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How to Specify Optical Modules

Optical modules are key components in networking equipment, and specifying the right modules can heavily influence overall system performance. Here, Erin Byrne of TE offers tips on key considerations.

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Optical modules are key components in networking equipment, and specifying the right modules can heavily influence overall system performance. There are several issues to consider when choosing an optical module.

To start, the supplier and the customer should agree on a set of specifications that can be tested and verified. Surprisingly, system designers sometimes want to buy technology in a format for which performance can’t be verified. For example, an equipment maker may inquire about the performance of an optical engine or subassembly, but such a subassembly should have features that allow adequate optical testing to measure and meet performance specifications. Typically, full-spec performance is only fully measured at the module level.

For optical modules, the most important design considerations are density and form factor. You can buy transceivers that plug into the faceplate, or you can buy embedded, mid-board optical modules. You may want to choose a mid-board module if you want more density at the faceplate or for greater electrical performance because you’re able to put the module closer to the IC on the circuit board and minimize electrical losses.

Standard choices determine bit-rate options, and the choices range from the small form-factor pluggable (SFP) module at 1Gb/s up to the quad small-form-factor pluggable 28 (QSFP28) module at 100Gb/s. Some parallel optical modules have incoming signal rates of 25Gb/s, and there are mid-board modules that use 12 lanes of 25Gb/s to deliver 300Gb/s. You can also choose the quad small-form-factor pluggable plus (QSFP+) module with four channels of 10 Gigabits each, or the small-form-factor pluggable plus (SFP+) module as a single 10Gb/s lane.

Another consideration is how far you want the optical signal to travel. This leads to a decision between an Active Optical Cable (AOC) and a transceiver. An AOC is a single unit that consists of two transceivers and a piece of optical fiber that joins them. With a transceiver, you take a passive fiber cable and connect it to the transceiver. For distances less than 20 to 30 meters, an AOC is probably the less expensive choice. If you want the signal to go more than 30 meters, you’d more likely use the transceiver with a passive fiber cable.

If you want to be able to source multiple vendors, you probably need transceivers that comply with interoperability standards, such as the IEEE 802.3ba Ethernet standard for 40Gb/s interfaces. AOCs need to meet only electrical standards because the optical signals are self-contained. Thus, AOCs offer more flexibility in terms of technology and they often can be a better value than transceivers.

Heat transfer and power consumption are two other considerations. Every optical module generates heat, but some modules run considerably cooler than others. Engineers need to assess how much power is being consumed and how much heat is being generated, as well as whether the system has the capability to remove that heat. With cooler optical modules, the equipment saves direct power but also can have a substantial impact on reducing air conditioning costs for the data center.

Finally, designers shouldn’t ignore the electrical connector in an optical solution. An optical module takes an electrical signal and converts it to optical for transport around the board or between the customer’s racks. Designers should consider the availability and suitability of the electrical connection as part of the total channel solution. You should think about how much room the electrical component is going to occupy, as well as the quality of the interface in terms of signal integrity.

By considering form factor, density, reach, bit rate, standards compliance, heat transfer, and electrical performance, designers can properly evaluate optical modules and specify the right one for the job.

Erin Byrne serves as director, optics product development engineering for TE Connectivity in Harrisburg, Penn., where she leads a global team of engineering professionals developing high-speed optical interconnects for data center applications. Prior to joining TE, Byrne was involved in commercializing leading-edge optical components for the telecommunications, defense/security, and oil/gas industries. She began her career at AT&T Bell Labs and holds a Ph.D. in inorganic chemistry from Cornell University.

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