Abstract: In order to enhance the light output from a side illuminating optical structure comprising a plurality of elongate optical fibers, means is provided for ensuring a permanent compressive stress in the optical structure, for example a permanent compressive stress in the radial direction and/or in the axial direction. A variety of means are disclosed, for example a transparent tape member wound with tension helically around the optical structure, an elongate heat-shrinkable sleeve surrounding the structure, a plurality of rings provided along the structure to keep it under permanent compressive stress. Embodiments using braids are also disclosed.