Abstract: An optical communications network includes a terminal which can simultaneously receive and modulate an optical signal. The terminal includes an optical modulator which is controlled by varying the bias voltage applied to it.

Abstract: An optical communications network includes a terminal which can simultaneously receive and modulate an optical signal. The terminal includes an optical modulator which is controlled by varying the bias voltage applied to it.

Abstract: An optical communications network includes a terminal which can simultaneously receive and modulate an optical signal. The terminal includes an optical modulator which is controlled by varying the bias voltage applied to it.

Abstract: An optical transmitter includes a directly modulated semiconductor laser and a non-linear optical intensity modulator which is connected in series with the output of the laser. High frequency analogue modulating signals are applied both to the laser and to the modulator. The modulator has a transfer characteristic such that it cancels intermodulation distortion in the output from the laser, to give a source with an improved dynamic range. The transmitter is suitable for use in an analogue optical distribution system for cellular radio.

Abstract: Improved single-modedness is achieved in distributed feedback (DFB) lasers by minimizing the feedback provided at or near the ends of the DFB grating. The feedback may be minimized at one or both ends of the grating. Feedback is controlled by reducing the coupling to the grating. Coupling is reduced by reducing the depth of the grating's teeth and/or by increasing the spacing of adjacent teeth.

Abstract: A method of undercutting mesa structures in which the lateral extent of the undercut is determined by a prior fabrication stage in which channels (5, 6) are etched and then infilled with a different material. The mesa is formed over the channels (5,6) and a selective etch is used to undercut the mesa, the extent of the undercut (11) being limited by the location of the infilled channels (5, 6). For mechanical stability and insulation the undercuts (11) are filled or partly filled with dielectric (10).

Abstract: A laser structure with highly p-doped active material which has a particularly low linewidth enhancement factor .alpha.. Hence the emission linewidth of the laser structure is relatively small. The structure shows a particular relationship between the photon energy, the band gap energy and the conduction band quasi Fermi level of the active material which minimizes .alpha., the valence band being degenerate. These conditions can be used to design low .alpha. laser structures. Such structures are of particular application to directly modulated lasers or absorption modulators and also find application in coherent optical detection.

Abstract: A semiconductor device is disclosed that comprises a base semiconductor portion and, thereon, first and second elevated semiconductor portions separated by a channel. The uppermost surface of the first elevated semiconductor portion carries a metal electrical contact layer and the uppermost of the second a dielectric layer. The surfaces defining the channel are substantially free of metal and dielectric. The structure can be used in a ridge waveguide laser, the first elevated semiconductor portion constituting the ridge. Distributed feedback corrugations may be incorporated in such devices or in other ridge waveguide structures.

Abstract: The emission linewidth of a semiconductor laser can be reduced by operating it at a wavelength which is less than the wavelength of maximum gain at the threshold current, .lambda. max. An assembly for carrying out such operation comprises a ridge waveguide laser provided with a distributed feedback (DFB) grating. The period of the grating is selected to give a predetermined emission wavelength to the laser which is shorter than .lambda. max.

Abstract: A gas mixture containing phosphine and R.sub.1 R.sub.2 R.sub.3 In X R.sub.4 R.sub.5 R.sub.6 or R.sub.1 R.sub.2 In X R.sub.4 R.sub.5 where the Rs are alkyl groups is passed over a semiconductor substrate comprising indium and phosphorus so as to deposit a semiconductor material comprising indium and phosphorus, and the exposure of the substrate to phosphine is controlled to avoid or reduce transport of the substrate material.Thus, for example, indium phosphide may be grown onto corrugations in gallium indium arsenide phosphide, the corrugations being non-deformed during this growth. Such a growth step may be used in the production of distributed feedback semiconductor lasers operating near 1.55 .mu.m.

Abstract: A semiconductor device comprises a base semiconductor portion and, thereon, first and second elevated semiconductor portions separated by a channel. The uppermost surface of the first elevated semiconductor portion carries a metal electrical contact layer and the uppermost of the second a dielectric layer. The surfaces defining the channel are substantially free of metal and dielectric.The structure can be used in a ridge waveguide laser, the first elevated semiconductor portion constituting the ridge (7", 8").Distributed feedback corrugations may be incorporated in such devices (6), or in other ridge waveguide structures.