JP4407742B2 - Inspection method of refractive index distribution of glass base material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of determining the distribution of refractive indexes of a glass preform by which the quality is reliably guaranteed only with a few points of measurement. <P>SOLUTION: A glass preform is obtained by vitrifying a glass particulate deposit by heat to obtain transparent glass. The method of determining the distribution of refractive indexes of a glass preform is characterized in that the distribution of the refractive indexes is determined on the glass preform after the vitrification step at the positions corresponding to the maximum and minimum points of the temperature distribution in the central axis direction of the glass particulate deposit. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a method for inspecting a refractive index distribution of a glass base material used for manufacturing, for example, an optical fiber.

  In the method of manufacturing a glass base material used for manufacturing an optical fiber or the like, glass raw material gas is flame-hydrolyzed to generate glass fine particles, and this is deposited on a starting glass rod or the like to form a glass fine particle deposit, which is dehydrated, It is known to sinter and form a transparent glass. In addition, a VAD method (vapor phase axis attaching method), an OVD method (external vapor phase vapor deposition method), and the like are known for producing a glass fine particle deposit.

In the OVD method, for example, a glass raw material gas such as SiCl ₄ is blown together with a combustion gas such as H ₂ gas and O ₂ gas on the outer periphery of a starting glass rod that rotates in a reaction vessel, and a flame hydrolysis reaction is performed. Glass particulates are produced and deposited to produce a glass particulate deposit. In the VAD method, a burner is disposed below a rotating starting glass rod, glass raw material gas and combustion gas are sprayed, and glass fine particles generated by a flame hydrolysis reaction are deposited in the axial direction to form a glass fine particle deposit. I am making it.
Thereafter, the glass fine particle deposit produced as described above is dehydrated and sintered in a heating furnace to become a transparent glass body (see, for example, Patent Document 1).

  However, the quality of the produced transparent glass body becomes unstable due to variations in the refractive index distribution characteristics in the central axis direction of the glass base material due to uneven heating during dehydration and sintering in the heating furnace. There is.

  In order to suppress the above-mentioned variation, for example, in Patent Document 2, after depositing an optical fiber porous preform in the axial direction of the starting preform, the optical fiber porous preform is heated at a predetermined temperature to be dehydrated and baked. In the manufacturing method in which the glass base material for optical fiber is manufactured by repeating and this process is repeated, the refractive index of the manufactured glass base material for optical fiber (the refractive index here is the core portion and the cladding portion) And the difference between this refractive index and the target refractive index is calculated, and this difference is converted into a correction value for the heating temperature of the optical fiber porous preform, and in the next manufacturing process. The heating temperature of the optical fiber porous preform is corrected with the correction value.

  Further, Patent Document 3 measures the refractive index distribution of the core rod in advance along the longitudinal direction prior to the deposition of the glass fine particles, and based on the measured refractive index distribution, the chromatic dispersion value is a predetermined value in the longitudinal direction. The target J magnification (glass base material outer diameter / starting glass rod outer diameter) distribution is calculated in the longitudinal direction so that the deposition amount of the glass fine particles is adjusted in the longitudinal direction so as to achieve the target J magnification. is there.

In Patent Document 4, a method for producing a glass base material in which a glass particulate deposit is housed in a furnace tube of a vacuum heating furnace, sintered, and turned into a transparent glass, provided on the side surface of the core tube at the time of the previous glass base material production. Measure the temperature of the base material surface of the glass particulate deposit or the glass base material and the core tube through the opening, and control the temperature of the core tube at the time of manufacturing the glass base material this time to the target heating temperature and heating time. The setting of the heating setting means is changed based on the previous temperature of the core tube and the base material surface temperature to control the heating temperature.
JP-A 63-206327 Japanese Patent Laid-Open No. 2001-19456 JP 2003-321239 A JP 2004-331414 A

  However, for example, when inspecting the refractive index distribution of a glass base material as in Patent Document 2, if a large number of measurement points are taken, there is a problem that productivity is lowered. Therefore, there has been a demand for a method for inspecting the refractive index distribution of a glass base material that can reliably guarantee the quality without unnecessarily increasing the number of measurement points and reducing productivity.

  The present invention has been made in view of the above, and an object of the present invention is to provide a method for inspecting the refractive index distribution of a glass base material that can reliably guarantee the quality with a small number of measurement points.

  The method for inspecting the refractive index distribution of the glass base material according to the present invention, which can solve the above-mentioned problems, after performing the transparent vitrification step of converting the glass fine particle deposit into a transparent glass by heating to form a glass base material, The refractive index distribution of the glass base material after vitrification is measured at a position corresponding to the maximum point and the minimum point of the temperature distribution in the central axis direction of the glass fine particle deposit in the transparent vitrification step.

The refractive index distribution of the glass base material is particularly susceptible to the thermal history during production, and is in the direction of the central axis (hereinafter also referred to as “longitudinal direction”) of the glass particulate deposit in the heating furnace during the transparent vitrification process. The refractive index distribution varies in the longitudinal direction due to the difference in temperature distribution. The inventor has found that the amount of deviation from the set value is maximized in the refractive index distribution at the maximum point and the minimum point of the temperature distribution in the longitudinal direction of the glass fine particle deposit in the transparent vitrification step, leading to the present invention. It is a thing.
That is, in the manufactured glass base material, if the refractive index distribution at the maximum point and the minimum point of the temperature distribution in the longitudinal direction of the glass fine particle deposit during the transparent vitrification step is measured, without increasing the measurement points unnecessarily, It becomes possible to guarantee the quality reliably with a small number of measurement points.

  The method for inspecting the refractive index distribution of a glass base material according to the present invention is a glass base material used for manufacturing an optical fiber, which is a core glass in a central part (the core glass has a core part and a part of a clad part. It can be applied to inspection of core clad glass obtained by synthesizing a porous clad portion around a transparent glass, or core glass produced in the production process. Moreover, it is effective not only for inspection of glass base materials for optical fibers but also for inspection of glass base materials that require stability of the refractive index.

  Furthermore, the method for inspecting the refractive index distribution of the glass base material according to the present invention is suitable when the transparent vitrification step is a step of heating the glass particulate deposit in a heating furnace having a plurality of heating zones. That is, the maximum point and the minimum point of the temperature distribution in the longitudinal direction of the glass base material in the transparent vitrification step correspond to the center part and the end part of each heating zone of the plurality of heating zones, respectively, and are measurement points. The maximum point and the minimum point can be easily specified, and the refractive index distribution can be inspected more efficiently.

  ADVANTAGE OF THE INVENTION According to this invention, the inspection method of the refractive index distribution of the glass base material which can ensure quality reliably with few measurement points can be provided.

  Hereinafter, an example of an embodiment of an inspection method for a refractive index distribution of a glass base material according to the present invention will be described with reference to the drawings.

Hereinafter, the transparent vitrification step will be described in detail.
FIG. 1 is a schematic cross-sectional view of a heating furnace that performs dehydration and sintering of a glass particulate deposit in a transparent vitrification step.
As shown in FIG. 1, the heating furnace 10 includes a cylindrical furnace core tube 12 surrounded by a furnace body 11. Around the core tube 12, there are provided a plurality of stages of heaters 13a, 13b, 13c (collectively referred to as heaters 13) arranged in the longitudinal direction.
In the present invention, regions heated by the individual heaters 13 are referred to as “heating zones”.
In this embodiment, the maximum point / minimum point of the temperature distribution is specified from the “heater temperature”, and the maximum point / minimum point of the temperature distribution is the “atmosphere temperature in the core tube” or “base material surface temperature”. It can be specified from either.
In this embodiment, the heater is provided in three stages, but the present invention is not limited to this.

The heaters 13 are shortly arranged in series in order to provide the thermal uniformity of the furnace core tube 12 or to have a desired temperature distribution, and are arranged with a gap therebetween.
The heaters 13 divided into a plurality of stages are connected to a control device 21, and the control device 21 can individually control heating. That is, the control device 21 can perform on / off and heating power adjustment for each heater 13. The heater 13 is, for example, a resistance heating type heater, and a strip-shaped resistance material having an annular shape surrounding the outer periphery of the core tube 12 is used.

A support device 14 is provided above the core tube 12, and the glass rod G <b> 1 (core glass having a core portion and a part of the clad portion) is formed in the core tube 12 by the support device 14. The glass particulate deposit G on which the particulates are deposited is suspended.
The heating furnace 10 is a vacuum or reduced pressure heating furnace in which a furnace core tube 12 is hermetically sealed with a furnace body 11. A desired gas can be appropriately supplied to and discharged from the inside of the furnace core tube 12, and heat treatment is performed. The gas used in is not configured to leak to the outside.

  On the outer peripheral side of the furnace body 11, a plurality of temperature measuring devices 22 arranged at intervals in the vertical direction are arranged toward each heater 13. These temperature measuring devices 22 are contact-type or non-contact-type temperature sensors, and are connected to the control device 21 and transmit measurement results to the control device 21.

  Further, the control device 21 has a power supply adjustment function for adjusting the power supplied to each heater 13, and can control the temperature of each heater 13 independently based on the measured temperature from each temperature measuring device 22. .

Next, the case where the glass particulate deposit G is sintered into a transparent glass by the heating furnace 10 to obtain a glass base material will be described.
First, a glass particulate deposit G formed by depositing glass particulates around the glass rod G1 by an OVD method or the like is suspended in a furnace core tube 12 by a support device 14 and supported in a vertical direction as shown in FIG. To do.
The temperature of the core tube 12 is set so that the heaters 13a to 13c are turned on all at once after inserting the glass particle deposit body into the core tube 12, and the temperature is changed from the preheating temperature (about 800 ° C.) to the dehydration temperature (about 1250 ° C.). To dehydration treatment. Thereafter, each of the heaters 13a to 13c is sequentially heated and controlled so as to become a predetermined temperature (about 1550 ° C.) to be transparent vitrified.

In Figure 1, L ₁ is the distance from the effective top end of the soot preform G to the upper heater 13a center. The “effective part” is a part that can be used as a product, and is arbitrarily determined depending on the state of the product. In FIG. 1, the upper end of the heater 13a and the upper end of the effective portion are at the same position, and the lower end of the heater 13c and the lower end of the effective portion are at the same position. L ₂ is the distance from the effective top end of the soot preform G to the boundary of the upper heater 13a and central heater 13b. L ₃ is the distance to central heater 13b from central effective part upper end of the glass particles deposit G. L ₄ is the distance from the upper end of the effective portion of the glass particulate deposit G to the boundary between the middle heater 13b and the lower heater 13c. L ₅ represents a distance from the effective top end of the soot preform G to the upper heater 13c center. _Lw is the effective part length of the glass particulate deposit G (distance from the upper end of the rear part to the lower end of the rear part).

In the present embodiment, the heating zone of the heater 13b is between L _{4 and} L ₂ , the center of the heating zone is the position of L ₃ , and the ends are the positions of L ₂ and L ₄ .
The heating zone of the heater 13a is because of the presence of effective top end is the base point in the heating zone of the heater 13a, the heating zone is equivalent to between L _2, the center portion of the heating zone the position of L ₁ , end is the position and effective top end position of L _2. Similarly, 13c are heating zones correspond to between L _w -L _4, the central portion of the heating zone is located in L _5, the end portion is located and an effective section lower end position of L _4.
Usually, in each heating zone of FIG. 1, L ₁ , L ₃ , and L _{5 that} are the center positions of the heaters 13 a, 13 b, and 13 c correspond to the maximum points of the temperature distribution in the longitudinal direction in the heating furnace 10. Further, the end of each heating zone, that is, the boundary positions (L ₂ , L ₄ ) of the heaters 13 a, 13 b, 13 c and the upper and lower positions of the effective portion are the temperature distribution in the longitudinal direction in the heating furnace 10. It corresponds to the minimum point.
In the case where the heater 13 is an integrated type, the upper end position of the heater 13 and the lower end position of the heater 13 correspond to the minimum points of the temperature distribution in the longitudinal direction in the heating furnace 10.

  2A and 2B are schematic views showing the appearance of the glass fine particle deposit G before and after sintering. FIG. 2A shows the glass fine particle deposit G, and FIG. 2B shows the sintered transparent glass G2. . In FIG.2 (b), lw represents the effective part length of the transparent glass body G2.

In this embodiment, the maximum points L ₁ , L ₃ , L _{5 of the} longitudinal temperature distribution determined by the positions of the heaters 13a, 13b, 13c in the transparent vitrification step of the transparent glass body G2 after sintering. The refractive index distribution is measured at positions corresponding to the minimum points L ₂ and L ₄ , near the upper end of the effective portion, and near the lower end of the effective portion.
That is, in the present embodiment, the vicinity of the effective portion upper end, L ₁ × lw / Lw, L ₂ × lw / Lw, L ₃ × lw / Lw, L ₄ × lw / Lw, L ₅ × lw / Lw, effective portion What is necessary is just to measure the lower end vicinity.

  The refractive index distribution can be measured with a refractive index distribution measuring device. FIG. 3 shows an example of a refractive index distribution measured by a refractive index distribution measuring device at a predetermined measurement point. The core diameter, the cladding diameter, and the core cladding outer diameter ratio are calculated from the refractive index distribution measured by the refractive index distribution measuring device. In FIG. 3, 20 is a core diameter and 21 is a clad outer diameter.

  In this embodiment, the ratio between the measured value of the above-mentioned core cladding outer diameter ratio (core diameter / cladding outer diameter) and the design value of the core cladding outer diameter ratio (measured value of the core cladding outer diameter ratio / core cladding outer diameter). (Design value of the ratio) is calculated, and a deviation amount between the manufactured glass base material and the design value is inspected. Based on the inspection result, the temperature condition of the transparent vitrification process is adjusted at the next production of the glass base material, and the glass base material having a core clad outer diameter ratio closer to the design value can be produced. By using the above inspection method, the characteristics of the manufactured glass base material can be efficiently inspected, and the quality can be reliably guaranteed without increasing the measurement points and reducing the productivity.

As described above, the method for inspecting the refractive index distribution of the glass base material according to the present invention has been described by taking as an example the case where the glass base material is a core clad glass used for optical fiber production or the like, but the present invention is not limited thereto. It is not something.
The method for inspecting the refractive index distribution of the glass base material according to the present invention is the same as described above when used for the inspection of the refractive index distribution of the core glass (glass rod G1) manufactured in the process of manufacturing the optical fiber glass base material. It can be implemented by a method. Based on the result of this inspection, by correcting the amount of glass fine particles deposited on the core glass and the heating temperature of the heating furnace 10 when synthesizing the clad portion later, the optical fiber glass mother with a constant refractive index distribution is corrected. It is possible to produce a material.

  Moreover, the method for inspecting the refractive index distribution of the glass base material according to the present invention is not limited to the glass base material for optical fibers, but is also effective for inspection of other glass base materials that require stability of the refractive index.

Example 1
As shown in FIG. 1, in a heating furnace 10 having three heaters 13 (13a, 13b, 13c from the top) in the length direction, a glass in which a porous clad portion is synthesized around a central core glass. The fine particle deposit G was dehydrated and sintered. Each heater 13 has a length of 500 mm, and adjacent heaters 13 are separated by 50 mm. The effective part length Lw of the glass fine particle deposit G was 1500 mm, but the effective part length lw of the transparent glass body G2 after transparent vitrification was 1000 mm. For this transparent glass body G2, the central portion of the heater 13 of the heating furnace 10 and the portion corresponding to the joint of the heater 13 are calculated from the relationship between Lw and lw at the stage of the glass particulate deposit G, and the refractive index at that point Distribution was measured. Table 1 shows each measurement position of the refractive index distribution. In this example, the measurement point 1 and the measurement point 7 were respectively shifted by 10 mm from the upper end of the effective portion and the lower end of the effective portion toward the center of the heating furnace 10.

FIG. 4 shows the shift amount of the core clad outer diameter ratio at a distance from the upper end of the effective portion of the transparent glass body G2 (x mark).
The deviation of the core clad outer diameter ratio in FIG. 4 is the difference between the measured value of the core clad outer diameter ratio (core diameter / cladding outer diameter) and the design value (actual measured value of the core clad outer diameter ratio / core clad outer). Design value of diameter ratio). The maximum point of the temperature distribution during vitrification of the glass particulate deposit G was 1400 ° C., and the minimum point was 1350 ° C.

(Comparative Example 1)
For the transparent glass body G2 described in Example 1 above, the refractive index distribution was measured at 100 mm intervals in the length direction of the effective portion, and the results indicated by the circles in FIG. 4 were obtained. Table 1 shows each measurement position of the refractive index distribution.

  As shown in FIG. 4, the location measured in Example 1 is a change point of the refractive index distribution. In the measurement of this comparative example, the change in the refractive index distribution was performed although there were as many as 11 measurement points. The result was that the points (maximum and minimum values) could not be captured.

(Example 2)
The temperature distribution in the heating furnace 10 is measured in advance, and at the longitudinal position of the transparent glass body G2 corresponding to the maximum and minimum points of the temperature distribution in the heating furnace 10 at the stage of the glass particulate deposit G. Even when the refractive index distribution was measured, the same result as in Example 1 was obtained.

FIG. 1 is a schematic cross-sectional view of a heating furnace 10 that performs dehydration and sintering of the glass particulate deposit G2 in the transparent vitrification step. It is a schematic diagram showing the external appearance before and behind sintering of the glass fine particle deposit G, FIG. 2A is the glass fine particle deposit G, and FIG. 2B is the transparent glass G2. It is an example of the refractive index distribution measured with the refractive index distribution measuring device at a predetermined measurement point. It is a figure which shows the deviation | shift amount of the core clad outer diameter ratio in the distance from the effective length upper end part of the transparent glass body G2 measured in Example 1 and Comparative Example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Heating furnace, 11 Furnace body, 12 Core tube, 13a, 13b, 13c Heating heater, G1 glass rod, G glass particle deposit body, 21 Control apparatus, 14 Support apparatus, 22 Temperature measuring device, L ₁ Glass particle deposit body G distance from the effective top end to the upper heater 13a center, L ₂ the distance from the effective top end of the soot preform G to the boundary of the upper heater 13a and central heater _13b, valid L ₃ glass particles deposit G distance from part top to middle heater 13b around the effective top end of L ₄ a distance from the effective top end of the soot preform G to the boundary of central heater 13b and the lower heater 13c, L ₅ glass particles deposit G To the center of the upper heater 13c, the effective part length of the _Lw glass particulate deposit G, and the effective part length of the lw transparent glass body G2.

Claims (3)

A method for inspecting the refractive index distribution of a glass base material,
After carrying out the transparent vitrification process which turns the glass particulate deposits into a transparent glass by heating to make a glass base material,
The refractive index distribution of the glass base material after the transparent vitrification is measured at a position corresponding to the maximum point and the minimum point of the temperature distribution in the central axis direction of the glass fine particle deposit in the transparent vitrification step, Inspection method of refractive index distribution of glass base material.   The method for inspecting a refractive index distribution of a glass base material according to claim 1, wherein the transparent vitrification step is a step of heating the glass fine particle deposit body in a heating furnace having a plurality of heating zones.   The maximum point and the minimum point of the temperature distribution in the central axis direction of the glass fine particle deposit in the transparent vitrification step are respectively a center part and an end part of each heating zone of the plurality of heating zones, The method for inspecting the refractive index distribution of the glass base material according to claim 2.

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