JP2000327361A - Method for drawing optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an optical fiber comprising such a practice that a preform having an optical propagation portion is brought into a drawing oven and held with a chuck part and then melted under heating while circumferentially rotating the preform normally/reversely followed by conducting a drawing so as to enable PMD minimization of the final optical fiber even if involving asphericity coat thickness deviation, periodical torsion, etc., in the optical fiber. SOLUTION: This method for producing an optical fiber 6 comprises the following practice: a preform 3 with a clad portion outside a core portion for optical transmission is held with a chuck part 2 via a dummy rod provided on the upper part of the preform and brought into an oven 4, which is subsequently raised in temperature to melt the preform 3 followed by centering and then spinning it; wherein the preform 3 is drawn while circumferentially rotating it normally/reversely by a motor 1 via the chuck part 2; in this case, the normal/reverse rotation is carried out pref. at a maximum angle of 30 deg. periodically pref. at a frequency of about 20 reciprocations/min; thereby applying anisotropic strain in a pseudorandom fashion to the core portion to effect substantial minimization of PMD of the resultant optical fiber which is then subjected to protective coating of silicone resin or the like using a resin coater 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber, and more particularly to a method for drawing an optical fiber so as to minimize polarization dispersion.

[0002]

2. Description of the Related Art FIG. 2 is an explanatory view showing a conventional optical fiber drawing method. Reference numeral 12 denotes a chuck portion, 13 denotes a preform, 14 denotes a drawing furnace, 15 denotes a resin coating device, 16 denotes an optical fiber, and 17 denotes a winding drum.

[0003] The preform 13 has a core portion and a clad portion for transmitting light therein. First, a method for manufacturing the preform 13 will be described below.

First, a porous preform is manufactured. This is S
A glass raw material such as iCl _{4 is} subjected to a flame hydrolysis reaction to generate glass fine particles SiO ₂ , and the glass fine particles are deposited on a rotating starting member, and are grown by growing a porous body in a rotation axis direction. The core of the porous base material contains GeO ₂ to increase the refractive index. The clad portion is formed only of glass fine particles SiO ₂ . The core portion and the clad portion are simultaneously deposited, and the clad portion is deposited from above the core portion.

Next, the SiO ₂ -GeO ₂ core / Si
The O ₂ clad porous base material is sintered in a high-temperature electric furnace to form a transparent glass, and then stretched. This stretched body is a glass rod for an optical fiber including a light propagation part, and is simply referred to as a glass rod. The surface of the glass rod is flame polished,
Impurities and the like are removed.

Then, while rotating the glass rod, glass fine particles of SiO ₂ are deposited on the outer periphery thereof, and sintered in a high-temperature electric furnace to form a transparent glass. Thus, the preform 13 (optical fiber preform) can be obtained. The reason for depositing the glass particles of SiO _{2 on} the glass rod is to make the size of the core portion a predetermined size when the optical fiber is drawn to an outer diameter of 125 μm. Note that the preform 13 can be manufactured collectively, that is, without manufacturing a glass rod in the middle depending on the manufacturing apparatus of the porous base material.

The preform 13 has a branch bar (dummy bar) fused to the upper part according to the size of the drawing furnace 14 and is held by the chuck unit 12. When the temperature of the drawing furnace 14 is increased and the necessary centering is completed, the drawing is started. The centering means that the preform 13 is drawn with the drawing furnace 14 and the resin coating apparatus 1.
5 is adjusted. The preform 13 is usually melted and spun into an optical fiber having an outer diameter of 125 μm, and coated with a silicone resin or the like by a resin coating device 15 for protection. The manufactured optical fiber 16 is wound on a winding drum 17, and the drawing step is completed. The preform 13 is fed into the drawing furnace 14 while being held by the chuck portion 12, but is drawn without being rotated.

[0008]

Since the preform is manufactured while rotating around the core, a uniform product can be obtained in the circumferential direction. This can be confirmed by measuring the eccentricity (the distance between the center of the core and the center of the clad) and the non-circularity of the core and the clad of the manufactured optical fiber. However, since it is uniform in the circumferential direction, there is anisotropic strain in the circumferential direction, for example, uneven thickness of the coating material (coating resin) at the time of drawing, or periodic twisting at the time of forming a cable causes an optical fiber. In addition, there is a problem that the polarization dispersion (Polarization Mode Dispersion, hereinafter abbreviated as PMD) of the optical fiber increases. Of course, if the core and cladding are non-circular, the PMD will increase, but even if the preform is made uniformly and the core and cladding are completely circular, the PMD will still be affected by the above-described anisotropic strain. The problem was getting bigger.

In this regard, in the conventional optical fiber drawing method, since the preform is drawn without rotating, it is impossible to solve the above-mentioned problem of PMD.

The PMD refers to the difference in arrival time at the optical fiber output end when two orthogonal linearly polarized lights are simultaneously incident on the optical fiber. The unit is psec / √km. That is, when anisotropic strain is applied in the circumferential direction, the refractive index distribution in the circumferential direction becomes non-uniform, and the arrival time of linearly polarized light depends on the incident azimuth. In other words, a phenomenon occurs in which linearly polarized light incident on an optical fiber at a specific azimuth angle propagates faster than linearly polarized light incident on other azimuth angles. The incident azimuth angle at which the arrival time becomes the shortest is referred to as an eigenaxis. The propagation time difference between linearly polarized light incident on this eigenaxis and linearly polarized light incident on an axis perpendicular to this eigenaxis is called polarization dispersion, and is proportional to the magnitude of anisotropic strain applied to the optical fiber. I do.

When the PMD becomes large, for example, when communication is performed using circularly polarized light, circularly polarized light is a combination of two orthogonal linearly polarized lights, and therefore reaches two orthogonally linearly polarized lights. Due to the time difference, the light is no longer circularly polarized at the emission end, and the signal light containing information is degraded, resulting in a narrow transmission band. Even when communication is performed using one linearly polarized light, mode conversion (transfer of power) occurs due to the presence of anisotropic distortion, and two orthogonal linearly polarized lights are generated.
Due to the difference in their arrival times, the signal light containing information is deteriorated in the same manner as described above, and as a result, the transmission band is narrowed. This phenomenon becomes a problem particularly when optical communication is performed over a very long distance such as a submarine cable.

Accordingly, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide an optical fiber having a non-circular core or cladding, and a coating material having uneven thickness at the time of drawing. Another object of the present invention is to provide a method of drawing an optical fiber in which PMD (polarization dispersion) is minimized even if the optical fiber is twisted periodically when the cable is formed.

[0013]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an optical fiber for holding and holding a preform having a light transmitting portion in a chuck portion and feeding the preform into a drawing furnace to melt and spin the preform. In the drawing method, the preform was drawn while rotating in the forward and reverse directions in the circumferential direction.

The rotation angle in the forward and reverse directions was a maximum of 30 degrees and the rotation was performed periodically.

The period was set to approximately 20 reciprocations per minute.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory view showing an embodiment of a method for drawing an optical fiber according to the present invention. 1 is a motor,
Reference numeral 2 denotes a chuck unit, 3 denotes a preform, 4 denotes a drawing furnace, 5 denotes a resin coating device, 6 denotes an optical fiber, and 7 denotes a winding drum. The method for producing the SiO ₂ —GeO ₂ core / SiO ₂ clad porous preform and the method for producing the preform 3 are the same as in the prior art.

The preform 3 has a branch bar (dummy bar) fused to the upper part according to the size of the drawing furnace 4 and is held by the chuck unit 2. When the required centering is completed by raising the temperature of the drawing furnace 4, the drawing is started. Preform 3
Is usually melted and spun into an optical fiber having an outer diameter of 125 μm, and is coated with a silicone resin or the like for protection.
Coated with. The manufactured optical fiber 6 is
It is wound on the winding drum 7 and the drawing process is completed.

During drawing, the preform 3 is drawn at a drawing speed of 200 m / min while the motor 1 performs a forward / reverse rotational movement at a maximum of 10 degrees in the circumferential direction at a rate of 20 reciprocations / min. The PMD of the optical fiber 6 actually obtained is 0.4 psec.
/ √km, which is about 30% lower than the PMD of an optical fiber manufactured by a conventional optical fiber drawing method. In addition, there was no particular difference in other characteristics such as eccentricity.

The reason why the PMD is reduced can be explained as follows. As described above, at the time of drawing, the preform 3 is rotated forward and backward at a rate of 20 reciprocations / minute at a maximum of 10 degrees in the circumferential direction. As a result, a twist is forcibly applied to the optical fiber in a pseudo-random manner. Although this twist does not affect characteristics such as eccentricity and transmission loss, it functions to apply anisotropic strain to the core in a pseudo-random manner. Then, mode conversion (power transfer) is positively and pseudo-randomly performed between two orthogonal linearly polarized waves. In such a case, since the mode conversion to the eigen axis where the PMD becomes the maximum becomes stochastically low, the PMD becomes substantially the minimum. This is proved by the theory of established statistics.

The angle at which the preform is rotated in the circumferential direction is 30 degrees at the maximum. Beyond this angle, the added twist will increase transmission losses. In the embodiment, the cycle is set to 20 reciprocations / minute,
Increasing the number of round trips affects transmission loss,
If less than this, the effect is reduced.

[0021]

According to the optical fiber drawing method of the present invention, since the preform is drawn while rotating the preform in the forward and reverse directions in the circumferential direction, even if the core portion or the clad portion of the optical fiber is noncircular. However, even if the coating material was uneven in thickness at the time of drawing, or the optical fiber was twisted periodically at the time of the cable, an optical fiber having the minimum PMD (polarization dispersion) could be obtained. As a result, it is possible to improve the product yield of the optical fiber and to reduce the optical fiber manufacturing cost.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing one embodiment of an optical fiber drawing method of the present invention.

FIG. 2 is an explanatory diagram showing a conventional optical fiber drawing method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Motor 2, 12 Chuck part 3, 13 Preform 4, 14 Drawing furnace 5, 15 Resin coating device 6, 16 Optical fiber 7, 17 Winding drum

Claims (3)

[Claims] 1. A method of drawing an optical fiber, comprising holding a preform having a light transmitting portion in a chuck portion and feeding the preform into a drawing furnace, and melting and spinning the preform. A method for drawing an optical fiber, characterized by drawing while rotating in the opposite direction. 2. The method for drawing an optical fiber according to claim 1, wherein the rotation angle in the forward and reverse directions is a maximum of 30 degrees and the rotation is performed periodically. 3. The method for drawing an optical fiber according to claim 2, wherein the period is about 20 reciprocations per minute.