Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-06-15
2004-11-02
Zarroli, Michael C. (Department: 2839)
Optical waveguides
Optical fiber waveguide with cladding
C385S123000
Reexamination Certificate
active
06813424
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a movable optical communications link having at least one optical fiber; in particular, for use in transmitting information or performing interferometric measurements.
BACKGROUND OF THE INVENTION
Optical fiber links used to transmit information via light have significant advantages, both for long transmission links in telecommunications, as well as for short transmission links inside buildings, vehicles, and machines, not to mention in electronic calculating machines, since they ensure high data transmission density accompanied by low power losses. Due to their thin, flexible, but mechanically very durable construction, incoming optical fiber lines and outgoing optical fiber lines are beneficial, particularly for connecting optical sensors for measuring physical parameters, such as pressure and temperature, etc. In addition, unlike electrical connections, such optical fiber links cannot cause any electrical sparkovers or short circuits. The high transmission capacity of the optical fibers makes it possible to modify or replace the sensors and measuring devices without having to replace the communication links. This can result in considerable cost savings in vehicles, buildings, machines, or production facilities. There is often the need for optical fiber links to be mechanically movable, such as when installed in robots. Further, in buildings and vehicles, one frequently encounters motion among components due to strain or expansion.
Therefore, optical fiber links for transmitting information are always of great benefit when there is a need to transmit high information densities and a mechanically flexible connection is required, since the distance between the sender and receiver of the information varies as a function of time.
A difficulty that arises that significant changes in the position of the transmitter and/or of the receiver, and, in particular, in their relative distance spanned by optical communication links constituted as simple cable, can cause the entire system, such as a remote-controlled robot, to be obstructed by the requisite reserved length of cable. Individual components, which communicate with one another via an optical communications link, can become mechanically blocked by loops of cable. Another difficulty is that one can end up with a “cable salad”.
Another difficulty encountered in response to variations in the position and distance of transmitters and/or receivers involves the nature of the optical transmission signal.
In communications transmissions of high quality and transmission frequency, it is necessary to control the polarization state of the optical information flow in the optical fiber, as well as in the other optical components. In the case of coherent transmissions, for example, phase-coherent mixing of the optical information flow with other light sources must be carried out. Such phase-coherent mixing is only optimal when the polarization states are substantially identical. When working with high bit-rate transmissions, the polarization mode dispersion of the fibers limits the reception quality, and transmission frequency can only be increased by carefully controlling the polarization. In many other optical components as well, the performance is a function of the polarization of the light.
Generally, the polarization state of the light in an optical fiber is not constant. Each glass fiber has a certain elliptical birefringence, so that the polarization of the light continually changes in the fiber. This variation propagates through to the end of the fiber, and, since it is dependent upon the spatial geometry of the fiber curve, the polarization state at the output end of a moving fiber varies with the motion.
In known methods this polarization effect can be avoided in that the optical communications transmission takes place in one of the intrinsic modes of a polarization-maintaining fiber. These polarization-maintaining fibers are characterized by pronounced birefringence, so that there is virtually no coupling over between the two polarization modes in the fiber. Since a change in the polarization of the light in an optical fiber is a phase shift effect between the intrinsic modes of the light, the polarization mode dispersion does not occur when the light in the fiber propagates through permanently in one intrinsic mode only.
The drawback of this method is that the polarization-maintaining fibers are expensive. Moreover, the light must be launched at the input ends of the polarization-maintaining fiber in a defined polarization state.
SUMMARY OF THE INVENTION
The present invention provides an optical communications link which can overcome the above-described difficulties and problems. To ensure a high transmission quality, the polarization state of the light should not depend substantially on changes in the form of the communications link and, therefore, on changes in the position of the transmitters and receivers. In addition, the communications link should be easily adaptable to changes in form, in particular to variations in length, but, it in this context, always be characterized by a straightforward arrangement.
In the present invention, the optical communications link having at least one optical fiber, in particular for communications transmission, where the optical fiber is repeatedly bent or curved and, in the process, can be wound in a helical shape, alternating as a right-hand and left-hand helix, fiber sections having a right and left curvature being distributed in such a way over the communications link that the average torsion of the fiber over the communications link is approximately zero.
The optical communications link of the present invention can be designed so that the sensitivity of the polarization state of the optical transmission signal to changes in the form of the communications link and, i.e., of the optical fibers, is substantially compensated. This is assured by the present invention in that the optical fiber is repeatedly bent, fiber sections having left-hand and right-hand curvature being distributed in such a way over the communications link that the average torsion of the fiber over the communications link is more or less zero. Preferably, this also holds for individual subsections of the fiber, so that left and right curvatures are uniformly distributed over the fiber. By preference, the fiber is wound in a helical shape, alternating with a right-hand and left-hand helix. Mixed forms having an even meander shape are also possible.
The present invention concerns the motion- and form-dependent birefringence of an optical fiber: the linear birefringence is heavily dependent upon the ellipticity are the fiber core, less heavily dependent upon the bend of the fiber, and hardly dependent upon the helical winding, given a large radius of the fiber. In contrast, the circular birefringence is hardly dependent upon the ellipticity of the fiber core and on the curve of the fiber, on the other hand, very heavily dependent upon the helical winding of the fiber. The main reason for the form dependency of the polarization state at the output end of an optical fiber is the considerable dependency of the fiber's optical activity upon the exact form of its helical windings. In the first approximation, this effect is achromatic and does not result in any polarization mode dispersion. It is caused by one of the so-called optical Berry phases, the “spin redirection phase” (R. Y. Chiao, Y. S. Wu, Phys. Rev. Lett. 57, 933 (1986)). This Berry phase (or geometric phase) is a phase effect produced by the structure of the fiber's space curve and not by an optical path, as is the case with the normal dynamic phase of the light. Nevertheless, with respect to interference of the light, geometric phases have the same properties as the normal dynamic phase.
The size of the spin redirection phase in a helically wound fiber is equivalent to the solid angle &OHgr; that the k vector (k corresponds to the propagation constant &bgr; in the technical literature) wraps around on
Dultz Gisela
Dultz Wolfgang
Frins Erna
Schmitzer Heidrun
Deutsche Telekom AG Bonn
Zarroli Michael C.
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