Introduction to 100G PSM4 Transceiver

Version 5

    Parallel Single Mode 4-channel (PSM4) is a type of single-mode transceiver that uses a parallel fiber design for reaches up to 2 km. For reaches up to 100-meter Short Reach 4-channel (SR4) multi-mode transceivers are used. PSM4s is the transceiver that enables single-mode fiber to become popular in next-generation data centers due to its low cost.





    PSM4 is built using one laser (instead of four), split into four paths or channels and separately modulated with electrical data signals. Each channel has its own fibers and is separated throughout the link. PSM4 uses eight-fibers with four-fiber for transmission and four-fibers for receiving. A parallel, eight fiber Multiple Push On (MPO) optical connector is used.




    The PSM4 transceiver is the lowest cost, 100 Gb/s transceiver on the market capable of using single-mode fiber for long reaches up to 2 km. The best use case is at reaches less than 500 meters. When tallying up the cost of longer reaches, the cost of the eight fibers adds up for each meter and the coarse wavelength division multiplexing (CWDM4), using two fibers only, becomes more economical.


    What Data Centers are 2 km (1.2 mi) Long?

    While most data centers are not 2 km (1.2 mi) long, the 2 km spec is another way of stating the optical power of the laser. Measured in powers of ten called dBm (decibels relative to 1 mW), the Mellanox PSM4 offers up to 2.0 dBm of optical power, which is enough to push through hundreds of meters of a dispersive fiber infrastructure consisting of dusty and/or misaligned optical connectors, jumpers, optical patch panels and other interferences to the light path. This is similar to needing a very powerful flash light to shine through a dense forest of twigs, branches and leaves in the way even though the distance is relatively short.


    Single Mode Fibers are Cheap but Transceivers Expensive – Reverse for Multi-Mode

    Interestingly, the multi-mode (large core 50ºµm diameter) fibers are more expensive than single-mode (9ºµm tiny core diameter) fibers but the transceivers are the reverse!  Single-mode fiber is used by the telecom industry and ordered in hundreds of thousands of miles per year – and so it is inexpensive. Multi-mode fiber is used exclusively in data centers and the amount made is relatively small so it is about three times more expensive.

    On the other hand, the multi-mode core diameter is large and easy to align with VCSEL lasers and detectors. The 9ºµm single-mode fiber is very hard to build and align to transceiver components and requires very expensive alignment equipment. Therefore, single-mode transceivers have always been significantly more expensive than multi-mode transceivers. Multi-mode transceivers are less expensive than single-mode transceivers and exactly the reverse for the fibers.

    Typically, single-mode transceivers use 10-20 different tiny parts that all need to be mechanically aligned to sub-micron tolerances. This requires a lot of manual labor, expensive test and alignment equipment and results in a low production yield.


    Silicon Photonics – Solves the Manufacturing Problems

    Silicon Photonics does away with most of these problems and integrates the optical components and waveguides into a silicon wafer – the same basic technology used to build CMOS semiconductor electronic chips. This is how single-mode transceivers will eventually become more price competitive with multi-mode transceivers over time. Mellanox builds PSM4 transceivers using its internally developed Silicon Photonics technology.


    Mellanox Designed PSM4 Silicon Photonics and control ASICs:






    PSM4 Solutions


    Numerous PSM4 Applications and Configurations for Any Need

    The PSM4 has many different configuration application uses. It can transfer 100 Gb/s point-to-point over 2 km or can be broken out into dual 50 Gb/s or quad 25 Gb/s links for connection to servers, storage and other subsystems. Additionally, the breakouts can be made using passive fiber breakout cables (splitter cables). The following diagram illustrates the Mellanox end-to-end system solutions consisting of switches and network adapters with cables and transceivers.





    PSM4 Breakouts to Servers & Storage

    Beside long reach 2 km point-to-point links, PSM4 channels can also be split out individually. The figure below shows a 100 Gb/s PSM4 transceiver split using a passive breakout cable with an MPO on one end and either dual MPOs (50 Gb/s) or quad LC connectors (25 Gb/s) on the other ends. CWDM4 does not support this feature and can only connect 100 Gb/s point-to-point.






    Not All MPO Connectors are the Same!

    One thing to note: the MPO used in the SR4 multi-mode MPO (colored aqua) is not the same as the MPO/APC (colored green or yellow) in the PSM4. Optical connectors pass through “most” of the light and some gets reflected back towards the laser from the inside surface of the fiber end in the connector. The PSM4 uses tiny core single-mode fiber and it concentrates any back reflections in the connector infrastructure and reflects it back to the laser degrading its performance. So, the single-mode fiber infrastructure polishes the fiber ends at an angle to divert the back reflections away from the laser. Hence, the name Angle Polished Connector (APC). Multi-mode fiber has a big 50ºµm core and the large area disperses the back reflections eliminating the problem.





    Mellanox PSM4-1550 nm Interoperates with Most Industry PSM4s

    Most PSM4 transceivers use a photodetector that has a wide bandwidth spanning both 1310 nm and 1550 nm. The Mellanox 1550 nm PSM4 can operate with most 1310 nm PSM4 transceivers even though the wavelengths are different. Many of our customers have tested more than ten different suppliers without any issue.

    At the Optical Fiber Conferences (OFC), Mellanox demonstrated its 100 Gb/s 1550 nm PSM4 interoperating with PSM4 transceivers from Innolight and AOI and in breakout configurations at 25 Gb/s with the LR transceivers with Oclaro, Hisense and Ligent.


    Interoperability Demo 1310 nm & 1550 nm PSM4s:




    The Mellanox PSM4 transceiver is the lowest-cost single-mode transceiver available today for use in next-generation data centers as it employs the low-cost and long reach features of single-mode fiber. It is a very flexible transceiver that can link 100 Gb/s point-to-point or be split out into individual channels combinations of 25  Gb/s or 50 Gb/s to servers, storage and other subsystems.

    While 100 Gb/s PSM4s are fairly new to the market, as the popularity climbs and volume manufacturing efficiencies kick in, the PSM4 has a chance of challenging the 100 Gb/s SR4 multi-mode transceiver in similar market prices when the transceiver and fiber link costs are added up. PSM4s will be the transceivers that enables single-mode fiber to become popular in next-generation data centers.