Abstract: Data is encrypted onto an electromagnetic beam by providing an electromagnetic beam having a signal component having a modal state, wherein the signal component is susceptible to accumulation of a geometric phase, and a reference component, transmitted along a path over at least part of which the signal component accumulates a geometric phase by transformation of its modal state from a first to a second modal state, from the second to at least one further modal state, and then back to the first modal state; and modulating with the data the geometric phase so accumulated, by modulating the modal state transformations. Data is decrypted from a received electromagnetic beam by corresponding processing of the received electromagnetic beam and by comparing an overall phase of the signal component with an overall phase of the reference component so as to retrieve the modulation.

Abstract: An apparatus and method for providing a delay to an optical signal including a Latin router having a plurality of input ports, including at least one signal-input port, and a plurality of output ports, including at least one signal-output port, and a plurality of optical waveguides, each connecting one of the input ports to one of the output ports that is not a signal-output port, in which the plurality of waveguides are connected between the input ports and the output ports in an arrangement such that a first signal entering the device and mapped to any one of the output ports experiences a delay that is different from a delay that is experienced by a second signal entering the device and mapped to a different one of the output ports.

Abstract: Data is encrypted onto an electromagnetic beam by providing an electromagnetic beam having a signal component having a modal state, wherein the signal component is susceptible to accumulation of a geometric phase, and a reference component, transmitted along a path over at least part of which the signal component accumulates a geometric phase by transformation of its modal state from a first to a second modal state, from the second to at least one further modal state, and then back to the first modal state; and modulating with the data the geometric phase so accumulated, by modulating the modal state transformations. Data is decrypted from a received electromagnetic beam by corresponding processing of the received electromagnetic beam and by comparing an overall phase of the signal component with an overall phase of the reference component so as to retrieve the modulation.

Abstract: An optical waveguide comprises a core and is characterised in that the core has a refractive index that includes a radial discontinuity and varies, with increasing azimuthal angle ?, from a first value n2 at a first side of the discontinuity to a second value n1 at a second side of the discontinuity.

Abstract: A fresenel zoned microstructured optical fiber is described. The fiber is constructed of concentric zones, each zone being defined by discontinuities in the refractive index. The refractive index within each zone may either be constant or may vary, for example each zone being a section of a parabola or hyperbola. The fiber may be used as a lens, for example for coupling light between fibers with different core sizes.

Abstract: A laser diode is driven by a first, integrating, amplifier receiving a d.c. reference voltage V.sub.ref and negative feedback from a back-facet photodiode, and by a second, wideband, amplifier receiving the data and the negative feedback.

Abstract: A bias controller for a semiconductor laser, for example, a laser having an light output which exhibits a threshold at a threshold value of an input drive current, is disclosed. The bias controller includes a test signal generator arranged to generate a low frequency test signal which is the superposition of a modulated control signal on a DC bias signal which is applied to the laser as a drive current superimposed on a high frequency data signal. A detector is arranged to determine the value of either the third harmonic component, or the second harmonic component, of the light output of the laser. A control means is arranged to control the DC bias current to have a value close to the threshold value by maintaining the detected value of the harmonic component of the light output of the laser near to a predetermined value.