1. Technical Field
The present invention relates to an optical fiber glass base material manufacturing apparatus or optical fiber drawing apparatus, particularly to a glass base material hanging mechanism that can be used to obtain a product with high precision and low curvature, eccentricity, and non-circularity.
2. Related Art
Optical fiber includes a core, which is a central portion with a high refractive index through which light passes, and a cladding, which has a low refractive index and surrounds the core. Quartz glass is the main material used in modern optical fiber to be used for optical communication, and this optical fiber usually has a core diameter of approximately 9 μm and a cladding diameter of approximately 125 μm.
The important characteristics desired for a communication optical fiber are low non-circularity for the cladding and low eccentricity for the core center relative to the cladding center (referred to hereinafter simply as “eccentricity”), for example. If the non-circularity and eccentricity are high, the cores of optical fibers are skewed from each other when the optical fibers are connected, and this results in high connection loss.
The optical fiber is manufactured by heating and elongating a cylindrical glass rod, known as optical fiber glass base material (referred to hereinafter simply as “glass base material”), from the bottom of the glass base material, and the non-circularity and eccentricity of this glass base material serving as the raw material are important for the resulting optical characteristics. Furthermore, if the glass base material is bent or skewed from a vertical orientation when being placed in the optical fiber manufacturing apparatus, the eccentricity and non-circularity of the resulting optical fiber will be even larger than the eccentricity and non-circularity of the glass base material serving as the raw material.
The optical fiber glass base material is manufactured through a process including a deposition step in which VAD or OVD is used to manufacture a porous glass base material, a sintering step in which the porous glass base material is sintered, and an elongation step in which the large glass base material obtained from the previous step is heated and elongated to be a size suitable for use as an optical fiber. This process is usually performed with the glass base material hanging vertically. For example, when VAD is performed, soot is deposited on the tip of a starting member that is hung vertically to manufacture the porous glass base material. Furthermore, when vertical OVD is performed, soot is deposited on the side surfaces of a starting member that is hung vertically to manufacture the porous glass base material. When the sintering is performed, the porous glass base material obtained from the VAD or OVD is hung vertically and undergoes transparent vitrification in a heating furnace. When the elongating is performed, the glass base material is hung vertically and heated.
In order to reduce the eccentricity and non-circularity, it is extremely important to hang the glass base material and starting member with a precisely vertical orientation. For example, when the porous glass base material undergoes transparent vitrification during the sintering, if the vertical orientation of the porous glass base material cannot be ensured, one side of the glass base material ends up nearer the heating furnace, and the temperature of this portion is higher than the temperature of other portions in the circumferential direction. This portion with a higher temperature experiences greater progression of the transparent vitrification, and this results in uneven contraction of the porous glass in the circumferential direction, thereby causing bending, eccentricity, and non-circularity.
Furthermore, if the glass base material is skewed from the vertical orientation during the elongation step, the heating state in the circumferential direction is uneven, and therefore bending and non-circularity occur when the pulling force for elongation is applied. Yet further, if the glass base material is skewed from the vertical orientation during the drawing step, the flow of gas in the furnace becomes asymmetrical, and this results in eccentricity and non-circularity of the fiber as well as bending known as “fiber curl” and diameter fluctuation in the optical fiber.
For the reasons shown above, it is extremely important that the glass base material and starting base material be hung vertically in each apparatus. Furthermore, if the connection portions between the glass base material or starting base material and the apparatus are not securely connected, the glass base material could be easily skewed from the vertical orientation by rotational force or pulling force, which results in the same effects as when the glass base material is skewed from the vertical position from the beginning.
A conventional glass base material hanging mechanism includes a wedge-shaped notch on a side surface of a hanging component (glass base material handle portion) and uses a hanging pin to align and connect a horizontal hole formed in a hanging shaft tube with the notch of the hanging component, such as shown in Patent Documents 1 and 2, for example. Furthermore, as shown in Patent Document 3, there is a glass base material hanging mechanism that includes a horizontal hole passing through the central axis of a hanging component and uses a hanging pin to align and connect a horizontal hole formed in a hanging shaft tube with the horizontal hole formed in the hanging component.