Patent GB-1356877-A (GB)
Title: Lasers
IPC: G02B6/122
Filed: 1972-04-05
Abstract: 1356877 Lasers WESTERN ELECTRIC CO Inc 5 April 1972 [5 April 1971 1 June 1971 (2)] 15574/72 Heading H1C The active medium of a laser forms or is associated with, a waveguide having a dimension which is of the same order of magnitude as the wavelength of the laser output and so constructed that emission is sustained by ring-like feedback. In Fig. 1A a thin light transmitting film 11 has an annular thicker region 15 containing the active medium. The thickening increases the phase propagation constant and a feedback ring is thus formed. A broad pumping beam is coupled into the film at 14, and output is coupled from the ring at 16. The output coupler is placed very close to the ring and frustrates the guiding in the ring; this coupler may be as in Specification 1,287,035. The ring may also be formed by creating an annular region of greater optical density in the film. The possible changes in phase propagation constant or refractive index are described in Figs. 2A-2D (not shown). A special case of one of the possibilities is shown in Fig. 5, where the thicker regions of the film form an optical device known as a Maxwell's fisheye. This device serves to confine the radiation to an annular region, as described with reference to Figs. 6A and 6B (not shown). The active medium may be a dye, e.g. rhodamine 6G, in a solid host material, a suitable pumping source being an argon laser. Inorganic laser ions, e.g. neodymium, may also be used. The optical density of the films may be altered if a dichromated gelatine host film is used and exposed to a suitable pattern of blue or ultraviolet light. The output coupling device may comprise a thin film guide positioned closely above or beside the ring, Fig. 4 (not shown), or a diffraction grating may be used. Fig. 3 shows a semi-conductor junction laser. The profile of the substrate of this is such that radiation is confined to an annular path which intersects the junction 31, gain occurring in this junction. It is also possible for the junction to be planar, in which case the profile of the alloyed region 33 would become such as to provide the annular guided path. Energy may be extracted by a coupling prism placed very close to the upper edge of the junction, or by making the device slightly asymmetric so that output leakage occurs. The active medium may also be a flat spiral, or a helix, provided the turns of the structure are close enough together to provide optical coupling to sustain oscillation, Figs. 7, 8 (not shown), the structure being formed by local thickening in a film. In these cases the pumping light may be a broad beam as in Fig. 1, or may be a narrow beam directed into the end of the spiral or helix. The structure includes a reflector at one end, and also contains a dichroic prism for energy extraction. The active medium may also be in the form of concentric rings of film material close enough to be coupled to one another, Figs. 9A, 9B (not shown). In this case, and in the case of the spiral (but not for the helix) the phase propagation constant preferably increases towards the centre of the structure for proper phase matching. In all the above embodiments the active medium can be separate from but close to the guiding structure, for example, a dye could be flowed over the guiding structure and be optically pumped from above. In Fig. 10 the active medium is a dye in a polyurethane film 111 on a glass rod 113. A prism 114 couples the output from the film as in Specification 1,287,035, two outputs being obtained (one for each direction of rotation about the rod). A diffraction grating coupler may be formed over part of the film to extract the output, as in Specification 1,334,140. Operating parameters and the forms of output modes obtainable are discussed; circumferential modes exist, corresponding to the Fabry-Perot longitudinal mode, and there are also radial and axial modes analagous to the Fabry-Perot transverse modes. The output coupling prism may have a curved base, Fig. 14 (not shown), to produce an output beam which has a Gaussian profile in the vertical direction; as in Fig. 10 the curvature of the film produces a Gaussian distribution in the horizontal direction. In another embodiment, Fig. 12 (not shown), frequency selective prisms formed by local thickening in the film are added to the device of Fig. 10. This restricts the laser output to a narrow band or to a single wavelength. The Fig. 12 embodiment uses a diffraction grating ruled on the film for output extraction. The frequency control is most desirable when a dye medium is used, but may also be applied when another medium, e.g. a Nd<SP>3+</SP> glass film, is used. A cross-sectional slice of the cylindrical film used in Figs. 10 and 12 may be used on its own, as in Fig. 13. Pumping light is coupled by prism 144 into a guide 145, which may contain the active medium, and thence into ring 141. Energy is extracted by an adjacent thin film region 143 which may contain a reflection grating 147 positioned so that only one output, anticlockwise in Fig. 13, is obtained. The ring-like path need not be circular, it being sufficient that the first derivative of its curvature be continuous. Ellipses are referred to. The polyurethane films may be formed by dip coating a solution containing the dye on to a rod and evaporating the solvent at temperatures low enough to preserve the dye. Spraying or brushing may be used. Other polymers may be used, e.g. polystyrene, polyester, pmma or epoxy films; sodium -fluorescein dyes give green outputs, and any soluble Nd<SP>3+</SP> compound may also be used. The light guiding film need not be a polymer, glasses being usable; a wet chemical method in which lead oxide and SiO 2  are mixed with a dye before mild heating on the rod is disclosed ; vacuum deposition or sputting of a glass or crystal on to the rod or substrate is also possible.