Telco grade fiber can be spliced a couple of different ways. The permanent method is indeed a form of weld. I'm not totally familiar with how it works but it can produce results so close to 100% that it is usually a non issue (when done properly). The challenge is lining up the fiber EXACTLY. As you can imagine, if the two pieces are not lined up properly, it will diminish the signal strength or alter it until it is unusable.
There are pieces of electronic gear that can do this automatically and do a very good job of lining up and splicing the fiber. These pieces of gear are expensive but tend to do a good job. I'm not familiar with their innards to explain how they work exactly, although I have seen them in action.
There are also manual devices that can assist the tech to line up the fibers prior to being spliced. They are not as good as the automated electronic versions but do a good job in the hands of an experinced technition.
The quick and dirty method is to use little plastic clips that attempt to line up the fiber and squeeze it together. They look like small plastic tubes about the size of a larger car fuse. They open on each side of the cylinder and the idea is that you put the end pieces of the fiber in each end and then close it and tighten a squeezing, locking mechanism. This locking mechanism squeezes the fiber into the dead center automatically and can often get you back in service.
These manual splicing clips do not normally give you 100% signal perfection, although the laws of chance sometimes makes it happen. However, they can get you back in service in a hurry until the van or trailer with the fancy splicing machine can get there. to do the permanent solution. The bonus is that anyone with 30 minutes of training can put these things on in a few minutes.
The real challenge is that in telco fiber, you are not talking about just one glass core. The cable is filled with 20, 50, 100, or possibly hundreds of smaller, individual fibers. In a large fiber cut, these all have to be sorted out so that you connect each side back together with it's proper match.
I wouldn't be to concerned if you heard about 'properly' spliced fiber in your telco plant. This is normal. A really good scope can spot a well done splice in a line. However, if it's done well enough, you might be very challenged to see it on a scope. Of course, a poorly done splice will be much more obvious to find.
As for fiber for home electronics, I know little about them but some reading tells me that older, poor quality cables with plastic cores can present issues over longer distances and sharper bends. A cable with a newer glass core is preferable.
If either are working in good conditions, both will be 100% effective and certainly won't sound different than the other as they are just passing ones and zeros. However, the plastic or otherwise inferior one is more likely to have stress related issues from distance or bending much quicker than the glass.
As stated. I'm not all that familiar with fiber interconnects for the home but going by the logic of the Telco manual splicing devices, I would have to say that there is an obvious logic that one long, well made fiber will be superior to two or more smaller fibers connected together.
I think that the common distance limitation for the TOSLink standard for optical home electronic connections is 5 meters. Since this distance in itself is not very far, adding any sort of interconnect between two TOSLink cables, you risk reducing this length or even making them unusable.
