Data Centers Rely on Optical Fiber Connector Performance

Qing Xu

An important part of new fiber infrastructure involves optical fiber connectors. By mechanically coupling and aligning fiber cores so light can pass through, optical fiber connectors can impact insertion loss and return loss levels, installation time and overall fiber system costs.

In fiber optic systems, LC, SC and MPO optical fiber connectors are often used.

  • SC (square connector) connectors have a push-pull coupling end face with a spring-loaded ceramic ferrule, and is ideal in data center applications.
  • LC (Lucent connector) connectors – also push-pull connectors – came along after SC connectors, and feature a smaller ferrule (for this reason, it’s known as a “small form-factor connector”). Its smaller size makes it ideal for dense data center racks and panels.
  • MPO connectors are used for ribbon cables with anywhere from eight to 24 fibers.

Optical Fiber Connectors’ Impact on Insertion Loss

Channel insertion loss in a fiber link depends on the length of the fiber and the number of connections. The initial connection to the most commonly used SFP and QSFP optical transceiver modules, as shown below, isn’t considered part of the channel link loss.



Transceiver module connections


The multimode fiber (MMF) attenuation parameter used to calculate the channel insertion loss budget is 3.5 dB/km. Currently, OM3/OM4/OM5 fiber cable specifications in ANSI/TIA-568.3-D have already been ratified to 3.0 dB/km, which allows additional headroom to the total link budget.


In MMF links, optical fiber connector loss usually dominates channel insertion loss, especially in short-reach applications where the fiber attenuation is very small. Many MMF connections installed in data centers are factory pre-terminated and professionally polished.


LC Duplex is an optical fiber connector with a pair of MMF or singlemode fiber (SMF) that transmits the data in two opposite directions, one in each fiber. It can be used for single-lane transceivers, such as 10G or 25G SFP, or used for wavelength multiplexing, such as CWDM4, SWDM4 or BiDi, in the same fiber pair.


MPO arrayed connectors support parallel fiber connectivity for MMF and SMF types. A typical use case for MMF MPO-12 (eight fibers) is the 40GBASE-SR4 and 100GBASE-SR4 transceiver, with four fibers as Tx (transmit) and the other four fibers as Rx (receive).  


High-fiber-count MPO-16, MPO-24 and MPO-32 connectors have also been developed to support permanent trunk cabling and transceivers like 100GBASE-SR10 (20 fibers) and 400GBASE-SR16 (32 fibers).


Such configuration has also been adopted for parallel-singlemode (PSM) transceivers, which typically support a 500 m reach in data centers. MPO trunk cables are also used as horizontal permanent links in data centers for fiber aggregation.


Historically, in an SMF system, insertion loss is considerably higher because links are designed for much longer reach; the attenuation in SMF is much lower than in MMF. Due to a much smaller fiber core size, insertion loss of each SMF connection is considerably higher than insertion loss of the MMF connection.



LC Duplex (single lane or wavelength multiplexing) and MPO-12, MPO-16, MPO-24 and MPO-32 (parallel fiber lane) optical fiber connectors


We use the overfilled launch condition model to compare connection insertion loss between MMF and SMF. Assuming that the light is evenly distributed in the fiber core, with a diameter of 2R, and the core-to-core vertical misalignment is Δr, then the connection insertion loss will be:


Connection Insertion Loss = 1 - (2α - sin2α) / π, with α = cos-1(Δr/2R).


As shown below, singlemode optical fiber connector performance is very prone to vertical misalignment. For instance, 1 µm vertical misalignment in an MMF connection (core size of 50 µm diameter) could only cause 2.5% loss (0.11 dB), while it could cause more than 14% loss (0.66 dB) in an SMF connection.


Optical fiber connector loss vs. fiber core vertical misalignment


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