Patent GB-1305872-A (GB)
IPC: H01S3/00
Filed: 1971-02-05
Abstract: 1305872 Lasers STANDARD TELEPHONES & CABLES Ltd 28 Oct 1971 [5 Feb 1971] 4026/71 Heading H1C A semi-conductor laser is pumped so that it lases in a circulatory mode, and its output is extracted by an optical fibre attached to an edge of the laser junction. In Fig. 1, a laser has a four sided resonant cavity formed by cleaving, and a circulatory resonance mode is promoted by the shape of the current supplying electrode. The output is from the plane of a P-N junction in GaAs 3, the remaining layers of the device forming a double heterostructure suitable for room temperature operation. An optical fibre is attached to the laser, and light couples into this as a function of the spacing of the fibre core 8 from the edge of the resonator to which the fibre cladding 9 is attached, and as a function of the resonator dimensions and output wavelength. To ease the tolerance requirements on the dimensions of the resonator, the reflector opposite the fibre may be formed of a region of lower refractive index, to provide total internal reflection. It is demonstrated in Fig. 3 (not shown), that if the resonator dimensions are varied with such a reflector, the effect on the angle of incidence of the output on the fibre is reduced, and hence dimensional accuracy requirements are reduced. An embodiment employing such an arrangement is shown in Fig. 4, where half of an optical fibre 43 is used as the totally internally reflecting resonator mirror. An anti-reflection layer 46 is also provided, and this, with the gap 42 in the electrode 41, ensures that no oscillation mode is possible unless it passes through the reflecting device 43. The reason for using half a fibre rather than a rectangular block is to compensate for diffraction fan-out at the junction edge. The main output fibre at the bottom of Fig. 4 has part of its cladding cut away so that the core is close enough to the junction edge to reduce the axial modes in the output. To prevent the output in this fibre coupling back into the non-lasing bulk of the semi-conductor, an interference layer 54, Fig. 5, preferably a ¥ wavelength layer, may be placed between the edge of the junction and the fibre. Operation.-Since there are normally two counter-rotating modes within the resonator, as in Fig. 2a, an output is obtained from each end of the fibre when the laser is used as a generator. A mirror may be placed at one end of the fibre, Fig. 2b (not shown), which reflects light back along the fibre. Since some of this light couples back into the laser the two modes become locked, and an in-phase output is obtained at the other end of the fibre. If the laser is maintained just below oscillation by choice of the pumping current, it will amplify any light passed along the fibre and having a wavelength such that it can enter the laser. A modification of this is shown in Fig. 2d where light is passed along a fibre at one side of the laser, and suitable wavelengths are coupled into the laser body. A second fibre is arranged at the other side of the laser body, and it is apparently possible to choose the coupling of this second fibre so that all the light of a given wavelength enters from the first fibre and leaves in one direction in the second fibre. This provides a highly selective device which extracts a selected wavelength but does not attenuate others.