Patent GB-1053166-A (GB)
IPC: H01S3/06
Abstract: 1,053,166. Lasers. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. April 17, 1964 [April 22, 1963], No, 15997/64. Heading H1C. At least one of the reflecting members. at each end of a laser active medium consists of two flat reflecting surfaces making an angle with each other which is smaller than 90 degrees so that a parallel beam of rays can repeatedly cover a closed resonant optical path. The angle should be at least 25 seconds of arc smaller than 90 degrees, and greater than 35 degrees, preferably lying between 80 degrees and 89 degrees. In Fig. 2, a laser body has a triangular cross-section and faces a, b, c are silvered, face a being arranged to transmit 5% of any incident radiation. Closed light paths such as GHKMNOG can be set up, all parts of which are parallel to one side of the pedal triangle DEF, which itself defines a possible light path. Roof angle A is 60 degrees so that points D, E, F lie midway along the sides. If angle A is larger than 60 degrees points E, F lie nearer corner A. As shown in Fig. 3, the reflecting members 12, 13 and 15 may be further apart, and faces 12, 13 need not be silvered since total internal reflection occurs. Points S, T, U are the pedal points of a triangle PQR and the roof angle Q is such that points S, U lie halfway along faces 12, 13. This occurs if the complement # of roof angle Q is approximately equal to the angle α = n 1 /n 2  . D/4L radians,   where D is width of the broadest rectangular sections of active medium lying between the reflecting members, L is the length, n 1  is the refractive index of active medium 16 and n 2  is the refractive index of the body 14. Bodies 14 and 16 may be integral, as shown, so that n 1 =n 2 . Angle # may differ from α by up to 60%, but preferably differs by not more than 40%. Body 14 may be rotated slightly about roof line 11. In a modification, Fig. 5 (not hown), the laser medium may be a gas or a s liquid in a quartz tube, having a dielectric reflecting coating at the lower end. The same relationship between n,n 2 , D and L holds. The lower surface of the triangular reflecting member may be coated to prevent reflections. Alternatively, reflections may be avoided by arranging the lower surface of the reflecting member at the Brewster angle to the lower reflecting member 37, Fig. 6. The arrangement is such that a ray 38 travelling along the bisector of roof angle 35 is refracted at surface 31 and is incident substantially normally on surface 37. In this case # should differ by not more than 60%, preferably by not more than 40%, from   α 1  = n 1 <SP>2</SP>/n 2 <SP>2</SP> . D/4L radians. In another arrangement, Figs. 7a and 7b, the upper reflecting surfaces define a roof line 53 such that a light ray travelling perpendicularly to surface 52 is refracted at 51 and meets the roof line at rightangles. Angles 56, 57 are equal to the complement of the Brewster angle #. The plane of Fig. 7b is preferably the bisector plane of roof angle 53. The complement of the roof angle should have the same value as in the arrangement shown in Fig. 3. In further embodiments both reflecting members consist of two reflecting surfaces. The reflecting members may be integral with the active medium and of the same material, or of different material, and each member may be of a different material to the other. In a first arrangement, corresponding to Fig. 3, two reflecting members with a triangular crosssection are mounted with their roof lines at right-angles to each other. The roof angle of each reflecting member is the same as in Fig. 3, D being measured perpendicular to the relevant roof line, and n 2  being the refractive index of the relevant reflecting member. The roof angles at the two ends may therefore differ. For a gas or liquid laser a reflecting member as in Fig. 6 may be provided at one end, and a member as in Figs. 7a and 7b at the other end (Figs. 10a, 10b, not shown). To obtain an output beam from the last two embodiments a prism is placed close to one of the reflecting surfaces to cause frustrated total reflection. The intensity of the output beam is controlled by adjusting the distance between the surface and the prism. In another arrangement the lower surface of the upper reflecting member is rotated so that radiation does not strike it exactly at the Brewster angle, and the reflected radiation is directed through a window in the tube. An arrangement similar to Fig. 3 is also described in which some rays do not undergo cyclic reflection. A roof angle of 80 degrees is quoted. The prisms need not be in contact with the laser medium. Instead of using solid prisms as reflecting members two plane mirrors may be used, making an angle equivalent to the roof angle with each other. The end surface of the laser body is cut to the Brewster angle. To reduce reflections from a Brewster angle surface, due to divergence of the incident rays, the surface may have a bend in it. Reference is made to ruby, uranium or samarium doped calcium fluoride and heliumneon laser materials. The solid lasers may be pumped by high-pressure mercury lamps, and the gaseous ones by an electrical discharge.