Abstract: A planar dispersion compensator for an optical signal is provided. The compensator decomposes an inputted optical signal into N component signals separated by a fractional wavelength &dgr;&lgr;. Each component signal has its path-length adjusted to induce a sufficient phase shift between input and output to change the group delay of the optical signal when recombined from each of the component signals. In this manner, pulse broadening can be compensated by selectively varying the induced phase shifts to produce the desired level of opposite group delay. Portions of the substrate of the planar waveguide are removed to improve thermal responsiveness of the path-length adjustment means.

Abstract: The saturation effects observed in an MQW electro-absorption modulator are reduced by modifying the composition of the MQW structure so that the barrier layers, that are interleaved with the quantum well layers, are in tension.

Abstract: An integrated optics polarization state converter comprises optically in series a first TM.sub.0 to TM.sub.1 mode converter that is substantially transparent to TE.sub.0, a concatenation of total internal reflectors and a second TM.sub.0 to TM.sub.1 converter, similarly substantially transparent to TE.sub.0, which is connected the way round so as to operate as a TM.sub.1 to TM.sub.0 converter. Each TM.sub.0 to TM.sub.1 converter may comprise a tandem arrangement of a 2.times.2 TE.sub.0 /TM.sub.0 polarization beam splitting coupler and a mismatched, 3 dB maximum, 2.times.2 beam splitting coupler. The place of the TM.sub.0 to TM.sub.1 converters substantially transparent to TE.sub.0 may be taken by TE.sub.0 to TE.sub.1 converters substantially transparent to TM.sub.0.

Abstract: A technique for achieving a substantially planar structure incorporating embedded vapor phase epitaxial growth involves the use of a window-frame shaped mask 40 of epitaxial growth resistant material to define a well in which embedded growth is to occur, and subsequently to ease mask registration problems in the location of a mask 60 employed while selectively removing unwanted material epitaxially grown on regions surrounding the well. Complementary format structures may also be formed in which epitaxial growth is provided up the sides of a mesa to substantially the same height as material grown on top of the mesa itself.

Abstract: A one-dimensional reflective-type diffraction grating is formed in a slab waveguide by a one-dimensional array of wells formed with substantially perpendicular walls.

Abstract: A photo-detector in the form of an optical field effect transistor comprig a semi-insulating InP substrate (1), a p.sup.+InP gate region ( 2) an n.sup.- InGaAs channel region (3) and source and drain contacts (5,6). Light incident on the bottom face of the substrate is detected. The channel region is thicker and reduced in doping in comparison with a normal JFET in order to achieve efficient light absorption and low gate to source capacitance. The source and drain contacts are interdigitated to increase the area for optical absorption (FIG. 1). Alternatively, (FIG. 2), the channel region is a composite structure including a lowly doped layer (11) for directing photogenerated carriers to a more highly doped layer (12) (active channel layer) of reduced dimensions of reduced area and upon which strip-like source and drain contacts (9,10) are disposed. The optical FET structures proposed facilitate integration with other circuit elements.

Abstract: A coupled waveguide injection laser is provided with two waveguiding ridges (36,37 or 60,61) that are configured such that the coupling between them produces a first order supermode for which a signal current modulation produces optical amplitude modulation with substantially no attendant frequency modulation. In a related structure the configuration affords the facility of providing for the zero order supermode optical frequency modulation with substantially no attendant amplitude modulation.

Abstract: A photoconductor comprising an optically sensitive FET in which an abrupt heterojunction (18) is inserted in the channel (11) at a certain distance from the gate contact (16). This provides a potential barrier (17, FIG. 4) in the valence band that accumulates minority carriers (carriers of the lower mobility type) and controls their release. A gate bias resistor which is conventionally used in a receiver circuit including the FET is no longer required, instead the potential barrier height determines the time constant and a response comparable in length with an input optical pulse is achieved. This overcomes the problems of integrated manufacture, and slow response, associated with the large value of the bias resistor which is needed to reduce noise.

Abstract: An injection laser array has 2.sup.n output elements (1) coupled by Y couplers (2, 4, 6) and (n-1) in sets of intermediate elements to a single element (7) in a tree configuration. The face end of the single element is provided with a high reflectivity coating (8) while those of the 2.sup.n output elements are provided with low a reflectivity coating (9).

Abstract: The gate of a gallium indium arsenide FET grown on an indium phosphide substrate comprises a top layer of GaInAsP (band gap 1.2 eV) a middle layer of GaInAs and a bottom layer of InP. This can be etched to produce an overhanging top layer which allows self-aligned gate contact metallization avoiding the registration problems of a further masking stage.

Abstract: A photo-detector includes a photoconductor comprised by a structure similar o a high electron mobility transistor (HEMT) but with the gate removed and the layer of high band gap thinned in order to reduce noise. On a semi-insulating substrate (8), an n-channel layer (9) is disposed and on channel layer (9) is disposed on n.sup.+ layer (7). Light incident on the n.sup.+ layer (7) causes electron-hole pairs to be generated in layer 9, the electrons and holes of which migrate to oppositely biased contact regions 10 and 11 respectively. The photoconductor is monolithically integrated with an HEMT (TI) (FIG. 3, FIG. 4 or FIG. 6) the latter comprising a pre-amplifier for a receiver circuit. In dependence on the materials chosen the photo-detector may be employed to detect wavelengths of the order of <8.88 .mu.m or <1.6 .mu.m.

Abstract: A single mode optical waveguide directional coupler is provided with enhanced wavelength selectivity by dividing the coupling region up into a number of identical sections (12) between which are interposed a set of identical decoupled sections (13) over which the two waveguides (10, 11) of the coupler are not optically coupled and have different optical path lengths.

Abstract: An integrated circuit incorporates a PIN diode connected to the gate of an FET. The semiconductor layers used to construct the diode are also the layers used to construct the FET, and provide a relatively low capacitance per unit area for the diode compared with that of the gate of the FET.

Abstract: A photodetector in the form of a JFET in which the gate is defined in a rib 5 which is also designed to function as an optically absorbing optical waveguide into which the optical input signal is launched.

Abstract: A transverse mode stabilized injection laser with a planar active layer is provided with a transverse waveguiding effect by the presence of a rib of intermediate refractive index material protruding through a blocking layer overlying the active layer. Optionally the blocking layer may include high refractive index material to provide additional waveguiding effect and controlled attenuation of higher order transverse modes.

Abstract: Porous silica doped with zinc is used as a p-type dopant source in the construction of rib lasers. Other modifications include allowing the zinc diffusion to go right through the active layer and to dimension the device so that lateral optical guidance is unaffected by the rib.

Abstract: A laser p-n junction is formed in a structure consisting of a pair of layers of opposite conductivity type grown by liquid phase epitaxy and having sandwiched therebetween a quantity of active material of higher refractive index and lower band gap. The layers are grown on a substrate having a groove in its surface under conditions such that the active material is thicker in the region overlying the center of the groove than elsewhere and where a dopant is diffused through one of the layers to translate the portions of the p-n junction not overlying the groove into the material of the other layer while leaving the portion of the p-n junction overlying the center of the groove bounded on at least one side by the lower band-gap active material.

Abstract: Porous silica doped with zinc is used as a p-type dopant source in the construction of rib lasers. This is preferable to the method used in the parent case of growing a zinc doped layer because epitaxial growth is liable to be accompanied by zinc diffusion into regions where it is not required. Other modifications include allowing the zinc diffusion to go right through the active layer and to dimension the device so that lateral optical guidance is unaffected by the rib.

Abstract: Porous silica doped with zinc is used as a p-type dopant source in the construction of rib lasers. Other modifications include allowing the zinc diffusion to go right through the active layer and to dimension the device so that lateral optical guidance is unaffected by the rib.

Abstract: A short length of step index fibre is placed in front of graded index fibre. Walls of the step index fibre form multiple images of its entrance face which are imaged by the graded index fibre to give a more nearly uniform illumination of the graded index fibre and thereby avoid the variable loss properties which can result from the repetitive focusing produced by point illumination or well collimated illumination.