JPS63195505A - Measuring method for shape of optical fiber base material - Google Patents

Abstract

PURPOSE:To measure the external diameter of a core, etc., with high accuracy even when an optical fiber base material and refractive index matching liquid has a difference in refractive index by multiplying the apparent core diameter of the optical fiber base material by a correction coefficient and determining the real core diameter. CONSTITUTION:The optical fiber base material 11 is dipped in the refractive index matching liquid 18 with a refractive index (n), laser luminous flux 15 is scanned at right angles to the center axis of the base material 11, and the time when the luminous flux 15 is cut off by the base material 11 is measured electrically. Then the center position of the base material 11 and the center position of the core are determined according to the obtained data and the observed distance between the center of the base material 11 and the center of the core is found from those data. Then, the external diameter and apparent core diameter are determined according to those data and the apparent core diameter is multiplied by the correction coefficient K=n/n1 (n1: refractive index of the clad of the base material 11) to determine the real core diameter. Thus, simple calculation or correction like this is added to accurately evaluate the shape of the base material with high accuracy.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to the shape of the optical fiber preform, that is, the outer diameter.

The present invention relates to a method of measuring outer diameter noncircularity, core diameter, core noncircularity, and core eccentricity with high precision.

(Prior Art) Conventionally, as a method for measuring the shape of an optical fiber preform, as shown in FIG. A container 3 having two front and rear windows 4 is disposed in a plane perpendicular to the object, and the central axis of the optical fiber base material 5 to be measured can be rotated in the container 3 in a direction perpendicular to the laser beam 2. The optical fiber base material 5
In an apparatus for measuring the shape of an optical fiber preform, which has a mechanism capable of rotating the base material at any angle about its central axis and moving it by any length in the axial direction, the container 3 satisfies the relationship that the refractive index is n. By filling the refractive index matching liquid 6 and scanning the laser beam 2 in a direction perpendicular to the central axis of the optical fiber preform 5, the time during which the laser beam 2 is blocked by the optical fiber preform 5 is electrically determined. There is a known method of measuring.

(Problems to be Solved by the Invention) In the conventional measuring method, the optical fiber preform 5 is immersed in a refractive index matching liquid 6 having a sufficiently small refractive index difference from the refractive index of the optical fiber preform 5. 5, that is, the boundary between the cladding and the refractive index matching liquid 6, there is no lens effect of the cladding part, and refraction of the laser beam 2 can be avoided. Although the core diameter and core non-circularity inside the optical fiber material 5 can be measured with extremely high precision, the refractive index of 1O-
There is a problem in that it is necessary to perform adjustment with high precision so as to satisfy the condition of 3 (n≠nt).

In view of the above-mentioned problems, an object of the present invention is to accurately adjust the outer diameter even if there is a slight difference in refractive index between the optical fiber base material and the refractive index matching liquid.
An object of the present invention is to provide a method for measuring the shape of an optical fiber preform by which the outer diameter noncircularity, core diameter, core noncircularity, and core eccentricity can be measured.

(Means for Solving the Problems) In order to achieve the above object, the present invention immerses an optical fiber preform in a refractive index matching liquid having a refractive index n, and immerses the optical fiber preform in a direction perpendicular to the central axis of the optical fiber preform. A first step of scanning a laser beam and electrically measuring the time during which the laser beam is intercepted by the optical fiber preform; and based on the data obtained in the first step, A second step of determining the center position and the center position of the core, and determining the observed distance between the center of the optical fiber preform and the center of the core from the data of the center position of the optical fiber preform and the center position of the core. The outer diameter and apparent core diameter are determined based on the data of the third step and the first step, and the correction coefficient K is added to the apparent core diameter.
= n / n 1 (where n represents the refractive index of the cladding of the optical fiber base material) to determine the true core diameter.

(Function) According to the above configuration, electrical data obtained by irradiating a laser beam perpendicularly to the central axis of the optical fiber base material immersed in a refractive index matching liquid with a refractive index n is used to adjust the optical fiber base material. The center position of the material and the center position of the core are determined, the distance between the center of the optical fiber base material and the center of the core is determined from each center position, and the outer diameter and apparent core diameter are determined based on the electrical data. The true core diameter is determined by multiplying the apparent core diameter by a correction factor of −n/n 1.

(Embodiment) FIG. 1 is a conceptual diagram showing an embodiment of an optical fiber preform shape measuring device used in the present invention. In the figure, reference numeral 10 denotes an optical system of a non-contact laser shape measuring section, which is formed by integrally connecting a laser beam transmitting section (scanning section) 10a and a light receiving section (system) 10b. 11 is a quartz-based optical fiber preform to be measured, 12 is a pair of chucks that hold the optical fiber preform 11, 13 is a glass container containing a refractive index matching liquid, and 14 is a laser beam 1
A transparent window material made of quartz is located in a plane perpendicular to 5, and 16 is a rotating shaft 17 connected to the chuck 12 and a glass container 13.
18 is a refractive index matching liquid filled in the glass container 13; 19 is a pulse motor having the function of rotating the rotary shaft 17 at fixed angles; 19' is a rotary shaft 17; A bearing, 20 is a pulse motor that moves the optical system 10 at regular intervals in the axial direction of the optical fiber base material 11, and 21 is a threaded rotating shaft that transmits the driving force of the pulse motor 20, which is a part of the optical system 10. The rotation of the rotating shaft 21 causes the optical system 10 to advance and retreat in the axial direction of the optical fiber preform 11. 22 is a bearing of the rotating shaft 21; 23 is an electric signal processing unit that processes signals measured by the optical system 10;
24 is a minicomputer that controls the entire device and performs arithmetic processing on the processed electrical signals; 25 is an XY block that displays the calculation results.

To operate this, first, a quartz-based single-mode optical fiber base material 11 is fixed with a chuck 12, and pure water (with a refractive index of 1.3308) at a temperature of 25° C. is placed inside a glass container 13.
Fill with refractive index matching liquid 18). Next, the optical system 10 is operated to scan the parallel light beam 15 of the He-Ne laser in a direction perpendicular to the central axis of the optical fiber matrix 11, and the received light signal at that time is sent to the electrical signal processing section 23. Fig. 3 schematically shows an example of measuring the received light signal, and points A and A' where the signal intensity decreases significantly are located at the optical fiber base material 11.
Similarly, the boundary between the cladding part and the refractive index matching liquid 18 is
and point B corresponds to the boundary between the cladding part and the core part of the optical fiber preform 11.

Further, the signal at the 0 point is a knife signal that serves as a reference for measurement. Here, the time between these pulsed signals 1,
The dimensions of the optical fiber preform 11 can be determined by electrically measuring 11.1 and t3. However, as will be described later, the refractive index n of the refractive index matching liquid 18 is nil so as to satisfy the following relationship. is affected by refraction at the boundary between the cladding portion and the refractive index matching liquid 18. Therefore, if the refractive index difference between the cladding part and the refractive index matching liquid 18 is sufficiently small (for example, (nl
-n)/n1O-3), pulses corresponding to the core position originally appear at positions D and D' on the time axis in FIG. 3, but in this example, the lens effect of the cladding part Therefore, a slight correction is required because it appears at the positions B and B'.

An example of an algorithm for determining dimensions is shown below.

Step-1: Measure t, tt, t2 and t3 using the method described above.

Step-2: Determine the center position of the core and optical fiber preform.

Center position of optical fiber base material (Tb) -(t + t3)/2...(1) Center position of core (Ta) -<t+t2)/2...(2) ■ Step-3, Optical fiber Find the observation distance between the center of the base material and the center of the core.

Observation distance between centers (△E,) −l Ta −Tb l −(3) Procedure-4,
Determine the core diameter and outer diameter.

Apparent core diameter (1) -1-1...(4)a
2 1 True core diameter (t) = (n/n,) Φt.

-(n/nl) ・ (12-1,)...(5) Outer diameter (1)-13-18...(6) The above are measurements in the time domain, but these should be calibrated. The actual dimensions can therefore be easily determined.

Note that the correction coefficient K W n /n l in equation (5) will be described later. In addition, since equations (1) to (3) are numerical determinations related to the center position, there is no need for correction and the outer diameter (t,
) also need not be corrected since it is not necessary to consider the bending of the light ray.

Next, these data are averaged several dozen to several thousand times in the electrical signal processing section 23, and then stored in a storage device built into the minicomputer 25. Next, a pulse signal is sent from the minicomputer 24 to the pulse motor 9 to rotate the rotary shaft 17 by 3 degrees, and the same measurement as above is performed.

At this time, the relative time positions of the points A, IM, B, and B'' in Figure 3 are also recorded as data.These data are
The amount of eccentricity can be used to further determine the non-circularity of the core diameter and outer diameter.

Hereinafter, the same measurements as above will be carried out at the full rotation angle (380' direction).
120 measurement points) and record the data on the minicomputer 24.

Here, two points that are 180° different from each other (for example, 30° and 2
10'), the average value thereof is taken as the actual value. This makes it possible to offset errors caused by lens effects when there is eccentricity.

Then, from the minicomputer 24, the pulse motor 20
The optical system 10 is moved by 5 mm in the axial direction of the optical fiber preform 11 by sending out a pulse and rotating the threaded rotating shaft 21, and the same measurement as above (120 lJI fixed points) is carried out. Hereinafter, by carrying out the above measurements while repeatedly moving the optical system 10 at intervals of 5 mm, and displaying the results on the plotter 25, the outer diameter,
The outer diameter non-circularity, core diameter, core non-circularity, core eccentricity, and straightness of the core center and clad center are determined with high precision. Further, the measurement interval can be arbitrarily selected within a required range.

Note that the outer diameter, core non-circularity, and core eccentricity are given by the following formula in the case of a single mode optical fiber preform.

Outer diameter non-circularity - (Maximum outer diameter - Minimum outer diameter) / (Standard outer diameter)
100 (%) Core noncircularity - (Maximum core diameter - Minimum core diameter) / (Standard core diameter) x 100 (%) Core eccentricity - (Distance between the center of the base material and the center of the core) / (
Standard outer diameter) x 100 (%) Straightness of the core center - Longitudinal dependence of the center position of the base metal Straightness of the cladding center - Longitudinal dependence of the center position of the core Figure 4 shows how to measure the core eccentricity. In the diagram schematically showing the principle, ΔE and ΔE2 are the observed distances between the center of the base material and the center of the core when the rotation angles of the base metal are deviated by 90 degrees from each other, and are obtained from the above formula (3). It is something. Here, the distance between the center of the base material and the center of the core (△E; time conversion)
According to the Pythagorean Theorem, the following (a) Also, the standard outer diameter and standard core diameter used in each calculation are based on the specified standard value for the base material of a certain standard. If not, it can be represented by the average value of the measured values of the outer diameter and core diameter.

Further, in this embodiment, the case where the optical fiber preform is held horizontally has been described, but it can also be held vertically depending on the device configuration.

Below, the refractive index conditions of the refractive index matching liquid and the principle for obtaining the signal shown in FIG. 3 will be explained.

FIG. 5 is a diagram illustrating the operation of this measuring device, showing a cross section of the optical system 10 shown in FIG. 1, in which 26 is a condenser lens and 27 is a light detector.

In FIG. 5, P-P-, Q-Q-, R-R.

SS'', T-T- indicate the He-Ne laser beam scanned in the X direction, and the relationship between the position of the beam in the X direction and the received optical signal is as follows when n < n 1. Here, FIG. 5 shows the case where the window material 14 is made of quartz with a refractive index n1, and by doing so, unnecessary reflection with the refractive index matching liquid 18 can be reduced. The deviation of the refractive index of the window material 14 from nt does not essentially affect the measurement.

In other words, He-N incident perpendicularly to the window material 14
When the e-laser beam comes into contact with the cladding and core of the optical fiber base material 11, it is refracted or reflected depending on the magnitude relationship of the refractive index between the media at the contact point and leaves the condensing lens 26, so that most of it is not received by the photodetector 27. It disappears. The conditions for refraction or reflection are as follows.

(1) Refraction and reflection on the outer periphery of the cladding of the optical fiber preform 11 (a) When n<nl, refraction occurs.

(b) If n>n1, it is reflected.

(2) Refraction at the outer periphery of the core of the optical fiber preform 11 (a) Since n < n O is always satisfied, the optical fiber is refracted.

■ Other than the above contact points, the difference in refractive index between each medium is small, so the light beam travels almost straight and is condensed by the condenser lens 26.
The light is received by the photodetector 27.

It has been explained that the pulsed light output as shown in FIG. 3 can be obtained for the above reasons. Here, this pulse has a certain width because the spot size of the incident light beam is finite and its intensity distribution is approximately Gaussian, and the light receiving surface of the photodetector 27 has a certain size. This is because it has In the measurement example shown in FIG. 3, the spot size is 9. 1 mmφ (1/e2), the aperture of the light receiver was 3 mm wide, and the focal length of the condenser lens 26 in front of the light receiver was 150 mm. Measurement errors due to such pulse broadening can be easily reduced to sufficient accuracy by calibrating using a reference sample.

Next, the coefficient for correcting the effect of refraction of the light beam on the cladding surface is determined as follows. In addition to Figure 5,
The following equation is obtained by Snell's law.

On the other hand, when the light beams S meet at point θ', the distance A in Fig. 5 is A-bsinθ1-(9) is given by

Also, in the triangle s'oo-, 0O--a, S"0-b,
Since o-s''0-02, the following equation is obtained.

bsinθ2-a (10
) From equations (8) to (10), the measured value A corresponding to the core radius when the light flux S contacts the core is given by:

That is, when determining the core dimensions, a-n/n
-A- to-A-(12) to-n/n1 ・
113), the effective value of the core can be determined. That is, the above formula (5) is obtained by multiplying the apparent core diameter t by the correction coefficient K -n/n to obtain the effective value, that is, the true core diameter t.

Here, K in this example is obtained as follows.

The cladding part of the base material 11 is made of pure quartz and is made of He-Ne.
The refractive index nt at the laser wavelength is n 1-1 , 4565 at room temperature (25° C.). Similarly, the refractive index n of pure water is n-1,3308 at room temperature (25°C). Therefore, we get K-0,9137.

It is generally known that the refractive index of a refractive index matching liquid changes with temperature as shown in Table 1.

As is clear from Table 1, the temperature change in the refractive index of the pure water used in this example is only 1/4 of that of silicone oil, which is commonly used as a refractive index matching liquid. , errors due to temperature fluctuations can also be sufficiently reduced. That is, K
Since the temperature change is expressed as T-35℃ (temperature difference ΔT
-10℃), obtain K-0,9132. At this time, the change in the correction coefficient is 0.05%, which indicates that practically sufficient measurement accuracy can be guaranteed.

Furthermore, pure water has a viscosity of 8.9 at a temperature of 25°C.
Because it is as small as
No noise is generated in the received light pulse as shown in the figure.

Next, the relationship between the refractive index conditions of the refractive index matching liquid and the measurement error will be described in detail in the case where the base material to be measured has eccentricity. As shown in Figure 6, the core eccentricity of the base material is d (mm)
(b/a = 15, outer diameter 15mm, core diameter a 1mm) Measurement error 12AoxK-2
When al/2aX100 (%) is determined by numerical calculation, the results shown in FIG. 7 are obtained. For example, when the refractive index matching liquid is water, it can be seen that at θ-0, π/2, π, and 3/2π, the core diameter measurement error is 0.3% or less when the eccentricity is 3%.

On the other hand, in the case of air without any refractive index matching liquid, that is, when the relative refractive index difference Δ-(nl-n)/nX is 100-31%, the error in core diameter measurement under the same conditions is 1.4%. It can be seen that this also applies. In addition. The core eccentricity of the single mode optical fiber base material, which is the main target of this measurement method, is that the refractive index n is 1.
．． In the case of 3308 pure water, it is usually 2% or less, and in this case, the measurement error is suppressed to 0.1% or less.

On the other hand, it can be seen from FIG. 7 that in the case of θ-0 and π, the larger the difference between nl and n, the larger the measurement error. The required measurement performance of this device is to measure the core diameter with a measurement error of 1% or less under the condition of maximum eccentricity of 5%.

. Figure 8 shows the same conditions as Figure 7, θ-0, eccentricity 5% (
d-0, 75 mm), the relationship between the core diameter measurement error and the difference between the refractive index of the refractive index matching liquid and the cladding is shown. From FIG. 8, the following conditions are required to reduce the measurement error of the core diameter to 1% or less.

In addition, another condition for the refractive index matching liquid of the present invention is required. This is stipulated to enable stable measurements even in locations with frequent temperature fluctuations, such as optical fiber factories. In other words, when considering ±2°C as the environmental temperature change for measurement, the variation in the refractive index difference between the cladding and the refractive index matching liquid is approximately 0 when using silicone oil with a large temperature coefficient.
．． It becomes 088%. Therefore, under the above conditions, in order to obtain an optical signal at the boundary between the refractive index matching liquid and the club and to satisfy the condition n≠n, a refractive index difference that exceeds the above variation range is required. .

FIG. 10 is a diagram showing the results of measuring the eccentricity of a single mode optical fiber preform by the measuring method of this example. Sample A is a base material produced by a jacket method, and sample B is a base material produced by a total synthesis method.

From this result, we found that in the jacket method, the eccentricity increases when a quartz tube is coated on the base material to produce a base material for a single mode optical fiber, whereas in the jacket method, the eccentricity increases when the core and cladding parts are simultaneously synthesized. In the method, the eccentricity is extremely small, with an average of 0.
It was confirmed that it was less than 3%.

(Effects of the Invention) As explained above, according to the present invention, an optical fiber base material is immersed in a refractive index matching liquid having a refractive index n, and a laser beam is manipulated in a direction perpendicular to the central axis of the optical fiber base material. A first step of electrically measuring the time during which the laser beam is interrupted by the optical fiber preform; and a center position and core of the optical fiber preform based on the data obtained in the first step. a second step of determining the center position of the optical fiber preform and a third step of determining the observed distance between the center of the optical fiber preform and the center of the core from the data of the center position of the optical fiber preform and the center position of the core;
The outer diameter and apparent core diameter are determined based on the data from the first step and the apparent core diameter, and the correction coefficient is -n/n 1 (where nt is the cladding of the optical fiber base material). The fourth step is to determine the true core diameter by multiplying it by the refractive index (representing the refractive index of In addition, it is possible to obtain highly accurate data without requiring strict adjustment of the refractive index of the refractive index matching liquid, which saves time, reduces costs, and creates an efficient optical fiber preform shape. It has the advantage of providing a measurement method.

[Brief explanation of the drawing]

Fig. 1 is a conceptual diagram of an optical fiber preform shape measuring device used in the present invention, Fig. 2 is a conceptual diagram of a conventional device of this kind, and Fig. 3 is a conceptual diagram of a conventional device of this type.
Figure 4 is an explanatory diagram showing an example of measuring a received light signal according to the present invention, Figure 4 is a principle diagram schematically showing a method for measuring eccentricity, and Figure 5 is an explanatory diagram of the operating principle and correction coefficient calculation of the present invention. , Fig. 6 is an explanatory diagram of core diameter measurement error calculation when there is eccentricity, Fig. 7
The figure shows the calculation result of the core diameter measurement error with respect to the core eccentricity, Figure 8 shows the relationship of the core diameter measurement error with the difference between the refractive index of the refractive index matching liquid and the cladding, and Figure 9 1 is a diagram showing the measurement results of the eccentricity of a preform for a single mode optical fiber. DESCRIPTION OF SYMBOLS 10... Optical system of laser shape n1 fixed part, 11... Optical fiber base material, 12... Chuck, 13... Glass container, 14... Window material, 15... Laser light flux, 16・
... Oil seal bearing, 17 ... Rotating shaft, 18
... Refractive index matching liquid, 19... Pulse motor for rotating the sample, 20... Pulse motor for moving the optical system, 21.
...Threaded rotating shaft, 22...Bearing, 23...
Electrical signal processing section, 24...Mini computer, 25.
...X-Y plotter, 26... Condensing lens, 27...
・Photodetector.

Claims (4)

[Claims] (1) An optical fiber preform is immersed in a refractive index matching liquid with a refractive index n, a laser beam is scanned in a direction perpendicular to the central axis of the optical fiber preform, and the laser beam is blocked by the optical fiber preform. a second step of determining the center position of the optical fiber preform and the center position of the core based on the data obtained in the first step; A third step of determining the observed distance between the center of the optical fiber preform and the center of the core from data on the center position of the optical fiber preform and the center of the core, and an external distance based on the data of the first step. Determine the diameter and apparent core diameter, and further determine the true core diameter by multiplying the apparent core diameter by a correction coefficient K = n/n_1 (where n_1 represents the refractive index of the cladding of the optical fiber base material). A method for measuring the shape of an optical fiber preform, comprising a fourth step of: (2) The refractive index matching liquid has a temperature coefficient of refractive index of 1.
2. A method for measuring the shape of an optical fiber preform according to claim 1, characterized in that a transparent liquid of 0^-^4 or less is used. (3) Highly purified water (H_2O) as the refractive index matching liquid
A method for measuring the shape of an optical fiber preform according to claim 1, characterized in that the method uses: (4) The measurement is performed while rotating the optical fiber preform at arbitrary angles and moving the laser beam at arbitrary lengths in the axial direction. A method for measuring the shape of an optical fiber preform according to any one of the above.