Single-mode transceivers now priced for high-volume data center use
Parallel Single Mode 4-channel (PSM4) is a type of single-mode transceiver that uses a parallel fiber design for reaches from up to 2 km and for reaches beyond the limits of 100-meter Short Reach 4-channel (SR4) multi-mode transceivers. PSM4s will be the transceiver that enables single-mode fiber to become popular in next-generation data centers due to its low cost and high configurability.
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.
100Gb/s PSM4 Transceiver
The PSM4 transceiver is the lowest cost, 100Gb/s transceiver on the market capable of using single-mode fiber for long reaches up to 2km. 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 CWDM4, using two fibers, becomes more economical.
What Data Centers are 2 km or 1.2 Miles Long?
While most data centers are not 2km (1.2 miles) long, the 2km spec is another way of stating the optical power of the laser. Measured in powers of ten called dBs (Decibels), the Mellanox PSM4 offers ~3.3 dBs of optical power which is enough to push through hundreds of meters of a lossy fiber infrastructure consisting of dirty 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? Huh?
Interestingly, the multi-mode (large core 50-um diameter) fibers are more expensive than single-mode (9-um 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 9-um single-mode fiber is very hard to build and align transceiver components with 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 high reject rate.
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 PSM4s using its internally developed Silicon Photonics technologies located in southern California which has been building and shipping Silicon Photonics products used as transceivers and Variable Optical Attenuators (VOAs) for nearly a decade
Mellanox Designed PSM4 Silicon Photonics and Control ASICs
Numerous PSM4 Applications and Configurations for Any Need.
The PSM4 has many different configuration application uses. It can bus 100Gb/s point-to-point over 2km or can be broken out into dual 50Gb/s or quad 25Gb/s links for linking to servers, storage and other subsystems. Additionally, the breakouts can be made using passive fiber splitter cables or a transceiver/AOC hybrid called a, “pigtail”. The following diagram illustrates the Mellanox, “end-to-end” system solutions consisting of switches and network adapters with cables and transceivers.
25G, 50G, 100G PSM4 Transceiver Applications
PSM4 Breakouts to Servers & Storage
Beside long reach 2km point-to-point links, PSM4 channels can also be split out individually. The diagram below shows a 100G PSM4 transceiver split using a passive breakout splitter cable with an MPO on one end and either dual MPOs (50G) or quad LC connectors (25G) on the other ends. CWDM4 cannot do this feature and can only bus 100Gb/s point-to-point.
Passive Fiber Breakout Configurations
PSM4 “Pigtail” Transceiver
Transceivers have their fibers attached to a detachable optical connector (MPO or LC). Active Optical Cables (AOCs) are two transceivers with the fibers permanently attached inside and not removable. A “Pigtail” (shown above) is a hybrid of both transceivers and AOCs and has the fibers attached to the transceivers with a short 1 meter length of fiber and one of three connector configurations:
While it only has 1 meter of fiber, it can reach 500 meters in the fiber infrastructure. Pigtails are used to link Top-of-Rack switches to 25Gb/s or 50Gb/s servers and storage subsystems or to plug into passive optical distribution patch panels that connect to other parts of the data center.
Bottom line, the Pigtail saves the material cost and maintenance for one optical connector as well as about 1 dB of optical loss gained back; small but when big data center builder orders tens of thousands, it all adds up.
Not All MPO Connector 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) for 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 aims it back at the laser which can destroy it. 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 50-um core and the large area disperses the back reflections making it less of a problem. In reality, the MPO connectors use 12 fibers with four unused.
MPO Optical Connectors
Flat and Angle Polished Fibers
Mellanox PSM4-1550nm Interoperates with Most Industry PSM4s
Most PSM4 transceivers use a PIN detector that has a wide bandwidth spanning both 1310nm and 1550nm. The Mellanox 1550nm PSM4 can talk to almost any 1310nm PSM4 transceiver and the reverse even though the wavelengths are different. Many of our customers have interoperate tested more than ten different suppliers without any issues.
At the Optical Fiber Conferences (OFC), Mellanox demonstrated our 100Gb/s 1550nm PSM4 interoperating with PSM4 transceivers from Innolight and AOI and in breakout configurations at 25Gb/s with the LR transceivers with Oclaro, Hisense and Ligent.
Interoperability Demo 1310nm & 1550nm PSM4s
The PSM4 will make another debut at 200Gb/s using the QSFP28 form-factors in late 2017 supporting 200Gb/s HDR for InfiniBand and also in a 1:2 splitter configuration split into two 100Gb/s HDR100 QSFP28.
The 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 100Gb/s point-to-point or be split out into individual channels combinations of 25Gb/s or 50Gb/s to servers, storage and other subsystems.
While 100G PSM4s are fairly new to the market, as the popularity climbs and volume manufacturing efficiencies kick in, the PSM4 has a chance at challenging the 100G 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.
LinkX is the Mellanox trademark and name for its cables and transceivers product line