Optical Reach Guide

Reach figures — 10km, 20km, 40km, 80km, 120km — are the most quoted numbers in optical networking, and the most misunderstood. A reach figure states the distance a module is designed to cover under stated conditions, over a stated fiber type, with a stated loss budget. It is not a guarantee for every installation at that distance, and it is not a property of the module alone. A link achieves its reach when the whole span — the fiber, the splices, the connectors and the module — stays within the loss budget. This guide explains what a reach figure means and what else a long link depends on.

The short end of the range is straightforward. Links up to a few hundred metres run over multimode fiber at 850nm, and the published SFP+ SR model states a 300m reach in that pattern. Links from a few hundred metres to a few kilometres run over single-mode fiber, and the published 1.25G SFP series lists a 2km multimode combination and 20km and 40km single-mode combinations at 1310nm, stated at series level. Those are campus and metro distances: between buildings, between sites in a city, and across a large site.

The middle of the range — 10km and 20km — is where most single-mode links sit. The published QSFP28 LR4 model states 10km and 20km reach options, and the published XFP LR model states a 20km reach, both at 1310nm over single-mode fiber. Those distances cover campus backbones, metro links and interconnections between buildings across a city. They are also the distances where the loss budget starts to matter: a 20km link with many splices and connectors can exceed its budget even though the distance alone looks comfortable.

The long end — 40km, 80km and 120km — is where planning becomes essential. The published 1.25G SFP series lists 1550nm combinations at 80km, 100km, 120km and 160km, stated at series level, and those distances are only achievable over single-mode fiber at 1550nm, where fiber loss is lowest. A long link depends on the fiber grade, the number of splices and connectors in the path, and the loss of each, not just the distance. A buyer planning a long link should state the actual span, the fiber type and the splice count during the inquiry, so the right combination can be confirmed for that specific path.

A final caution applies to every reach figure on this site. A figure stated for one model belongs to that model alone, and a figure stated at series level belongs to the series range, not to every model in it. This site does not transfer a reach figure from one model to another, even within a series, because the published material does not support that transfer. Before ordering, confirm quantity, price and delivery time during the inquiry, together with the span and the fiber plant your link will use. Certificates are available upon request.