as the cladding to be added is placed outside the starting base material. A method for measuring the refractive index distribution of an optical fiber preform, wherein the measurement is carried out while coaxially covering the optical fiber.