Abstract: A coupler (FIG. 4) comprising a pair of interwined monomode fibers optically coupled along a portion of their length. The diameter of one fiber is significantly smaller than that of the other and this fiber is thus wound helically. A helical pitch not exceeding 10 mm is chosen. For such pitch, the smaller diameter fiber is appreciably longer. The following inequality thus holds:.lambda..sub.c .multidot.L/.DELTA.L.ltoreq.L.sub.c :where.lambda..sub.c is the design wavelength,.DELTA.L/L is the difference in fiber lengths per unit length, and L.sub.c is the interaction length of the coupler. The signal coupling of the coupler is than a strongly peaked function of wavelength. The coupler is suited to wavelength multiplex/demultiplex applications.

Abstract: A fibre optic modulator comprises an optical fibre 2 and a waveguide 10 of active material. The waveguide 10 is dimensioned such that any mode of the waveguide has a propogation constant which is separated from the propogation constant of the optical fibre mode by an amount equal to or greater than .lambda./L where .lambda. is the wavelength of transmitted electromagnetic radiation in the mode of the optical fibre and L is the length of the effective portion of the waveguide of active material.

Abstract: An optical receiver comprises a local oscillator which produces an output signal having a frequency which differs by a predetermined amount from the frequency of an incoming phase-modulated coherent light signal having a "1" and "0" signal bits, mixing means for mixing the local oscillator output with the incoming signal and switching means effective for switching the mixed signal to respective detectors and the mixed signal frequency whereby the major proportion of the power content of the mixed signal during a "1" bit period is fed to one of the detectors and the major proportion of the power content of the mixed signal during a "0" bit period of the mixed signal is fed to the other detector.In an alternative arrangement the output from the local oscillator is phase-modulated at the intermediate frequency before the mixing operation.

Abstract: A method for accurately aligning an optical fibre with a light source comprises firstly approximately aligning the axis of the fibre with the source (e.g. a laser) and locating two prismatic elements between the end of the fibre and the source. On or both of the prismatic elements is then moved to focus the image of the light source on to the end of the fibre and secured in position. The method can be described as an "Optical lever" as the movement of one of the prismatic elements causes a much smaller movement of the image and the method is thus very accurate. The prismatic elements can be any transparent members which produce an output beam which is angularly displaced from the input beam such as conventional prisms, gratings, diffused lenses or weak decentered lenses. Preferably they are cylindrical transparent rods with one end of each arranged at an acute angle to its axis.

Abstract: Apparatus for measuring the spot size of light emerging from an optical fibre comprises a rotating radial grating which is moved relative to a monochromatic beam of light, passing through the fibre, to vary the spatial frequency of the light entering a photodetector situated at the output end of the optical fibre, thereby obtaining a variation in the modulation amplitude of the light received by the detector, allowing measurement of the spot size to be obtained.

Abstract: A coupler for an optical fibre system includes a retroreflector which because of its properties allows considerable relaxation in the positioning of the fibre in relation to the retroreflector. The principle may be used to provide switches or couplers.

Abstract: Optical apparatus for determining for example by display on an oscilloscope the refractive index profile across an optical fibre including an ellipsoidal reflector and a scanning system for a laser beam directed towards one focus of the ellipsoidal reflector at which point the fibre to be examined is placed and including a detector which is relatively small in size situated at the other focus of the ellipsoidal reflector.

Abstract: It is important that in an optical fibre connector the ends of the fibres should contact or very nearly contact to avoid excessive optical losses. They must be accurately aligned and lateral movements, causing varying optical output, be minimized. In this invention, the fibre or fibres are secured in grooves formed in flat surfaces in two abutting blocks and the blocks are clamped together with their flat surfaces in contact with a further flat surfaced member. The clamp consists of a cam surface formed in a housing which is adapted to contact a portion of the further member when the blocks are introduced into the housing.

Abstract: An optical fibre connector comprising in respect of each of the optical fibres to be connected, a connector body part formed at one end with a recess which accurately locates a sphere lens relative to an optical fibre receiving hole extending through the body part and terminating at the center of the recess whereby the axis of the optical fibre received by the hole and preferably bonded to the surface of the sphere lens. Connecting means is provided for connecting together in axial alignment two of the body parts with the sphere lenses of the parts arranged in opposed relationship.

Abstract: An optical fibre terminal block assembly and a method of connecting an optical fibre to a piece of material by pressing the fibre and the material together so that the fibre becomes embedded in the material, the structure so formed being used to provide connectors for connecting optical fibres in accurate alignment.

Abstract: A connector for optical fibres uses the thickness of a piece of metallic foil to space an optical fibre from a known plane and utilizes the metallic foil to hold the optical fibre. Two portions are provided, each with an optical fibre retained by metallic foil, and a third member is used to align the optical fibres in one plane by abuttment against the metallic foil and in a further plane by use of, for example, a groove. Since grooves may be accurately machined and metallic foil thickness may be accurately controlled, an accurate and cheap connector may be produced.

Abstract: The end of a glass fibre of the graded or at least stepped, index type, or in the case of a clad fibre, the end of its core, is etched with an etchant which varies in etching power according to the composition, and thus in a graded or stepped - index fibre according to the index, of the fibre - core material, thereby producing at the end of the fibre a recess or "well" whose depth increases from the edge towards the axis of the fibre.

Abstract: A press control system in which multiple pushbuttons are employed for initiating a press cycle with each pushbutton including a normally closed and a normally open blade and with each switch blade having connected in parallel therewith a monitoring circuit which produces logic signals in conformity with the position of the respective switch blade. A logic circuit analyzes the logic signals from the several monitoring circuits and permits operation of the press only when all of the switches operate properly during a press cycle.