Abstract: A method and an apparatus are provided for launching light into an entrance facet of a MMF of an optical MMF link in a way that excites one or more targeted higher-order mode groups in the MMF. The light is launched into the entrance facet of the MMF as a line of phase-modulated spots, referred to herein as a “line launch”. The line launch causes one or more targeted higher-order mode groups to be excited in the MMF. The use of the line launch to excite one or more higher-order mode groups in the MMF increases the bandwidth of the link and allows overall link lengths to be increased. In addition, the use of the line launch is reliable and robust despite defects in the MMF and despite connector offsets. Thus, the use of the line launch ensures that a sufficient increase in link bandwidth will be achieved despite the existence of defects in the MMF and even if there is some amount of optical misalignment due to the connector being offset relative to the corresponding receptacle.

Abstract: Optical radiation is applied to a multimode optical fibre, either from another fibre or from a laser, in such a way as to control the mode distribution in the multimode optical fibre. The mode distribution is selected in such a way as to improve performance, for example to avoid transmission nulls or to reduce noise due to reflections back to a laser.

Abstract: A method of transmission of radio signals over all types of graded-index multimode fibre is provided. The method comprises launching optical radiation into the core of the multimode fibre with a specified restricted launch to allow multiple trans-verse mode lasers transmitters to be used in low cost radio over fibre links. The launch technique allows a reduction in modal dispersion and modal interference, thus greatly improving the transmission performance of radio over fibre signals over multimode fibre as well as reducing system impairments such as outages and link failures.

Abstract: A vertical cavity surface emitting laser device (10) has a discontinuity (11) formed within the body of the device. When the device is in use, the direction of polarization of light emitted from the device is substantially aligned with a boundary of the discontinuity.