Optical Connectors

Several optical connector types are used to link optical fibers together which are inserted into transceiver ends. A fiber cable consists of multiple optical fibers and two optical connectors for a straight cable and up to 5 optical connectors for 4x splitter cables.

Optical connector types typical in high-speed data centers are:

  • MPO-12/APC

  • LC duplex

  • MPO-12/UPC

  • MPO-16/APC

8-fiber
2-fiber
8-fiber
16-fiber

50G-PAM4, 100G-PAM4
25G-NRZ, 50G-PAM4
25G-NRZ, 50G-PAM4
50G-PAM4

The Multiple-Push-On (MPO) optical connector is a ceramic block with holes that contain the ends of multiple optical fibers epoxied in as either single-mode or multimode types. The ceramic blocks are made with different numbers of holes 8, 12, 16, 24, etc. but data centers typically use the 8-fiber but labeled MPO-12 with 4 unused, or 16-fiber MPO-16 versions.

Some of the light sent into a fiber reflects backwards from the fiber end face. Slower speed electronics and optics are less sensitive to back reflection created inside the optical fiber. Hence, 25G-NRZ (4x25G-NRZ SR4) and some 50G-PAM4 transceivers (4x50G-PAM4 SR4) use the MPO-12/UPC or Ultra-flat Polished Connector.

8x 50G-PAM4 400G and all 100G-PAM4 transceivers are more sensitive to back reflections and use the MPO-12/APC or Angled Polished Connector. This has an 8-degree polish on the end that causes the back reflections to be diverted into the fiber side cladding and away from the transmitter.

MPO/UPC (flat) cannot be mated together with MPO/APC (angled) connectors.

The 2-fiber Lucent Connector or LC duplex is typically used for a single-channel links using a transmit fiber and a receive fiber. This applies to multimode and single mode optics.

Running parallel fibers over long reaches becomes expensive so transceivers use multiple lasers with different wavelengths (colors) which are combined or “multiplexed” into a single fiber for transmission and filtered and separated back out at the receiver. One fiber for transmission and one for receiving but carrying 4 or 8 channels.

The LC duplex optical connector is used for single-channel multimode SR transceivers and using single-mode optics, the 1-channel, LR transceiver, and multiplexed transceivers such as 100G DR1, 200G FR4, 400G FR4, 2xFR4, LR4 and ER transceivers. These are used to link InfiniBand and Spectrum Ethernet switches together and linking clusters across campuses by using only two fibers in each optical connector to save on fiber costs over long runs

The 800G twin-port OSFP 2xDR4 transceivers use two MPO-12/APC optical connectors. The 2xFR4 transceiver uses two LC optical connectors.

MPO-12/APC, MPO-12/UPC and duplex LC optical connectors

MPO-12/APC

MPO-12/UPC

Duplex LC

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