JP5655304B2 - Optical fiber drawing furnace and optical fiber drawing method - Google Patents

Description

  The present invention relates to a drawing furnace and a drawing method for drawing an optical fiber from an optical fiber preform.

  In recent years, as the cost of optical fibers has been reduced, the optical fiber preform has become larger and the drawing speed has been increased. Further, the drawing furnace itself cannot avoid some unevenness along the circumferential direction structurally due to the presence of electrodes and gas flow paths. For this reason, temperature unevenness occurs in the optical fiber preform that is heated and melted, and the cross section of the optical fiber that is drawn after being melted and contracted does not become a perfect circle but becomes an oval shape. In general, the non-circularity “(major axis−minor axis) / average diameter” is used to represent the degree of non-circularity of the optical fiber. When the non-circularity is large, there is a problem that connection loss increases due to connection by an optical connector or fusion connection.

  In order to reduce the non-circularity of the optical fiber, for example, Patent Document 1 discloses adjusting the temperature distribution of a heater unit that heats an optical fiber preform through a furnace core tube. Patent Document 2 discloses a structure in which a heater for heating an optical fiber preform through a core tube is shaped so that the heating temperature is uniform along the circumferential direction. For example, as shown in FIG. 4, a cylindrical heater 101 having a heating part 102 meandering in the vertical direction with a slit 103 is provided, and a plurality of electrode connection parts 104 to 107 for power supply are provided, thereby generating heat in the circumferential direction. As an example, a method for making the thickness uniform is shown.

JP-A-8-208262 JP-A-9-71433

  Normally, the heating unit that heats and melts the optical fiber preform in the drawing apparatus includes, for example, a cylindrical heater 4 outside the core tube 3 in which the optical fiber preform 2 is accommodated, as shown in the schematic diagram of FIG. Arranged. The cylindrical heater 4 is assumed to be a heat generating portion 4a that zigzags in a zigzag manner by alternately inserting slits 4b similar to those shown in FIG. A pair of terminal portions 9a and 9b are provided at one end (lower end) of the cylindrical heater 4 so as to face each other with a 180 ° gap therebetween, and an attachment position 10a with respect to the pair of electrode portions 5a and 5b for supplying electric power. In 10b, the fixing member 11 is used for connection and fixing at the terminal portion interval L, and connection to the power supply circuit and attachment and fixing are performed.

  Since the cylindrical heater 4 is circular and is assembled so as to be concentric with the furnace core tube, at normal temperature, the interval S between the outer surface of the optical fiber preform 2 to be stored and the inner surface 4c of the cylindrical heater 4 is the circumference. Uniform in the direction, the outer diameter D of the cylindrical heater 4 is constant. However, since the cylindrical heater 4 has a temperature of up to about 2000 ° C., the cylindrical heater 4 is deformed in the radial direction in order to release strain caused by thermal expansion due to this temperature rise.

  At this time, deformation in the XX direction connecting the terminal portions 9a, 9b connected and fixed to the electrode portions 5a, 5b is suppressed, and the outer diameter D of the heater 7 does not change. However, the heat generating portion 4a between the terminal portions 9a and 9b swells in the YY direction and changes from the outer diameter D to E as indicated by a chain line. As a result, the distance S between the outer surface of the optical fiber preform 2 and the inner surface 4c of the cylindrical heater 4 is S = S1 in a portion where the outer diameter does not change, and S2 in a portion where the outer diameter E changes, and S = S1 < It becomes non-uniform in S2, the temperature distribution in the circumferential direction of the optical fiber preform 2 becomes non-uniform, and the heated and melted state is biased. For this reason, the non-circularity of the optical fiber to be drawn increases.

  In order to make this non-uniform temperature distribution uniform, the above-mentioned Patent Document 1 adjusts the heat retaining characteristics by partially moving the heat insulating material surrounding the heater, or Patent Document 2 has a plurality of electrode connection portions for fixing the heater. For example, the heater cross-sectional area is partially changed. However, these methods complicate the structure of the drawing furnace, and need to be significantly modified to be applied to an existing drawing furnace, resulting in high cost facilities.

  The present invention has been made in view of the above-described circumstances. An optical fiber drawing furnace and an optical device that can easily achieve uniform temperature distribution in the circumferential direction of heater heating using an existing cylindrical heater. The object is to provide a fiber drawing method.

An optical fiber drawing apparatus and a drawing method according to the present invention include a core tube supplied with an optical fiber preform, a cylindrical heater surrounding the core tube, an electrode unit for supplying power to the cylindrical heater, and a cylindrical heater. Is an optical fiber drawing using an optical fiber drawing furnace provided with a heat insulating material surrounding the outside and a furnace casing surrounding the whole. By this drawing, the temperature of the cylindrical heater is raised to a temperature at which the optical fiber preform is heated and melted, and the terminal portion of the cylindrical heater is used as an electrode portion at a position where the cylindrical heater is thermally expanded to a perfect circle. connect fixed, cylindrical heater was set to be elliptical in a state of not being heated, and the cylindrical heater during heating and melting of the optical fiber preform is set to be perfectly circular, heating the optical fiber preform It is characterized by melting.

The dimension between the terminal portions of the cylindrical heater at room temperature t0 is L, the outer diameter dimension of the cylindrical heater cylindrical portion is D, the temperature when the cylindrical heater is heated is t1, and the temperature of the cylindrical heater terminal is increased. When the temperature at the time is t2 and the thermal expansion coefficient of the cylindrical heater is α, the dimension L ′ between the terminal portions when the cylindrical heater is connected and fixed to the electrode portion,
L ′ = L + [D × (t1−t0) × α] + [(LD) × (t2−t0) × α].

  According to the present invention, a cylindrical heater manufactured in a normal shape is simply connected and fixed to the electrode portion at the position where the cylindrical heater is displaced in advance to a dimension in which the cylindrical heater is heated and thermally expanded. With this simple method, the non-circularity can be reduced. For this reason, since it is not necessary to modify the existing equipment, it can be easily carried out without increasing the cost.

It is a figure explaining the outline of the drawing furnace in this invention. It is a figure explaining an example of the cylindrical heater used for the drawing furnace of this invention. It is a schematic diagram of the cylindrical heater explaining the subject of this invention. It is a figure explaining a prior art.

  An embodiment of the present invention will be described with reference to FIGS. In the figure, 1 is a drawing furnace, 2 is an optical fiber preform, 3 is a core tube, 4 is a cylindrical heater, 4a is a heat generating part, 4b is a slit, 4c is an inner surface of the heater, 5a and 5b are electrode parts, and 6 is heat insulation. Material, 7 is a furnace casing, 7a is an upper cylinder, 7b is a lower cylinder, 8 is a base material supply mechanism, 9a and 9b are terminal portions, 10a and 10b are attachment positions, 11 is a fixing member, 12 is an insulating member, 13 , 14 is a gas inlet, and 15 is a gas outlet.

  As shown in FIG. 1, the drawing of the optical fiber is performed by heating the lower part of the optical fiber preform 2 that is supported in a suspended state, and melting and dropping the optical fiber 2a from the lower end that has become a small diameter by heating and melting. It is done by drawing to a diameter. For this purpose, the drawing furnace 1 is provided with a heating cylindrical heater 4 so as to surround the furnace core tube 3 to which the optical fiber preform 2 is supplied, and is insulated so that the heat of the cylindrical heater 4 is not dissipated to the outside. It is surrounded by a material 6 and the entire outside thereof is surrounded by a furnace casing 7.

  The optical fiber preform 2 is suspended and supported by a simulated preform feeding mechanism 8 and is sequentially controlled to move downward as the optical fiber is drawn. The furnace casing 7 is made of a metal having excellent corrosion resistance such as stainless steel, and a cylindrical furnace core tube 3 made of high-purity carbon is arranged at the center. The upper part of the furnace housing 7 is formed by an upper cylinder 7a, and the optical fiber preform 2 is introduced in a sealed state. In addition, the upper cylinder 7a is provided with a gas inlet 13 so that an inert gas such as helium gas or argon gas can flow in to prevent oxidation and deterioration in the furnace. This inert gas passes through the gap between the optical fiber preform 2 and the furnace core tube 3 and is discharged from the lower tube 7 b of the furnace casing 7.

  The furnace casing 7 can be cooled by providing a cooling water channel or the like, although not shown in the figure, except that the temperature of the furnace casing 7 is not increased by the heat of the cylindrical heater 4 by the heat insulating material 6. As a result, the furnace casing 7 can be kept near room temperature even during operation, and can be in a state in which there is substantially no variation in dimensions due to thermal expansion. In addition to the above gas inlet, a gas inlet 14 and a gas outlet 15 are provided in the furnace casing 7 so that the cylindrical heater 4 and the heat insulating material 6 do not deteriorate due to oxidation. An inert gas such as a gas is supplied.

In addition, a pair of electrode portions 5 a and 5 b are attached and fixed to the furnace casing 7 via an insulating member 12 in order to supply electric power to the cylindrical heater 4. The electrode portions 5 a and 5 b have a function of fixing and supporting the cylindrical heater 4 in addition to supplying power to the cylindrical heater 4.
The cylindrical heater 4 is provided with a pair of terminal portions 9a and 9b for supplying power at one end portion (the lower end portion in this example) at opposing positions separated by 180 °, and its attachment position (the center of the attachment hole). 10a and 10b are fixed to the electrode portions 5a and 5b by a fixing member 11 such as a screw.

  The present invention has the above-described configuration of the optical fiber drawing furnace 1 and is characterized by the mounting structure of the cylindrical heater 4, and the details thereof will be described with reference to FIG. The cylindrical heater 4 shown in FIG. 2 has the same structure as that described with reference to FIG. The cylindrical heater 4 is configured as a heating element using the electrical resistance of carbon. In this structure, the slits 4b are alternately inserted from the upper and lower directions of the cylinder, meandering in a zigzag manner to form a heat generating portion 4a having a predetermined cross section and length, and a predetermined resistance value is obtained. Moreover, the cylindrical heater 4 becomes a shape which is easy to expand-contract in a radial direction by inserting the slit 4b.

  A pair of terminal portions 9a and 9b are provided at one end (lower end) of the cylindrical heater 4 so as to face each other with a 180 ° separation, and are attached to a pair of electrode portions 5a and 5b for supplying power. The positions 10a and 10b are connected and fixed at a predetermined terminal portion interval L ′. Thereby, the connection and attachment of the cylindrical heater 4 to the power supply circuit are fixed. In the cylindrical heater 4 connected to the electrode portions 5a and 5b, current flows from one terminal portion 9a through the heat generating portions 4a on both sides in parallel to the other terminal portion 9b to cause the heat generating portion 4a to generate heat. .

  Here, in the state of room temperature before the cylindrical heater 4 is installed in the drawing furnace, for example, as described with reference to FIG. 3, the outer diameter D of the heat generating portion 4 a is in a perfectly circular state without deformation. It is assumed that the interval between the terminal portions 9a and 9b is L. If the cylindrical heater 4 is heated to a predetermined heat generation temperature (for example, 2000 ° C.) in a free state without being fixed to the electrode portions 5a and 5b, it is uniform in the radial direction due to the thermal expansion coefficient of the heater. Thermal expansion. This also increases the distance L between the terminal portions 9a and 9b.

In the present invention, when the cylindrical heater 4 is thermally expanded uniformly in the radial direction and the terminal portion interval L between the terminal portions 9a and 9b is expanded, the dimension L ′ is defined as L ′. As the interval, it is connected and fixed to the electrode portions 5a and 5b. That is, the cylindrical heater 4 is heated to a temperature at which the optical fiber preform is heated and melted, and the terminal portions 9a, 9b of the cylindrical heater are connected to the electrode portions 5a, Connect and fix to 5b.
As a result, when the drawing furnace 1 is not operated (not heated), the cylindrical heater 4 has an elliptical shape extended in the direction of the terminal portions 9a and 9b as shown in FIG. Thus, the outer diameter Dx in the XX direction of the terminal portions 9a and 9b is larger than the original diameter D, and the outer diameter Dy in the YY direction, which is 90 ° different from this, is smaller than the original diameter D. Retained.

  In this state, when the cylindrical heater 4 attached and fixed generates heat to a predetermined temperature and is heated, the cylindrical heater 4 expands in the radial direction in order to release distortion due to thermal expansion due to the temperature increase. However, in the XX direction of the terminal portions 9a and 9b, the terminal portions 9a and 9b are fixed to the electrode portions 5a and 5b and are prevented from being deformed, and thus swell in the YY direction. As a result, when the temperature is raised to a predetermined temperature, the outer diameter in the YY direction becomes F as shown by the chain line. At this time, if L ′ is set so that the outer diameter Dx in the XX direction in which the deformation is suppressed and the outer diameter F in the YY direction are equal, the cylindrical heater 4 becomes a perfect circle. It transforms to become.

  If the cylindrical heater 4 is operated at a predetermined heating temperature while maintaining a perfect circular state, the distance S1 between the optical fiber preform 2 and the inner surface 4c of the cylindrical heater 4 in the XX direction, and YY The distance S2 between the optical fiber preform 2 and the inner surface 4c of the cylindrical heater 4 in the direction is S1 = S2. Thereby, the optical fiber preform 2 is heated at a uniform interval S with respect to the cylindrical heater 4. As a result, the temperature distribution in the circumferential direction of the optical fiber preform 2 is uniform, and the non-circularity of the drawn optical fiber can be reduced.

Note that the dimension between the terminal portions of the cylindrical heater 4 at room temperature t0 is L, and the outer diameter dimension of the cylindrical heater cylindrical portion is D. The temperature when the cylindrical heater is heated is t1, the temperature when the terminal portion 9a (9b) of the cylindrical heater 4 is heated is t2, and the thermal expansion coefficient of the cylindrical heater 4 is α. In this case, the dimension L ′ between the terminal portions when the cylindrical heater 4 is connected and fixed to the electrode portions 5a and 5b is as follows.
L ′ = L + [D × (t1−t0) × α] + [(LD) × (t2−t0) × α]
It can be calculated by the following formula. When the cylindrical heater 4 is made of carbon, the thermal expansion coefficient of the isotropic carbon is a value of about (4-6) × 10 ⁻⁶ / ° C. in the operating temperature range.

  In recent years, in order to improve the transmission characteristics of optical communication, a dispersion-compensating optical fiber is used when optical communication is performed in a wavelength of 1.55 μm using a general single mode optical fiber for wavelength of 1.3 μm. Has been done. This dispersion compensating optical fiber has a structure in which the relative refractive index difference with respect to the cladding portion is increased by adding a dopant to the core portion, and the core diameter is 8 μm to 10 μm for a general optical fiber, whereas It is 2 μm to 6 μm.

  In an optical fiber having a core portion with a high refractive index such as a dispersion compensating optical fiber, polarization mode dispersion (PMD) is likely to occur. Further, when a dopant is added to the core portion, the glass forming the core has a low viscosity, so that the core portion is likely to be non-circular. Since the PMD of the dispersion compensating optical fiber increases in proportion to the non-circularity, it is particularly necessary to reduce the noncircularity in an optical fiber having a large non-refractive index difference such as a dispersion compensating optical fiber.

  In the present invention, as described above, the position where the pair of terminal portions of the commonly used cylindrical heater are fixedly connected to the electrode portions can be realized by a simple method with a slight change. For this reason, it is not necessary to modify the heating part or change the shape of the heater, and therefore can be easily carried out without increasing the cost. In particular, non-circularity is likely to occur, and the non-circularity needs to be kept small. It is useful when applied to the manufacture of certain dispersion compensating optical fibers.

DESCRIPTION OF SYMBOLS 1 ... Drawing furnace, 2 ... Optical fiber preform, 3 ... Core tube, 4 ... Cylindrical heater, 4a ... Heat generating part, 4b ... Slit, 4c ... Heater inner surface, 5a, 5b ... Electrode part, 6 ... Heat insulation material, 7 ... furnace casing, 7a ... upper cylinder, 7b ... lower cylinder, 8 ... base material supply mechanism, 9a, 9b ... terminal part, 10a, 10b ... attachment position, 11 ... fixing member, 12 ... insulating member, 13,14 ... Gas inlet, 15 ... gas outlet.

Claims (3)

A core tube to which an optical fiber preform is supplied, a cylindrical heater surrounding the core tube, an electrode portion for supplying power to the cylindrical heater, a heat insulating material surrounding the outside of the cylindrical heater, and the whole An optical fiber drawing furnace with a furnace casing surrounding
The cylindrical heater is heated to a temperature at which the optical fiber preform is heated and melted, and the cylindrical heater terminal portion is connected to the electrode portion at a position where the cylindrical heater is thermally expanded to a perfect circle. An optical fiber drawing furnace which is fixed and has an elliptical shape in a state where the cylindrical heater is not heated, and has a perfect circle shape in the heated state after the thermal expansion . The dimension between the terminal portions of the cylindrical heater at room temperature t0 is L, the outer diameter dimension of the cylindrical heater cylindrical portion is D, the temperature when the cylindrical heater cylindrical portion is heated is t1, and the cylindrical heater terminal When the temperature at the time of heating of the part is t2, and the thermal expansion coefficient of the cylindrical heater is α,
The dimension L ′ between the terminal portions when the cylindrical heater is connected and fixed to the electrode portion is:
L ′ = L + [D × (t1−t0) × α] + [(LD) × (t2−t0) × α]
The optical fiber drawing furnace according to claim 1, wherein A core tube to which an optical fiber preform is supplied, a cylindrical heater surrounding the core tube, an electrode portion for supplying power to the cylindrical heater, a heat insulating material surrounding the outside of the cylindrical heater, and the whole An optical fiber drawing method using an optical fiber drawing furnace provided with a furnace casing surrounding
The cylindrical heater is heated to a temperature at which the optical fiber preform is heated and melted, and the terminal portion of the cylindrical heater is connected to the electrode portion at a position where the cylindrical heater is thermally expanded to a perfect circle. fixed to the cylindrical heater was set to be elliptical in a state of not being heated, the cylindrical heater is set to be a true circle when heating and melting of the optical fiber preform, the optical fiber preform An optical fiber drawing method characterized by heating and melting the glass.

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