INTEGRATED OPTICS RING RESONATOR

Abstract: A ring resonator waveguide for a laser gyroscope comprises a ring of waveguide material (2) formed around the inside or outside surface of a hollow cylinder (3) or ellipsoid (4). Such a ring resonator waveguide is made by providing a hollow cylindrical substrate (1), depositing waveguide material (2) on the inside or outside surface of the hollow cylindrical substrate (1), heating the substrate (1) and blowing it to form a cylindrical or ellipsoidal (4) member, and cutting a circular section (5, 6) from the cylindrical member (3) or from a central portion of the ellipsoidal member (4).

Description: As an alternative to a coil of optical fibre, a ring resonator for a ring-resonator laser gyroscope say, may comprise an integrated optics endless loop waveguide formed on a planar substrate or, as shown in the perspective view of Figure 1 of the accompanying drawings, around the surface 5 of a cylindrical substrate 3, for example as disclosed in PS WO 86/02171. The planar waveguide relies on total internal reflection to contain light in both the plane of the planar waveguide and in the radial direction. Total internal reflection tends to cause losses especially in the radial direction at the lateral edges of the planar waveguide as a result of the roughness of this interface. The waveguide integrated in a surface of a cylinder can be likened to a planar version of a laser cavity with two plane mirrors. A laser cavity with two plane mirrors is a difficult cavity to align with any misalignment stopping resonance. Equally, with the waveguide formed around a cylinder any cone angle in the cylinder geometry causes misalignment and prevents resonance. According to this invention, there is provided a ring resonator for a laser gyroscope comprising an integrated optical waveguide formed at a surface of a substrate and extending around said substrate, characterised in that the waveguide extends around an inner or an outer surface of a hollow substrate, which inner and outer surface is ellipsoidal. With such an arrangement total internal reflection confines the light only in the radial direction and, in the axial direction of the ellipsoid, laser modes are generated with different transverse modal patterns. The lowest order modes that can be carried by such a waveguide are a TE₀ mode with the E vector in the plane of the waveguide and a TM₀ mode with the E vector radially oriented which, in general, has a different resonant frequency to the TE₀ mode. The next two highest modes are the TE₁ and TM₁ modes which have a single node and two antinodes. By polarising the incident light it is possible to select the TE or TM modes separately. A different optical path length between the TE and TM modes is equivalent to birefringence in planar optical waveguides or optical fibres and this implies that the polarisation along the principal axes will be maintained. The ring resonator may be made by providing a preform comprising a hollow cylindrical substrate, having waveguide material on its inside or outside surface, heating the preform and blowing it to form an ellipsoidal member, and cutting a circular section from a central portion of the ellipsoidal member to provide the ring resonator. Preferably the preform is of the kind from which low loss (1db/km) clad optical fibre is drawn. The waveguide material may have been provided by chemical vapour deposition processing. The waveguide may be formed as an equatorial slice of an oblate or prolate ellipsoid. The resulting waveguide is curved when considered in a plane containing the axis of the ellipsoid with the curve being formed by a portion of an ellipse adjacent its major or minor axis. A sphere or spheroid is a special case of an ellipsoid in which the major and minor axes are substantially equal, and the waveguide may be formed by an equatorial slice from a sphere or spheroid. By analogy when the waveguide is formed around an ellipsoid this is similar to the case of a linear laser cavity with concave mirrors so that light is encouraged towards the central, substantially cylindrical portion of the ring. The ellipse may be arranged so that the curvature of the waveguide corresponds, by analogy, to a confocal cavity for a linear laser. It is preferred that the waveguide material is deposited upon the inside of the ellipsoidal surface, or the inside of the substrate and, in this case, total internal reflection takes place substantially only around the radially outer surface of the waveguide material. The waveguide material may be buried below the surface by virtue of having a silica overlayer. This will help to reduce the scatter loss. To couple light into and out of the waveguide material, the resonator may include a coupler formed by winding a clad optical fibre around a former which is circular in cross-section, polishing the cladding away from part of the winding to leave only a small portion of the cladding remaining, and then placing the polished portion against the waveguide. Evanescent coupling then takes place between light carried by the winding and the waveguide resonator with a coupling coefficient of typically 1000 ppm. Typically the cladding is polished back to a few micrometres away from the core on the winding. Alternatively a prism can be coupled to the waveguiding film. A silica overlayer may be used to reduce the coupling efficiency to the required value by varying its thickness to separate the prism from the waveguide. A detailed example of an apparatus in accordance with this invention will now be described with reference to the remaining figures of the accompanying drawings in which:- Figure 2 is a perspective view of a preform; Figure 3 is a perspective view of a preform after it has been blown into an ellipsoid and indicating the portion of ellipsoid to be used as the resonator; and Figures 4A and 4B are an end elevation showing the complete ring resonator with light coupler and an enlarged detail. A fibre optic preform has the form of a tube 1, see Figure 2, with the waveguide layer 2 typically formed from germanium oxide deposited on the inside of the tube 1 which itself is typically formed from silica. Normally, this preform is drawn to provide an optical fibre but, in the present invention this preform is heated and blown to provide an ellipsoid 4 as shown in Figure 3. As the preform is blown it expands but the germanium oxide 2 provides a continuous waveguide layer around its inside surface. A central section 6 is then cut from the ellipsoid to form the ring resonator. An optical coupler 7 is provided to couple light into and out of the waveguide 2 on the ring resonator. The coupler 7 comprises a clad optical fibre 8 wound around a substantially cylindrical former 9 and then its cladding is polished from one side of the winding until only a few microns of cladding remain. This polished portion of the winding is then placed immediately adjacent the inner surface of the waveguide layer 2 of the ring resonator and the light is evanescently coupled across the remaining few microns of cladding to provide a very small coupling ratio of the order of 1000 ppm.

Claims: A ring resonator for a laser gyroscope comprising an integrated optical waveguide formed at a surface of a substrate and extending around said substrate, characterised in that the waveguide (2) extends around an inner or an outer surface of a hollow substrate (4), which inner and outer surface is ellipsoidal. A ring resonator according to claim 1, the resonator having been made by providing a hollow cylindrical substrate (1) having waveguide material (2) on its inside or outside surface, heating the substrate and blowing it to form an ellipsoidal (4) member, and cutting an equatorial slice (6) from the ellipsoidal member to provide the ring resonator. A ring resonator according to claim 1 or 2, in which the waveguide material (2) is buried beneath a layer of silica. A ring resonator according to claim 1, 2 or 3, in which the ring resonator also includes a coupler (7) to couple light into and out of the waveguide material (2), the coupler (7) having been formed by winding a clad optical fibre (8) around a former (9) which is circular in cross-section, polishing the cladding away from part of the winding to leave only a small portion of the cladding remaining, and then placing the polished portion against the waveguide (2).

Inventors: MALVERN, ALAN RICHARD C/O BRITISH AEROSPACE PLC
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: G01C19
IPC codes: H01S3, G02B6, G01C19