This is how I built my first ever product, an optical switchbox, at my first employer. It was part of an automated transceiver test solution alongside and BERT and an optical sampling scope (both singlemode and multimode). Amongst the things I did, I spliced, joined the fibers, assembled the whole system, and then tested the complete circuit and made sure that we had low insertion loss. The first box I built had some of my blood in it, and I personally designed the system in and delivered it to the system integrator myself. I can genuinely write a whole article about it.
Fusion splicing two fiber optic cables