Issue link: https://www.e-digitaleditions.com/i/1386104
July/August/September 2021 I 21 REFLECTANCE MATTERS NOW TOO Insertion loss, which is the amount of energy that a signal loses as it travels along a cable link and the loss caused by any connection point along the way, has long been the primary performance factor for optical fiber applications. Industry standards specify the amount of insertion loss allowed for fiber applications to function properly, and data center designers determine their fiber loss budgets based on distances between the functional areas and number of connection points along the way to ensure that they stay within these requirements. As data centers expand, migrate to higher spaces, and the use of cross-connects for flexibility in highly virtu- alized environments becomes commonplace, insertion loss remains a critical performance factor. For example, changes like adding extra connections into a channel by implementing a cross-connect for convenient patching and rapid expansion can add loss into the channel. Elim- inating an intermediate switch to reduce latency by joining two permanent links together into one longer link can also adversely impact insertion loss and the ability to support higher-speed applications. Furthermore, the more reconfigurations occur in the data center, the greater the chance for dirty fiber endfaces that also increase insertion loss. Reflectance, which measures the amount of reflection created by a reflective event (e.g., a connector) compared to the amount of light injected, has typically only been a performance parameter that connector manu- facturers needed to worry about to comply with industry component standards. However, new single- mode applications being deployed in the data center now mean that reflectance has also become an important testing consideration. At a fiber connector, there is always a small air gap between the two fibers, and the difference in refractive index between the glass and the air causes light to be reflected back. While multimode transceivers are extremely tolerant of reflection, singlemode trans- ceivers are not. In fact, with high power singlemode lasers, too much reflection can actually destroy the trans- ceiver. As data centers undergo transformation, new short-reach singlemode applications like 100GBASE-DR, 200GBASE-DR4, and 400GBASE-DR4 are gaining ground. While these applications enable the use of lower cost, low-power singlemode transceivers, these transceivers are more susceptible to reflectance. For these applications, industry standards actually now specify insertion loss limits based on the number and reflectance of connections in the channel. As shown in Table 2, in a 100GBASE-DR4 application with four connectors that have a reflectance between -45 and -55 dB, the insertion loss is 3.0 dB (highlighted in red). With four connectors that have a reflectance between -35 and -45 dB, the insertion loss goes down to 2.7 dB (highlighted in yellow). TABLE 2: Short-reach singlemode maximum insertion loss is based on the number and reflectance of connections. 100GBASE-DR Maximum channel insertion loss (dB) Number of connections where the reflectance is between -45 and -55 dB Number of connections where the reflectance is between -35 and -45 dB