5G rollouts are on the horizon, with major telecom companies set to rollout limited network access in the US and Europe. Most folks pay attention to the wireless requirements in these networks, but local antennas will still need to be connected to the telephone network and the Internet with high bandwidth optical fibers or wireless backhaul connections.
All this requires fiber optic transceivers to support fiber networking equipment. Choosing the right transceiver for fiber networks depends on multiple factors, although in 5G the principal factors to consider are bandwidth, data rate, conversion loss, and fiber type. Before you can choose the correct fiber transceiver, the first step is to determine what type of fiber the network is using, or what type of fiber cable the application will require to achieve optimal speed and bandwidth.
Which Type of Fiber are You Using?
There are two main types of fiber cable, each of which is appropriate for different applications and will require different transceivers:
Multimode Fiber (MMF): this type of fiber can be used to transmit multiple channels simultaneously. Greater mode density leads to greater modal dispersion that accumulates over the distance of the fiber, thus these fibers are best used for short-run links, such as in MAN and LAN networks.
Single-mode Fiber (SMF): This fiber is designed for longer distances and will provide faster data transmission rates in a single channel with the correct transceivers. These fibers are often bundled in a single cable for massive data transmission over long distances.
Within SMF and MMF classes of fiber, there are different fiber types that provide different data rates and are rated for use over different distances under TIA/EIA standards for fiber optics. Your optical power budget will also determine the limit transceiver you can use for a given link length, and your output on the transmitting side may need to increase the output from your transmitting transceiver to compensate losses in a link.
Clearly, there are several important systems design points to consider, but the first important points to consider in a real network are link length and required data rate. Newer portions of fiber to support upcoming 5G rollouts require multi-Gbps data transmission over long distances to support connections between base stations and cell towers, and to provide fiber-to-the-home and fiber-to-the-premises.
Some municipalities are already installing dark fiber that is capable of up to 40 or 100 Gbps, and networking equipment to support these dark fiber networks will need to include transceivers to support these data rates. Ideal link lengths can range anywhere from hundreds of meters (MMF will be used here) up to be dozens of kilometers (SMF will be used here) in order to support existing cellular infrastructure. If you’re working with SMF fiber over long distances, expect to drop bundles of fiber and deploy scalable networking equipment that includes swappable transceivers with standard form factors. QSFP+ or CFP will be the dominant form factors, especially CFP as it already supports 40 and 100 Gbps systems.
The Finisar FTL4C1QM1C fiber optic transceiver has QSFP+ form factor that supports 39.8 to 44.6 Gbps data rates with low power dissipation (<3.5 W). This transceiver is hot-swappable and supports up to 10 km links over SMF. This transceiver also provides a number of built-in digital diagnostic functions, including transmit and receiver power monitoring.
The Finisar FTLC9558REPM fiber optic transceiver is one option for 100 m links over at 103.1 Gbps over MMF. Just like the previous product, this transceiver module is hot-swappable and runs at low power (<2.5 W). Data is transferred in 4 lanes at 25 Gbps with a VCSEL-based transmitter at 850 nm, while the receive side operates with a 4x25G electrical interface over I2C:
They are compliant with the QSFP28 MSA and IEEE 802.3bm 100GBASE-SR4 and CAUI-4. Digital diagnostics functions are available via the I2C interface, as specified by the QSFP28 MSA and Finisar Application Note AN-2141. The optical transceiver is compliant per the RoHS Directive 2011/65/EU. See Finisar Application Note AN-2038 for more details.
The Avago AFBR-79EQDZ 40 Gbps transceiver can be used in up to 100 m links with OM3 MMF, or in 150 m links using OM4 MMF (both fiber types operate at 850 nm). Note that each lane operates at 10.3125 Gbps. It also supports 10GBase-SR modules in compliance with the IEEE 802.3ae standard, as long as the 10G receiver can sustain 2.4 dBm maximum input optical power. The optical interface on the transmit and receive sides both use standard optics for high speed fiber:
The optical transmitter portion…incorporates a 4-channel VCSEL (Vertical Cavity Surface Emitting Laser) array, a 4-channel input buffer and laser driver, diagnostic monitors, control and bias blocks. The optical receiver portion…incorporates a 4-channel PIN photodiode array, a 4-channel TIA array, a 4 channel output buffer, diagnostic monitors, and control and bias blocks.
Note that, in some cases, you can get away with using an SMF with a fiber optic transceiver designed for MMF as the core in an SMF fiber is about 20% the value required in the receiver. This provides easy coupling and the fiber will be insensitive to alignment, but this is not recommended and many not work over longer distances. In the ideal case, you should choose a transceiver that will support the data rates and fiber type you are using in your particular application.
Telecommunications systems aren’t the only application where fiber will see greater use. The insensitivity of fiber to EMI and ESD, as well as the low weight of fiber compared to copper, makes fiber ideal for use in aerospace applications and other environments where noise is a problem. If you’re looking for a fiber transceiver for your next telecommunications system or other specialized application, you can find the components you need on fiber-mart.com.