Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-11-09
2004-05-18
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S477000, C356S073100
Reexamination Certificate
active
06738146
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for measuring the spin in an optical fibre by irradiating an optical fibre with light so as to form an interference pattern.
DESCRIPTION OF RELATED ART
Such a method is known per se from European patent no. 0 785 913 which has been granted to the present applicants. According to the method disclosed therein, as described in embodiment 4, the outer surface of the preform from which the fibre is drawn is provided with a short score-line extending substantially parallel to the preform's longitudinal axis. Subsequently, a test optical fibre is drawn from this scored portion of preform by heating the preform to a temperature above the plasticizing temperature thereof. Since a rotation is imparted to the fibre during said drawing process, the score-line will rotate along with the fibre material when spin is being imparted to the fibre. When the protective coating is chemically removed from such a test optical fibre and the fibre is subsequently transversely irradiated by a laser source, for example a HeNe laser, the laser light will produce a diffraction pattern on a screen disposed behind the irradiated portion of the fibre. The presence of lateral asymmetry, viz. the score-line which is also present in the optical fibre that is formed in this manner, produces a characteristic diffraction pattern with a clear visually detectable intensity maximum. Said intensity maximum undergoes a visible shift upon slow manual rotation of the fibre around its longitudinal axis. By slowly moving the laser source slowly along a given length of the fibre and subsequently monitoring the angle through which the fibre is to be hand-rotated so as to keep the diffraction pattern constant, it is possible to measure the spin amplitude, viz. the maximum spin angle which has been imparted to the fibre, and the so-called spatial period. Thus it is possible to determine the number of rotations per unit length of the optical fibre on the basis of the interference pattern or diffraction pattern. Such a method has appeared to be labour-intensive and time-consuming in practice. Moreover, such a method must be carried out separately for each drawing tower, because each drawing tower possesses characteristic process parameters. Another drawback is the fact that such a measurement is not carried out during the actual drawing process, so that any undesirable deviations in the spin of the optical fibre cannot be directly corrected.
The term Polarisation Mode Dispersion (PMS) relates to the dispersion of a signal that propagates through an optical fibre, in particular a so-called single mode fibre, as a result of birefringence in the core portion of the fibre. This birefringence is generally caused by imperfections in the fibre, such as a so-called ovality or non-circularity of its core cross-section, asymmetrical lateral stress, etc. In a single mode fibre, the light can propagate in two orthogonal modes (two polarisation directions). If the fibre core possesses the aforesaid imperfections, one of said modes will propagate through the fibre faster than the other mode. This results in a difference in delay, as a result of which dispersion will occur. The PMD value indicates the difference in delay between the two polarisation directions. In general it obtains that the higher the PMD, the poorer the fibre quality. Thus it is desirable to enhance the PMD, for example by rotating the fibre, which is still slightly plastic, as it is drawn from the heated preform, so that a spin becomes “frozen” into the fibre as it cools, The resulting stress in the fibre produces continuous mode-coupling between the orthogonal polarisation modes of a carried signal, thereby inhibiting the accumulation of a significant phase lag between the two modes, which thus causes a significant reduction in the fibre's PMD.
In practice it has become apparent that an optical fibre's PMD is too high at some positions in the optical fibre. Analysis of this problem has shown that the overall PMD, measured over the entire fibre length, can meet the desired low value requirement indeed, but that the PMD of the individual parts formed by cutting the fibre into short lengths may no longer range within the desired specifications. From this it can be concluded that the effects that lead to a higher PMD can possibly be averaged out when the length of the optical fibre increases.
Methods for manufacturing optical fibres having a low PMD are generally known from the prior art, for example from U.S. Pat. No. 5,298,047, European patent application no, 0 795 521, European patent application no. 0 744 636, International patent application no. WO 97/26221 and International patent application no. WO 98/46536, which optical fibres can all be subjected to the present method for measuring the spin.
Thus it is desirable to develop a method which measures the spin in an optical fibre continuously, so that the optical fibre's PMD will fall within the required specification for any fibre length. In addition, it is desirable to develop a method for measuring the spin in the optical fibre wherein the method does not required a so-called test run on a drawing tower but which allows direct use in any drawing tower.
In addition to that it is desirable to develop a method for measuring the spin in an optical fibre which can be used during the fibre production.
SUMMARY OF THE INVENTION
According to the invention, the method as referred to in the introduction is characterized in that the ovality of the optical fibre, which results in a continuously changing interference pattern, is used for determining the spin in the optical fibre.
Since the optical fibre possesses an inherently small ovality or non-circularity, the rotation or spin that is imparted to the optical fibre will result in a continuous diameter change when using an interferometric measuring method, wherein the fibre to be measured is continuously irradiated with light so as to produce the interference pattern. This optical technique comprises the irradiation of the fibre in a direction perpendicularly to the direction of movement of the fibre, thus producing an interference pattern as a result of the superposition of light being reflected from the fibre surface and light being deflected from the fibre body. The interference pattern will in fact be a function of the wavelength of the incident light and of the refractive index values and the diameters both of the fibre core and of the fibre cladding.
According to the present method, the shift of the interference pattern is preferably measured at an angle of 48-72° to the incident light beam.
This can be followed in real time, as it were, without complicated calculations being required, as is the case with U.S. Pat. No. 5,309,221. By using this real time information, which is available upon measurement of the shifts in the interference pattern, it is possible to determine the amount of rotations or spin in the fibre, even at high velocities of movement of the optical fibre. Thus it is possible with the present invention to measure the spin in an optical fibre at the time of the production thereof.
The term “spin” as used in connection with the present invention relates to the rotations or twists that have been imparted to the optical fibre. Said terms are interchangeable within the framework of the present introduction to the disclosure.
Although a method for measuring the diameter of a transparent filament on the basis of an interference pattern is known from the aforesaid U.S. Pat. No. 5,309,221, it can be established that the method that is known therefrom is unsuitable for use at high throughput speeds of the optical fibre. The reason for this is that the detected interference pattern undergoes a number of mathematical calculations, as a result of which the response time of such a measurement is relatively long, which makes it unsuitable for high throughput speeds.
It is especially preferable to measure the interference pattern during the drawing process for producing the optical fibre f
Fianen Jozef Wilhelmus Quirinus
Groenewoud Marco
Draka Fibre Technology B.V.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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