22:17 Jul 6 

Thanks for your help. Actually I also have a "tight-buffered" fiber, beside the "bare buffered" fiber: does it help ?

22:13 Jul 6 

...of my ground here, but this might give you a lead for further research.

I think a lot of fibre optics are terminated in connectors etc. 'bare', i.e. without any filler material; but cetainly in the early days of fibre optics, for greater optical efficiency (= reducing losses at boundaries), certain connections used to be 'buffered' with a gel-like substance that improved the optical transmittance of the interface between fibre and connector. I would assume those would have been described as 'not bare'...