A Guide to the Interfaces of Optical Transceiver Modules

In today's optical communications market, there are a variety of transceiver modules with various types of interfaces. Because different types of cables/connectors/adapters are required for different interfaces, we need to pay more attention when selecting the relevant assemblies. This article will give you a detailed introduction to the mainstream transceiver module interfaces on the market, so that everyone has a clearer understanding of the transceiver modules.

First of all, we use the following table to list all transceiver modules' interfaces.

Form Factor	Transmission Mode	Interface	Example
QSFP-DD	Parallel	MPO	200G QSFP-DD SR8/PSM8
QSFP-DD	Multiplexing	Dual CS	200G QSFP-DD CWDM8
QSFP28	Parallel	MPO	100G QSFP28 SR4/PSM4
QSFP28	Multiplexing	Duplex LC	100G QSFP-DD LR4/CLR4/CWDM4/ER4
QSFP+	Parallel	MPO	40G QSFP+ SR4/PSM4
QSFP+	Multiplexing	Duplex LC	40G QSFP+ LR4
SFP28	Dual Fiber	Duplex LC	25G SFP28 SR/LR
SFP28	Single Fiber Bidirectional	Simplex LC	25G SFP28 BiDi
SFP+	Dual Fiber	Duplex LC	SFP+ 10GBASE-SR/LR
SFP+	Single Fiber Bidirectional	Simplex LC/SC	SFP+ BiDi
SFP+	2-channel Bidirectional	Dual LC	SFP+ 2-channel BiDi
SFP+	Electrical Copper Cable	RJ-45	SFP+ 10GBASE-T
SFP	Dual Fiber	Duplex LC	SFP 1000BASE-SX/LX
SFP	Single Fiber Bidirectional	Simplex LC/SC	SFP BiDi
SFP	2-channel Bidirectional	Dual LC	SFP 2-channel BiDi (CSFP)
SFP	Electrical Copper Cable	RJ-45	SFP 1000BASE-T
CXP	Parallel	MPO	120G CXP SR10
CFP	Parallel	MPO	100G CFP SR10
CFP	Multiplexing	Duplex LC	100G CFP LR4/ER4
CFP2	Parallel	MPO	100G CFP SR10
CFP2	Multiplexing	Duplex LC	100G CFP2 LR4/ER4
CFP4	Parallel	MPO	100G CFP4 SR4
CFP4	Multiplexing	Duplex LC	100G CFP4 LR4/ER4

As the table shows, although there are more than a dozen types of transceiver modules, there are only a few types of interfaces. These types of optical interfaces are LC, SC, MPO, and CS. And there are also electrical copper transceiver modules using the RJ-45 interface. Among these interfaces, the LC interface can be divided into duplex and simplex, and there are dual-simplex LC interface (such as CSFP). For BiDi optical transceivers, there are also simplex SC interface, in addition to the simplex LC. We will introduce each of these interfaces one by one, according to the transmission modes of the transceiver modules.

LC/SC

As we konw, a transceiver module consist of a transmiter and a receiver. This means that the transmission has two directions. For the common single-channel optical transceivers, such as SFP28, SFP+, and SFP, the transmitting terminal is connected to one optical fiber and the receiving terminal is also connected to one optical fiber. That's why the common optical transceivers are called dual-fiber transceiver generally. The dual-fiber transceiver has a duplex LC interface connected to a duplex LC patch cable. (The XENPAK, X2, and GBIC dual-fiber transceivers, not listed in the table, have a duplex SC interface connected to a duplex SC patch cable.)

The single-fiber bidirectional transmission mode is called BiDi for short. The BiDi signals in both directions are combined in a single fiber. The bidirectional transmission means that the light is directional and will not interference each other. The BiDi optical transceiver, such as BiDi SFP+ and BiDi SFP, have a simplex LC or SC interface connected to a simplex LC or SC patch cable. And for high-density BiDi transmission networks, there are 2-channel BiDi SFP+/SFP (CSFP+/CSFP) optical transceivers using dual simplex LC interface.

MPO

For multi-channel optical transceivers, such as 4-channel QSFP+, 4-channel QSFP28, and 8-channel QSFP-DD, there are several Tx and several Rx. Some of them (such as 100G QSFP28 SR4 and 100G QSFP28 PSM4) have MPO interfaces, that is, multi-fiber pull on/off, using multiple optical fibers for the parallel transmission shown as the figure below.

There are also dual-fiber 4-channel optical transceivers using the multiplexing transmission mode, that is, multiple Tx multiplexing and Rx demultiplexing. These optical transceivers, such as 40G QSFP+ LR4 and 100G QSFP28 CWDM4, use two optical fibers for long-distance transmission, saving more optical fiber resources than using multi-core optical fibers. Like the common single-channel optical transceiver, the dual-fiber 4-channel optical transceiver also has a duplex LC interface connected to a duplex LC patch cable.

CS

The QSFP-DD MSA specification defines an 8-channel module, cage and connector system. The cage and connector system provides backward compatibility to the 4-channel QSFP28 modules. Doubling the number of duplex optical links with the QSFP-DD specification requires a new smaller optical interconnect to fit in the same physical cage form factor. For the eight-channel QSFP-DD optical transceivers using the multiplexing transmission mode, a new type of optical interface called dual CS is used to replace the duplex LC. The dual CS interface is connected to the CS connector, a miniature single-position plug which is characterized by duo cylindrical, springloaded butting ferrule(s) of a 1.25 mm typical diameter, and a push-pull coupling mechanism. The CS connector provides the characteristics and simplicity of the duplex LC connector into a smaller footprint to allow 2 pairs of CS connectors to fit within the physical constraints of the QSFP-DD form factor.

RJ-45

The RJ-45 interface is used in copper transceiver modules, such as 10G copper SFP+, 1G copper SFP and 100M copper SFP. The copper SFP+ transceivers transmit electrical signals over Category 6a or Category 7 copper cables with RJ-45 connectors, while the copper SFP transceivers transmit electrical signals over Category 5 or Category 5e copper cables with RJ-45 connectors.

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