1. Field of the Invention
The present invention relates to controlling birefringence in an optical waveguide, and in particular in a silicon rib waveguide structure.
2. Description of the Related Art
As is well known, birefringence represents a significant problem in optical waveguides. Birefringence can result from a number of different sources each of which cause light polarised in a different manner to be subjected to different refractive indices. This results in light of different polarisations being transmitted differently by the waveguide with the result that the behaviour of a device receiving light with a random polarisation, and in particular transmission losses, become unpredictable. Some well known sources of birefringence are the crystalline structure of waveguides, the shape of the waveguide (in terms of its light guiding cross section), and stress and strain induced as a result of any bends, substrate discontinuations etc. in the path of the waveguide.
Rib waveguide structures manufactured on a silicon on insulator chip are known. One such arrangement is described for example in PCT Patent Specification No. WO95/08787. This form of waveguide provides a single mode, low loss (typically less than 0.2 dB/cm for the wavelength range 1.2 to 1.6 microns) waveguide typically having dimensions in the order of 3 to 5 microns which can be coupled to optical fibres and which is compatible with other integrated components. This form of waveguide can also be easily fabricated from conventional silicon-on-insulator wafers (as described in WO95/08787 referred to above) and so is relatively inexpensive to manufacture. That waveguide already exhibits lower birefringence than some other waveguide structures used in integrated optics, such as LiNbO3. Nevertheless, it is an aim of the invention to further reduce or remove birefringence in structures of this type.
According to one aspect of the present invention there is provided a method of controlling birefringence in a rib waveguide structure manufactured in silicon, the rib waveguide structure comprising an elongated rib element having an upper face and two side faces, the method comprising: forming a blanket layer of silicon nitride to a predetermined thickness over said rib waveguide structure directly abutting said upper face and side faces.
Preferably, the blanket layer of silicon nitride extends over the substrate flanks on either side of the rib waveguide structure.
Another aspect of the invention provides a method of controlling birefringence in a rib waveguide structure manufactured in silicon, the rib waveguide structure comprising an elongated rib element having an upper face and two side faces, the method comprising: growing a layer of oxide over the upper face and side faces; stripping the oxide layer to reveal the upper face and side faces; and forming a layer of silicon nitride to a predetermined thickness over said rib waveguide structure directly abutting said upper face and side faces.
Another aspect of the present invention provides use of a layer of silicon nitride in a method of fabricating a rib waveguide structure in silicon to control birefringence by depositing said layer to a predetermined thickness over said rib waveguide structure.
Still further aspects of the present invention provide a method of manufacturing a silicon rib waveguide structure comprising: forming an elongated rib element in a silicon substrate, the elongated rib element having an upper face and two side faces; and forming a layer of silicon nitride to a predetermined thickness over said elongated rib element directly abutting said upper face and side faces, the predetermined thickness being selected such as to control birefringence in the rib waveguide structure.
A still further aspect of the invention provides a method of manufacturing a silicon rib waveguide structure, the method comprising: forming an elongated rib element having an upper face and two side faces in a silicon substrate; growing a layer of oxide over the upper face and side faces; stripping the oxide layer to reveal the upper face and side faces; and forming a layer of silicon nitride to a predetermined thickness over said rib waveguide structure directly abutting said upper face and side faces.
A yet further aspect of the invention provides a silicon rib waveguide structure comprising an elongated rib element having an upper face and two side faces formed of silicon and a layer of silicon nitride directly abutting said upper face and side faces and having a predetermined thickness selected to control birefringence in the silicon rib waveguide structure.
A yet further aspect of the invention provides an evanescent coupler structure comprising first and second silicon rib waveguides each comprising an elongated rib element having an upper face and two side faces formed of silicon and a layer of silicon nitride directly abutting said upper face and side faces and having a predetermined thickness selected to control birefringence in the evanescent coupler.
It has been determined that for a 4 micron rib waveguide, the optimum thickness of the silicon nitride layer is 1000 A.
For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: