Method and Apparatus for Fabricating an Oval Cross-Sectional Optical Waveguide

The invention concerns a method and apparatus for fabricating an optical waveguide with an oval cross-section and wherein the core and the cladding are vapor deposited to the inside surface of a quartz glass substrate tube.

Optical waveguides fabricating processes of the above type are known, (Essay: "Production of preforms for optical fibers" Vol. 62, no. 3/4, Electrical Communications of 1988).

The effort in known optical waveguides is concentrated on producing an optical waveguide core with a precisely circular cross-section. Any deviation from the circular geometry of the optical waveguide leads to problems with the propagation of light, and therefore to problems with the transmission of the information.

The light that is used to transmit the information may have two perpendicular directions of polarization, which have equal use in the transmission of information through an optical waveguide with a precisely circular core.

However, with the transmission of information through optical waveguides, instances of application can be envisioned in which only one direction of polarization is desired, and the other direction of polarization is suppressed, because in this way a signal can be routed over a long distance without major interference.

The basic technical problem of the invention consists of an apparatus and method to produce an optical waveguide in which the light dispersion is preferred in one direction of polarization, while most of the light dispersion is suppressed in the other direction of polarization.

This technical problem is solved by the invention, in that the core and the cladding have an oval cross-section. In such an optical waveguide, the transmission of information is preferred in the direction of polarization that coincides with the direction of the major axis of the oval.

A method and apparatus for fabricating such an optical waveguide is disclosed. The method comprises vapor deposition of glass layers to the inside of a substrate tube of quartz glass, wherein two diametrically opposed surface areas of the substrate tube are cooled during the glass deposition. This cooling greatly inhibits glass deposition on the cooled inside surface areas of the substrate while the remaining inside areas further away from such cooling have correspondingly greater glass deposition, hence resulting in the core and cladding of the optical waveguide produced as having a generally oval or elliptical cross-section. The apparatus for carrying out this method uses a glass lathe to rotatably hold the substrate tube, wherein a gas burner is mounted on a support movable along the substrate tube and consists of two opposite nozzles directed toward the substrate tube. A cooling device supplies cooling gas through the nozzles toward the substrate tube. The cooling device rotates synchronously with the substrate tube as the tube is rotated by the lathe.

The following explains the invention by means of FIGS. 1 to 3, where:

FIG. 1 schematically shows part of a device for producing an optical waveguide preform according to the modified chemical vapor deposition (MCVD) process, with the cooling device according to the invention;

FIG. 2 shows a cross-section of the cooling device according to the invention;

FIG. 3 shows a longitudinal section of the device in FIG. 2.

FIG. 1 depicts the part of a device that is essential for understanding the invention, which is used for the production of optical waveguide preforms according to the modified chemical vapor deposition (MCVD) process. It consists of a spindle 1 of a glass lathe, in which the substrate tube 2 is clamped in the chuck, and the artificial glass layers for the core and the outer cladding of the optical waveguide are deposited on the inside surface. The oxyhydrogen gas burner 4 is located on the support 3 and heats the rotating substrate tube 2, thus triggering the `thermophoretic` process for the production of the artificial glass layers. The cooling device 5 is also located on the support, next to the oxyhydrogen gas burner 4, and cools two diametrically opposed surface areas of the substrate tube 2, as explained below.

FIG. 2 illustrates the principal construction of the cooling device 5, which contains the fixed ring-shaped housing 6. The wheel 7 can rotate in the housing 6; it has a toothed rim on its periphery, which engages the pinion gear 8. The wheel 7 has an annular channel 9 on the inside, which ends at the nozzles 10 that are directed toward the periphery of the substrate tube 2. Cooling gas, for example nitrogen, is supplied through the annular channel 9 to the nozzles 10. Since the wheel 7 and the substrate tube 2 always rotate synchronously at the same speed, e.g. 30 min.sup.-1 (30 revolutions/min), the cooling gas from the nozzles 10 is always directed to the same surface areas of the substrate tube 2. The heating of the substrate tube 2 and the simultaneous cooling of certain surface areas produces an elliptical temperature profile in the substrate. For that reason, the artificial glass layers have a corresponding distribution, i.e. an artificial glass layer is produced on the hot inside surface of the substrate tube, and none, or only a small amount of artificial glass, is produced on the cooled inside surface. The core and cladding of an optical waveguide produced with such a preform have an oval or generally elliptical cross-section. In a first approximation, an elliptical optical core only permits the passage of the light in one direction of polarization.

Details of the cooling device 5 can be found in FIG. 3. The wheel 7 passes through a web 11 in a groove of housing 6. The housing 6 is closed by a housing cover 12, which also contains an annular channel 13. On one side, this annular channel 13 is connected with a cooling gas tank through a tube line 14, and on the other it is connected with the annular channel 9 through branch connection 15. The pinion gear 8 is coupled to the motor 17 by shaft 16.