Sep 29, 2022 — Traditionally, only UPC connectors have been used with multimode fiber cables for several reasons.

The lasers used to transmit signals over multimode fiber are more resilient to back reflections than those used in single mode fiber links. Usually, there are fewer connections in a multimode fiber link than a single mode fiber link due to the reduced reach and therefore, fewer sources of multipath interference. And multimode fiber links do not use fiber amplifiers or have unterminated connectors that may be present in some single mode fiber links.

In standards, and in the operations of the largest data center operators in the world, APC multimode connectivity is featuring more in the plan for 400G and beyond. Optical transceivers compliant to 400G-SR8 have an APC MPO16 interface. Most 200G-SR4 transceivers designed for hyperscale data centers mate with APC MPO8 connectors, and 400G-SR4 (IEEE 802.3db) will use the same. It is anticipated that 800G-SR8 (IEEE 802.3df) will also use an APC MPO16 interface.

Experimental results from CommScope’s labs can shed some light on why some data center operators want to use APC connectors in multimode fiber cables. APC connectors offer insurance against poor performance due to the presence of air gaps between connectors often found in problematic UPC systems. In addition, new high-speed multimode fiber applications are more sensitive to back reflection than those with slower speeds. Next these new applications use 4-level Pule Amplitude Modulation (PAM4) instead of the traditional Non-Return to Zero (NRZ) modulation. PAM4 modulation is more sensitive to intensity noise since the bit decision levels are much closer together, and light reflected back into a laser can introduce unwanted intensity noise to the transmitted signal.