As it turns out, the planar splitters we bought use bend‑insensitive fiber — G.657 standard. Based on what I could see on the splicer screen, the input fiber looks like G.657.B, while the output fibers look like G.657.A. That's not a definitive identification, of course — the splitter documentation doesn't specify the fiber types, only the insertion loss and return loss measurements. The "B" type fiber is recognizable on screen by a clear gradient from the core to the cladding — sharp transitions that are easy to see. This is due to a smaller mode field diameter and special doping. Standard fiber, by contrast, has a core that's almost all one color, with barely visible stripes running along it. The "A" type fiber looks more like G.652 — the stripes at the edges of the core are a bit more noticeable, but the mode field diameter matches that of standard fiber. So splicing standard single‑mode fiber to "bend‑insensitive" G.657.A shouldn't cause any problems. And that's exactly what we saw in practice. The only thing is, because of the different materials, you can see a thin vertical line at the splice point. The splicer estimates loss about the same as for identical fibers, and in most cases we got splice losses around 0.01‑0.03 dB.
When splicing standard fiber to "bend‑insensitive type B," though, we ran into trouble. The splicer identifies G.657.B as non‑zero dispersion‑shifted fiber (NZDSF — it shows up as "NZ" on the screen). That said, the actual splice process looked pretty clean — the arc was stable, and any bubbles we got were mostly due to poor cleaves, which happens with standard fiber too. What bothered me was that no matter how good the splice looked, the splicer always threw up some kind of error at the end. And of course, you could easily see the splice point on the screen. So I spent the evening browsing forums to see how others were handling this. In the end, I settled on using the "SM Auto" mode (standard single‑mode with automatic calibration) and evaluating splice quality visually — looking for a clean, stable arc with no spots or flashes, and ignoring the splicer's loss estimate altogether. In theory, you could measure the loss with an OTDR, but as I mentioned earlier, we're splicing the input fiber to the splitter — which is less than a meter long — so the splice wouldn't even show up on the trace; it'd be buried in the OTDR's dead zone and reflections. That leaves only one option: measuring the optical signal level before and after the splice. But unfortunately, for various reasons, that's not always practical.
While searching for information, I came across an article that reported splice losses for these fiber types. On average, they were around 0.6 dB — which is quite high compared to the 0.1 dB maximum typically allowed for a splice. I'm planning to run some tests with actual signal level measurements in the next few days, and I'll definitely post the results.
This article is a translation of the original Russian-language post.My journey of learning GPON