If you’re a hobbyist looking to splice two separate pieces of fiber optic filament together and retain 100% of the transmitted light, you most likely will be disappointed.
The ‘design’ of end glow filament is such that the inner wall ‘reflects’ the ‘light waves’ internally. Any breach or break in the path will cause the light to emit out. This attenuation (reduction) or ‘light leaks’, while desired sometimes, depending on the application, will reduce the amount of light waves available to travel the balance of the filament.
To successfully splice together two or more lengths of fiber optic filament as to maintain minimal light loss, requires expensive splicing equipment along with the training on how to use it. Candidly, this is beyond the scope of our site.