JP2945131B2 - Optical fiber preform refractive index 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] FIG. 5 schematically shows a conventional refractive index distribution measuring system (an upper side view and a lower side plan view).

20 is a light source, 22 is an optical system on the incident side, 24 is an optical system on the emitting side, 26 is a detecting device for detecting the position of a light beam, and 28 is an analyzing device.

The base material 10 to be measured is immersed in a matching oil 16 having a refractive index substantially equal to that of the clad.

The light beam 32 is incident from the z direction (see arrow 30) perpendicular to the axis (this is the x axis), and the refraction angle φ of the emitted light beam is determined by the detection device 26 and the analysis device 28. Ask.

The incident position of the light beam 32 is sequentially moved in the y direction to obtain the distribution of the refraction angle φ.

 The refractive index distribution is calculated from the refractive angle distribution by calculation.

[Problems to be Solved by the Invention] The optical fiber preform 10 is immersed in the matching oil 16
In the atmosphere, the difference in refractive index between the air as the medium and the quartz optical fiber preform as the object to be measured is too large, so that the refraction angle φ is too large to perform accurate measurement.

Therefore, a matching oil 16 having a refractive index substantially equal to that of the clad member 14 of the optical fiber preform is used so that the refraction angle φ does not become too large. However, many of the matching oils are made of saturated fatty acids or the like, and are expensive and difficult to remove after the measurement.

In addition, when the optical fiber preform becomes large recently, handling for immersion in matching oil becomes very difficult.

[Means for Solving the Problems] As shown in FIG. 1, a refraction angle distribution is obtained by a method based on the incidence of the light beam only in the range of the core portion 12 of the optical fiber preform 10 and the cladding portion 14a in the vicinity thereof. .

Then, the outer cladding portion 14b is complemented by estimation to obtain the entire refraction angle distribution.

[Description] The optical fiber preform 10 has a core portion arranged concentrically.
12 and a clad portion 14.

The ratio of the outer diameter of the core portion 12 to the outer diameter of the clad portion 14 is about 13 in the case of a single mode optical fiber.

Usually, the clad portion 14 has a flat refractive index distribution.

From the above facts, for measuring the internal refractive index distribution of the optical fiber preform, it is possible to know almost everything by measuring only the core portion 12 and the cladding portion 14a near the center of the preform. Can, its outer cladding part
About 14b, it may be presumed that it is almost the same as the cladding part 14a near the core.

In addition, as shown in FIG. 2, near the center of the optical fiber preform, there is almost no difference in the magnitude of the refraction angle between when measured in air and when measured in matching oil (outside the center). In those areas, the refraction angle in the atmosphere becomes very large.

Therefore, even if only the central part of the preform is measured in the air without immersing the optical fiber preform in the matching oil,
The refractive index distribution of the entire base material can be obtained with sufficient accuracy.

It is known that if the clad / core ratio is 13, the measurement range y may be up to about 15% of the diameter of the optical fiber preform 10.

FIG. 3 is a schematic explanatory view of a measuring device, which differs from the conventional case of FIG. 5 in that the matching oil 16 is not provided. In this case, the side view and the plan view are also shown.

The solid line in FIG. 4 is an example of the refraction angle distribution measured by this apparatus.

In this case, about 20% of the diameter was measured.

The remaining broken line portion is obtained by estimating the deposition of the light beam by numerical calculation and connecting the solid line portion to complement it.

After the refraction angle distribution is obtained in this way, the refraction index distribution can be obtained by calculation in the same manner as in the conventional case.

[Function and Effect of the Invention] The refractive angle distribution is obtained only for the core portion of the optical fiber preform and the range of the cladding portion near the core portion, and then the outer cladding portion is complemented by estimation to obtain the entire refractive angle distribution. (1) As mentioned above, do not use matching oil,
Even when measured in the atmosphere, sufficient accuracy can be obtained.

(2) As a result, the above-mentioned problems associated with the use of the matching oil (particularly difficult handling in the case of a large base material, washing after measurement, etc.) are solved.

According to the method of the present invention, the measurement, which took about 40 minutes by the conventional method, could be completed in 5 minutes while securing approximately the same accuracy.

[Brief description of the drawings]

1 to 4 relate to an embodiment of the present invention. FIG. 1 is an explanatory diagram of a measurement range of a refraction angle, and FIG. 2 is a diagram showing a difference in a refraction angle measured in the atmosphere and in a matching oil. FIG. 3 is a model explanatory view of a measuring apparatus, FIG. 4 is a refraction angle distribution diagram, and FIG. 5 is a model explanatory view of a prior art. 10: optical fiber preform, 12: core part 14: clad part, 20: light source 22, 24: optical system 26: angle and position detection device 28: analysis device

Claims (1)

(57) [Claims] 1. A light beam is incident on an optical fiber preform from a z-direction perpendicular to its axis (x-axis), the refraction angle of an outgoing light beam is measured, and the incident position of the light beam is measured. Are sequentially moved in the y-direction to determine the distribution of the refraction angle, and determine the refractive index distribution from the refraction angle distribution.
In the method for measuring the refractive index of the optical fiber preform, only for the core portion of the optical fiber preform and the range of the clad portion in the vicinity thereof, the refraction angle distribution is obtained by the method by incidence of the light beam, and then the outer clad portion is obtained. Is a method for measuring the refractive index of an optical fiber preform, which obtains the entire refraction angle distribution by complementing by estimation.