1. Field of Invention
The present invention relates to a method for drawing an optical fiber having reduced polarization mode dispersion (PMD). The present invention also relates to the single mode optical fiber made in accordance with the presently disclosed method.
Single mode optical fiber commonly used in communication systems is not purely single mode. Rather, two modes, with perpendicular polarizations, exist in single mode fiber. These two polarizations form an orthogonal basis set. Accordingly, any configuration of light that propagates through a single mode fiber can be represented by a linear superposition of these two modes.
If the fiber has perfect circular symmetry in both geometry and internal applied stress, the two polarization modes propagate with the same group velocity. They have no time delay difference after traveling the same distance in the fiber. But a practical fiber does not have perfect circular symmetry. Imperfections such as geometric deformation and stress asymmetry makes the two polarization modes to propagate with different velocities (a function of propagation constant). The difference between the propagation constants is termed as birefringence. The differential time delay between the two polarization modes is called PMD (polarization mode dispersion). This limits the high bit rate transmission in the communication system.
2. Description of Prior Art
According to prior art of spinning the fiber mentioned in U.S. Pat. No. 5,418,881; the fiber is subjected to a sinusoidal spin function, which varies with the spatial distance of the fiber drawn. However, this patent does not teach about the discrete spin rates and a trapezoidal wave function of the spin rate with the spatial distance of the fiber.
In another published PCT application bearing No. WO/97/26221, the authors have identified the disadvantage of above-cited U.S. Pat. No. 5,418,881. The major disadvantage identified is being in the difference of beat lengths along the different lengths of the fiber. A particular combination of spin amplitude and spin frequency will lower the PMD effectively for a particular beat length. However, this published PCT application WO/97/26221 does not teach the manner for spinning the fiber, particularly after coating and twisting the fiber so as to achieve effective lowering of the PMD of the fiber, particularly cabled fiber. The author of this published PCT application has not visualized that the use of combination of discrete spin rates of different values, which repeats itself in a periodic fashion can effectively lower the PMD of the cabled fiber.
The present invention now makes a disclosure of a method by which it is possible to achieve the reduced PMD in optical fiber as low as less than about 0.1 ps/√km and the post cabling PMD to be as low as less than about 0.3 ps/√km when the spinning is performed on a fiber, particularly on a drawn, coated and twisted fiber by employing the spin function, which is a combination of four discrete spin rates which repeat itself in a periodic fashion after a certain length of the fiber drawn and the spin rate of each spin function varies in a trapezoidal manner along the drawn length of fiber. The fibers drawn from performs in accordance with the method of the present invention have ovality at max about 3%.