Unintuitive behaviour of fibre coupled collimating optics used for plasma emission observations

The collection of light emitted from a plasma or other volumetric light source is often collected using an optical collimator coupled to an optical fibre. In this work the spatial dependence of transmission from a light source into a fibre via a variety of collimating lens systems is investigated using a table top optical setup and a full 3D ray tracing simulation. The solid angle emitted from a point source that is accepted into the fibre is found to depend in a complex and unintuitive manner on the spatial position of the source relative to the optical system. Back-illumination is often used to focus the optical system such that it produces a well defined beam, which is then assumed as a ‘line of sight’. However even in these cases there are non-monotonic behaviours within this ‘line of sight’ that mean the often-made assumption of a single acceptance angle or cone of observation is not valid in these systems. This has significant implications for the application of these optical systems to the observation of non-uniform volumetric light sources where variations of light intensity or spectrum occur within the defined line of sight.