What is attached to the bag using a coupler ?
A prism coupler is a prism designed to couple a substantial fraction of the power contained in a beam of light ( e.g. , a laser beam ) into a thin film to be used as a waveguide without the need for precision polishing of the edge of the film , without the need for sub-micrometer alignment precision of the beam and the edge of the film , and without the need for matching the numerical aperture of the beam to the film .